Leflunomide Applicability in Rheumatoid Arthritis: Drug Delivery Challenges and Emerging Formulation Strategies

Abstract

Rheumatoid arthritis (RA) is a chronic systemic inflammatory disorder primarily targeting joints, leading to pain, swelling, and stiffness. RA results from the body’s own immune system attacking its own tissues. Currently, there are various treatments available for RA including disease-modifying antirheumatic drugs (DMARDs) and NSAIDs. Leflunomide (LEF) is a USFDA-approved synthetic DMARD which is being widely prescribed for the management of RA; however, it faces several challenges such as prolonged drug elimination, hepatotoxicity, and others. LEF exerts its therapeutic effects by inhibiting dihydroorotate dehydrogenase (DHODH), thereby suppressing pyrimidine synthesis and modulating immune responses. Emerging nanotechnology-based therapies help in encountering the current challenges faced in LEF delivery to RA patients. This review enlists the LEF’s pharmacokinetics, mechanism of action, and clinical efficacy in RA management. A comparative analysis with methotrexate, biologics, and other targeted therapies, highlighting its role in monotherapy and combination regimens and the safety concerns, including hepatotoxicity, gastrointestinal effects, and teratogenicity, is discussed alongside recommended monitoring strategies. Additionally, emerging trends in novel formulations and drug delivery approaches are explored to enhance efficacy and minimize adverse effects. Overall, LEF remains a perfect remedy for RA patients, specifically individuals contraindicated with drugs like methotrexate. The therapeutic applicability of LEF could be enhanced by developing more customized treatments and advanced drug delivery approaches.

1. Introduction

Rheumatoid arthritis (RA) is a chronic autoimmune indication in which autoantibodies are produced against immunoglobulin G (IgG) or rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPAs) in the body, causing serious damage to limbs joints [1]. A recent epidemiological estimate depicts that around 0.5 to 1% of the world’s population is affected by RA, while, in India, nearly 14–20% of the population is suffering from RA and associated disorders. The prevalence for RA is higher in developing nations and females are more prone to RA as compared to males [2]. A bilateral inflammation of joints manifests in initial symptoms, specifically in the small joints of fingers and toes. Proliferative synovitis in RA causes severe damage to cartilage and bone of the upper and lower limbs. RA is characterized by a range of symptoms, which also includes joint swelling, pain, stiffness, and disability in movement of the joints. Inflammation of the synovium causes the activation of macrophages and fibroblast to occur, and production of inflammatory cytokines, tumor necrosis factor (TNF-α), interleukin-1 (IL-1), interleukin-6 (IL-6), interleukin-17 (IL-17), macrophage colony-stimulating factor (M-CSF) was also observed. Inflammation leads to the activation of osteoclasts, which ultimately causes bone destruction [3]. Diagnostic techniques for RA include MRI, radiography, and hematological markers such as ACPAs, erythrocytes sedimentation rate (ESR), C-reactive protein (CRP), and RF. The pathogenesis involves autoimmunity, chronic inflammation, macrophage and cytokine production, metabolic syndrome, genetic causes, and environmental factors. A number of U.S. Food and Drug Administration (USFDA)-approved treatments are currently available for the management of RA. These treatments are composed of biological and synthetic disease-modifying anti-rheumatic drugs (DMARDs), and non-steroidal anti-inflammatory drugs (NSAIDS). NSAIDs are mostly used to relieve pain and inflammation in the mild-to-moderate RA patients. Synthetic DMARDs includes methotrexate, sulfasalazine, hydroxychloroquine, chloroquine, and leflunomide (LEF), while biological DMARDs consists of drugs that specifically target inflammatory mediators such as tocilizumab, adalimumab, rituximab, infliximab, and others. Due to the complex heterogeneity of this disease and various unclear pathogenic factors, it remains an unturned stone to find a particular therapeutic cure for RA.

Leflunomide (LEF) is a pyrimidine inhibitor, which was registered and first licensed in 1998 by the US FDA for the treatment of RA. Later, it was approved in Europe by the European medicines agency (EMA) in 1999 and afterwards in other countries. LEF is a synthetic DMARD derived from the isoxazole compound. An active metabolite known as teriflunomide (A77 1726) belongs to LEF and has a role in the reversible inhibition of enzyme dihydro-orotate dehydrogenase (DHODH). DHODH plays a key role in the de novo synthesis of pyrimidines. The proliferation and cell division of T lymphocytes depends upon the de novo synthesis of pyrimidines, which is blocked by the active metabolite teriflunomide. There are some serious concerns with LEF such as long half-life, hepatotoxicity, gradual, elimination, and risk of opportunistic infections due to effect on lymphocytes proliferation. These drawbacks of LEF call for researchers to find novel formulation and drug delivery approaches with lower and minimal adversity to the patients. Numerous strategies are being investigated to unravel potential options for safer delivery of LEF to RA patients, which could be novel drug delivery systems or other types of evolving techniques [4].

The current review covers the various challenges encountered in the delivery of LEF and aims to provide a broad overview of emerging formulation strategies for the safer delivery of LEF to RA patients. The clinical outcomes of ongoing research in newer formulation strategies for LEF are also discussed. A section dedicated to advanced drug delivery systems and topical and transdermal delivery of LEF is enlisted with a conclusion and future prospects.

2. Overview of Rheumatoid Arthritis (RA)

The chronic autoimmune illness known as RA is characterized by articular tissue damage, vague inflammation of peripheral joints, and joint abnormalities. Since RA does not affect a single area of the body, it can be referred to as systemic. RA is idiopathic, meaning that a combination of genetic predisposition, environmental factors, and chance appears to cause the illness to develop [5]. Joint erosion, symmetric synovitis, chronic polyarthritis, and the breakdown of cartilage and bone are the hallmarks of RA. Multisystem-extra-articular symptoms typically appear gradually over several months. 15–20% may experience acute or sudden symptoms [5,6].

2.1. Pathophysiology and Current Treatment Landscape

Various pathological factors are responsible for RA generation including environmental and genetic factors. RA begins with small peripheral joints and is generally symmetric; if left untreated, it can progress to proximal joints. Long term inflammation of joints will lead to deterioration of joint condition, thereby causing bone erosion and loss of cartilage [5,7]. Different risk factors are responsible for the generation of RA including epigenetic, reproductive, genetic, and comorbid host factors and neuroendocrine factors. The first step in the progression of RA is immune tolerance, when autoreactive B and T cells are activated. Over-activation of immune responses could also be due to other factors such as gut microbiota dysbiosis and various infections. Autoantibodies, RF, and ACPAs manifest in RA patients even before the appearance of clinical symptoms. Citrullinated proteins, which are produced during inflammation due to post-translational alterations, are the target of ACPAs. Antibody RF and ACPAs increase synovial inflammation by forming immunological complexes which activate molecular mechanisms responsible for causing inflammation. Figure 1 shows an illustrative summary of pathophysiological events involved in RA. A number of molecular pathways which are interconnected are involved in the development of RA (

Table 1. Various bioactive molecules playing a role in RA development.

