Targeting tumor microenvironment-mediated radiopharmaceutical therapy resistance in prostate cancer - PROJECT SUMMARY Radiopharmaceutical therapy (RPT) is a clinically validated approach to deliver therapeutic radiation precisely to tumor tissue. PSMA-targeting RPT (Pluvicto) is effective for the treatment of metastatic castration-resistant prostate cancer. However, therapy resistance and relapse uniformly occur. Therefore, there is an urgent need to define targetable mechanisms that limit durable RPT responses. In tumors, RPT induces DNA damage, cancer cell death, and production of the immuno-regulatory small molecule cGAMP. cGAMP drives the expression of type I interferon (IFN) that promotes anti-tumor adaptative immune responses and contributes to reversing the immune suppression observed in the microenvironment of prostate cancer bone metastases. Alternatively, cGAMP-mediated signaling is corrupted in a subset of prostate cancer tumors with high expression of the ectonucleotidase ENPP1. ENPP1 degrades cGAMP to produce adenosine, which has potent immuno-suppressive effects. The proposed research will test the hypothesis that in ENPP1-low tumors, PSMA- RPT will trigger IFN production and anti-tumor immune responses. We further hypothesize that in ENPP1-high tumors, PSMA-RPT will drive adenosine-mediated immunosuppressive effects that will be prevented by pharmacological ENPP1 inhibition. Two UCLA investigators with complementary expertise in innate immune signaling and RPT therapy will guide the proposed research. In Specific Aim 1, the role of ENPP1 in regulating 177Lu-PSMA-RPT-driven IFN signaling in pre-clinical models will be defined. Isogenic PSMA+ prostate cancer cell lines will be engineered to model the heterogeneity of ENPP1 expression observed in human tumors. In implantable tumor models, the effect of 177Lu-PSMA-RPT on tumor IFN signaling will be evaluated in the context of high or low tumor ENPP1 expression. A luciferase-linked IFN response reporter and bio- luminescence imaging will be applied to track IFN signaling in tumors. ENPP1 blockade using a small molecule inhibitor will be tested to modulate 177Lu-PSMA-RPT -induced IFN signaling responses in tumors expressing high levels of ENPP1. In Specific Aim 2, the efficacy of combination 177Lu-PSMA-RPT and ENPP1 inhibitor treatment will be tested using 2 syngeneic prostate cancer models in immunocompetent mice. Tumor growth will be monitored by µCT, and tolerability will be assessed by multi-parametric blood marker analysis. 177Lu- PSMA-RPT and ENPP1 inhibitor-induced alterations in the tumor immune microenvironment will be evaluated using flow cytometry and single-cell RNAseq analysis of tumor-infiltrating lymphocytes. The anti-tumor activity of the 177Lu-PSMA-RPT and ENPP1 inhibitor combination therapy will be confirmed in mouse models of prostate cancer bone metastasis. This research will apply genetic, pharmacological, and imaging approaches to define new tumor microenvironment-driven RPT resistance mechanisms. If successful, these studies will provide the rationale for new combination treatment strategies and patient stratification approaches to unleash the potential immune-activating properties of RPT.
