Genetic variability in nicotine metabolism. Conclusions. . to extrahepatic metabolism; compared with the liver, P isozymes in. Metabolism Hepatic Metabolism Extrahepatic Metabolism Metabolism of Cannabidiol Elimination Terminal Elimination Half-Lives of. No significant differences in metabolism between men and women have been Extrahepatic Metabolism Other tissues, including brain, intestine, and lung.
Metabolism 2.3.2. Extrahepatic
THC produced schizophrenia-like positive and negative symptoms and euphoria, and altered aspects of cognitive function. Plasma cortisol concentrations were not affected. THC produced a broad range of transient symptoms, behaviors, and cognitive deficits in healthy individuals that resembled endogenous psychoses. The investigators suggested that brain-cannabinoid-receptor function could be an important factor in the pathophysiology of psychotic disorders. Cannabidiol CBD is a natural, non-psychoactive [ 49 ][ 50 ] constituent of Cannabis sativa , but possesses pharmacological activity, which is explored for therapeutic applications.
CBD has been reported to be neuroprotective [ 51 ], analgesic [ 37 ][ 38 ][ 52 ], sedating [ 37 ][ 38 ][ 53 ][ 54 ], anti-emetic [ 54 ], anti-spasmodic [ 55 ], and anti-inflammatory [ 56 ]. In addition, it has been reported that CBD blocks anxiety produced by THC [ 57 ], and may be useful in the treatment of autoimmune diseases [ 53 ]. These potential therapeutic applications alone warrant investigation of CBD pharmacokinetics.
Further, the controversy over whether CBD alters the pharmacokinetics of THC in a clinically significant manner needs to be resolved [ 58 ][ 59 ]. Recently, Nadulski et al. The authors suggest that identification and quantification of CBD could be an additional proof of cannabis exposure and could improve interpretation of THC effects considering the potential ability of CBD to modify THC effects.
When comparing sublingual administration of THC 25 mg alone vs. The only statistically significant difference was in the time of maximum THC concentration. All three analytes were detectable ca. High intra- and inter-subject variability was noted.
THC Plasma concentrations decrease rapidly after the end of smoking due to rapid distribution into tissues and metabolism in the liver. THC is highly lipophilic and initially taken up by tissues that are highly perfused, such as the lung, heart, brain, and liver.
Tracer doses of radioactive THC documented the large volume of distribution of THC and its slow elimination from body stores. In animals, after intravenous administration of labeled THC, higher levels of radioactivity were present in lung than in other tissues [ 64 ]. Studies of the distribution of THC into brain are especially important for understanding the relationships between THC dose and behavioral effects. Plasma concentrations were of similar magnitude to those measured in men exposed to marihuana smoke.
Kreuz and Axelrod were the first to describe the persistent and preferential retention of radiolabeled THC in neutral fat after multiple doses, in contrast to limited retention in brain [ 66 ].
The ratio of fat to brain THC concentration was approximately With prolonged drug exposure, THC concentrates in human fat, being retained for extended periods of time [ 69 ]. In addition, these investigators found that tolerance to the behavioral effects of THC in pigeons was not due to decreased uptake of cannabinoids into the brain.
Tolerance also was evaluated in humans. Pharmacokinetic changes after chronic oral THC administration could not account for observed behavioral and physiologic tolerance, suggesting rather that tolerance was due to pharmacodynamic adaptation. Adams and Martin studied the THC dose required to induce pharmacological effects in humans [ 73 ]. In a recent, highly interesting study, Mura et al. There was no correlation between blood and brain concentrations; brain levels were always higher than blood levels, and in three cases measurable drug concentrations remained in the brain, when no longer detectable in the blood.
Blood concentrations were lower than in the two-paired brains. The authors postulate that long-lasting effects of cannabis during abstinence in heavy users may be due to residual THC and OH-THC concentrations in the brain. Storage of THC after chronic exposure could also contribute to observed toxicities in other tissues. After single intramuscular administration of radioactive THC in rats, only 0.
The authors suggest that the blood—brain and blood—testicular barriers limit storage of THC in brain and testis during acute exposure; however, during THC chronic exposure, pharmacokinetic mechanisms are insufficient to prevent accumulation of THC in tissues, with subsequent deregulation of cellular processes, including apoptosis of spermatogenic cells. In one of the latest investigations on THC distribution in tissues, the large-white-pig model was selected due to similarities with humans in drug biotransformation, including enzymes and isoenzymes of drug biotransformation, size, feeding patterns, digestive physiology, dietary habits, kidney structure and function, pulmonary vascular bed structure, coronary-artery distribution, propensity to obesity, respiratory rates, and tidal volume [ 75 ].
THC Plasma pharmacokinetics was found to be similar to those in humans. At 30 min, high THC concentrations were noted in lung, kidney, liver, and heart, with comparable elimination kinetics in kidney, heart, spleen, muscle, and lung as observed in blood.
