Synthesis, Radiolabeling, and Preclinical Evaluation of 68Ga/177Lu-Labeled Leuprolide Peptide Analog for the Detection of Breast Cancer

Objectives: The expansion of novel and potent tumor receptor binding peptides is a promising approach for the precise targeting of various cancer. Leuprolide is a 9-residue peptide analog of gonadotropin-releasing hormone and is extensively used in the treatment of sex hormone-dependent tumors, including prostate, breast, and ovarian cancer. This preclinical study was undertaken to prepare a new radiolabeled leuprolide peptide for the detection of breast carcinoma.Methods: A 1,4,7,10-tetraazacyclododecane-N,N ',N '',N '''-tetraacetic acid (DOTA)-coupled 9-amino acid leuprolide peptide was synthesized after typical 9-fluorenylmethyl-oxycarbonyl-based solid-phase peptide synthesis and radiolabeled with both Ga-68 and Lu-177 radionuclides for theranostic use. The systemic pharmacokinetics was done in healthy balb/c mice. The in vitro tumor cell binding affinity was determined on MCF7, T47D, and MDA-MB-231 breast cancer cell lines. In vivo tumor targeting and micro positron-emission tomography imaging was performed on nude mice with MCF7 breast tumor xenografts.Results: The leuprolide peptide was conveniently synthesized by solid-phase synthesis strategy and its identity and purity were validated by mass spectrometry and high-performance liquid chromatography. The peptide radiolabeled efficiently (>94%) with both diagnostic (Ga-68) and therapeutic (Lu-177) radionuclides and displayed nanomolar binding potency to all three tested MCF7, T47D, and MDA-MB-231 cell lines. Fast and favorable pharmacokinetics was observed for Ga-68/Lu-177-leuprolide in healthy Balb/c mice. In nude mice, Ga-68-leuprolide peptide exhibited rapid clearance from the blood circulation with low to moderate (up to 5% ID/g) uptake/retention by the major body organs. The accumulation in the estrogen receptor-positive MCF7 tumor was 2.24% +/- 0.62% ID/g at 45 min p.i, with good tumor to blood and muscle uptake ratios. The radiolabeled peptide was excreted primarily through the renal pathway.Conclusion: The encouraging results of this initial study demonstrate that additional testing of this leuprolide peptide seems to be indicated because of its convincing potential to be a new agent for the management of breast carcinoma.