Design, Synthesis, and Biological Evaluation of Novel Investigational Nonapeptide KISS1R Agonists with Testosterone-Suppressive Activity

Metastin/kisspeptin is a 54 amino acid peptide ligand of the KISS1R receptor and is a critical regulator of GnRH secretion. The N-terminally truncated peptide, metastin(45–54), possesses a 10-fold higher receptor-binding affinity than full-length metastin and agonistic KISS1R activity but is rapidly inactivated in rodent plasma. We have developed a decapeptide analog [d-Tyr⁴⁵,d-Trp⁴⁷,azaGly⁵¹,Arg­(Me)⁵³]­metastin­(45–54) with improved serum stability compared with metastin(45–54) but with decreased KISS1R agonistic activity. Amino acid replacements at positions 45–47 led to an enhancement of KISS1R agonistic activity and metabolic stability. N-terminal truncation resulted in a stable nonapeptide, [d-Tyr⁴⁶,d-Pya­(4)⁴⁷,azaGly⁵¹,Arg­(Me)⁵³]­metastin­(46–54), compound <b>26</b>, which displayed KISS1R binding affinities comparable to metastin(45–54) and had improved serum stability. Compound <b>26</b> reduced plasma testosterone in male rats and is the first short-length metastin analog to possess testosterone suppressive activities. Compound <b>26</b> has led to the elucidation of investigational analogs TAK-683 and TAK-448, both of which have undergone clinical evaluation for hormone-dependent diseases such as prostate cancer

Physicochemically and Pharmacokinetically Stable Nonapeptide KISS1 Receptor Agonists with Highly Potent Testosterone-Suppressive Activity

Modifications of metastin(45–54) produced peptide analogues with higher metabolic stability than metastin(45–54). N-terminally truncated nonapeptide <b>4</b> ([d-Tyr⁴⁶,d-Pya­(4)⁴⁷,azaGly⁵¹,Arg­(Me)⁵³]­metastin­(46–54)) is a representative compound with both potent agonistic activity and metabolic stability. Although <b>4</b> had more potent testosterone-suppressant activity than metastin, it possessed physicochemical instability at pH 7 and insufficient in vivo activity. Instability at pH 7 was dependent upon Asn⁴⁸ and Ser⁴⁹; substitution of Ser⁴⁹ with Thr⁴⁹ reduced this instability and maintained KISS1 receptor agonistic activity. Furthermore, [d-Tyr⁴⁶,d-Trp⁴⁷,Thr⁴⁹,azaGly⁵¹,Arg­(Me)⁵³,Trp⁵⁴]­metastin­(46–54) (<b>14</b>) showed 2-fold greater [Ca²⁺]_i-mobilizing activity than metastin(45–54) and an apparent increase in physicochemical stability. N-terminal acetylation of <b>14</b> resulted in the most potent analogue, <b>22</b> (Ac-[d-Tyr⁴⁶,d-Trp⁴⁷,Thr⁴⁹,azaGly⁵¹,Arg­(Me)⁵³,Trp⁵⁴]­metastin­(46–54)). With continuous administration, <b>22</b> possessed 10–50-fold more potent testosterone-suppressive activity in rats than <b>4</b>. These results suggested that a controlled release of short-length KISS1 receptor agonists can suppress the hypothalamic–pituitary–gonadal axis and reduce testosterone levels. Compound <b>22</b> was selected for further preclinical evaluation for hormone-dependent diseases