177Lu-AMBA Biodistribution, Radiotherapeutic Efficacy, Imaging, and Autoradiography in Prostate Cancer Models with Low GRP-R Expression

Lu-177-DO3A-CH2CO-G-4-aminobenzoyl-Q-W-A-V-G-H-L-M-NH2 (Lu-177-AMBA) is a radiolabeled bombesin derivative that is bound and internalized by cells expressing the G-protein-coupled gastrin-releasing peptide receptor (GRP-R) and is currently in phase I clinical trials. In previous radiotherapy studies with PC-3 xenografted mice, Lu-177-AMBA treatment significantly increased survival and reduced tumor growth rates. The PC-3 tumor cell line has an elevated expression of GRP-Rs (2.5 x 10(5)/cell), whereas LNCaP-a prostate cancer metastatic cell line representing the early androgen-sensitive stage of prostate cancer-and DU145-an androgen-insensitive metastatic line-express lower receptor numbers (5.9 x 10(3) and 1.2 x 10(4)/cell, respectively). Because of tumor heterogeneity, the high number of receptors in the PC-3 line may not represent the clinical situation, and little definitive work on the GRP-R status of primary prostate tumors and metastases exists. We sought to evaluate the tumor binding and imaging potential of Lu-177-AMBA in low GRP-R models of prostate cancer and determine how reduced expression affects Lu-177-AMBA radiotherapy efficacy. Methods: The LNCaP and DU145 cell lines were used to determine the binding (K-d), retention, and efflux of Lu-177-AMBA. Biodistribution radiotherapy, imaging, and autoradiography studies were performed in LNCaP, DU145, or PC-3 tumor-bearing male nude mice. Immunohistochemistry was used to determine the proliferative state in LNCaP and DU145 models and the vascular phenotype of LNCaP radiotherapy tumors. Results: Lu-177-AMBA binds to GRP-R in these cell lines with high affinity (K-d of LNCaP, 0.65 +/- 0.2 nM; K-d of DU145, 0.53 +/- 0.1 nM). The uptake of Lu-177-AMBA is at least 10-fold less in LNCaP and DU145 cell lines than it is in the PC-3 cell line. Autoradiography identifies activity concentrated in areas of viable tumor tissue, and gamma-images of Lu-177-AMBA identify tumors in vivo. Despite having lower uptake, Lu-177-AMBA demonstrated radiotherapeutic efficacy and decreased proliferation in the LNCaP and DU145 xenografts; in the LNCaP model, Lu-177-AMBA normalized the phenotype of microvasculature, reducing tumoral blood pooling. Conclusion: Lu-177-AMBA is a single radiolabeled agent that combines targeted radiotherapy after imaging dosimetry with the potential for single-agent or multimodality therapy for prostate cancer.