It's been a long road for Exelixis (NASDAQ:EXEL) to get a drug through the marathon of drug development. On Thursday, the Food and Drug Administration approved Exelixis' cancer treatment cabozantinib, which will go by the brand name Cometriq. After being in existence for 18 years, the biotech can finally label itself as a commercial-stage biotech.

Technically speaking, of course.

Cometriq is approved to treat medullary thyroid cancer that has spread to other parts of the body. The indication is so small -- each year, just 500 to 700 patients in the U.S. reach the stage when they'd use the drug -- that Exelixis is only using five sales reps to promote the drug to patients. And it's not even hiring the sales reps directly; they'll be employees of inVentiv Health, a sales force outsourcing firm, although they'll be a dedicated sales force that only sells Cometriq.

At $9,900 per month and figuring that patients stay on the drug for 10 months, the maximum market is about $70 million. And that assumes it takes all the market from AstraZeneca's (NASDAQ:AZN) Caprelsa, which is also approved to treat the rare indication.

Clearly, Exelixis needs to expand into other tumor types to make it to profitability. The next major test will come in late-stage prostate cancer where Cometriq is in phase 3 trials. It'll have more competition -- Dendreon (NASDAQ: DNDN), Johnson & Johnson (NYSE:JNJ), and Medivation (NASDAQ:MDVN) all have drugs for metastatic prostate cancer -- but there are substantially more than 700 patients with the disease.

The current label comes with a boxed warning about potential for issues with bleeding. In theory, the side effects could pose a problem as Exelixis moves onto other types of tumors where doctors have multiple treatment options with fairly tolerable drugs. But the biotech is testing Cometriq at a lower dose -- 60 mg in prostate cancer compared to 140 mg, which was used as the staring dose in the thyroid cancer trial. So far, the side effects haven't been an issue at the lower dose.

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