Characterization and Evaluation of CD24 and NPY as Biomarkers for Metastatic Castration-Resistant Prostate Cancer

<b>Background/Objectives</b>: Prostate cancer is the most diagnosed and third most deadly cancer among men in Europe. Metastatic castration-resistant prostate cancer (mCRPC) is incurable and resistant to standard androgen ablation therapy. More biomarkers are needed to select patients for novel personalized treatments. Previous whole-genome RNA sequencing results indicated a possible role for cluster of differentiation 24 (CD24) and neuropeptide Y (NPY) as diagnostic or prognostic biomarkers in androgen receptor-positive (AR⁺) mCRPC. <b>Methods</b>: We analyzed tissue microarrays representing 127 primary prostate cancers (with matched adjacent benign prostatic glands) and 124 metastases (from 34 patients) using immunohistochemistry to detect CD24 or NPY. <b>Results</b>: CD24 was more highly expressed in primary prostate cancer than in adjacent benign tissue for nuclear (<i>p</i>: <0.001), cytoplasmic (<i>p</i>: <0.001), and membranous staining (<i>p</i>: <0.001), while NPY showed no difference. Average NPY scores were lower in prostate cancers that later recurred (geometric mean 17.6, 95% CI: 9.5-32.5) compared to those that did not (38.7, CI: 23.2-64.4; <i>p</i>: 0.044, d: 0.773). In mCRPC, CD24 was detectable in 76% of cores at the cell membrane and in 58% in the nucleus. NPY was detectable in the cytoplasm of 17%. Scores for NPY and nuclear (but not membranous) CD24 were higher in AR⁺ mCRPC. In the RNA sequencing results, <i>CD24</i> did not correlate with <i>AR</i> or <i>kallikrein-related peptidase 3</i> (<i>KLK3)</i>, while <i>NPY</i> positively correlated with <i>AR</i> (r_s: 0.313; <i>p</i>: <0.004) and <i>KLK3</i> (r_s: 0.400; <i>p</i>: <0.004). NPY and CD24 scores did not correlate with neuroendocrine mCRPC markers. Nuclear and membranous CD24 showed differential expression by metastatic site. <b>Conclusions</b>: We did not find strong evidence to support the use of CD24 or NPY alone as clinical biomarkers. Membranous and nuclear CD24 were expressed in the majority of mCRPC specimens, while NPY expression was more limited. NPY and nuclear CD24 were more highly expressed in AR⁺ mCRPC than AR^- neuroendocrine disease, and nuclear CD24 displayed site-specific expression, suggesting a potential role for nuclear CD24 in promoting AR⁺ mCRPC.