Exelixis (NASDAQ:EXEL) already markets cabozantinib as a treatment for kidney cancer and liver cancer; its latest results suggest it may secure approval for its use in prostate cancer someday, too.

Interim results from a phase 1b trial showed that 32% of 44 metastatic castration-resistant prostate cancer (CRPC) patients responded to a combination of cabozantinib, which is marketed under the brand name Cabometyx, and Roche's (OTC:RHHBY) Tecentriq, a PD-1 inhibitor.

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The responses include 12 partial responses and two complete responses. Including patients with stable disease, the disease control rate was 80%. Safety doesn't appear to be concerning either, given that just 7% of patients discontinued the study because of adverse events.

The findings suggest Cabometyx may help delay the need for chemotherapy in advanced patients who have few other treatment options. According to the American Cancer Society, nearly 200,000 men are diagnosed with prostate cancer annually, and 33,000 die from it every year. In 2020, approximately 43,000 prostate cancer patients will progress to metastatic CRPC, a diagnosis with a median survival time of less than two years.

Exelixis is expanding the number of patients enrolled in its metastatic CRPC trial; if data from the new patients is similarly positive, then management hopes to file for accelerated FDA approval of Cabometyx in this indication as soon as 2021. Net sales of Cabometyx totaled $800 million in 2019; Exelixis estimates that approvals in metastatic CRPC could add $600 million annually to net product sales by 2025, making this biotech stock worth watching.

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