Preclinical Evaluation of 177Lu-Labeled PSMA and FAP Bispecific Radioligand for Prostate Cancer

Background/Objectives: During prostate cancer progression, downregulation or loss of prostate-specific membrane antigen (PSMA) expression, together with interlesional heterogeneity, may limit the efficacy of PSMA-targeted radioligand therapy. Fibroblast activation protein (FAP), expressed mainly by activated cancer-associated fibroblasts in the tumor microenvironment, represents a potential complementary target. We developed the PSMA/FAP bispecific radioligand with a biphenyl scaffold and evaluated its feasibility and therapeutic potential. Methods: The radiochemical purity and in vitro stability were evaluated by Radio-HPLC. Cellular uptake was evaluated in 22Rv1 (PSMA+), U87MG (FAP+), and PC-3 (PSMA−/FAP−) cells. SPECT/CT imaging and biodistribution studies were performed using mice bearing separate 22Rv1 (PSMA+) and U87MG (FAP+) xenografts. Therapeutic efficacy was assessed after a single administration of 30 or 60 MBq [177Lu]Lu-PSFA-1, together with an acute toxicity evaluation. Results: [177Lu]Lu-PSFA-1 showed PSMA- and FAP-associated uptake. SPECT/CT imaging and biodistribution studies showed persistent uptake in both tumor types, with higher uptake in U87MG tumors and increasing tumor-to-background ratios over time. A single administration of 30 or 60 MBq delayed tumor growth and prolonged survival, with more durable tumor control at 60 MBq. No evident acute toxicity was observed. Conclusions: [177Lu]Lu-PSFA-1 specifically targeted both PSMA- and FAP-positive tumors, with sustained tumor retention and antitumor activity. These findings support further evaluation of [177Lu]Lu-PSFA-1 as a PSMA/FAP bispecific radioligand, although its potential benefit in heterogeneous prostate cancer requires further validation.