Sipuleucel-T is an autologous cellular immunotherapy indicated for patients with asymptomatic or minimally symptomatic metastatic castration resistant prostate cancer (mCRPC). Since radiation therapy (RT) can suppress bone marrow function and immune responses, previous studies evaluating sipuleucel-T excluded patients who received RT less than or equal to 28 days prior to sipuleucel-T therapy. Recent evidence suggests that RT may act synergistically with immunotherapy to enhance and broaden antitumor immune response.

Patients who met standard criteria for sipuleucel-T were randomized to receive sipuleucel-T alone (Arm A) or sipuleucel-T initiated 1 week after completing sensitizing RT to single metastatic site (Arm B). RT was delivered at 300cGy/day to 3000 cGy total. The primary endpoint was the ability to safely combine sipuleucel-T preceded by RT and generate sipuleucel-T with adequate product immune activation parameters. Secondary endpoints included the measurement of systemic immune responses to prostatic acid phosphatase (PAP), a target for sipuleucel-T immune therapy and PA20204 (recombinant fusion protein utilized in the generation of sipuleucel-T).

51 pts were enrolled, 2 did not receive any sipuleucel-T because of vascular access problems and were excluded. 24 were treated on Arm A, 25 on Arm B. 47/49 patients received all 3 sipuleucel-T infusions. Median age was 66 yrs (range 45-90). Sipuleucel-T product parameters including: total nucleated cell (TNC) count, antigen presenting cell (APC) count were similar in both groups. Cumulative APC upregulation was higher in Arm A. 1 patient in Arm A demonstrated PSA response. Median progression free survival (PFS) was 2.46 months on Arm A and 3.65 months on Arm B (p = 0.06). Both arms showed similar increases in humoral responses to PA2024 and PAP. IFN-ƴ ELISPOT T-cell activation responses to PA20204 were observed in both arms, but were more robust in the Arm A (p = 0.028). Both arms were well-tolerated, with fatigue as the most common grade 2 adverse event (1 patient in Arm A and 3 patients in Arm B).

Sensitizing RT completed 1 week before generation of sipuleucel-T did not affect the majority of product parameters and the ability to deliver sipuleucel-T therapy. RT did not enhance the humoral and cellular responses associated with sipuleucel-T therapy.