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‐Trp 6 ‐GnRH‐Ι‐scaffold, and its potency was increased by the replacement Gly‐NH 2 by NH‐NH 2 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 IC 50  = 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.