Combination Therapy Should Be Reserved as Second-Line Treatment of Onychomycosis: A Systematic Review of Onychomycosis Clinical Trials

Onychomycosis is the most common nail disease encountered in clinical practice. Its importance extends well beyond aesthetics, often causing pain, difficulty with ambulation and performing daily activities, and impairing quality of life. Many patients fail to achieve cure with antifungal monotherapy and recurrences are common. Combination therapy has therefore gained considerable interest, given the potential for drug synergy and prevention of antifungal resistance, but it has not been well studied. A systematic review of onychomycosis medication only, as well as medication and procedural (laser, debridement, photodynamic therapy), clinical or randomized controlled trials evaluating combination vs. monotherapies was performed. After exclusions, 30 studies were included in the final analysis. There were conflicting results for medication-only trials, with some showing significant benefit of combination therapy over monotherapy, however, trials were not robustly designed and lacked sufficient follow-up. Procedural studies also lacked long-term follow-up, and failed to demonstrate efficacy in some severe onychomycosis cases. Considering the high cure rates demonstrated in pivotal antifungal monotherapy trials, and conflicting results, costs, and safety concerns associated with combination therapy, we recommend that combination therapy be reserved as second-line treatment options in patients with poor prognostic factors or for those who failed monotherapy for onychomycosis.


Introduction
Onychomycosis is a fungal nail infection due to dermatophytes, nondermatophytes, and yeast [1], clinically presenting with nail plate onycholysis, thickening, and subungual hyperkeratosis [2], with significant physical, aesthetic, and psycho-social consequences. Currently, oral terbinafine, itraconazole, and griseofulvin, as well as topical ciclopirox, efinaconazole, and tavaborole, are United States (US) Food and Drug Administration (FDA) approved for onychomycosis treatment, and oral fluconazole is often used offlabel [3,4]. In a systematic review and meta-analysis of 26 randomized controlled trials (RCT) investigating systemic monotherapy for toenail onychomycosis [5], there was a significantly greater odds ratio (OR) of achieving mycological cure for all monotherapy treatments vs. placebo. Onychomycosis treatment is challenging, and is individualized based on disease severity, co-morbidities, and infecting organism(s), with consideration of associated medication adverse events, drug-drug interactions, and cost [1,6]. Some disadvantages of onychomycosis monotherapy include potential antifungal resistance, and difficulty of achieving high concentrations of biologically effective drug in affected

Materials and Methods
The objective of this systematic review was to examine combination therapy for onychomycosis treatment, and is reported in accordance with PRISMA guidelines [17]. This review was not registered and a protocol was not prepared. PubMed, Scopus, and Web of Science databases were searched for articles on onychomycosis combination therapy on 1 July 2021, for all peer-reviewed, English-language, human subject onychomycosis clinical and RCTs with no date ranges, and using search terms "onychomycosis treatment", "onychomycosis therapy", and "onychomycosis combination therapy". Articles were independently screened by two authors (R.L. and S.R.L.) based on abstracts. Both authors then independently reviewed full-text articles for eligibility and extracted data for eligible studies. Inclusion required investigation of a combination therapy versus monotherapy for onychomycosis treatment, and mycological confirmation with microscopy, culture, or another validated laboratory-based testing method prior to treatment initiation. Duplicate, non-English, non-randomized, non-clinical trials, and studies investigating monotherapy treatment regimens, diagnoses other than onychomycosis (i.e., tinea pedis), lacking monotherapy control groups, or control group medications that differed from both drugs in the combination group were excluded. Outcomes, including number of subjects, treatment protocol, treatment success rate, and adverse effects, were extracted from each study. Data that was not available was stated (N/A). The PRISMA flow diagram ( Figure 1) provides additional information regarding the systematic search.

Results
There was a total of 726 studies from the initial search, with 30 clinical trials (2531 participants) meeting inclusion criteria and included in the final analysis ( Figure 1). Half (15/30) of the studies investigated medications only (Table 1) and half studied procedures (debridement, photodynamic therapy, lasers) in combination with medication ( Table 2).