Mechanistic Molecule/Enzymes	Role in RA Development	Reference
Cyclooxygenases 1 and 2 (COX 1 and 2)	Produce prostanoids involved in inflammatory and physiological processes	[11]
Tumor necrosis factor (TNF α and TNF- β)	Activation of macrophage, chondrocytes, endothelial cells, synovial fibroblasts, and osteophytes, resulting in cell division and elevated MMP and adhesion molecule overregulation	[12]
TNF-α converting enzyme (TACE)	Converts the membrane bound form of TNF-α into its soluble form	[13]
Interleukin-1	Pro-inflammatory initiator of various inflammatory factors like MMPs, eicosanoids, and inducible nitric oxide synthase	[14]
Interleukin-1β	Cartilage degradation and expression of pro-inflammatory chemokine receptors, thus facilitating the recruitment and retention of inflammatory cells in RA synovium	[14]
Interleukin-6	Production of autoantibodies by stimulating B-cell differentiation and activating auto-reactive T-cells and promotes bone reabsorption	[15]
Interleukin-8	Encourages the harmful stimulation of immune and stromal cells in the tendons, blood vessels, lungs, and synovial membrane of RA, resulting in extra-articular issues	[16]
Interleukin-10	Significant immunoregulatory element that controls the production of monocytes and, in certain situations, T cell cytokines in the cytokine network of RA	[17]
Interleukin-12	Enhances Th1 responses, raises inflammation, and encourages joint damages, which contributes to the advancement of RA	[18]
Interleukin-15	Onset of serious inflammatory arthritis	[19]
Interleukin-17	Stimulates neutrophil infiltration, cartilage destruction, and chronic inflammation	[16]
Interleukin-18	Induces RA synovial fibroblasts to emit chemokines, endothelial cell adhesion proteins are upregulated, and monocytes, lymphocytes, and neutrophils directly function as chemo-attractants to induce leukocyte extravasation.	[20]
Matrix metalloproteinases (MMPs)	Group of 25 zinc- and calcium-dependent proteinases, having a role in breaking down the extracellular matrix leading to bone and cartilage destruction	[21]
NOD-like receptor family pyrin domain-containing 3 (NLRP-3)	Stimulates the release of pro-inflammatory cytokines, which in turn causes synovial inflammation, joint damage, and autoimmune response.	[22]
TLRs (toll-like receptors)	Promotes the release of pro-inflammatory cytokines leading to a reduction in autoimmunity, increases joint degradation, and synovial inflammation.	[22]
Bruton’s tyrosine kinase (BTK)	Overexpression of BTK can lead to abnormal B cells proliferation	[23]
Spleen tyrosine kinase (SYK)	Synovial inflammation, immune cell signaling, and joint degeneration	[24]
Phosphatidylinositol 3-kinases (PI3K)	Upregulation of PI3K causes abnormal cell growth, overexpressed cell survival, and intracellular trafficking	[25]
Janus kinase (JAK)-Signal transducer and activator of transcription (STAT)	Inflammation, cytokine signaling, and immune cell stimulation	[26]
Phosphodiesterase-4 (PDE-4)	PDE-4 degrades cAMPs/cGMPs, which control the activity of multiple immune cells	[27]
Cytotoxic T-lymphocyte-associated antigen 4 (CTLA4)	Suppression of autoimmunity and excessive T cell proliferation	[28]

). All molecules involved in these pathways contribute to joint damage and inflammation. The pathogenesis of RA is unclear, since it is an idiopathic disease, meaning that several biological mechanisms are involved and eventually lead to chronic inflammation, metabolic syndrome, and autoimmunity [8]. A number of treatments are available for the management of RA, but they have not been proven fruitful due to various adverse effects and efficacy issues. The most commonly used medication for alleviating the symptoms of RA such as pain, inflammation, and stiffness are NSAIDS, which causes serious side effects on long-term use to the patient. Synthetic DMARDs including LEF, sulfasalazine, methotrexate, hydroxychloroquine, and chloroquine are being most frequently prescribed by rheumatologists for treatment of RA [9]. In recent years, biological DMARDs have been significantly employed for RA treatment because of their specific targeting on various molecular pathways. These include TNF Inhibitors, T cell co-stimulation Inhibitors, CD-20 depleting antibodies, IL-1 Inhibitors, IL-6 Inhibitors, and other types of biological DMARDs [10]. NSAIDs are commonly used for treatment but have significant adverse effects, necessitating the development of novel therapeutic strategies. Figure 2 shows the currently employed pharmacological treatments for the treatment of RA.

2.2. Rationale for Reviewing Leflunomide’s Utility

The assessment of LEF in current dosage form is required to deliver it more safely to patients with lower dosage, more efficacy, and minimum adverse effects. There is a lot of research on LEF conducted in combination with other drugs such as methotrexate, sulfasalazine, and others, but these combinations are associated with a number of adversities such as elevated hepatic transaminase enzymes, diarrhea, hypertension rash, nausea, and alopecia. However, if the secondary complications to patient’s highly perfused organs are not too serious and can be manageable by a rheumatologist, clinical translation must be addressed, if possible. The use of LEF with biological DMARDs is still unanswered and potential toxicities are never challenged. Research on LEF in triple therapies has not been evaluated by scientists to a significant level. The tolerability and utility of LEF across a range of optional dosing regimens needs to be reconsidered. The cause of the majority of the toxicities linked to LEF use is still not fully understood. The pathological mechanisms of common side effects linked to LEF such as hypertension, diarrhea, and some other extremities are not understood clearly, including why some patients experience these symptoms mostly with the loading dose. The other routes of administration for LEF must be explored to minimize the major adverse effect that is hepatotoxicity. Trials employing a reduction in maintenance dose and frequency of LEF could help in achieving optimal treatment for RA patients [29]. Cholestyramine is used to flush out the longer retention of LEF, as it has an elimination half-life of almost 14 days [30].

The clinical use of LEF is also a matter of concern due to safety and pharmacokinetic concerns. The extensive metabolism of teriflunomide is a major challenge which leads to potential deposition and prolonged drug exposure to various organs. The high lipophilicity and albumin binding of LEF in blood plasma (~99%) limits its capacity for the formulation of controlled release or targeted drug delivery systems. The wide inter-patient variability in bioavailability due to the low aqueous solubility of LEF is one the issue that needs to be addressed. The ability of LEF to inhibit DHODH and suppress tyrosine kinases involved in T-cell proliferation encourages scientists to reveal its full potential for safer and more effective delivery. LEF has shown efficacy similar to other DMARDs like methotrexate and has been explored in other indications as well such as psoriatic arthritis and systemic lupus erythematosus. The anti-inflammatory and immunosuppressive effects of LEF which have been proven in various studies make it a perfect drug candidate to unravel its potential for safer delivery in patients through various emerging formulation strategies [31].

2.3. Mechanism of Action of Leflunomide

Lymphocytes in RA enter their replicative phase when antigen-presenting cells (APCs) activate T-cells. Dihydroorotate is subsequently converted to orotate (a pyrimidine precursor) in the mitochondria by the enzyme DHODH, which is overexpressed during the G1 phase of the cell cycle. The rate-limiting step in pyrimidine de novo synthesis, mitochondrial dihydroorotate dehydrogenase, is reversibly inhibited when DHODH is inhibited, depriving cells of UMP (uridine-5′-monophosphate), a crucial component for RNA and DNA synthesis. LEF is available in 10, 20, and 100 mg dosage strength and the patient are given an initial dose of 100 mg followed by a maintenance administration dosage of 10 or 20 mg [32].

Table 2. A list of main manufacturers and their market brands of leflunomide.

Manufacturer	Brand Name	Common Dosage Strengths	100 mg Strength Availability (Used as Loading Dose)	Region	Reference
Sanofi-Aventis	Arava®	10 mg, 20 mg	Yes	Global/USA/EU	[33]
Apotex Inc.	Leflunomide Apotex®	10 mg, 20 mg	Not available	Canada/Australia	[34,35]
Zydus Cadila	Lefumide®, Rumalef ®	10 mg, 20 mg	Yes	India/UK	[36]
Torrent Pharmaceuticals	Lefra®	10 mg, 20 mg	Yes	India	[37]
Lupin Pharmaceuticals	Lefno®	10 mg, 20 mg	Yes	India	[38]
Sun Pharmaceuticals	Cleft®	10 mg, 20 mg	No	India/USA	[39]
Teva Pharmaceuticals	Leflunomide-Teva®	10 mg, 20 mg	Yes (select markets only)	EU/UK	[40]
Mylan/Viatris	Leflunomide Mylan®	10 mg, 20 mg	Yes	Europe/Australia/UK	[41]
Sandoz	Leflunomide Sandoz®	10 mg, 20 mg	Yes (limited availability)	Europe	[42]
Ratiopharm GmbH	Leflunomide Ratiopharm	10 mg, 20 mg	Yes	Europe/UK	[43]

shows the list of main manufacturers and their market brands of LEF.

3. Pharmacokinetics of Leflunomide

LEF is widely prescribed by rheumatologists for the management of RA. The body extensively metabolizes it, and its pharmacokinetic profile is essential for assessing both the safety and effectiveness of its treatment [44]. The drug LEF has an oral bioavailability of approximately 80%, leading to it being an extensively used systemic treatment [32].

3.1. Absorption, Metabolism, and Elimination

LEF is an orally administered drug which undergoes metabolism in the liver and intestinal wall. Food consumption has a minimal effect on the absorption of the drug, allowing patients to adjust dosing schedules. After the conversion of LEF to an active metabolite in the body, the metabolite immediately binds to protein (almost 99% protein binding) leading to longer retention time of LEF and stable plasma concentrations. The volume of distribution of LEF is nearly 11 L per kg, indicating the deeper entry of the drug into the tissues. Additionally, a good protein binding also has an impact on the LEF interactions with other drugs as well, mainly when it comes to other medications that are strongly protein-bound. The complete conversion of LEF to its active metabolite is performed by the liver metabolism using two enzymes, oxidoreductases and cytochrome P450. The enterohepatic circulation of LEF is responsible for the prolonged half-life of 14 to 18 days. A major portion of the drug is excreted through renal elimination (nearly 43%) and biliary elimination (nearly 38%). Due to the prolonged half-life of LEF, it requires a loading dose of 100 mg for 3 days to rapidly reach drug plasma levels [30,45].

