PF-07321332 is an antiviral drug developed by Pfizer which acts as an orally active 3CL protease inhibitor.

Clinical data
ATC code
  • None
  • (1R,2S,5S)-N-[(1S)-1-cyano-2-[(3S)-2-oxopyrrolidin-3-yl]ethyl]-3-[(2S)-3,3-dimethyl-2-[(2,2,2-trifluoroacetyl)amino]butanoyl]-6,6-dimethyl-3-azabicyclo[3.1.0]hexane-2-carboxamide
CAS Number
  • 2628280-40-8
PubChem CID
Chemical and physical data
Molar mass499.535 g·mol−1
3D model (JSmol)
Melting point192.9[1] °C (379.2 °F)
  • CC1([C@@H]2[C@H]1[C@H](N(C2)C(=O)[C@H](C(C)(C)C)NC(=O)C(F)(F)F)C(=O)N[C@@H](C[C@@H]3CCNC3=O)C#N)C
  • InChI=1S/C23H32F3N5O4/c1-21(2,3)16(30-20(35)23(24,25)26)19(34)31-10-13-14(22(13,4)5)15(31)18(33)29-12(9-27)8-11-6-7-28-17(11)32/h11-16H,6-8,10H2,1-5H3,(H,28,32)(H,29,33)(H,30,35)/t11-,12-,13-,14-,15-,16+/m0/s1
Xray crystal structure PDB:7si9
Xray crystal structure (PDB:7SI9 and 7VH8) of the SARS-CoV-2 protease inhibitor PF-07321332 bound to the viral 3CLpro (Mpro) protease enzyme. Ribbon diagram of the protein with the drug shown as sticks. The catalytic residues (His41, Cys145) are shown as yellow sticks.

The combination of PF-07321332 with ritonavir is in phase III trials for the treatment of COVID-19[2][3][4][5][6] and is expected to be sold under the brand name Paxlovid.[7][8][9] In this combination, ritonavir serves to slow down metabolism of PF-07321332 by cytochrome enzymes to maintain higher circulating concentrations of the main drug.[10] In November 2021, Pfizer announced positive phase II/III results, including 89% reduction in hospitalizations when given within three days after symptom onset.[7] Despite not being fully approved yet in either country, the UK has already placed an order for 250,000 courses.[11][12] and Australia has preordered 500,000 courses of the drug.[13]


Coronaviral proteases cleave multiple sites in the viral polyprotein, usually after glutamine residues. Early work on related human rhinoviruses showed that the flexible glutamine side chain could be replaced by a rigid pyrrolidone.[14][15] These drugs were then further developed for other diseases including SARS.[16]

The utility of targeting the 3CL protease in a real world setting was first demonstrated when GC376 (a prodrug of GC373) was used to treat the previously 100% lethal cat coronavirus disease, feline infectious peritonitis, caused by FIPV.[17] The Pfizer drug is an analog of GC373, where the aldehyde covalent cysteine acceptor has been replaced by a nitrile.[18][19]

PF-07321332 was developed by modification of an earlier clinical candidate lufotrelvir,[20][21] which is also a covalent inhibitor but its warhead is a phosphate prodrug of a hydroxyketone. Lufotrelvir needs to be administered intravenously limiting its use to a hospital setting. Stepwise modification of the tripeptide mimetic led to PF-07321332 which is suitable for oral administration.[1] Key changes include a reduction in the number of hydrogen bond donors, and the number of rotatable bonds by introducing the rigid bicyclic non-canonical amino acid, which mimics the leucine residue found in earlier inhibitors. This residue had previously been used in the synthesis of boceprevir.[22]

Chemistry and pharmacologyEdit

Full details of the synthesis of PF-07321332 were first published by scientists from Pfizer.[1]


In the penultimate step, a synthetic homochiral amino acid is coupled with a homochiral amino amide using the water-soluble carbodiimide EDCI as coupling agent. The resulting intermediate is then treated with Burgess reagent, which dehydrates the amide group to the nitrile of the product.

PF-07321332 is a covalent inhibitor, binding directly to the catalytic cysteine (Cys145) residue of the cysteine protease enzyme.[23]

Society and cultureEdit

Misleading comparison with ivermectinEdit

Conspiracy theorists on the internet have claimed that Paxlovid is merely a "repackaged" version of the antiparasitic drug ivermectin, which has been erroneously promoted as a COVID-19 "miracle cure". Their claims are based on a narrative that Pfizer is suppressing the true benefits of ivermectin and rely on superficial correspondences between the drugs and a misunderstanding of their respective pharmakokinetics.[24] Paxlovid is not structurally related or similar to ivermectin, and while both are 3C-like protease inhibitors, Paxlovid is much more potent with an IC50 around 10,000 times lower, allowing for effective oral dosing within the therapeutic margin.[25]

Legal statusEdit

In November 2021, Pfizer signed a license agreement with the United Nations–backed Medicines Patent Pool to allow PF-07321332 to be manufactured and sold in 95 countries.[26] 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". However, the deal excludes several countries with major COVID-19 outbreaks including Brazil, China, Russia, Argentina, and Thailand.[27]

On 16 November, Pfizer submitted an application to the U.S. Food and Drug Administration (FDA) for emergency authorization for PF-07321332 in combination with ritonavir.[28][29][30] The US government is expected to sign a contract to buy around 10 million courses of the combination treatment.[31][32]