Results
There was a total of 726 studies from the initial search, with 30 clinical trials (2531 participants) meeting inclusion criteria and included in the final analysis ( Figure 1). Half (15/30) of the studies investigated medications only (Table 1) and half studied procedures (debridement, photodynamic therapy, lasers) in combination with medication ( Table 2).        were the most commonly studied oral and topical medications, respectively. Most studies (11/15, 73.3%) investigated an oral medication in combination with a topical medication, with oral terbinafine and topical amorolfine (4/11, 36.4%), and oral terbinafine and topical ciclopirox (3/11, 27.3%) being most common, with more than half (7/11, 63.6%) designed with the topical medication administered for longer than the oral medication. Common endpoints assessed were mycological cure rates in 13 (86.7%), complete cure rates in 10 (66.7%), and clinical cure rates in 6 (40.0%) studies, with 5 studies (33.3%) reporting all 3 cure rates.
In seven medication-based studies that reported adverse events, 57.1% reported greater adverse events in monotherapy groups compared to combination groups. Cost (treatment cost per cured patient or cost per cure ratio) for oral terbinafine and topical amorolfine vs. oral terbinafine [22] and oral itraconazole and topical amorolfine vs. oral itraconazole [28,29] was lower for combination therapy than monotherapy.
In studies assessing laser and oral itraconazole combination therapy, efficacy rate (cure (new clear nail growth with less than 5% nail dystrophy) plus significant efficacy (60% new clear nail growth) rates) were significantly higher at 8 (20% vs. 13%) and 24 weeks (21% vs. 11%) (p < 0.05, both) compared to laser monotherapy, only for patients with severe onychomycosis, with no significant differences across groups in patients with mild/moderate onychomycosis (p > 0.05) [37]. In another study, overall clinical cure rate was significantly greater (p = 0.001) and mean OSI score after treatment significantly lower (p < 0.01) in laser and oral itraconazole combination group vs. oral itraconazole monotherapy, however mycological responses did not significantly differ [38]. Laser and oral terbinafine combination therapy resulted in significantly greater mycological and clinical cure rates at 4, 8, 12, 16, and 24 weeks vs. both laser and terbinafine monotherapies (p < 0.05, all) [41].
Adverse events were reported in seven studies, with almost half (3/7, 42.9%) not differentiating between procedural/medication and control treatment groups. Three studies (75%) reported greater adverse events in combination vs. monotherapy groups. Information on cost was not provided in any study.
We found that seven medication-based studies yielded conflicting results, with similar quality across studies. Two studies showed conflicting results for oral itraconazole and topical amorolfine therapy, with one study including 131 patients, multicenter, and 5.5 months of follow-up [28], versus 90 patients, single-center, and 9 months of followup [29]. Three studies showed conflicting results for oral terbinafine and topical ciclopirox combination therapy, with one study including 80 patients, single-center, 9 months of follow-up, and non-blinded [23], another including 73 patients, multi-center, 11 months of follow-up, and single-blinded [24], and a final study including 96 patients, single-center, 8.3 months of follow-up, and single-blinded [25]. Two studies had conflicting results on oral itraconazole and oral terbinafine combination therapy, with one study including 190 patients, multicenter, 16.6 months of follow-up, and single-blinded [30], versus 106 patients, multicenter, and single-blinded [31]. Given that these trials with conflicting results were comparable in terms of quality, without a clearly superior trial demonstrating greater efficacy for combination vs. monotherapy treatments, large, multicenter, double-blinded trials with sufficient follow-up are necessary to determine the efficacy of combination therapy for onychomycosis treatment.
