Fixing the Achilles Heel of Pfizer's Paxlovid for COVID-19 Treatment

Abstract

Nirmatrelvir (PF-07321332), a first-in-class inhibitor of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) main protease (M^pro), was developed by Pfizer under intense pressure during the pandemic to treat COVID-19. A weakness of nirmatrelvir is its limited metabolic stability, which led to the development of a combination therapy (paxlovid), involving coadministration of nirmatrelvir with the cytochrome P450 inhibitor ritonavir. However, limitations in tolerability of the ritonavir component reduce the scope of paxlovid. In response to these limitations, researchers at Pfizer have now developed the second-generation M^pro inhibitor PF-07817883 (ibuzatrelvir). Structurally related to nirmatrelvir, including with the presence of a trifluoromethyl group, albeit located differently, ibuzatrelvir manifests enhanced oral bioavailability, so it does not require coadministration with ritonavir. The development of ibuzatrelvir is an important milestone, because it is expected to enhance the treatment of COVID-19 without the drawbacks associated with ritonavir. Given the success of paxlovid in treating COVID-19, it is likely that ibuzatrelvir will be granted approval as an improved drug for treatment of COVID-19 infections, so complementing vaccination efforts and improving pandemic preparedness. The development of nirmatrelvir and ibuzatrelvir dramatically highlights the power of appropriately resourced modern medicinal chemistry to very rapidly enable the development of breakthrough medicines. Consideration of how analogous approaches can be used to develop similarly breakthrough medicines for infectious diseases such as tuberculosis and malaria is worthwhile.

Figure 1

Like nirmatrelvir, ibuzatrelvir inhibits SARS-CoV-2 M^pro via reversible covalent reaction. (a) Ibuzatrelvir (PF-07817883; 2) is structurally related to nirmatrelvir (PF-07321332; 1) and inhibits SARS-CoV-2 M^pro via reversible covalent reaction with the nucleophilic thiolate of Cys145 (deprotonated by His41). (b) Ensitrelvir (S-217622; 3) inhibits M^pro by tight binding to the active site via noncovalent interactions. (c) View from a crystal structure of SARS-CoV-2 M^pro (gold cartoon and gray surface) in complex with ibuzatrelvir (orange carbon backbone; PDB ID: 8V4U(15)); the nitrile-derived thioimidate is located in an oxyanion hole (formed by the main chain amide NHs of Cys145 and Gly143). (d) Ibuzatrelvir (orange carbon backbone; PDB ID: 8V4U(15)) and nirmatrelvir (green carbon backbone; PDB ID: 7TE0(19)) adopt similar conformations when bound to SARS-CoV-2 M^pro. P1-P4-equivalent positions are in blue; S1-S4 subsites are in red.