Down-staging of locally advanced prostate cancer with anti-CTLA-4 monoclonal antibody prior to radical prostatectomy

e16103 Background: At present, androgen ablation (AA) therapy ± radiation therapy represents a first-line treatment for patients with unresectable locally advanced prostate cancer (CAP), typically resulting in palliative disease control. Immunotherapy, in the form of CTLA-4 blockade, has been shown to potentiate T-cell responses, representing a potential antitumoral immunotherapy for treating CAP. Here, we discuss the outcomes of 5 locally advanced CAP patients treated with the monoclonal antibody MDX-CTLA-4 (MDX-010) + AA who subsequently underwent radical prostatectomy (RP), 2 of whom exhibited striking down-staging of their disease. Methods: A Phase II trial testing AA + MDX-010 therapy for treatment of locally or systemically advanced CAP is currently ongoing at our institution. To date, 5/85 patients have electively opted for off-study RP with extended pelvic lymphadenectomy after documentation of robust treatment response by PSA, staging digital rectal exam, and 3-D imaging. Results: All men had unresectable, locally advanced disease (stage T2c-T4) prior to treatment with AA + MDX-010 and subsequently underwent RP. Median blood loss was 400mL (400–900). All patients had negative surgical margins. Of the 5 patients, 2 exhibited dramatic and indisputable down-staging of their disease. These two patients presented with 80% of biopsies involved with cancer, extraprostatic extension, seminal vesicle (SV) invasion, and bladder base involvement. On final pathology, both exhibited extensive treatment effect, remission of SV involvement, and robust disaggregation of solid tumors resulting in only microscopic, non-contiguous foci of residual intraprostatic cancer. Both had T4 disease pre-treatment, and had stage T2c on final pathology. One patient is free of disease with undetectable PSA for 19 months following adjuvant radiation treatment but no further therapy, while the other patient is awaiting follow-up. Conclusions: We have been able to demonstrate true down-staging of locally advanced prostate cancer with neoadjuvant AA + MDX-010 therapy. This therapeutic modality exhibits significant promise for men with unresectable disease and may provide durable and potentially curative cancer control. No significant financial relationships to disclose.