Cocaine Half-life Research Articles

Effects of a recombinant humanized anti-cocaine monoclonal antibody on the metabolism and distribution of cocaine in vitro and in mice.

The anti‐cocaine monoclonal antibody, h2E2, is a candidate for treating cocaine‐use disorder. h2E2 binds to and sequesters cocaine in the plasma compartment, effectively decreasing cocaine concentrations in the brains of rats and mice. Despite the binding of cocaine to h2E2, plasma cocaine concentrations decline rapidly in rodents over time, but there was a drastic decrease in the urinary elimination of cocaine in the presence of h2E2. Since cocaine is not being renally excreted, the apparent disappearance of cocaine from the plasma must be explained by either metabolism or distribution. However, binding of cocaine to h2E2 may restrict the availability of cocaine for hydrolysis by endogenous esterases. Therefore, the antibody would be expected to extend the elimination half‐life of cocaine. In contrast, previous studies reported h2E2 as having no effect on the rate of cocaine clearance. It is important to examine the ultimate clearance of the cocaine to ascertain its half‐life and potential for re‐intoxication. Therefore, we investigated the effects of h2E2 on cocaine hydrolysis in vitro and on cocaine metabolism and disposition in vivo over a 6‐h time course. The spontaneous and enzyme‐mediated in vitro hydrolysis of cocaine was drastically decreased in the presence of h2E2 in vitro. Additionally, in mice, h2E2 significantly increased the distribution and elimination half‐lives of cocaine relative to vehicle controls over an extended time course. Therefore, we concluded that h2E2 slowing the distribution and elimination of cocaine is the most appropriate explanation for the initial disappearance of cocaine from the plasma in vivo.

Effects of a Recombinant Humanized Anti‐Cocaine Monoclonal Antibody on the Metabolism and Distribution of Cocaine In Vitro and in Mice

The anti‐cocaine monoclonal antibody (mAb), h2E2, is a candidate for treating cocaine‐use disorder. h2E2 binds to and sequesters cocaine in the plasma compartment, effectively decreasing cocaine concentrations in the brains of rats and mice. Despite the binding of cocaine to h2E2, plasma cocaine concentrations decline rapidly in rodents over time, but there was a drastic decrease in the urinary elimination of cocaine in the presence of h2E2. Since cocaine is not being renally excreted, the apparent disappearance of cocaine from the plasma must be explained by either metabolism or distribution. However, binding of cocaine to h2E2 may restrict the availability of cocaine for hydrolysis by endogenous esterases. Therefore, the antibody would be expected to extend the elimination half‐life of cocaine. In contrast, previous studies reported h2E2 as having no effect on the rate of cocaine clearance. It is important to examine the ultimate clearance of the cocaine to ascertain half‐life and potential for re‐intoxication. Therefore, we investigated the effects of h2E2 on cocaine hydrolysis in vitro and on cocaine metabolism and disposition in vivo over a six‐hour time course. The spontaneous and enzyme‐mediated in vitro hydrolysis of cocaine was drastically decreased in the presence of h2E2 in vitro. Additionally, in mice, h2E2 significantly increased the distribution and elimination half‐lives of cocaine relative to vehicle controls over an extended time course. Therefore, we concluded that h2E2 slowing the distribution and elimination of cocaine is the most appropriate explanation for the initial disappearance of cocaine from the plasma in vivo.

Human Carboxylesterase 2 in Cocaine Metabolism

Increased hydrolysis of cocaine to nontoxic compounds is a promising way to prevent cocaine-induced toxicity. However, the short half-life of cocaine in the blood and the rapid conversion in the body to the hydrolysis-resistant metabolite benzoylecgonine, limits the therapeutic potential of serum proteins. Therefore, hydrolysis by tissue-specific hydrolases that do not generate benzoylecgonine deserves further investigation. Here, we report for the first time the mechanism of cocaine hydrolysis by the human carboxylesterase 2. We have combined conventional and accelerated Molecular Dynamics, which allowed us to identify the structural motions of the α1 and α10’ helices that act as a putative lid. Quantum Mechanics/Molecular Mechanics calculations on the full cycle showed that the rate-limiting step is the formation of benzoic acid (deacylation step) with a ΔG of 19.5 kcal mol−1 when calculated with B3LYP-D3/6–311++G(d,p). This ΔG value is in close conformity with the experimental value of 19.7 kcal mol−1.

