TC-5214 (S-(+)-mecamylamine): a neuronal nicotinic receptor modulator with antidepressant activity.

Both clinical and preclinical data support a potential therapeutic benefit of modulating the activity of CNS neuronal nicotinic receptors (NNRs) to treat depression and anxiety disorders. Based on the notion that the depressive states involve hypercholinergic tone, we have examined the potential palliative role of NNR antagonism in these disorders, using TC-5214 (S-(+) enantiomer of mecamylamine), a noncompetitive NNR antagonist. TC-5214 demonstrated positive effects in a number of animal models of depression and anxiety. TC-5214 was active in the forced swim test in rats (minimum effective dose (MED)=3 mg/kg i.p.), a classical depression model. It was also active in the behavioral despair test in mice (0.1-3.0 mg/kg i.p.), another model of depression. In the social interaction paradigm in rats, a model of generalized anxiety disorder (GAD), TC-5214 was active at a dose of 0.05 mg/kg s.c. In the light/dark chamber paradigm in rats, a model of GAD and phobia, TC-5214 was also active at a dose of 0.05 mg/kg s.c. Although TC-5214 shows modest selectivity among NNR subtypes, the antidepressant and anxiolytic effects seen in these studies are likely attributable to antagonist effects at the alpha4beta2 NNRs. This is supported by the observation of similar effects with alpha4beta2-selective partial agonists such as cytisine and with alpha4beta2-selective antagonists such as TC-2216. TC-5214 was well tolerated in acute and chronic toxicity studies in mice, rats, and dogs, showed no mutagenicity and displayed safety pharmacology, pharmacokinetic and metabolic profiles appropriate for therapeutic development. Overall, the results support a novel nicotinic cholinergic antagonist mechanism for antidepressant and anxiolytic effects and highlight the potential of NNR antagonists such as TC-5214 as therapeutics for the treatment of anxiety and depression.

Ispronicline: a novel alpha4beta2 nicotinic acetylcholine receptor-selective agonist with cognition-enhancing and neuroprotective properties.

To date, the primary treatments for Alzheimer's disease with proven efficacy have been acetylcholinesterase inhibitors that prevent the hydrolysis of acetylcholine (ACh) in the synaptic cleft, thereby prolonging its activity. Although these agents have some benefit in alleviating cognitive impairment, they have limited clinical utility because of insufficient efficacy and marginal tolerability. Within the last decade, there has been much experimental support for the use of therapeutics that directly target nicotinic ACh receptors (nAChRs) to improve cognitive function and slow neurodegenerative disease progression. These findings have spurred considerable research efforts to develop ligands selective for nAChRs, such as ABT-418 (Arneric et al., 1995), SIB-1553 (Bontempi et al., 2001), TC-2403 (Lippiello et al., 1996), and TC-2559 (Bencherif et al., 2000). There is abundant evidence that nAChR modulators have the potential to alleviate cognitive impairment in demented states. In addition to improving cognitive function, a large body of research implicates a role for nAChRs in neuroprotection, suggesting potential for disease modification. An impact of nAChR agonists on disease progression would provide an advantage over currently available treatments for memory loss. The profile of previous nAChR-targeted clinical candidates has not been adequate to warrant further development owing to poor oral bioavailability, side effects, and/or lack of efficacy. Thus, a challenge in nAChR drug design and development has been the reduction of undesirable effects that result from activity at specific nAChRs in the CNS and PNS, including cardiovascular toxicity, emesis, seizures, and hypothermia.

TC-1734: an orally active neuronal nicotinic acetylcholine receptor modulator with antidepressant, neuroprotective and long-lasting cognitive effects.

The development of selective ligands targeting neuronal nicotinic acetylcholine receptors to alleviate symptoms associated with neurodegenerative diseases presents the advantage of affecting multiple deficits that are the hallmarks of these pathologies. TC-1734 is an orally active novel neuronal nicotinic agonist with high selectivity for neuronal nicotinic receptors. Microdialysis studies indicate that TC-1734 enhances the release of acetylcholine from the cortex. TC-1734, by either acute or repeated administration, exhibits memory enhancing properties in rats and mice and is neuroprotective following excitotoxic insult in fetal rat brain in cultures and against alterations of synaptic transmission induced by deprivation of glucose and oxygen in hippocampal slices. At submaximal doses, TC-1734 produced additive cognitive effects when used in combination with tacrine or donepezil. Unlike (-)-nicotine, behavioral sensitization does not develop following repeated administration of TC-1734. Its pharmacokinetic (PK) profile (half-life of 2 h) contrasts with the long lasting improvement in working memory (18 h) demonstrating that cognitive improvement extends beyond the lifetime of the compound. The very low acute toxicity of TC-1734 and its receptor activity profile provides additional mechanistic basis for its suggested potential as a clinical candidate. TC-1734 was very well tolerated in acute and chronic oral toxicity studies in mice, rats and dogs. Phase I clinical trials demonstrated TC-1734's favorable pharmacokinetic and safety profile by acute oral administration at doses ranging from 2 to 320 mg. The bioavailability, pharmacological, pharmacokinetic, and safety profile of TC-1734 provides an example of a safe, potent and efficacious neuronal nicotinic modulator that holds promise for the management of the hallmark symptomatologies observed in dementia.