Durable biochemical and radiographic response during toripalimab treatment in MSI-high/TMB-high mCRPC: a case report

We present a 79-year-old male patient with MSI-H/TMB-H (91.5 mutations/Mb) metastatic castration-resistant prostate cancer who progressed following abiraterone and enzalutamide treatment. When toripalimab was started, his serum PSA was 102 ng/mL. We maintained androgen-deprivation therapy with leuprorelin and stopped enzalutamide. Toripalimab 240 mg was given intravenously every 21 days. His PSA dropped below 0.09 ng/mL after three treatment cycles. We retrospectively applied RECIST version 1.1 criteria and selected the locally invasive prostate-soft-tissue mass as our only measurable non-nodal target lesion. Its maximal diameter shrank from 119 mm at baseline to 43 mm after six cycles (63.9% reduction), and further to 31 mm after 25 cycles (73.9% reduction), satisfying criteria for partial response. Pelvic lymph nodes could only be classified as non-target disease because short-axis diameters were unavailable in imaging reports, though these nodes resolved on imaging after six cycles. Bone lesions were not included in RECIST response evaluation and were interpreted separately on serial bone scans. At cycle 31 (roughly 21 months after toripalimab commencement), PSA stayed undetectable without clinical or radiological signs of progression; the biochemical response persisted for about 19 months starting from cycle 3. As of manuscript revision, the patient remains on toripalimab plus leuprorelin with routine follow-up. No treatment-related adverse events were identified using CTCAE version 5.0. We also conducted a targeted narrative review of mCRPC cases treated with PD-1/PD-L1 inhibitors other than pembrolizumab. This single-case observation highlights the value of genomic profiling for mCRPC patients with relevant biomarkers. Readers should interpret these findings as hypothesis-generating and not proof of universal toripalimab activity.