Ontogeny of cocaine-induced behaviors and cocaine pharmacokinetics in male and female neonatal, preweanling, and adult rats.

Ontogenetic differences in the behavioral responsiveness to cocaine have often been attributed to the maturation of dopaminergic elements (e.g., dopamine transporters, D2High receptors, receptor coupling, etc.). The purpose of this study was to determine whether ontogenetic changes in cocaine pharmacokinetics might contribute to age-dependent differences in behavioral responsiveness. Male and female neonatal (PD 5), preweanling (PD 10 and PD 20), and adult (PD 70) rats were injected (IP) with cocaine or saline and various behaviors (e.g., locomotor activity, forelimb paddle, vertical activity, head-down sniffing, etc.) were measured for 90min. In a separate experiment, the dorsal striata of young and adult rats were removed at 10 time points (0-210min) after IP cocaine administration. Peak cocaine values, cocaine half-life, and dopamine levels were determined using HPLC. When converted to percent of saline controls, PD 5 and PD 10 rats were generally more sensitive to cocaine than older rats, but this effect varied according to the behavior being assessed. Peak cocaine values did not differ according to age or sex, but cocaine half-life in brain was approximately 2 times longer in PD 5 and PD 10 rats than adults. Cocaine pharmacokinetics did not differ between PD 20 and PD 70 rats. Differences in the cocaine-induced behavioral responsiveness of very young rats (PD 5 and PD 10) and adults may be attributable, at least in part, to pharmacokinetic factors; whereas, age-dependent behavioral differences between the late preweanling period and adulthood cannot readily be ascribed to cocaine pharmacokinetics.

Oral fluid cocaine and benzoylecgonine concentrations following controlled intravenous cocaine administration

Limited oral fluid (OF) pharmacokinetic data collected with commercially available collection devices after controlled cocaine administration hinder OF result interpretations. Ten cocaine-using adults provided OF, collected with Oral-Eze® (OE) and StatSure Saliva Sampler™ (SS) devices, an hour prior to and up to 69h after 25mg intravenous (IV) cocaine administration. Cocaine and benzoylecgonine (BE) were quantified by a validated 2D-GC-MS method. Large inter-subject variability was observed. Cocaine was detected in OF in the first 0.17h sample after IV administration, with much more rapid elimination than BE. OE observed Cmax median (range) concentrations were 932 (394–1574)μg/L for cocaine and 248 (96.9–953)μg/L for BE. SS observed cocaine and BE Cmax median (range) concentrations trended lower at 732 (83.3–1892)μg/L and 360 (77.2–836)μg/L, respectively. OE and SS cocaine OF detection times were 12.5 and 6.5h and for BE 30.5 and 28.0h, respectively at 1μg/L. There were no significant pharmacokinetic differences between OE and SS OF collection devices, except cocaine half-life was significantly shorter in SS OF specimens. This difference could be attributed to differences in stabilizing buffers present in OF collection devices, which may affect cocaine stability in OF specimens, or decreased recovery from collection pads. Both OE and SS OF collection devices were effective in monitoring cocaine and metabolite concentrations with similar detection windows. Furthermore, we demonstrated that different confirmatory OF cutoffs can be selected to produce shorter or longer cocaine and metabolite detection windows to address specific needs of clinical and forensic drug testing programs.

