Dimethocaine Hydrochloride (DMC, larocaine), a synthetic derivative of cocaine, is a widely distributed "legal high" consumed as a "new psychoactive substance" (NPS), originally was used in the 1930s as an anesthetic, primarily in dentistry, ophthalmology, and otolaryngology. This drug completely inhibits dopamine transporter and has had the potential for abuse. Dimethocaine Hydrochloride is intended for forensic and research purposes.

Selected local anesthetics on the dopamine transporter

The effects of selected local anesthetics on in vitro and in vivo measurements of dopamine transporter activity were determined to investigate the role of local anesthetic activity in the neuronal actions of cocaine. Cocaine inhibited [3H]2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane 1.5-naphthalenedisulfonate (CFT) binding and [3H]dopamine uptake with estimated Ki and IC50 values of 0.6 microM and 0.7 micorM, respectively. Of the local anesthetics tested, only Dimethocaine Hydrochloride showed full displacement of CFT binding (0-30 microM tested) and full inhibition of dopamine uptake (0-100 microM tested). Dimethocaine Hydrochloride was only slightly less potent than cocaine with an estimated Ki of 1.4 micorM and an IC50 value of 1.2 microM for [3H]CFT binding and dopamine uptake.[1]

At a maximum concentration of 100 microM, the ester containing local anesthetics procaine, tetracaine, piperocaine and the amide containing local anesthetic dibucaine and bupivacaine partially inhibited dopamine uptake by 47-70%. The ester containing local anesthetic propoxycaine and the amide containing local anesthetics prilocaine, etidocaine, procainamide, and lidocaine inhibited dopamine uptake by 8-30% at 100 microM. A 10 min administration of cocaine, Dimethocaine hydrochloride, or procaine in the dialysis solution produced dose-dependent, reversible increases in endogenous dopamine efflux from the striata of awake rats. Cocaine and dimethocaine hydrochloride produced similar 12-fold increases in dialysate dopamine at concentrations of 0.1 mM and 1 mM respectively. Procaine (10 mM) produced a 6-fold increase in dialysate dopamine while lidocaine (1 mM) produced a reproducible and reversible decrease (30%). These results show that the cocaine-like actions of certain local anesthetics such as dimethocaine hydrochloride and procaine result from their direct actions of dopamine uptake inhibitors.

Dimethocaine Hydrochloride metabolism catalyzed by P450s and NAT2

Dimethocaine Hydrochloride (DMC, larocaine, 3-diethylamino-2,2-dimethylpropyl)-4-aminobenzoate) was marketed as local anesthetic in the 1930s and used in dentistry and ophthalmology. Besides local anesthetic effects, Dimethocaine Hydrochloride has also effects on the central nervous system acting as dopamine-reuptake-inhibitor. It is listed by the EMCDDA under the category “synthetic cocaine derivatives” and offered in numerous online shops. The abuse is described to produce feelings of euphoria and sometimes relaxed feeling accompanied with side effect such as a strong hangover and fatigue. The in vivo pharmacological properties of DMC were compared to cocaine after intraperitoneal injection into rats. Dimethocaine Hydrochloride was shown to have high affinity for the DA transporter mainly in the nucleus accumbens stimulating the reward system. Furthermore, Woodward et al. (1995) have shown that DMC is nearly as potent as cocaine concerning the DA-reuptake-inhibitor efficiency. Therefore, the authors investigated in previous studies the affinity of DMC to human ABC transporter P-glycoproteine (P-gp) and the metabolic pathway of DMC in rat. Main observed metabolic steps were the N-acetylation, hydroxylation, and N-deethylation. Therefore, the aim of the presented study was to elucidate in vivo contribution of human N-acetyltransferases (NATs) and cytochrome P450s (P450 s) to the hepatic metabolism of Dimethocaine Hydrochloride using the relative activity factor (RAF) approach.[2]

It seemed likely that isozyme NAT1 should be of importance for Dimethocaine Hydrochloride N-acetylation because of its p-aminobenzoic acid structure, which was described to be a substrate of this enzyme (Butcher et al., 2002). However, initial activity screening with conditions adequate to make a statement on the general involvement of the NAT isozymes revealed that only NAT2 was capable catalyzing the N-acetylation of DMC. This was further supported by enzyme kinetic studies, where data could only be acquired for NAT2 due to a very low product formation in NAT1 incubations. The MS2 mass spectrum of acetylated DMC together with its analytical separation is shown. The kinetic profile of Dimethocaine Hydrochloride acetylation by NAT2 best fits into Michaelis–Menten kinetic as shown. To check proper incubation conditions, sulfamethazine was used as suitable test substrate with Km values available for both NAT enzymes. The Km value for Dimethocaine Hydrochloride acetylation was determined to be 102 μM, the Vmax value to be 1.1 units/min/pmol.