The fastest THC elimination was noted in liver, where concentrations fell below measurable levels by 6 h. Mean brain concentration was approximately twice the blood concentration at 30 min, with highest levels in the cerebellum, and occipital and frontal cortex, and lowest concentrations in the medulla oblongata.
THC Concentrations decreased in brain tissue slower than in blood. The slowest THC elimination was observed for fat tissue, where THC was still present at substantial concentrations 24 h later.
The authors suggest that the prolonged retention of THC in brain and fat in heavy cannabis users is responsible for the prolonged detection of THC-COOH in urine and cannabis-related flashbacks. The author of this review hypothesizes that this residual THC may also contribute to cognitive deficits noted early during abstinence in chronic cannabis users. THC accumulation in the lung occurs because of high exposure from cannabis smoke, extensive perfusion of the lung, and high uptake of basic compounds in lung tissue.
Lung tissue is readily available during postmortem analysis, and would be a good matrix for investigation of cannabis exposure. Other possible explanations include lower plasma-protein binding of OH-THC or enhanced crossing of the blood—brain barrier by the hydroxylated metabolite. The distribution volume V d of THC is large, ca. More recently, with the benefit of advanced analytical techniques, the steady state V d value of THC was estimated to be 3. THC-COOH was found to be far less lipophilic than the parent drug, whose partition coefficient P value at neutral pH has been measured at 6, or higher , and more lipophilic than the glucuronide [ 78 ].
The fraction of THC glucuronide present in blood after different routes of administration has not been adequately resolved, but, recently, the partition coefficient of this compound indicated an unexpectedly high lipophilicity, ca. THC rapidly crosses the placenta, although concentrations were lower in canine and ovine fetal blood and tissues than in maternal plasma and tissues [ 79 ]. Blackard and Tennes reported that THC in cord blood was three to six times less than in maternal blood [ 82 ].
Transfer of THC to the fetus was greater in early pregnancy. THC also concentrates into breast milk from maternal plasma due to its high lipophilicity [ 83 ][ 84 ]. THC Concentration in breast milk was 8. They also documented that THC can be metabolized in the brain. Conjugation with glucuronic acid is a common Phase-II reaction.
Side-chain hydroxylation was common in all three species. THC Concentrations accumulated in the liver, lung, heart, and spleen. Hydroxylation of THC at C 9 by the hepatic CYP enzyme system leads to production of the equipotent metabolite OH-THC [ 89 ][ 90 ], originally thought by early investigators to be the true psychoactive analyte [ 64 ]. More than THC metabolites, including di- and trihydroxy compounds, ketones, aldehydes, and carboxylic acids, have been identified [ 21 ][ 70 ][ 91 ].
Less than fivefold variability in 2C9 rates of activity was observed, while much higher variability was noted for the 3A enzyme. THC-COOH and its glucuronide conjugate are the major end products of biotransformation in most species, including man [ 91 ][ 95 ]. The phenolic OH group may be a target as well.
Addition of the glucuronide group improves water solubility, facilitating excretion, but renal clearance of these polar metabolites is low due to extensive protein binding [ 72 ]. No significant differences in metabolism between men and women have been reported [ 27 ]. After the initial distribution phase, the rate-limiting step in the metabolism of THC is its redistribution from lipid depots into blood [ 98 ].
However, later studies did not corroborate this finding [ 8 ][ 91 ]. More than 30 metabolites of CBD were identified in urine, with hydroxylation of the 7-Me group and subsequent oxidation to the corresponding carboxylic acid as the main metabolic route, in analogy to THC [ ]. Other tissues, including brain, intestine, and lung, may contribute to the metabolism of THC, although alternate hydroxylation pathways may be more prominent [ 86 ][ - ].
An extrahepatic metabolic site should be suspected whenever total body clearance exceeds blood flow to the liver, or when severe liver dysfunction does not affect metabolic clearance [ ]. Within the brain, higher concentrations of CYP enzymes are found in the brain stem and cerebellum [ ]. Metabolism of THC by fresh biopsies of human intestinal mucosa yielded polar hydroxylated metabolites that directly correlated with time and amount of intestinal tissue [ ].
In a study of the metabolism of THC in the brains of mice, rats, guinea pigs, and rabbits, Watanabe et al. Hydroxylation of C 4 of the pentyl side chain produced the most common THC metabolite in the brains of these animals, similar to THC metabolites produced in the lung.
These metabolites are pharmacologically active, but their relative activity is unknown. CBD Metabolism is similar to that of THC, with primary oxidation of C 9 to the alcohol and carboxylic acid [ 8 ][ ], as well as side-chain oxidation [ 88 ][ ].