3.2. Conversion to Active Metabolite (Teriflunomide)

LEF is a prodrug that, after being metabolized in the body to its metabolite, teriflunomide, binds to proteins and casues immunosuppressive effects in the body by inhibiting the DHODH enzyme, which is vital for the production of pyrimidines. The conversion of LEF to teriflunomide occurs mainly through first-pass metabolism via microsomal and cytosolic enzymes. This conversion of LEF to an active metabolite does not involve any specific enzyme-controlled reaction. Consequently, there is little variation in the conversion efficiency between patients, guaranteeing steady drug activation [46]. Teriflunomide is licensed under the brand name Aubagio^® [47]. This metabolite is undergoing several clinical trials to check its potential in other autoimmune diseases such as in multiple sclerosis [48]. It was discovered that teriflunomide inhibits the NF-κB transcription element. Similarly, tyrosine kinase enzymes are inhibited by it, but only at high levels that are not employed in therapeutic settings [46]. Figure 3 shows the conversion of LEF into its active metabolite, teriflunomide in vivo.

3.3. Drug Interactions and Bioavailability Considerations

LEF is frequently prescribed alongside other medications such as other DMARDs, NSAIDs, corticosteroids, etc., which increases the threat of pharmacokinetic and pharmacodynamic interactions. For instance, the coadministration of methotrexate and LEF may cause serious liver enzyme elevation, causing hepatotoxicity and an increased risk of bone marrow suppression. Clinical guidelines in such cases suggest regular monitoring through complete blood count (CBC) and liver function test (LFT) to mitigate hematologic and liver toxicity. The administration of LEF with NSAIDs may increase the risk of gastrointestinal and renal toxicity. Hence, regular assessment of renal activity (serum creatinine, eGFR) is recommended, especially in elderly patients or those with pre-existing kidney disease. Sometimes, corticosteroids are prescribed with LEF to manage acute inflammation—this co-administration may cause a toxicity risk in the patient; therefore, vigilance for signs of infections is generally required. The combination of biological DMARDs with LEF raises the risk of severe infections. Hence, a surveillance of the patient prior to LEF administration is recommended. Also, a washout period of the body with cholestyramine before switching to biologics is required. The concomitant administration of LEF with warfarin causes an increase in international normalized ratio (INR). The increased INR may induce increased bleeding risk in patient. Routine blood monitoring and dose adjustment in diabetic patients is required before LEF administration because LEF interactions may alter the plasma levels of antidiabetic drugs [52,53]. The metabolite of LEF suppresses CYP2C9, which affects how treatments including NSAIDs, sulfonylureas, and warfarin are metabolized. Likewise, by interfering with enterohepatic recirculation, simultaneous dosage administration of cholestyramine or activated charcoal might expedite drug clearance considerably. It has been already reported that LEF is teratogenic; hence, it is contraindicated in pregnancy. Therefore, females planning to conceive must quit the LEF intake prior to conception with the detailed advice of a doctor [54,55].

4. Challenges in LEF Delivery to RA Patients

LEF has its established efficacy as a DMARD for RA, but it has a number of drug delivery issues that may restrict its effectiveness and patient compliance. There are various types of challenges which are encountered in the safe delivery of LEF to RA patients. Such challenges arise due to its formulation constraints, adverse effects profile, and pharmacokinetic characteristics, all of which call for the advancement of improved drug delivery systems. The best selling point of LEF for RA management is that it helps in controlling the hyperactive cells of the immune system which produce inflammation in joints. However, its cost is higher in comparison to other DMARDs, so LEF is prescribed by doctors or rheumatologists when other DMARDs are not working well, or inflammation is severely damaging the joints [56]. A number of side/adverse effects are involved with LEF administration to RA patients, as have appeared in clinical studies, which should be managed by doctors in a balanced approach [31].

Table 3. Approaches to tackle the various side/adverse effects of LEF that occurred during clinical studies of RA.

Adverse/Side Effect	Prevention	Management
Hepatotoxicity	✓ Baseline liver function test (LFT) before initiation ✓ Patients with pre-existing liver disease should not be administered ✓ Steer clear of concurrent hepatotoxic medications	✓ Consistent LFT surveillance throughout treatment ✓ reduction in dose or stop if ALT/AST > 2× ULN ✓ Activated charcoal or cholestyramine should be given to increase drug elimination
Gastrointestinal complications (e.g., diarrhea)	✓ Must be taken after food consumption ✓ enteric-coated dosage form should be used	✓ Symptomatic treatment (e.g., loperamide) ✓ Modification of dosage amount or withdrawal for short duration if persistent
Skin rash and allergic reactions	✓ Examining the patient’s past allergy record ✓ Use with patients in atopic individuals after proper consultations from medical professionals	✓ Stop administration in severe cases ✓ Administer antihistamines or corticosteroids ✓ Consult a dermatologist if required
Teratogenicity	✓ Before beginning, a compulsory negative pregnancy test is required ✓ Reliable contraception during and after treatment ✓ Avoid use in pregnancy	✓ Use of cholestyramine/activated charcoal to completely washout LEF before planning pregnancy ✓ Patient counseling and monitoring
Alopecia	✓ Inform the patient of the possibility of short-term hair loss. ✓ Check early signs	✓ Usually self-limiting ✓ Decrease dose if cosmetically unpleasant
Respiratory infections	✓ Check for infection threat ✓ Guide the patient about preventing infections. ✓ Vaccination where applicable	✓ Stop administration during active infection ✓ Continue only after complete recovery ✓ Supportive treatment for infection
Hypertension	✓ Check blood pressure both prior-to and throughout treatment. ✓ Manage underlying hypertension	✓ Start or adjust antihypertensive therapy ✓ Stop LEF in severe or uncontrolled cases

shows the approaches to tackling the various side/adverse effects of LEF that occurred during clinical studies of RA [31,57,58,59].

4.1. Safety and Tolerability Issues

LEF has been a promising immunomodulatory option when other conventional/synthetic DMARDs have not responded in RA patients due to severe inflammation and joint damage. However, some of the serious safety and tolerability issues deter rheumatologists from recommending LEF, hence making its use limited. These issues range from mild to severe and require routine examination to track patient-specific adverse events. A huge database is already available on LEF safety and efficacy, which must be utilized to address these issues, as most of the adverse events take place in the initial phases of treatment. Most of the adverse/side effects are common such as skin rashes and allergy, diarrhea, reversible alopecia, etc., while the most serious adversity with LEF is hepatotoxicity, which raises a deep concern for patients who have had past liver complications. Some of the rarer adverse events include hypertension, teratogenicity, interstitial pneumonitis, diabetes, bone marrow suppression, and others [60].

4.1.1. Hepatotoxicity and Monitoring Requirements

A major serious side effect that is linked to LEF is hepatotoxicity. LEF causes a rise in liver transaminase levels, leading to severe liver damage, mainly in patients with a history of liver complications or who are consuming other types of hepatotoxic drugs. Liver function tests (LFTs) should be regularly monitored, particularly in the first six months of treatment. The significance of early detection and management in preventing permanent harm has been emphasized in recent studies. A study conducted by Razak and colleagues reported the incidence and risk factors responsible for the LEF-induced liver injury in RA patients. They collected data from 73 patients from a local tertiary hospital and observed that 35 out of 73 patients had elevated levels of hepatic enzymes. However, the mean onset of hepatic adverse events began after 6.44 ± 8.11 months from the first dosing of LEF in RA patients. The most common reason for the discontinuation of treatment was 15.1% (n = 11) because of severe liver injury, while 12.3% (n = 9) was due to various adverse drug events. The type of rheumatoid factor (p = 0.005) and dyslipidemia (p = 0.029) have been found to be prognostic of the degree of hepatic damage based on alterations in liver enzymes [61]. A case report from 2010 also showed that a 27-year-old patient with RA, when administered with LEF, developed acute liver failure after 9 months of treatment. The patient was treated for liver failure but later he was referred to liver transplantation [62]. Another recent study conducted by Nandan and coworkers in 2025 reported on the association of metabolic syndromes (such as dyslipidemia, diabetes, and obesity) with hepatotoxicity, leading to discontinuation or the change of a particular DMARD to another. An analysis of 341 patients showed that 20 patients changed their DMARD due to hepatotoxicity. Also, hepatotoxicity had appeared mainly due to the methotrexate and LEF [63].

