Introduction of d-Phenylalanine enhanced the receptor binding affinities of gonadotropin-releasing hormone peptides

The purpose of this study was to examine whether the introduction of d-Phe could improve the GnRH receptor binding affinities of DOTA-conjugated d-Lys ⁶-GnRH peptides. Building upon the construct of DOTA-Ahx-(d-Lys ⁶-GnRH1) we previously reported, an aromatic amino acid of d-Phe was inserted either between the DOTA and Ahx or between the Ahx and d-Lys ⁶ to generate new DOTA-d-Phe-Ahx-(d-Lys⁶-GnRH) or DOTA-Ahx-d-Phe-(d-Lys⁶-GnRH) peptides. Compared to DOTA-Ahx-(d-Lys⁶-GnRH1) (36.1 nM), the introduction of d-Phe improved the GnRH receptor binding affinities of DOTA-d-Phe-Ahx-(d-Lys⁶-GnRH) (16.3 nM) and DOTA-Ahx-d-Phe-(d-Lys⁶-GnRH) (7.6 nM). The tumor targeting and pharmacokinetic properties of ¹¹¹In-DOTA-Ahx-d-Phe-(d- Lys⁶-GnRH) was determined in MDA-MB-231 human breast cancer-xenografted nude mice. Compared to ¹¹¹In-DOTA-Ahx-(d-Lys ⁶-GnRH1), ¹¹¹In-DOTA-Ahx-d-Phe-(d-Lys⁶-GnRH) exhibited comparable tumor uptake with faster renal and liver clearance. The MDA-MB-231 human breast cancer-xenografted tumors were clearly visualized by single photon emission computed tomography (SPECT) using ¹¹¹In-DOTA- Ahx-d-Phe-(d-Lys⁶-GnRH) as an imaging probe, providing a new insight into the design of new GnRH peptides in the future.