[I-125]-S36057: a new and highly potent radioligand for the melanin-concentrating hormone receptor

1 Shortened, more stable and weakly hydrophobic analogues of melanin-concen trating hormone (MCH) were searched as candidates for radioiodination. Star ting from the dodecapeptide MCH6 (17), we found that: (1) substitution of T yr(13) by a Phe residue; (3) addition of a 3-iodo-Tyr residue at the N-term inus; and (3) addition of a hydrophilic spacer 8-amino-3.6-dioxyoctanoyl be tween the 3-iodo-Tyr and MCH, 17 (compound S36057), led to an agonist more potent than MCH itself in stimulating [S-35]-GTP gammaS binding at membrane s from HEK293 cells stably expressing the human MCH receptor. 2 Specific binding of [I-125]-S36057 was found in HEK293 and CHO cell lines stably expressing the human MCH receptor. This radioligand recognized a si milar number of binding sites (c cr. 800 fmol mg(-1)) than [I-125]-[3-iodo Tyr(13)]-MCH. 3 However, the KI, for [(12)5T]-S36057 obtained from saturation studies (0. 037 nhl) or from binding kinetics (0.046 nhl) was at least 10 fold higher t o that of [I-125]-[3-iodo Tyr(13)]-MCH (0.46 nhl). 4 Affinities determined for a series of MCH analogues were similar with bot h radioligands, S36057 being the most potent compound tested (K-1 = 0.053 n M). 5 Finally, [I-125]-S36057 also potently labelled the MCH receptor in membra nes from whole rat brain (K-D 0.044 nh l. B-max= 11 fmolmg(-1)). 6 Tn conclusion, [I-125]-S36057 is a more potent and more stable radioligan d than [I-125]-[3-iodo Tyr(13)]-MCH that will represent a reliable tool for binding assays in the search of novel MCH ligands. It should also provide great help for autoradiographic studies of the MCH receptor distribution in the central nervous system.