Triazolines .27. Delta(2)-1,2,3-triazoline anticonvulsants: Novel 'built-in' heterocyclic prodrugs with a unique 'dual-action' mechanism for impairing excitatory amino acid L-glutamate neurotransmission

The Delta(2)-1,2,3-triazoline anticonvulsants (1) may be considered as representing a unique class of 'built-in' heterocyclic prodrugs where the active 'structure element' is an integral part of the ring system and can be identified only by a knowledge of their chemical reactivity and metabolism. Investigations on the metabolism and pharmacology of a lead triazoline, ADD17014 (1a), suggest that the triazolines function as 'prodrugs' and exert their anticonvulsant activity by impairing excitatory amino acid (EAA) L-glutamate (L-Glu) neurotransmission via a unique 'dual-action' mechanism. While an active beta-amino alcohol metabolite, 2a, from the parent prodrug acts as an N-methyl-D-aspartate (NMDA)/MK-801 receptor antagonist, the parent triazoline impairs the presynaptic release of L-Glu. Various pieces of theoretical reasoning and experimental evidence led to the elucidation of the dual-action mechanism. Based on the unique chemistry of the triazolines, the potential metabolic pathways and biotransformation products of 1a were predicted to be the beta-amino alcohols 2a and 2a', the alpha-amino acid 3a, the triazole 4a, the aziridine 5a, and the ketimine 6a. In vivo and in vitro pharmacological studies of 1a and potential metabolites, along with a full quantitative urinary metabolic profiling of 1a, indicated the beta-amino alcohol 2a as the active species. It was the only compound that inhibited the specific binding of [H-3]MK-801 to the MK-801 site, 56% at 10 mu M drug concentration, but itself had no anticonvulsant activity, suggesting 1a acted as a prodrug. Three metabolites were identified; 2a was the most predominant, with lesser amounts of 2a', and very minor amounts of aziridine 5a. Since only 5a can yield 2a', its formation indicated that the biotransformation of la occurred, at least in part, through 5a. No amino acid metabolite 3a was detected, which implied that no in vivo oxidation of 2a or oxidative biotransformation of la or 5a by hydroxylation at the methylene group occurred. While triazoline la significantly decreased Ca2+-dependent, K+-evoked L-Glu release (53% at 100 mu M drug concentration), triazolines 1a-1c showed an augmentation of 50-63%, in the Cl- channel activity, a useful membrane action that reduces the excessive L-Glu release that occurs during epileptic seizures. The high anticonvulsant activity of 1a may be due to its unique dual-action mechanism whereby 1a and 2a together effectively impair both pre- and postsynaptic aspects of EAA neurotransmission, and has clinical potential in complex partial epilepsy which is refractory to currently available drugs.