Discovery, Development, Inventions and Patent Review of Fexinidazole: The First All-Oral Therapy for Human African Trypanosomiasis

Human African trypanosomiasis (HAT or ‘sleeping sickness’) is a neglected tropical disease. If untreated, it is always fatal and leads to death. A few treatments are available for HAT, but most of them require a skilled professional, which increases the financial burden on the patient. Recently, fexinidazole (FEX) has been approved by the European Medicine Agency (EMA) and the United States Food and Drug Administration (USFDA) as the first all-oral therapy for the treatment of stage-1 (hemolymphatic) as well as stage-2 (meningoencephalitic) of HAT. Before the FEX approval, there were separate treatments for stage-1 and stage-2 of HAT. This study reviews the discovery, development timeline, inventions, and patent literature of FEX. It was first approved by EMA and USFDA in 2018 and 2021, respectively. FEX was also added to the World Health Organization’s list of essential drugs in 2019. The patent literature search revealed many types of patents/patent applications (compound, salt, process, method of treatment, drug combinations, and compositions) related to FEX, which have been summarized in this article. The authors foresee a great scope to develop more inventions based on FEX (novel salts, polymorphs, drug conjugates, cyclodextrin complex, etc.) for the treatment of many protozoal diseases (Leishmaniasis and Chagas disease), inflammatory diseases, and other microbial infections. New combinations of FEX with other treatments of HAT may also provide fruitful results. This review might be useful to the scientists working on the HAT and other neglected diseases to develop novel inventions and innovations of therapeutic relevance.

. Current treatment option for human African trypanosomiasis.

Drug (Dosage Form and Administration) Comments
Treatment of Stage 1 of Human African Trypanosomiasis Pentamidine (Solution for inhalation/Injection) A skilled and trained professional is needed for drug administration. It is administered as a single daily intramuscular/intravenous injection for seven days. It can cause severe hypotension after intramuscular/intravenous administration, hypoglycemia, acute pancreatitis, and cardiac arrhythmias, and is effective against stage-1 of g-human African trypanosomiasis only because it does not cross the blood-brain barrier efficiently [9].

Suramin (Intravenous, injection)
A skilled and trained professional is needed for drug administration. It is mainly used for stage-1 of r-human African trypanosomiasis, and rarely used for stage-1 g-human African trypanosomiasis. It can cause renal toxicity and anaphylactic reactions [10].

Drug (Dosage Form and Administration) Comments
Treatment of Stage 2 of Human African Trypanosomiasis Nifurtimox (Tablet, Oral) The combination of nifurtimox with eflornithine is the first-line treatment for stage-2 of human African trypanosomiasis. It has potential for genotoxicity, carcinogenicity, fetal toxicity, worsening of neurological and psychiatric conditions, hypersensitivity, decreased appetite and weight loss, and porphyria [11].

Eflornithine (Intravenous, injection)
A skilled and trained professional is needed for drug administration and requires long therapy. It can cause fever, pruritus, hypertension, cough, anorexia, nausea, vomiting, diarrhea, abdominal pain, headaches, and is the second-line treatment for stage-2 of g-human African trypanosomiasis [12].

Melarosoprol (Intravenous, injection)
A skilled and trained professional is needed for drug administration and is effective for stage-2 g-human African trypanosomiasis. Its administration is painful and toxic. The adverse events may be life-threatening including encephalopathic syndrome [13].
Nifurtimox-eflornithine combination therapy (Oral Nifurtimox + Intravenous Eflornithine) A skilled and trained professional is needed for drug administration. It needs systematic hospitalization and is mainly used for stage-2 of g-human African trypanosomiasis [14].
It is evident from Table 1 that few treatments are available for HAT, wherein nifurtimox and pentamidine are USFDA approved drugs. Most of these treatments are injectable and are effective either for stage-1 or stage-2 of HAT [9][10][11][12][13][14]. None of the available treatments are effective in treating stage-1 as well as stage-2 of HAT. Most of the treatments require a skilled person for IV administration, which increases the financial burden on the patient. In 2018, 2019, and 2021, FEX was approved by the EMA, the Democratic Republic of Congo, and USFDA for the treatment of both first-stage (hemolymphatic) and secondstage (meningoencephalitic) of HAT due to gHAT in patients 6 years of age and older and weighing at least 20 kg [15][16][17] (Table 2). Some reviews on FEX have been published [14,[17][18][19]. However, none of them is directed to the inventions and patent literature of FEX. To make new inventions and innovations related to any drug, it is crucial to understand the prior inventions related to the drug. Therefore, the authors intended to write the titled review on FEX that will be helpful to the scientists of the pharmaceutical industry/academia to generate new ideas and develop inventions related to FEX.

