Antivirals Targeting Coronavirus RNA-Dependent RNA Polymerase and Main Protease: From Mechanisms of Action to Outcomes in COVID-19 Clinical Trials.

The rapid global spread of SARS-CoV-2 triggered an unprecedented effort to develop effective antivirals. Among the first approved agents was remdesivir, an injectable nucleoside analogue developed by Gilead Sciences, that led to chain termination of viral RNA synthesis and showed broad antiviral activity against RNA viruses. Early clinical results were mixed: The US ACTT-1 trial reported an accelerated recovery and reduced mortality in treated patients, while the WHO Solidarity and a European trial revealed no impact of remdesivir on mortality. In contrast, a US trial in outpatients demonstrated a clear clinical benefit when treatment was administered early. Molnupiravir, an orally applicable nucleoside analogue developed by Merck, induces lethal mutations in the viral genome rather than chain termination. Molnupiravir showed in vivo antiviral activity against coronaviruses in different animals. In MOVe-OUT trials, molnupiravir reduced the rate of hospitalisation in treated outpatients. In the PANORAMIC trial, molnupiravir reduced the time to recovery in outpatients but not their rate of hospitalisation. No drug effect of molnupiravir was seen in the RECOVERY trial with hospitalised COVID-19 patients. Using structural biology and medicinal chemistry approaches, Pfizer developed nirmatrelvir, an oral inhibitor of the major coronavirus protease. In high-risk but not in standard-risk COVID-19 patients, the combination nirmatrelvir/ritonavir reduced the rate of hospitalisation (EPIC HR and SR trials). Retrospective cohort studies showed treatment effects in defined patient groups. This review compares the efficacy and clinical performance of different antivirals, including emerging drugs such as obeldesivir and alternative protease inhibitors (lopinavir, simnotrelvir). It further examines their roles in prophylaxis, treatment of long covid symptoms, pharmacological considerations and antiviral resistance. Particular attention is given to factors underlying variable outcome of the trials, including viral variant evolution, population immunity increases, disease severity changes and timing of therapy initiation.