In three medication-only studies that provided cost information, combination therapy was more cost effective than monotherapy, considering duration of usage and efficacy. However, this data cannot be extrapolated to include combination oral therapy with the newer topicals. In a review of data from the National Average Drug Acquisition Cost Medicaid Pharmacy Pricing database, 2013-2018 [72], inflation-adjusted costs changed at an annual rate of −18.2% and −3.4% for generic oral itraconazole and terbinafine, respectively, while brand name medications Kerydin, Jublia, and Diflucan increased 3.7%, 4.5%, and 17.2%, respectively. This data suggests that branded topical and oral antifungals are costly, and that prescribing them as part of combination regimens will increase health care costs. In a study evaluating the cost of topical efinaconazole 10% solution [73], a 48-week treatment course for one great toenail ($8057) was nearly 35 and 12 times more expensive than projected costs (using data from goodrx.com, accessed on 1 July 2021) of 3 months of treatment with oral terbinafine (250 mg/day, $233) and oral itraconazole (200 mg/day, $683), respectively, without consideration of relative efficacy. In an analysis of Medicare provider utilization and payment data, part D, 2013-2018 [74], total costs and costs per supply day increased yearly by 3091% and 144%, respectively, for topical efinaconazole, and decreased and increased by 12.5% and 42.4%, respectively, for topical tavaborole. Therefore, more research is necessary to assess cost vs. benefits of combination treatments compared to monotherapy.
We found that almost all studies examining combination procedural and medication therapy showed significant benefit compared to monotherapy. However, trials were relatively short, with an average follow-up of 32.5 weeks (SD: 14.3; range: 13.04-62.0 weeks). RCTs on photodynamic and laser monotherapies had a similar average follow-up of 35.3 weeks (SD: 11.5; range: 24.0-52.0 weeks) [75][76][77][78][79][80][81][82][83][84][85], which suggests that procedural studies, in general, lack the long-term follow-up that is sufficient to determine efficacy of onychomycosis treatment. Furthermore, in our review, some monotherapy and combination arms did not demonstrate efficacy for severe onychomycosis cases [35]. In a review of 24 laser trials on onychomycosis [86], there was limited evidence supporting lasers for onychomycosis cure, and only 30% and 20% of RCTs described methods of randomization or utilized blinding in their experimental design, respectively. Furthermore, in a systematic review of 25 RCTs investigating laser monotherapy for toenail onychomycosis [87], mycological cure was evaluated in only one study, and complete cure was not reported in any study. Mean OSI changes from baseline were minimal (range: −3.6. to +1.4), and efficacies of control and treatment groups were similar, thereby failing to demonstrate improvements in the US FDA approved endpoint "temporary increase of clear nail" [3]. Methodology of current laser trials is inadequate, both for monotherapy trials and for combination trials reported in this review. Therefore, laser monotherapy efficacy should be confirmed in rigorous RCTs, before it can be considered for use in conjunction with other onychomycosis medications.
While cost was not reported in any procedural and medication combination trials evaluated in this review, laser treatments are expensive, not covered by insurance, and typically require multiple monthly sessions [3]. Considering that laser therapies are less efficacious than topical or oral therapeutic options [6] and are costly, we caution against use of lasers in combination with antifungals until more robust RCTs are conducted, demonstrating superior efficacy with favorable cost-benefit profiles.
We found that combination therapy was generally well tolerated across medicationonly studies. Notably, in a systematic review and meta-analysis of 26 RCTs investigating monotherapy for toenail onychomycosis [5], the OR of adverse events in any treatment group did not significantly differ from placebo, except in the case of efinaconazole 10% solution (OR 1.28; 95% CI: 1.02-1.61), for transient application-site reactions. Therefore, since monotherapy for onychomycosis treatment is well tolerated, we recommend single medication therapy to limit adverse effects. In procedural and medication studies, we found greater reports of adverse events with combination therapy than monotherapy, however, sample sizes were small, and in all three studies, monotherapy was a topical or placebo treatment, rather than a procedural treatment. It is therefore uncertain whether the adverse event in the combination group was due to the procedure itself (i.e., skin irritation from laser treatment), or due to the combination of treatments. In a systematic review of 35 RCTs (1723 patients and 4278 nails with onychomycosis) [88], the majority of patients reported a mild-to-moderate burning sensation during laser treatment, with some reporting bleeding.
