Abstract 4633: Theranostic evaluation of 68Ga-PMSA, 177Lu PSMA therapy, and biomarker guided immunotherapy and DDR inhibitor combinations in LNCaP prostate cancer xenografts

Abstract Background and Rationale. PSMA-targeted radioligand therapy with 177Lu-PMSA is a promising treatment for metastatic castration-resistant prostate cancer. To optimise its clinical translation, preclinical models are required to link to diagnostic imaging with dosimetry, assess biomarkers or response, and evaluate sensitising combination. We conducted an in vivo study using LNCaP xenograft model comparing 177Lu-PSMA monotherapy with immunotherapy, DDR inhibition and combinations, integrating multimodal imaging, pharmacokinetics, toxicity, and ex vivo biomarker analyses. Methods. Male immunodeficient mice bearing LNCaP tumours (100-150mm3) underwent baseline 18F-PSMA PET/CT to confirm uptake and lesion dosimetry, then were randomised into 6 cohorts (n 8-10). 1. Vehicle; 2. 177Lu-PSMA monotherapy (minimal effective dose); 3. anti-PD-1; 4. 177Lu plus anti-PD-1; 5. 177Lu plus PARP inhibitor, 6. Triple combination. Longitudinal 18F-PSMA at 7, 14, and 28 days monitored early metabolic response, alongside tumour volume and body weight. Blood samples post 177Lu (1-96 h) informed PK and clearance, with concurrent CBC and serum chemistry to assess systemic toxicity. At endpoint, tumours were harvested for immunostaining and molecular analysis for markers of proliferation, immune response and DDR inhibition. In addition, exploratory γH2AX in PBMCs and ctDNA profiling were assessed as translational biomarkers. Results. 18F-PSMA-PET early uptake and diagnostic assessment correlated with 177Lu efficacy. Minimal dose 177Lu maintained significant anti-tumour activity and reduced haematologic toxicity. Combination with anti-PD-1 enhanced efficacy via radio sensitisation. Triple therapy produced maximal tumour suppression with manageable toxicity. Ex vivo biomarkers provided mechanistic insight and predictive correlates across cohorts. Conclusions. This integrated preclinical study demonstrates that 177Lu-PSMA efficiency is enhanced by immunotherapy and DDR inhibition. Multimodal imaging, PK, toxicity, and biomarker analyses inform rational dosing, combination design, and translational strategies for future clinical trials in prostate cancer. Citation Format: Juliana Maynard, Anita Liu, Benedetta Arno, Tracy Hall, William Drewe, Kerry Shea, Andrzej Rutkowski, Grace Haydon, Tiffany-Jane Allen, Fiona Yau, Liviu Lucaciu, Mikaela Griffiths, Gayle Marshall, . Theranostic evaluation of 68Ga-PMSA, 177Lu PSMA therapy, and biomarker guided immunotherapy and DDR inhibitor combinations in LNCaP prostate cancer xenografts 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 4633.