Kinetic profiles of all P450 catalyzed metabolite formations followed classic Michaelis–Menten behavior with enzyme affinities (Km values) between 3.6 and 220 μM. Using the relative activity factor approach, the net clearances for deethylation of Dimethocaine Hydrochloride were calculated to be 3% for P450 1A2, 1% for 2C19, <1% for 2D6, and 96% for 3A4. The net clearances for hydroxylation of DMC were calculated to be 32% for P450 1A2, 5% for 2C19, 51% for 2D6, and 12% for 3A4. Furthermore, these data were confirmed by chemical inhibition tests in human liver microsomes. As Dimethocaine Hydrochloride is metabolized via two main steps and different P450 isoforms were involved in the hepatic clearance of Dimethocaine Hydrochloride, a clinically relevant interaction with single P450 inhibitors should not be expected. However, a slow acetylation phenotype or inhibition of NAT2 could lead to decreased N-acetylation and hence leading to an increased risk of side effects caused by this arylamine.

Dimethocaine Hydrochloride, a synthetic cocaine analogue

Dimethocaine Hydrochloride (DMC, larocaine, 3-diethylamino-2,2-dimethylpropyl)-4-aminobenzoate) was used in the 1930s as a local anesthetic in dentistry and ophthalmology. It also acts as dopamine-reuptake inhibitor with stimulating effects similar to those of cocaine and was, therefore, removed from the market because of these psychoactive effects. In the meantime Dimethocaine Hydrochloride attracted the attention of the worldwide drugs scene and it is distributed as a legal cocaine replacement, as so-called “new psychoactive substance” (NPS). It is even listed by the European Monitoring Centre for Drugs and Drug Addiction under the category “synthetic cocaine derivatives” and offered in numerous online shops. After consumption of DMC, consumers describe feelings of euphoria but also dyspnea and complaints of angina pectoris. The in-vivo pharmacological properties of DMC and cocaine have been compared after intraperitoneal injection into rats. Levels of dopamine (DA) and its metabolites have been measured, and Dimethocaine Hydrochloride has been shown to stimulate the reward system by elevating the concentration of DA in the nucleus accumbens. Woodward et al. have shown that DMC is similar in potency to cocaine in inhibiting DA reuptake. Wilcox et al. compared the pharmacological potency of different local anesthetics, and concluded that for inhibiting DA uptake the order of potency was cocaine?>?DMC?>?tetracaine?>?procaine?>?chloroprocaine. They also showed, in another study, that DMC increases DA levels, and is more likely than cocaine to induce self-administration by monkeys.[3]

Dimethocaine Hydrochloride is a widely distributed “l(fā)egal high” consumed as a “new psychoactive substance” (NPS) without any safety testing, for example studies of metabolism. Therefore, the purpose of this work was to study its in-vivo and in-vitro metabolism by use of liquid chromatography–(high resolution) mass spectrometry (LC–HRMSn). Dimethocaine Hydrochloride was administered to male Wistar rats (20 mg kg?1) and their urine was extracted either by solid-phase extraction after enzymatic cleavage of conjugates or by use of protein precipitation (PP). The metabolites were separated and identified by LC–HRMSn. The main phase I reactions were ester hydrolysis, deethylation, hydroxylation of the aromatic system, and a combination of these. The main phase II reaction was N-acetylation of the p-aminobenzoic acid part of the unchanged parent compound and of several phase I metabolites. The metabolites identified were then used for identification of DMC in rat urine after application of a common user’s dose. By use of GC–MS and LC–MSn standard urine-screening approaches (SUSAs), Dimethocaine Hydrochloride and its metabolites could be detected in the urine samples.

References

[1]Woodward JJ, Compton DM, Balster RL, Martin BR. In vitro and in vivo effects of cocaine and selected local anesthetics on the dopamine transporter. Eur J Pharmacol. 1995 Apr 13;277(1):7-13.

[2]Meyer MR, Lindauer C, Maurer HH. Dimethocaine, a synthetic cocaine derivative: studies on its in vitro metabolism catalyzed by P450s and NAT2. Toxicol Lett. 2014 Feb 10;225(1):139-46.

[3]Meyer MR, Lindauer C, Welter J, Maurer HH. Dimethocaine, a synthetic cocaine analogue: studies on its in-vivo metabolism and its detectability in urine by means of a rat model and liquid chromatography-linear ion-trap (high-resolution) mass spectrometry. Anal Bioanal Chem. 2014 Mar;406(7):1845-54.