Drug Biotransformation PDF

DRUG BIOTRANSFORMATION Seth K. Amponsah (PhD) Department of Medical Pharmacology UGMS  DRUG BIOTRANSFORMATION Humans come into contact with scores of foreign chemicals or xenobiotics. Drugs are also considered xenobiotics. Humans have developed a means to r...

DRUG BIOTRANSFORMATION Seth K. Amponsah (PhD) Department of Medical Pharmacology UGMS  DRUG BIOTRANSFORMATION Humans come into contact with scores of foreign chemicals or xenobiotics. Drugs are also considered xenobiotics. Humans have developed a means to rapidly eliminate these xenobiotics. 3/16/2022 S. K. AMPONSAH 2  Drug Biotransformation Drug elimination is important otherwise drug would stay in the body indefinitely. Drugs are eliminated from the body by the aid of two processes- Biotransformation (Metabolism) and Excretion. 3/16/2022 S. K. AMPONSAH 3  Drug Biotransformation Some drugs after having their pharmacological actions are eliminated solely by renal excretion. For renal excretion to occur, the drug should: – have a small molecular mass – not bind easily to plasma proteins – be polar in nature Most drugs are complex and do not have all the above properties. 3/16/2022 S. K. AMPONSAH 4 3/16/2022 S. K. AMPONSAH 5  Drug Biotransformation Most drugs are lipophilic in nature thus readily pass across biological barriers or membranes. They also bind strongly to plasma proteins. These properties reduce the extent to which they are eliminated by the kidney. Hence the need for biotransformation. 3/16/2022 S. K. AMPONSAH 6  Drug Biotransformation Biotransformation is essential for the termination of the biological activity of some drugs or xenobiotics in the body. Biotransformation often generates more polar, less active or inactive metabolites which are readily excreted from body. 3/16/2022 S. K. AMPONSAH 7  Drug Biotransformation Although drugs are biotransformed into excretable forms (polar), some may still have potent biological activity (or may have toxic properties). There are also pharmacologically inactive drugs which can be biotransformed into active metabolites. These drugs are known as prodrugs. 3/16/2022 S. K. AMPONSAH 8  Drug Biotransformation Sometimes, drug biotransformation can occur before a drug is distributed to tissues. This can occur in the intestinal endothelium or the liver before it reaches the systemic circulation ie after oral administration. This process reduces oral bioavailability of certain drugs. This phenomenon is known as first pass effect or first pass metabolism. 3/16/2022 S. K. AMPONSAH 9 3/16/2022 S. K. AMPONSAH 10  Drug Biotransformation Drug Biotransformation reactions can be classified as: – Phase I reactions – Phase II reactions Phase I – functionalization reactions (reactive group) Phase II – conjugation reactions (endogenous substrate) 3/16/2022 S. K. AMPONSAH 11  Phase I reactions They convert the parent drug to a more polar metabolite by introducing or unmasking functional groups such as-OH, -NH₂, -SH. Phase I metabolites may be: – More active (most often) – Toxic – Inactive Also leads to activation of prodrugs. After Phase I, some metabolites can be excreted by the kidneys. 3/16/2022 S. K. AMPONSAH 12  Phase II (conjugation reactions) Covalent linkage is formed between the functional groups on the parent compound or Phase I metabolites and endogenous substrates such as glucuronic acid, sulfate, acetate, or an amino acid. Metabolites of Phase II are highly polar, less active and are rapidly excreted in urine. If after Phase II the metabolite is not too polar to undergo renal excretion, it undergoes biliary excretion. 3/16/2022 S. K. AMPONSAH 13 3/16/2022 S. K. AMPONSAH 14  Enterohepatic circulation Enterohepatic cycling occurs by biliary excretion and intestinal reabsorption of a drug. This happens as a result of hepatic conjugation and intestinal deconjugation. Cycling is often associated with a longer apparent half-life of the drug. 3/16/2022 S. K. AMPONSAH 15  Enterohepatic circulation 3/16/2022 S. K. AMPONSAH 16 SITE OF BIOTRANSFORMATION Most tissues have the ability to biotransform drugs. The liver is ideally placed to intercept ingested xenobiotics and has a major role in biotransformation. Other organs involved in biotransformation include GIT, Lungs, Skin and Kidneys. 