A Glance at the Development and Patent Literature of Tecovirimat: The First-in-Class Therapy for Emerging Monkeypox Outbreak

Monkeypox disease (MPX) is currently considered a global threat after COVID-19. European Medicines Agency (EMA) approved Tecovirimat in capsule dosage form (200 mg) as the first treatment for MPX in January 2022. This article highlights Tecovirimat’s development and patent literature review and is believed to benefit the scientists working on developing MPX treatments. The literature for Tecovirimat was gathered from the website of SIGA Technologies (developer of Tecovirimat), regulatory agencies (EMA, United States Food and Drug Administration (USFDA), and Health Canada), PubMed, and freely accessible clinical/patent databases. Tecovirimat was first recognized as an anti-orthopoxvirus molecule in 2002 and developed by SIGA Technologies. The USFDA and Health Canada have also recently approved Tecovirimat to treat smallpox in 2018 and 2021, respectively. The efficacy of Tecovirimat was verified in infected non-human primates (monkeys) and rabbits under the USFDA’s Animal Rule. Most clinical studies have been done on Tecovirimat’s safety and pharmacokinetic parameters. The patent literature has revealed inventions related to the capsule, injection, suspension, crystalline forms, amorphous form, and drug combinations (Tecovirimat + cidofovir) and process for preparing Tecovirimat. The authors foresee the off-label use of Tecovirimat in the USA and Canada for MPX and other orthopoxvirus infections. The authors also trust that there is immense scope for developing new Tecovirimat-based treatments (new drug combinations with other antivirals) for orthopoxvirus and other viral diseases. Drug interaction studies and drug resistance studies on Tecovirimat are also recommended. Tecovirimat is believed to handle the current MPX outbreak and is a new hope of biosecurity against smallpox or orthopoxvirus-related bioterrorism attack.

nologies (developer of Tecovirimat), regulatory agencies (EMA, United States Food and Drug Administration (USFDA), and Health Canada), PubMed, and freely accessible clinical/patent databases. Tecovirimat, TPOXX, SIGA-246, SIGA246, ST-246, ST246, and SIGA Technologies, alone or in combination with other keywords (smallpox/monkeypox), were used to collect the literature from the sources mentioned earlier.

Worldwide Approval
Tecovirimat monohydrate has been approved by the EMA [16], USFDA [17], and Health Canada [20,21] to treat smallpox (Table 1). However, only EMA has approved Tecovirimat to treat monkeypox [16]. The chemical structure of Tecovirimat has some chiral carbons. Therefore, it exists in many stereoisomeric forms. However, only one stereoisomer is used in the approved dosage form of Tecovirimat ( Figure 1) [18,19]. Tecovirimat monohydrate (a non-hygroscopic white crystalline solid) is insoluble in water at a pH range of 2.0-6.5 [19] but demonstrates high permeability in CaCo-2 cells. Accordingly, Tecovirimat monohydrate is classified as a BCS-II drug [18,19]. Tecovirimat exists in different polymorphic forms, including crystalline Tecovirimat monohydrate Form I, amorphous form, hemihydrate Form V, and other crystalline forms. The crystalline Tecovirimat monohydrate Form I is used to develop the marketed dosage form (capsule). Furthermore, the crystalline Tecovirimat monohydrate Form I can be prepared consistently by recrystallization from a mixture of ethyl acetate and water [18].

Development
Tecovirimat was recognized in 2002 as an effective molecule against orthopoxvirus-induced cytopathic effects by a high throughput screening process of 356,240 compounds [25,26]. The chemical structure of Tecovirimat has been provided first time in Patent Cooperation Treaty (PCT) Patent Application Publication Number WO2004112718A2 (previously owned by Viropharma) [27]. The crystalline Tecovirimat monohydrate used as the active ingredient of TPOXX was disclosed in 2011 for the first time in the United States Patent Application Publication number US2011236434A1, which has now been granted as US9339466B2 (SIGA Technologies) [28]. SIGA Technologies have developed TPOXX (capsule and intravenous dosage forms of Tecovirimat) in collaboration with the Biomedical Advances Research and Development Authority (BARDA) of the US Department of Health and Human Services. Tecovirimat was mainly developed to combat smallpox-related bioterror concerns [17,20,21]. SIGA Technologies is also developing Tecovirimat's oral (liquid) formulations [17]. The development timeline of Tecovirimat is provided in Figure 2 [17,25].

