The first step in the bioactivation of GS-441524 is the rate-limiting step, something that remdesivir was designed to avoid. But that doesn’t matter clinically because of remdesivir’s rapid transformation to GS-441524 in the bloodstream. Remdesivir’s lackluster results in patients with advanced Covid-19 in the NIAID-sponsored trial and the finding that it provided no statistically significant benefit in a clinical trial conducted in China among patients with severe Covid-19 symptoms are likely due to the suboptimal level of active GS-441524 triphosphate in the lungs. Patients with advanced or severe Covid-19 generally have a high viral load in their lungs and would...