Preclinical evaluation of new GnRH‐I receptor radionuclide therapy with 177Lu‐peptide tracer

The purpose of this study was to develop preclinical evaluation of a novel radiolabeled gonadotropin‐releasing hormone (GnRH) receptor targeting peptide for prostate cancer therapy. The new antiproliferative agent of GnRH‐I analogue was developed on the basis of the D‐Trp⁶‐GnRH‐I scaffold, and in vivo pharmacokinetics and receptor binding affinity were enhanced by the substitution of Gly‐NHNH₂ for Gly‐NH₂ at position 10 in D‐Trp⁶‐GnRH‐I. To evaluate ¹⁷⁷Lu‐DOTA‐triptorelin‐hydrazide as radionuclide therapy of tumor, the quality control tests and preclinical stage assessment were carried out. Solid‐phase method was used to synthesize new peptide. Characterization and purity of peptide were done by mass spectroscopy and high‐performance liquid chromatography (HPLC). In order to be utilized in targeted therapy, the new GnRH‐I agonist was coupled with pSCN‐Bn‐DOTA. The precipitate crude of DOTA‐triptorelin‐hydrazide was then purified via preparative HPLC. At optimal conditions of time, temperature, ligand amount, and lutetium content, DOTA‐triptorelin‐hydrazide was labeled with ¹⁷⁷Lu (specific activity not less than 925 GBq/mg). Investigation of the in vivo biodistribution and in vitro studies for ¹⁷⁷Lu‐DOTA‐TRPHYD was performed in three different ways, and the binding of radiopeptide to GnRH receptors was expressed on the human cell lines using ¹²⁵I‐labeled D‐TRP⁶GnRH‐I as a tracer, respectively. Synthesized novel GnRH‐I was obtained with purity greater than 98%. Paper chromatography was found to be the most suitable with R_f of the complex and observed radiochemical purity of RTLC and HPLC greater than 97%. For in vivo studies, ¹⁷⁷Lu‐DOTA‐triptorelin‐hydrazide showed promising results with fast clearance from the blood and resulted in good T/NT ratios at 1, 4, and 24 hours postinjection and satisfactory biodistribution with no significant activity seen in normal tissue. The values of internalization efficiency and receptor affinity of new radiopeptide binding were IC₅₀ = 0.47 ± 0.06 vs 0.13 ± 0.01 nM for triptorelin and cellular uptake: 3.4 ± 0.7% at 1 hour and 6.8 ± 1.17% at 4 hours of the internal reference. The results showed a good stability and radiochemical purity of the obtained radioconjugate. For in vivo and in vitro studies, new radiopeptide showed a high uptake of ¹⁷⁷Lu conjugate in tumor and rapid clearance from the blood stream almost entirely via the renal/urinary pathway and binding to the GnRH receptors with high specificity and affinity, respectively.