The influence of carboxylesterase 1 polymorphism and cannabidiol on the hepatic metabolism of heroin

Highlights

• Hepatic carboxylesterase 1 contributes to 3.66% of heroin hydrolysis in the liver.

• Carboxylesterase 1 G143E variant unlikely to clinically impact heroin metabolism.

• Cannabidiol is a potent in vitro inhibitor of the two-step hydrolysis of heroin.

• Cannabidiol can potentially interact with heroin in various clinical settings.

Abstract

Heroin (diamorphine) is a highly addictive opioid drug synthesized from morphine. The use of heroin and incidence of heroin associated overdose death has increased sharply in the US. Heroin is primarily metabolized via deacetylation (hydrolysis) forming the active metabolites 6-monoacetylmorphine (6-MAM) and morphine. A diminution in heroin hydrolysis is likely to cause higher drug effects and toxicities. In this study, we sought to determine the contribution of the major hepatic hydrolase carboxylesterase 1 (CES1) to heroin metabolism in the liver as well as the potential influence of one of its known genetic variants, G143E (rs71647871). Furthermore, given the potential therapeutic application of cannabidiol (CBD) for heroin addiction and the frequent co-abuse of cannabis and heroin, we also assessed the effects of CBD on heroin metabolism. In vitro systems containing human liver, wild-type CES1, and G143E CES1 S9 fractions were utilized in the assessment. The contribution of CES1 to the hydrolysis of heroin to 6-MAM was determined as 3.66%, and CES1 was unable to further catalyze 6-MAM under our assay conditions. The G143E variant showed a 3.2-fold lower intrinsic clearance of heroin as compared to the WT. CBD inhibited heroin and 6-MAM hydrolysis in a reversible manner, with IC₅₀s of 14.7 and 12.1 μM, respectively. Our study results suggested only minor involvement of CES1 in heroin hydrolysis in the liver. Therefore, the G143E variant is unlikely to cause significant impact despite a much lower hydrolytic activity. CBD exhibited potent in vitro inhibition toward both heroin and 6-MAM hydrolysis, which may be of potential clinical relevance.

Introduction

Heroin (diamorphine, diacetylmorphine) is a highly addictive and widely abused opioid drug synthesized from morphine, a natural substance originating in the opium poppy. The use of heroin in the US has been rapidly rising in the last decade (Substance Abuse and Mental Health Services Administration, 2018). Accompanying this increased use is a nearly 6.5-fold increase in heroin associated drug overdose deaths constituting the widely recognized “opioid crisis” in the US (National Institute on Drug Abuse, 2019).

Upon intravenous administration, heroin is rapidly converted to its first hydrolytic metabolite, 6-monoacetylmorphine (6-MAM), which is further hydrolyzed (deacetylated) to the major active metabolite, morphine (Fig. 1) [[1], [2], [3], [4]].

Given its lower affinity for the mu-opioid receptor [5,6] and short-lived presence in both plasma and brain [4,7], the drug effects of heroin are thought to be primarily mediated through its metabolites [1]. 6-MAM achieves its maximum concentration within 0.3–2.7 min after the intravenous administration of heroin and is believed to contribute to the acute drug effects [2,3,8], while morphine is believed to responsible for the sustained effects. However, since heroin is more lipophilic than its metabolites, it may penetrate the blood-brain barrier more easily [9], and thereby cause higher brain 6-MAM concentrations and stronger rewarding effects. Recent studies using animal models that compared equimolar doses of heroin and its metabolites suggest that this is the case [7,10]. Therefore, if the hydrolysis of heroin is impaired by certain conditions or substances such as genetic polymorphisms and xenobiotic inhibitors, respectively, it is possible that this increased exposure to heroin and its metabolites may pose an unrecognized risk of heroin-related toxicity.

Several different esterases contribute to the hydrolysis of heroin to 6-MAM, namely butyrylcholinesterase (BChE), carboxylesterase 1 (CES1), and carboxylesterase 2 (CES2) [[11], [12], [13]], while erythrocyte acetylcholinesterase (AChE), CES1, and CES2 are responsible for further hydrolysis of 6-MAM to morphine [12,14,15]. Both BChE and AChE are primarily present in the plasma/blood, while CES2 is mainly expressed in the gastrointestinal tract and far less in the liver. CES1 is primarily expressed in the liver and lung, and unlike rodents and most animal models, CES1 is not found in the plasma of humans. The human CES1 gene encodes for the enzyme CES1, a serine esterase governing both metabolic deactivation and activation of numerous therapeutic agents and chemicals [16,17]. Although significant hydrolysis of heroin in blood had been suggested based on the high clearance of heroin in human (i.e. half-life ~3 min) [2,3,18,19], organs are believed to play a main determinant role in the depletion of heroin in systemic circulation as evidenced by an apparently longer half-life (9–15 min) of heroin when it was incubated in human blood [1,14]. Since both liver and lung are well-perfused organs that may potentially affect the overall heroin hydrolysis, it is rational to investigate the impact of CES1 in a hepatic assay system. In addition, although some previous studies have investigated the role of BChE, CES1, and CES2 in heroin metabolism [[11], [12], [13]], they have not been confirmed in a hepatic assay system.

A recent double-blind, randomized study reported the reduction of cue-induced craving and anxiety by cannabidiol (CBD) in individuals with heroin use disorder [20]. These findings suggest that CBD, a phytocannabinoid naturally present in cannabis plants, could represent a viable treatment for heroin addiction. Furthermore, a rodent study also suggested a beneficial effect on drug-seeking behavior [21]. We have previously shown CBD as a potent inhibitor of CES1 [22], and in an earlier study, the inhibition of esterase was found to be associated with altered heroin effects in mice [23]. Therefore, it is important to evaluate the potential of drug-drug interactions (DDI) between CBD and heroin, because individuals may be exposed to heroin and CBD concomitantly as a treatment intervention, or potentially, through recreational use of cannabis.

In the present study, we sought to determine the potential contribution of hepatic CES1 to the metabolism of heroin, and the potential for altered heroin metabolism by G143E (rs71647871), a well-characterized loss-of-function CES1 variant [24]. Lastly, we also evaluated the influence of CBD on the hepatic hydrolysis of heroin to 6-MAM and of 6-MAM to morphine, respectively.