  1. ^ a b c Owen DR, Allerton CM, Anderson AS, Aschenbrenner L, Avery M, Berritt S, et al. (November 2021). "An oral SARS-CoV-2 Mpro inhibitor clinical candidate for the treatment of COVID-19". Science: eabl4784. doi:10.1126/science.abl4784. PMID 34726479. S2CID 240422219.
  2. ^ Vandyck K, Deval J (August 2021). "Considerations for the discovery and development of 3-chymotrypsin-like cysteine protease inhibitors targeting SARS-CoV-2 infection". Current Opinion in Virology. 49: 36–40. doi:10.1016/j.coviro.2021.04.006. PMC 8075814. PMID 34029993.
  3. ^ Şimşek-Yavuz S, Komsuoğlu Çelikyurt FI (August 2021). "Antiviral treatment of COVID-19: An update". Turkish Journal of Medical Sciences. doi:10.3906/sag-2106-250. PMID 34391321. S2CID 237054672.
  4. ^ Ahmad B, Batool M, Ain QU, Kim MS, Choi S (August 2021). "Exploring the Binding Mechanism of PF-07321332 SARS-CoV-2 Protease Inhibitor through Molecular Dynamics and Binding Free Energy Simulations". International Journal of Molecular Sciences. 22 (17): 9124. doi:10.3390/ijms22179124. PMC 8430524. PMID 34502033.
  5. ^ "Pfizer begins dosing in Phase II/III trial of antiviral drug for Covid-19". Clinical Trials Arena. 2 September 2021.
  6. ^ Nuki P (26 April 2021). "Pfizer is testing a pill that, if successful, could become first-ever home cure for COVID-19". National Post. Archived from the original on 27 April 2021.
  7. ^ a b "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". Pfizer Inc. 5 November 2021.
  8. ^ Mahase E (November 2021). "Covid-19: Pfizer's paxlovid is 89% effective in patients at risk of serious illness, company reports". BMJ. 375: n2713. doi:10.1136/bmj.n2713. PMID 34750163. S2CID 243834203.
  9. ^ Robbins R (5 November 2021). "Pfizer Says Its Antiviral Pill Is Highly Effective in Treating Covid". The New York Times. ISSN 0362-4331. Archived from the original on 8 November 2021. Retrieved 9 November 2021.
  10. ^ Woodley M (19 October 2021). "What is Australia's potential new COVID treatment?". The Royal Australian College of General Practitioners (RACGP). Retrieved 6 November 2021.
  11. ^ "Pfizer Covid pill 'can cut hospitalisations and deaths by nearly 90%'". The Guardian. 5 November 2021. Retrieved 17 November 2021.
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  13. ^ "What are the two new COVID-19 treatments Australia has gained access to?". ABC News (Australia). 17 October 2021. Retrieved 5 November 2021.
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  15. ^ 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. PMID 10197965.
  16. ^ Pillaiyar T, Manickam M, Namasivayam V, Hayashi Y, Jung SH (July 2016). "An Overview of Severe Acute Respiratory Syndrome-Coronavirus (SARS-CoV) 3CL Protease Inhibitors: Peptidomimetics and Small Molecule Chemotherapy". Journal of Medicinal Chemistry. 59 (14): 6595–6628. doi:10.1021/acs.jmedchem.5b01461. PMC 7075650. PMID 26878082.
  17. ^ Pedersen NC, Kim Y, Liu H, Galasiti Kankanamalage AC, Eckstrand C, Groutas WC, et al. (April 2018). "Efficacy of a 3C-like protease inhibitor in treating various forms of acquired feline infectious peritonitis". Journal of Feline Medicine and Surgery. 20 (4): 378–392. doi:10.1177/1098612X17729626. PMC 5871025. PMID 28901812.
  18. ^ Halford B (7 April 2021). "Pfizer unveils its oral SARS-CoV-2 inhibitor". Chemical & Engineering News. 99 (13): 7. doi:10.47287/cen-09913-scicon3. S2CID 234887434.
  19. ^ Vuong W, Khan MB, Fischer C, Arutyunova E, Lamer T, Shields J, et al. (August 2020). "Feline coronavirus drug inhibits the main protease of SARS-CoV-2 and blocks virus replication". Nature Communications. 11 (1): 4282. doi:10.1038/s41467-020-18096-2. PMC 7453019. PMID 32855413.
  20. ^ 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
  21. ^ Boras B, Jones RM, Anson BJ, Arenson D, Aschenbrenner L, Bakowski MA, et al. (February 2021). "Discovery of a Novel Inhibitor of Coronavirus 3CL Protease for the Potential Treatment of COVID-19". bioRxiv: 2020.09.12.293498. doi:10.1101/2020.09.12.293498. PMC 7491518. PMID 32935104.
  22. ^ 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. PMID 18193821. S2CID 2629035.
  23. ^ Pavan M, Bolcato G, Bassani D, Sturlese M, Moro S (December 2021). "Supervised Molecular Dynamics (SuMD) Insights into the mechanism of action of SARS-CoV-2 main protease inhibitor PF-07321332". J Enzyme Inhib Med Chem. 36 (1): 1646–1650. doi:10.1080/14756366.2021.1954919. PMC 8300928. PMID 34289752.
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  30. ^ "Pfizer Will Allow Its Covid Pill to Be Made and Sold Cheaply in Poor Countries". The New York Times. 16 November 2021. Retrieved 17 November 2021.
  31. ^ "U.S. to Buy Enough of Pfizer's Covid Antiviral Pills for 10 Million People". The New York Times. 17 November 2021. Retrieved 17 November 2021.
  32. ^ Pager, Tyler; McGinley, Laurie; Johnson, Carolyn Y.; Taylor, Adam; Parker, Claire. "Biden administration to buy Pfizer antiviral pills for 10 million people, hoping to transform pandemic". The Washington Post. Retrieved 16 November 2021.

External linksEdit