3/16/2022 S. K. AMPONSAH 17  Site of biotransformation With respect to drug metabolizing reactions, two sub-cellular organelles are quantitatively the most important: Smooth Endoplasmic Reticulum and the Cytosol. Enzymes (microsomal) involved in Phase I are almost exclusively localized in the Smooth endoplasmic reticulum (SER). Enzymes (non-microsomal) involved in Phase II are located predominantly in the cytosol. 3/16/2022 S. K. AMPONSAH 18  Site of biotransformation Microsomes are vesicle-like artifacts formed from the endoplasmic reticulum (ER) when eukaryotic cells are broken-up (homogenized) and separated by differential centrifugation. RER contains enzymes but are involved in protein synthesis. SER contain enzymes involved in drug biotransformation, hence the name microsomal enzymes. 3/16/2022 S. K. AMPONSAH 19 3/16/2022 S. K. AMPONSAH 20  Site of biotransformation Principal microsomal enzymes are CYP 450 system that are involved in Phase I reactions. There is one unique microsomal enzyme which carries out a Phase II reaction ie UDP glucuronyl transferase (glucuronidation). 3/16/2022 S. K. AMPONSAH 21  Site of biotransformation Non-microsomal enzymes are present in the cytosol (mainly) and mitochondria of cells. They catalyze mostly conjugation reactions (Phase II) and a few oxidative, reductive and hydrolytic reactions (Phase I) also. 3/16/2022 S. K. AMPONSAH 22 PHASE I BIOTRANSFORMATION Oxidation, reduction, hydrolysis, and hydration Many drugs undergo a number of these reactions. Phase I metabolism often prepares the drug for Phase II. CYP450 enzyme system found in microsomes of many cells performs many different functionalization reactions in Phase I. 3/16/2022 S. K. AMPONSAH 23  PHASE I BIOTRANSFORMATION CYP450 Enzyme System A superfamily of heme containing proteins. Involved in metabolism of diverse endogenous and exogenous compounds; − Steroid hormones − Drugs – Environmental chemicals – Other xenobiotics 3/16/2022 S. K. AMPONSAH 24  PHASE I BIOTRANSFORMATION ~1000 currently known CYP450s, about 50 active in humans. categorized into families (CYPs) – identified by an Arabic number, CYP1, CYP2 further into sub-families – identified by a letter, CYP1A, CYP1B may have different individual isoforms − identified by another Arabic number, CYP1A1, CYP1A2 3/16/2022 S. K. AMPONSAH 25  PHASE I BIOTRANSFORMATION 1. Oxidative reactions Oxidation is the most common of Phase I biotransformation reactions. Involves mostly microsomal enzymes such as CYP450. Commonest reactions by CYP450 include aliphatic and aromatic hydroxylation , N- dealkylation and O-dealkylation. 3/16/2022 S. K. AMPONSAH 26  Oxidative reactions Not all oxidative reactions involve the CYP450 system or microsomal enzymes. Ethanol is oxidized by a non-microsomal enzyme, alcohol dehydrogenase. Other cytosolic enzymes involved in oxidation include xanthine oxidase and monoamine oxidase. 3/16/2022 S. K. AMPONSAH 27  2. Reductive reactions Are less common than oxidative ones. An example is the inactivation of warfarin by the conversion of a ketone to a hydroxyl group by CYP2A6. Nitroglycerin also undergoes reductive hydrolysis. 3/16/2022 S. K. AMPONSAH 28  Reductive reaction involving a ketone 3/16/2022 S. K. AMPONSAH 29  3. Hydrolytic reactions Often involves the hydrolysis of amide and ester bonds. There are a number of drugs that possess ester or amide linkages and these are vulnerable to the activity of esterases and amidases. There are many esterase enzymes in various tissues and plasma responsible for the rapid metabolism of certain drugs. 3/16/2022 S. K. AMPONSAH 30  Hydrolytic reactions Examples of esterases include cholinesterases and other plasma esterases that inactivate choline esters, local anesthetics and other drugs. Amidases also hydrolyze amides and lead to the release of amine. 