Excretion of the unchanged form of the drug is one way to terminate the action of the drug. Another way is to chemically alter it. This process is called biotransformation and is primarily for lipid-soluble drugs that cannot be eliminated unchanged by the kidney (as they readily cross the cell membranes and have considerable tubular reabsorption). In this article, we will study about various type of metabolic reactions of drugs and factors affecting the biotransformation reactions.
Cytochrome P450 Oxidase

Image: “Cytochrome P450 Oxidase” by Jag123. License: CC BY-SA 3.0

Introduction to Biotransformation

Drugs can be eliminated from the systemic circulation unchanged by renal excretion (e.g. benzylpenicillin, aminoglycosides, metformin etc.); fully transformed and excreted only as metabolites (e.g. phenothiazines, chloramphenicol etc.); or both unchanged and as metabolites (e.g. salicylates, acetaminophen etc.).

The site of biotransformation is primarily the liver. Other tissues such as intestinal mucosa, colon, placenta, adrenals, etc. also contribute to biotransformation. Biotransformation can make the drug inactive, but can also produce another pharmacologically active drug that has a more or less or equal activity as the parent drug; it can even produce compounds that are toxic. Some examples are as under:

  • Drugs that become inactive: Phenytoin, salicylic acid, phenobarbital, amphetamine
  • Drugs that produce pharmacologically active metabolites: Amitriptyline (nortriptyline), imipramine (desipramine), diazepam (temazepam), digitoxin (digoxin)
  • Drugs that produce toxic metabolites: Paracetamol, isoniazid, valproic acid

The pathways of drug metabolism can be categorized into phase I and phase II (and sometimes phase III) reactions:

  • Phase I reactions involve attachment (or unmasking) of polar groups to the drugs.
  • Phase II reactions involve conjugation of compounds such as organic acids to the polar group to make the compound highly water soluble.
  • Phase III reactions refer to the active transport of the compound into urinary or hepatobiliary system to facilitate elimination.

Phase I Reactions

Drug metabolism in liver

Image: “Drug metabolism in liver. transferases are : glutathione, sulfate, acetate, glucoronic acid. P-450 is cytochrome P-450 enzymes. 3 different pathways are depicted for Drugs A,B and C.” by Drriad. License: CC BY-SA 3.0

Phase I reactions, also known as functionalization reactions, involve the transformation of lipophilic drugs into a more water-soluble compound. This occurs by introduction of a polar group, such as hydroxyl (-OH), amine (-NH2), and sulfhydryl (-SH)groups.

For example, phenylbutazone is a benzene compound. An -OH group is added to one of the benzene rings, yielding a para-hydroxylated product known as oxyphenbutazone. Phase I reactions are primarily catalyzed by cytochrome P450 (CYP) enzymes. Other enzymes involved in phase I reactions and examples of their substrates are as below.


CYP-catalyzed reactions Drugs
C-hydroxylation Amphetamines, Phenytoin, Warfarin, Barbiturates, Propranolol, Tolbutamide, Phenylbutazone
N-hydroxylation Dapsone
N-dealkylation Morphine, Theophylline, Caffeine, Lidocaine, Amitriptyline
O-dealkylation Codeine, Dextromethorphan
N-oxidation Acetaminophen
S-oxidation Chlorpromazine, Thioridazine
Deamination Amphetamine, Diazepam
Flavin-containing monooxygenase (FMO)-catalyzed reactions Drugs
S-oxidation Cimetidine
N-oxidation Nicotine
Monoamine oxidases (MAO)-catalyzed reactions Dopamine, Norepinephrine, Serotonin
Molybdenum-containing oxidase-catalyzed reactions Hypoxanthine, Allopurinol, Benzaldehyde, 6-Mercaptopurine
Alcohol/aldehyde dehydrogenase-catalyzed reactions Methanol, Ethanol, Ethylene glycol


Reaction Drug
Azo-reduction Tartrazine, Prontosil
Nitro-reduction Nitrobenzene, Chloramphenicol, Dantrolene, Clonazepam
Carbonyl-, aldehyde- and aldose-reduction Chloral hydrate, Methadone


