De Novo Design, Synthesis, and Pharmacology of α-Melanocyte Stimulating Hormone Analogues Derived from Somatostatin by a Hybrid Approach

A number of α-melanotropin (α-MSH) analogues have been designed de novo, synthesized, and bioassayed at different melanocortin receptors from frog skin (fMC1R) and mouse/rat (mMC1R, rMC3R, mMC4R, and mMC5R). These ligands were designed from somatostatin by a hybrid approach, which utilizes a modified cyclic structure (H-D-Phe-c[Cys-Cys]-Thr-NH₂) related to somatostatin analogues (e.g. sandostatin) acting at somatostatin receptors, CTAP which binds specifically to μ opioid receptors, and the core pharmacophore of α-MSH (His-Phe-Arg-Trp). Ligands designed were H-D-Phe-c[XXX-YYY-ZZZ-Arg-Trp-AAA]-Thr-NH₂ [XXX and AAA = Cys, D-Cys, Hcy, Pen, D-Pen; YYY = His, His(1′-Me), His(3′-Me); ZZZ = Phe and side chain halogen substituted Phe, D-Phe, D-Nal(1′), and D-Nal(2′)]. The compounds showed a wide range of bioactivities at the frog skin MC1R; e.g. H-D-Phe-c[Hcy-His-D-Phe-Arg-Trp-Cys]-Thr-NH₂ (6, EC₅₀ = 0.30 nM) and H-D-Phe-c[Cys-His-D-Phe-Arg-Trp-D-Cys]-Thr-NH₂ (8, EC₅₀ = 0.10 nM). In addition, when a lactam bridge was used as in H-D-Phe-c[Asp-His-D-Phe-Arg-Trp-Lys]-Thr-NH₂ (7, EC₅₀ = 0.10 nM), the analogue obtained is as potent as α-MSH in the frog skin MC1R assay. Interestingly, switching the bridge of 6 to give H-D-Phe-c[Cys-His-D-Phe-Arg-Trp-Hcy]-Thr-NH₂ (5, EC₅₀ = 1000 nM) led to a 3000-fold decrease in agonist activity. An increase in steric size in the side chain of D-Phe⁷ reduced the bioactivity significantly. For example, H-D-Phe-c[Cys-His-D-Nal(1′ )-Arg-Trp-D-Cys]-Thr-NH₂ (24) is 2000-fold less active than 9. On the other hand, H-D-Phe-c[Cys-His-D-Phe(p-I)-Arg-Trp-D-Cys]-Thr-NH₂ (23) lost all agonist activity and became a weak antagonist (IC₅₀ = 1 × 10^-5 M). Furthermore, the modified CTAP analogues with a D-Trp at position 7 all showed weak antagonist activities (EC₅₀ = 10^-6 to 10^-7 M). Compounds bioassayed at mouse/rat MCRs displayed intriguing results. Most of them are potent at all four receptors tested (mMC1R, rMC3R, mMC4R, and mMC5R) with poor selectivities. However, two of the ligands, H-D-Phe-c[Cys-His-D-Phe-Arg-Trp-Pen]-Thr-NH₂ (9, EC₅₀ = 6.9 × 10^-9 M, 6.4 × 10^-8 M, 2.0 × 10^-8 M, and 1.4 × 10^-10 M at mMC1R, rMC3R, mMC4R, and mMC5R, respectively) and H-D-Phe-c[Cys-His(3′ -Me)-D-Phe-Arg-Trp-Cys]-Thr-NH₂ (16, EC₅₀ = 3.5 × 10^-8 M, 3.1 × 10^-8 M, 8.8 × 10^-9 M, and 5.5 × 10^-10 M at mMC1R, rMC3R, mMC4R, and mMC5R, respectively) showed significant selectivities for the mMC5R. Worthy of mention is that neither of these two ligands is potent in the frog skin MC1R assay (EC₅₀ = 10^-7 M for 9 and EC₅₀ = 10 ^-5 M for 16). These results clearly demonstrated that binding behaviors in rodent MCRs are quite different from those in the classical frog skin (R pipiens) assay.