Co-administration of CBD did not significantly affect the total clearance, volume of distribution, and terminal elimination half-lives of THC metabolites.
Numerous acidic metabolites are found in the urine, many of which are conjugated with glucuronic acid to increase their water solubility. Another common problem with studying the pharmacokinetics of cannabinoids in humans is the need for highly sensitive procedures to measure low cannabinoid concentrations in the terminal phase of excretion, and the requirement for monitoring plasma concentrations over an extended period to adequately determine cannabinoid half-lives.
The slow release of THC from lipid-storage compartments and significant enterohepatic circulation contribute to a long terminal half-life of THC in plasma, reported to be greater than 4. Isotopically labeled THC and sensitive analytical procedures were used to obtain this drug half-life. No significant pharmacokinetic differences between chronic and occasional users have been substantiated [ ].
An average of This represents an average of only 0. Prior to harvesting, cannabis plant material contains little active THC. When smoked, THC carboxylic acids spontaneously decarboxylate to produce THC, with nearly complete conversion upon heating. Pyrolysis of THC during smoking destroys additional drug. Drug availability is further reduced by loss of drug in the side-stream smoke and drug remaining in the unsmoked cigarette butt.
These factors contribute to high variability in drug delivery by the smoked route. It is estimated that the systemic availability of smoked THC is ca. THC Bioavailability is reduced due to the combined effect of these factors; the actual available dose is much lower than the amount of THC and THC precursor present in the cigarette. Another factor affecting the low amount of recovered dose is measurement of a single metabolite.
Following controlled oral administration of THC in dronabinol or hemp oil, urinary cannabinoid excretion was characterized in 4, urine specimens [ ][ ]. THC Doses of 0. The two high doses 7. The availability of cannabinoid-containing foodstuffs, cannabinoid-based therapeutics, and continued abuse of oral cannabis require scientific data for the accurate interpretation of cannabinoid tests.
These data demonstrate that it is possible, but unlikely, for a urine specimen to test positive at the federally mandated cannabinoid cutoffs, following manufacturer's dosing recommendations for the ingestion of hemp oils of low THC concentration. An average of only 2. Specimen preparation for cannabinoid testing frequently includes a hydrolysis step to free cannabinoids from their glucuronide conjugates.
Alkaline hydrolysis appears to efficiently hydrolyze the ester glucuronide linkage. Mean THC concentrations in urine specimens from seven subjects, collected after each had smoked a single marijuana cigarette 3.
Using a modified analytical method with E. We found that OH-THC may be excreted in the urine of chronic cannabis users for a much longer period of time, beyond the period of pharmacodynamic effects and performance impairment. Compared to other drugs of abuse, analysis of cannabinoids presents some difficult challenges.
Complex specimen matrices, i. Care must be taken to avoid low recoveries of cannabinoids due to their high affinity to glass and plastic containers, and to alternate matrix-collection devices [ - ].
Whole-blood cannabinoid concentrations are approximately one-half the concentrations found in plasma specimens, due to the low partition coefficient of drug into erythrocytes [ 96 ][ ][ ].
THC Detection times in plasma of 3. In the latter study, the terminal half-life of THC in plasma was determined to be ca. This inactive metabolite was detected in the plasma of all subjects by 8 min after the start of smoking. The half-life of the rapid-distribution phase of THC was estimated to be 55 min over this short sampling interval. The relative percentages of free and conjugated cannabinoids in plasma after different routes of drug administration are unclear.
Even the efficacy of alkaline- and enzymatic-hydrolysis procedures to release analytes from their conjugates is not fully understood [ 24 ][ 77 ][ 93 ][ ][ ][ ][ - ]. In general, the concentrations of conjugate are believed to be lower in plasma, following intravenous or smoked administration, but may be of much greater magnitude after oral intake.
There is no indication that the glucuronide conjugates are active, although supporting data are lacking. Peak concentrations and time-to-peak concentrations varied sometimes considerably between subjects. Most THC plasma data have been collected following acute exposure; less is known of plasma THC concentrations in frequent users.
No difference in terminal half-life in frequent or infrequent users was observed. There continues to be controversy in the interpretation of cannabinoid results from blood analysis, some general concepts having wide support. It is well-established that plasma THC concentrations begin to decline prior to the time of peak effects, although it has been shown that THC effects appear rapidly after initiation of smoking [ 15 ]. Individual drug concentrations and ratios of cannabinoid metabolite to parent drug concentration have been suggested as potentially useful indicators of recent drug use [ 24 ][ ].
This is in agreement with results reported by Mason and McBay [ 96 ], and those by Huestis et al. Measurement of cannabinoid analytes with short time courses of detection e. This correlates well with the suggested concentration of plasma THC, due to the fact that THC in hemolyzed blood is approximately one-half the concentration of plasma THC [ ]. Accurate prediction of the time of cannabis exposure would provide valuable information in establishing the role of cannabis as a contributing factor to events under investigation.