3/16/2022 S. K. AMPONSAH 31 3/16/2022 S. K. AMPONSAH 32 PHASE II BIOTRANSFORMATION Except for UDP glucuronyl transferase enzyme, all enzymes are non-microsomal. Gives products that are generally inactive, water soluble and easily excreted. Includes conjugation reactions such as; glucuronidation, sulfation, methylation, acetylation, and glutathione conjugation. 3/16/2022 S. K. AMPONSAH 33  PHASE II BIOTRANSFORMATION 1. Glucuronidation Most common and important of the all conjugation reactions. Uses the cofactor UDP–glucuronic acid. UDP-glucuronyl transferase catalyses these reactions. Bilirubin and other endogenous substances are also conjugated by the same system. Examples of drugs – CPL, acetaminophen 3/16/2022 S. K. AMPONSAH 34  2. Sulfation Major conjugation pathway for phenols, alcohols and amines. Involves sulfotransferase enzymes. Compounds that can be glucuronidated can also be sulfated e.g acetaminophen. In general, sulfate conjugation predominates at low substrate concentration and glucuronide conjugation predominates at high substrate concentration. 3/16/2022 S. K. AMPONSAH 35  3. Glutathione Conjugation Glutathione is a protective compound (tripeptide, Gly-Cys-Glu) within the body for removal of potentially toxic electrophilic compounds. Many drugs are metabolized in phase I to strong electrophiles. These react with glutathione to form non- toxic conjugates. The enzyme involved is glutathione-S- transferase. 3/16/2022 S. K. AMPONSAH 36  4. Acetylation Accomplished by N-acetyltransferase (NAT) enzyme. The enzyme utilizes acetyl CoA as a source of acetate. NATs are among the most polymorphic of human xenobiotic drug-metabolizing enzymes. There are two functional NAT genes in humans, NAT1 and NAT2. 3/16/2022 S. K. AMPONSAH 37  Acetylation NAT1 is ubiquitously expressed in human tissues. NAT2 is found in liver and the GI tract. Enzymes from both genes have the capacity to form N-acetylated metabolites. 3/16/2022 S. K. AMPONSAH 38 FACTORS THAT AFFECT DRUG BIOTRANSFORMATION Diet Gender Smoking Alcohol Disease Factors Age Genetic Variation 3/16/2022 S. K. AMPONSAH 39  DIET Food contains a vast array of chemicals, such as polyphenols, plant toxins and preservatives. Polyphenols such as naringenin and bergamottin found in grape juice can affect biotransformational activity. It has become apparent that these polyphenols can induce glucuronyltransferase enzyme. 3/16/2022 S. K. AMPONSAH 40  Diet Barbecued meat has also been found to contain polycyclic aromatics, nitrosoamines and heterocyclic aromatic amines which all induce CYP1A2 and CYP1A1. Increased intake of this meat could increase the metabolism of drugs such as amitriptyline, clozapine, haloperidol and naproxen. 3/16/2022 S. K. AMPONSAH 41  Diet Broccoli and its sister vegetables (cabbage) can also induce CYP1A2. But you need to consume about 500 g/day to get that effect. It appears unlikely that many would consume that amount of cruciferous vegetables per day. 3/16/2022 S. K. AMPONSAH 42  Diet Some drugs which are metabolized by CYP1A2 include clozapine and theophylline. It means that these drugs could have their metabolism accelerated if that much of cruciferous vegetables is taken. Caffeine is also entirely metabolized by CYP1A2. It acts as a competitive inhibitor of substrates that are metabolized by this CYP. 3/16/2022 S. K. AMPONSAH 43  GENDER In female epileptics, phenytoin metabolism is significantly accelerated just before the beginning of menstruation. This can cause the drug levels to fall out of the therapeutic window. Also, many CYP3A4 substrates such as erythromycin are metabolized more rapidly in women than men. 3/16/2022 S. K. AMPONSAH 44  Gender Ciprofloxacin is metabolized significantly more slowly in women. Peak drug levels are more than a third higher in females than in males. Women thus suffer more adverse effects from this drug than men. 3/16/2022 S. K. AMPONSAH 45  SMOKING Tobacco smoke contains around 4000 chemicals. Most of the aromatic hydrocarbon found in tobacco have been found to induce CYP1A1 and CYP1A2. CYP1A1 contributes very little to the drug metabolism. 