BACKGROUND AND PURPOSEThe) is an antagonist at the CXC chemokine receptor CXCR4, which plays a role in human immunodeficiency virus infection, cancer and stem cell recruitment. Binding modes for FC131 in CXCR4 have previously been suggested based on molecular docking guided by structure-activity relationship (SAR) data; however, none of these have been verified by in vitro experiments. EXPERIMENTAL APPROACHHeterologous 125 I-12G5-competition binding and functional assays (inhibition of CXCL12-mediated activation) of FC131 and three analogues were performed on wild-type CXCR4 and 25 receptor mutants. Computational modelling was used to rationalize the experimental data. KEY RESULTS CONCLUSIONS AND IMPLICATIONSLigand modification, receptor mutagenesis and computational modelling approaches were used to identify the binding mode of FC131 in CXCR4, which was in agreement with binding modes suggested from previous SAR studies. Furthermore, insights into the mechanism for CXCR4 activation by CXCL12 were gained. The combined findings will facilitate future design of novel CXCR4 antagonists. Abbreviations2-Nal, 3-(2-naphthyl)alanine; 7TM, 7 transmembrane helix; Aib, 2-aminoisobutyric acid; Cit, citrulline; ECL, extracellular loop; HIV, human immunodeficiency virus; IP, inositol phosphate; SAR, structure-activity relationship; WT, wild type BJP British Journal of Pharmacology

The readily available N-Boc-protected delta-amino alpha,beta-unsaturated gamma-keto ester 1 was diastereoselectively reduced to the corresponding alcohols 2 and 3, using boron- and aluminum-based reducing reagents. Reduction reactions were successful and resulted in anti/syn ratios of alcohols of >95:5 (80% yield), using LiAlH(O-t-Bu)(3) in EtOH at -78 degrees C under chelation control, and 5:95 (98% yield), using NB-Enantride in THF at -78 degrees C under Felkin-Anh control.

In the absence of an experimentally determined binding mode for the cyclopentapeptide CXCR4 antagonists, we have rationally designed conformationally constrained analogues to further probe the small peptide binding pocket of CXCR4. Two different rigidification strategies were employed, both resulting in highly potent ligands (9 and 13). The information provided by this cyclopentapeptide ligand series will be very valuable in the development of novel peptidomimetic CXCR4 antagonists.

The diffusion coefficient (also known as diffusivity) of an active pharmaceutical ingredient (API) is a fundamental physicochemical parameter that affects passive diffusion through biological barriers and, as a consequence, bioavailability and biodistribution. However, this parameter is often neglected, and it is quite difficult to find diffusion coefficients of small molecules of pharmaceutical relevance in the literature. The available methods to measure diffusion coefficients of drugs all suffer from limitations that range from poor sensitivity to high selectivity of the measurements or the need for dedicated instrumentation. In this work, a simple but reliable method based on time-resolved concentration measurements by UV-visible spectroscopy in an unstirred aqueous environment was developed. This method is based on spectroscopic measurement of the variation of the local concentration of a substance during spontaneous migration of molecules, followed by standard mathematical treatment of the data in order to solve Fick's law of diffusion. This method is extremely sensitive and results in highly reproducible data. The technique was also employed to verify the influence of the environmental characteristics (i.e., ionic strength and presence of complexing agents) on the diffusivity. The method can be employed in any research laboratory equipped with a standard UV-visible spectrophotometer and could become a useful and straightforward tool in order to characterize diffusion coefficients in physiological conditions and help to better understand the drug permeability process.

Abstract:We here report an experimentally verified binding mode for the known tripeptidomimetic CXCR4 antagonist KRH-1636 (1). A limited SAR study was first conducted based on the three functionalities of 1, 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