4.1.2. Gastrointestinal (GI) and Hematological Effects

The common side effects of GI disturbances of LEF are nausea, diarrhea, and abdominal pain, which mostly leads to initial treatment stoppage. These side effects can be managed by adjusting the dose and consuming the drug after a meal. However, some of the patients need symptomatic management or complete discontinuation of drug administration. Alongside this, hepatotoxicity is one of the abnormalities that proceeds the GI side effects [64].

4.1.3. Skin Rash and Wound Healing

LEF therapy in patients with RA is frequently associated with common but frequently disregarded skin-related side effects, such as allergic reactions and rashes. About 10–15% of individuals experience mild-to-moderate maculopapular rashes, pruritus, or localized erythema; these conditions usually go away on their own or with medical attention. But in cases of serious hypersensitivity reactions, which include fever, eosinophilia, and an extensive rash, LEF should be discontinued immediately. Systemic corticosteroids may also be necessary. Also, administration of LEF should be discontinued if ulcerative colitis is evident. LEF should be stopped as soon as skin and/or mucosal events occur which increase the possibility of such severe responses. Cholestyramine or activated charcoal should also be used immediately to lower the plasma levels of LEF. A complete elimination of the drug from the body is a must in these situations, and re-administration of LEF is contraindicated. Patients may develop skin ulcers while taking LEF and whenever skin ulcers are suspected, drug administration should be discontinued depending upon the clinical decision of proper wound healing. An individualized evaluation in the perioperative period must be performed so as to avoid any adverse effects, and a washout period must follow for the complete removal of the drug [65].

4.1.4. Bone Marrow Suppression

LEF rarely causes cytopenia; however, bone marrow suppression has been confirmed, typically in conjunction with other identified reasons, including concomitant medicine use and aliments. Some patients who had been administered with LEF reported respiratory tract infections. The infection was classical pulmonary tuberculosis, and no reactivation of latent tuberculosis was documented [66]. There can be a rise in the vulnerability to infections. The overall population of RA patients treated with nonbiologic DMARDs does not show any strong proof that these side effects are more common when taking LEF [31].

4.1.5. Hypertension

Hypertension is one of the major side effects of LEF therapy. Hypertension as a side effect of LEF raises concerns for patients who already have increased levels of cardiovascular risks, because hypertension will further increase these threats. Hence, it is crucial to track and regulate blood pressure throughout LEF treatment. In certain circumstances, discontinuing LEF or administering antihypertensive medicines may be indicated. The management of hypertension in RA patients is a very challenging task due to a number of issues such as polypharmacy, inflammation, physical inactivity, and many others [67].

4.1.6. Weight Loss and Diabetes

Although weight loss was observed in the initial Phase II research, further studies have not verified this [68]. According to an observational study, 7% of LEF-treated individuals lost a large amount of weight (8–20 kg), which was unexpected [69]. LEF may raise metabolic requirements by uncoupling oxidative phosphorylation, which lowers adenosine triphosphate (ATP) synthesis, as DHODH is a mitochondrial enzyme. The data sheet lists weight loss as a usual adverse effect. According to the LEF data sheet, 1% to 3% of people using the drug in clinical trials developed diabetes. Very few studies have been published that report diabetes as a side effect. LEF has shown promise as a protective agent in preclinical models of autoimmune diabetes. Although it does not appear to be a clinically important issue, patients may inquire about this potential risk [65].

4.1.7. Teratogenicity

If effective contraception is not used, teratogenicity is a serious contraindication for usage in women of reproductive potential. Since teriflunomide, the active metabolite of LEF, has a lengthy half-life, cholestyramine is needed for drug clearance if exposure or pregnancy are desired. Teratogenic repercussions from LEF’s ability to cross the placenta have been observed in animal models of rats and rabbits. When administered between days seven and seventeen of pregnancy, these teratogenic outcomes include skull schisis and exencephaly. Furthermore, insufficient bone ossification and other head, vertebral, and limb abnormalities may manifest. A recent study reported the most toxic dose of LEF as 70 mg/Kg in rat models, which caused severe teratogenicity and led to the death of all fetuses. Furthermore, a dose of 30 mg/kg caused a substantial rise in intrauterine growth restriction-related abnormalities, but fewer fetuses were lost [70]. A new DMARD with a less severe teratogenic profile may be prescribed to women to avoid these teratogenic impacts. Women are recommended to cease taking LEF before pregnancy to achieve proper elimination and lower chances of teratogenic results due to the drug’s exceptionally long half-life in its active metabolite. Nevertheless, some pregnant women may be able to discontinue LEF therapy totally because of immune system modifications that reduce RA symptoms [30].

4.2. Patient Counseling

Successful patient counseling is the foundation for any successful treatment of the patient, and it remains a significant hurdle in regular clinical practice. Since LEF is linked to various types of adverse events such as hepatotoxicity, gastrointestinal tract (GIT) disturbances, skin rashes and allergies, teratogenicity, and others, it therefore becomes essential to guide the patient in drug administration to avoid any confusion and safer delivery. The main challenge encountered in LEF-delivery to patients is associated with its long half-life, which is more than 14 days. This prolonged retention of LEF in the body after discontinuation causes serious secondary complications in patients; hence, all thorough information about potential toxicity issues must be given to patients. Particularly women who are taking LEF and are planning to conceive should be counseled properly about the contraindications. Such women should use effective contraception till treatment, and a complete washout period should follow to avoid any toxicity, as this may cause severe teratogenicity. Written materials and symbolic pamphlets can be used for these types of counseling. Incomplete information should not be passed in any way to patients, so that they do not lose trust in the medical professional handling their case. Regular LFTs, complete blood counts tests, etc., must be recommended to patients during their treatment. Other challenges include linguistic hurdles, unavailability of counseling resources, and routine clinical visits by patients; these will further impede efficient counseling [71,72].

4.3. Restricted Use in Specific Subject Populations

LEF is contraindicated in many of the population groups due to the serious adverse effects and toxicities discussed in Section 4.1. A number of treatment-related adverse events (TRAE) are reported in different clinical studies, which restricts the use of LEF in a variety of populations. Most of the trials conducted have shown that women of child-bearing potential are contraindicated for treatment with LEF, as it causes severe teratogenicity and hepatotoxicity. Therefore, other novel options are required for the safer delivery of LEF to the larger population group. A study with 407 RA patients was conducted and 378 patients were included in the study, with 78.7% being females. The mean age was 57.7 ± 12.0 years, and the mean disease time period was 9.7 ± 8.5 years. After administration of 20 mg LEF to all patients for 6 months, premature trial termination was attributed to TRAE in 15.9% of individuals, while 2.4% of patients experienced serious adverse events that may have been caused by their treatment [73]. Another retrospective study involved 90 Italian patients who were examined for LEF activity in RA and psoriatic arthritis (PsA) patients. Among the 90 patients, 50 belonged to the age bracket of ≤65 and 40 patients were >65 years. In total, 21 patients (23.3%) quit using LEF due to side effects, whereas 10 patients (11.1%) stopped taking the drug due to ineffectiveness (one patient withdrew due to both ineffectiveness and side symptoms). Additionally, there was no discernible variation in leflunomide therapy survival rates between patients aged 65 and those aged < 65 [74]. Newer drug delivery strategies and new drug combinations should be unraveled to address the current challenge of restricted use in specific patient segments.

4.4. Differing Clinical Responses

LEF presents differing clinical responses in different patients due to diverse interrelated factors such as disease condition, secondary complications, drug metabolism heterogeneity, and genetic predisposition. Early RA has been best treated with LEF loading doses, but also serious toxicity issues have been observed during initial treatment. Therefore, disease duration and its stage often affect the clinical responses in patients. Furthermore, the sex, age, and body weight of the patients has been a determinant for clinical response. Normally, patients who are obese or have a higher body mass index (BMI) may have lower plasma drug concentrations, which could result in more severe than optimal clinical results. Comorbid diseases and polypharmacy may cause aged people to have decreased tolerance, requiring meticulous dose adjustment and surveillance. Secondary complications in a patient such as kidney dysfunction, hepatic injury, or cardiovascular abnormality affect LEF metabolism and safety. They may also require dose adjustments, which can impact the efficacy of therapy. In certain events, doctors may choose to administer combination treatments or lower dosages, which could change the patient’s response pattern. Variations in genes can modify metabolic enzymes, mainly the DHODH that converts LEF to its active metabolite, teriflunomide. These variations in metabolic enzymes affect both drug dosage and the duration of action, leading to an increase in adverse effects and low therapeutic results. The efficacy and tolerability of LEF may be affected by concurrent drugs when used in combination with other synthetic or biological DMARDs. Although combination therapy has the potential to improve the control of diseases, it also increases the risk of side effects, which can affect patient compliance and treatment outcomes over time [65,75].