Pharmacokinetics of disappearance of cocaine from hair after discontinuation of drug use

Methods that employ detection of drugs of abuse in hair are important for monitoring compliance with drug abstinence. Understanding the mechanisms and timeline of drug disappearance from hair is critical for clinical and forensic application of hair testing. We aimed to evaluate the kinetics of disappearance of cocaine and its metabolite, benzoylecgonine (BE), from hair after discontinuation of drug use. Methods The Motherisk laboratory at the Hospital for Sick Children in Toronto routinely receives hair samples for toxicology analysis. Cocaine and BE hair results were obtained from the Motherisk Database for calculation of half-life of these compounds in hair. Subjects were included in the study if they had gradually decreasing concentrations of cocaine and/or BE in sequential hair samples, with higher levels in the 1–3 cm distal segments (i.e. earlier in time) and low or non-measurable levels in the segment closest to the scalp (i.e. closer to the date of sampling). Elimination half-life of cocaine and BE in hair was calculated using standard kinetics calculations. The study was anonymous, and received ethics approval by the Ethics Review Board of our institution. Results 137 subjects met the inclusion criteria for the study. The median half-life of cocaine in hair was 1.5 months (95% CI 1.2–1.8) in females and 1.5 months (95% CI 1.1–1.8) in males. The median half-life of BE was 1.5 months (95% CI 1.1–2) in females and 1.5 months (95% CI 0.8–1.8) in males. Half lives of cocaine or BE were not statistically different between males and females (Mann–Whitney U-test; P = 0.93 for cocaine, P = 0.99 for BE). Half lives of cocaine and BE were strongly correlated (Spearman rank rho = 0.73; P < 0.001). Conclusion Cocaine and BE could be detected in hair of former drug users for several months after abstinence. The calculated half-life of over 1 month for cocaine implies that, assuming first order elimination, approximately 3–4 months have to pass for hair testing to become negative in the segment proximal to the scalp. This finding should be incorporated in interpreting compliance with abstinence of former drug users, and suggests that caution has to be exerted when evaluating potential breaches of abstinence.

Activation of Endothelial Cells by Cocaine.

Cocaine use is associated with cardiac ischemia, sudden death, and stroke in patients with few additional risk factors for cardiovascular disease. Despite the very short halflife of cocaine in circulation (less than 1 hr), the risk of these cardiovascular events remains elevated for up to one week after cocaine ingestion. Pathologic findings following cocaine-associated cardiac events include platelet-rich microthrombi and accelerated atherosclerosis. In addition, observational studies have identified increased levels of von Willebrand Factor (VWF) at steady state in plasma of chronic cocaine abusers, which increase following cocaine challenge. Plasma VWF is derived primarily from endothelial cells, with a smaller contribution from platelets. Thus, cocaine may activate platelets and/or endothelial cells contributing to tissue ischemia, in addition to its well-known effects of vasoconstriction and increased tissue metabolic demand. When first released, VWF is hyperadhesive and capable of spontaneously binding platelets. This form, termed ultralarge VWF (ULVWF) because some of the multimers are extremely large, is normally rapidly processed by the metalloprotease ADAMTS13 into the smaller, less reactive forms usually found in plasma. Failure of this process, due to congenital or acquired deficiency of ADAMTS13, leads to accumulation of ULVWF, and in its most severe form produces the devastating microthrombotic disease, thrombotic thrombocytopenic purpura (TTP). We hypothesized that cocaine could activate the endothelium to secrete ULVWF, and thus promote thrombosis and/or vaso-occlusion. To test this possibility, we exposed cultured human umbilical vein endothelial cells (HUVEC) to various concentrations of cocaine. We found that 0.5 mg/ml cocaine, a concentration comparable to peak blood levels detectable in cocaine abusers, induced secretion of ULVWF almost as efficiently as did 25 μM histamine, a potent ULVWF secretogogue. If platelets were then perfused over the endothelium in a parallel plate flow chamber at 2.5 dyne/cm2, they spontaneously formed beads-on-a-string structures on the attached ULVWF strands. We also evaluated the ability of cocaine metabolites to induce ULVWF release. Neither norcocaine nor ecoginine methyl ester (both minor metabolites) stimulated ULVWF release. However, benzoylecgonine, the major cocaine metabolite (detectable in plasma up to 6–8 days after cocaine use) potently induced ULVWF release, as detected by the formation of ULVWF strings and accumulation of higher molecular weight VWF multimers in the HUVEC supernatant on non-reducing agarose gels. These effects of cocaine are likely to contribute to cardiovascular syndromes associated with cocaine use, including the triggering of vasoocclusive crises in sickle cell anemia, and may explain the observed association of cocaine use with TTP. Furthermore, these results suggest that clinical management of cocaineinduced ischemia or vaso-occlusion may benefit from therapies aimed at disrupting ULVWF.

