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Abiraterone acetate improved OS in metastatic castration-resistant prostate cancer

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2011 ASCO Annual Meeting

CHICAGO — For the first time, results from a prospective, phase 3 study showed that the selective CYP17 inhibitor abiraterone acetate extended OS for men with metastatic castration-resistant prostate cancer.

Howard I. Scher, MD, chief of the genitourinary oncology service at Memorial Sloan-Kettering Cancer Center, presented results from the COU-AA-301 trial at the 2011 ASCO Annual Meeting. The findings not only demonstrate that abiraterone (Zytiga, Centocor Ortho Biotech) prolongs survival, but also represents a new treatment option after docetaxel chemotherapy.

The FDA in April approved abiraterone combined with prednisone for the treatment of metastatic castration-resistant prostate cancer. A study published by The New England Journal of Medicine in May showed that abiraterone was associated with 4-month improvement in median survival.

In this preplanned analysis, 1,195 patients who had undergone treatment with docetaxel were assigned to 1,000 mg daily abiraterone and prednisone (n=797) or placebo plus prednisone (n=398).

After 775 events, OS increased from 3.9 months in the placebo group to 4.6 months in the experimental arm (HR=0.74; 95% CI, 0.638-0.859). Median OS was 15.8 months for abiraterone vs. 11.2 months for placebo.

“The results establish definitively that castration-resistant prostate cancers are not uniformly hormone refractory,” Scher said.

Researchers enumerated circulating tumor cells (CTC; the number of cells per 7.5 mL of blood) at baseline and again at 4, 8 and 12 weeks to determine whether those cells could serve as a surrogate biomarker for survival. Circulating tumor cell conversion using the standard definition for unfavorable (CTC≥5) and favorable (CTC<5) counts predicted OS as early as 4 weeks after treatment began.

Scher said abiraterone improved median OS for patients with both favorable (22.1 months vs. 19.7 months) and unfavorable (10.9 months vs. 8.2 months) baseline CTC counts compared with placebo. Lactate dehydrogenase at baseline (HR=3.036; 95% CI, 2.276-4.048) and CTC conversion rate (HR=0.386; 95% CI, 0.284-0.527) were also statistically significant predictors for survival. – by Jason Harris

For more information:

• Scher HI. #LBA4517. Presented at: the 2011 ASCO Annual Meeting; June 3-7; Chicago.

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