X-ray crystal structure (PDB:7SI9 and7VH8) of the SARS-CoV-2 protease inhibitor nirmatrelvir bound to the viral3CLpro protease enzyme.Ribbon diagram of the protein with the drug shown as sticks. The catalytic residues (His41, Cys145) are shown as yellow sticks.
Coronaviralproteases cleave multiple sites in the viralpolyprotein, usually after there areglutamine residues. Early work on related humanrhinoviruses showed that the flexible glutamine side chain in inhibitors could be replaced by a rigidpyrrolidone.[9][10] These drugs had been further developed prior to theCOVID-19 pandemic for other diseases includingSARS.[11] The utility of targeting the 3CL protease in a real world setting was first demonstrated in 2018 whenGC376 (aprodrug of GC373) was used to treat the previously 100% lethal cat coronavirus disease,feline infectious peritonitis, caused byfeline coronavirus.[12] Nirmatrelvir and GC373 are bothpeptidomimetics, share the aforementionedpyrrolidone in P1 position and are competitive inhibitors. They use anitrile and analdehyde respectively to bind the catalyticcysteine.[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 usingcombinatorial chemistry (hybrid combinatorial substrate library technology).[20][21] The leucine-like residue resulted in loss of a nearby contact with aglutamine on the 3C-like protease.[15] To compensate, Pfizer tried adding methanesulfonamide,acetamide andtrifluoroacetamide, discovering that of the three, trifluoroacetamide resulted in superior oral bioavailability.[15]
In the co-packaged medicationnirmatrelvir/ritonavir,ritonavir serves to slow the metabolism of nirmatrelvir viacytochrome enzyme inhibition, thereby increasing the circulating concentration of the main drug.[23] This effect is also used inHIV therapy, where ritonavir is used in combination with anotherprotease inhibitor to similarly enhance their pharmacokinetics.[24]
In November 2021, Pfizer signed a license agreement with theUnited Nations–backedMedicines 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 itsCambridge, 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 apharmacokinetic 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'sGroton, Connecticut site.[29]
A study published in March 2023 reported that treatment with nirmatrelvir within five days of initial infection reduced the risk oflong 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]
^Dragovich PS, Prins TJ, Zhou R, Webber SE, Marakovits JT, Fuhrman SA, et al. (April 1999). "Structure-based design, synthesis, and biological evaluation of irreversible human rhinovirus 3C protease inhibitors. 4. Incorporation of P1 lactam moieties as L-glutamine replacements".Journal of Medicinal Chemistry.42 (7):1213–1224.doi:10.1021/jm9805384.PMID10197965.
^Clinical trial numberNCT04535167 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 " atClinicalTrials.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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