X-ray crystal structure (PDB:7SI9 and 7VH8) of the SARS-CoV-2 protease inhibitor nirmatrelvir bound to the viral 3CLpro protease enzyme. Ribbon diagram of the protein with the drug shown as sticks. The catalytic residues (His41, Cys145) are shown as yellow sticks.
Coronaviral proteases cleave multiple sites in the viral polyprotein, usually after there are glutamine residues. Early work on related human rhinoviruses showed that the flexible glutamine side chain in inhibitors could be replaced by a rigid pyrrolidone.[9][10] These drugs had been further developed prior to the COVID-19 pandemic for other diseases including SARS.[11] The utility of targeting the 3CL protease in a real world setting was first demonstrated in 2018 when GC376 (a prodrug of GC373) was used to treat the previously 100% lethal cat coronavirus disease, feline infectious peritonitis, caused by feline coronavirus.[12] Nirmatrelvir and GC373 are both peptidomimetics, share the aforementioned pyrrolidone in P1 position and are competitive inhibitors. They use a nitrile and an aldehyde respectively to bind the catalytic cysteine.[13][14] Pfizer investigated two series of compounds, with nitrile and benzothiazol-2-yl ketone as the reactive group, respectively, and in the end settled on using nitrile.[15]
Tert-leucine (abbreviation: Tle) used in the P3 position of nirmatrelvir was identified first as optimal non-canonical amino acid in potential drug targeting SARS-CoV-2 3C-like protease using combinatorial chemistry (hybrid combinatorial substrate library technology).[20][21] The leucine-like residue resulted in loss of a nearby contact with a glutamine on the 3C-like protease.[15] To compensate, Pfizer tried adding methane sulfonamide, acetamide and trifluoroacetamide, discovering that of the three, trifluoroacetamide resulted in superior oral bioavailability.[15]
Chemistry and pharmacology
Full details of the synthesis of nirmatrelvir were first published by scientists from Pfizer.
In the co-packaged medication nirmatrelvir/ritonavir, ritonavir serves to slow the metabolism of nirmatrelvir via cytochrome enzyme inhibition, thereby increasing the circulating concentration of the main drug.[23] This effect is also used in HIV therapy, where ritonavir is used in combination with another protease inhibitor to similarly enhance their pharmacokinetics.[24]
Society and culture
Licensing
In November 2021, Pfizer signed a license agreement with the United Nations–backed Medicines Patent Pool to allow nirmatrelvir to be manufactured and sold in 95 countries.[25] Pfizer stated that the agreement will allow local medicine manufacturers to produce the pill "with the goal of facilitating greater access to the global population". The deal excludes several countries with major COVID-19 outbreaks including Brazil, China, Russia, Argentina, and Thailand.[26][27]
The research that led to nirmatrelvir began in March 2020, when Pfizer formally launched a project at its Cambridge, Massachusetts site to develop antiviral drugs for treating COVID-19.[15] In July 2020, Pfizer chemists were able to synthesize nirmatrelvir for the first time.[15] In September 2020, Pfizer completed a pharmacokinetic study in rats which suggested that nirmatrelvir could be administered orally.[15] The actual synthesis of the drug for laboratory research and for clinical trials was carried out at Pfizer's Groton, Connecticut site.[29]
A study published in March 2023 reported that treatment with nirmatrelvir within five days of initial infection reduced the risk of long COVID relative to patients who did not receive Paxlovid.[31]
A 2024 study found that "the time to sustained alleviation of all signs and symptoms of Covid-19 did not differ significantly between participants who received nirmatrelvir–ritonavir and those who received placebo."[32]
^Clinical trial number NCT04535167 for "First-In-Human Study To Evaluate Safety, Tolerability, And Pharmacokinetics Following Single Ascending And Multiple Ascending Doses of PF-07304814 In Hospitalized Participants With COVID-19 " at ClinicalTrials.gov
^Njoroge FG, Chen KX, Shih NY, Piwinski JJ (January 2008). "Challenges in modern drug discovery: a case study of boceprevir, an HCV protease inhibitor for the treatment of hepatitis C virus infection". Accounts of Chemical Research. 41 (1): 50–59. doi:10.1021/ar700109k. PMID18193821. S2CID2629035.
^World Health Organization (2022). "International nonproprietary names for pharmaceutical substances (INN): recommended INN: list 88". WHO Drug Information. 36 (3). hdl:10665/363551.
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