Baseline PSA linked with immunotherapy outcomes

Chicago-Findings from a phase IV registry study are consistent with phase III study data in showing an association between baseline PSA and outcomes in men with asymptomatic/minimally symptomatic metastatic castrate-resistant prostate cancer (mCRPC) treated with sipuleucel-T (Provenge).

The research was presented at the American Society of Clinical Oncology annual meeting in Chicago. It included data from 1,886 men enrolled in PROCEED, a multicenter, open-label observational study of patients being treated with the autologous cellular immunotherapy in the real-world clinical practice setting. Overall survival, time to first anticancer intervention, and time to death due to disease progression were analyzed after stratifying patients into quartiles according to baseline PSA.

For men in the lowest PSA quartile (≤5.27 ng/mL), which was used as the reference group, median (range) time to overall survival was 48 months (range, 44-51 months), median time to first anticancer intervention was 10 months (range, 9-12 months), and median time to death due to disease progression was 57 months (range, 49-not estimable). Pairwise comparisons showed that for each endpoint, the outcome was significantly worse for men in the second (>5.27 to ≤15.08 ng/mL), third (>15.08 to ≤46 ng/mL), and fourth (>46 ng/mL) PSA quartiles than in the reference group.

“PROCEED is not a randomized trial, and there is lead time bias inherent in the registry’s design. Therefore, we cannot use the findings from this analysis to make any conclusive statements about the prognostic significance of PSA for outcomes with sipuleucel-T,” said lead author Oliver Sartor, MD, C.E. & Bernadine Laborde Professor for Cancer Research, Tulane Medical School, New Orleans. “This analysis has strengths, however, in that it includes long-term follow-up for a large patient cohort, and the trend it found for longer overall survival among men in the lowest PSA quartile is concordant with that seen in an exploratory analysis of data from the phase III controlled IMPACT (Immunotherapy for Prostate Adenocarcinoma Treatment) trial.

“Intriguingly, the current analysis also showed that some men in the lowest quartile did very well after starting sipuleucel-T. About 30% of patients in that subgroup went 2 years without requiring additional anticancer treatment and about 40% were still alive after 5 years,” he told Urology Times.

Dr. Sartor pointed out that PSA values for men enrolled in the phase III IMPACT study were much higher than those of men in PROCEED. Cut-offs for the four PSA quartile groups in IMPACT were ≤22.1, 22.1 to 50.1, 50.2 to 134.1, and >134.1 ng/mL, respectively. The analysis of IMPACT data showed that sipuleucel-T extended median overall survival compared with control by 13 months among men in the lowest PSA quartile, 7.1 months among those in the second PSA quartile, and 2.8 months in the highest PSA group.

Patients enrolled in PROCEED received sipuleucel-T per label recommendations-three infusions at 2-week intervals. The patients included in the analysis presented were enrolled in PROCEED between January 2011 and January 2017. They had a median age of 72 years, and the majority were Caucasian (87%). Median follow-up was approximately 47 months, approximately two-thirds of the enrolled men died during available follow-up, and approximately three-fourths of the deaths were due to mCRPC progression.

Few differences among PSA quartiles

Comparisons of patient demographics, baseline disease characteristics, and history of prior prostate cancer treatment identified few major differences among PSA quartile groups. The highest PSA quartile had a higher percentage of African-Americans compared with the lowest PSA quartile group as well as a higher number of men with bone and other metastases. Compared with the lower PSA quartile, a lower percentage of men in the higher PSA quartile group had prior radical prostatectomy and radiation therapy. Patients with a higher baseline PSA more often had prior anticancer therapy.

Identifying factors that correlate with better overall survival in men treated with sipuleucel-T is the subject of ongoing research. Dr. Sartor said that in a paper presented at the 2017 AUA annual meeting in Boston, he reported an analysis of PROCEED data that showed median overall survival for sipuleucel-T-treated men was significantly longer among African-Americans compared with matched Caucasian patients, and the race-related difference was particularly seen among men in the lowest PSA quartile.

Disclosures:

Dr. Sartor is a consultant to Dendreon and other companies that market treatments for prostate cancer. For a full list of disclosures, go to bit.ly/PROCEEDdisclosures.