Visualizing viral transduction of a cocaine-hydrolyzing, human butyrylcholinesterase in rats

Human plasma butyrylcholinesterase (BChE) is essential for cocaine detoxification even though its catalytic efficiency for that substrate is relatively poor. Site-directed mutagenesis of this protein has recently been used to obtain much-improved cocaine esterases, one of which we designate CocE. We previously showed that adenoviral transduction of such esterases caused up to 50,000-fold increases in circulating cocaine hydrolase activity, led to drastically shortened cocaine half-life, and blunted the cardiovascular responses to cocaine in rats. In those experiments, gene transduction of cocaine esterase was sustained at high levels for up to 1 week but then declined steeply. Our eventual goal is to use long-term esterase expression as a means of reducing drug reward and extinguishing intake in models of cocaine-addiction. Therefore, we investigated the site of enzyme transduction for clues to the local reactions that may limit the duration of CocE expression. Histological and immunohistochemical observations demonstrated that hepatocytes were the primary focus for transduction of modified human BChE. Rats were administered 2.2 × 10 10 plaque forming units of a replication-incompetent, type-5 adenoviral vector incorporating CocE cDNA. Within days the livers showed intense thiocholine staining for BChE activity. Selective immunohistochemistry for human BChE proved that this activity represented CocE transgene. By 5 days, however, pockets of mononuclear cells had invaded the hepatic parenchyma, and a meshwork of IgM-like immunoreactivity had lined the hepatic sinusoids. These phenomena probably represent early responses of the immune system, either to the transduced CocE or to the hepatocytes producing this protein.

The Effect of Lovastatin and Thioridazine on the Degradation of Cocaine in Human Serum In Vitro

The effect of lovastatin and thioridazine on the degradation of cocaine in human serum was studied by incubating therapeutic and toxic concentrations of either drug with cocaine in human serum at 37 degrees C. Without these other drugs, cocaine concentrations decreased by an average of 88% in 120 minutes. Lovastatin at all concentrations studied showed a negligible effect on the degradation of cocaine in human serum and did not alter the pseudocholinesterase activity of the serum. In contrast, in the presence of a therapeutic concentration of thioridazine, cocaine concentrations decreased by only 71% during this period of time, and, with a toxic concentration of thioridazine, cocaine concentrations decreased by only 55%. Thioridazine suppressed the pseudocholinesterase activity of human serum by up to 19% during the 120-minute incubation period, and this effect was more pronounced at higher thioridazine concentrations. These findings suggest that thioridazine may prolong the serum half-life of cocaine by inhibiting the pseudocholinesterase-mediated catabolism of cocaine to ecgonine methyl ester. This may be important in cocaine users who are treated with phenothiazines and other structurally similar drugs. They also may be of interest in cocaine-abusing patients who are treated with phenothiazines for schizophrenic disorders.