Fexinidazole (FEX)
FEX ( Figure 1), a 2-substituted-5-nitroimidazole antiprotozoal agent, chemically is 1-methyl-2-{[4-(methylthio)phenoxy]methyl}-5-nitro-1H-imidazole (molecular formula: C 12 H 13 N 3 O 3 S: molecular weight: 279.3; CAS registry number: 59729-37-2; Other identifiers: HOE-239) [14,[17][18][19]. It is a yellow crystalline, non-hygroscopic powder, which is practically insoluble in water, sparingly soluble in acetone and acetonitrile, very slightly soluble in ethanol, and slightly soluble in methanol. It is an achiral molecule and does not have optically active isomers [14,[17][18][19]. It is marketed as a base, and only one polymorphic form-I is identified to date. FEX exhibits solubility and permeability characteristics consistent with BCS class II drugs [20]. The structure of FEX was disclosed by Hoechst AG (Germany) in 1977 [21]. However, the company abandoned this compound due to its strategic planning. The Drugs for Neglected Diseases initiative (DNDi) was established in 2003. In 2005, the DNDi scientists and the scientists of Sanofi screened about 700 nitroheterocyclic compounds against HAT. This led to the identification of FEX (HOE239) as a promising candidate for further development [18,19]. The positive pre-clinical studies [22] on FEX led to an agreement between Sanofi and DNDi, where DNDi was responsible for the preclinical/clinical/pharmaceutical development and Sanofi was accountable for the industrialization/production/registration/distribution of the drug. FEX is the first new chemical entity developed by DNDi. Its Phase I study was started in 2009, whereas the Phase II/III trial was started in 2012 at the Democratic Republic of Congo and the Central African Republic. The promising results of the Phase III study of FEX were declared by DNDi in 2017. A summary of the clinical trial data of FEX is mentioned in Table 3. The European Medicines Agency, Democratic Republic of Congo, and the United States Food and Drug Administration approved FEX for the treatment of HAT in 2018, 2019, and 2021, respectively. The drug approval process in other endemic countries is also underway. The development timeline of FEX is provided in Scheme 1 [14,[17][18][19].

Pharmacology of FEX
The nitroheterocyclics require bio-activation of their nitro groups to become biologically active. The nitro-reductase (NTR) enzyme is shown to activate the nitroimidazoles FEX and nifurtimox in the parasites [23]. The NTR enzyme of the parasite activates FEX and its active metabolites (FEX sulfoxide and FEX sulfone) (Scheme 2) to generate reactive amines that damage the DNA and proteins of the parasite. However, the precise mechanism by which FEX, FEX sulfoxide, and FEX sulfone demonstrate action against T. brucei is still unknown [16]. One study has stated that the action of the NTR enzyme on nitroaromatic compounds (Nifurtimox and Benznidazole) leads to the generation of different toxic fragments (open chain nitrile and hydroxylamine) that exert their effects against T. brucei [24]. The chemical structure of Benznidazole is similar to FEX [24]. Therefore, the authors believe that there is a possibility that the NTR enzyme converts the nitro group of FEX to nitroso intermediate followed by the generation of hydroxylamine intermediate. The hydroxylamine intermediate may be responsible for the activity of FEX against T. brucei.

