Properties and pharmacokinetics an oral drug candidate for COVID-19 treatment

In a study posted to the bioRxiv* preprint server, researchers studied the preclinical pharmacokinetics (PK) and in vitro absorption, distribution, metabolism, and elimination (ADME) properties of GS-441524, a potential oral drug for the coronavirus disease 2019 (COVID-19) treatment.

Study: Preclinical Pharmacokinetics and In Vitro Properties of GS-441524, A Potential Oral Drug Candidate for COVID-19 Treatment. Image Credit: Dragon Claws/ShutterstockStudy: Preclinical Pharmacokinetics and In Vitro Properties of GS-441524, A Potential Oral Drug Candidate for COVID-19 Treatment. Image Credit: Dragon Claws/Shutterstock

Despite several approved COVID-19 vaccines being distributed and administered at a large scale globally,  ongoing gene mutations in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative organism of COVID-19, give rise to new and more dangerous variants with vaccine escape abilities.

Although remdesivir has been proven to reduce hospitalization or mortality by 87% in severe COVID-19 patients, it can only be administered intravenously, which is a major limitation of the drug. Thus, safe and effective orally administered antiviral therapeutics are urgently needed for treating COVID-19. To this end, researchers assessed if GS441524 could be a potential oral drug for COVID-19 treatment.


The researchers determined the oral bioavailability of GS-441524 in animal species and evaluated the in vitro and in vivo ADME properties of GS-441524, including its interactions with human nucleoside transporters.

PK studies were performed after intravenous (IV) and PO administrations of GS-441524 in C57BL/6 mice, Cynomolgus monkeys, SpragueDawley (SD) rats, and Beagle dogs to analyze its bioavailability and disposition parameters.

All PK data were released to the public as soon as they were available. They used allometric scaling to measure human plasma clearance (CLp) and distribution volume at steady state (Vdss) from the preclinical PK data. They also confirmed GS-441524’s inhibitory activity in a 3D human airway epithelial (HAE) model of SARS-CoV-2 infection and Vero E6 cells derived from an African green monkey’s kidney.


The results showed that GS-441524 was stable in cytosols, liver microsomes, and hepatocytes of mice, monkeys, dogs, rats, and humans. In all the species studied, the plasma-free fractions of GS-441524 ranged between 62-78%, and the blood-to-plasma partitioning ratio was approximately 1.2.

According to plasma protein binding studies, GS-441524’s unbound fraction was similar in mice, rats, dogs, and monkeys in the range of 62-64%, whereas it was higher at about 78% in humans. Overall, GS-441524 was found to be a low-plasma protein-bound drug compound.

As per the results of the in vitro transporter study, GS-441524 was a substrate of BCRP, MDR1, ENT1, CNT3, and ENT2 nucleoside transporters, while it was not a substrate of ENT4, CNT1, and CNT2. The findings confirmed that GS-441524 has similar inhibitory activity against SARS-CoV-2 to remdesivir in vitro in the 3D HAE tissue model; however, it had slightly more strong inhibitory activity in Vero E6 cells.

GS-441524 showed low to moderate plasma clearance (CLp) of 4.1 and 26 mL/min/kg in dogs and mice, respectively. The Vdss was 0.9 and 2.2 L/kg in dogs and mice, respectively, after IV administration. GS-441524 was mainly eliminated via urinary excretion in all species studied. Using an accurate and selective ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method, the researchers quantified GS-441524 in urine, plasma, and bile samples. They found that GS-441524 showed poor retention on regular C18 columns due to its hydrophilicity.

After oral administration of GS-441524, its bioavailability was 8.3% in monkeys, 39% in mice, 33% in rats, and 85% in dogs. The researchers simulated the PK profile of GS-441524 in humans using a simple one-compartment model along with the sophisticated GastroPlus compartmental absorption and transit model. They found that 300, 600, and 1000 mg of GS-441524 twice daily will be ideal for achieving plasma concentrations above 0.5, 1, and 2 µM, respectively.

Based on the observations, the PK and ADME properties of GS-441524 endorse its development as an oral therapy candidate for COVID-19.


The study results demonstrated that GS-441524, an active metabolite of remdesivir, could be suitable as an oral antiviral agent for treating SARS-CoV-2 infection. In the experiments conducted by the researchers, GS-441524 showed low to moderate plasma clearance in rats, mice, dogs, and cynomolgus monkeys. The free unbound fractions of GS-441524 were in the range 62-78% in all species studied, and renal excretion was the main mode of elimination of GS-441524.

Based on the in vitro PK and ADME results obtained from this study, the authors proposed human oral dosing of GS-441524 compatible with a twice-daily dose regimen. They found that a simulation of 1000 mg twice daily dose helped achieve a 2 μM plasma concentration above EC50 in the 3D HAE tissue model and Vero E6 cells.

*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.

Journal reference:
Susha Cheriyedath

Written by

Susha Cheriyedath

Susha has a Bachelor of Science (B.Sc.) degree in Chemistry and Master of Science (M.Sc) degree in Biochemistry from the University of Calicut, India. She always had a keen interest in medical and health science. As part of her masters degree, she specialized in Biochemistry, with an emphasis on Microbiology, Physiology, Biotechnology, and Nutrition. In her spare time, she loves to cook up a storm in the kitchen with her super-messy baking experiments.


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