4.5. Relative Efficiency Compared to Other DMARDs

Methotrexate (MTX) remains the first choice of treatment compared to other DMARDs like LEF. Similarly, when other DMARDs do not work against the inflammation or manage RA, LEF is prescribed by the medical professional. Initially, conventional synthetic DMARDs are preferably prescribed over LEF due to their hepatotoxicity and prolonged retention time in the body. However, LEF has a rapid onset of action compared to other DMARDs including MTX, which is better tolerated and prevents against long term effects from structural damage. Some of the clinical trials have depicted that LEF has better functional capacity when compared to sulfasalazine in terms of the alleviation of symptoms like RA inflammation. Biological DMARDs (bDMARDs) have been proved more efficacious compared to LEF, but cost and easy accessibility have been major issues for bDMARDs. Additionally, if a combination of any bDMARDs and LEF are used, this raises concerns of the occurrence of serious adverse effects when in combination.

Table 4. A relative comparison of efficacy of LEF with other types of DMARDs.

LEF Comparison with	Relative Efficacy	Therapeutic Remarks	Reference
Methotrexate (MTX)	Comparable (MTX is slightly higher for long-term structural results)	Similar symptomatic alleviation and physical function. Long-term tolerance was higher for MTX. Leflunomide has slow onset of action.	[76]
Hydroxychloroquine (HCQ)	LEF has greater efficacy than HCQ	HCQ used in combination regimens or for mild RA. LEF is more efficacious than HCQ.	[77]
Sulfasalazine (SSZ)	SSZ have lesser efficacy than LEF	Leflunomide-based triple therapy is not less effective than sulfasalazine-based therapy in methotrexate-refractory RA, and it has a similar safety profile.	[78]
Biological DMARDs (bDMARD)	bDMARDs have higher efficacy than LEF	In general, bDMARDs are more successful in treating moderate-to-severe RA. Biosimilars provide a more affordable option with higher quality-adjusted life years than leflunomide.	[79]

shows the relative comparison efficacy of other DMARDs with respect to other types of DMARDs.

4.6. Slow Onset and Prolonged Retention

As already discussed in earlier sections, LEF causes severe hepatotoxicity and other adverse effects due to longer elimination half-life. The onset of action to produce a desired therapeutic effect in the body after LEF administration is about 4 to 6 weeks. This duration of action is very long, as symptomatic relief will not appear in patients for up to 4 to 6 weeks. Since the elimination half-life of LEF is almost ~2 weeks, the rate of elimination is very slow and will take longer to achieve steady-state plasma concentrations in the patient’s body. In total, 4–5 half-lives will be required to completely flush out the drug from the body. To speed up the elimination process, a washout period with cholestyramine or activated charcoal recommended by a doctor should be followed. The slow onset of action and prolonged retention of LEF have been unturned stones for researchers [80]. Novel formulation strategies could be a gamechanger in such challenges; therefore, serious focus should be given to address slow onset and prolonged retention of LEF in the body.

4.7. Further Research Requirements

There are various gaps that limit its use in clinical practice that need to be addressed for a safer and more effective delivery of LEF. All the potential challenges discussed above in Section 4 are hurdles in the safe and effective delivery of LEF to RA patients. A detailed investigation on the rectification of its therapeutic positioning, increasing tolerability, and recognition of selective predictors of response in patients is needed. The unavailability of solid biomarkers for the estimation of adverse effects or responses associated with LEF is another milestone yet to be established. Pharmacogenomic profiling is one personalized health care technique that may aid in customizing therapies and minimizing trial-and-error prescriptions. Future research should also explore the LEF efficacy in combination with other DMARDs and mainly also with newer biologics. The well-planned randomized controlled trials to evaluate LEF in newer combination modules are still not accomplished. Hence, the design of well-stabilized randomized controlled trials stands as a challenge in the way of safe delivery of LEF to RA patients. Additionally, a focus on researching dosing adjustments, co-therapies for protection against LEF-induced adverse effects, and alternative formulations should be given. Future studies must involve real-world pharma-economic analysis to evaluate treatment affordability and results over prolonged time among clinical set-ups.

Although LEF has proven strong anti-inflammatory and immunosuppressive effects, all challenges discussed above limit its clinical versatility. A safer LEF delivery to RA patients is still a herculean task for both scientists and rheumatologists. Various issues include low aqueous solubility and high albumin binding in blood plasma, which leads to a longer retention time of the drug and thereby causes accumulation in major organs like the liver, leading to severe side effects like hepatotoxicity. The treatment-related adverse events such as gastrointestinal irritation, teratogenicity, and immunosuppression further restricts its use, especially in long-term RA treatment. Conventional drug delivery approaches are not able to balance efficacy with safety, causing suboptimal patient therapy results. All challenges in LEF delivery to RA patients underscore the requirement of drug delivery strategies that could enhance bioavailability, improve solubility, reduce off-target drug effects, and reduce systemic toxicity. The next section explores emerging strategies, including nanocarriers and surface-functionalized nano-platforms, stimuli-responsive drug delivery approaches, photothermal and photodynamic therapies, and personalized LEF-therapy designed to overcome the challenges in LEF delivery to RA patients.

5. Emerging Drug Delivery and Formulation Strategies to Overcome LEF Challenges

Diverse types of strategies have been employed in last few years to improve the overall efficacy and safety of LEF in RA patients. Nanotechnology-based drug delivery systems (DDSs) have emerged as a promising option for LEF-based formulations. Nanoscale drug delivery systems such as polymeric nanoparticles (PNPs), liposomes, nanostructured lipid carriers (NLCs), solid lipid nanoparticles (SLNs), and others could be fundamental in addressing the challenges discussed in Section 4. A few of the nanotechnology-based formulations have cleared preclinical assessments and entered clinical trials which can emerge as newer treatment option for RA patients. All these strategies aim to increase the overall efficacy of LEF with minimal adverse effects. Other approaches like microneedles, stimuli-responsive DDSs, surface functionalization, microsponge DDSs, physical methods such as electroporation, and sonophoresis and photothermal therapy have also been rising as emerging formulation approaches for LEF.

5.1. Novel Formulations and Drug Delivery Systems

Novel drug delivery strategies involve the use of nanotechnology-based formulation to resolve the current challenges encountered in LEF delivery to RA patients. A variety of other drug carriers such as hydrogels for transdermal and topical formulations, etc., also play a vital role in safer delivery of LEF to RA patients.

5.1.1. Nanocarrier-Based Drug Delivery Systems

Nanoparticle drug delivery systems are sophisticated technologies that utilize nanoscale pathways for the precise administration and sustained release of drug substances.

