What happened

Shares of Exelixis (NASDAQ:EXEL), a biotech company with a keen focus on developing therapies to treat cancer, skyrocketed 32% in June, according to data from S&P Global Market Intelligence. The reason for the move can almost entirely be traced to a positive press release on June 19 regarding its Cabosun trial.

So what

As some quick background, Exelixis' lead drug is Cabometyx, a Food and Drug Administration-approved treatment for second-line advanced renal cell carcinoma (RCC). In the late-stage Meteor trial, Cabometyx wound up hitting the trifecta of statistically significant improvements in progression-free survival (the primary endpoint), median overall survival, and response rate, compared to the placebo.

Biotech lab researchers examining data.

Image source: Getty Images.

Cabosun was a midstage study that Exelixis had designed to compare Cabometyx in a head-to-head against the first-line advanced RCC standard-of-care drug, Pfizer's (NYSE:PFE) Sutent. This exploratory study yielded some phenomenal results. As we found out at the European Society for Medical Oncology in October, Cabometyx led to a 31% reduction in the median rate of progression or death compared to Sutent (8.2 months versus 5.6 months). It also offered a significantly higher response rate of 46% compared to Sutent's 18%. 

The reason Exelixis exploded higher in June was it announced that an analysis of the Cabosun trial data by a blinded independent radiology review committee confirmed the primary progression-free survival efficacy of the phase 2 study. As my colleague Brian Orelli pointed out, Cabosun wasn't designed as a trial to lead to a label expansion. But given its strong results, and now the support of the independent review committee, it seems all the more likely that the FDA could grant Exelixis an expansion into first-line RCC, perhaps even supplanting Pfizer's Sutent in the process.

It's also worth mentioning that Exelixis and Roche announced the initiation of a phase 1b trial involving Cabometyx and cancer immunotherapy Tecentriq in patients with locally advanced or metastatic solid tumors. Chalk this up as more opportunities for Exelixis to expand the label indications of its lead drug.

A person holding a sign that reads "FDA Approved."

Image source: Getty Images.

Now what

At the moment, the chances of the FDA approving a label expansion for Cabometyx in perhaps the first quarter of next year looks promising. But what's really next up on the docket is the Celestial study data.

Celestial is the company's ongoing phase 3 trial involving Cabometyx as a treatment for advanced hepatocellular carcinoma. Practically every trial involving Cabometyx has hit its progression-free survival target, but this trial's primary endpoint is a statistically significant improvement in median overall survival. Cabometyx hasn't always hit the mark when overall survival is the primary endpoint (see the Comet-1 study for advanced prostate cancer). Nonetheless, the first planned interim analysis gave Exelixis a green light to continue its trial, which is good news.

In the meantime, Cabometyx appears to have a pretty clear path to perhaps $1 billion or more in annual sales -- and that's before we take into account whether or not any of its cancer immunotherapy combination studies yield positive results. Even following its massive run-up over the past two years, Exelixis could still be a bargain, depending on what happens with Celestial and a handful of ongoing trials. As a long-term shareholder, I see no reason to cash in my chips at this point.

Sean Williams owns shares of Exelixis. The Motley Fool owns shares of and recommends Exelixis. The Motley Fool has a disclosure policy.