We currently have some successful antiviral drugs. Acyclovir is potent against herpes, particularly genital herpes, taken orally and used locally as a cream.
So why, you may ask, do we not have an antiviral drug to treat Covid? Well, we could in fact have one by the autumn.
Finding one hasn’t been easy, says David Smith, clinical research fellow in infection and community, and Dipender Gill, clinical lecturer in clinical pharmacology at St George’s University of London.
In its search for orally acting drugs that curb multiplication and help eliminate Covid from the body, the Government has launched a Covid-19 antivirals taskforce with the aim of finding drugs for home treatment by autumn this year. These drugs have the potential to reduce viral transmission, illness and mortality.
And we need them. Current Covid-19 treatments are largely limited to anti-inflammatory drugs (dexamethasone, tocilizumab, and sarilumab) that save only around one in three patients who would otherwise have died.
Remdesivir, the most commonly used antiviral in the UK for Covid, must be given intravenously and doesn’t seem to improve survival.
Of the available oral drugs with broad spectrum antiviral activity, only ribavirin is licensed to treat several viruses, and usually in combination with other drugs. Its toxic effects rule out its use for Covid-19 prevention and clinical trials are still under way.
Our experience with anti- HIV drugs is illuminating. In the 1980s, trials showed that azidothymidine improved survival in people with HIV infection, then viral resistance emerged.
It became clear that patients needed a combination of three drugs to avoid resistance. More recently, the pandemic influenza strain H1N1 has developed resistance to oseltamivir. If Covid shares this capacity to become resistant, then we’re going to need combinations of several antiviral drugs for effective treatment or prevention.
So it’s complicated. One of the most promising lines of research is preventing the virus from entering cells by clinging to their surface. One such drug, Camostat, works this way and early trials are currently in progress.
A second point of entry for the virus may be blocked by the drug apilimod, and early studies are encouraging.
Covid also produces an enzyme, which it uses for building viruses, so an enzyme inhibitor could prevent virus assembly.
Nitazoxanide may work by inhibiting both cell entry and virus assembly and seemed to improve viral clearance in patients with Covid-19 in one small trial.
Although the Government favours oral treatments, medications given by nasal spray may be as effective – iota-carrageenan, a sugar molecule derived from seaweed, is in early trials after encouraging results too.