Two mathematical models for the prediction of time of cannabis use from the analysis of a single plasma specimen for cannabinoids were developed [ ]. More recently, the validation of these predictive models was extended to include estimation of time of use after multiple doses of THC and at low THC concentrations 0. Some 38 cannabis users each smoked a cigarette containing 2. The predicted times of cannabis smoking, based on each model, were then compared to the actual smoking times.
The most accurate approach applied a combination of models I and II. All time estimates were correct for 77 plasma specimens, with THC concentrations of 0.
The models provide an objective, validated method for assessing the contribution of cannabis to accidents or clinical symptoms. These models also appeared to be valuable when applied to the small amount of data from published studies of oral ingestion available at the time.
Additional studies were performed to determine if the predictive models could estimate last usage after multiple oral doses, a route of administration more popular with the advent of cannabis therapies. Each of twelve subjects in one group received a single oral dose of dronabinol 10 mg of synthetic THC. In another protocol, six subjects received four different oral daily doses, divided into thirds, and administered with meals for five consecutive days.
There was a d washout period between each dosing regimen. The daily doses were 0. The actual times between ingestion of THC and blood collection spanned 0. These results provide further evidence of the usefulness of the predictive models in estimating the time of last oral THC ingestion following single or multiple doses.
Detection of cannabinoids in urine is indicative of prior cannabis exposure, but the long excretion half-life of THC-COOH in the body, especially in chronic cannabis users, makes it difficult to predict the timing of past drug use. This individual had used cannabis heavily for more than ten years. However, a naive user's urine may be found negative by immunoassay after only a few hours following smoking of a single cannabis cigarette [ ].
Assay cutoff concentrations and the sensitivity and specificity of the immunoassay affect drug-detection times. A positive urine test for cannabinoids indicates only that drug exposure has occurred.
The result does not provide information on the route of administration, the amount of drug exposure, when drug exposure occurred, or the degree of impairment. THC-COOH concentration in the first specimen after smoking is indicative of how rapidly the metabolite can appear in urine.
Thus, THC-COOH concentrations in the first urine specimen are dependent upon the relative potency of the cigarette, the elapsed time following drug administration, smoking efficiency, and individual differences in drug metabolism and excretion. The mean times of peak urine concentration were 7. Although peak concentrations appeared to be dose-related, there was a twelvefold variation between individuals. Drug detection time, or the duration of time after drug administration in which the urine of an individual tests positive for cannabinoids, is an important factor in the interpretation of urine drug results.
Detection time is dependent on pharmacological factors e. Mean detection times in urine following smoking vary considerably between subjects, even in controlled smoking studies, where cannabis dosing is standardized and smoking is computer-paced. During the terminal elimination phase, consecutive urine specimens may fluctuate between positive and negative, as THC-COOH concentrations approach the cutoff concentration.
It may be important in drug-treatment settings or in clinical trials to differentiate between new drug use and residual excretion of previously used cannabinoids. After smoking a cigarette containing 1. This had the effect of producing much longer detection times for the last positive specimen. Normalization of cannabinoid concentration to urine creatinine concentration aids in the differentiation of new from prior cannabis use, and reduces the variability of drug measurement due to urine dilution.
Due to the long half-life of drug in the body, especially in chronic cannabis users, toxicologists and practitioners are frequently asked to determine if a positive urine test represents a new episode of drug use or represents continued excretion of residual drug. Random urine specimens contain varying amounts of creatinine, depending on the degree of concentration of the urine.
Hawks first suggested creatinine normalization of urine test results to account for variations in urine volume in the bladder [ ]. Whereas urine volume is highly variable due to changes in liquid, salt, and protein intake, exercise, and age, creatinine excretion is much more stable.
If the increase is greater than or equal to the threshold selected, then new use is predicted. This approach has received wide attention for potential use in treatment and employee-assistance programs, but there was limited evaluation of the usefulness of this ratio under controlled dosing conditions.
Huestis and Cone conducted a controlled clinical study of the excretion profile of creatinine and cannabinoid metabolites in a group of six cannabis users, who smoked two different doses of cannabis, separated by weekly intervals [ ]. As seen in Fig. Being able to differentiate new cannabis use from residual THC-COOH excretion in urine would be highly useful for drug treatment, criminal justice, and employee assistance drug testing programs.
The ratio times of the creatinine normalized later specimen divided by the creatinine normalized earlier specimen were evaluated for determining the best ratio to predict new cannabis use.
The most accurate ratio To further substantiate the validity of the derived ROC curve, urine-cannabinoid-metabolite and creatinine data from another controlled clinical trial that specifically addressed water dilution as a means of specimen adulteration were evaluated [ ]. Sensitivity, specificity, accuracy, and false positives and negatives were These data indicate that selection of a threshold to evaluate sequential creatinine-normalized urine drug concentrations can improve the ability to distinguish residual excretion from new drug usage.