Modafinil Influences the Pharmacokinetics of Intravenous Cocaine in Healthy Cocaine-Dependent Volunteers

To determine if modafinil, a putative treatment for cocaine dependence, influences the pharmacokinetics of intravenous cocaine in otherwise healthy cocaine-dependent volunteers. Cocaine 20 or 40 mg was administered intravenously on consecutive days over 1 minute at baseline and after modafinil administration at each of two dosages of 400 and 800 mg/day for 7 days. Twelve subjects completed the clinical protocol. Compared with baseline, the cocaine peak plasma concentration was decreased after both the 20 and 40 mg cocaine infusions, but the reduction was only statistically significant after the 40 mg cocaine infusion (p < 0.01 after modafinil 400 mg/day; p < 0.05 after modafinil 800 mg/day). The area under the cocaine plasma concentration-time curve from 0 to 180 minutes (AUC180) was significantly decreased by modafinil administration (p < 0.01 and p < 0.001 for modafinil 400 and 800 mg/day, respectively, for the cocaine 20mg dose; p < 0.001 for the cocaine 40 mg dose at both modafinil levels). There were no significant changes in total AUC, clearance or elimination half-life of cocaine. This study did not find evidence for a harmful pharmacokinetic interaction between modafinil and cocaine. In contrast, long-term administration of modafinil significantly decreased systemic exposure to cocaine during the first 180 minutes following intravenous cocaine administration.

Gene Transfer of Cocaine Hydrolase Suppresses Cardiovascular Responses to Cocaine in Rats

We previously found that injection of a cocaine hydrolase (CocE) engineered from human butyrylcholinesterase will transiently accelerate cocaine metabolism in rats while reducing physiological and behavioral responses. To investigate more extended therapeutic effects, CocE cDNA was incorporated into a replication-incompetent type-5 adenoviral vector with a cytomegalovirus promoter. In rats dosed with this agent (2.2 x 10(9) plaque-forming units), the time course of expression was characterized by reverse transcription polymerase chain reaction for CocE mRNA and by radiometric assay for enzyme activity. Liver and plasma showed comparable expression, beginning 2 days after vector administration and peaking between 5 and 7 days. Plasma CocE content was up to 100 mU/ml, with total cocaine hydrolyzing activity 3000-fold greater than in "empty vector" or untreated controls. This level of expression approximated that found immediately after i.v. injection of purified hydrolase, 3 mg/kg, a dose that shortened cocaine halflife and blunted cardiovascular effects. Sucrose density gradient analysis showed that 96% of the circulating CocE activity was associated with tetrameric enzyme forms, expected to be stable in vivo. Consistent with this expectation, CocE from vector-treated rats showed a plasma t(1/2) of 33 h when reinjected into naive rats. Transduction of another mutant butyrylcholinesterase, Applied Molecular Evolution mutant 359 (AME(359)), caused plasma cocaine hydrolase activity to rise 50,000-fold. At the point of peak AME(359) expression, cocaine was cleared from the blood too rapidly for accurate measurement, and pressor responses to the injection of drug were greatly impaired.

Gender differences in cocaine pharmacokinetics in CF-1 mice

Hepatocellular damage is thought to occur as a result of cytochrome P450-mediated oxidation of cocaine to norcocaine (NC), a precursor of the hepatotoxic nitrosonium ion. However, this damage occurs only in male mice, with females exhibiting minimal biochemical and histological signs of hepatocellular stress. The objective of this study was to determine the plasma time course and tissue disposition of cocaine and its metabolites to further investigate the role that metabolism may play in the gender difference observed. Male and female CF-1 mice were orally administered 20 mg/kg cocaine hydrochloride once daily for 7 days. Blood samples were withdrawn at various time points post-injection and analyzed for cocaine and its metabolites benzoylecgonine (BE), norcocaine, ecgonine methyl ester (EME), and ecgonine (E). In addition, tissue concentrations of cocaine and its metabolites were determined in liver, heart, brain, and kidney tissue. The results demonstrated that the plasma elimination half-life of cocaine is nearly three times longer in males versus females. Non-hepatotoxic hydrolysis metabolites BE, EME, and E were higher in female tissues while norcocaine was detected in tissues of male animals only. This study revealed that differences in cocaine pharmacokinetics and the resultant differences in the biodisposition of cocaine and its metabolites in tissues contribute to the mechanism of gender difference seen in cocaine hepatotoxicity.

Cocaine metabolism accelerated by a re-engineered human butyrylcholinesterase.

