The goal of the Early Career Award in Chemistry of Drug Abuse and Addiction (ECHEM) (R21/R33) is to help applicant establish an independent and highly-competitive chemistry research program applied to drug abuse and addiction, with a major focus of developing radiopharmaceuticals for PET imaging study of monoacylglycerol lipase (MAGL) in the endocannabinoid signaling system. MAGL is the principle enzyme for metabolizing endogenous cannabinoid ligand: 2-arachidonylglycerol (2-AG). Blockade of MAGL increases 2-AG levels, resulting in anti-nociceptive, anxiolytic and anti-emetic responses, and has emerged as a therapeutic strategy to treat drug addiction and substance-use disorders (SUDs). Human genetic studies have identified the link between MAGL mutations and increased risk for addiction. Abnormal MAGL expression has been detected in the post-mortem brain of alcoholic subjects. However, these results discovered via ex vivo (destructive) analysis cannot provide a direct and real-time correlation between MAGL activity and human disease stages, particularly for tissues such as the brain. In this context, a PET radiotracer can fill this void and provide a noninvasive tool for quantifying MAGL activity and possible aberrant eCB function in drug abuse and addiction. However, the foremost barrier holding back PET applications for this purpose is the scarcity of radiotracers targeting MAGL, representing a significant deficiency in our understanding of this enzyme. The first brain penetrant MAGL radiotracer, [11C]SAR127303 recently developed by the PI and others, showed saturable binding but this compound also binds a second 2-AG degrading enzyme, ABHD6, thereby undermining the specific binding in the brain. To overcome this selectivity problem, we will utilize a novel proteomic technology, activity based protein profiling (ABPP) to perform target selectivity screening based on an array of novel MAGL inhibitors developed in house. An azetidine carbamate (IC50 0.4 nM) exhibits >500-fold selectivity to MAGL over FAAH, CB1 and CB2 receptors, and >30-fold MAGL over ABHD6. Preliminary studies indicate the 11C-labeled azetidine crosses the blood brain barrier in rat and nonhuman primate (NHP), and shows >65% specific binding. Although it is not clear if this radiotracer will be satisfactory for human use, it shows promise as a first generation of selective MAGL radiotracers. In addition to further evaluation of this tracer, we will use this scaffold to concurrently prepare a series of carefully chosen MAGL tracers with further improved selectivity, and evaluate their ability to quantify MAGL activity using rodents and NHPs. The impact of this work is not only to develop the first potent and selective MAGL neurotracer for basic eCB research, but also ultimately to progress this imaging tool for translational human imaging studies and investigate underlying mechanisms of MAGL-linked diseases including SUDs. Relevance: This proposal has the potential to improve public health and help patients suffering from addiction through advancement of PET neuroimaging using MAGL radiotracers.
Positron Emission Tomography (PET) is a medical technique that creates images of biochemical processes occurring in vivo, where radioactive molecules are injected and the distributions of those molecules are measured using a PET scanner. Data can be acquired in animals, in research subjects and eventually in patient care. As the burden of psychiatric illnesses in the United States and worldwide is high, there is a critical need to develop novel PET radiopharmaceuticals to image illness-related processes in the brain. This work will develop key tools designed to advance the scientific understanding of monoacylglycerol lipase in the cannabinoid system in the living brain of preclinical models, for the potential human transition into in vivo imaging of patients with drug addiction and other neuropsychiatric diseases.