Cyclic Melanotropins. 9.¹7-D-Phenylalanine Analogues of the Active-Site Sequence

The cyclic melanotropin Ac-Ser¹-Tyr²-Ser³-Cys⁴-Glu⁶-His⁶-Phe⁷-Arg⁸-Trp⁹-Cys¹⁰-Lys¹¹-Pro¹²-Val¹³-NH₂is a highly potent agonist as determined in several melanocyte bioassays. In linear melanotropins, a D-Phe⁷substitution leads to increased potency and often prolonged biological activity. In order to determine if this substitution would have the same effect in cyclic melanotropins, we have prepared a series of these analogues. The D-Phe⁷-substituted cyclic melanotropins Ac-[Cys⁴,D-Phe⁷,Cys¹⁰]-α-MSH_{4_10}-NH2 and Ac-[Cys⁴,D-Phe⁷,Cys¹⁰]-α-MSH_{4_n}-NH₂were both more potent than their cyclic L-Phe⁷-containing counterparts in either the frog or lizard skin bioassay by more than a factor of 10. Neither peptide, however, exhibited prolongation of biological activity in either assay. Substitution of D-Phe⁷into the cyclic 4–12 and 4~13 sequences led to a slight or no increase in potency in both assays relative to the L-Phe⁷counterparts, but the activity of the melanotropins was ultraprolonged in each assay. Ac- [Cys⁴,D-Phe⁷,Cys¹⁰]-α-MSH_{4_12}-NH₂was about equipotent to Ac-[Cys⁴,D-Phe⁷,Cys¹⁰]-α-MSH_{4_13}-NH₂, again demonstrating as with certain linear and cyclic L-Phe⁷-containing melanotropins, that the C-terminal amino acid valine is not required for biological activity or for superpotency. Similar to the linear D-Phe⁷analogues that possessed ultraprolonged melanotropic activity, the 4–12 and 4–13 cyclic D-Phe⁷analogues also displayed the phenomenon of superagonism, which is a time-dependent increase in efficacy over that produced by an equipotent concentration of the native hormone. Cyclization of certain linear melanotropins resulted in analogues with increased resistance to biological degradation by serum enzymes or purified proteolytic enzymes. Further, incorporation of a D-Phe⁷into in the cyclic analogues led to melanotropins that were totally resistant to enzymatic inactivation by trypsin.