If the trial is successful, it is just possible a home cure for Covid-19 will become available later this year. U.K. Prime Minister Boris Johnson, who announced the formation of an “Antivirals Taskforce” last week specifically to invest in such products, will no doubt be scanning his text messages for early updates.
The molecule being tested is a bespoke antiviral code-named PF-07321332. Classed as a “protease inhibitor”, it has been formulated to attack the “spine” of the SARS-CoV-2 virus and stop it replicating in our nose, throats and lungs. It was protease inhibitors that turned the tide on the spread of HIV in the UK and around the world. Now researchers hope they may be on the brink of a similar pandemic-busting breakthrough.
The antiviral pill was developed from scratch during the current pandemic, Dafydd Owen, director of medicinal chemistry at Pfizer, told a private symposium of the Division of Medicinal Chemistry last month.
The first seven milligrams of the compound – no more than a raindrop — were made in late July 2020. By late October, they’d made 100 grams.
Just two weeks later, they had more than a kilogram in the bag. It took 210 researchers to do it, said Owen.
“We have designed PF-07321332 as a potential oral therapy that could be prescribed at the first sign of infection, without requiring that patients are hospitalised or in critical care”, said Mikael Dolsten, chief scientific officer and president of worldwide research, development and medical at Pfizer, in a statement released last month.
According to Ward, Pfizer’s scientists will have most likely established the drug’s “potent” action against SARS-CoV-2 by deploying it against infected human tissue cultures, including lung tissue, in a laboratory. “Once you know it works in vitro, it’s all about establishing its tolerance in animals and then humans,” she said.
“To date, the study drug has not been administered to humans,” say the documents which were formally approved on February 8 this year.
“The safety of the study drug has been studied in animals. In these animal studies, no significant risks or safety events of concern were identified, and the study drug did not cause side effects at any of the dose levels that will be used in clinical studies.”
Those who have signed up for the trial are in for an intensive few months.
The trial is split into three phases and will run for 145 days, with another 28 days added for “screening and dosing”. For all participants, there will be several overnight stays.
“Taking part in this study is voluntary… If you are not completely honest about your health history, you may be harmed by being in this study.”
The “randomised, double-blind, placebo-controlled, single- and multiple-dose escalation study” is designed to see how well or otherwise different dosing regimens are tolerated in humans while the active compound is maintained in the body.
PF-07321332 will be administered in combination with low doses of ritonavir, an antiviral used to treat HIV. It acts as a “booster” to increase the amount of PF-07321332 in participants’ blood.
Phase 2 will do the same but with “multiple doses”, while in Phase 3, tablet and liquid forms of the drug will be tested, as will the impact of eating on top of it.
Every detail has been specified in advance. In Phase 3, for example, a high-fat breakfast is defined as: “2 eggs fried in butter, 2 strips of pork bacon, 2 slices of toast with butter, 4 oz. of hash brown potatoes, and 8 oz. of whole milk… eaten in 20 minutes”.
Bringing a new drug to market is a long and difficult process and even if PF-07321332 is found to be well tolerated in humans, formal Phase 3 trials would need to follow to establish whether the drug worked against people exposed to SARS-CoV-2.
Prof Ward has also warned of more practical problems. The antiviral Tamiflu that she helped create costs about pounds 25 a course and is still not widely prescribed in the UK against seasonal flu because of its price tag, despite some 20,000 people dying of the disease in Britain each year.
For Pfizer and PF-07321332, it is a “race against time”, she said. They not only need to produce a drug that works but need to do it while SARS-CoV- 2 still presents a major public health threat.