Synthesis of the Four Isomers of 4-Aminopyrrolidine-2,4-dicarboxylate:  Identification of a Potent, Highly Selective, and Systemically-Active Agonist for Metabotropic Glutamate Receptors Negatively Coupled to Adenylate Cyclase

Abstract

The four isomers of 4-aminopyrrolidine-2,4-dicarboxylate (APDC) were prepared and evaluated for their effects at glutamate receptors in vitro. (2R,4R)-APDC (2a), an aza analog of the nonselective mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (1S,3R)-ACPD, 1), was found to possess relatively high affinity for metabotropic glutamate receptors (mGluRs) (ACPD-sensitive [3H]glutamate binding IC50 = 6.49 +/- 1.21 microM) with no effects on radioligand binding to NMDA, AMPA, or kainate receptors up to 100 microM. None of the other APDC isomers showed significant mGluR binding affinity, indicating that this interaction is highly stereospecific. Both 1 and 2a were effective in decreasing forskolin-stimulated cAMP formation in the adult rat cerebral cortex (EC50 = 8.17 +/- 2.21 microM for 1; EC50 = 14.51 +/- 5.54 microM for 2a); however, while 1 was also effective in stimulating basal tritiated inositol monophosphate production in the neonatal rat cerebral cortex (EC50 = 27.7 +/- 5.2 microM), 2a (up to 100 microM) was ineffective in stimulating phosphoinositide hydrolysis in this tissue preparation, further supporting our previous observations that 2a is a highly selective agonist for mGluRs negatively coupled to adenylate cyclase. Microelectrophoretic application of either 1 or 2a to intact rat spinal neurons produced an augmentation of AMPA-induced excitation (95 +/- 10% increase for 1, 52 +/- 6% increase for 2a). Intracerebral injection of 1 (400 nmol) produced characteristic limbic seizures in mice which are not mimicked by 2a (200-1600 nmol, ic). However, the limbic seizures induced by 1 were blocked by systemically administered 2a in a dose-dependent manner (EC50 = 271 mg/kg, ip). It is concluded that (2R,4R)-APDC (2a) is a highly selective, systemically-active agonist of mGluRs negatively coupled to adenylate cyclase and that selective activation of these receptors in vivo can result in anticonvulsant effects.

Conformationally Constrained Analogues ofL-Glutamate as Subtype-Selective Modulators of Metabotropic Glutamate Receptors

“…These observations led them to conclude that NMDA receptors are stimulated by the folded form of L-glutamate, while mGluRs, at least group II mGluRs, are activated by the extended form of L-glutamate. As noted before, this hypothesis was further supported by the results of (2R,4R)-APDC (15,16).…”

“…Based on these observations, several groups have carried out a modification to the structure of (1S,3R)-ACPD in order to find more potent or selective ligands. Lilly's researchers reported that an aza-substituted analogue, (2R,4R)-APDC, had about two times greater potency (EC 50 ϭ 3.5 M at human mGluR2) of activating group II mGluRs than (1S,3R)-ACPD but had little effect on group I mGluRs (15,16). Conformation studies showed that this compound preferentially adopted a fully extended glutamate conformation in the vacuum phase, which gave an additional support for the hypothesis that L-glutamate interacts with group II mGluR protein in a fully extended form (16).…”

“…Lilly's researchers reported that an aza-substituted analogue, (2R,4R)-APDC, had about two times greater potency (EC 50 ϭ 3.5 M at human mGluR2) of activating group II mGluRs than (1S,3R)-ACPD but had little effect on group I mGluRs (15,16). Conformation studies showed that this compound preferentially adopted a fully extended glutamate conformation in the vacuum phase, which gave an additional support for the hypothesis that L-glutamate interacts with group II mGluR protein in a fully extended form (16). Benzylation of (2R,4R)-APDC led to 1-benzyl-APDC, which was found to have weak antagonist activity for group I and II mGluRs (IC 50 ϭ 600 and 200 M at mGluR5 and mGluR2, respectively) but to activate mGluR6 selectively (EC 50 ϭ 20 M) ( Fig.…”

Molecular targets for the rational design of antiepileptic drugs and related neuroprotective agents

“…This represents the first example documenting the effects of a mGluR agonist; due to its potency, selectivity, and systemic activity, 32 should prove to be a useful pharmacological tool for studying the functional consequences of group II mGluR activation in vivo. 130 …”

Activation of spinal metabotropic glutamate receptors elicits cardiovascular responses in pentobarbital anesthetized rats

“…Although DCG IV is a potent agonist at both NMDA and Group II mGlu receptors (Wilsch et al 1994;Uyama et al 1997), 2R,4R-APDC is highly selective for Group II mGlu receptors (Monn et al 1996). Thus, it seems unlikely that the results obtained in rats pretreated with APV were related to the blockade of some action of Group II mGlu receptor agonists at NMDA receptors.…”