Targeting Glutamate Transporters in the Brain for Neurodegenerative Disease Treatment

Alzheimer’s disease (AD) is a significant global health concern that exerts a dramatic burden on patients and their caregivers. To date, there are no effective drugs that can prevent, delay, or reverse AD. As such, identification and development of drugs that can modulate multiple pathological pathways are considered. One such target is solute carrier family 1 member 2 (SLC1A2; also known as excitatory amino acid transporter 2 (EAAT2) or glutamate transporter 1 (GLT-1), which is expressed in glial cells (i.e., astrocytes, microglia) and is regulator of both synaptic glutamate concentrations and microglial phagocytosis of Abeta1-42. This symposium presentation will provide state-of-the-art data in support of the mechanistic hypothesis that novel small molecule compounds targeting the SLC1A2 transporter can be developed into AD therapeutics. Using primary cultures of human neurons and primary cultures of human microglia, our laboratory has demonstrated that N-(5-ethoxy-4-methyl-1,3-thiazol-2-yl)-2-[(6-methyl-5-oxo-4,5-dihydro-1,2,4-triazin-3-yl)sulfanyl]propenamide (designated AD41) can provide neuroprotection and stimulate microglial phagocytosis. Cellular exposure to AD41 (10 uM) can significantly increase (p < 0.05) the rate of [3H]glutamic acid uptake by primary cultures of human microglia. Interestingly, the EC50 for increased SLC1A2-mediated glutamic acid transport by AD41 (1.4 nM) was considerably lower than that for riluzole (59.2 nM), an FDA-approved drug that targets glutamatergic neurotransmission, thereby demonstrating our ability to identify and evaluate compounds with superior potency in enhancing SLC1A2 transport activity. Specificity for glutamic acid transport by SLC1A2 was confirmed using WAY213613 (10 uM), a competitive pharmacological inhibitor of SLC1A2. Additionally, WAY213613 blocked AD41-stimulated uptake of Abeta1-42 in primary cultures of human microglia, suggesting that SLC1A2 transport activity is a primary regulator of phagocytosis, a physiological process that can promote both neural repair and blood-brain barrier (BBB) protection. In vivo experiments were performed using well-established 3xTg mouse model of AD. These animals display pathological features of AD that correlate with age including neuroinflammation, elevated amyloid beta protein levels, tau tangles, and associated cognitive deficits. Following a 28-day treatment course, AD41 (0.25 mg/kg, i.p.) was shown to improve working memory as determined by a modified conditioned place preference (CPP) paradigm and reduce brain levels of amyloid beta proteins including ABeta1-40 and ABeta1-42 in 3xTg mice of both sexes. Pharmacokinetic and nonclinical toxicology studies in male and female C57BL/6J mice showed measurable AD41 brain penetration as early as 30 min post-injection and lack of liver/kidney toxicity. Overall, this work demonstrates that the SLC1A2 transporter can be targeted in the brain for discovery and development of novel compounds for treatment of neurodegenerative disease states. These findings are translationally relevant given the current clinical experience with Abeta targeting drugs such as aducanumab (Aduhelm) and lecanemab-irmb (Leqembi), which have shown limited ability to improve cognitive performance and have revealed significant CNS adverse events. Indeed, SLC1A2 represents an opportunity to evaluate novel AD therapeutic targets, which will lead to more effective pharmacotherapy.