Nanoparticles

Nanoparticles (NPs) are tiny particles that lies within size range 1–1000 nm and hold applications in drug delivery, diagnostics, imaging, and biomaterials. This system has been very useful in the delivery of LEF to RA patients, providing a controlled release and delivering drugs directly to desired site of action. Nanoparticles like PLGA-based systems can offer regulated and targeted administration to reduce issues like rapid metabolism and prolonged retention of LEF in the body. This lowers systemic exposure while maintaining therapeutic concentrations in inflamed areas. Different types of NPs currently being investigated for LEF delivery include metallic NPs, mesoporous NPs, carbon nanotubes, liposomes, polymeric NPs, and dendrimers. Shareef and coworkers formulated transferosomes loaded with LEF (LEF-TSF) incorporated in a hydrogel matrix for transdermal delivery in RA. The LEF-loaded formulation in vitro showed excellent biocompatibility, high permeability, and sustained release action. The in vivo evaluation demonstrated the reduction in RA symptoms when LEF-TSF-incorporated hydrogel was applied on the RA-induced mice model [81]. Singh and coworkers designed poly(ε-caprolactone) PLGA nanoparticles loaded with LEF for the symptomatic management of RA. The prepared NPs were optimized using the full factorial design of the Quality by Design (QbD) approach. In vivo assessment of PLGA NPs was carried out using the adjuvant induced-arthritis (AIA) rat model, where the drug showed a sustained release for up to 168 h in simulated synovial fluid [82]. In a research study, Siddiqui and colleagues prepared chitosan–chondroitin sulfate nanoparticles conjugated with folate and loaded with LEF (FA-LEF-NPs) for transdermal administration in RA. The NPs were further incorporated into the hydrogel for transdermal application. The in vivo evaluation of NP-fabricated hydrogel demonstrated the deposition of FA-LEF-NPs in inflamed parts of joints as compared to other organs in the body. FA-LEF-NP-incorporated hydrogel also exhibited an improvement of inflammatory cytokine expression with reduced adversities [83]. Zewail and coworkers prepared nanostructured lipidic carriers (NLCs) loaded with LEF, coated with hyaluronic acid, and incorporated in hydrogel for intra-articular delivery in RA. An extended release up to 51 days was observed from formulated NLCs, and faster recovery was noted after intra-articular injection in arthritis-induced rat model [84]. Krishnan and coworkers prepared LEF-loaded NLCs, mainly focusing on increasing the lymphatic delivery through chylomicron production to enhance bioavailability with minimal systemic side effects. The NLCs loaded with LEF depicted an optimal decrement in inflammation scores in vivo as compared to drug-free Sprague-Dawley rats induced with arthritis. The anti-arthritic efficacy of the drug in formulation was assessed by estimating the reduction in knee thickness. Histopathological investigation and radiographic evaluation revealed a healthy cartilage formation after treatment in arthritis-induced rats [85]. Abd-El-Azim and their associated researchers developed solid lipid nanoparticles (SLNs) encapsulated with teriflunomide for transdermal administration via microneedles for the treatment of RA. An in vivo assessment in an antigen-induced arthritis rat model showed SLNs loaded with LEF administered with the use of hollow microneedles had a great impact in increasing teriflunomide anti-arthritic efficacy compared with teriflunomide suspension, with no specific distinction from the negative control treated group [86]. Various types of NPs offer site-specific drug delivery with minimal adverse effects, leading to increased efficacy and safety. These could be advantageous in the coming era for safer LEF delivery to RA patients.

5.1.2. Lipid-Based Formulations

Lipid-based formulations have attracted the attention of scientists because of their capacity to improve the bioavailability of drugs with poor water solubility. Lipid-based formulations involve emulsions, suspensions, oil solutions, or self-micro or self-nano emulsifying drug delivery systems (SMEDDS/SNEDDS). Lipid-based formulations provide a biocompatible nanocarrier system that can improve drug encapsulation, bioavailability, and controlled release to decrease peak plasma concentrations and LEF-associated side effects, especially considering LEF’s poor aqueous solubility and potential for systemic toxicity. These drug delivery systems have the ability to modify formulations in different ways to meet the variety of clinical requirements for RA patients. These drug delivery systems could be instrumental in delivering the LEF safely through various other routes such as transdermal, topical, intra-articular, parenteral, etc., contributing to product stability with minimized side effects and toxicity [87]. Zewail and coworkers developed a clove oil-loaded nanoemulsion (NE) coated with chitosan (CS) for oral administration of LEF in RA. The administration of LEF-CS-NE in complete Freund’s adjuvant (CFA)-induced arthritis rodent model in vivo successfully reduced remarkable edema levels (46.68%) and decreased IL-6, TNF-α, and rheumatoid factor (RF) to a significant extent. Also, elevated levels of serum glutamic oxaloacetic transaminase (SGOT) and serum glutamic pyruvic transaminase (SGPT) were also reported to be reduced with LEF-CS-NE treatment. Hence, the safety and efficacy of LEF-CS-NE demonstrated by in vivo outcomes position it as a potential drug delivery approach for RA [88]. Abbas and associated researchers formulated supramagnetic nanoparticles (SPIONs) loaded with LEF incorporated into emulsomes for intra-articular administration in RA. In vivo testing in adjuvant-induced arthritis in rodent models showed the retention of LEF-loaded SPIONs in arthritic knees after articular administration in RA (Figure 4) [89].

In another study, Nashaat and coworkers developed phytosomes for the co-administration of LEF and curcumin (CUR) via the oral route of administration in RA. An in vivo assessment demonstrated the anti-arthritic activity of CUR/LEF-phytosomes in an arthritis-induced rat model. A remarkable reduction in inflammatory mediators and pain swelling was observed for formulation with respect to the drug-free group. There was no signature of arthritic symptoms seen in histopathological studies when CUR/LEF phytosomes were given to rats. These results showed that the phytosomal system could be a promising option for the treatment of RA [90]. Verma and his coworkers formulated a nanoemulgel loaded with LEF for topical delivery in RA using the spontaneous emulsification technique. The prepared nanoemulgel exhibited a drug release rate of 98.13% ± 1.20% and was found to be stable in the range from 25 to 45 °C [91].

5.1.3. Micelle-Based Systems

Micelle-based systems have emerged as a promising drug delivery approach in recent years due to their capacity to solubilize hydrophobic drugs. Polymeric micelles are nanosized (10–200 nm), comprising self-aggregating amphiphilic block co-polymers in an aqueous medium. The micelles formation takes place at a particular concentration known as critical micelle concentration. LEF encounters low aqueous solubility and systemic toxicities, which are barriers in the safer delivery to RA patients; therefore, a micellar system for LEF delivery could be an exemplary strategy to resolve the issues faced in currently available dosage forms. The inner hydrophobic core of micellar systems aids in the solubilization of LEF, and the external core stabilizes the micelles in biological fluids and thereby increases the circulation time, leading to enhanced bioavailability. Although specific studies on LEF-loaded polymeric micelles have not been explored, there has been a lot of research performed for other conventional DMARDs. Dedicated research on micelle specific to LEF delivery in RA will be more helpful to understand and unravel the paradigm in this delivery system [92].

5.1.4. Transdermal and Topical Formulations

Transdermal and topical formulations for LEF delivery can be a potential route of administration for enhanced efficacy with reduced systemic toxicity to the RA patients. These routes of administration prevent first-pass metabolism and reduce enzymatic degradation in GIT, which leads to an increase in bioavailability and minimized side effects. Transdermal patches give long-term, sustained drug release, while topical strategies offer focused treatment, particularly for symptoms like pain and inflammation. Current developments in permeation enhancers, polymeric systems, and nanotechnology-based drug carriers have also led to significant advancements in LEF-based transdermal and topical formulations. Shewaiter and coworkers developed a microemulsion-based gel loaded with LEF and diclofenac sodium for transdermal administration in active RA. The prepared formulation exhibited a cumulative in vitro release after 24 h of 89.90% for diclofenac sodium and 77.36% for leflunomide. A 28-day in vivo assessment showed excellent anti-arthritic activity in an arthritis-induced rodent model. The promising outcomes from the microemulsion gel evaluation such as sustained release, anti-arthritic activity, etc., make it a suitable candidate for transdermal administration in RA [93]. Jurca and coworkers were successful in developing a topical gel containing both synthetic and natural anti-inflammatory agents for RA treatment. A triple combination of diclofenac sodium (a non-steroidal anti-inflammatory drug), a topical analgesic called methyl salicylate, and a lyophilized powder of Calendula officinalis as the antioxidative agent were employed in the formulation development. The therapeutic properties of the developed topical gel were confirmed via a prospective study on 115 RA patients and an anti-oxidative assay. The gel demonstrated a local anti-inflammatory activity and analgesic properties [94]. In similar ways, LEF can also be delivered via topical and transdermal routes to RA patients. In a similar fashion, Bae and park evaluated the accumulation of teriflunomide (an active metabolite of LEF) in local tissues and its anti-inflammatory activity through topical application of LEF in adjuvant-induced arthritis rat model. The effective delivery of LEF was achieved through topical application and deposition of its active metabolite, teriflunomide, was also found. The results showed that topical application of LEF can offer the patient compliance with reduced side effects [95]. Despite the various advantages of the topical delivery of LEF, potential side effects must be considered. In preclinical models, studies examining the topical delivery of LEF have shown mild to severe skin irritation, such as dermatitis, dryness, or erythema, especially when transdermal flux is facilitated by penetration enhancers. Furthermore, impaired skin barriers, which are common in inflammatory diseases like psoriasis or RA, may make systemic absorption more likely, resulting in low but detectable plasma concentrations of LEF and its active metabolite, teriflunomide. If not properly monitored, this could reintroduce systemic adverse effects such as immunosuppression or hepatotoxicity. In order to reduce local toxicity, formulation techniques including the use of non-irritating carriers (such hydrogels or liposomes) and avoiding harsh solvents or surfactants are important [96]. A lot of efforts have been made to accomplish the safer delivery of LEF through topical and transdermal administration to RA patients, but a milestone after addressing key challenges is yet to be achieved.

