Effect Of Psa-Tricom, A Pox-Viral Vaccine In Prostate Cancer (Pca), On Tumor Growth Rates Within 80 Days After Initiation In Nonmetastatic Pca

57 Background: Our understanding of immunotherapies for prostate cancer (PSA-TRICOM, sipuleucel-T, ipilimumab) is incomplete in that such therapies have improved overall survival (OS) without changes in time to progression (TTP) in randomized trials. In an effort to better understand this discrepancy, we evaluated data from studies of PSA-TRICOM. A pox viral vaccine expressing PSA and 3 T-cell co-stimulatory molecules, PSA-TRICOM has demonstrated PSA-specific immune responses and evidence of clinical activity that supported initiation of a currently accruing Phase III trial. An analysis of NCI PCa trials (including a PSA-TRICOM trial) suggests that immune therapies may eventually slow the growth rate (GR) of tumors, leading to unaltered short term TTP, yet improved OS (Stein et al. Clin Can Res. 2011).PSA-TRICOM was administered to 50 hormone-naïve patients (pts.) with non-metastatic, castration naive PCa in a multi-center trial (ECOG 9802). Pts were treated every 4 weeks for 3 months, then every 12 weeks (preliminary data previously reported, DiPaola, RS et al. ASCO GU 2009). PSA values were used to calculate tumor GR within the first 100 days of treatment. (Pts were given no additional therapies during this time.) As previously described, a two-phase mathematical equation yielded concomitant PSA GR and regression rate constants.(Stein et. al., 2011) Results: See Table.These data suggest that PSA-TRICOM can alter GR significantly within 3 months. If confirmed in future trials, it could explain why vaccines have demonstrated improved OS without improved TTP. A slowing of the GR may not lead to substantial differences in short term TTP, but may enhance OS in the long term. This concept will be evaluated in an international Phase III trial of PSA-TRICOM in minimally symptomatic, metastatic castration-resistant PCa that is currently recruiting pts.NCT00108732. [Table: see text].