Uptake of radiolabeled morphiceptin and its analogs by experimental mammary adenocarcinoma: in vitro and in vivo studies

Morphiceptin (Tyr-Pro-Phe-Pro-NH2) and its analogs modified at position 3: [D-Phe3]morphiceptin, [D-ClPhe3]morphiceptin and [D-Cl2Phe3]morphiceptin were synthesized and labeled with [125I] or [131I]. Their binding to membranes isolated from experimental adenocarcinoma was examined in vitro with the use of a cross-linking assay followed by the Western blot technique. The radioactive complex had molecular weight of about 65 kDa and was detectable by anti-μ-opioid receptor polyclonal antibody. Expression of the μ-opioid receptor in mouse mammary adenocarcinoma was confirmed by reverse transcriptase-polymerase chain reaction. The binding studies showed the highest affinity and capacity for [D-Phe3]morphiceptin (Kd 0.39 and Bmax 1112) and [D-ClPhe3]morphiceptin (Kd 1.8 and Bmax 220). Morphiceptin and its D-Cl2Phe analog had significantly lower Bmax values (131 and 83, respectively). Biodistribution experiments in tumor-bearing C3H/Bi mice with the use of the 131I-labeled peptides confirmed the results of our in vitro studies. The highest accumulation of radioactive peptides in the tumor tissue was also found for peptides with D-Phe and D-ClPhe