Plasma butyrylcholinesterase (BChE) is important in the metabolism of cocaine, but natural human BChE has limited therapeutic potential for detoxication because of low catalytic efficiency with cocaine. Here we report pharmacokinetics of cocaine in rats treated with A328W/Y332A BChE, an excellent cocaine hydrolase designed with the aid of molecular modeling. Compared with wild-type BChE, this enzyme hydrolyzes cocaine with 40-fold improved k(cat) (154 min(-1) versus 4.1 min(-1)) and only slightly increased K(M) (18 microM versus 4.5 microM). In rats given this hydrolase (3 mg/kg i.v.) 10 min before cocaine challenge (6.8 mg/kg i.v.), cocaine half-life was reduced from 52 min to 18 min. Mirroring the reductions of plasma cocaine were large increases in benzoic acid, a product of BChE-mediated cocaine hydrolysis. All other pharmacokinetic parameters confirmed a large, dose-dependent acceleration of cocaine removal by the injected cocaine hydrolase. These results show that A328W/Y332A, an efficient cocaine hydrolase in vivo as well as in vitro, might promote cocaine detoxication in a clinical setting.

Satiety threshold during maintained cocaine self-administration in outbred mice.

Male Swiss Webster mice maintained cocaine self-administration in a regular and dose-dependent manner. These characteristics made it possible to apply the satiety threshold model of drug self-administration developed recently for cocaine self-administration in rats. Non-linear regression analysis revealed that cocaine satiety threshold was 1.3 +/- 0.6 mg/kg and the functional half-life of the cocaine was 8.1 +/- 2.2 min. Whether the self-administration of cocaine was maintained by lever presses or nose pokes did not influence the inter-injection intervals. The results are consistent with the pharmacological model of maintained cocaine self-administration. The ability to determine addiction-relevant phenotypes (the satiety threshold and functional half-life of cocaine) in inbred strains of mice may help to identify the genetic determinants of cocaine self-administration behavior.

Cocaine and metabolite elimination patterns in chronic cocaine users during cessation: plasma and saliva analysis.

Several reports suggest a prolonged elimination of cocaine and metabolites after chronic use compared with single or occasional use. This study was designed to measure the half-lives of cocaine in plasma and saliva of individuals who consumed cocaine on a frequent basis. The disposition and elimination patterns of cocaine and metabolites in the body fluids of chronic high-dose cocaine users during acute cessation of use were investigated. Plasma and saliva specimens were collected over a 12-h period during cessation and analyzed by gas chromatography-mass spectrometry. Pharmacokinetic parameters were derived by noncompartmental analysis of plasma and saliva data. Results indicated a cocaine terminal T(1/2) of 3.8 h in plasma and 7.9 h in saliva. The terminal T(1/2) of benzoylecgonine was 6.6 h in plasma and 9.2 h in saliva. Compared with prior studies of acute low-dose cocaine administration, these findings suggest that cocaine's half-life is longer in active street users than in occasional users though the half-life of its main metabolite benzoylecgonine remains similar (as do cocaine saliva-to-plasma ratios). Thus, regular use of cocaine appears to alter the disposition and elimination of cocaine when compared to single or occasional use.

Cocaine metabolism in hyperthermic patients with excited delirium

The half-life of cocaine in clinical experiments has been reported to range from 60 to 90 min. It has been previously suggested that elevated temperature may accelerate the metabolism of cocaine. However, there is no clinical data to indicate the presence of hyperthermia like that seen in excited delirium alters the half-life of cocaine. We report the results of half-life determinations from serial cocaine concentrations in two patients with excited delirium. Both patients presented to the emergency department with classic findings of excited delirium that included hyperthermia, agitation, and cardiovascular aberrations. One patient died despite aggressive therapeutic intervention. Cocaine and metabolite concentrations were determined by an extractive alkylation mass spectrometry procedure. Presenting cocaine concentrations in patient 1 and patient 2 were 0.387 and 0.266 mg/L respectively. Results from pharmacokinetic modeling of the serial concentrations show that the half-life of cocaine was not significantly accelerated, despite the presence of hyperthermia. Data from these two cases provide further evidence that catastrophic reactions to cocaine are independent of amount or route of administration, and that the metabolism of cocaine, at least in these patients, was not altered by hyperthermia.

