| Summary: | Objective: Small cell prostate carcinoma (SCPC) is a rare and highly malignant subtype of prostate cancer. SCPC frequently lacks androgen receptor (AR) and prostate-specific antigen (PSA) expression, and often responds poorly to androgen deprivation therapy (ADT). AR splice variant-7 (AR-V7) is a truncated AR protein implicated in resistance to AR-targeting therapies. AR-V7 expression in castration-resistant prostate cancers has been evaluated extensively, and blood-based detection of AR-V7 has been associated with lack of response to abiraterone and enzalutamide. However, whether AR-V7 is expressed in SCPC is not known. Methods: Using validated antibodies, we performed immunohistochemistry (IHC) assay for the full-length AR (AR-FL) and (AR-V7) on post-ADT surgical SCPC specimens. Results: Seventy-five percent (9/12) of the specimens showed positive staining for the AR-FL with various intensities. Thirty-three percent (4/12) of the specimens showed positive staining for AR-V7. Among the specimens with positive AR-V7 staining, two samples displayed very weak staining, one sample showed weak-to-moderate staining, and one sample showed strong staining. All positive specimens displayed a heterogeneous pattern of AR-FL/AR-V7 staining. All specimens positive for AR-V7 were also positive for AR-FL. Conclusion: The study findings support the existence of measurable AR-FL and AR-V7 proteins in SCPC specimens. The results also have implications in detection of AR-V7 in specimens obtained through systemic sampling approaches such as circulating tumor cells. A positive AR-V7 finding by blood-based tests is not impossible in patients with SCPC who often demonstrate low PSA values. Keywords: Small cell prostate carcinoma, Androgen receptor, Androgen receptor splice variant-7, Immunohistochemistry
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