Men with lower PSA levels at the start of sipuleucel T immunotherapy for metastatic castration-resistant prostate cancer (mCRPC) have better survival odds, new findings suggest.

Sipuleucel T is designed to stimulate an immune response against prostatic acid phosphatase, an antigen expressed in most prostate tumors.

Researchers led by Philip W. Kantoff, MD, of Dana-Farber Cancer Institute and Harvard Medical School in Boston, analyzed data from 512 mCRPC patients who were randomized to receive sipuleucel-T or control infusions as part of the phase 3 Immunotherapy for Prostate Adenocarcinoma Treatment (IMPACT) trial. They subdivided patients according to PSA quartiles.

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Compared with controls, sipuleucel-T recipients in the lowest quartile (22.1 ng/mL or less) had a 49% decreased risk of death, whereas those in the highest quartile (greater than 134 ng/mL) had a 16% decreased risk, according to findings published online ahead of print in Urology. The estimated improvement in median survival associated with sipuleucel T treatment was 13 months for men in the lowest PSA quartile versus 2.8 months for men in the highest quartile. The estimated three-year survival for those in the lowest quartile was 62.6% among the sipuleucel T recipients and 41.6% for controls, a 50% relative increase in survival.

The study also showed that patients with other favorable baseline predictors of overall survival, such as, serum lactate dehydrogenase, and Eastern Cooperative Oncology Group performance status, experienced a greater treatment effect from sipuleucel T.

“These findings provide a rationale for immunotherapy as an early step in sequencing treatment algorithm for mCRPC and also suggest a greater benefit when immunotherapy is used earlier in the treatment paradigm,” the authors concluded.

 The investigators pointed out that the overall finding of a larger treatment effect in patients with generally more favorable prognostic features is consistent with the proposed mechanism of action of sipuleucel T. “Patients with a lower cancer burden tend to experience less immunosuppression systemically and in the tumor microenvironment. Therefore, patients earlier in the natural history of the disease would be anticipated to have a more robust and effective immunologic response to sipuleucel T treatment.”