Together with cardiovascular and cerebrovascular diseases, infectious diseases caused by bacteria, viruses, parasites and fungi are the leading cause of death worldwide [1]. According to the World Health Organization, the global spread of coronavirus infection, which began in 2019 in China, has infected more than 600 million and killed more than 6.5 million people over three years [2]. The cause of the COVID-19 pandemic was a new coronavirus, SARS-CoV-2. Previously, members of the Coronaviridae family SARS-CoV and MERS-CoV caused outbreaks of severe acute respiratory syndrome (SARS) in 2002 and Middle East respiratory syndrome (MERS) in 2012 [3].

Several decades of studies of the family Coronaviridae have shown that the viral RNA genome is translated into two large polyproteins, pp1a and pp1ab, which, through their internal peptidase activity, are cleaved into several non-structural proteins (Nsps) required to enable transcription and replication of the viral genome [4]. Two cysteine proteases, papain-like peptidase (PLP) [5] and chymotrypsin-like peptidase (3CL), also known as the major coronavirus protease (M^Pro), are critical for proteolytic degradation of polyproteins [6]. MPro peptidase consists of three domains: domains I and II form a chymotrypsin-like fold containing a substrate-binding site located in the cleft between the two domains, while domain III is required for homodimer formation and plays a critical role in the catalytic activity of the protease as the M^Pro monomer is inactive [7]. M^Pro of different coronaviruses share highly conserved substrate-binding sites recognizing the amino acid sequence of the polyprotein (Leu-Gln)↓(Ser/Ala/Gly), where the peptide bond after the glutamine residue is hydrolyzed [7, 8].

The development of inhibitors of cysteine proteases involved in coronavirus (CoV) replication represent an effective strategy against COVID-19 and other diseases caused by coronaviruses. M^Pro is a promising target for the development of antiviral drugs targeting SARS-CoV-2 and other CoV because of its important role in post-translational processing of polyproteins. Moreover, the absence of human proteases cleaving proteins after the Gln residue is one of the advantages of M^Pro as a target for inhibitor development, as it increases their specificity and limits the undesirable side-effects. Since the epidemic outbreaks caused by CoV in 2002 and 2012, various M^Pro inhibitors have been proposed [9], but not until 2021 that the first drug candidates that successfully passed clinical trials have appeared [10, 11].

Another interesting strategy for antiviral drug development is the use of proteolysis to activate prodrugs [12]. Prodrugs are inactivated derivatives of drug molecules that can undergo enzymatic transformation to release the active compound in vivo [12]. A number of protease-activated prodrugs (PAPs) have been developed and successfully used in cancer treatment to improve drug delivery to malignant neoplasms, where protease expression is higher than in healthy tissues [13]. However, the application of PAPs is not limited to the development of anti-cancer drugs; recent publications show that this approach can also be used to treat bacterial and viral infections [14, 15].

CONCLUSION

The combination of the two strategies could be a promising avenue in the development of drugs for the treatment of COVID-19. The use of inactivated cytotoxic and cytostatic drugs conjugated with both irreversible and reversible selective M^Pro protease inhibitors can provide the targeted delivery and release of the active agents in infected cells and reduce the systemic toxicity of the developed drugs.