Association Between Black Race And Improved Survival Following Sipuleucel-T Immunotherapy In Metastatic Castrate-Resistant Prostate Cancer: Implications For Immune Biology And Integration Of Radiation Therapy With Immunotherapy

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The biology of prostate cancer in black patients may differ from white patients. For example, recent data suggests that black patients may harbor tumors that are more sensitive to radiation or immune-directed therapy. Sipuleucel-T, a cell-based immunotherapy approved for use in men with metastatic castrate-resistant prostate cancer (mCRPC), is currently also being studied in early-stage non-metastatic prostate cancer. To explore possible immunologic differences and opportunities for synergy with radiation therapy, we examined the association between race and overall survival (OS) among patients receiving sipuleucel-T therapy for mCRPC using a novel real-world database. Using the CancerLinQ database, we identified 474 patients with mCRPC managed with sipuleucel-T. We used Cox modeling to examine the association between race (black vs white) and OS, with or without adjustment for other prognostic variables (age, prior systemic therapy, and prostate-specific antigen [PSA]), and performed subgroup analyses stratifying by the median PSA (16.6 ng/mL). In secondary analyses, we used Cox modeling to estimate treatment-free survival following sipuleucel-T and we used competing risks regression to model time to initiation of a subsequent systemic therapy. Median follow-up was 38.8 months. Black patients (n = 68) experienced significantly improved OS compared to white patients (n = 406) (median 43.1 months vs 30.2 months, hazard ratio [HR] = 0.638, 95% confidence interval [CI] 0.443-0.920, p = 0.016). These results persisted on multivariable analysis (adjusted HR 0.542, 95% CI 0.371-0.792, p = 0.002). Among patients with PSA > 16.6 ng/mL, median OS was 37.7 months vs 20.3 months (HR = 0.463, 95% CI 0.277-0.775, p = 0.003). For patients with PSA < = 16.6 ng/mL, there was no significant difference in median OS (43.1 months vs 40.6 months, HR = 0.842, 95% CI 0.487-1.456, p = 0.538). There were no significant differences in treatment-free survival (HR = 0.777, 95% CI 0.581-1.040, p = 0.090) or initiation of a subsequent systemic therapy (HR = 1.287, 95% CI 0.802-2.065, p = 0.295) by race. Black patients were more likely to receive subsequent cabazitaxel therapy compared to white patients (26.5% vs 15.0%, p = 0.019) but there were no significant differences in the receipt of other therapies (docetaxel, abiraterone, enzalutamide, radium-223). Among men with mCRPC who received sipuleucel-T, black patients had significantly longer OS compared to white patients but no difference in the time to subsequent systemic therapy, mirroring the results of prior immunotherapy studies. This finding may reflect differences in the underlying immune biology of prostate cancer in black versus white patients and may have implications for the integration of radiation therapy and immunotherapy in prostate cancer.