Tc-99m labeled small-molecule inhibitors of prostate-specific membrane antigen (PSMA): New molecular imaging probes to detect metastatic prostate adenocarcinoma (PC).
173 Background: Sensitive and specific imaging remains a clinically-relevant problem for men with PC. PSMA is a well established target for imaging of PC with therapeutic implications. We have recently developed novel 99mTc-labeled small molecule inhibitors of the enzymatic domain of PSMA based on glutamate-urea-glutamate and glutamate-urea-lysine pharmacophores, and contain a bis-imidazole chelator to complex Tc-99m. Preclinical studies with PSMA positive LNCaP cells and xenografts demonstrate that 99mTc-MIP-1404 and 99mTc-MIP-1405 bind to PSMA with high affinity and localize in tumors rapidly. This study reports the first human data in men with metastatic PC and in healthy male subjects. Methods: Under an exploratory IND, using a cross-over design, the pharmacokinetics, biodistribution, and tumor uptake of 99mTc-MIP-1404 and 99mTc-MIP-1405 were compared in 6 healthy men and 6 men with radiographic evidence of metastatic PC. Whole body images were obtained at 10 min, 1, 2, 4 and 24 hr. Single photon emission computed tomography (SPECT) was performed between 3–4 hours post injection. Results: Both agents cleared the blood rapidly with MIP-1404 demonstrating significantly lower urinary activity (7%) compared to MIP-1405 (26%). Both agents showed persistent uptake in the salivary, lacrimal and parotid glands. Uptake in liver and kidney was acceptable for imaging at 1-2 hr post injection (PI). In men with PCa, both agents rapidly localized in bone and lymph node lesions as early as 1 hr PI. SPECT demonstrated excellent lesion contrast. Good correlation was seen with bone and CT scans, In majority of patients, more lesions including sub-cm lymph nodes were seen with 99mTc-MIP-1404 and 99mTc-MIP-1405. The high contrast images exhibited signal:noise ratios from 3:1 to 28:1 at 4 and 24 hr. Conclusions: 99mTc-MIP-1404 and 99mTc-MIP-1405 identified a greater number of lesions than bone scans and rapidly detected soft tissue PC lesions including sub-cm lymph nodes. Since 99mTc-MIP-1404 has minimal activity in the bladder, further work is planned to correlate imaging findings with histopathology in patients with high risk clinically-localized PC.