On November 4, 2021, the Medicines and Healthcare Products Regulatory Agency (MHRA) granted marketing approval for Molnupiravir (trade name: Lagevrio), an oral COVID-19 drug co-developed by Merck and Ridgeback, for the treatment of patients with mild to moderate COVID-19. This is the first oral antiviral drug approved globally for the treatment of mild to moderate COVID-19 in adults.
Clinical data showed that 775 patients recently infected with COVID-19 were treated with molnupiravir or placebo, respectively. There were 7.3% hospitalizations and no patient deaths in the molnupiravir-treated group and 14.1% hospitalizations and 8 deaths in the placebo group, p=0.0012. The risk of hospitalization and death was reduced by 50% in the molnupiravir-treated group compared to the placebo group. 
It is important to note that Molnupiravir must be taken within five days of the onset of symptoms of viral infection, and is not effective if the patient is already hospitalized. In addition, patients with mild to moderate COVID-19 taking Molnupiravir should also include at least one of the following risk factors: e.g., obesity, old age, diabetes, or heart disease.
Just one day later, on November 5, 2021, Pfizer made an announcement that PAXLOVID (PF-07321332; ritonavir), Pfizer's proprietary COVID-19 oral drug, met the primary study endpoint. The risk of death and hospitalization was reduced by 89% in the PAXLOVID treatment group compared to the placebo control group.
This announcement is based on an interim analysis of the Phase 2/3 EPIC-HR (Evaluation of Protease Inhibition for COVID-19 in High-Risk Patients) randomized, double- blind study of non-hospitalized adult patients with COVID-19, who are at high risk of progressing to severe illness.
The scheduled interim analysis showed that 0.8% of patients treated with PAXLOVID were hospitalized within day 28 after randomization grouping (3/389 hospitalizations, no patient deaths), compared with 7.0% of patients treated with placebo who were hospitalized or died (27/385 hospitalizations, 7 deaths). The statistical significance of these results was high (p<0.0001).
A similar reduction in hospitalizations or deaths associated with COVID-19 was observed in patients treated with PAXLOVID within five days of symptom onset. 1.0% of patients randomized to PAXLOVID were hospitalized within day 28 (6/607 hospitalizations, no deaths) compared to 6.7% of patients treated with placebo (41/612 hospitalizations, 10 subsequent deaths), which was highly statistically significant (p<0.0001). In the entire study population, no deaths were reported in patients treated with PAXLOVID through day 28. In contrast, there were 10 (1.6%) deaths in patients taking placebo .
From the above data, we can roughly see that Pfizer's Paxlovid seems to have better efficacy than Merck's Molnupiravir. So are there any differences in the development mechanism of these two drugs?
How The Coronavirus Infects Cells?
Coronaviruses (SARS-CoV-2) are enveloped viruses that contain a positive, single-stranded RNA genome, which is packaged within a capsid. The capsid consists of the nucleocapsid protein N and this is further surrounded by a membrane, that contains three proteins: the membrane protein (M) and the envelope protein (E), which are involved in the virus budding process, and the spike glycoprotein (S), which is a key player in binding host receptor and mediating membrane fusion and virus entry into host cells.
Structure of COVID-19 Virus
Following, let's us find the mechanism of COVID-19 entry and viral replication and viral RNA packing in the human cell.
(a) The S-protein on the surface of the virus binds to the ACE2 (angiotensin-converting enzyme 2) receptor in human lung cells, allowing entry of the coronavirus into human cells through endocytosis (membrane fusion).
(b) After the virus enters the cytoplasm, the proteasome of the human cell hydrolyzes the S-protein, further activating membrane fusion in vivo.
(c) The proteasome hydrolyzes the viral nucleocapsid protein and the viral genetic material, single-stranded RNA, is completely released into the cytoplasm.
(d) RNA is translated into polypeptide chains with the help of ribosomes and hydrolyzed into RNA-dependent RNA polymerase (RdRp) by 3CLpro enzyme (3 (a) RdRp uses the genome as a template to generate full-length negative-sense RNA, which subsequently serves as a template to generate additional full-length genomes.
(e) Viral membrane proteins, S-proteins and envelope proteins are synthesized in the cytoplasm and then inserted into the endoplasmic reticulum and transferred to the Golgi intermediate compartment of the endoplasmic reticulum.
(f) In the cytoplasm, nucleocapsids are formed by capsidization of replicating genomes by nucleocapsid proteins, which then aggregate within the endoplasmic reticulum-Golgi intermediate compartment membrane to self-assemble into new viral particles.
(g) Finally, the new viral particles are transported to the cell membrane via smooth-walled vesicles, which are then secreted via exocytosis and thus exported from the infected cell, thereby infecting other cells. At the same time, the virus generates pressure on the endoplasmic reticulum eventually leading to cell death.
The schematic diagram of the mechanism of COVID-19 entry and viral replication and viral RNA packing in the human cell.
Treatment Strategies for COVID-19
From the process of entry of the coronavirus into human cells and some historical experience with coronaviruses, we can broadly derive the following ways to block the replication of the virus and treat COVID-19.
(1) Direct attack on the S-protein target of the virus with the help of monoclonal antibodies or plasma from recovered patients, which is also the mechanism of action of the vaccine, keeping the virus completely out of human cells.
