Domain-specific inhibition of angiotensin-converting enzyme as a therapeutic strategy for opioid use disorders

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ABSTRACT This project combines the mutual expertise of Drs. Patrick Rothwell and Swati More (Principal Investigators) in nucleus accumbens opioid signaling and medicinal chemistry. As part of an ongoing collaboration supported by NIDA (R21 DA050120), we have found that angiotensin-converting enzyme (ACE) has a non-canonical function in the nucleus accumbens: it degrades Met-enkephalin-Arg-Phe (MERF) and thereby regulates endogenous opioid signaling. Conventional ACE inhibitors block the degradation of MERF, leading to an enhancement of endogenous opioid signaling in the nucleus accumbens. This causes a selective reduction of glutamate release onto medium spiny projection neurons that express the Drd1 dopamine receptor (D1-MSNs), which express ACE at a higher level than other neurons. This mechanism of action has great therapeutic potential, as our preliminary data indicate the decrease in excitatory drive to D1-MSNs can diminish the rewarding effects of fentanyl. Previously published enzymatic assays using recombinant protein suggest that MERF is efficiently degraded by the catalytic N-domain of ACE, though this has not been examined in brain tissue. This raises the exciting possibility of a double-dissociation between catalytic domains of ACE that degrade angiotensin (C-domain) and MERF (N-domain). The goal of this project is to independently evaluate the contribution of each ACE catalytic domain to MERF degradation and endogenous opioid signaling in the nucleus accumbens, in order to generate new domain-specific ACE inhibitors with optimized properties for treatment of opioid use disorders. We will use mice as an experimental system to separately manipulate each catalytic domain of ACE, through a combination of complementary genetic and pharmacological manipulations. AIM 1 is to determine which catalytic domain of ACE degrades MERF in nucleus accumbens tissue. We will directly quantify extracellular levels of MERF using liquid chromatography-tandem mass spectrometry, and measure excitatory synaptic transmission using whole-cell patch-clamp recordings from nucleus accumbens neurons. AIM 2 is to determine the behavioral impact of domain-specific ACE inhibition on fentanyl CPP and self-administration. This will build on our preliminary experiments using non-contingent fentanyl exposure (CPP), by incorporating parallel analysis of intravenous fentanyl self-administration on an intermittent access schedule. AIM 3 is to optimize the central activity and drug-like properties of domain-specific ACE inhibitors. We will perform systematic chemical iterations involving (but not limited to) prodrug and drug delivery systems, with the goal of improving permeability across the blood-brain barrier. These experiments should result in the identification and early optimization of compounds that inhibit degradation of MERF by ACE in the brain. This novel mechanism could form the basis of a viable new therapeutic strategy for treating opioid use disorders.