Scheme 2. Transformation of fexinidazole into its metabolites.
FEX is well absorbed orally. After oral administration, FEX rapidly converts to its active metabolites, FEX sulfoxide (M1) and FEX sulfone (M2) (Scheme 2) [16,25]. FEX tablet must be swallowed (not chewed) during or just after a solid meal (not liquid meal) to increase its absorption (2-3 times), especially for stage-2 HAT. A dose administered without food is considered as a dose skipped [16]. The apparent volume of distribution of FEX is 3222 ± 1199 L [16], whereas the protein binding of FEX, M1, and M2 are 98%, 41%, and 57%, respectively [16,25]. FEX is rapidly metabolized via hepatocytes to the M1 and M2 metabolites (Scheme 2) which provide most if not all the trypanocidal activity against Trypanosomia brucei gambiense strains of the parasite. The sulfone metabolite (M2) does not seem to go through further metabolism [16,22,25,26]. The elimination of FEX is almost completely extra-renal, wherein the majority of the excreted metabolites are M1 and M2 [16,25]. The reported half-life (hours) of FEX, M1, and M2 are 15.0, 16.0, and 23.0 h, respectively [16,25], whereas the clearance (liter/hour) of FEX is 161.0 [16]. The adverse effects of FEX include QT prolongation, CNS and psychiatric-related adverse reactions, risk of infection, hepatotoxicity, and disulfiram-like reactions [16]. The toxic (overdose) effects of FEX in adults (3600 mg/day for 14 days) comprise elevated levels of transaminases, panic attack, and vomiting. The safety and efficacy of FEX for the treatment of both stages of HAT have been established in children ≥ 6 years old and weight ≥ 20 kg (National clinical trial numbers 02169557 and 02184689, Table 3). However, such pediatric patients may be more sensitive to vomiting. The safety and efficacy of FEX have not been established in children <6 years old and weight <20 kg [16]. The overdose in pediatric HAT patients causes elevated potassium, reduced calcium levels, and vomiting. The treatment of FEX toxicity is supportive therapy with monitoring due to the lack of its antidote [16,22]. The metabolism of FEX involves many enzymes including CYP1A2, CYP2B6, CYP2C19, CYP2D6, CYP3A4, CYP3A5, and flavin mono-oxygenase-3 (FMO-3). Therefore, drugs, including herbal medicines, which are metabolized by these enzymes may interfere with the FEX treatment [16].
In clinical trials, a difference in the efficacy of FEX has been detected in both stages of HAT (National clinical trial number 01685827; Table 3). In the first stage of HAT, a treatment success rate of 98.7% and 97.6% was observed. However, a decrease in the efficacy of FEX (86.9%) was seen in a comparison of nifurtimox-eflornithine combination therapy (98.7%) in the second stage of HAT. Furthermore, all-cause mortality was higher in FEX treated patients (4.4%) than nifurtimox-eflornithine combination therapy (0%). Accordingly, the patients of the second stage of HAT can be treated with FEX in the absence of other treatment options like nifurtimox-eflornithine combination therapy [16]. The safety and efficacy of FEX have not been fully established in certain populations (children below 6 years, pregnant/lactating women, and hepato-compromised patients) [16].

Clinical Trials on FEX
A search on the clinicaltrial.gov database [27] was done utilizing keywords "fexinidazole, HOE-239, and HOE239". A total of 12 clinical studies were found, which are listed in Table 3.
It is evident from Table 3 data that the Drugs for Neglected Diseases initiative is developing FEX for the treatment of other neglected diseases also (Chagas disease, South American Trypanosomiasis, Leishmaniasis, and Trypanosomiasis). These clinical trials indicate the foreseeable potential of FEX to treat many neglected diseases, and make FEX an important molecule for neglected diseases.

