Preparation of a radiolabeled GnRH-I analogue derivative with 111 In as a new anti-proliferative agent.

The new GnRH-Ιanalogue developed in this paper was based on the D-Trp6 -GnRH-Ι-scaffold, and its potency was increased by the replacement Gly-NH2 by NH-NH2 binding to the Gly at position 10. Triptorelin-Hydrazide analogue was synthesized using solid phase. For 111 In labeling, synthesized peptide was followed by conjugation with DOTA using pSCN-Bn-DOTA. The conjugated Triptorelin-Hydrazide was labeled with 500-550 MBq of 111 In-chloride (in 0.2 M HCl). At optimized conditions after labeling, radio-chromatography showed radiochemical purity of approximately equal to 98% (RTLC) and greater than 95% (HPLC). The serum stability of the tracer was determined up to 24 hr. Binding affinities of Triptorelin-Hydrazide analogue were determined in a binding assay for both human and rat GnRH receptors. For in vivo studies, 111 In-peptide was injected intravenously via the tail vein into rats and significant ovaries uptake consist with reported GnRH receptor mappings. In vitro radioligand binding assays performed with GnRHR-expressing human cell lines using 125 I-Triptorelin as the standard radioligand. The quantities of internalization efficiency and receptor affinity of the new radioligand were IC50  = 0.20 ± 0.04 nM vs 0.13 ± 0.08 nM for Triptorelin and internalization: 3.5 ± 0.9% at 1 hr and 12.8 ± 1.8% at 4 hr of the internal reference.