Satiety threshold: a quantitative model of maintained cocaine self-administration

The intervals between self-injections of cocaine by rats are defined by an equation that contains only three parameters: the dose of cocaine administered, the elimination half-life of cocaine, and an amount of cocaine in the body, which we have termed the cocaine satiety threshold. This latter parameter is defined as the maximal level of cocaine at which the probability of self-administration approximates one and above which the probability of self-administration is low. The mathematical model generated mean values for the satiety threshold and the functional elimination half-life of cocaine of approximately 1.7 mg/kg (i.v.) and 8.2 min, respectively. Therefore, the simple equations presented here permit the measurement of the pharmacokinetics and pharmacodynamics of cocaine using self-administration behavior as a bioassay. Our satiety model predicts that when cocaine levels are maintained above the satiety threshold, rats would not self-administer cocaine. The elimination rate of cocaine at the satiety threshold was calculated to be approximately 2 μg kg −1 s −1. Therefore, an infusion of cocaine at this rate should maintain cocaine levels fractionally above the satiety threshold. A continuous infusion of cocaine at this rate prevented cocaine self-administration for the duration of the infusion, thereby confirming the validity of the satiety model. These equations provide a quantitative description of cocaine self-administration and contain no subjective terms, implying that concepts such as “craving”, drug “wanting” and “liking” and “reinforcement”, used in psychologically oriented models, are not necessary for descriptions of this behavior in rats.

The influence of plasma butyrylcholinesterase concentration on the in vitro hydrolysis of cocaine in human plasma.

In humans, the plasma enzyme butyrylcholinesterase, BChE (EC 3.1.1.8), mediates the in vivo plasma hydrolysis of cocaine to the pharmacologically inactive metabolite ecgonine methyl ester, EME. This enzyme has been purified from human plasma to investigate the potential as a treatment for cocaine intoxication. Cocaine (2.1 micrograms mL-1) was incubated in plasma with a BChE concentration in the normal range (3.02 micrograms mL-1) and in plasma with enhanced BChE concentrations of 9.14, 20.8 and 37.8 micrograms mL-1, respectively for time periods up to 120 min. Cocaine and the hydrolytic products, ecgonine methyl ester and ecgonine, were quantified simultaneously by gas chromatography-mass spectrometry (GC-MS). The enhancement of plasma BChE concentration resulted in a dramatic increase in the rate of hydrolysis of cocaine. There was a stoichimetric conversion of cocaine to the inactive hydrolysis product, ecgonine methyl ester. Accordingly, the half-life of cocaine in plasma decreased significantly with enhanced BChE concentration. At plasma BChE concentrations of 3.02, 9.14, 20.8 and 37.8 micrograms mL-1, half-life values of 116, 35.8, 21.4 and 9.0 min, respectively were observed. The marked reduction in cocaine half-life provides evidence supporting the potential therapeutic use of BChE for the treatment of cocaine intoxication.

Pharmacokinetic Considerations for the Use of the Ovine Model in Cocaine Research