Patent Searching
The patents searching was done on 6 November 2021, utilizing different patent databases, namely, Espacenet [28], USPTO [29], Patentscope [30], and Sci-finder [31]. The patent searching methodology is described in Scheme 3. The bibliographic data of the patents/patent applications (Table 4) was obtained from Espacenet, Patentscope, and USPTO. The patents/patent applications that explicitly or implicitly cover FEX or any of its synonyms in the claim section were included in this review and analyzed.   US4042705A claims 1-methyl-2-(phenyloxymethyl)-5-nitro-imidazole derivatives of formula I (Figure 2), which generically covers FEX and FEX sulfoxide (M1 metabolite of FEX) [21]. These compounds were developed based on the chemical structure of metronidazole of formula II (Figure 2). US4042705A does not provide any experimental data about the antiprotozoal activity of any compound. However, it claims a pharmaceutical composition (oral or local route) of the claimed compound ( Figure 2) for treating protozoal diseases as well as diseases caused by bacteria and fungi in admixture or conjunction with a pharmaceutically acceptable carrier and/or excipient.
This patent provides two methods for preparing FEX, which have been summarized in Schemes 4 and 5. As per Scheme 4, FEX is synthesized by mixing a solution of 4-methylmercapto-phenol (0.1 mol; 14 g) in dimethylformamide (DMF) with potassium carbonate powder (0.1 mol, 13.8 g). To this mixture is added, a solution of 1-methyl-2chloromethyl-5-nitro-imidazole (0.1 moL, 17.6 g) in DMF gradually with stirring at an ambient temperature. The resulting mixture is stirred for 1 hr, maintaining the temperature of this exothermic reaction below 35 • C. The precipitate obtained after pouring the reaction mixture over crushed ice is filtered and crystallized with methanol/water to obtain a light yellow crystalline solid in 70% yield. FEX can alternatively be prepared from 1-methyl-2-(4thiocyanatophenyloxymethyl)-5-nitro-imidazole. The starting material is added slowly to the mixture of concentrated sulfuric acid and dimethyl sulfate under nitrogen atmosphere at room temperature and then kept aside for 12 h followed by heating at 60 • C for 30 min.
The resulting mixture is poured over crushed ice to yield precipitates which can be purified using column chromatography to obtain pure crystals of FEX.  US4042705A also mentions one method for preparing FEX sulfoxide which has been depicted in Scheme 6 [21]. A solution of FEX (0.1 mol; 27.9 g) in chloroform (200 mL) is added slowly to a solution of m-chloroperbenzoic acid (0.1 moL; 17.3 g) in chloroform and then stirred for 60 min at room temperature. Dilute sodium carbonate solution is added to the reaction mixture and the organic layer is separated, dried over anhydrous sodium sulfate, and finally evaporated to obtain a residue which upon recrystallization with ethanol produces pure yellow crystals of 1-methyl-2-(4-methyl-sulfinylphenyl-oxymethyl)-5-nitroimidazole in 73% yield.

Process Patents
CA1079738A claims the preparation of FEX sulfone metabolite (M2) by the oxidation of FEX employing an oxidizing agent (hydrogen peroxide, peracetic acid, pertrifluoroacetic acid, chloroperbenzoic acid, nitric acid, chromic acid, chromic acid anhydride, permanganates, hypochlorites, chlorates, perchlorates, periodates, and nitric oxides) and an inert solvent (acetic acid, trifluoroacetic acid, methylene chloride, or chloroform) or dispersing agent at a temperature of 0-100 • C [32]. Example 1 of CA1079738A provides preparation of FEX sulfone metabolite (M2) (yellowish crystals recrystallized from isopropanol; Yield = 93%; m.p. = 157 • C) from FEX using 35% hydrogen peroxide solution and acetic acid at a temperature ranging from room temperature to 60 • C. The patent states that M1 can also be converted to M2 utilizing the same reaction, but employing half the amount of the oxidizing agent [32].
US9758488B2 discloses a method of preparing FEX (Scheme 7) with a limited number and amounts of process-related impurities (Figure 3). The patent also discloses hydrochloride salt of FEX [33]. According to this method, FEX of high purity grade and in substantial yield in industries can be obtained in four simple and easy steps. In step 1 of this nucleophile substitution reaction, 1-methyl-2-hydroxymethyl-5-nitro-imidazole is reacted with a solution of methanesulfonyl chloride in acetone. The reaction mixture is stirred at room temperature in the presence of a suspension of powdered alkaline carbonate in an anhydrous polar aprotic organic solvent such as acetone or acetonitrile to produce the intermediate 1-methyl-5-nitro-1H-imidazol-2-yl)methyl methanesulfonate.
Step 2 involves the reaction of the resulting reaction mixture with a solution of 4-methylmercapto-phenol in anhydrous acetone. After stirring, heating, and quenching with water, the acetone phase is separated and mixed with aqueous hydrochloric acid over 1 h to obtain FEX hydrochloride salt (72% yield) in step 3. Finally, FEX hydrochloride salt is converted into a free FEX base by neutralization with an aqueous ammonia solution. The process of this patent is distinct from the prior art process because a single solvent (acetone) is used in all the steps; a single catalyst (potassium carbonate) is used in the first two steps; synthesis of FEX hydrochloride takes place in one step, and the FEX is obtained in good yield and purity. Two processes for preparing FEX are also provided in non-patent literature [34,35]. Since our review focuses on patent literature, we have not discussed it here in detail.

