In a fascinating new preprint paper published on bioRxiv* in August 2020, the researchers report “the synthesis and antiviral evaluation of three novel lipophilic prodrugs of RVn-monophosphate that are substantially more active than Remdesivir in cells infected with SARS-CoV-2.” These could be of immense use in treating COVID-19 patients early and helping to end transmission.
Three major coronaviruses have caused havoc in the human population in the recent past, namely, SARS CoV, MERS CoV, and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). These had varying case fatality rates, but while epidemiological interventions contained the first two to a large extent, the present COVID-19 outbreak caused by SARS-CoV-2 has failed to be contained. Currently, it has caused over 25 million confirmed infections, with nearly 850,000 deaths globally.
Colorized scanning electron micrograph of a cell (green) infected with SARS-CoV-2 virus particles (purple), isolated from a patient sample. Image captured at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID
Unsustainable Physical Distancing Measures
Most countries that were hit early and badly by the virus attempted to control its spread by social distancing, quarantine and isolation, and school/ workplace closures. While these are demonstrably effective in the short-term containment of the pandemic, they have an unsustainable economic impact. Moreover, when the measures are relaxed, the virus returns aggressively.
The most intensively researched option at present is a vaccine to induce long-term immunity against the virus. However, there are many challenges, including the many and diverse strains of the virus, the uncertainty about the duration of the elicited immunity against reinfection, the possibility of lower efficacy of vaccination in people as young as 30 years, and the risk of antibody-dependent enhancement.
Despite recently obtained proof that reinfection does occur, casting doubts on the protection afforded by natural infection, hope springs from the successful control of HIV infection by cost-effective, widely accessible, and highly effective medication to prevent HIV transmission and treat it when already acquired. A similar strategy could be the preferred one for SARS-CoV-2 as well.
Remdesivir Approved for COVID-19
One of the few drugs which have shown moderate antiviral activity against the virus is remdesivir (RD, a polymerase inhibitor. RDV is a prodrug that contains three ester groups and requires to be converted to RVn triphosphate (remdesivir nucleoside triphosphate (RVn triphosphate)), where it has a potent inhibitory effect on the polymerase of all coronaviruses. This broad range of inhibition is owing to the relatively well-conserved polymerase sequence of the coronavirus RNA dependent RNA polymerase among all these viruses.
Problems with RDV
RDV was created to provide a detour around the slow first phosphorylation step of RVn, since this was the bottleneck in the synthesis of RVn-triphosphate, the active metabolite. When tested in Vero cells, RDV failed to live up to its promise, with both the current research and earlier studies showing that RVn actually has greater activity against viral infection in these cells than RDV.
RDV does not act equally well in all types of cells, perhaps because the conversion of RDV to RVn triphosphate takes place in four steps, using four enzymes, all of which are not equally expressed in all cells. During in vitro experiments using three types of cells, namely, Vero cells, Calu3 cells, and human alveolar epithelial cells, the EC50 (effective concentration) varies by 165 times.
Moreover, its beneficial effects are most apparent when given before or soon after exposure to the virus. Secondly, RDV is poorly bioavailable following oral administration and is therefore given intravenously. This has limited its use in COVID-19 to hospitalized cases, particularly those with progressive disease.
RDV also has a very short half-life of only about half an hour or less.
Highly Potent, Lipid-Soluble Compounds
The current study is focused on deriving an active form of RVn which can be orally administered, and which will yield prolonged adequate plasma levels. The researchers report that they have synthesized three new drugs (represented in brief as ODBG-P-RVn, ODE-P-RV, and HDP-P-RVn ), which are lipophilic, and have much higher anti-CoV activity than RDV when tested in Vero cells infected with the virus. They predict that on the basis of their similarity with earlier antivirals of the same general structure, these compounds will be orally bioavailable. Such prodrugs could make it possible to offer earlier and more potent antiviral treatment at the point of diagnosis.
One of these prodrugs also seems to target the lung rather than the liver specifically. This is important because RDV is toxic to the liver at higher doses, which limits its dosage regimen.
Comparing the dose-response curves, they found that while RDV and RVn have effective antiviral concentrations of 4.6 and 1.7 µM, respectively, the lipid prodrugs denoted by ODBG-P-RVn, ODE-P-RV, and HDP-P-RVn are much more active, with EC50 being between 0.19 to 0.96 µM. In fact, the first-mentioned is 24 times as active as RDV, and almost 10 times more so than RVn. Concerning cytotoxicity, the first two are the most selective, especially ODBG-P-RVn, with a cytotoxic concentration that is 240 times higher than the effective concentration.
Advantages of the Lipophilic Prodrugs
In order to overcome the many concerns of RDV use, including a very rapid inactivation in plasma, need for intravenous administration, poor lung concentrations and liver toxicity, the researchers designed three lipid prodrugs which bypass the three enzymatic steps required to activate RDV. It uses just one of two enzyme option to cleave the lipid ester group and become active.
Secondly, these are orally bioavailable and plasma-stable. Thirdly, it is simpler to synthesize and scale up the manufacture of these prodrugs compared to RDV. Also, at least one of them is likely to be preferentially delivered to the lung than to the liver, avoiding dose-limiting hepatotoxicity.
The researchers point out, “It will be important to evaluate the antiviral activity of RDV, RVn and these 3 novel lipid prodrugs of RVn in cells representing various tissues [intestine, spleen, kidney, brain, lymph nodes endothelium and intestine] which are infected by SARS-CoV-2,” since the virus will need to be cleared from all of them for successful treatment.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.