crystal structure analyses of azasulfuryltripeptides reveal potential for γ-turn mimicry†

Diversity-Oriented A3-Macrocyclization for Studying Influences of Ring-Size and Shape of Cyclic Peptides: CD36 Receptor Modulators

Cyclic peptide diversity has been broadened by elaborating the A³-macrocyclization to include various di-amino carboxylate components with different N^ε-amine substituents. Triple-bond reduction provided new cyclic peptide macrocycles with Z-olefin and completely saturated structures. Moreover, cyclic azasulfurylpeptides were prepared by exchanging the propargylglycine (Pra) component for an amino sulfamide surrogate. Examination of such diversity-oriented methods on potent cyclic azapeptide modulators of the cluster of differentiation 36 receptor (CD36) identified the importance of the triple bond as well as the N^ε-allyl lysine and azaPra residues for high CD36 binding affinity. Cyclic azapeptides which engaged CD36 effectively reduced pro-inflammatory nitric oxide and downstream cytokine and chemokine production in macrophages stimulated with a Toll-like receptor-2 agonist. Studying the triple bond and amine components in the multiple-component A³-macrocyclization has given a diverse array of macrocycles and pertinent information to guide the development of ideal CD36 modulators with biomedical potential for curbing macrophage-driven inflammation.

Synthesis and Biomedical Potential of Azapeptide Modulators of the Cluster of Differentiation 36 Receptor (CD36)

The innovative development of azapeptide analogues of growth hormone releasing peptide-6 (GHRP-6) has produced selective modulators of the cluster of differentiation 36 receptor (CD36). The azapeptide CD36 modulators curb macrophage-driven inflammation and mitigate atherosclerotic and angiogenic pathology. In macrophages activated with Toll-like receptor-2 heterodimer agonist, they reduced nitric oxide production and proinflammatory cytokine release. In a mouse choroidal explant microvascular sprouting model, they inhibited neovascularization. In murine models of cardiovascular injury, CD36-selective azapeptide modulators exhibited cardioprotective and anti-atherosclerotic effects. In subretinal inflammation models, they altered activated mononuclear phagocyte metabolism and decreased immune responses to alleviate subsequent inflammation-dependent neuronal injury associated with retinitis pigmentosa, diabetic retinopathy and age-related macular degeneration. The translation of GHRP-6 to potent and selective linear and cyclic azapeptide modulators of CD36 is outlined in this review which highlights the relevance of turn geometry for activity and the biomedical potential of prototypes for the beneficial treatment of a wide range of cardiovascular, metabolic and immunological disorders.

Azasulfurylpeptide Modulation of CD36-Mediated Inflammation Without Effect on Neovascularization

Modulation of the cluster of differentiation-36 receptor (CD36) has proven promising for dampening pro-inflammatory macrophage signaling. For example, azapeptides (e.g., 1 and 2) bind CD36 selectively with high affinity, mitigate Toll-like receptor (TLR) agonist-induced overproduction of nitric oxide (NO), and reduce pro-inflammatory cytokine and chemokine production in macrophages. Moreover, semicarbazides 1 and 2 inhibit microvascular sprouting mediated through CD36 in the choroid explant. Seeking a selective CD36 modulator that mediated inflammation without influencing neovascularization, a set of azasulfurylpeptides (e.g., 3a–e) were synthesized in which the semicarbazide was replaced by an N-aminosulfamide residue using a novel solid-phase approach. Notably, azasulfurylpeptide 3c diminished selectively CD36-mediated TLR-2-triggered inflammatory response without affecting neovascularization. Subtle chemical modification at the peptide backbone from a carbonyl to a sulfuryl residue has had a selective effect on biological activity providing a valuable probe for studying CD36 chemical biology.

Urotensin II((4-11)) Azasulfuryl Peptides: Synthesis and Biological Activity

Cyclic azasulfuryl (As) peptide analogs of the urotensin II (UII, 1, H-Glu-Thr-Pro-Asp-c[Cys-Phe-Trp-Lys-Tyr-Cys]-Val-OH) fragment 4-11 were synthesized to explore the influences of backbone structure on biological activity. N-Aminosulfamides were inserted as surrogates of the Trp(7) and Lys(8) residues in the biologically relevant Trp-Lys-Tyr triad. A combination of solution- and solid-phase methods were used to prepare novel UII((4-11)) analogs 6-11 by routes featuring alkylation of azasulfuryl-glycine tripeptide precursors to install various side chains. The pharmacological profiles of derivatives 6-11 were tested in vitro using a competitive binding assay and ex vivo using a rat aortic ring bioassay. Although the analogs exhibited weak affinity for the urotensin II receptor (UT) without agonistic activity, azasulfuryl-UII((4-11)) derivatives 7-9 reduced up to 50% of the effects of UII and urotensin II-related peptide (URP) without affecting their potency.