3/16/2022 S. K. AMPONSAH 46  Smoking CYP1A2 metabolizes drugs such as theophylline, fluvoxamine, warfarin, clozapine and olanzapine. Rate of fluvoxamine metabolism has been found to be double in a smoker than a non-smoker. With clozapine, it is reckoned that smokers require about 1.5 times the dose of non-smokers. 3/16/2022 S. K. AMPONSAH 47  ALCOHOL Alcohol is metabolized in the liver by the enzyme alcohol dehydrogenase (ADH) to acetaldehyde, which is extremely toxic. The acetaldehyde formed by ADH is further metabolized by aldehyde dehydrogenase (ALDH). Around 90% of alcohol is metabolized by the ADH/ALDH system, with the remainder oxidized by CYP2E1. 3/16/2022 S. K. AMPONSAH 48  Alcohol Alcohol is primarily an inducer of CYP2E1, although CYP1A1 and CYP3A are also affected. The degree of induction is dependent on the individual’s usual consumption. For those chronically dependent on alcohol, their CYP2E1 levels can be ten-fold higher than non-drinkers. 3/16/2022 S. K. AMPONSAH 49  Alcohol These individuals would metabolize CYP2E1 drug substrates faster. This can affect a drug like warfarin. There are also a list of drugs that inhibit the metabolism of alcohol. The drugs inhibit especially aldehyde dehydogenase (ALDH). 3/16/2022 S. K. AMPONSAH 50  Alcohol Examples include metronidazole, griseofulvin, CPL, nitrofurantoin and sulphamethoxazole. Inhibiting metabolism of acetaldehyde causes severe flushing, vomiting, sweating and nausea (exceedingly unpleasant). Hence patients are advised not to take alcohol when on any of the above drugs. 3/16/2022 S. K. AMPONSAH 51  DISEASE FACTORS The major location of drug metabolizing enzymes is the liver. Dysfunction can lead to impaired drug metabolism due to decreased enzyme activity. Liver diseases such as cirrhosis, alcoholic liver disease or liver carcinoma can affect drug biotransformation. 3/16/2022 S. K. AMPONSAH 52  Disease factors The main effects of cirrhosis include reduction in blood flow and loss of functional hepatocytes. Cirrhosis leads to reduced activity in mainly the CYPs 3A and 1A sub-families. Sulfation activity is also much reduced. CYP2 family and glucuronidation activity often survives. 3/16/2022 S. K. AMPONSAH 53  Disease factors During cirrhosis, metabolism of high extraction drugs, where liver blood flow is the major determinant are severely compromised. Metabolism of drugs such as propranolol, metroprolol, labetalol, lidocaine and opiates can be affected. 3/16/2022 S. K. AMPONSAH 54  Disease factors Chronic Hepatitis C has also been found to reduce enzymatic activity of CYP2D6 and 2A6. Individuals with Type II diabetes have been found to metabolize a CYP2E1 substrate, chlorzoxazone, more than 2.5 times faster than healthy individuals. 3/16/2022 S. K. AMPONSAH 55  Disease factors This effect has been reported in especially obese Type II diabetic patients. This is however not seen in well controlled Type I diabetics. 3/16/2022 S. K. AMPONSAH 56  AGE Age is a major factor that affects drug metabolism. It is clear that at the extremes of life (neonates and geriatrics) drug metabolism can differ from the rest of population. Neonates have CYP activities less than half of adult activity. 3/16/2022 S. K. AMPONSAH 57  Age This is reflected in the inability of neonates to metabolize many drugs that rely on the major CYPs. Drug half-lives in neonates can be up to 10 times that of adults. There is even a marked difference between preterm and full-term neonates. 3/16/2022 S. K. AMPONSAH 58  Age Preterm neonates metabolize drugs less than half as quickly as full-term neonates. Neonate levels of CYP2C, CYP2E1, and especially CYP1A2 are very low. The difference between adult and neonate drug metabolism is rapidly eroded 6-12 months in age. 3/16/2022 S. K. AMPONSAH 59  CYP450 Enzyme activity 3/16/2022 S. K. AMPONSAH 60  Age Due to this variation, drug dose should be adjusted appropriately in neonates. Doses for neonates and infants are generally based on weight and age. This reduces the chances of toxicity. 3/16/2022 S. K. AMPONSAH 61  Age Glucuronidation systems also mature more slowly than other biotransforming systems. In human fetuses, at 20 weeks gestation, there is no evidence of any expression of glucuronyl transferase enzyme. This explains the high incidence of jaundice in newborns (↑ bilirubin levels). 