Cannabinoids were detectable for 93 d after cessation of smoking, with a decreasing ratio of cannabinoids to creatinine over time. An excretion half-life of 32 d was determined.
When cannabinoid concentrations had not been normalized to creatinine concentrations, a number of false positive indications of new drug use would have occurred. Within this range, cannabinoid excretion is more variable, most likely based on the slow and variable release of stored THC from fat tissue.
The factors governing release of THC stores are not known. Additional research is being performed to attempt to determine appropriate ratio cutoffs for reliably predicting new drug use in heavy, chronic users. Oral fluid also is a suitable specimen for monitoring cannabinoid exposure, and is being evaluated for driving under the influence of drugs, drug treatment, workplace drug testing, and for clinical trials [ - ].
The oral mucosa is exposed to high concentrations of THC during smoking, and serves as the source of THC found in oral fluid. Only minor amounts of drug and metabolites diffuse from the plasma into oral fluid [ ]. Following intravenous administration of radiolabeled THC, no radioactivity could be demonstrated in oral fluid [ ].
Oral fluid collected with the Salivette collection device was positive for THC in 14 of these 22 participants. Several hours after smoking, the oral mucosa serves as a depot for release of THC into the oral fluid. In addition, as detection limits continue to decrease with the development of new analytical instrumentation, it may be possible to measure low concentrations of THC-COOH in oral fluid.
Detection times of cannabinoids in oral fluid are shorter than in urine, and more indicative of recent cannabis use [ ][ ]. Oral-fluid THC concentrations temporally correlate with plasma cannabinoid concentrations and behavioral and physiological effects, but wide intra- and inter-individual variation precludes the use of oral-fluid concentrations as indicators of drug impairment [ ][ ].
THC may be detected at low concentrations by radioimmunoassay for up to 24 h after use. After these times, occasional positive oral-fluid results were interspersed with negative tests for up to 34 h. They suggested that the ease and non-invasiveness of sample collection made oral fluid a useful alternative matrix for detection of recent cannabis use.
Oral-fluid samples also are being evaluated in the European Union's Roadside Testing Assessment ROSITA project to reduce the number of individuals driving under the influence of drugs and to improve road safety.
The ease and non-invasiveness of oral-fluid collection, reduced hazards in specimen handling and testing, and shorter detection window are attractive attributes to the use of this specimen for identifying the presence of potentially performance-impairing drugs. They determined that, with a limit of quantification of 0. As mentioned above, oral-fluid specimens tested positive for up to 34 h. Positive oral-fluid cannabinoid tests were not obtained more than 2 h after last use, suggesting that much lower cutoff concentrations were needed to improve sensitivity.
Detection of cannabinoids in oral fluid is a rapidly developing field; however, there are many scientific issues to resolve. One of the most important is the degree of absorption of the drug to oral-fluid collection devices. Recently, there has been renewed interest in oral-fluid drug testing for programs associated with drug treatment, workplace, and driving under the influence of drugs. Small and inconsistent specimen volume collection, poor extraction of cannabinoids from the collection device, low analyte concentrations for cannabinoids, and the potential for external contamination from environmental smoke are limitations to this monitoring method.
Recently, independent evaluations of the extraction of cannabinoids from the collection device [ - ] and measurement of oral-fluid THC-COOH in concentrations as low as picograms per milliliter appear to adequately address these potential problems.
The extraction efficiency of the buffer was reported to be between Recently, a nomenclature system for the superfamily of cytosolic SULTs has been established analogously to those of other drug metabolising enzymes such as cytochromes P or UDP—glucuronosyltranferases Blanchard et al. The superfamily of cytosolic sulfotransferases is subsequently divided into families and subfamilies according to the amino acid sequence identity among individual SULTs.
These SULT families include at least 13 different members. Cytosolic sulfotransferases exert relatively broad tissue distribution. The members of SULT1A family were found in liver, brain, breast, intestine, jejunum, lung, adrenal gland, endometrium, placenta, kidney and in blood platelets.
The mean expression values for each enzyme are displayed as percentages of the total sum of immunoquantified SULTs maximum five enzymes present in each tissue. SULT1A1 has been shown to be one of the most important sulfotransferases participating in metabolism of xenobiotics in humans.
SULT1A1 also takes part in transformation of hydroxymethyl polycyclic aromatic hydrocarbons, N —hydroxyderivatives of arylamines, allylic alcohols and heterocyclic amines to their reactive intermediates which are able to bind to nucleophilic structures such as DNA and consequently act as mutagens and carcinogens Glatt et al. SULT1A2 also plays an important role in the toxification of several aromatic hydroxylamines Meinl et al. SULT1E1 plays a key role in estrogen homeostasis.