Reaction Drug
Alkaline phosphatase Fosphenytoin, Fospropofol, Clindamycin phosphate (examples of pro-drugs)
Paraoxonase Statins, Spironolactone
Carboxylesterases/Butyrylcholinesterase Succinylcholine, Mivacurium, Procaine, Tetracaine, Cocaine, Esmolol, Meperidine, Remifentanil, Enalapril
Microbial hydrolase (in the colon) Bisacodyl

Phase II Reactions

Many phase I metabolites are not sufficiently polar to be directly excreted by the kidneys. These compounds are then subjected to phase II reactions, also known as conjugation or substitution reactions, which involve the conjugation of compounds such as organic acids (glucuronic acid, sulfuric acid, acetic acid, etc.) to the polar group to make the compound highly water soluble and, usually, pharmacologically inactive.

If a drug already possesses a polar group, it may directly undergo phase II reaction, thus bypassing the phase I reaction. The reactions are catalyzed by a group of transferase enzymes that are often non-specific. The most dominant enzymes are uridine 5′-diphosphate (UDP)-glucuronosyl transferases (UGTs).

The most important conjugation pathway is glucuronidation, and perhaps the most notable example is morphine: it undergoes glucuronidation to become morphine-6-glucuronide, which is more potent than morphine.

Other pathways and examples are listed in the following table:

Glucuronidation Acetaminophen, Diazepam, Digoxin, Chloramphenicol, Oxazepam, Phenolphthalein, Propofol, Diethylstilbestrol, Estradiol, Thyroxine, Diclofenac, Furosemide
Sulfation Acetaminophen, Methyldopa, Minoxidil
Glycine conjugation Deoxycholic acid, Nicotinic acid (Niacin), Salicylic acid, Benzoic acid
Glutathione conjugation Ethacrynic acid, reactive phase I metabolite of acetaminophen
Acetylation Clonazepam, Dapsone, Isoniazid, Mescaline, Sulfonamides
Methylation Dopamine, Epinephrine, Histamine, Norepinephrine, Thiouracil


Some drugs are administered in the inactive form and become pharmacologically active after they have undergone phase I or II reaction. Such drugs are called as prodrugs and are administered to improve the drug’s bioavailability.

A typical example is enalapril, which gets converted by hydrolysis into its active form enalaprilat. In fact, most other ACE inhibitors are prodrugs—Benazepril, fosinopril, moexipril, perindopril, quinapril, ramipril, and trandolapril—and get converted to their active form in the liver via hydrolysis.

Other examples include L-dopa, cyclophosphamide, bacampicillin, bambuterol, chloramphenicol succinate, dipivefrin, fosphenytoin, pralidoxime, etc. In the presence of CYP inhibitors, a prodrug that gets converted to its active form by CYP enzymes will show considerably diminished activity because of reduced amount of conversion.

Toxic Metabolism

drug-induced hepatitis

Image: “Low magnification micrograph of an adverse drug reaction leading to a hepatitis, also known as drug-induced hepatitis, with non-caseatinggranulomata. Liver biopsy. H&E stain.” by Nephron. License: CC BY-SA 3.0

CYPs are present mainly in the liver. Metabolism of certain drugs by CYPs leads to production of toxic metabolites that can cause hepatotoxicity.

Acetaminophen (also known as paracetamol) and halothane are well-known examples of hepatotoxic drugs, by the production of toxic metabolites N-acetyl-p-benzoquinone imine and 1-chloro-2,2,2-trifluoroethyl radical, respectively. Other NSAIDs such as diclofenac also induce hepatotoxicity by CYP-medicated mechanisms.

Valproic acid also shows hepatotoxicity when its main metabolite (2-propyl-4-pentenoic acid) undergoes further metabolism. Compounds such as ethanol, carbon tetrachloride, chloroform, N-nitrosodimethylamine also involve CYP-mediated hepatotoxicity (by generation of free radicals). CYP2E1 is involved in anti-tubercular drug hepatotoxicity.