Patents/Applications Related to the Method of Treatment
US9585871B2 claims a method for curing a canine animal of a Leishmania infantum parasite infection employing an effective amount of FEX, which is metabolized in the canine animal to produce a parasiticidal effective amount of FEX sulfone (M2) and sulfoxide (M1) metabolites that are responsible for killing the Leishmania infantum parasite [36]. The metabolites are present in the canine animal's plasma in concentrations exceeding 2 ng/mL for at least one hour of each day throughout the FEX administration. The invention demonstrated that FEX treatment can substantially eliminate or cure L. infantum infection in canines. No relapse was observed 180 days after treatment initiation. The patent exemplifies the efficacy of FEX in dogs Infected with L. infantum [36].
WO9912547A1 states that bio-reductive compounds are a class of compounds requiring metabolic reduction to generate cytotoxic metabolites [37]. This patent application claims the use of a bio-reductive compound (FEX, metronidazole, benznidazole, tinidazole, ornidazole, misonidazole, secnidazole, carnidazole, nimorazole, panidazole, or satranidazole) capable of targeting tissues having an enhanced reductase activity in the manufacture of a medicament for use in the treatment of rheumatoid or osteoarthritis, Crohn's disease, or periodontitis. The bio-reductive compound can also be used in combination with a second medicament such as non-steroidal anti-inflammatory agent (NSAID), a corticosteroid or glucocorticoid, an alkylating agent, an antimalarial, a gold compound, penicillamine, sulphasalazine, methotrexate, or azathioprine [38]. WO9912548A1 [38] is a family member of WO9912547A1 [36]. It also claims the use of a bioreductive compound in the manufacture of a medicament for use in the treatment of an inflammatory condition associated with hypoxia, ischemia, diabetes, stroke, sepsis, Alzheimer's disease, cancer, kidney disease, digestive diseases, liver disease, tissue transplantation, the healing of wounds, fibrotic disorders, cardiovascular reperfusion injury, cerebral reperfusion injury, cystic fibrosis, psoriasis, para-psoriasis, ulcers, AIDS, and inflammatory bowel disease [38]. US2021220335A1 claims a method for the prevention and/or treatment of an infectious disease (diarrhea, colitis) using a nitroimidazole (metronidazole, bamnidazole, carnidazole, dimetridazole, FEX, ipronidazole, ornidazole, panidazole, secnidazole, ternidazole, tinidazole), wherein the infectious disease is caused by bacteria from the genus Clostridium (antibiotic-resistant bacterial strain) [39]. The nitroimidazole can also be co-administered with a berberine alkaloid for a synergistic effect. The patent application provides biological activity data of metronidazole and berberine, but it is silent about the biological activity data for FEX and its combination with berberine [39].

