A precursor to remdesivir shows therapeutic potential for COVID-19

In the pursuit of effective antiviral drugs to counter the current coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), few stones have been left unturned. One of the earliest drugs to be approved by the US Food and Drug Administration (FDA) was remdesivir, even though its clinical trial results showed non-remarkable efficacy, and came from a single study.

A new study, released on the bioRxiv* preprint server, points to the potential to use the remdesivir precursor, GS-441524, as a treatment choice against COVID-19.

Remdesivir

Remdesivir is a phosphoramidate prodrug, and requires conversion through enzymatic action to its active form, the nucleoside triphosphate of remdesivir (NTP; GS-443902). This is incorporated into the viral RNA and ends transcription as well as replication by inhibiting the viral RNA-dependent RNA polymerase.

This is accomplished by enzymes that are highly expressed in liver tissue but poorly in alveolar type 2 cells (AT2). This may explain why only a single double-blind, placebo-controlled randomized clinical trial (RCT) showed even slight efficacy against SARS-CoV-2, since the lungs are the primary site of attack in this condition.

Other RCTs failed to demonstrate significant improvements in mortality with the use of this drug.

The metabolism of remdesivir by the liver may cause toxicity to the liver cells, which limits dose escalation to potentially more effective dosage ranges. Moreover, remdesivir is a hydrophobic drug, and must be formulated with excipients in complex form. The latter could impair renal function.

And finally, remdesivir can only be given via the intravenous route, making it non-feasible for outpatient use. This means it cannot be tested in early COVID-19, for its efficacy in preventing disease progression or the development of clinical disease following SARS-CoV-2 infection.

Finally, escape mutations on RdRp may allow the virus to surmount the inhibitory effects of remdesivir.

GS-441524 may be more effective

In view of the obvious drawbacks of remdesivir, its parent compound GS-441524 has been suggested to be a better choice in treating this condition. One reason is that it is the dominant metabolite found in plasma after the intravenous infusion of remdesivir, and persists the longest in preclinical studies. Its half-life in humans in is over 24 hours.

GS-441524 is converted to GS-443902 in the body by nucleoside kinases (probably adenosine kinase), enzymes found in a range of tissues. In this, too, it is unlike remdesivir. Besides, its safety profile is superior, allowing for the use of higher doses without dose-related liver toxicities. This enables higher concentrations of the final bioactive metabolite GS-443902.

In cell cultures, the activity of GS-441524 against SARS-CoV-2 infection has been found to be powerfully inhibitory. The 50% effective concentrations are comparable to that of remdesivir itself, in vitro.

In cats, naturally infected with the feline coronavirus (FCoV), and showing feline infectious peritonitis (FIP), the use of GS-441524 subcutaneously has shown a cure rate of 96%. In mouse studies, using either SARS-CoV-2 or murine hepatitis virus (MHV), which is closely related to the former, GS-441524 has been found to reduce viral loads in affected organs with encouragingly low toxicity levels.

These results have driven the current study on the pharmacokinetics of GS-441524 in dogs, since they indicate that this drug could potentially be developed as an orally available agent against COVID-19 on an outpatient basis.

Low liver metabolism

The researchers established that when dogs were given either oral GS-441524 or oral excipient-free remdesivir, the plasma concentrations of the former were 25-fold higher. Since the former is the metabolite of the latter, its concentration peaked at one hour following administration, but at three hours after administration of remdesivir.

The shift in values may have been due to the systemic release of remdesivir following the model described with this class of compounds. That is, the prodrug is rapidly removed from circulation by the liver, and then accumulates in two forms – the active nucleoside triphosphate and the hydrolyzed nucleoside. The latter form is slowly released into the systemic circulation.

Even then, the concentrations of GS-441524 following remdesivir administration are below the EC50 values required against SARS-CoV-2. Moreover, this dose cannot be used long-term in humans due to the associated liver toxicity. This rules out the possibility of using oral remdesivir to attain therapeutic levels of GS-441524 in COVID-19.

Oral bioavailability

Conversely, when directly administered, GS-441524 reaches EC50 values within an hour, and were maintained for eight hours or more, indicating high oral absorption. This confirms earlier studies showing the oral bioavailability of 85% in dogs. Even when used without excipients, at a slightly higher dosage, the high solubility of GS-441524 allowed a high oral bioavailability of 76%.

These data hint at the feasibility of using an excipient-less pill formulation for GS-441524 for outpatient treatment.”

What are the implications?

The number of dogs to which excipient-free GS-441524 was administered did not capture individual variability in formulation-derived dosage. Also, dogs typically have higher oral bioavailability compared to other early experimental species. This may be due to their possessing a paracellular nuclear transporter lacking in humans and non-human primates.

This caveat means that humans may have a lower oral bioavailability than indicated from the findings of this study. Despite this, “As a prodrug inhibitor of the SARS-CoV-2 RNA-dependent RNA polymerase (RdRp), GS-441524 is aptly poised to demonstrate consistent efficacy among new mutations of SARS-CoV-2, as RdRp is much less susceptible to efficacy-altering mutations than is the spike protein.”

*Important Notice

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.

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.