A novel dual-action small molecule developed by a team led by scientists at Kyungpook National University (KNU) in Daegu, South Korea, has been shown to improve the neuropathological features of Alzheimer's disease (AD) in mice, the authors reported in the Jan. 18, 2022, edition of Proceedings of the National Academy of Sciences.

AD is a progressive neurodegenerative disease with complex underlying pathologies, which extend far beyond its characteristic accumulation of amyloid beta (Abeta) plaque and tau protein neurofibrillary tangles in the brain.

For example, multiple studies have shown that AD memory impairment is driven by the interaction of several pathologic processes, suggesting that effective AD drug development should focus on therapeutic strategies targeting these complex neuropathological features.

"Considering the multiple complex pathologic processes involved in AD, multitargeted therapeutic approaches should be advantageous for effective AD treatment," said study co-leader Hee Kyung Jin, a professor in the KNU Alzheimer's Disease Research Institute.

In this regard, several sphingolipid-metabolizing enzymes, most notably the acid sphingomyelinase (ASM), have been reported to be abnormally expressed in both AD patients and in mouse models of the disease.

ASM inhibition

Increased ASM activity in the blood and brain of AD mice is known to contribute to various AD neuropathological features, suggesting that ASM inhibition could be an important therapeutic target.

However, while direct or indirect ASM functional inhibitors have been identified, their nonspecificity has been associated with off-target effects and unclear mechanisms of action in AD, hence the need to develop new selective ASM inhibitors.

In the new PNAS study co-led by Jin and KNU professor Jae-sung Bae, "we focused on identifying an effective direct inhibitor of ASM activity based on studies showing the importance of this enzyme for the development of various neuropathological features in AD," Jin told BioWorld Science.

Screening two large libraries of putative ASM inhibitor compounds using cell-based ASM activity assays in AD patient fibroblasts then led to discovery of the KARI series of ASM inhibitors.

These candidate molecules were structurally optimized by the addition of carbon chains to enhance their lipophilicity to enable effective blood-brain barrier penetration in AD patients.

Interestingly, a longer carbon chain length in one of the molecules, designated KARI-201, was found to correlate with higher ASM binding and inhibitory effects.

"We also observed that the bioavailability, brain distribution and microsomal stability depended on carbon chain length," noted Jin.

"These findings indicate that the carbon chain length might play a critical role for stable binding to ASM and enhanced pharmacokinetic properties, especially regarding brain distribution, leading us to select KARI-201 as the optimal compound with these properties," she said.

Unexpectedly, besides being a direct ASM Inhibitor, KARI-201 was shown to be an agonist of the growth hormone secretagogue receptor 1alpha (GHSR1alpha), commonly known as the ghrelin receptor, in KARI-201-treated AD mice, which may have AD treatment implications.

"This [ghrelin receptor] activity is critical for hippocampal synaptic physiology and may also impact neuropathological features in AD," Jin said.

"The dual action in neurons of KARI-201 as a direct ASM inhibitor and GHSR1alpha agonist led to outstanding, synergetic therapeutic effects in AD mouse models on neuropathological features involving learning and memory impairment.

In addition to being a direct, highly selective and competitive ASM inhibitor with excellent brain distribution and druggability, KARI-201 was further shown to lack off-target effects in AD mice, with excellent pharmacokinetic properties, especially regarding brain distribution.

KARI-201 was also shown to normalize autophagy dysfunction by improving lysosomal biogenesis in KARI-201-treated mice, leading to enhanced Abeta phagocytic microglial function and improved Abeta deposition, neuroinflammation, synaptic plasticity and memory dysfunction.

These results suggest that KARI-201 might be useful for targeting several of the neuropathological features in AD, acting as both an ASM inhibitor and ghrelin receptor agonist to improve the systemic pathological features of AD.

Moreover, "combining KARI-201 with current AD therapies such as anti-Abeta antibodies might synergistically improve several AD-related neuropathological features," suggested Jin.

However, she cautioned that to date, the use of such AD mouse and cell models for drug development has been limited, "with many drugs that showed efficacy in such models having failed to translate into efficacy in humans."

Nevertheless, "our findings suggest that KARI-201 may be a promising treatment for AD and possibly other neurologic disorders with increased ASM activity or dysregulated ghrelin receptor signaling, although this awaits confirmation in future preclinical and clinical studies."