Favipiravir in the Spotlight: In Search of Treatment Against COVID-19

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), also referred to as the novel coronavirus (2019-nCoV), is currently a worldwide threat to public health [1, 2]. According to the Johns Hopkins University monitor, there have been more than 10 million reported cases of coronavirus 2019 disease (COVID-19) and 500,000 deaths as of June 29, 2020 [3]. However, it is interesting to mention that the incidence and mortality rates vary widely around the globe and among different populations, and there is an ongoing debate on the possible reasons, with authors usually deeming it to various biological, environmental or social factors [4 - 6]. Nevertheless, the infection has now reached 188 countries, making the identification of a drug candidate and treatment regimen imperative [3].

Favipiravir (Fig. 1) is a pyrazinecarboxamide derivative with potent activity against RNA viruses, but not against DNA viruses. It has demonstrated relevant activity against the Orthomyxoviridae family, including Influenza A, B, and C, with EC50 values ranging from 0.014 to 0.55 μg/mL, being active against oseltamivir-resistant strains [7]. Favipiravir was first approved as a treatment for Influenza viruses in Japan in 2014. Moreover, this drug has been considered promising in treating Ebola virus patients during the Ebola outbreak in 2014. However, researchers felt it was unethical to conduct randomized trials in that situation, and preliminary, non-randomized data, concluded that although this drug prolonged survival, it was unlikely for favipiravir to be active on patients with very high viremia as monotherapy [8]. Favipiravir has also been tested against Lassa Virus, Rabies Virus, Novovirus, among others [7, 9].

We reviewed all relevant press releases, scientific articles, and official documents to compile information about Favipiravir and its status as a potential drug to treat COVID-19. Press releases from 01/11/2019 to 29/06/2020 were retrieved from google search by using search terms such as “Favipiravir COVID” and “Favipiravir Approval.” Scientific articles were collected similarly from SciFinder, Google Scholar, and PubMed. Finally, major ongoing clinical trials were searched in clinicaltrials.gov, and other databases (search entry: “Favipiravir”), and all clinical trials related to COVID-19 were compiled. In the case of press releases, we narrowed the information to its primary source or a reputable source before reviewing. Moreover, we have compiled all clinical trial results on Favipiravir published by 29/06/2020.

2.2. Exemplified Synthetic Pathways

The original synthetic approach to Favipiravir from Toyama Chemicals consists of a 5-step synthesis from 3-amino-6-bromopyrazine-2-carboxylate 1 (Scheme 1). The first step consists of a diazotization reaction followed by nucleophilic aromatic substitution with methanol, in concentrated sulfuric acid, to furnish 2 in low yields [12].

The next step is the introduction of an amino group by Pd catalyzed imine substitution in the presence of (S)-(-)-2,2′-Bis(diphenylphosphino)-1,1′-binaphthalene ((S)-BINAP) and tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) to furnish 3. Amide 4 is obtained after aminolysis with aqueous ammonia, and, finally, intermediate 4 goes through fluorination and demethylation to yield Favipiravir [12].

Other procedures were published later in order to improve the original protocol. Scheme 2 displays the second-generation route from Toyama company to Favipiravir [13,14], which affords much higher yields in the synthesis of this drug, by using the nitration and POCl3 chlorination steps for the synthesis of 7 and 8, respectively. Intermediate 8 is then fluorinated with KF, followed by selective hydrolysis of product 9 to form key intermediate 10, which is subsequently hydrolyzed to the carboxamide derivative Favipiravir (Scheme 2) [13,14].

The third-generation route from Toyama Chemicals is shown in Scheme 3. This route is much longer than the second-generation one, comprising nine synthetic steps; however, the synthetic conditions and inexpensive starting materials make this procedure attractive to the industry. In this synthesis, key intermediate 17 is reached from ethyl diethoxyacetate in five steps. Hydrolysis of the starting ester followed by reaction with amino-acetonitrile furnishes 13, which reacts with dimethyl oxalate in the presence of potassium tert-butoxide and then with hydroxylamine hydrochloride to furnish the isoxazole derivative 15 [15].

Finally, 15 is submitted hydrolysis reaction of the acetal, followed by the formation of the pyrazine ring and chlorination to afford 17. From key intermediate 17, reaction with KF and subsequent opening of the isoxazole ring furnish intermediate 10, which is hydrolyzed to Favipiravir [15].

Fig. (2). Formation of the RNA-Favipiravir-RdRp complex and chain termination [6, 7].

Scheme 1. Original Synthesis of Favipiravir.

Scheme 2. Second-generation synthesis of Favipiravir.

Scheme 3. Third-generation synthesis of Favipiravir.

Many minor variations of the three main routes published by Toyama have been attempted and published. However, in the past three years, two major contributions have been made. Protocol A aims to synthesize Favipiravir from a cheaper starting material while avoiding the use of harsh conditions. The authors reported moderate to high yields in all reaction steps (55 to 82%) (Scheme 4). Finally, Protocol B attempts to innovate by reaching the desired molecule straight from compound 8, in a one-pot procedure. The authors report a 60% overall yield for this last step, which is considered an advantage, as it avoids two reaction steps, saving time and reducing costs (Scheme 4) [16 - 17].

