Purpose/Objective(s) Prostate-specific membrane antigen (PSMA), also known as folate hydrolase 1 (FOLH1), is a highly specific biomarker and therapeutic target for prostate cancer (PC). J591, a monoclonal antibody specific to PSMA, is the primary method used to detect PSMA expression. It is currently under investigation as a vehicle to deliver targeted therapies to PC cells. Androgen deprivation therapy (ADT) upregulates PSMA. Preclinical models of androgen resistant (AR) xenografts demonstrate PSMA upregulation by ADT increases the therapeutic index of J591-drug conjugates, suggestive of enhanced drug uptake. We sought to evaluate the effect of external beam radiation (EBRT) on PSMA expression in PC cells ± ADT. Methods Androgen sensitive LNCaP and AR CWR22Rv1 PC cell lines were cultured in standard and charcoal-stripped media for two weeks. Both cell lines were grown in both media conditions, and received either no EBRT, EBRT in 2 Gray (Gy) daily fractions to a maximum of 10 Gy, or 7.5 Gy in 1 fraction. Cell surface PSMA expression was measured by flow cytometry as mean fluorescence intensity 24 hours after each 2 Gy fraction, or on days 1, 3, 5, 7, and 9 after 7.5 Gy. Results LNCaP in standard media upregulated PSMA by 2.97-fold (SEM ± 0.72) with fractionated EBRT to a cumulative dose of 4 Gy, peaking on days 2 and 3 after 4 Gy and 6 Gy cumulative doses, respectively. CWR22Rv1 in standard medium, upregulated PSMA expression by 3.21-fold (SEM ± 0.44) after a cumulative dose of 8 Gy, peaking on day 4. Under ADT (charcoal-stripped media), EBRT did not further increase LNCaP cell PSMA expression (maximal fold-change of 1.01 (SEM ± 0.02)) relative to ADT alone. Compared to CWR22Rv1 with ADT alone, ADT plus EBRT upregulated PSMA by 3.73-fold (SEM ± 0.44) after a cumulative fractionated dose of 8 Gy, and peaking on days 4 and 5. However, 7.5 Gy in one fraction did not significantly increase PSMA expression of LNCaP or CWR22Rv1 in standard media. LNCaP showed a maximal fold-change of 1.14 (SEM ± 0.29) on day 9. CWR22Rv1 showed a maximal fold-change of 1.27 (SEM ± 0.02) on day 1. Conclusions We show PSMA expression is independently enhanced by ADT and by low-dose fractionated EBRT, but not with single-fraction ablative EBRT. These observations warrant additional studies using in vitro and in vivo PC models to validate our findings. PSMA upregulation may aid in optimizing J591 targeting for treatment of local and disseminated or micrometastatic disease. Validation of PSMA upregulation by ADT and EBRT supports use of J591-drug conjugates as an adjunct treatment to these standard therapies for PC. Furthermore, if fractionated EBRT upregulates PSMA, then targeted brachytherapy using appropriately selected J591-radionucleotide conjugates may increase its own therapeutic index in a time-dependent manner as a result of regional low-dose rate beta or alpha emission.

Citation Format: Marigdalia K. Ramirez-Fort, Sean S. Mahase, M. Junaid Niaz, He Liu, Vincent Navarro, Goran Rac, Harry S. Clarke, Scott T. Tagawa, Joseph M. Jenrette, Neil H. Bander, Christopher S. Lange. External beam radiation therapy and androgen deprivation therapy increase PSMA expression in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1774.