Remdesivir and chloroquine effectively inhibit the recently emerged novel coronavirus (2019-nCoV) in vitro

Picture the last days of 2019. A cluster of strange pneumonia cases pops up in Wuhan, a city of about eleven million. Within weeks, the case count grows, and by January twenty-seventh, Chinese authorities report two thousand eight hundred thirty-five confirmed infections and eighty-one deaths, with cases already spilling into other regions. The culprit is a new coronavirus, closely related to the one behind the two thousand three SARS outbreak. No specific treatment exists. And when you don't have time to invent a new drug from scratch, you reach for an old playbook: repurpose medicines that might already work. Because this virus sits in the Betacoronavirus family alongside SARS and MERS, compounds with activity against those relatives jump to the front of the line. Wang and colleagues moved fast on that idea. They took a clinical isolate of the new virus—labeled BetaCoV Wuhan WIV zero four two thousand nineteen—and infected Vero E6 cells, a standard African green monkey kidney line cataloged as ATCC-1586. They kept the infection modest, using a multiplicity of infection of zero point zero five. Then they bathed the cells in one of seven drugs that were either Food and Drug Administration approved or well characterized. Dimethyl sulfoxide, or DMSO, served as the vehicle control. Forty-eight hours later, they asked two questions: how much virus was produced and how many cells looked infected? For viral yield, they measured RNA copies in the culture supernatant by quantitative reverse transcription polymerase chain reaction. For infection status, they stained the viral nucleoprotein, or NP, by immunofluorescence. At that forty-eight-hour mark, the cells didn't yet show an obvious cytopathic effect—the cell damage you can see under a microscope—so those molecular readouts carried the load. Everything was run in triplicate. They also made sure the drugs weren't just killing the cells. Cytotoxicity was measured with the Cell Counting Kit eight assay, or CCK-eight, a simple colorimetric readout of cell viability. With those pieces in hand, they built dose-response curves and pulled out three pharmacology numbers for each compound. EC50 is the concentration that cuts infection by half. CC50 is the concentration that kills half the cells. And the selectivity index is defined as CC50 divided by EC50—how far apart those two thresholds are. In words, the selectivity index asks: how much room do you have to suppress the virus before you start hurting the host? A large ratio hints at a useful therapeutic window. It doesn't prove a drug will work in people, but it's an encouraging first filter. Two compounds immediately separated from the pack. Remdesivir, a nucleotide analog originally developed for Ebola and related RNA viruses, posted an EC50 of zero point seven seven micromolar, with no measurable cytotoxicity up to one hundred micromolar. That puts its selectivity index above one hundred twenty-nine. And if you push harder, the EC90—the concentration that knocks down infection by ninety percent—was one point seventy-six micromolar. Chloroquine, the old antimalarial, also looked strong in this system. Its EC50 was one point thirteen micromolar, CC50 again above one hundred micromolar, and the selectivity index landed above eighty-eight. The EC90 was six point ninety micromolar. Those are low micromolar potencies with a lot of separation from toxicity in this cell line. In plain terms: they shut down the virus well before they started hurting the cells. The rest of the panel formed a clear second tier. Nitazoxanide, a broad-spectrum antiviral, inhibited at low micromolar levels too, with an EC50 of two point twelve micromolar. But its CC50 was only a bit higher, over thirty-five point five micromolar, leaving a selectivity index just north of sixteen. That's decent, but not in the same league. Nafamostat, a serine protease inhibitor known to block MERS-CoV entry, required more drug to make a dent here: EC50 at twenty-two point fifty micromolar, CC50 over one hundred micromolar, and a selectivity index above four point four. It did something, just not with the headroom you'd want. Then there were the familiar nucleoside analogs that many hoped would carry over from other viral diseases. Ribavirin needed one hundred nine point fifty micromolar to reach its EC50; penciclovir landed at ninety-five point ninety-six micromolar; favipiravir at sixty-one point eighty-eight micromolar. All three showed no cytotoxicity until concentrations above four hundred micromolar, but their selectivity indices—three point sixty-five, four point seventeen, and six point forty-six—were modest. You can read those numbers two ways. On the optimistic side, they do inhibit. On the realistic side, you need a lot more drug to get there, and the safety buffer in this assay is narrow compared to the top performers. Numbers tell us who won the screen. Timing tells us how they're winning. Wang's team ran a classic time-of-addition experiment to map when in the viral life cycle each drug exerts its effect. They set up three dosing windows plus a full-time condition. In the full-time setup, cells were pretreated with the drug for an hour, exposed to the virus for two hours, then maintained in drug-containing medium until the end. In the entry window, the drug was present around the time of viral attachment and then washed away at two hours post-infection. In the post-entry