3/16/2022 S. K. AMPONSAH 62 3/16/2022 S. K. AMPONSAH 63  Age Babies who are not jaundiced can have their low glucuronyl transferase capability revealed when a drug is administered. The use of CPL for infections in the 1950’s led to the “grey baby syndrome”. CPL is metabolized by CYP2C19 to hydroxylamine. The hydroxylamine is further conjugated to glucuronic acid. 3/16/2022 S. K. AMPONSAH 64  Age Poor glucuronyl transferase capability causes the hydroxylamine (oxidant) to accumulate leading to methemoglobin formation. Methemoglobin cannot bind oxygen and this often leads to reduced oxygen supply to tissues. Symptoms include ashen gray color of the skin (Grey baby), cyanosis, hypotension. 3/16/2022 S. K. AMPONSAH 65  Age In geriatrics, some but not all Phase I rxns diminish as one ages. This affects certain drugs like diazepam, chlordiazepoxide and flurazepam. Phase II (conjugation) metabolic pathways do not appear to diminish with age in geriatrics. 3/16/2022 S. K. AMPONSAH 66 3/16/2022 S. K. AMPONSAH 67  Age There is also a significant change in the liver with age. Decreases in mass and blood flow occurs as we age. This is at a rate of around 0.5-1.5% per year. By age 60-70 yrs, a 40% decline in liver blood flow compared to a 30 year old is expected. 3/16/2022 S. K. AMPONSAH 68  GENETIC VARIATION There is wide variability in the response to drugs between individuals. The consequence of such variation could result in therapeutic failure or an adverse drug reaction. Allelic variants of enzymes with different catalytic activities from that of the wild-type have been identified. 3/16/2022 S. K. AMPONSAH 69  Genetic Variation Single nucleotide polymorphism leads to sub- populations of enzymes. The enzymes have different drug metabolizing abilities; − lack of activity − reduction in catalytic ability − enhanced activity Frequency of the polymorphism often varies according to the ethnic ancestry of the individual. 3/16/2022 S. K. AMPONSAH 70  Genetic variation CYP2D6 has been found to be highly polymorphic. It’s expression leads to 3 phenotypes − Extensive metabolizers (EMs) have functional enzyme activity − Intermediate metabolizers (IMs) have diminished enzyme activity − Poor metabolizers (PMs) have little or no activity 5-10% of Caucasians and 1-2% of Asians exhibit the PM phenotype. 3/16/2022 S. K. AMPONSAH 71  Genetic variation Codeine (prodrug) is oxidized to morphine by CYP2D6. The analgesic effect of codeine lies in the morphine. It thus implies PMs may have no therapeutic effect. 3/16/2022 S. K. AMPONSAH 72  Genetic variation Debrisoquine, formerly used in the treatment of hypertension, is metabolized by CYP2D6 to 4-hydroxydebrisoquine. It is usually used for phenotyping the CYP2D6 enzyme. This is based on assaying parent drug and the metabolite. Polymorphism in CYP2C9 and CYP2C19 also exist in a number of populations. 3/16/2022 S. K. AMPONSAH 73  Genetic variation Individuals may also exhibit slow or fast acetylation of some drugs such as isoniazid. The slow acetylator phenotype have been identified among ethnic groups: 40- 70% of Caucasians and African-Americans, 10-20% of Japanese and Canadian Eskimo. 3/16/2022 S. K. AMPONSAH 74  Genetic variation More than 80% of Egyptians, and certain Jewish populations are also slow acetylators. Peripheral neuropathy have been observed in slow acetylators due to high plasma concentration of isoniazid. 3/16/2022 S. K. AMPONSAH 75  Genetic variation Succinylcholine or suxamethonium is a NM blocker used for muscle relaxation. Suxamethonium is metabolized by plasma cholinesterase. About 1 in 3000 individuals exhibit an abnormal plasma cholinesterase (atypical) that cannot metabolize succinylcholine. 3/16/2022 S. K. AMPONSAH 76  Genetic variation Individuals with the atypical enzyme are subject to prolong apnea after receiving normal dose of succinylcholine. Due to this, patients are screened (Dibucaine number) for atypical cholinesterase before given succinylcholine. 3/16/2022 S. K. AMPONSAH 77