5.1.5. Hybrid and Multifunctional Systems

Hybrid and multifunctional drug delivery systems offer a combination of multiple therapeutic approaches or technologies into one robust system. Properties like targeting ligands, stimuli-responsive release, imaging capabilities, and dual drug loading are involved in these systems, enabling increased therapeutic accuracy and synergistic benefits. Hybrid and multifunctional systems are highly appealing for chronic conditions like RA because they hold capacity to treat inflammation, immunological regulation, and joint deterioration simultaneously. Kalashnikova and coworkers developed albumin–cerium oxide nanoparticles loaded with methotrexate via a biomineralization process and subsequently conjugated it with near-infrared indocyanine green (ICG) dye. In vitro evaluation was performed in monocyte cell lines, which depicted the elevated efficacy for reactive oxygen species (ROS) scavenging and enzymatic-like functionality for converting pro-inflammatory mediators into the anti-inflammatory ones. An in vivo collagen-induced arthritic mice model showed the deposition of nanoparticles in RA joints and gave a therapeutic response similar to free drug methotrexate [97]. Nanocomposite hydrogel provides a better sustained release of drugs and prevents unwanted drug interactions that occur in individual drug carrier systems. In a similar effort, Zewail and coworkers formulated LEF- and Dex-encapsulated NLCs and PLGA NPs and further incorporated them into chitosan/β glycerophosphate (CS/βGP) thermo-sensitive hydrogels. The administration of dual-drug-loaded nanocomposite hydrogel in an RA-induced rodent model showed a desirable therapeutic effect through a raised joint resident time of both drugs, leading to arthritic joint healing [98]. There has been less work undertaken in leflunomide delivery specifically using the hybrid and multifunctional systems; hence, a sophisticated and detailed investigation is required to explore the potential of this drug delivery system in LEF delivery to RA patients.

5.1.6. Hydrogel and Injectable Depot Systems

Three-dimensional frameworks of hydrophilic polymers, known as hydrogels, have the capacity to hold plenty of water and facilitate extended drug release. Hydrogels can help in offering prolonged drug release, precision targeting to inflamed joints in RA, and reducing dosing frequency and unwanted drug exposure of LEF to other organs. Shareef and his team formulated transferosome-based hydrogel loaded with LEF for the treatment of RA. They were successful in establishing the therapeutic profile of the hydrogel in reducing the inflammation and other symptoms of RA. Also, an improved pharmacodynamic profile was found for transferosome-based gel as compared to free LEF [81]. Alhelal and coworkers developed a LEF-loaded SLN incorporated in hydrogel and showed good potential for anti-inflammatory properties in vitro. LEF-SLN-incorporated hydrogel was also found to increase the photostability of encapsulated LEF along with decreasing its skin irritation, and it improved the topical application properties [99]. Injectable depot systems utilize biodegradable polymers such as poly (lactic-co-glycolic acid) (PLGA) to encapsulate drugs and give them the capacity to provide delayed release of the drug substance over a prolonged period of time. Generally, subcutaneous and intra-articular routes of administration are employed for drug delivery through injectable depot systems. In the case of LEF, injectable depot systems could be a promising strategy for reducing dosing frequency and minimizing adverse effects. Studies specific to LEF delivery using injectable depot systems are still emerging, and many advancements in the research of such strategies are required to uncover their potential in LEF delivery in RA [100].

5.2. Stimuli Responsive-Based Drug Delivery Systems (SRDDSs)

SRDSSs are known for their ability to release drugs in the presence of certain stimuli, which can be external or internal. These stimuli can be physical (ultrasound, light, or temperature), chemical (pH, redox potential), and biological (enzymes and receptors).

Table 5. Various stimuli responsive-based drug delivery systems with their instances in RA.

Stimuli	Stimuli Source	Research Instance	In Vitro and In Vivo Outcomes	Reference
pH	Low pH aids in swelling and degradation of polymer matrix	LEF-loaded chitosan and chondroitin sulfate NPs fabricated with folic acid (FA) and NPs further loaded in carbopol hydrogel for transdermal delivery	In vivo evaluation showed the deposition of LEF-FA-NPs in inflamed joints with minimum adverse effects	[83]
Ultrasound	Drug release takes place when ultrasound assistance applied externally	Biocompatible drug microneedles (DMNs) coated with hyaluronic acid (HA) embedded with ultrasound-responsive NPs to increase drug penetration	Synergistic drug effects appeared in collagen-induced arthritis model in rats in vivo	[102]
Receptor mediated	Upregulated CD44 receptors active targeting to CD44 receptors overexpressed in the articular tissue	A receptor-mediated synergistic hydrogel loaded with LEF-nanocarriers for delivery through intra-articular (IA) administration in RA	In vivo IA administration in arthritis-induced rodent model showed speedy recovery after HA-conjugated nanostructured lipid carrier (NLC) injection	[84]
Redox	Raised levels of reactive oxygen species (ROS) causing inflammation acts as stimuli for NPs	Ibuprofen (IBF)- and curcumin (CUR)-loaded HA-fabricated NPs for RA	Combination of CUR and IBF decreased the pro-inflammatory factors, ROS concentration, and COX-2 in vitro. Alas, dual drug-loaded HA-NPs alleviated foot tumefaction and lowered the expression of proinflammatory mediators in the RA-model mice in vivo	[103]
External magnetic field	An external magnetic field applied on inflamed joint after IA injection	LEF-loaded emulsomes (EMLs) loaded with supramagnetic nanoparticles (SPIONs) for IA administration in RA	In vivo evaluation depicted deposition of EMLs in the intra-articular cavity upon administration, providing sustained release and amelioration of inflammation in joints	[89]
Temperature	Local irradiation of the plasmonic laser produces thermal effect to promote drug release from NPs	Gold–thiol-beaded albumin nanoparticles (GTBA-NP-L) loaded with LEF for chemo-combined pulsatile plasmonic laser treatment for RA	Both in vitro and in vivo, GTBA-NP-L treatment demonstrated reduced inflammation and decreased pro-inflammatory cytokines	[101]

shows the various stimuli responsive-based drug delivery systems with their instances in RA. Siddiqui and coworkers formulated LEF-loaded chitosan and chondroitin sulfate NPs fabricated with folic acid (FA) and NPs further loaded in carbopol hydrogel for RA treatment. In vivo evaluation demonstrated the deposition of LEF-loaded NPs in inflamed joints with minimum adverse effects [83]. Zewail and coworkers developed a receptor-mediated synergistic hydrogel loaded with LEF-nanocarriers for delivery through intra-articular (IA) administration in RA. In vivo, IA administration in an arthritis-induced rodent model showed speedy recovery after HA-conjugated nanostructured lipid carrier (NLC) injection [84]. In another study, Gadeval and coworkers formulated gold–thiol-beaded albumin nanoparticles (GTBA-NP-L) loaded with LEF for chemo-combined pulsatile plasmonic laser treatment for RA. In vitro, laser stimuli-regulated LEF release increased the anti-inflammatory effect in activated macrophages cells, while in vivo tests demonstrated promising anti-arthritic activity in an RA-induced rat model [101]. Stimuli-responsive properties incorporated in nanocarriers have been emerging as a drug delivery system and are being intensively researched, showing the potentially tremendous value of this specific therapy. Figure 5 shows the various stimuli used in SRDDSs for drug release in RA therapy.

5.3. Photothermal and Photodynamic Therapy

Currently, photothermal therapy (PTT) is also emerging for different types of disorders including cancer and RA. In this therapy, a light of particular wavelength is irradiated on photothermal substances and light is converted into heat or produces ROS to kill RA cells and tissues causing inflammation in the patient. In comparison to healthy tissues, the inflammatory cells have a large number of blood vessels that restrict loss of heat, which limits their heat resistance. This facilitates the local destruction of inflammatory cells through PTT’s hyperthermic actions. To optimize formulation effectiveness, substances exhibiting photothermal properties can be combined with different anti-rheumatic drugs and administered via an efficient delivery system that provides a regulated release pattern with minimal therapeutic adverse events [104]. Conventional RA treatment can reduce symptoms and delay its advancement, but frequent and large dosages produce side effects. A combination of conventional treatments including DMARDs with PTT will be a precise therapy due to high selectivity and its capacity to decrease the intrusive nature of inflammatory cells. However, when a light of near-infrared (NIR) (750–2500 nm) is incident on a photothermal agent, there is a chance that it could damage nearby tissues. Various nanocarriers are utilized to increase the localized targeting of inflammatory cells through photothermal ablation [105].

