Advantages of the Parent Nucleoside GS-441524 over Remdesivir for Covid-19 Treatment

Abstract

While remdesivir has garnered much hope for its moderate anti-Covid-19 effects, its parent nucleoside, GS-441524, has been overlooked. Pharmacokinetic analysis of remdesivir evidences premature serum hydrolysis to GS-441524; GS-441524 is the predominant metabolite reaching the lungs. With its synthetic simplicity and in vivo efficacy in the veterinary setting, we contend that GS-441524 is superior to remdesivir for Covid-19 treatment.

Figure 1

McGuigan prodrugs on remdesivir are prematurely hydrolyzed in serum. (A) The ideal bioactivation of remdesivir predominately occurs in vitro. (B) The presence of serum enzymes in vivo predominately results in premature hydrolysis of the phosphate prodrugs, followed by dephosphorylation to the nucleoside, GS-441524.

Figure 2

McGuigan prodrugs on remdesivir are preferentially bioactivated in the liver. (A) Labile prodrug moieties on remdesivir with corresponding bioactivation enzymes. (B) Relative tissue mRNA expression of initial prodrug bioactivating enzymes for RDV (CES1/CTSA/HINT1) adapted from the HPA data set on the Human Protein Atlas reported as median-centered protein-coding transcripts per million (pTPM). Overall, McGuigan prodrug bioactivating enzymes are more highly expressed in the liver than in the lungs. (C) Immunohistochemistry images from the Human Protein Atlas indicating expression for ProTide bioactivating enzymes. Brown regions indicate enzyme expression while blue regions indicate absent expression. For the lung, pneumocytes—cells frequently infected by Covid-19—are characterized by a threadlike appearance. Expression in the liver is generally higher compared to lung for all enzymes. For CTSA, darkly stained regions are associated with macrophages. IHC images for the skin are included to show lack of enzyme expression. Antibodies used: CTSA (CAB024930), CES1 (HPA046717), HINT1 (HPA044577).

Figure 3

Unlike remdesivir, GS-441524 persists in serum at concentrations above the EC₅₀ value required against SARS-CoV-infected primary HAE cells for long durations. (A) In vitro potency data replotted from Agostini et al. mBio, 2018. Primary HAE cells were infected with either MERS-CoV or SARS-CoV and treated with either GS-441524 (open squares) or remdesivir (closed circles). Mean EC₅₀ of GS-441524 for SARS-CoV-infected HAE cells was found to be 0.18 ± 0.14 μM (note large standard deviations, red arrows). A study by Murphy et al. shows that GS-441524 has an EC₅₀ value of 0.78 μM against FCoV-infected CRFK cells (red dashed line), which is higher than the EC₅₀ value for GS-441524 against SARS-CoV-infected primary HAE cells. (B) Estimated metabolite concentrations for a PK experiment in a SARS-CoV-2 primate model replotted from Williamson et al. Nature, 2020. Primates were initially injected IV with 10 mg/kg of remdesivir 12 h postinoculation with SARS-CoV-2 and then 5 mg/kg of remdesivir every 24 h after. Throughout the experiment, GS-441524 is present in serum at concentrations ∼1000-fold higher than remdesivir; the concentration of GS-441524 is consistently above the EC₅₀ value in SARS-CoV-infected primary HAE cells (red dashed line) at all time points taken in the experiment. In contrast, the concentration of remdesivir in serum never exceeds that required to give the EC₅₀ value against SARS-CoV-infected primary HAE cells (gray dashed line). (C) PK data replotted from Warren et al. Nature, 2016 following IV injection (10 mg/kg) of remdesivir in NHP. Dashed lines indicate the approximate EC₅₀ values of GS-441524 (red) or remdesivir (gray) needed to reach EC₅₀ in SARS-CoV primary HAE cells obtained in (A). Unlike remdesivir, the concentration of drug required to give the EC₅₀ value against SARS-CoV primary HAE cells is maintained for significantly longer with GS-441524 than with remdesivir.