This year, breast cancer (BC) will attack approximately 210,000 and will take the lives of 40,000 women in the U.S. Standard screening with breast self-examination and mammography, recommended to minimize BC morbidity and mortality, miss 10-20% (up to 40% in young women) of breast cancers. Moreover, if an abnormality is found, an invasive diagnostic procedure is required to determine if the breast contains hyperplasia, atypia, or cancer. Approximately 80% of invasive procedures detect a benign process. BC cells express a gene product, cell surface receptor VPAC1, so named because the endogenous growth hormones Vasoctive Intestinal Peptide (VIP) and Pituitary Adenylate Cylcase Activating Peptide (PACAP) bind to VPAC1 receptors with affinity. We have labeled two VIP analogs with Tc-99m (Tc-99m- TP3654, EC50, 15 nM and Tc-99m-TP3652, EC50, 0.8 nM) and shown in a pilot clinical study that these agents correctly and unequivocally imaged all tumors which overexpress VPAC1 receptors, including precancerous lesions in the breast, supporting our hypothesis that potent analogs of VIP and PACAP candetect cancer at an early stage. We have prepared two additional analogs (Tc-99m-TP3871, EC50, 0.45 Nm and Tc-99m-TP3475, EC50, 1.5 nM) with improved receptor affinity and more favorable structural conformation for better in vivo stability. Furthermore, we have labeled VIP analog (TP3654) with p+ emitter Cu-64, which improved BC uptake by 85-fold. These results, compounded by the high resolution and sensitivity of PET, have prompted us to undertake a systematic investigation to develop a PET molecular imaging technique that will 1) target early BC, 2) distinguish between hyperplastic ductal epithelium (normal image) and atypical hyperplasia and invasive cancer (positive image), 3) minimize the number of unnecessary invasive procedures, and 4) permit clinicians to treat precancerous mastopathy (atypia and in situ cancer) or invasive cancer with preventive or therapeutic agents, respectively, to decrease morbidity and mortality associated with progressing disease. The study will 1) systematically evaluate four analogs for critical imaging parameters, 2) determine the specificity of a lead agent in differentiating pre- and invasive BC from normal breast tissue ex vivo by autoradiography and RT-PCR, 3) perform preclinical toxicity studies in animals, and 4) examine its ability to correctly identify tumors which over express VPAC1 receptors by PET imaging.