AMPA-glutamate receptors (AMPARs) are expressed throughout the CNS and mediate the majority of fast excitatory synaptic transmission. Ampakines are orally available small molecules that bind allosterically to AMPARs and enhance excitatory currents elicited by the endogenous agonist glutamate. In preclinical studies, ampakines are effective in ameliorating symptoms in a battery of neurodegenerative and neuropsychiatric diseases in which excitatory transmission is compromised. However, the development of ampakines as medicines was slowed by the emergence of neurotoxicity and seizures in rodents due to some ampakines. Here, we describe the preclinical pharmacology of a novel ampakine, N-methyl-N-(tetrahydro-2H-pyran-4-yl)benzo[c][1,2,5] oxadiazole-5-carboxamide (CX1739), that does not induce seizures in animals or humans at efficacious doses. CX1739 dose-dependently enhanced long-term potentiation in vivo in rats, a process thought to be a molecular substrate of learning and memory. Correspondingly, CX1739 dose-dependently enhanced performance in assays that probed multiple aspects of cognition—the novel object recognition test, the win shift radial arm maze, and the five-choice serial reaction time task in rats. CX1739 also abrogated amphetamine-induced locomotor activity, demonstrating that it may be given in conjunction with stimulants for pro-cognitive gains while mitigating the side effects of stimulant-based ADHD medications. CX1739 also rapidly reversed opioid-induced respiratory depression. While efficacy in these tests occurred at doses of 0.03–18 mg/kg, there were no adverse events detected in safety studies in rats up to 2000 mg/kg. These preclinical findings suggest that CX1739 can be translated safely into the clinical setting to potentially treat dementia, neuropsychiatric disorders, and the life-threatening complication of opiate-induced suppression of endogenous inspiratory breathing rhythms.

Ampakines are a class of orally available positive allosteric modulators of the AMPA-glutamate receptor (AMPAR) and have therapeutic implications for neurological/neuropsychiatric disorders in which AMPAR signaling is compromised. Low-impact ampakines are a distinct subclass of drugs that only modestly offset receptor desensitization and do not alter agonist binding affinity and thus lack the neurotoxicity and epileptogenic effects associated with other AMPAR modulators. In these studies, we describe the pre-clinical pharmacology of ampakine 1-(benzofurazan-5-ylcarbonyl)morpholine (CX717). CX717 modestly offsets desensitization in hippocampal patches and augments synaptic transmission in vivo. CX717 also enhances long-term potentiation in rats, which is crucial for learning and memory. CX717 enhances performance in the eight-arm radial maze and abrogates amphetamine-induced locomotor activity while being devoid of cataleptic activity in rats. CX717 also ameliorates alfentanil-induced respiratory depression in rats and is not toxic to cultured rat neurons. CX717 is active at doses of 0.3–10 mg/kg and lacked serious adverse events in safety studies in mice up to 2000 mg/kg. CX717 was also previously shown to be safe in humans and effective in reversing opiate-induced respiratory depression and hyperactivity and inattentiveness in adults with ADHD. These findings support the continued clinical investigation of CX717 in the treatment of ADHD, dementia, and opiate-induced respiratory depression.

Ampakines—positive allosteric modulators of AMPA-type glutamate receptors (AMPARs)—are drug candidates that have shown substantial promise in pre-clinical models of various neurodegenerative and neuropsychiatric diseases. Much of the study of ampakines has focused on how these drugs modulate neuronal AMPARs to achieve certain therapeutic effects. However, astrocytes also express functional AMPARs and their physiology may be sensitive to modulation by ampakines. Herein, we investigate the effects of multiple ampakines on calcium levels in cortical astrocytes. We find that ampakines augment cytosolic calcium elevations in astrocytes to an extent far greater than that achieved by AMPA alone. This effect is amenable to competitive AMPAR blockade. Furthermore, calcium induction is sensitive to phospholipase C_β antagonism and blockade of inositol triphosphate receptors located on the endoplasmic reticulum. Low-impact ampakines exerted weaker effects on cytosolic calcium levels in astrocytes and higher concentrations were required to observe an effect. Furthermore, high doses of the low-impact ampakine, CX717, were not toxic to cortical astrocytes at high concentrations, which may serve to differentiate low-impact ampakines from classical AMPAR positive modulators like cyclothiazide. As ampakines are further developed for clinical use, it would be prudent to determine the extent to and manner by which they affect astrocytes, as these effects may also underpin their therapeutic utility in CNS pathologies.