Pharmacology
The in vitro antiviral activity-based concentration of Tecovirimat inhibiting the virusinduced cytopathic effect (CPE) by 50% (EC 50 in µmol/L) was promising against different orthopoxvirus (Variola = 0.016-0.067; Monkeypox = 0.014-0.039; Rabbitpox = 0.015; Vaccinia = 0.009). It completely inhibited the CPE of the wild-type cowpox virus and extracellular vaccinia virus formation but had little effect on the intracellular formation of the vaccinia virus [17]. The clinical trials in humans with smallpox are neither feasible nor ethical. Therefore, the efficacy of Tecovirimat was verified in infected rabbits and nonhuman primates (monkeys) under the USFDA's Animal Rule [17,[29][30][31][32][33]. The effectiveness of Tecovirimat in the non-human primate monkeypox model utilizing MPX strain Zaire 79 (V79-I-005) infected cynomolgus monkeys is reported [29][30][31][32]. Tecovirimat (3, 10, 30, and 100 mg/Kg once a day) was administered orally to different groups of infected monkeys after three days of infection (stage of secondary viremia). The treatment duration was 14 days. Tecovirimat administration demonstrated 100% protection to the infected animal and reduced the viral load and lesion formation. The animal not receiving Tecovirimat died or required euthanasia after 33 days of the infection. An equivalent drug concentration level in animals and humans was observed at a dose of 10 mg/Kg/day and 400 mg/day, respectively. This observation suggested an adequate oral dose of 400 mg/day for 14 days to treat monkeypox or smallpox infections in humans [29][30][31][32]. The study of Tecovirimat in the rabbitpox virus model also demonstrated a 90-100% survival rate for rabbits when Tecovirimat (20 mg/Kg, 40 mg/Kg, 80 mg/Kg, and 120 mg/Kg) was administered for 14 days after starting the dose on day four after the infection [30][31][32].
The mechanism of action of Tecovirimat is well explained in the literature [17,25], documents released by SIGA Technologies [20,21], and the approved drug label of TPOXX [23, 30,31]. Tecovirimat is an orthopoxvirus-specific VP37 protein inhibitor that inhibits the virus's systemic spread to other cells [17,20,21,23,25,30,31]. VP37 protein was recognized as a target of Tecovirimat by the genetic mapping of Tecovirimat-resistant mutant viruses [25]. The orthopoxvirus-specific VP37 protein is responsible for forming an envelope around the orthopoxvirus-like MPXV. Creating this envelope is essential for exiting the virus from the cell and spreading the virus to other cells. The mechanism of action of Tecovirimat is also depicted in Figure 3.  The important pharmacological parameters of Tecovirimat are provided in Table 2.

Human Cell
Entry of MPXV into other human cells Orthopox virus-specific VP37 protein Tecovirimat Figure 3. Mechanism of action of Tecovirimat.
The important pharmacological parameters of Tecovirimat are provided in Table 2.

Contraindications
None for capsule dosage form [30,31]. The intravenous injection should not be administered to individuals with serious renal impairments/injury [34].

Clinical Studies
The information about past and ongoing clinical studies on Tecovirimat was collected from the clinical database [36], employing different keywords. This search was done on 11 July 2022, and revealed many clinical studies on Tecovirimat (Tecovirimat = 11; SIGA-246 = 10; SIGA246 = 10; ST-246 = 11; ST246 = 11; TPOXX = 11). The overlapping results were removed, and the summary of the finalized 12 studies is provided in Table 3. The

