Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase activating peptide (PACAP) receptor specific peptide analogues for PET imaging of breast cancer: In vitro/in vivo evaluation

Vasoactive intestinal peptide and pituitary adenylate cyclase activating peptide have high affinity for VPAC1, VPAC2 and PAC1 receptors overexpressed on human cancer cells. Four potent analogues of these peptides, TP3939, TP3982, TP4200 and TP3805 were labeled with 64Cu and evaluated ex vivo and in vivo to asses their biological activity and receptor specificity. The ultimate goal is to utilize 64Cu analogues for positron emission tomography (PET) imaging of breast cancers in humans.Radiochemical purity of each analogue was > 92%. The muscle relaxivity assay revealed IC50 to be 5.3 × 10–8 M, 4.4 × 10–8 M, 8.1 × 10–8 M, 8.1 × 10–9 M and Kd values determined by receptor specific cell binding assays were 3.3 nM, 0.33 nM, 0.2 nM and 0.72 nM for TP3805, TP3939, TP3982, and TP4200 respectively. The receptor affinity, using human breast cancer tissues, was 10.93 times greater than normal breast tissues. RT-PCR confirmed increased VPAC1 receptor expression on human breast tumor cells over normal cells and corroborated with autoradiography data. The blood clearance was rapid and in vivo translocation of 64Cu to plasma protein was < 15%. Data demonstrate that these analogues are potent, have uncompromised biological activity and are worthy of further evaluation for accurate PET imaging of human breast cancers and in determining malignant and benign lesions.