Paxlovid enhances treatment options for COVID-19 patients

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In a recently published review Pharmaceuticals, A group of authors investigated the design, synthesis and mechanism of action of paxlovide, a protease inhibitor (PI) drug combination for the treatment of coronavirus disease 2019 (COVID-19).

Study: Design, synthesis and mechanism of action of Paxlovide, a protease inhibitor drug combination for the treatment of Covid-19. Image credit: Tobias Arhelger/


The COVID-19 pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus has significantly challenged healthcare systems and medical science worldwide.

In response, researchers worldwide have developed vaccines with innovative mechanisms and small-molecule antivirals targeting key viral proteins.

Among these, paxlovidTM, a combination of nirmatrivir and ritonavir PIs, stands out for its efficacy in the treatment of Covid-19.

Nirmatrelvir inhibits the major protease of SARS-CoV-2, essential for viral replication, while ritonavir increases the effectiveness of nirmatrelvir by inhibiting cytochrome P450 3A4 (CYP3A4), an enzyme that otherwise rapidly degrades nirmatrelvir.

Despite the success of the nimatrelivir-ritonavir combination, further research is needed to develop alternative major protease (Mpro) inhibitors to ensure continued efficacy against COVID-19.

PIs as antivirals for hepatitis C virus (HCV) and human immunodeficiency virus (HIV)

PI drugs for HCV and HIV infection

PIs are important in the treatment of HCV and HIV infections. HCV, a small ribonucleic acid (RNA) virus that causes hepatic disease, is targeted by PIs such as asunaporevir, telaprevir, and boceprevir, which target the nonstructural (NS)3/4A serine protease.

These inhibitors are peptidomimetics, containing a peptide bond and a ‘warhead’ group that binds covalently but reversibly to the enzyme’s active site.

HIV PI targets the viral aspartic acid protease, which is crucial for viral replication. They are used in antiretroviral therapy, converting HIV from fatal to chronic.

Development and mechanism of nirmatrelvir

Nirmarelvir, developed from Pfizer’s previous SARS-CoV-1 PI .. PF-00835231, challenges oral absorption.

Modifications such as altering the warhead and substituting different molecular components increased its binding affinity and antiviral activity, eventually leading to nimitrelvir with a nitrile warhead, improving solubility and synthesis.

Despite the different warhead, its structural similarity to boceprevir and its role as a covalent inhibitor of SARS-CoV-2 Mpro make it significant in the treatment of COVID-19.

Synthesis of nimarelvir

The synthesis of nirmatrelvir involves the coupling of the P1 building block and the P2-P3 dipeptide, with the final step being the formation of the nitrile warhead.

The process starts with protected amino acid derivatives, proceeds through steps such as bok-deprotection, ester cleavage and dipeptide formation.

The synthesis produced nirmatrelvir with high efficiency and introduced a new approach involving an Ugi-type three-component reaction for superior diastereoselectivity.

Synthesis and structure-activity relationship (SAR) studies of nirmarelvir analogues

Research by Chia and coworkers led to the synthesis of nirmatrivir analogs with different P1′ moieties examining the role of the warhead in antiviral activity.

These studies revealed varying levels of protease inhibition and antiviral activity efficacy, with some derivatives showing similar or superior effects to nimarelvir. However, the cell penetration and specificity challenges of SARS-CoV-2 have limited the broad application of these analogs.

Novel covalent and non-covalent inhibitors of SARS-CoV-2 Mpro

Recent developments in SARS-CoV-2 Mpro inhibitors have introduced both peptidomimetic and non-peptidic inhibitors.

These include warheads, such as epoxide rings and fluoromethyl groups, providing alternative mechanisms of covalent binding to the enzyme.

Non-covalent inhibitors, such as encitrelvir, show lower reactivity but better selectivity due to the nature of their secondary interactions. These developments represent important steps in diversifying therapeutic options against COVID-19 and its emerging strains.

Ritonavir as a pharmacokinetic enhancer

Structure, activity and interactions of ritonavir

Originally an HIV protease inhibitor, ritonavir is known to be effective at low doses (~100 mg) in inhibiting the CYP3A4 enzyme, an important component of drug metabolism.

Although high doses of ritonavir are poorly tolerated, its low-dose efficacy in combination therapy with other HIV protease inhibitors increases their half-life and thus reduces the required dose.

This unique use of ritonavir has even been explored in early COVID-19 treatments. However, its use poses the risk of significant drug-drug interactions, particularly with drugs metabolized by CYP3A4, potentially elevating their levels to toxic concentrations.

In addition, effects of ritonavir on other enzymes and transport proteins can be observed, although of lesser importance in paxlovide treatment.

Synthesis of ritonavir

Developed at Abbott Laboratories, the synthesis of ritonavir involves complex chemical processes, combining chiral amine and carboxylic acid building blocks.

The synthesis begins with a cyclocondensation reaction involving thioformamide and ethyl 2-chloroacetate, followed by several steps leading to the formation of ritonavir.

This complex process involves various intermediate compounds and chemical reactions, including triethylamine and 4-dimethylaminopyridine, highlighting the sophistication required in pharmaceutical synthesis.

The production of ritonavir demonstrates the complex chemical engineering required to develop effective pharmaceutical agents.

Application and activity against paxlovid-mutant variants

Paxlovide, in combination with nimatrelivir and ritonavir, has shown significant efficacy in reducing COVID-19-related hospitalizations and mortality.

Although it has been approved for emergency use in several regions, its efficacy against emerging strains and mutant variants is constantly being verified.

The evolving landscape of SARS-CoV-2 mutations requires ongoing monitoring to ensure the sustained efficacy of treatments such as paxlovide.

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