Clinical Studies
The information about past and ongoing clinical studies on Tecovirimat was collected from the clinical database [36], employing different keywords. This search was done The overlapping results were removed, and the summary of the finalized 12 studies is provided in Table 3. The readers can open the page of the clinical database [36] and get the complete clinical study data using the National Clinical Trial number (NCT number) for each study mentioned in Table 3.  The clinical studies of Tecovirimat cited in Table 3 are related to smallpox (six studies), vaccinia (one study), healthy volunteers (one study), vaccinia (one study), orthopox viral disease (two studies), and monkeypox (two studies). No study has been conducted on children and pregnant women. In summary, the clinical studies of Tecovirimat were mainly related to its pharmacokinetic and safety parameters. Tecovirimat was well tolerated without any serious side effects in the phase I studies (NCT00728689 and NCT00431951). The phase II study (NCT00907803) revealed headache and nausea as Tecovirimat's most frequent side effects without any serious effect on patients' wellbeing. The phase III clinical study (NCT02474589) did not identify any safety concerns with the oral administration of Tecovirimat and demonstrated that Tecovirimat's bioavailability increases up to 50% when taken with food. Our search on PubMed also revealed some pharmacokinetic studies on Tecovirimat [37][38][39][40][41]. The pharmacokinetic parameters of Tecoviramat are now well  Table 2. Accordingly, the authors have not elaborated on the studies related to pharmacokinetic parameters of Tecovirimate.

Patent Searching and Summary
Tecovirimat's patents/patent applications were searched on 11 July 2022, through freely available patent databases (Espacenet, Patentscope, and the United States Patent and Trademark Office) [42][43][44][45][46]. A preliminary search employing different keywords and combinations thereof (Tecovirimat; TPOXX; SIGA246; ST-246 + Monkeypox; ST-246 + Smallpox; ST246 + Monkeypox; ST246 + Smallpox; SIGA-246 + Smallpox; SIGA-246 + Monkeypox; SIGA + Smallpox; SIGA + Monkeypox) provided hundreds of hits, which were not possible to discuss in this review. Tecovirimat has been developed by SIGA Technologies and is the marketing authorization holder for Tecovirimat in the USA, Europe, and Canada. Accordingly, the authors are providing a summary of patents/patent applications assigned to SIGA Technologies, including the patents listed in the Orange Book (OB) of the USFDA [47] and other important patents/patent applications explicitly claiming inventions of Tecovirimat (Table 4). It should be noted that most of the patent numbers mentioned in Table 4 belong to the same patent family. Therefore, they have identical authors and titles but different claims, patent numbers, and grant dates. This OB-listed patent claims a method of preventing/treating orthopoxvirus infection utilizing an adequate amount of a mixture of Tecovirimat (95% w/w or more) and one of its isomers (5% or less w/w) [50].

US8530509B2 (Patented case)
This OB-listed patent claims a pharmaceutical composition comprising an adequate amount of a mixture of Tecovirimat (95% w/w or more) and one of its isomers (5% or less w/w) for preventing/treating orthopoxvirus infection [51].

