What happened

Shares of Arcus Biosciences (NYSE:RCUS), a clinical-stage biopharmaceutical company, are on the move following a progress report for the company's most valuable asset at the moment. Investors hoping Gilead Sciences (NASDAQ:GILD) will exercise an option to license domvanalimab have driven the stock 14% higher as of 11:29 a.m. EDT on Thursday.

So what 

Earlier this year, Gilead Sciences signed a 10-year agreement to co-develop and co-commercialize a handful of Arcus Biosciences' next-generation cancer therapies. Gilead Sciences still has an option to pay Arcus $275 million for exclusive rights to domvanalimab, one of several anti-Tigit antibodies in clinical-stage testing. 

Scientist in the laboratory looking at a bottle.

Image source: Getty Images.

In theory, inhibiting Tigit will make it harder for tumors to hide from the immune system. Early clinical trial results from Arcus Biosciences' peers suggest combining anti-Tigit drugs with other immunotherapies can boost their effectiveness. 

Interest in anti-Tigit antibodies has been high ever since Roche (OTC:RHHBY) shared compelling data for one called tiragolumab in January. Treatment with tiragolumab plus Tecentriq shrank tumors for 31% of lung cancer patients compared to just 16% of patients who received Tecentriq on its own.

The gain that Arcus Biosciences stock made today wasn't very big for a biotech that could have a blockbuster cancer drug in its late-stage pipeline. That's because the company's being unusually cagey about sharing key details from the ongoing Arc-7 trial.

Now what

Arcus Biosciences described the response rate data as encouraging but declined to share the actual percentages. Clinical-stage biotechnology companies never sit on positive data, so today's nondisclosure should set off alarm bells.

According to the company, patients who received the company's experimental PD-1 blocker, zimberelimab, as a monotherapy performed as well as patients who received Tecentriq and similar immunotherapies in the same setting. Without the actual numbers, though, investors have to assume adding domvanalimab to zimberelimab didn't double patient response rates.

The disturbing nondisclosure Arcus Biosciences made today doesn't necessarily mean that Gilead Sciences won't exercise its option to fully license domvanalimab. Without response rates to be proud of, though, it sure doesn't seem likely. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.