Photodynamic therapy (PDT) is another rising therapeutic approach for arthritis, including RA, which combines oxygen molecules, photosensitizers, and NIR light sources for treatment. PDT employs NIR light with the right wavelength (700–1300 nm) to excite photosensitizers that are absorbed by tissue cells, resulting in photochemical reactions that eliminate inflammatory cells. Considering systemic toxicities of conventional therapies, PDT could be a newer, safer therapy for RA that can be directed only towards affected joints. The specific targeting of cells causing inflammation and pain in RA is possible in PDT without affecting the other parts of the body. The accumulated photosensitizers inside inflamed cells via endocytosis when irradiated with a beam of NIR light promote apoptosis or necrosis to relieve pain and inflammation due to RA. The production of ROS due to PDT allows the elimination of inflammatory cells, leading to a decrease in the synthesis of inflammatory mediators and an elevation in the regulation of bone damage and articular cartilage, thereby relieving the symptoms of RA [106]. Various research has been conducted to test the applicability of different DMARDs with PDT and PTT for finding new potential therapies that may be the optional one for RA, but no such efforts have been fruitful up to now. Research on a combination of PDT or PTT with LEF has not been explored on a good scale, limiting LEF’s capability to find an alternative administration pathway for safer and more effective drug delivery to RA patients. Further such studies on LEF using PDT and PTT must be performed to explore the potential of such optional treatments.

5.4. Surface Functionalization of Nanocarriers for LEF Delivery

There are still several drawbacks to nanocarriers, such as inadequate targeting, restricted drug loading, and low drug permeation. The surface functionalization of nanocarriers can be a solution to such drawbacks, and this can yield tailored drug delivery, improved permeation efficiency, and regulated drug release-like outcomes. Surface functionalization is the active targeting and can be receptor- or antibody-mediated. Receptor-mediated active targeting involves the modification of a surface using ligands that can identify and bind to specific receptors which are overexpressed on the target site. In RA, various receptors such as CD44, folate, and integrins are upregulated in endothelial cells, synoviocytes, and macrophages in affected joints. Surface modification of nanocarriers reduces the off targets, leading to increased drug effectiveness [107,108]. Some biopolymers such as hyaluronic acid (HA), chondroitin sulfate (CHS), and chitosan (CS), which target overexpressed CD44 receptors, can be utilized for the surface functionalization of nanocarrier-based drug delivery systems in RA. Furthermore, because HA and CS are found naturally in articular and synovial tissues, they will improve joint lubrication, reduce the mechanical load on inflammatory tissues, and promote cartilage repair [109,110]. In a study by Zewail and coworkers, they developed NLCs coated with CHS or CS for oral delivery of LEF in RA. The surface functionalization with CHS or CS of LEF-loaded NLCs enabled a sustained release profile up to 21 days. In vivo results in RA-induced rat model showed that therapy with LEF-NLCs, as compared to LEF suspension, resulted in better joint recovery and less hepatotoxicity; CHS–NLCs had the highest Cmax, AUC, and lowest TNF-α level. The coating of CHS over LEF-NLCs aided in achieving the receptor-mediated targeting to CD44 receptors, and thereby increased the LEF amount at the site of action, along with a synergistic effect on joint healing (Figure 6A). Also, LEF-NLC suspension showed normal hepatocytes organization in terms of shape and size in the liver (Figure 6B(a)), whereas no toxicity was seen for NLC-treated groups in the kidneys (Figure 6B(b)) [111].

Polyethylene glycol (PEG) is another moiety that is widely used as a hydrophilic polymer for the surface modification of nanocarriers for targeted delivery in different disorders like cancers [112] and RA [113]. PEGylated nanocarriers are successful in attaining more safety and efficacy for targeted drug delivery in RA. In the case of LEF, PEGylation could offer a lower level of LEF in the spleen and liver, leading to reduced toxicity. This strategy would also allow more accumulation of LEF in inflammatory synovium and increase the efficacy of the drug [114]. In a similar effort, Zewail and coworkers formulated PEGylated-lecithin chitosan nanoparticles loaded with LEF for oral delivery in RA. An improved release profile was seen for PEGylated chitosan nanoparticles compared to non-PEGylated ones and free LEF. An in vivo complete Freund adjuvant-induced arthritis model in rats demonstrated the inhibition of inflammatory factors such as TNF-α (55.6%), AST (39.83%), RF (40.76%), ALT (80.59%), and IL-6 (47.42%). Additionally, a histopathological evaluation showed the safety profile of PEGylated nanoparticles coated with chitosan where a normal-appearing lung, heart, kidney, and liver were seen [115].

As per reports, the PEGylation of nanocarriers can give a massive immune reaction in the body of a patient, leading to blood clearance [116]. Hence, a biomimetic drug delivery system to counter the blood clearance was proposed which makes nanoparticles camouflage as a dissimilar moiety to restrict their ability to induce a strong immune response, increase the circulation time, facilitate deposition at the site of action, improve therapeutic efficiency, prevent immune defense, and reduce off-target and adverse effects [117]. Biomimetic drug delivery systems have immense potential for the safer delivery of LEF to RA patients by increasing its solubility through using various cell membranes resembling a natural system. A list of various cell membrane sources and their benefits in RA therapeutics with supporting research studies are shown in

Table 6. Various cell membrane sources and their benefits in RA therapeutics with supporting research studies.

Coating Membrane Origin in Biomimetic DDS	Benefits in RA Therapeutics	Supporting Instance	Reference
Macrophage	Inherently targets inflamed joints; neutralizes cytokines	Macrophage membrane-camouflaged biomimetic nanoparticles for RA treatment via modulating macrophage polarization	[118]
Platelets	Interacts with endothelial damage at inflamed joints	Biomimetic platelet membrane-coated nanoparticles for targeted therapy	[119]
Neutrophils	Migrates to inflamed synovia; inflammatory tropism	Peptide-anchored neutrophil membrane-coated biomimetic nanodrug for targeted treatment of RA	[120]
Exosomes	Natural nanocarriers with communication signals	M2-type exosome nanoparticles for rheumatoid arthritis therapy via macrophage re-polarization	[121]
Stem cells	Biocompatible, with regenerative cues	A biomimetic adipocyte mesenchymal stem cell membrane-encapsulated drug delivery system for the treatment of rheumatoid arthritis	[122]

.

5.5. Personalized LEF Therapy

Since RA is a very complex and heterogeneous disorder, patients show significant variability in terms of disease severity, underlying molecular causes, and response towards treatment. The current research approaches for LEF revolve around symptom-based management for patients, similar a trial-and-error approach. Therefore, personalized LEF therapy by optimizing the specific dose and duration of treatment for each patient depending upon their condition and symptoms could be a solution to current challenges like treatment response variability among others. Personalized medicine strategies like biomarker-guided therapy and pharmacogenomics can support clinicians in designing specific dosages of drugs or finding patients who are at higher risk of side effects. Adoption of these strategies can increase the treatment results and reduce the unwanted drug exposure to harmful treatment [123].

6. Conclusions and Future Prospects

LEF is widely used clinically for the treatment of RA and has been a standard therapy over other conventional DMARDs. When used in monotherapy, LEF has shown greater efficacy when the patients do not respond to other synthetic DMARDs like methotrexate or biologic DMARDs. LEF is associated with several adverse effects such as hepatotoxicity among others, which makes the safe delivery of drugs difficult to patients. Additionally, prolonged retention and long elimination half-life of LEF is still a challenge which needs to be addressed on a serious note. If LEF is used as a combination therapy with other DMARDs such as methotrexate or sulfasalazine, it will raise the efficacy as compared to monotherapy, but the potential risk of serious adverse effects or toxicity will also be increased. However, if the concerns related to efficacy and adverse effects associated with LEF are resolved, RA treatment can achieve significant milestone both in terms of safety and efficacy. Novel drug delivery strategies including nanocarrier-based systems such as polymeric nanoparticles, SLNs, NLCs, lipid-based systems, micellar systems, multifunctional and hybrid drug delivery systems, hydrogel and injectable depot systems, and others could be instrumental in achieving the goal of safer and more effective delivery of LEF. Personalized or tailored therapy of LEF could also be gamechanger in cases of varying clinical responses in RA patients. Various clinical studies for LEF combinations with other DMARDs are ongoing and may yield the desired outcomes to unravel newer potential drug combinations. A detailed post-marketing surveillance should be undertaken to check the patient dependency on the LEF when other DMARDs do not work in a patient. Despite various challenges, LEF has been the most prescribed synthetic and non-biologic DMARD by rheumatologists for RA.