Shares in PharmaMar (PHMR.MC) jumped almost 20% on Tuesday after the Spanish drugmaker said lab trials suggested its Plitidepsin drug was effective against the main COVID-19 variants, including the highly contagious Omicron.

Results from in-vitro tests published in the Life Science Alliance journal showed that Plitidepsin, also known as Aplidin, had a potent antiviral effect in all variants and decreased the viral load detected in animal lung tissue by 99%, the company said.

The same paper also included previously published positive effects in Phase I and II clinical trials carried out on patients who were hospitalised with COVID-19.

The drug, originally developed as a cancer treatment, is now undergoing Phase III trials.

“All data we have seen so far with Plitidepsin corroborate our initial hypothesis about its antiviral activity,” PharmaMar Chairman Jose Maria Fernandez Sousa said in a statement.

Shares in the company rose 19% in early afternoon trade, on track for their best day in nearly a year. The shares had gained about 20% in January last year, also following encouraging news on Plitidepsin.

Alvaro Lenze, an analyst with Spanish brokerage Alantra, sounded a note of caution, however, saying it is too soon to draw definitive conclusions about the drug’s effectiveness against coronavirus variants.

“We also believe that PharmaMar’s opportunity in COVID is very limited, as its potential treatment would likely arrive too late,” he wrote in a note to clients.

Early data suggests that PharmaMar’s drug is less effective than rival treatments while its intravenous administration is less convenient than the pills produced by Pfizer (PFE.N) and Merck & Co. (MRK.N), Lenze added.

“We believe that the market reaction is disproportionate and see the rally as an opportunity to sell the stock.”