Start the clock. Exelixis (Nasdaq: EXEL) looks like it'll have a drug on the market in a few months. The biotech announced today that it has completed the filing of its marketing application for cabozantinib as a treatment for advanced medullary thyroid cancer.

Technically, we won't know how much time to put on the countdown timer until the FDA accepts the application. Exelixis requested a priority review that cuts the review time down from 10 months to six months, which I'd expect the agency to grant. In fact, investors should be a little worried if the agency gives the drug a standard review because it's a sign that the agency doesn't think cabozantinib is all that important.

Nothing is a sure thing in drug development, but an approval for cabozantinib looks fairly likely. Exelixis ran a phase 3 trial under a Special Protocol Assessment, meaning the FDA signed off on the trial design. Since the trial was positive, an approval seems likely with the usual caveats that not every piece of data, especially surrounding manufacturing, is available to investors.

Thyroid cancer will be the first indication that cabozantinib is approved for, but it isn't the one with the most potential. Cabozantinib won't have much competition in advanced medullary thyroid cancer; AstraZeneca's (NYSE: AZN) Caprelsa is approved for the indication as well, but there aren't that many patients out there.

Cabozantinib has a better chance of becoming a blockbuster in prostate cancer. Exelixis is running a dual strategy, looking at pain in prostate cancer patients with advanced disease, which should read out quicker, and in a separate trial it's looking at overall survival in patients who have failed Johnson & Johnson's (NYSE: JNJ) Zytiga and/or Medivation's (Nasdaq: MDVN) MDV3100. The second trial just began, with results not expected until the first half of 2014.

Prostate cancer is considerably more crowded -- there's also Dendreon's (Nasdaq: DNDN) Provenge and chemotherapy agents from Sanofi -- but there are lots of patients to be had, which will only increase as the population ages. Exelixis just needs to find its niche, potentially even by combining the drug with other agents; an investigator recently started a trial testing cabozantinib combined with Zytiga.

Exelixis is a pick of David Gardner's Rule Breakers newsletter service. You can see his latest health-care find in the Fool's new free report "Discover the Next Rule-Breaking Multibagger." Get your copy for free.

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.