Down regulation or loss of SULT1E1 could be to a certain extent responsible for growth of tumor in hormone sensitive cancers such as breast or endometrial cancer Cole et al. Clinically relevant substrates for other cytosolic sulfotransferases have not been identified yet. Several authors have claimed that SULT1A1 polymorphism might play a role in the pathophysiology of lung cancer Arslan et al. Since the first detection of glutathione transferase activity in rat liver cytosol by Booth in the early s, the family of glutathione transferases synonymously glutathione S —transferases; GSTs has been studied in detail.
Undoubtedly, the members of glutathione transferase family play an important role in metabolism of certain therapeutics, detoxification of environmental carcinogens and reactive intermediates formed from various chemicals by other xenobiotic—metabolising enzymes. Furthermore, GSTs constitute an important intracellular defence against oxidative stress and they appear to be involved in synthesis and metabolism of several derivatives of arachidonic acid and steroids van Bladeren, On the other hand, various chemicals have been shown to be activated into potentially dangerous compounds by these enzymes Sherratt et al.
In general, these enzymes catalyze a nucleophilic attack of reduced glutathione on lipophilic compounds containing an electrophilic atom C —, N — or S —. In addition to nucleophilic substitutions, these transferases also account for Michael additions, isomerations, and hydroxyperoxide reductions. In most cases, more polar glutathione conjugates are eliminated into the bile or are subsequently subjected to other metabolic steps eventually leading to formation of mercapturic acids.
Figure 4 shows the sequential steps in the synthesis of mercapturic acids. Mercapturic acids are excreted from the body in urine Commandeur et al. For instance, industrial chemicals such as acrylamide or trichloroethylene are detoxified via mercapturic acids Boettcher et al.
Up to now, two different superfamilies of GSTs have been described. The first one includes soluble dimeric enzymes localized mainly in cytosole but certain members of this superfamily have been also identified in mitochondria Robinson et al.
The superfamily of human soluble GSTs is further divided into eight separate classes: Formation of mercapturic acid. Glutathione S—transferase 1 catalyzes the conjugation between glutathione and various endogenous or xenobiotic electrophilic compounds.
Finally, cysteine S—conjugate N—acetyltransferase 4 catalyses formation of mercapturic acid. Various electrophilic compounds act as substrates for GSTs. They include a broad spectrum of ketones, quinones, sulfoxides, esters, peroxides, and ozonides van Bladeren et al. Chemotherapeutics such as busulfan, cis—platin, ethacrynic acid, cyclophosphamide, thiotepa ; industrial chemicals, herbicides, pesticides acrolein, lindane, malathion, tridiphane are detoxified by GSTs Hayes et al.
Epoxides and other reactive intermediates formed from environmental procarcinogens mostly as a result of metabolism by cytochromes P aflatoxin B 1 , polycyclic aromatic hydrocarbons, styrene, benzopyrene, heterocyclic amines also undergo detoxification by soluble GSTs.
Besides their enzymatic activity, cytosolic GSTs such as class Alpha exhibit an ability to bind various hydrophobic ligands xenobiotics as well as hormones and thus contribute to their intracellular transport and disposition. GSTs play an essential role in the fight against products of oxidative stress which unavoidably damage cell membrane lipids, DNA, or proteins.
Reactive intermediates resulting from lipid peroxidation 4—hydroxynonenal , nucleotide peroxidation adenine propenal or catecholamine peroxidation aminochrome, dopachrome, adrenochrome are particularly inactivated by GSTs Dagnino—Subiabre et al.
Several specific substrates for GSTs have been identified. For instance, ethacrynic acid has been found to be predominantly metabolised by GSTP1, whereas trans—stilbene oxide is a specific substrate for GSTM1 van Bladeren, The GSTT1 enzyme is responsible for conjugation of halogenated organic compounds such as dichlormethane or ethylene—dibromide Landi, This step leads to activation of these compounds to their reactive electrophilic metabolites with potential mutagenic and cancerogenic effect.
Ethylene—dibromide, a gasoline additive and a fumigant, is presumed to be potential human carcinogen because it is transformed by GSTs to DNA—reacting episulfonium ion van Bladeren, The glucocorticoid response element and the xenobiotic response element activated by glucocorticoids and planar aromatic hydrocarbons respectively might play a role in the induction of expression of GSTs Talalay et al.
Most members of both glutathione transferase superfamilies have been found to be genetically polymorphic. Several genetic variants of particular GSTs are supposed to contribute to the development of certain cancers or other diseases. Furthermore, genetic polymorphism in GSTs is pressumed to influence metabolism and disposition of various anticancerogenic drugs Crettol et al.