In pivotal RCTs investigating oral terbinafine monotherapy, adverse effects were transient, mild to moderate in severity, and not significantly different from placebo groups [89]. There were no reported laboratory abnormalities [90]. In the terbinafine package insert, liver enzyme abnormalities and taste disturbances were only reported in 3.3% and 2.8% of patients, respectively, with a discontinuation rate of 0.2% for both (https://www.accessdata. fda.gov/drugsatfda_docs/label/2012/020539s021lbl.pdf, accessed on 18 January 2022). Oral itraconazole monotherapy is well tolerated, with the most common reported adverse events being gastrointestinal discomfort and headache, both mild and transient [66], and with safety profiles similar between placebo and itraconazole treatment groups in a phase-III RCT [64]. In two phase-III multicenter RCTs assessing topical efinaconazole monotherapy [68], rates of adverse events were similar between treatment and vehicle groups in both studies, with most mild or moderate in severity, low rates of treatmentrelated discontinuation, and no clinically meaningful laboratory or vital sign changes from baseline. Therefore, when used as monotherapy, FDA-approved onychomycosis treatments have limited side effects, with none that are life threatening, and rarely lead to medication discontinuation. However, it is important to note that oral itraconazole and fluconazole are contraindicated with certain medications. In addition, oral itraconazole, fluconazole, and terbinafine, can alter the plasma concentration of select medications [6]. Considering the frequency of drug interactions of oral onychomycosis therapies with other medications, initiation of oral combination therapy would therefore require close monitoring of patients on multiple medications, with medication adjustments or dose reductions more likely than for patients on monotherapy treatment.
In a meta-analysis of 26 RCTs investigating systemic monotherapy for toenail onychomycosis [5], there was significantly greater OR of achieving mycological cure for all treatments vs. placebo, with continuous itraconazole 200 mg (OR: 18.61; 95% CI: 7.40-46.81) and continuous terbinafine 250 mg (OR: 16.41; 95% CI: 6.49-41.47) the most efficacious treatments. Monotherapy therefore is effective in treating onychomycosis. Therefore, taken together with our analysis of combination therapy for onychomycosis, monotherapy should be considered as a first-line treatment option prior to initiating combination therapy.
Predisposition to mucocutaneous fungal infections may be due to specific genotypes. For example, Tyr238X dectin-1 or caspase recruitment domain containing protein-9 mutations cause impaired β-glucan recognition and cytokine responses, and defects in major histocompatibility complexes may interfere with initial fungi recognition and prevent T-cell activation. Alterations in intercellular adhesion molecule-1 may prevent immune cells from migrating to infected tissues, and elevated levels of T-regulatory cells may modify T-cell behavior, all of which may increase susceptibility to dermatophyte and Candida spp. infections [91,92].
In patients with poor prognostic factors, the advantages of combination therapy may outweigh the risks and costs, and should be considered especially in patients who failed previous treatments. Importantly, we found that in many combination trials, patients with poor prognostic factors or risk factors for recurrence were excluded. In medication-only studies, six provided no information on exclusion criteria, while 66.7% (6/9) of studies with exclusion criteria included a risk factor for poor prognosis or high risk of recurrence. There was an upper age limit for inclusion in the study in 20% (3/15) of studies. In procedural and medication studies, all studies included information on exclusion criteria, with 66.7% (10/15) including a risk factor for poor prognosis or high risk of recurrence. Upper age limits for inclusion were reported in 26.7% (4/15) of studies. Medication-based monotherapy trials showed good efficacy in some subgroups, including older adults [52], diabetics [75,[93][94][95], and those with severe onychomycosis [66] or dermatophytoma [96]. Therefore, published combination trials excluded many patients who may have benefited most from combination therapy, such as older patients and the immunocompromised, and future trials must be conducted in these patient populations to determine the efficacy of combination therapy in difficult-to-treat onychomycosis cases.
