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In the midst of a pandemic like Covid-19, for which there are no FDA-approved drug treatments, hope is important. That’s one reason why remdesivir, an antiviral drug that Gilead Sciences originally made to fight Ebola, has been propelled into the spotlight with the hope that it can stop, or at least curtail, the ravages of SARS-CoV-2, the virus that causes Covid-19.

Data from the open-label SIMPLE trial, sponsored by Gilead, and the randomized controlled Adaptive Covid-19 Treatment Trial, sponsored by the National Institute of Allergy and Infectious Diseases, show that remdesivir may accelerate recovery rates among patients with advanced Covid-19. The drug’s modest effects are a far cry from the strong antiviral activity it demonstrated in preclinical primate models of coronavirus (both MERS and SARS-CoV-2). Yet that has been enough for the Food and Drug Administration to grant emergency use authorization for remdesivir and for the Japanese Ministry of Health, Labour, and Welfare to approve it for the treatment of Covid-19.


As chemists, we are troubled by the challenges to mass producing remdesivir. We aren’t alone. On the day that results from the two trials emerged, Gilead CEO Daniel O’Day praised the chemists behind the drug, saying he is “proud of the team because this is a complicated chemical process. It takes many, many steps.”

But does it really have to be that complicated? O’Day’s admission is interesting given that Gilead has another compound in its pipeline that is easier to make, has been shown to be effective against coronavirus in animal models, and is potentially as effective as remdesivir, if not more so.

Some background: Remdesivir works by interfering with the cellular machinery that allows viruses to replicate inside a human host. It is a pro-drug, meaning it must be metabolized and undergo a sequence of five bioactivation steps before it becomes GS-441524 triphosphate, the active compound that impedes viral replication.


Remdesivir isn’t Gilead’s only antiviral nucleoside analogue. The company has also developed GS-441524, another pro-drug that, as its name suggests, the body also converts into GS-441524 triphosphate, but in just in three steps. GS-441524 is easier to synthesize than remdesivir, requiring three steps instead of the seven needed for remdesivir.

Researchers initially thought that remdesivir would be activated more quickly than GS-441524 in human cells infected with the SARS and MERS coronaviruses. Yet data from primary human airway epithelial cells — one of the most clinically relevant cell-based models of the human lung — showed no statistically significant difference in potency between the two compounds. These data align with previous reports on the similar effectiveness of remdesivir and GS-441524 in coronavirus-infected cat cells. When GS-441524 was used to treat cats with feline infectious peritonitis, a progressive and usually fatal disease caused by a coronavirus, it displayed remarkable safety and therapeutic efficacy, with 96% of cats recovering after treatment.

Recent research in coronavirus-infected nonhuman primates demonstrated problems with remdesivir that inadvertently showed the antiviral effectiveness of GS-441524. In multiple studies testing remdesivir in coronavirus-infected mice or rhesus macaques, it was rapidly converted to GS-441524 in the bloodstream.

Take the latest controlled study conducted in rhesus macaques infected with SARS-CoV-2: After remdesivir was administered intravenously, GS-441524 was present in serum samples at concentrations 1,000-fold greater than remdesivir. Upon completion of the study, the researchers found that only GS-441524 — not remdesivir — was detected in the macaques’ lungs, yet they exhibited no signs of respiratory disease, significantly reduced viral loads, and a distinct reduction in damage to lung tissue. Such results reinforce those obtained from a prior study, also in macaques, and data from other species that GS-441524 exhibits strong antiviral activity.

Data in cats and primates have pointed to GS-441524’s safety. In the study using GS-441524 to treat feline coronavirus, the researchers noted its “impressive” safety profile when administered at high doses, and reported that no systemic signs of toxicity were observed over 12 to 30 weeks of treatment. In primates, GS-441524 was found to be present at high concentrations in the blood (1,000-times higher than remdesivir) with no apparent adverse effects.