Overall, it is clear that even though Favipiravir is a tiny molecule, its synthesis is complex and involves many steps. Therefore, one can expect a low overall yield of Favipiravir, which will lead to a higher aggregate price in the final product. Recently, Glenmark Pharmaceuticals has launched Fabiflu in India at 103 INR each 200mg tablet (1,36 USD) with an estimate of 185 USD for the full treatment course [18]. However, this price would still make Favipiravir cheaper than other antiviral candidates, such as Remdesivir (estimated 66-80 USD each 100mg dose in India for the generic drug) [19].

2.5. Pharmacokinetics

Favipiravir has a median t_max of 1h to 2h following a single or multiple doses, respectively, with a half-life t1/2 of 2 to 4h. Reported bioavailability in mice is 97.6%. It is essential to mention that Cmax in US patients was 50% lower than in Japanese patients [25].

Scheme 4. Some protocols for the synthesis of Favipiravir..

The metabolism carries elimination with M1 (Fig. 3), formed via Aldehyde Oxidase (AO), being the primary metabolite identified. The formation of a glucuronide in Phase 2 metabolism has also been detected. Moreover, it is unlikely for CYP isoenzymes to play a part in this elimination mechanism. Some food effect was observed in low doses, but it is irrelevant when the approved regimen is applied since plasma concentration will increase due to the irreversible inhibition of AO [21].

Fig (3). Main Favipiravir metabolite detected.

Favipiravir is known for its teratogenicity and embryotoxicity, and it should not be administered to pregnant women. Common adverse effects include diarrhea (4.1%) and increased serum uric acid (5.6%) [21]. Moreover, this drug reportedly inhibits the formation of acetaminophen sulfate, increasing systemic exposure to paracetamol by 20%, according to Zhao and co-workers [26]. Even though the authors conclude that the increased exposure is unlikely to be of clinical importance, we believe researchers should closely monitor this possible interaction in clinical trials. Favipiravir also causes the irreversible inhibition of Aldehyde Oxidases (AO) and precautions for concomitant use with AO inhibitors (Cimetidine, Estradiol, Chlorpromazine, among others) or substrates (Allopurinol, Methotrexate, Pyrazinamide, and others) must be taken [21].

Interest in Favipiravir for the treatment of COVID-19 began to rise after the report of two non-randomized clinical trials conducted by the Clinical Medical Research Center and the Third People's Hospital of Shenzhen, which indicated that this drug would be more potent than the antiretrovirals lopinavir/ritonavir without any significant side effect. Patients were treated with oral favipiravir (1600mg twice a day on day one and 600mg twice a day from day 2 to the end) and inhaled interferon. The median shedding of the virus was four days (11 for the antiretrovirals), and radiographic improvement was seen in 91% of the subjects (62% for lopinavir/ritonavir) [27]. It is also interesting to mention that Favipiravir exhibited the EC50 of 61.88 μM against SARS-CoV-2 in Vero E6 cells, and, though this in vitro result may seem underwhelming, the Ebola experience has shown that Favipiravir was able to inhibit the infection in mice, even if it seemed to lack potency in vitro [28]. After these initial data were made public, various reliable sources reported that Chinese officials approved the mass production of the drug in order to ensure a stable supply, as the drug had shown “obvious efficacy.” However, there is no mention of Favipiravir in the available translation of the “Chinese Clinical Guidance for COVID-19 Pneumonia Diagnosis and Treatment (7th edition)” [29 - 31].

A randomized clinical trial of Favipiravir versus Arbidol (Umifenovir), the recommended drug in the Chinese guideline for COVID-19, was reported (April 08, 2020) and the safety and efficacy of these drugs were compared. In this study, 116 patients were assessed in the Favipiravir group (1600mg twice a day on day one and 600mg twice a day from day 2 to the end) and 120 patients in the Arbidol group (200mg each time, three times a day, from the first day to the end). The clinical recovery rate was 61% for Favipiravir after seven days, versus 51% for Arbidol, and hence no statistically superior efficacy was found. Moreover, the clinical recovery rate for critical patients was 0% for Arbidol and 5% for Favipiravir, showing that the use of Favipiravir may be limited to moderate cases, but the drug may be useful in halting disease progress into acute respiratory distress syndrome, shock and multiple organ failure [9]. Some side effects observed included abnormal liver function test (8%), psychiatric symptom reactions (4%), digestive tract reactions (14%), and raised serum uric acid (14%) [32]. It is important to mention that this study has not been peer-reviewed yet.

Another exploratory study not yet peer-reviewed was made public on May 5, by Qiu and co-workers. In this study, the authors evaluated the inclusion of Favipiravir in the standard of care protocol containing Lopinavir/Ritonavir or Darunavir/Cobicistat plus Arbidol and α-interferon. No additional benefits in the inclusion of Favipiravir (First dose of 1600 or 2200 mg, followed by 600mg three times a day) to the standard of care protocol were reported. However, due to the exploratory nature of the study, robust statistical analysis was not performed [33].