Discussion
MPX is the currently emerging global outbreak. The MPX is closely related but less fatal than smallpox disease [1][2][3][4][5][6]. Smallpox vaccination is thought to control MPX because the MPXV and smallpox virus are genetically close relatives and share common signs and symptoms. However, smallpox was eradicated long ago, and the routine administration of the smallpox vaccine has not been done by a large part of the global population [17]. This has increased the chances of individuals getting infected with MPX, making it a currently unmet medical need.
In January 2022, the EMA approved the Tecovirimat capsule (200 mg), developed by SIGA Technologies, as the first treatment for MPX [16]. Tecovirimat was mainly invented for treating smallpox because of bioterrorism concerns. The US Strategic National Stockpile has reserved about 2 million doses of Tecovirimat and other vaccines/drugs (JYNNEOS, ACAM2000, cidofovir, and vaccinia immunoglobulin intravenous (VIGIV)) in case of any bioterrorist attack [17]. The USFDA and Health Canada approved Tecovirimat for treating smallpox in 2018 and 2021, respectively, but not MPX [17,20,21]. Based on the EMA approval, the off-label use of Tecovirimat and the post-exposure prevention of MPX is foreseeable in the USA and Canada. These countries may also approve Tecovirimat for MPX shortly. The regulatory approval of Tecovirimat in other MPX-affected countries is also predictable soon. The EMA-, Health Canada-, and USFDA-approved capsules of Tecovirimat contain the crystalline Form I of Tecovirimat monohydrate because of its thermodynamic stability over other polymorphic forms [18,19]. Tecovirimat has been developed under the Animal Rule, which does not require efficacy-based clinical trials in humans for life-threatening, unnatural, and severe diseases. Accordingly, the efficacy of Tecovirimat was established in non-human primates (monkeys) and rabbits [17,[29][30][31][32][33]. Tecovirimat also demonstrated high efficacy in different animal models against all recognized pathogenic orthopoxviruses at suggested human doses or its equivalents [81]. This is also evident from the clinical study data (Table 3), wherein most clinical studies were based on Tecovirimat's pharmacokinetic and safety parameters. The drug-drug interaction, drug-disease interaction, and drug-food interaction studies are essential parameters concerning the safety and efficacy of a drug [82]. The interaction of Tecovirimat with repaglinide and midazolam has been established [30][31][32]. The contraindication of Tecovirimat injection in patients with renal injury is also reported [34]. Tecovirimat with milk, yogurt, applesauce, etc., is also recommended [30,31]. Tecovirimat has not shown any teratogenic effect in mice but showed non-teratogenic adverse events in maternal rabbits [18]. The authors believe that further studies must be done concerning drug interactions and teratogenic effects of Tecovirimat in higher animal models to ensure patient safety. Accordingly, the patient treated with Tecovirimat must be monitored, which may help identify its new benefits/side effect. Particular emphasis must be given while monitoring pregnant/pediatric/geriatric patients, patients suffering from chronic diseases such as diabetes, hypertension, cancer, etc., and patients on polypharmacy treatment. SIGA Technologies collaborates with many organizations to tackle the MPX outbreak [83]. Hopefully, this collaboration will also take care of the points mentioned earlier.
A DNA virus like MPXV has a lower mutation rate, but a mutation in the virus may develop resistance to a drug [2,81]. A nucleotide and frameshift mutation has been reported in the currently circulating strain of MPXV, which may increase its transmissibility [2]. Furthermore, mutations in MPXV are also possible. The authors opine to assess the efficacy of Tecovirimat against the MPXV that demonstrated nucleotide and frameshift mutation. This study may help to understand the possibility of Tecovirimat-resistance development by the MPXV. The case of Tecovirimat resistance among unresponsive patients to Tecovirimat must also be considered [3]. Tecovirimat is an inhibitor of the VP37 protein of orthopoxviruses [17,20,21,23,25,30,31]. The authors believe that drug repurposing-based and artificial intelligence-empowered drug development programs on the existing marketed antiviral drugs may also help to identify VP37 protein inhibitors for further development.
Developing commercially important inventions requires an understanding of existing patented inventions. SIGA Technologies owns many patents related to Tecovirimat (Table 4). These patents claim the marketed pharmaceutical compositions (capsule and intravenous injection) and suspension; method of treating orthopoxvirus infection; different polymorphic forms (crystalline and amorphous); and process for preparing Tecovirimat. SIGA Technologies is also developing an oral suspension formulation of Tecovirimat in collaboration with BARDA [17]. The synergistic drug combination patent application (Tecovirimat + cidofovir) has also been filed [78]. Tecovirimat does not interfere with the DNA replication process of MPXV and is unable to prevent the formation of intracellular MPXV ( Figure 3). Accordingly, the authors trust that combining Tecovirimat with existing DNA replication inhibitors may provide synergistic effects. The effect of Tecovirimat may be reduced in immunocompromised patients (HIV/AIDS, leukaemia, lymphoma, generalized malignancies, etc.) [30][31][32]. Combining immunity boosters such as zinc, quercetin, vitamin C, thymoquinone, and other antioxidants [84][85][86] may increase Tecovirimat's efficacy for MPX among immunocompromised patients. The authors believe that there is an excellent scope for developing more and better combinations of Tecovirimat against orthopoxvirus infections.

Conclusions
Tecovirimat is a new hope against the MPX outbreak and biosecurity against smallpox or orthopoxvirus-related bioterrorism attacks. At the time this article was written, Tecovirimat was only approved by the EMA to treat MPX. We expect the regulatory approval of Tecovirimat in other MPX-affected countries shortly. The efficacy of Tecovirimat in MPXV demonstrating nucleotide and frameshift mutation is warranted to assess the possibility of Tecovirimat resistance. Tecovirimat has been established as a safe and effective MPX treatment, but further Tecovirimat interaction-based pharmacological studies are also necessary to provide safe and effective treatment. Finally, the research programs on Tecovirimat-based drug combinations are also suggested to develop better MPX treatments.