Efforts of the global scientific community to develop easy-to-use oral pills to treat Covid-19 patients is nearing success. Paxlovid, Pfizer's drug under development, has reported 90% safety and efficacy in preventing serious illness, hospitalisations and death.

The positive news from Pfizer comes as another much-hyped Covid-19 oral pill under development - Molnupiravir by U.S.-based Merck (known as MSD outside of the U.S. and Canada) and Ridgeback Biotherapeutics – stares at a possible failure. While earlier trial results had shown about 50% reduction in deaths and hospitalisation, the latest data submitted to the regulator a few days ago suggests only 30% efficacy. Roche and its partner Atea Pharmaceuticals were also developing an oral antiviral pill to treat Covid-19, but had terminated the project a month ago due to poor results in the second phase trials.

The U.S. Food and Drug Administration (US FDA) is yet to allow sales of any oral pills to treat Covid-19. Pfizer's pill is a combination of antivirals Nirmatrelvir with Ritonavir. If approved, Pfizer plans to make nearly two million doses this year and 80 million next year. Pfizer has a licensing agreement with the international public health group Medicines Patent Pool (MPP) to entrust generic manufacturers (likely mainly from India) to make the drug cheap for 95 low- and middle-income countries. Analysts expect the pill to generate $10-15 billion in revenues.

MSD's plans were to make 10 million doses of Molnupiravir this year and at least 20 million in 2022. Despite lower efficacy, the US FDA's advisory committee had recommended Molnupiravir for Emergency Use Authorisation (EUA) with a narrow 13–10 votes in favour. Britain's Medicines and Healthcare products Regulatory Agency (MHRA) gave clearance to the drug a month ago, for use in people only with mild to moderate Covid-19 symptoms at the onset of disease.

Molnupiravir, a re-purposed old molecule, was seen as a possible cheap option to treat the disease and MSD had entered into license deals with generic makers, mostly from India, for global supplies. Trials were done in India by Dr Reddy’s Laboratories for Sun Pharmaceuticals, Cipla and Emcure Pharmaceuticals, Optimus Pharma, Aurobindo Pharma, Natco Pharma, Hetero Drugs, and BDR Pharmaceuticals.

Early this month, the subject expert committee (SEC) of India’s drug regulator deferred its approval in the country, citing submission of final trial data and lower efficacy in interim trials.

Experts believe Molnupiravir’s benefits outweigh its largely unknown risks, as the pill works by getting itself into the virus’s RNA and creating error codes to stop the ability of the virus to replicate. This may render vaccines ineffective, cause the virus to mutate further, and may lead to birth defects. Further, they doubt it failed to kill the fast-spreading delta variant.

As against this, Nirmatrelvir, which originated in Pfizer laboratories, blocks the activity of the SARS-CoV-2-3CL protease, an enzyme that the coronavirus needs to replicate. Co-administration with a low dose of Ritonavir helps slow the metabolism, or breakdown of Nirmatrelvir. The much tested and proven protease inhibitors are currently used to treat HIV, Hepatitis C and other viruses.

Pfizer said final data available from all high-risk patients enrolled in a Paxlovid trial, involving 2,246 candidates, confirmed prior results of an interim analysis – which reduced the risk of hospitalisation or death by 89% within three days of symptom onset, and 88% within five days of symptom onset. Once approved, it can be prescribed at the first sign of infection or at first awareness of an exposure – potentially helping patients avoid severe illness.

Pfizer also said the drug has shown consistent efficacy against the Omicron variant, as well as previously identified variants of concern like alpha, beta, delta, gamma, lambda, and mu. Once approved, it will be administered in doses of 300 mg (two 150 mg tablets) of Nirmatrelvir with one 100 mg tablet of Ritonavir, given twice daily for five days. One box contains five blister packs.

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