GSTP1 is responsible for metabolism of alkylating agents, topoisomerase inhibitors, antimetabolites, or tubulin inhibitors used in treatment of cancer.
Cyclophosphamide is biotransformed by GSTA1. On the other hand, several drugs activated by GSTs require a well—functioning enzyme.
Patients with the active GSTM1 gene treated for acute myeloid leukemia with doxorubicin had a superior survival rate compared to patients with at least one null allele Autrup et al. Genetically—based defects of these enzymes are also noteworthy because of their partial responsibility for increased risk of asthma, allergies, atherosclerosis, and rheumatoid arthritis van Bladeren, , Hayes et al. Compared to sulfonations and glucuronidations, acetylations are modest in terms of the number and variety of substrates, but remain significant in a toxicological perspective.
Drugs and other foreign compounds that are acetylated in intact animals are either aromatic amines or hydrazines, which are converted to aromatic amides and aromatic hydrazides Parkinson, Acetylation reactions are characterized by the transfer of an acetyl moiety, the donor generally being acetyl coenzyme A, while the accepting chemical group is a primary amino function.
As far as xenobiotic metabolism is concerned, three general reactions of acetylation have been documented, namely N — Fig. N —acetylation of aromatic amine is recognized as a major detoxification pathway in arylamine metabolism in experimental animals and humans Hein et al. However, O — and N,O —acetylations occur in alternative metabolic pathways following activation by N —hydroxylation. NATs are cytosolic enzymes found in many tissues of various species. NAT1 and NAT2 have distinct substrate specificities and differ markedly in terms of organ and tissue distribution.
NAT2 protein is present mainly in the liver Grant et al. NAT1 appears to be ubiquitous. Expression of human NAT1. Reactions catalysed by N—acetyltransferases. These reactions use acetyl—coenzyme A as acetyl donor.
A notable difference between the two isoenzymes is the presence of NAT1 activity in fetal and neonatal tissue, such as lungs, kidneys, and adrenal glands Pacifici et al. NAT1 has also been detected in cancer cells, in which it may not only play a role in the development of cancers through enhanced mutagenesis but may also contribute to the resistance of some cancers to cytotoxic drugs Adam et al.
NATs are involved in the metabolism of a variety of different compounds to which we are exposed on a daily basis. In humans, acetylation is a major route of biotransformation for many arylamine and hydrazine drugs, as well as for a number of known carcinogens present in diet, cigarette smoke, car exhaust fumes, and environment in general.
Substrates of NAT1 include p —aminobenzoic acid, p —aminosalicylic acid, the bacteriostatic antibiotics sulfamethoxazole and sulfanilamide, 2—aminofluorene and caffeine Ginsberg et al. Moreover, it has been proposed by Minchin that human NAT1 plays a role in folate metabolism through the acetylation of the folate metabolite p —aminobenzoylglutamate Minchin, Human NAT2 is a xenobiotic—metabolising enzyme that provides a major route of detoxification of drugs such as isoniazid an anti—tuberculotic drug , hydralazine and endralazine anti—hypertensive drugs , a number of sulphonamides anti—bacterial drugs Kawamura et al.
N —acetylation polymorphism represents one of the oldest and most intensively studied pharmacogenetic traits and refers to hereditary differences concerning the acetylation of drugs and toxicants. The genetic polymorphism in NAT activity was first recognised in tuberculosis patients treated with isoniazid, which is metabolised principally by N —acetylation.
The polymorphism causes individual differences in the rate of metabolism of this drug. Individuals with a faster rate are called rapid acetylators and individuals with a slower rate are called slow acetylators.
Rapid acetylators were competent in isoniazid acetylation but the drug was cleared less efficiently in the slow acetylator group, which resulted in elevated serum concentration and led to adverse neurologic side effects due to an accumulation of unmetabolized drug Brockton et al. Consistent with the toxicity of isoniazid in slow acetylators, there is an increased incidence of other drug toxicities in subjects carrying defective NAT2 alleles, such as lupus erythematosus in patients treated with hydralazine or procainamide Sim et al.
The high frequency of the NAT2 and also NAT1 acetylation polymorphism in human population together with ubiquitous exposure to aromatic and heterocyclic amines suggest that NAT1 and NAT2 acetylator genotypes are important modifiers of human cancer susceptibility. Methylation is a common but generally minor pathway of xenobiotic biotransformation. Unlike most other conjugative reactions, methylation often does not dramatically alter the solubility of substrates and results either in inactive or active compounds.
Methylation reactions are primarily involved in the metabolism of small endogenous compounds such as neurotransmitters but also play a role in the metabolism of macromolecules for example nucleic acids and in the biotransformation of certain drugs. A large number of both endogenous and exogenous compounds can undergo N — Fig.