Patents/Applications Related to the Combination of FEX with Other Molecules
WO2017072523A1 claims use of delamanid and its derivatives alone or in combination with one or more additional compounds of therapeutic utility (FEX, FEX sulfone, nifurtimox, etc.) to treat parasitic disease caused by or associated with parasites selected from the order Kinetoplastida consisting of species belonging to the genera Leishmania and Trypanosoma including visceral leishmaniasis, mucocutaneous leishmaniasis, cutaneous leishmaniasis, African trypanosomiasis, and Chagas' disease [40]. This patent application does not exemplify the treatment of any parasitic diseases using a combination of delamanid and its derivatives with FEX [40].
US10392363B2 [41], US10562880B2 [42], and US10752606B2 [43] are members of the same patent family. US10392363B2 claims a method of treating or preventing protozoan colonization or infection (trypanosomosis, amoebiasis, Chagas disease, giardiasis, leishmaniasis, malaria, toxoplasmosis, etc.) using a therapeutically effective amount of carbonimidic dihydrazides and their salt alone or in combination with an antiprotozoal agent like FEX [41]. US10562880B2 claims a method of treating or preventing a protozoan infection using a therapeutically effective amount of a pyrimidine derivative alone or in combination with nitroimidazoles (FEX) [42]. US10752606B2 claims a method of treating a protozoan infection using NCL258 and NCL261 alone or in combination with nitroimidazoles (FEX) [43]. The US10392363B2 [41], US10562880B2 [42], and US10752606B2 [43] do not provide any experimental data about the efficacy of the combination of carbonimidic dihydrazides, pyrimidine derivatives, NCL258, and NCL261 in combination with FEX.
US2021322329A1 claims a lipid nanoparticle-based pharmaceutical composition containing particles of one or more active substances (benzimidazole, nifurtimox, ertanidazole, buthionine sulfoximine, eflornithine, melarsoprol, pentamidine, suramin, and FEX) for the treatment of diseases produced by trypanosomes and for the treatment of tumors of neural origin. This patent application is silent about the biological effect of the lipid nanoparticles of FEX [46]. US2021052498A1 claims a nanocarrier for treating Chagas disease comprising a poly(ethylene glycol)-block-poly(propylene sulfide) copolymer; and a therapeutic agent (benznidazole, nifurtimox, FEX) for treating Chagas disease and Trypanosoma cruzi infection [47]. The patent application states that the subject receiving such nanocarrier formulation does not experience one or more side effects associated with the therapeutic agent because the therapeutic agent can be administered at substantially lower levels than the free form drug. The patent application exemplifies the biological effects of the nanocarrier of benznidazole, but is silent about the effects of the nanocarrier of FEX [47].
WO2021123775A2 relates to engineered platelets with chimeric platelet receptors (CPR) with the desired target specificity. Furthermore, the engineered platelets may encompass a therapeutic agent which may be released upon activation of the platelet. The patent application mentions the names of hundreds of therapeutic agents including FEX. However, this application does not provide any experimental data for FEX [48].
US9016221B2 claims a polymeric article having a surface topography for resisting bioadhesions of organisms comprising a biologically active agent [49]. The claims mention a large number of biologically active agents (antibiotics, anti-proliferative/anti-mitotic/ alkylating agents, antiplatelet agents, anti-metabolites, anti-coagulants, anti-migratory agents, antisecretory agents, anti-inflammatory agents, and anti-sense oligonucleotides) including FEX. However, no example of a polymeric article comprising FEX has been provided in the patent. The claimed polymeric article may find utility in biomedical implants, biomedical instruments, hospital surfaces, clothing/protective personal wear, biomedical packaging, cleanroom surfaces, food packaging, food preparation surfaces, etc. [49].

Conclusions
The approval of the orally effective FEX tablets by EMA and USFDA is a landmark for the treatment of HAT. Some patents/patent applications related to salt, process, method of treatment, drug combinations, and compositions of FEX have been filed. However, there remains a great scope to develop more inventions based on FEX for the treatment of other protozoal and microbial infections. New combinations of FEX with other treatments of HAT may also provide fruitful results. The development of FEX has motivated scientists to develop more oral treatments and vaccines against HAT and other protozoan diseases. Accordingly, new oral treatments for HAT are also anticipated in the future.