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 need a high concentration of GS-441524 triphosphate to combat it. The benefit of using GS-441524 over remdesivir is that GS-441524 can almost certainly be given at much higher doses due to its lower toxicity. This would result in more conversion to the active compound, GS-441524 triphosphate, in the lungs.

When viewed through a different lens, the initial results from the NIAID-sponsored trial are more encouraging than they would seem. The active agent, GS-441524 triphosphate, clearly exerts antiviral activity against SARS-CoV-2 in humans, as supported by the accelerated recovery rates in advanced Covid-19 patients enrolled in the trial. Our analysis of preclinical and clinical trial data strongly suggests that early and direct administration of GS-441524 should be considered as a synthetically simpler and potentially more effective alternative to remdesivir, especially as GS-441524’s remarkable safety would enable higher dosing.

We see numerous advantages to using GS-441524 rather than remdesivir as an anti-Covid-19 therapy. GS-441524 is easier to synthesize and dissolves in water, which can speed manufacturing and enable higher dosing. It is a smaller molecule than remdesivir, which would make it easier to produce an aerosolized formulation for inhalable therapeutic and prophylactic treatment — this would be particularly attractive for achieving a high concentration of the drug in lung cells while minimizing systemic toxicity or side effects. And it is also less toxic than remdesivir. For these reasons, we do not see the point of making a significantly more complex drug like remdesivir when what actually reaches infected lungs is GS-441524.

The attractive profile of GS-441524 from both manufacturing and clinical perspectives raises this question: Why hasn’t Gilead opted to advance this compound to the clinic? We would be remiss for not mentioning patents, and thus profits. The first patent on GS-441524 was issued in 2009, while the first patent for remdesivir was issued in 2017.

We aren’t the only ones questioning Gilead’s strategy. We have spoken with a number of chemists, biochemists, veterinarians, and others who are also surprised that GS-441524 has remained out of the spotlight. Veterinarians we spoke to have noted that the strong antiviral activity of GS-441524 has resulted in a “miraculous turn of events” for cats infected with feline coronavirus, which was once considered a death sentence.

Given GS-441524’s optimal properties, we — along with the millions of people awaiting an effective treatment for Covid-19 — are left to wonder why Gilead isn’t giving it the same attention it is giving remdesivir. The world can only hope it isn’t for the sake of protecting its intellectual property.

Victoria C. Yan is a graduate research assistant specializing in phosphonate chemistry at the University of Texas MD Anderson Cancer Center in Houston. Florian L. Muller is an assistant professor specializing in cancer drug development in MD Anderson’s Department of Cancer Systems Imaging.

  • I have read many of the references linked in this opinion piece, and followed some of them back to earlier publications to better understand why Gilead would be likely to be putting their resources into investigating GS-5734 (‘remdesivir’) rather that GS-441524 (‘nuc’). Both GS-5734 and GS-441524 exhibit antiviral activity against a wide variety of human viruses in human cell cultures (Warren et al. 2016, Lo et al. 2017); however, the 50% effective concentration (EC50) values were consistently 10 to 100 fold lower for GS-5734 than for GS-441524. When tested against the Ebola virus in human cell culture (HeLa cells), the EC50 for GS-5734 was 0.086 µM but GS-441524 showed no antiviral activity at 20 µM, the highest concentration tested (Warren et al. 2016). Therefore, it is clear why Gilead would have continued to study GS-5734 against Ebola in primate and human studies and not have pursued further efforts with GS-441524. The result was that GS-5734 (‘remdesivir’) was tested in a clinical trial in humans against Ebola, and a record of clinical safety in humans was established prior to the outbreak of SARS-CoV-2 (the virus causing Covid-19). What I could not find was any study that showed a comparison of efficacy between GS-5734 and GS-441524 in vivo in either animal or humans against any virus. On the aspect of cellular toxicity, Warren et al. (2016) reported that GS-5734 showed no cell toxicity at 20 µM, and Lo et al. (2017) reported that the EC50 values for GS-5734 were 16 to 150 fold lower than the concentration causing 50% cell toxicity (CC50).
    In the papers cited regarding efficacy against Feline Infectious Peritonitis (FIP), I found the statement that the EC50 values for the two prodrugs in cat cell cultures were similar and the decision was made to pursue the chemically simpler GS-441524. Murphy et al. (2018) found that the plasma concentration of GS-441524 over 24 hours was identical whether the prodrug was administered intravenously or subcutaneously at 5 mg/kg body weight. They also found that the intracellular concentration of the nucleoside triphosphate (NTP) was maintained at 8 ‒ 20-fold higher than the 0.78 µM concentration necessary to inhibit FIPV replication by 50%. In fact, the intracellular NTP concentrations were consistently double following subcutaneous injection.
    The Murphy et al. (2018) observations with cats does give some support for the idea that GS-441524 could be effective against Covid-19 in humans when administered subcutaneously. The simpler chemical synthesis of GS-441524, and potentially simpler method of administration, do hold promise for easier treatment of Covid-19 vs intravenous administration of remdesivir. However, that by no means suggest that Gilead should ditch remdesivir for GS-441524.
    Lo et al. Nature/Scientific Reports (2017) 7:43395
    Murphy et al. Veterinary Microbiology 219 (2018) 226-233
    Warren et al. Nature 531 (2016) 381-384

