Abstract 1073: Monitoring androgen receptor and neuroendocrine marker expression dynamics in CTCs in patients with mCRPC

Abstract Androgen receptor (AR) remains important treatment target for prostate cancer and androgen receptor signaling inhibitor (ARSI), such as abiraterone acetate (AA) or enzalutamide have already been widely used in daily practice. However, approximately 30% of patients show primary refractory and almost all patients eventually resist ARSI within several years. One of the resistant mechanisms is AR amplification and/or mutation which is detected by ctDNA after ARSI treatment. Another one is phenotypic transformation from adenocarcinoma to neuroendocrine prostate cancer (NEPC) by treatment pressure with ARSI. This study investigated dynamic changes of AR and neuroendocrine markers expression during abiraterone treatment by longitudinal Circulating Tumor Cells (CTCs) collection. We have developed a method using molecular imaging flow cytometry to detect CTCs and their AR and Synaptophysin (SYP) expression. For CTC enrichment from whole blood, cell size-based approach using microfluidics technology was adapted. Our focus was on AR nuclear localization status of CTCs in patients with metastatic castration resistant prostate cancer (mCRPC). To validate the analytical performance of our method, the AR and SYP expression of detected CTCs was compared with the pathological results of tissue biopsy obtained within 28 days before or after blood sampling. One or more CTCs were detected in 89.4% (17/19) of patients, and 13 of 17 patients had available pathological results. The sensitivity and specificity of CTC detection for the pathological results were 67% and 100% in AR, and 75% and 89% in SYP, respectively. Additionally, SYP showed relatively high positive predictive value of 75% and negative predictive value of 89%. Furthermore, we have studied the change in AR nuclear localization of AR-positive CTCs before and one month after the start of AA treatment. The degree of AR nuclear localization of each CTC was quantified by calculating the similarity between the nucleus staining and AR immunofluorescent image. Our results demonstrated the tendency with lower degree of AR nuclear localization in patients who responded to AA. In the AA-treated cohort, PSA responders showed a decrease in AR nuclear localization in average and non-responders did not (AR nuclear localization positive rate one month after treatment (0% vs 33.0%, p=0.206, Fisher’s exact tests). In contrast, in the control cohort (non-AA-treated group), AR nuclear localization scores remained unchanged or increased in both PSA responders and non-responders (33.0% vs 33.3%, p=1.0). Our observations suggest that CTC SYP may guide indication for tissue re-biopsy to detect NEPC and the AR nuclear localization in CTCs may serve as a pharmacodynamic marker of AA for mCRPC. It represents an important finding for clinical applications such as predicting treatment efficacy, monitoring treatment and selection for anti-prostate cancer drug. Citation Format: Nobuaki Matsubara, Kentaro Shirai, Hiroyuki Obinata, Hiromichi Nakajima, Chikako Funasaka, Chihiro Kondoh, Kana Kawasaki, Eri Katsumata, Taiga Ajiri, Maharjan Bishnu, Fumie Kato, Masatoshi Yanagida, Reiko Watanabe, Toru Mukohara. Monitoring androgen receptor and neuroendocrine marker expression dynamics in CTCs in patients with mCRPC abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1073.