Cyclic RGD Peptidomimetics Containing Bifunctional Diketopiperazine Scaffolds as New Potent Integrin Ligands

Abstract

The synthesis of eight bifunctional diketopiperazine (DKP) scaffolds is described; these were formally derived from 2,3-diaminopropionic acid and aspartic acid (DKP-1-DKP-7) or glutamic acid (DKP-8) and feature an amine and a carboxylic acid functional group. The scaffolds differ in the configuration at the two stereocenters and the substitution at the diketopiperazinic nitrogen atoms. The bifunctional diketopiperazines were introduced into eight cyclic peptidomimetics containing the Arg-Gly-Asp (RGD) sequence. The resulting RGD peptidomimetics were screened for their ability to inhibit biotinylated vitronectin binding to the purified integrins α(v)β(3) and α(v)β(5), which are involved in tumor angiogenesis. Nanomolar IC(50) values were obtained for the RGD peptidomimetics derived from trans DKP scaffolds (DKP-2-DKP-8). Conformational studies of the cyclic RGD peptidomimetics by (1)H NMR spectroscopy experiments (VT-NMR and NOESY spectroscopy) in aqueous solution and Monte Carlo/Stochastic Dynamics (MC/SD) simulations revealed that the highest affinity ligands display well-defined preferred conformations featuring intramolecular hydrogen-bonded turn motifs and an extended arrangement of the RGD sequence [Cβ(Arg)-Cβ(Asp) average distance ≥8.8 Å]. Docking studies were performed, starting from the representative conformations obtained from the MC/SD simulations and taking as a reference model the crystal structure of the extracellular segment of integrin α(v)β(3) complexed with the cyclic pentapeptide, Cilengitide. The highest affinity ligands produced top-ranked poses conserving all the important interactions of the X-ray complex.

Synthesis and Biological Evaluation of RGD Peptidomimetic–Paclitaxel Conjugates Bearing Lysosomally Cleavable Linkers

“…[ [17][18][19] Notably, the RGD-PTX conjugate 7, besides showing the worst inhibition of biotinylated vitronectin binding to integrin a v b 3 in the series, did not allow us to obtain reproducible binding curves to a v b 5 integrin. This unusual behavior is possibly due to the low solubility of conjugate 7 in the medium, which might interfere with the binding process, especially at the highest concentration (10 À5 m).…”

“…The cell antiproliferative activity of the RGD-drug conjugates 6-8 was tested against two isogenic cancer cell lines expressing different a v b 3 levels, in order to investigate the a v b 3 -targeted delivery of the cytotoxic agent. [18] the functionalized analogue 2, [19] and the SMDC cyclo[DKP-RGD]-PTX (3). [19] Bz: benzoyl.…”

Effects of a novel cyclic RGD peptidomimetic on cell proliferation, migration and angiogenic activity in human endothelial cells

“…The peptidomimetic cyclo [DKP-RGD] 1 was prepared according to a published procedure [16]. Annexin V-FITC Apoptosis Detection Kit I was purchased by Biosciences (BD, Italy).…”

“…Recently, some of us reported a new class of cyclic RGD peptidomimetics containing a bifunctional diketopiperazine (DKP) scaffold, showing a low nanomolar affinity for integrins α V β 3 and α V β 5 [15,16]. The present study is aimed at characterizing the ability of the cyclic RGD peptidomimetic cyclo [DKP-RGD] 1 (Figure 1) to affect cell viability, proliferation, migration and capillary network formation in human umbilical vein endothelial cells (HUVEC).…”

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“…The functionalization was exemplarily carried out with the well investigated cRGD [151][152][153] , a small molecule which acts as a ligand for specific binding to the    5 integrin receptor, involved in neoangiogenic and metastasis processes [154][155][156] . The characteristics of the used DAR inv -allowed a bioorthogonal click reaction strategy [157][158][159] for a bridge formation between inorganic surfaces and biological active molecules 160 .…”