Professor of Anesthesiology and Obstetrics/Gynecology (Morishima); Assistant Professor of Anesthesiology (Whittington), Departments of Anesthesiology and Obstetrics and Gynecology, College of Physicians and Surgeons, Columbia University, New York, New York 10032.Supported, in part, by NIDA grants RO1 DA06648 and RO1 DA07588 and NIH grant T32 GM08464.To the Editor:-In a recent study by Bernards et al., the authors report that chronic cocaine administration reversibly increases isoflurane minimum alveolar concentration in sheep. [1]This study illustrates the dilemma often encountered by researchers when trying to answer a clinically relevant question with a less than ideal animal model.The authors do acknowledge that no animal metabolizes cocaine in a manner identical to humans, and that the clearance of cocaine in sheep is significantly more rapid than in humans. However, there are other pharmacokinetic considerations that were not mentioned by the authors that are essential whenever using this species for cocaine and drug interaction research. Specifically, the metabolism of cocaine in sheep is quite different from its metabolism in humans and other commonly used laboratory animals. [2]The primary metabolite of cocaine in sheep is ecognine methyl ester, whereas in other species (e.g., human, subhuman primate, rat, guinea pig), benzoylecgonine is the major metabolite. These metabolites are cleared more slowly than the parent compound, and, consequently, would be expected to accumulate in a chronic situation. Because it is unclear as to whether the accumulation of these metabolites alters minimum alveolar concentration, ideally, an animal model in which benzoylecgonine, rather than ecognine methyl ester, would be expected to accumulate would be preferable.In addition, the investigators used a subcutaneous osmotic pump in this study to deliver cocaine continuously so as to maintain a steady, low background plasma cocaine concentration, ranging from 15 to 69 ng/ml. Although the authors state that this background cocaine infusion was intended to mimic heavy cocaine use, the pharmacodynamic rationale behind this “chronic exposure” with such pharmacologically ineffective low plasma concentrations is unclear. In addition, because of the drug's short elimination half-life in this species, a cocaine infusion rate of 0.4 mg/kg/min over 10 min without an initial loading dose would not produce a pattern of plasma drug concentrations or hemodynamic responses similar to that observed in human “binge” abusers. As previously demonstrated in our continuous intravenous administration study in sheep, the plasma cocaine concentration decreased rapidly when the cocaine infusion was terminated, whereas the benzoylecgonine concentration continued to increase (note that ecognine methyl ester was not determined in this early study). The elimination half-life of cocaine in this study was 5 min.* In addition, in this species, “binge” cocaine doses at 1-h intervals probably do not mimic one important aspect of human “binge” abuse, namely cocaine accumulation. It is unfortunate that the authors did not measure cocaine concentrations during the “binge,” because this might have provided more information in regard to drug accumulation.Based on this knowledge, the investigators similarly should have kept plasma cocaine concentrations in a range occurring in human “binge” abusers by administering a programmed constant intravenous infusion at a high initial rate followed by a gradual decrease in rate.Hisayo O. Morishima, M.D., Ph.D.Professor of Anesthesiology and Obstetrics/GynecologyRobert A. Whittington, M.D.Assistant Professor of AnesthesiologyDepartments of Anesthesiology and Obstetrics and Gynecology College of Physicians and Surgeons; Columbia University, New York, New York 10032(Accepted for publication December 12, 1996.)*Morishima HO, Chao CR, Matsuo M, Abe Y, Cooper TB: Placental transfer and disposition of cocaine in the pregnant sheep (abstract). Anesthesiology 1992; 77:A1038.

Plasma butyrylcholinesterase activity and cocaine half-life differ significantly in rhesus and squirrel monkeys

In vitro studies have implicated butyrylcholinesterase (Bche, B.C.3.1.1.8) as the major enzyme for metabolizing cocaine in humans, but little is known about endogenous BChE activity in monkeys and other animals often used in preclinical studies of cocaine. We compared BChE activity in 18 rhesus and 11 squirrel monkeys, using the colorimetric method of Ellman with butyrylthiocholine as substrate, and in vitro cocaine half-life in pooled plasma samples measuring cocaine concentrations over 60 minutes by GC-MS. Rhesus monkeys had a significantly higher plasma BChE activity than squirrel monkeys (8.2 ± 0.5 U/L vs. 2.8 ± 0.5 U/L), and a three-fold shorter in vitro cocaine half-life (20.1 min vs. 60.2 min). BChE activity in rhesus monkeys was comparable to the activity reported in humans. There was no significant influence of age, weight, or prior cocaine exposure. These results indicate that BChE level can vary between species of non-human primates, a factor that should be taken into account when studying drugs such as cocaine which are metabolized by BChE.