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

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. A 1,4,7,10-tetraazacyclododecane-''''''-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 and Lu radionuclides for theranostic use. The systemic pharmacokinetics was done in healthy balb/c mice. The tumor cell binding affinity was determined on MCF7, T47D, and MDA-MB-231 breast cancer cell lines. tumor targeting and micro positron-emission tomography imaging was performed on nude mice with MCF7 breast tumor xenografts. 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) and therapeutic (Lu) 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/Lu-leuprolide in healthy Balb/c mice. In nude mice, Ga-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. 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.