Probing the Molecular Interactions between CXC Chemokine Receptor 4 (CXCR4) and an Arginine-Based Tripeptidomimetic Antagonist (KRH-1636).

We here report an experimentally verified binding mode for the known tripeptidomimetic CXCR4 antagonist KRH-1636 (1). A limited SAR study based on the three functionalities of 1 was first conducted, followed by site-directed mutagenesis studies. The receptor mapping showed that both the potency and affinity of 1 were dependent on the transmembrane residues His(113), Asp(171), Asp(262), and His(281) and also suggested the involvement of Tyr(45) and Gln(200) (potency) and Tyr(116) and Glu(288) (affinity). Molecular docking of 1 to an X-ray structure of CXCR4 showed that the l-Arg guanidino group of 1 forms polar interactions with His(113) and Asp(171) and the (pyridin-2-ylmethyl)amino moiety is anchored by Asp(262) and His(281), whereas the naphthalene ring is tightly packed in a hydrophobic subpocket formed by the aromatic side chains of Trp(94), Tyr(45), and Tyr(116). The detailed picture of ligand-receptor interactions provided here will assist in structure-based design and further development of small-molecule peptidomimetic CXCR4 antagonists.

Design, synthesis, and biological evaluation of scaffold-based tripeptidomimetic antagonists for CXC chemokine receptor 4 (CXCR4).

Structure-activity relationship studies of the cyclopentapeptide CXCR4 antagonists (cyclo(-l-/d-Arg(1)-Arg(2)-2-Nal(3)-Gly(4)-d-Tyr(5)-)) suggest that the l-/d-Arg(1)-Arg(2)-2-Nal(3) tripeptide sequence contained within these cyclopentapeptides serves as a recognition motif for peptidic CXCR4 antagonists. Starting by dissecting the cyclopentapeptide structure and reintroducing cyclic constraints in a stepwise manner, we here report a novel class of scaffold-based tripeptidomimetic CXCR4 antagonists based on the d-Arg-Arg-2-Nal motif. Biological testing of the prototype compounds showed that they represent new peptidomimetic hits; importantly, the modular nature of the scaffold provides an interesting starting point for future ligand optimization.

Structure-activity relationship studies of the aromatic positions in cyclopentapeptide CXCR4 antagonists.

The cyclopentapeptide CXCR4 antagonist FC131 (cyclo(-Arg(1)-Arg(2)-2-Nal(3)-Gly(4)-D-Tyr(5)-), 2; 2-Nal = 3-(2-naphthyl)alanine) represents an excellent starting point for development of novel drug-like ligands with therapeutic potential in HIV, cancer, stem-cell mobilization, inflammation, and autoimmune diseases. While the structure-activity relationships for Arg(1), Arg(2), and Gly(4) are well established, less is understood about the roles of the aromatic residues 2-Nal(3) and D-Tyr(5). Here we report further structure-activity relationship studies of these two positions, which showed that (i) the distal aromatic ring of the 2-Nal(3) side chain is required in order to maintain high potency and (ii) replacement of D-Tyr(5) with conformationally constrained analogues results in significantly reduced activity. However, a simplified analogue that contained Gly instead of D-Tyr(5) was only 13-fold less potent than 2, which means that the D-Tyr(5) side chain is dispensable. These findings were rationalized based on molecular docking, and the collective structure-activity data for the cyclopentapeptides suggest that appropriately designed Arg(2)-2-Nal(3) dipeptidomimetics have potential as CXCR4 antagonists.

Anticancer activity of small amphipathic ß²,²-amino acid derivatives.

We report the anticancer activity from screening of a series of synthetic ß(2,2)-amino acid derivatives that were prepared to confirm the pharmacophore model of short cationic antimicrobial peptides with high anti-Staphylococcal activity. The most potent derivatives against human Burkitt's lymphoma (Ramos) cells displayed IC(50) values below 8 µM, and low toxicity against human red blood cells (EC(50) > 200 µM). A more than 5-fold preference for Ramos cancer cells compared to human lung fibroblasts (MRC-5 cells) was also obtained for the most promising ß(2,2)-amino acid derivative 3-amino-N-(2-aminoethyl)-2,2-bis(naphthalen-2-ylmethyl)propanamide (5c). Screening of 5c at the National Cancer Institute (NCI, USA) confirmed its anticancer potency and revealed a very broad range of anticancer activity with IC(50) values of 0.32-3.89 µM against 59 different cancer cell lines. Highest potency was obtained against the colon cancer cell lines, a non-small cell lung cancer, a melanoma, and three leukemia cell lines included in the NCI screening panel. The reported ß(2,2)-amino acid derivatives constitute a promising new class of anticancer agents based on their high anticancer potency, ease of synthesis, mode-of-action, and optimized pharmacokinetic properties compared to much larger antimicrobial peptides.