(2) At the cellular level, the transmembrane protease serine 2 (TMPRSS2) preemptively contacts the S-protein of coronaviruses and promotes viral entry and infection, so TMPRSS2 would be a potential target for drug development to inhibit COVID-19. TMPRSS2 inhibitors, such as camostat mesylate, are considered to be potential antiviral agents against COVID-19. Several clinical trials of camostat mesylate for the treatment of COVID-19 have been carried out. The results of one randomized controlled trial showed no improvement in clinicals .
(3) Chloroquine (CQ) and hydroxychloroquine (HCQ) have demonstrated positive results indicating a potential antiviral role against SARS-CoV-2 （HCQ is preferred due to its higher water solubility, lower toxicity and also feasibility for prolonged use with increased tolerance）. Its mechanism of action (MOA) includes the interference in the endocytic pathway, blockade of sialic acid receptors, restriction of pH mediated spike (S) protein cleavage at the angiotensin-converting enzyme 2 (ACE2) binding site and prevention of cytokine storm. Unfortunately, its adverse effects like gastrointestinal complications, retinopathy and QT interval prolongation are evident in treated COVID-19 patients. But there is a lack of quality evidence to demonstrate CQ and HCQ are effective in the treatment of COVID-19.
(4) The viral 3-chymotrypsin-like cysteine protease (3CLpro), playing pivotal roles in coronavirus replication and polyprotein processing, is essential for its life cycle. Therefore, inhibiting 3CLpro enzyme is also an important way to block viral replication. The HIV inhibitor ritonavir/lopinavir can inhibit the 3CLpro enzyme action of COVID-19, and the oral drug PF-07321332 developed by Pfizer has a similar mechanism. The addition of ritonavir reduces the rate of metabolism of PF-07321332 by human cells and enhances therapeutic concentrations.
(5) RNA-dependent RNA polymerase (RdRp) can synthesize new viral RNA using viral single-stranded RNA as a template, therefore, viral replication can also be blocked by inhibiting the action of RdRp. Remdesivir is a phosphoramidate prodrug that is metabolized in cells to yield an active NTP analog21 that we refer to as remdesivir triphosphate (RTP). RdRp can be inserted into the RNA strand being extended using RTP as a substrate, after which the replication process of RdRp is blocked. The nucleoside analogue Remdesivir can avoid proofreading correction during RNA replication because its incorporation does not immediately terminate the extension, but will block RdRp only after the addition of three nucleotides .
(6) The oral drug Molnupiravir, developed by Merck as a substrate alternative to cytidine/uridine triphosphate , can incorporates either A or G when RdRp uses viral RNA as a template for replication, resulting in a mutated RNA. Structural analysis of RdRp-RNA complexes containing mutated products showed that Molnupiravir could form stable base pairs with either G or A in the RdRp active center, which explains how the polymerase could escape proofreading and synthesize mutated RNA. Unlike Remdesivir, Molnupiravir does not block the action of RdRp, but reduces COVID-19 activity by producing mutant viral RNA by a mechanism similar to that of Favipiravir .
Paxlovid and Molnupiravir: Difference In Mechanism of Action
In principle, Pfizer's oral drug PAXLOVID directly inhibits the action of COVID-19 RNA polymerase, blocking the process of viral replication; while Mercer's oral drug Molnupiravir does not block the replication of RNA, but generates mutated genetic material by replacing C and G bases with fake nucleoside analogues. However, it is still worth investigating whether the mutated RNA is still virulent.
Vaccines, Remdesivir, Molnupiravir and PAXLOVID, despite their different mechanisms of development, are all based on a deep understanding of viral structure and physiological processes, and are effective weapons in our response to COVID-19. Different R&D strategies may lead to different results. As of now, Pfizer's oral drug works better than Merck Sharp & Dohme's, or even other drugs, while Camostat Mesylate has been proven to be ineffective in COVID-19 patients, but none of this can deny the exploration and efforts made by the scientists behind it.
Huateng Pharma can offer pharmaceutical intermediate contract manufacturing services. With comprehensive technical expertise and current GMP facilities in China, we can provide custom synthesis for R&D and commercial scaleup of these Paxlovid intermediates.
▶ CAS NO.67911-21-1 | Caronic anhydride
▶ CAS NO.194421-56-2 | 6,6-Dimethyl-3-Azabicyclo[3.1.0]hexane-2,4-dione
▶ CAS NO.565456-77-1 | (1R,2S,5S)-Methyl 6,6-dimethyl-3-azabicyclo[3.1.0]hexane-2-carboxylate hydrochloride
▶ CAS NO.943516-54-9 | 6,6-Dimethyl-3-azabicyclo[3.1.0]hexane
 Molnupiravir: What is the COVID-19 pill and how does it work?, https://www.sciencefocus.com/news/molnupiravir-covid-pill/
 Pfizer’s Novel COVID-19 Oral Antiviral Treatment Candidate Reduced Risk of Hospitalization or Death by 89% in Interim Analysis of Phase 2/3 EPIC-HR Study, https://www.pfizer.com/news/press-release/press-release-detail/pfizers-novel-covid-19-oral-antiviral-treatment-candidate
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