window, the drug wasn't added until two hours after infection and stayed on thereafter. All infections were still done at a multiplicity of infection of zero point zero five. For the timing assay, they quantified viral RNA in supernatants by quantitative reverse transcription polymerase chain reaction, and they also examined NP expression by immunofluorescence early—at fourteen hours post-infection—when entry effects are easiest to spot. The patterns were striking. Remdesivir worked when added after the virus had already entered the cell, consistent with its chemistry. Inside the cell, remdesivir is converted to an active triphosphate that the viral RNA polymerase mistakes for a real building block; once incorporated, it stalls further replication. That's a post-entry job. Chloroquine, in contrast, blunted infection both when present during viral attachment and when added later. That split personality fits with its known biology. Chloroquine can raise endosomal pH and interfere with entry for viruses that depend on acidified compartments, and it can also perturb post-entry processes like glycosylation of host receptors. In Vero E6 cells infected with this clinical SARS-CoV-2 isolate, it looked like both doors were open to inhibition. Let's pause on what those timing results mean in practical terms. You can imagine two kinds of antiviral defenses. One primes the walls of the house so invaders have a hard time getting through the door. The other waits inside and jams their tools once they're in. Remdesivir is the second kind. Chloroquine does a bit of both in this system. Which is useful, because in a real treatment scenario, you don't always get to the virus before it enters cells. Drugs that still work after entry give you a fighting chance even if you start late. The selectivity numbers matter too, and Wang's group is careful about how to use them. A high selectivity index in Vero E6 cells suggests you might have a workable dose range in a living organism, but it doesn't guarantee it. Vero cells are not human lung tissue. They're a handy, permissive lab model. Pharmacokinetics—how the body absorbs, distributes, and clears a drug—can tighten or widen that window in people. At the forty-eight-hour time point here, cytotoxicity in Vero cells wasn't obvious, which supports the interpretation of a favorable index under these conditions. But it's a starting point, not a finish line. There's one more detail worth mentioning. In supplementary data, remdesivir also showed preliminary activity in a human cell line, Huh-seven. That doesn't overhaul the main picture, which is rooted in Vero E6, but it nudges the finding toward broader relevance. It's a reminder that what looks potent in a monkey kidney cell isn't automatically a fluke. Methodologically, the study is disciplined and transparent. They ran everything in triplicate. They kept the infection at a multiplicity of infection of zero point zero five to let differences in viral amplification emerge over forty-eight hours. They measured two independent faces of antiviral effect—viral genomes in the supernatant and the fraction of cells expressing NP—and paired those with an orthogonal cell viability assay. They summarized potency with EC50 and EC90, safety with CC50, and pulled them together with a simple ratio, the selectivity index. If you like equations spoken out loud, it's this: selectivity equals cytotoxic threshold divided by antiviral threshold. The higher that ratio, the more room you have to operate. If you lay the full panel side by side, a rank order writes itself. Remdesivir sits at the top with an EC50 under one micromolar and a selectivity index above one hundred twenty-nine. Chloroquine follows closely with an EC50 just over one micromolar and an index above eighty-eight. Nitazoxanide is the next strongest, but with a much slimmer safety margin in this assay. Nafamostat does less at the tested ranges. The classic nucleoside analogs are in the mix only at high concentrations, with narrow windows. That ladder of potency and selectivity is the core result. Now, what do you do with that in January two thousand twenty? You don't treat patients based on one cell line. But you do take the strongest signals—remdesivir and chloroquine in this case—and move them forward. You ask whether the pharmacology that looked generous in Vero E6 translates in primary human airway cells, in animals, and ultimately in people. You ask whether the timing insights hold in more complex biology. And you weigh all of that against safety profiles and the realities of dosing. With hindsight, we know how some of that story unfolded. But if you put yourself back in the lab with Wang and colleagues, this was a crisp, early x-ray of where the signal lived. It said: in a head-to-head screen against a clinical SARS-CoV-2 isolate, remdesivir and chloroquine shut down infection at low micromolar concentrations with wide safety margins in vitro. Remdesivir worked post-entry. Chloroquine hit entry and post-entry. Nitazoxanide showed a respectable but tighter window. Others trailed. And the data were strong enough, and clear enough, to justify taking the next steps. That mix of speed and rigor matters. In a crisis, you can't wait for perfect models or five layers of validation before you act. But you also can't skip the basics. Define your readouts. Control your timing. Anchor your claims to numbers, not vibes. Wang's team stayed inside those guardrails and handed the field a short, readable map of where to look next.