SYNTHESIS OF UNNATURAL LIPOPHILIC N-(9H-FLUOREN-9-YLMETHOXY)CARBONYL-SUBSTITUTED ALPHA-AMINO-ACIDS AND THEIR INCORPORATION INTO CYCLIC RGD-PEPTIDES - A STRUCTURE-ACTIVITY STUDY

The alpha(v) beta(3) integrin is implicated in human tumor metastasis and angiogenesis. It has been shown that structures of the sequence cy clo(-Arg(1)-Gly(2)-Asp(3)-D-Phe(4)-Xaa(5)-) (I) and cyclo(-Arg(1)-Gly( 2)-Asp(3)-Phe(4)-D-Xaa(5)-) (II) bind with high affinity and the latte r with high selectivity to this receptor. The residues Xaa and D-Xaa a ccept a broad variety of amino acids. Here, we report on the synthesis , activities, and conformational analysis of cyclic Arg-Gly-Asp (RGD) peptides containing lipophilic amino acids Xaa or D-Xaa in position 5. For I, these were (2S)-2-aminohexadecanoic acid (Ahd) and N-hexadecyl glycine (Hd-Gly) and in II, D-Ahd and Hd-Gly, and, for control purpose s, Ahd were incorporated (Fig. 1). The enantiomerically pure a-amino a cids were obtained by non-enantioselective synthesis and subsequent en zymatic separation of isomers using acylase I (Scheme). Hd-Gly was pre pared in a modified procedure according to Stewart from ethyl bromoace tate and hexadecylamine (Scheme). The synthesis and physicochemical pr operties of the corresponding (9H-fluoren-9-ylmethoxy)carbonyl (Fmoc) derivatives, compatible with solid-phase peptide synthesis, are descri bed. Structure elucidation by NMR reveals that the lipid modification has no significant impact on the template structures when incorporated into them. For peptides I with Xaa = Ahd or Hd-Gly (1 or 2), a beta I I'/gamma-turn-like arrangement with D-Phe in i+1 position of the beta- turn is found. Peptides II with D-Xaa = D-Ahd or Hd-Gly (3 or 4) exhib it a beta II'/gamma-turn conformation with Gly in i+1 position of the beta-turn, whereas II with Ahd instead of D-Xaa, i.e., lacking a D-ami no acid in position 4 or 5 (5), adopts no defined conformation. Howeve r, in assays of receptor specificity employing human alpha(v) beta(3) integrin, the compounds exhibit IC50 values ranging from nanomolar to less than millimolar. These results indicate that although the arrange ment of the pharmacophoric groups is preserved in the target compounds , the biological activity is highly dependent on spatial requirements of the lipid anchor in the receptor binding pocket. Obviously, only ce rtain positions do not affect the binding.