Background: Ampakines are a family of molecules that enhance the functioning of AMPA-glutamate receptors (AMPAR). High-impact ampakines completely offset receptor desensitization and enhance agonist binding affinity, while low-impact ampakines only modestly affect receptor desensitization and do not alter agonist binding affinity. Nonetheless, little is known about AMPAR recovery following ampakine treatment. Methods: Herein, we study the effects of ampakines on AMPAR recovery and the interaction between high- and low-impact ampakines. Results: The high-impact ampakine CX729 did not induce any current in the absence of glutamate, but it dramatically increased glutamate-induced steady-state inward currents. Recoveries from the enhancement were significantly slower than those for the low-impact ampakine CX516, as was also seen on miniature synaptic currents. Electrophysiological interaction studies suggest that high- and low-impact ampakines may have different binding sites. We further investigated the induction of the potentiated response by measuring glutamate-induced responses after transient applications of CX729 or CX729 plus glutamate. Under both circumstances, subsequent application of glutamate yielded comparably potentiated responses. Furthermore, the recovery time was not different if saline was substituted for glutamate during the recovery period. Conclusions: These observations show that AMPAR potentiation by CX729 does not require the simultaneous presence of glutamate, nor is the slow reversal of the effects of the ampakine altered by subsequent receptor activation. Hence, the slow recovery from the effects of these select ampakines on the AMPAR may be the result of slow dissociation kinetics. We posit that the slow recovery of AMPAR from high-impact ampakines may contribute to the seizurogenic effects of this drug class and that high-impact ampakines that allow for more rapid AMPAR recovery may be safer and more clinically viable candidates.

Background: The GABA_A receptor (GABAAR) potentiator, KRM-II-81, is being developed as a novel antiseizure medication with reduced potential for sedation, tolerance development, and abuse liability. Although KRM-II-81 has been shown to provide antiseizure protection against a broad array of seizure induction paradigms, seizures induced by viral vectors have not been previously studied. GABAARs with specific α subunit compositions have been studied in relation to the reduced side-effect liability of KRM-II-81; however, the role of β subunit composition has yet to be determined. Methods: In the present study, KRM-II-81 was studied against handling-induced seizures in Theiler’s murine encephalomyelitis virus (TMEV)-infected mice. Results: An intracerebral infusion of TMEV on day 0 increased the cumulative seizure burden in mice when assessed for handling-induced seizures on days 3–7. KRM-II-81 (15 mg/kg, p.o., bid) nearly completely suppressed seizures in TMEV-infected mice over the course of daily treatments. The number of the most severe seizures (stage 5, tonic/clonic seizures) in the mice was suppressed to zero by KRM-II-81. Although the selectivity of KRM-II-81 for GABAAR α2/3 receptor subtypes might imbue KRM-II-81 with a reduced side-effect liability, other mechanisms are possible, and the potentiation of β1-containing GABAARs has been implicated in inducing sedation. The role of β subunit composition has yet to be determined for KRM-II-81. In electrophysiological studies with cells transfected with α_xβ₁γ₂ or α_xβ₃γ₂, KRM-II-81 preferentially potentiated GABA responses in cells containing β3 subunits in α2/3-containing GABAARs. Conclusions: The present findings confirm the robust antiseizure activity of KRM-II-81, now extended to a virus-induction model, and suggest a possible role of reduced β1-potentiation in the low side-effect profile of KRM-II-81.