Considering the limited number of studies available for review, conflicting results in medication-only based studies, limited efficacy of procedural combination treatments, and cost considerations, we recommend that combination therapy be considered as a second-line treatment option for patients with resistant cases of onychomycosis or poor prognostic features. Our data showed that combinations of oral griseofulvin and topical tioconazole or bifonazole, or oral terbinafine and topical amorolfine, demonstrated significant improvements in combination vs. monotherapy group in all trials. Oral terbinafine, itraconazole, and griseofulvin are US FDA approved for onychomycosis treatment, however, griseofulvin is no longer commonly used, considering its inferior efficacy compared to other treatment options, lengthier treatment courses, and higher risk of adverse effects [3].
Topical efinaconazole 10% and tavaborole 5% solution are US FDA approved for toenail onychomycosis, with ciclopirox 8% nail lacquer approved for both fingernail and toenail onychomycosis [6], and topical tioconazole, bifonazole, and amorolfine are not currently available in the US. A panel of expert dermatologists, podiatrists, and a microbiologist [4], recommended that terbinafine and efinaconazole 10% solution should be used as first-line oral and topical onychomycosis treatments, respectively. Considering these recommendations, the results from our study, and accessibility of antifungal treatments in the US, we recommend oral terbinafine as first line, used alone for straightforward moderate to severe onychomycosis cases, and in combination with efinaconazole 10% solution, tavaborole 5% solution, or ciclopirox 8% nail lacquer for patients who failed previous treatments or who have poor prognostic factors. Nonetheless, medication selection must be tailored to each individual patient with consideration of the extent of nail involvement, infecting pathogen, comorbidities, concomitant medications, expense, and patient preferences [6].
Retinoids in combination with antifungals may be effective for onychomycosis treatment. Retinoids have both in vitro and in vivo antimicrobial activity against fungi, with tretinoin and isotretinoin most effective against M. furfur, tazarotene against dermatophytes, and tretinoin against A. fumigatus and C. albicans. By preventing hyphal germination necessary for biofilm formation, all-trans retinoic acid may also be effective against Candida biofilm-related infections, which are typically difficult to treat due to multidrug resistance [97]. In a study performed after our systematic search on 135 patients with toenail and/or fingernail onychomycosis receiving oral itraconazole pulse monotherapy, oral acitretin monotherapy, or combined pulsed itraconazole and acitretin for 3 months [98], mycological cure was 51.1%, 28.9%, and 80%, respectively, and complete cure was 20%, 28.9%, and 53.3%, respectively (p ≤ 0.05). OSI scores significantly improved in the combination group compared to itraconazole monotherapy (p = 0.005) and acitretin monotherapy (p = 0.006) groups, with no difference observed between the monotherapy groups (p = 0.95). Therefore, retinoids combined with antifungals may increase efficacy compared to monotherapy treatments. Large RCTs are needed to corroborate these findings.
There are several limitations to our study. While we performed an exhaustive search of combination studies for onychomycosis treatment, only a small number met inclusion criteria for our review. Different endpoints and medication combinations were used across studies, limiting the ability to make direct comparisons of study findings. It is also difficult to draw comparisons about studies that use keratolytic agents in combination with topical antifungals. Furthermore, study inclusion/exclusion criteria differed across studies (Table 3). Nail characteristics variably specified fingernail vs. toenail, number of affected nails, matrix involvement, and nail plate surface area and thickness. T. rubrum was the etiological agent in most studies, while others did not list the fungal organisms, or reported mixed infections. Vehicles differed, including nail lacquers, creams, and solutions, likely contributing to differences in efficacy. The majority of studies were conducted in Europe or North America, limiting the diversity of participants.
Future research should be directed at conducting combination clinical trials that are large, multicenter, and randomized, with sufficient follow-up, and including diverse patient populations across different geographic locations, to determine the value of combination therapy for treatment of onychomycosis, and to establish standard treatment regimens. Until such studies are conducted, and a clear clinical benefit of combination therapy is demonstrated, we recommend combination therapy as a second-line option in patients with difficult-to-treat cases or poor prognostic factors.