CHICAGO—Retreatment with sipuleucel-T, an autologous cellular immunotherapy, is safe and may benefit some patients with metastatic castration-resistant prostate cancer (mCRPC), even years after receiving the first treatment for an earlier stage of prostate cancer (PCa), according to new data presented at the 2013 American Society of Clinical Oncology annual meeting.

Researchers presented data from P10-1, an open-label phase 2 study of sipuleucel-T in men with mCRPC previously treated with sipuleucel-T in the PROTECT (PROvenge Treatment and Early Cancer Treatment) trial. They evaluated antigen-presenting cell (APC) activation, a measure of product potency.  The study included men who received one or more infusions of sipuleucel-T in PROTECT and progressed to mCRPC. In the current study, all men were retreated with three infusions of sipuleucel-T.

Seven men with a median age of 70 years as of October 23, 2012 have been enrolled in the trial. The median time from the first infusion to retreatment in the current trial was 9.2 years (range 7.8–10.0 years). APC activation was greater at the first P10-1 treatment compared with the last retreatment.

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“Retreatment is feasible and it appears immune memory is long-term following treatment with this agent,” said investigator Tomasz Beer, MD, of Oregon Health & Science University Knight Cancer Institute in Portland. “More studies are needed to determine if retreatment should be something we do for patients outside of a clinical trial.”

This is the first trial to demonstrate the feasibility of sipuleucel-T retreatment following treatment for an earlier stage of PCa. Sipuleucel-T is designed to stimulate humoral and cellular immune responses against prostatic antigen phosphatase, which is an antigen expressed in most prostate tumors.

Also at the meeting, the researchers reported that retreatment with sipuleucel-T was well tolerated, and most of the adverse events were mild to moderate in severity. “Side effects are largely transient,” Dr. Beer said. “They include fever, aches, etcetera that occur with the infusion and for a couple days later. We don’t see long-term side effects.”

In the pivotal phase 3 IMPACT study, treatment with sipuleucel-T resulted in a 4.1 month improvement in median overall survival relative to control treatment in patients with asymptomatic or minimally symptomatic mCRPC.  The Phase 3 PROTECT study investigated sipuleucel-T for men with rising PSA after three to four months of androgen deprivation therapy following radical prostatectomy. The primary endpoint of time to biochemical failure (PSA level 3 ng/mL or higher) was not statistically significantly different between the treatment group and the control arm (median 18.0 and 15.4 months, respectively). Men treated with sipuleucel-T, however, had a 48% increase in PSA doubling time following testosterone recovery relative to the control group (155 vs. 105 days).