Optimization of Covalent Inhibitors of Human CD33

Microglia are innate immune cells resident in the brain responsible for maintaining a clean intercellular environment, supporting neurons, and shaping the synaptic connections between them. CD33 is a receptor protein that resides on the cell surface of microglia. When CD33 interacts with sialic acid sugars (sialoglycans) on proteins in the cell’s environment, signaling by CD33 inhibits the microglial cell’s “clean-up” function, the engulfment of microorganisms and debris such as the misfolded beta-amyloid and tau proteins, hallmarks of AD. Sialoglycans bind to a specific part of the CD33 protein, the IgV domain. In AD models, if binding by sialoglycans to CD33 is reduced, microglia clear more beta amyloid and fewer beta amyloid plaques accumulate, making CD33 modulation an appealing therapeutic target. In genome-wide association studies, variants of CD33 have a strong impact on late-onset (typical) AD risk. Variants that increase CD33 expression and thus allow more binding with sialoglycans increase AD risk, while a CD33 variant that yields a short CD33 protein lacking the IgV domain, which thus cannot interact with sialoglycans, is associated with decreased risk.

Dr. Subhash Sinha and colleagues have studied the structure and function of CD33 to develop small-molecule drug candidates that leverage these observed changes in CD33-associated AD risk. In previous grant cycles, they identified compounds that block human CD33, increase microglial uptake of amyloid, and reduce harmful inflammatory signals without toxicity. They hypothesize that CD33 inhibition will restore protective microglial functions and reduce their vulnerability to AD-related changes and impairment.

In the new grant cycle, the team will continue pursuing drug discovery and development for their CD33 inhibition strategy through two aims. In the first aim, they will optimize the molecules they previously identified as CD33 inhibitors and identify chemically similar analogs to their initial candidates. This will provide the team with more potential candidates for further testing and help establish and compare the efficacy of different inhibitor profiles based on their chemical mechanisms. They will screen potential new inhibitors for CD33 specificity, stability, and predicted ability to reach the brain at sufficient levels. The second aim will test these candidate molecules in cell culture models to determine their efficacy in engaging CD33 and define each inhibitor’s functional impact on the microglia, including their ability to engulf amyloid beta. The goal of this aim is to identify one or two lead candidates with the most favorable pharmacological profiles for future testing in animal models.

Overall, this project continues an effort to understand the function of CD33 and translate that understanding into a therapy aimed at inhibiting CD33 to restore protective microglial function in AD.