Teaching old molecules new tricks: repurposing investigational drugs as chemical probes

Abstract

This thesis revolves around two investigational drugs with remarkable biological activity, MCC950 and PD-132301. MCC950 inhibits the NOD-like receptor protein 3 (NLRP3) inflammasome – an emerging drug target implicated in a diverse array of diseases, including several neurological conditions. PD-132301 is an inhibitor of sterol O-acyltransferase 1 (SOAT1) – an enzyme previously thought to modulate atherosclerotic plaque formation. The development of both compounds was abandoned due to toxicity concerns and limited knowledge of the biological pathways. By understanding the shortcomings of these compounds, they can be repurposed and redesigned as chemical probes of diseases with unmet clinical needs.Chapter 1 introduces chemical probes, including photoaffinity and positron emission tomography (PET) probes, and how they can be used in target identification and biodistribution studies. The NLRP3 inflammasome pathway and the rationale behind developing NLRP3 inhibitor drugs are also discussed.Chapter 2 focuses on identifying the molecular target of MCC950. The design and synthesis of two MCC950-based photoaffinity probes are described. These probes are evaluated in cell-based assays, as both NLRP3 inhibitors and target identification tools. Both photoaffinity probes suggest a direct interaction with NLRP3 itself. This finding, together with independent and collaborator research, furthers our understanding of how NLRP3 inhibitors ameliorate inflammatory diseases.Chapter 3 describes the evaluation of MCC950 brain uptake using PET imaging. Carrier added [11C]CO2 fixation chemistry was used to synthesise [11C]MCC950. [11C]MCC950 brain uptake was evaluated in healthy mouse, rat, and monkey PET studies. At microdoses, low molar activity [11C]MCC950 exhibited poor brain uptake and rapid washout and therefore appears to be a poor neuroimaging agent in healthy subjects. Although low molar activity [11C]MCC950 does not appear useful for in vivo imaging in healthy subjects, this work facilitates 11C-labelling of similar analogues. Attempts to synthesise [18F]Fluoro-MCC950 are also described.Chapter 4 investigates the repurposing of SOAT1 inhibitors for atherosclerosis PET imaging. [11C]PD-132301 was synthesised via [11C]methylation, while [18F]4.1 was synthesised via aromatic [18F]fluorination. Evaluation in ex vivo rat biodistribution studies suggests that [11C]PD-132301 is more selective for SOAT1. Using PET imaging, [11C]PD-132301 identified aortic atherosclerosis in apolipoprotein E deficient mice. This work suggests SOAT1 inhibitors are worthy of further investigation as atherosclerosis imaging probes.Chapter 5 summarises this work in the context of repurposing compounds as chemical probes.