What's happening: Shares of the cancer drug specialist Exelixis (NASDAQ:EXEL) briefly jumped by double-digits this morning after the company released a clinical update for its ongoing midstage trial for cabozantinib in patients with advanced RET-rearranged lung cancers. What peaked investors' interest was the fact that the drug has already met its primary endpoint, with an objective response rate of 38% (6/16) and a median response duration of 8 months, in just the first-stage of the study. The second leg of the trial is now under way, where another nine patients will be enrolled. 

Why it's happening: While this early result in RET-rearranged lung cancers is certainly encouraging, the bigger issue is that it shows that cabozantinib does have potent anti-tumor properties, at least in some cases, and may yet prove to be a broadly applicable cancer treatment.

Last year, the market seemed to lose faith in the drug after it failed to improve clinical outcomes in a large late-stage trial dubbed "COMET-1" for metastatic castration-resistant prostate cancer. As a result, investors haven't been particularly optimistic about cabozantinib's forthcoming late-stage data readout as a potential second-line treatment for kidney cancer. Following today's clinical update in another hard-to-treat cancer, however, we might be seeing the beginning of a shift in sentiment.

If cabozantinib can meet its primary endpoint in kidney cancer, it would be indicated for a market that's expected to reach $2.7 billion by 2019. Even a small chunk of this growing market would cause revenues to skyrocket from current levels, making this top-line data release a major catalyst to keep your eyes peeled for in the weeks ahead.