Brian Edward Kornberg

AMPA Receptor antagonists

AMPA Receptor antagonists have received considerable attention in recent years. Within the class of excitatory amino acid receptor antagonists AMPA receptor antagonists have shown excellent neuroprotection in several models of cerebral ischemia and neuronal injury. However, poor physical properties have been a major limiting factor in developing these as viable drug candidates. The quinoxaline-2,3-dione template has been the backbone of various competitive AMPA receptor antagonists such as NBQX, PNQX, YM-90K and more recently ZK200775. The SAR learned from these have been valuable for developing the AMPA pharmacophore model (Fig. 2) and has been discussed in detail in this review. There have been efforts in this area to design very selective AMPA receptor antagonists by minimizing the interaction at the NMDA associated GlyN receptors. Compounds designed by BASF and Yamanouchi have been successful in these efforts and their compounds show excellent affinity for the AMPA receptors. Efforts by Warner-Lambert and Novartis also highlight significant success in developing balanced AMPA and GlyN receptor antagonists. Non-competitive AMPA receptor antagonists are also being pursued for various neurological disorders including neuroprotection and are divided in two major classes, viz. positive and negative allosteric modulators. The physical properties of negative allosteric modulators such as GYKI 52466, which belong to the 2,3-benzodiazepinyl structural class have been significantly better. However, the in vitro activity of these compounds has been in the micromolar range and the overall class has the disadvantage of not having a high throughput assay. Other classes of compounds such as phthalazines and quinazolines are being developed and have raised hopes for the second generation of compounds in this area.

Metabolism-dependent mutagenicity of a compound containing a piperazinyl indazole motif: Role of a novel p450-mediated metabolic reaction involving a putative oxaziridine intermediate.

Compound 1a (6-chloro-5-{3-[4-(1H-indazol-3-yl)-piperazin-1-yl]-propyl}-3,3-dimethyl-1,3-dihydro-indol-2-one) was mutagenic to Salmonella typhimurium TA98 in the presence of rat liver S9 subcellular fraction. The metabolism of 1a in rat liver S9 or microsomes demonstrated that it underwent a P450-mediated N-deindazolation (loss of indazole ring) as a predominant metabolic pathway. To investigate a possible link between metabolism and mutagenicity, a structural analogue 1b (6-chloro-5-{3-[4-(1H-indazol-3-yl)-piperidin-1-yl]-propyl}-3,3-dimethyl-1,3-dihydro-indol-2-one), the cleaved product 2a (6-chloro-3,3-dimethyl-5-(3-piperazin-1-yl-propyl)-1,3-dihydro-indol-2-one), and the core motif 3a (3-piperazinyl indazole) were evaluated in the Ames assay. It was found that 1b was not mutagenic to Salmonella typhimurium TA98 in the absence or presence of a metabolic activating system. In contrast to 1a, 1b did not undergo the metabolic cleavage (loss of indazole ring). Marginal mutagenicity of 2a to TA98 was observed with rat liver S9, whereas 3a was shown to be a promutagen. It was further demonstrated that 1a inactivated P450 3A, the principle enzyme catalyzing the N-deindazolation reaction, in an NADPH-, time-, and concentration-dependent manner. The kinetics of inactivation was characterized by a K(I) of 8.1 microM and k(inact) of 0.114 min(-1). The differences in mutagenicity between 1a and 1b suggest that a chemical bond extending from the 3-position of the indazole to a heteroatom (as part of another cyclic ring) is a prerequisite for the toxicity. The metabolic process leading to the elimination of the indazole from the rest of the molecule apparently plays a key role in causing mutagenicity. It is postulated that the N-deindazolation of 1a proceeds via an oxaziridine intermediate, the formation of which is indirectly inferred from the presence of benzoic acid in microsomal incubations. Benzoic acid is thought to be derived from the hydrolysis of 3-indazolone, an unstable product generated from the oxaziridine. Evidence suggests that the electrophilic oxaziridine intermediate may be responsible for the mutagenicity and inactivation of P450 3A.