    • Thank you for reading the references and taking the time to understand our rationale. Remdesivir may offer an immediate triage to the problem, but multiple studies have demonstrated the stronger antiviral benefits of early or even prophylactic intervention. With some of the qualities mentioned in the article, GS-441524 would seem to fit the bill here.

      With regards to cell culture work: remdesivir may exhibit greater potency compared to GS-441524 in some models, but a couple of things. 1.) Note the EC50 values/stdvs against human airway epithelial cells derived directly from patients ( “among the most clinically relevant in vitro models of the lung”; Figure 2a, Table 1). These differences in EC50 values are not statistically significant. 2.) More importantly: if GS-441524 is the predominant species that reaches the lungs (Supplementary Figure S1 in Williamson et al. bioRxiv 2020), then differences observed in vitro potency are moot.

      Based on IN VITRO data alone, it may be apparent why Gilead would choose to advance remdesivir over GS-441524. However (especially for the purpose of MERS/SARS), the moment that they had IN VIVO PK data and realized that intact remdesivir was rapidly hydrolyzed and converted to GS-441524, I believe they should have reconsidered their strategy and begun comparing remdesivir and GS-441524 IN VIVO.

      Key references:

      Pedersen et al. JFMS (2019)
      Agostini et al. mBio (2018)
      Williamson et al. bioRxiv (2020)
      Warren et al. Nature (2016)
      Sheahan et al. Sci. Transl. Med. (2017)

  • So layout the consequences of Gilead focusing on Remdesivir: how much cheaper and faster is it to make GS-441524? If you want people to be concerned, give them an idea of how many lives are going to be lost because Gilead wants to massage the stats and get a leg up on illicit producers in China!

    • Remdesivir is an immediate triage to this problem, since it had already cleared phase 1/2 for EBOV. Multiple studies have documented the strong antiviral effects of early/prophylactic treatment (eg: PrEP for HIV). The concern here is that Gilead will focus on remdesivir to the exclusion of GS-441524.

      I can’t speak to cost. From a synthesis standpoint, I can only say that GS-441524 takes 3 steps to make. Remdesivir takes 7 steps (see: Seigel et al. J. Med. Chem. 2017). As far as I can see, mass manufacturing of GS-441524 should not be an issue. If there are any issues with regards to supply I doubt it will come from Gilead’s ability to synthesize the drug…but rather business/patent/legal matters.

  • Thank you for all your dedication and hard work in finding treatments, not only for this and past viruses but also for those to come. I understand the testing involved…. all you need is one bad result to start a conspiracy theory. I read about new ideas in health and never cease to be amazed at the changes in medicine and care since I was a child over 70 years ago. Keep up the good work!

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