Syntheses and affinities of novel organometallic-labeled estradiol derivatives: a structure-affinity relationship.

Two series of novel estradiol derivatives, including cationic species, labeled with organometallic fragments Cr(CO)3, Cp*Ru+, or Cp*Rh2+ [Cp* = eta 5-C5(CH3)5] either in the 17 alpha-position or on the A-ring were synthesized, and their relative binding affinities (RBA) for the estradiol receptor were determined. The Ru(II) and the Rh(III) cationic derivatives were obtained as stable salts with the following counter anions (BF4-, PF6-, CF3SO3-). The satisfactory RBA values obtained for most complexes belonging to the 17 alpha series confirm that this position tolerates the presence of bulky neutral species. For instance, complex 4, in which the organometallic fragment Cr(CO)3 was attached to the phenyl ring of the 17 alpha-phenylethynyl fragment, exhibited an RBA value of 24%, very similar to that of the uncomplexed estrogen derivative 3. Surprisingly, the analogous cationic species 6 had no affinity for the estradiol receptor. This unprecedented result shows that the hormone binding site of the estrogen receptor does not tolerate the presence of a positive charge in the 17 alpha-position of the steroid. On the other hand, the alpha-face of the A-ring of estradiol did tolerate positively charged organometallic fragments bearing bulky substituents although the RBA value tended to decrease with increasing charge. The counterion in these cationic derivatives also affected binding affinity. For instance, the Ru(II) species 7a containing an CF3SO3- ion exhibited a reasonable RBA value (5.8%) compared to analogous species 13 with a PF6- ion (RBA of only 0.1%). Moreover, the triflate counteranion preserved the phenolic form of the A-ring of the estrogen derivative whereas the PF6- derivative was unstable and rapidly converted into the dienonylic form in buffer. The compared RBAs of the neutral and cationic species illustrate the preferences of the receptor hormone binding site in accepting or rejecting species of hydrophobic or hydrophilic character.

Preferred antagonist binding state of the NMDA receptor: synthesis, pharmacology, and computer modeling of (phosphonomethyl)phenylalanine derivatives.

A series of substituted [phosphono-, sulfo-, carboxy-, and (N-hydroxycarbamoyl)methyl]phenylalanines were synthesized as probes for the investigation of the preferred antagonist state of the NMDA receptor antagonists. The potency of these compounds was evaluated by measuring electrophysiological responses induced by NMDA in cultured mouse cortical neurons. 3-(Phosphonomethyl)phenylalanine [1(m)] a formal AP7 analogue, has been shown to be the most potent antagonist in this study with an IC50 of around 5 microM. The isomeric 2-(phosphonomethyl)phenylalanine [1(o)] was about half as active as 1(m) and as active as compound 5(3), a derivative which is cis-hydrogenated on the phenyl ring of 1(m). Replacement of a phosphono by a sulfo group led to a large reduction in the ability of these compounds to antagonize NMDA responses, although the ortho and meta isomers retained some activity in their reduced forms. In both series the para isomers were almost completely inactive at 100 microM. Introduction of a carboxyl or a bidentate HONHCO group in place of the phosphono moiety in the 3-position results in compounds devoid of activity. The active and inactive compounds of this study were used in conjunction with the most potent linear and cyclic phosphono-containing NMDA antagonists reported to date to determine, via computer modeling techniques, a three-dimensional model corresponding to a antagonist preferring state of the NMDA binding site. This structure defines a pharmacophore which is characterized by (i) well-defined distances between the central atoms of the polar groups PO3H-, NHn+, (n = 2, 3), and COO- (P-N = 5.89 +/- 0.12 A, P-C = 6.66 +/- 0.08 A, and N-C = 2.28 +/- 0.01 A), (ii) a sterically allowed region between the C5 methylene and the PO3H- group, and (iii) a molecular electrostatic field in which the positive, neutral, and negative potential zones are self-contained--with the negative potential zone connecting the PO3H- and COO- groups as the largest. We have compared our results to a preliminary model of the NMDA antagonist site by Hutchison et al. and to a topological model of the NMDA-glycine receptor site by Cordi et al. Our proposed steric-electrostatic pharmacophore which refines, simplifies, and improves these models has now to be validated by the design of new NMDA antagonists.