Factors Affecting Drug Metabolism

In addition to drug distribution and elimination, drug metabolism also affects the dose and frequency of drug administration. Many factors can affect the rate of biotransformation, including biological factors relating to variation among individuals. The study of interindividual variation in drug response is known as pharmacogenetics. Some of the inter- and intra-individual factors are as follows:

  • Demographics factors such as age (gender is not an important factor)
  • Diet
  • Physiological states such as pregnancy, hormones
  • Diseases
  • Ethnicity
  • Temporal factors such as circadian rhythm
  • Genetic factors

Genetic factors

Influence of genes takes place by a phenomenon known as genetic polymorphism. A variant allele of a gene can affect the enzyme levels or the functional activity of the enzyme, or both, thereby leading to variation in the expected response of the drug (in terms of efficacy and/or adverse effects). Obviously, this requires titration of the dose, especially for drug with narrow therapeutic windows.

An individual may be a poor or slow metabolizer of a drug because of the allele (e.g. slow acetylators of isoniazid). Such individuals may be hyperreactive to the drug. On the other hand, higher levels of the enzymes my cause the individuals to be a fast metabolizer (e.g. fast acetylators of isoniazid), sometimes leading to non-reactivity to the drug. This genetic variation also shows ethnic diversity: while almost equal number of whites and blacks are slow and fast acetylators, slow acetylators are predominant in Japanese and Eskimo populations.

Succinylcholine is usually rapidly metabolized by pseudocholinesterase, and this activity lasts for 5 min. In rare cases, this is metabolized very slowly, to the extent that the duration of action of one dose can last for a few hours.

Another example is codeine, which is metabolized by the CYP2D6 enzyme into its active metabolite morphine. However, some individuals carry two inactive copies of the CYP2D6 gene (poor metabolizers); therefore, they are unable to derive any pharmacological effect from codeine. On the other head, there are those who are ultrarapid metabolizers of codeine (because they carry more than two copies), thereby leading to symptoms of drug overdose even from a usual dose of codeine. (Note that most individuals are extensive metabolizers, with two CYP2D6 copies, and some are intermediate metabolizers, with one copy.)

Other factors include physical and chemical properties of the drug as well as factors that inhibit or enhance drug metabolizing enzymes.

Cytochrome P450

Ribbon diagram of human cytochrome P450 isozyme 2C9

Image: “Ribbon diagram of human cytochrome P450 isozyme 2C9” by Fvasconcellos. License: Public Domain

Cytochrome P450 (or CYP450) are a group of proteins. Although there are many enzymes in this family, about six of them (CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2D6, CYP2E1 and CYP3A4) metabolize most of the drugs. In fact, CYP3A4 alone metabolizes around 50 % of all drugs metabolized by the liver.

CYPs are the important targets for drug interactions. Knowledge of these interactions is crucial when more than one drug is being administered to a patient, especially, for example, in older people who are often subjected to polypharmacy.

Some drugs are metabolized largely or exclusively by only one CYP. E.g. phenytoin (by CYP2C9) and halothane (by CYP2E). Some other drugs can be metabolized by several CYPs. E.g. acetaminophen (by CYP2E1, CYP1A2, and CYP3A4) and dextromethorphan (by CYP2D6 and CYP3A4).

Interactions of CYP450: Induction and Inhibition

Interaction with CYP450 can be through one of the following mechanisms:

  • Two drugs can compete for the same enzyme, thereby leading to slight prolongation of the duration of action of both drugs.
  • One drug can inhibit the metabolism of the other.
  • One drug can induce the metabolism of the other.


Consequences of increased drug metabolism are as follows:

  • Concentration of the drug in the blood decreases
  • Activity of the drug decreases or increases (depending on whether the metabolite is inactive or active, respectively)
  • Pharmacological effect, and the duration of action, of the drug is diminished

E.g., rifampicin, an antitubercular drug, induces the metabolism of the anti-HIV drug class protease inhibitors; this considerably reduces their efficacy. As mentioned, many drugs are metabolized by CYP3A4. Therefore, it is crucial to know at least the most common inducers of this enzyme. These are barbiturates, carbamazepine, corticosteroids, efavirenz, phenytoin, rifampin, pioglitazone, St. John’s wort.

