MicroPET imaging of breast cancer using radiolabeled bombesin analogs targeting the gastrin-releasing peptide receptor

Mammography is a well-established method for detecting primary breast cancer; however, it has some limitations that may be overcome using nuclear imaging methods. Current radiopharmaceuticals have limited sensitivity for detecting small primary lesions and it has been suggested that novel radiopharmaceuticals are necessary for detection of primary breast cancer, as well as for detecting metastases and recurrence, or for monitoring therapy. The gastrin-releasing peptide receptor (GRPR) is a seven-transmembrane G-protein coupled receptor that is overexpressed on primary breast cancer and lymph node metastases. Bombesin (BN) is a tetradecapeptide that binds with high affinity to GRPR and can be radiolabeled with the positron-emitter, copper-64 ((64)Cu) for imaging with positron-emission tomography (PET). The goal of this study was to evaluate BN analogs that could be radiolabeled with (64)Cu for PET imaging of breast cancer. A series of BN analogs containing 4, 5, 6, 8, and 12- carbon linkers were evaluated with regard to their binding and internalization into T-47D human breast cancer cells. The (64)Cu-labeled analogs were then evaluated in mice bearing T-47D xenografts by tissue biodistribution and microPET imaging. These studies showed that all of the analogs had IC(50) values