Several N —methyltransferases have been described in humans and other mammals; including indolethylamine N—methyltransferase INMT , which catalyzes the N —methylation of tryptamine and structurally related compounds. The INMT exhibits wide tissue distribution. Human INMT was expressed in the lung, thyroid, adrenal gland, heart, and muscle but not in the brain Thompson et al.
Since INMT is predominantly present in peripheral tissues, its main physiological function is presumably non—neural. NNMT is predominantly expressed in the liver; expression has been also reported in other tissues such as the kidney, lung, placenta, and heart Zhang et al. NNMT was one of the potential tumor biomarkers identified in a wide range of tumors such as thyroid, gastric, colorectal, renal, and lung cancer Zhang et al. Histamine N—methyltransferase HNMT , the second most important histamine inactivating enzyme, is a cytosolic enzyme specifically methylating the imidazole ring of histamine and closely related compounds in the intracellular space of cells.
Levels of HNMT activity in humans are regulated genetically. Common genetic polymorphisms for HNMT might be related to a possible role for individual variation in histamine metabolism in the pathophysiology of diseases such as allergy, asthma, peptic ulcer disease, and some neuropsychiatric illnesses Preuss et al.
Phenylethanolamine N—methyltransferase PNMT plays a role in neuroendocrine and blood pressure regulation in the central nervous system. PNMT, the terminal enzyme of the catecholamine biosynthesis pathway, catalyzes the N —methylation of the neurotransmitter norepinephrine to form epinephrine Ji et al. PNMT is a cytosolic enzyme that is present in many tissues throughout the body, with particularly high concentration in the adrenal medulla, adrenergic neurons in the amygdala and retina and the left atrium of the heart Haavik et al.
Its activity increases after stress in response to glucocorticoids and neuronal stimulation Saito et al. Phosphatidylethanolamine N—methyltransferase PEMT converts phosphatidylethanolamine to phosphatidylcholine in mammalian liver. Phosphatidylcholine is nutrient critical to many cellular processes such as phospholipid biosynthesis, lipid—cholesterol transport, or transmembrane signaling.
The process of O —methylation of phenols and catechols is catalyzed by two different enzymes known as phenol O —methyltransferase and catechol O —methyltransferase. POMT is a localized in the microsomes of the liver and lungs of mammals but is also present in other tissues.
It is an enzyme that plays a key role in the modulation of catechol—dependent functions such as cognition, cardiovascular function, and pain processing. It has also been suggested that a common functional genetic polymorphism in the COMT gene may contribute to the etiology of alcoholism Wang et al. Thiol methylation is important in the metabolism of many sulfhydryl drugs.
Human tissues contain two separate genetically regulated enzymes that can catalyze thiol S —methylation. Thiol methyltransferase TMT is a membrane—bound enzyme that preferentially catalyzes the S —methylation of aliphatic sulfhydryl compounds such as captopril and D—penicillamine, whereas thiopurine S—methyltransferase TPMT is a cytoplasmic enzyme that preferentially catalyzes the S —methylation of aromatic and heterocyclic sulfhydryl compounds including anticancer and immunosuppressive thiopurines such as 6—mercaptopurine, 6—thioguanine, and azathioprine.
Thiopurine drugs have a relatively narrow therapeutic index and are capable of causing life—threatening toxicity, most often myelosuppression Sahasranaman et al. TPMT genetic polymorphism represents a striking example of the clinical importance of pharmacogenetics. Subjects homozygous for low TPMT activity have a high risk of myelosuppression after treatment with standard dose of azathioprine.
TPMT shows the highest level of expression in liver and kidney and the physiological role of this enzyme, despite extensive investigation, remains unclear. Arsenic is a well—known naturally occurring metaloid which is considered as a multiorgan human carcinogen. Occupational exposure to arsenic occurs in the smelting industry and during the manufacture of pesticides, herbicides, and other agricultural products. Arsenic plays a dual role as environmental carcinogenic pollutant and as a successful anticancer drug against promyelocytic leukemia Wood et al.
Its metabolism proceeds via a complicated enzymatic pathway, acting both as detoxification and producing more toxic intermediates. Methylation is an important reaction in the biotransformation of arsenic.
Methylated and dimethylated arsenic are the major urinary metabolites in human and many other species Li et al. Two reaction schemes Fig.
Alternative schemes for the conversion of inorganic arsenic iAs into methylated metabolites.
Phase II Drug Metabolism
Pharmacokinetics and Metabolism of the Plant Cannabinoids, sº- Tetrahydrocannabinol Hepatic Metabolism.. Extrahepatic Metabolism. Chemistry and Drug Metabolism, Intramural Research Program, National Institute on Extrahepatic Metabolism. Metabolism of Cannabidiol. Metabolites With Low Circulating Concentrations and/or Extrahepatic .. Metabolism in the Male Reproductive System. Reports of TCE exposure affecting .