Expert Opinion
HAT is caused by two subspecies of T. brucei called T.b. gambiense (causes gHAT) and T.b. rhodesiense (causes rHAT). This disease is prevalent in sub-Saharan countries. The gHAT is a chronic illness of endemic areas and constitutes about 85-90% of the total cases of HAT. The rHAT is an acute systemic illness of local populations/travelers and constitutes about 10-15% of the total cases of HAT. Only a few patient non-compliant treatments (mainly IV treatments) were available for this illness until 2018 [50]. None of the treatments was effective against both stages (stage-1 and stage-2) of HAT [50,51]. The efforts to develop oral and patient-compliant interventional therapy have resulted in the development of FEX. The EMA approved FEX in 2018 as the first all-oral treatment for HAT, which could be used in both stages of HAT. FEX became part of WHO's list of essential drugs in 2019, and USFDA also approved FEX in 2021 (Figure 1).
The patent searching of FEX resulted in the identification of 17 patents/patent applications belonging to 14 patent families (Table 4). It can be observed that no patent/patent application related to FEX was published from 1980-1998 and 2000-2014 ( Figure 4). This may be because, during 1980-1998, the development of FEX was not part of the strategic planning of Sanofi/Hoechst AG, and during 2000-2014 FEX was in the initial phase of the clinical trial for an orphan disease. It is quite interesting to note that Sanofi/Hoechst, the innovator of FEX, has only three patents to its credit (Table 4) ( Figure 5). The first patent was the compound patent of FEX, which has expired [21]. The second expired patent belongs to a process for preparing FEX-sulfone (M2) [32]. The third patent relates to a process for preparing FEX, which is still enforceable [33]. The majority of the published patents/patent applications were filed in Europe (Germany and United Kingdom) followed by the USA and Australia (Table 4). The identified 17 patents/patent applications of 14 patent families claimed different inventions of FEX ( Figure 6). The compound patent of FEX [21] has expired long ago and is also not listed in the orange book of the USFDA [52,53]. Since this patent has expired, it is not eligible for any patent term extension [54,55]. Accordingly, we trust that the generic entry in the USA will be after the expiry of the orphan drug exclusivity (16 July 2028) ( Table 2). Our search revealed one enforceable patent related to the process for preparing FEX [33]. However, it can easily be designed around by following the process for preparing FEX provided in the compound patent [21]. One patent discloses the hydrochloride salt of FEX [33]. This indicates that acid addition salts of FEX are possible, and can be developed to check their utility as a therapeutic agent and to develop a novel dosage form of FEX. The base of FEX shows polymorphism, wherein crystalline Form-I is known [20]. Accordingly, patent filings related to new salts and polymorphs of FEX are foreseeable. Some patents/patent applications claiming the use of FEX to treat kinetoplastid diseases, inflammatory diseases, and other microbial infections alone or in combination with other compounds were also identified [36][37][38][39][40][41][42][43][44]. However, most of these patents/patent applications did not provide any experimental data to support their claims. Accordingly, we foresee a scope to carry out such experiments to develop a new combination of FEX with other therapeutic agents. It has been observed that most of the treatments are given by IV routes. There is a possibility to make new oral dosage forms of such treatments with fewer side effects, for example, the drugs may be encapsulated into a cyclodextrin molecule. This strategy has been used on melarsoprol [56]. The cyclodextrin complex of FEX in combination with other IV HAT treatments may provide promising results. FEX is not considered as the first-line treatment of the second stage of HAT in the presence of nifurtimox-eflornithine combination therapy [16]. Accordingly, the authors see a patentable opportunity to identify a novel drug combination therapy of FEX that may be a patient compliant therapy for the patients suffering from the second stage of HAT. The authors also foresee the development of FEX to treat Chagas disease and Leishmaniasis along with other neglected diseases (Table 3).
HAT has been targeted by the WHO, which is committed to terminating its transmission by 2030. The efforts of the WHO have led to a promising decrease in the number of cases at the global level [2,58,61]. Some gaps still exist and need to be addressed to fulfill the commitment of the WHO. These gaps include eliminating the transmission of tsetse flies through asymptomatic, but infected domestic animals and travelers [61]. The authors trust that the introduction of FEX therapy is a welcome advancement for HAT treatment, which in combination with the vector control would help to eliminate HAT in the future.