On May 26, 2020, a Japanese observational study of Favipiravir against COVID-19 was released. A total of 2158 cases of compassionate Favipiravir use were registered from many hospitals in Japan, and survey data were analyzed. Clinical improvement was high after seven days of treatment in mild and moderate cases (73.8% and 66.6%, respectively); however, the drug did not seem to show the same efficacy for severe cases. Moreover, this study does not directly compare the Favipiravir-receiving patients with others who have not received the medication, and therefore more studies are necessary, despite the promising initial results [34].

After the approval of Favpiravir by the Russian authorities on May 30, the Russian Direct Investment Fund has disclosed in a press release some Avifavir results against COVID-19 in clinical trials. According to the release, this drug was evaluated for ten days in clinical trials, and the median elimination of the virus decreased from nine days with the standard therapy to four days with Favipiravir. Avifavir has also shown high efficacy and minimum side effects in their study [23]. Glenmark has also mentioned a randomized, multi-centric clinical trial being conducted with 150 subjects in India in their recent press brief (June 20, 2020). In both cases, no further data was provided for scientific scrutiny as of June 26, 2020 [18].

Fujifilm, the company behind the development of Favipiravir (Avigan), has announced the start of Phase II clinical trial in the U.S., in collaboration with Brigham and Women's Hospital, Massachusetts General Hospital, and the University of Massachusetts Medical School, which will enroll approximately 50 patients with COVID-19 [35]. The company also started a Phase III clinical trial in Japan [36]. These studies are of the utmost importance since previous data show that the plasma concentration of Favipiravir in US patients is 50% of that in Japanese patients, which cannot be ignored [21]. Another limitation for the use of this drug is its reported teratogenicity, a fact that should be emphasized, despite the promising initial results. Table 1 shows other ongoing clinical trials, according to clinicaltrials.gov (search term, Favipiravir).

Nine studies use the same regimen approved for Influenza use (1600mg twice a day on day one and 600mg twice a day from day 2 to the end); however, the treatment regimen varies widely in the other protocols. Loading doses may range from 1800mg BID (bis in die) to 2400 mg BID, followed by maintenance doses up to 1000mg BID. There is no available data on planned dosages for entries (5), (6), (7), (12), (13) as of June 29, 2020. We should also highlight entry (20), as the study has now been completed and results will probably be published soon; entry (21), which will investigate the use of Favipiravir in prophylaxis; and entry (11), the study being sponsored by Fujifilm Pharmaceuticals in the USA that is now recruiting.

Search in clinicaltrialsregister.eu (search term, Favipiravir) yields three different studies (EudraCT Numbers: 2020-001904-41; 2020-001435-27; 2020-001528-32), in the UK, France, and Italy, and search in globalclinicaltrialdata.com yields Fujifilm's multicenter clinical trials in Japan (GCT03205238, source clinicaltrials.jp). Fujifilm's Japanese trials have a primary completion date of 06/30/2020; therefore, results will be available soon. Fig. (4) shows the chemical structure of some synthetic drugs that will be evaluated in combination with Favipiravir in some listed clinical trials.

Favipiravir is one of the most promising drug candidates against COVID-19, mainly due to its unique mechanism of action, and we expect to see more clinical trials listed soon. Teratogenicity is a substantial side effect that may limit the use of Favipiravir for COVID-19, but on the other hand, all other adverse effects reported in the early trials were mild, manageable, and there are only a few critical drug interactions to watch out for in-hospital use. So far, this drug has shown promising results when used in combination with inhaled interferon in early Chinese clinical trials; however, these trials were not randomized. A recently published randomized clinical trial has shown that Favipiravir's monotherapy may have only a slight clinical advantage over Umifenovir's and could be ineffective in fighting severe COVID-19 cases; still, it significantly improved time-to-relief for fever and cough symptoms. The two most recent studies published displayed conflicting conclusions. While Qiu and co-workers' exploratory study did not see an advantage in adding Favipiravir to the standard of care protocol, the preliminary report released by Fujita Health University in Japan highlights promising results for treating mild and moderate COVID-19 cases. Meanwhile, two studies not yet made public, conducted by the Russian Direct Investment Fund and Glenmark Pharma, have led The Ministry of Health of the Russian Federation to grant a temporary registration certificate to Favipiravir (Avifavir) on May 30, 2020, and the Central Drugs Standard Control Organization in India to grant emergency approval to this drug (FabiFlu) on June 19, 2020.

It still seems too early to recommend a clinical protocol with Favipiravir, as more robust, less biased data will be submitted to scientific scrutiny in a couple of months. There are currently 24 ongoing clinical trials throughout the world; thirteen Phase 3 trials. It is also important to mention that FUJIFILM Corporation, the developer of Favipiravir, is backing two new trials, one in Japan and another in the USA. The FDA has greenlit this first US clinical trial of Avigan on April 7, 2020. Furthermore, since Favipiravir is to be included in the Japanese, Russian, and Indian therapeutic arsenals, data provided by these countries will be of the utmost importance to reveal the true potential of Favipiravir in the fight against COVID-19.