Other examples of CYP inducers are as follows:

CYP1A2 Carbamazepine, Phenobarbital, Rifampin, Omeprazole, St John’s wort
CYP2E1 Ethanol, Isoniazid
CYP2C9 Barbiturates, especially Phenobarbital, Phenytoin, Primidone, Rifampin
CYP2B6 Phenobarbital, Cyclophosphamide


Contrary to CYP induction, inhibition of CYP enzymes would lead to increase in the drug’s concentration, pharmacological activity, and duration of action (of course, if the drug is activated by CYP enzymes, then CYP inhibition will reduce the drug’s activity). Inhibition is usually due to competition; however, certain drugs such as ketoconazole inhibit CYP without being metabolized by it.

Erythromycin, ketoconazole, and ritovarin are important inhibitors as they inhibit several isoenzymes. Natural substances can also inhibit CYP enzymes, e.g. furanocoumarins present in grapefruit inhibit CYP3A4. The inhibitors of CYP3A4 and other CYP enzymes are indicated in the following table:

CYP3A4 Itraconazole, Clarithromycin, Ketoconazole, Erythromycin, Diltiazem
CYP1A2 Ciprofloxacin, Fluvoxamine
CYP2E1 4-Methylpyrazole, Disulfiram
CYP2C9 Tienilic acid, Sulfaphenazole
CYP2B6 Ticlopidine, Clopidogrel
CYP2D6 Fluoxetine, Paroxetine, Methadone, Quinidine

Certain drugs (or their metabolites) bind irreversibly to the CYP enzymes, thereby inhibiting them. Such drugs are known as suicide inhibitors. Important examples of suicide inhibitors are spironolactone, ethinyl estradiol, allopurinol, propylthiouracil, fluroxene, and secobarbital.

P-glycoprotein (MDR-1) Inhibitors

P-glycoprotein (P-gp), also known as multidrug resistance protein 1 (MDR1) or ATP-binding cassette sub-family B member 1 (ABCB1) or cluster of differentiation 243 (CD243), is an important modulator of intestinal drug transport, i.e. its function is to pump drugs out of the cells in the intestinal mucosa back into the lumen. In this way, it participates in presystemic elimination. Therefore, if a drug inhibits intestinal P-gp, it would act like a drug metabolism inhibitor and increase the drug activity and bioavailability.

P-gp inhibitors include verapamil and grapefruit (furanocoumarin). Important drugs that are normally expelled by P-gp (and are therefore potentially more toxic when given with a P-gp inhibitor) include digoxin, cyclosporine, and saquinavir. If P-gp inhibitors are administered along with a CYP (or other enzyme) inhibitor, then the concentration of the drug may reach toxic levels.

Popular Exam Questions

The correct answers can be found below the references.

1. Which of the following is true for individuals who are slow acetylators?

  1. About 50 % of Japanese population is slow acetylators.
  2. They are likely to develop hepatotoxicity after halothane anesthesia.
  3. They are likely to suffer from peripheral nerve damage after isoniazid treatment.
  4. They are unlikely to respond to standard doses of hydralazine therapy for hypertension.
  5. The parents of an individual with slow acetylator phenotype must be slow acetylators too.

2. A 35-year-old woman presented to the clinic with history of suicidal tendencies. To treat her depression, the psychiatrist started her on 75 mg venlafaxine, a serotonin-norepinephrine reuptake inhibitor that is primarily metabolized by CYP2D6 into its active metabolite. However, no mental improvement occurred. The dose was gradually increased to 225 mg/day, but no improvement was document; in fact, her mental situation worsened. She started experiencing tachycardia and anxiety, which are adverse effects of venlafaxine. Serum venlafaxine level was 1300 μg/L (recommended range: 195–400 μg/L), which was very high. Which of the following situations might explain the cause of her refractoriness to venlafaxine?

  1. She drinks a lot of grapefruit juice.
  2. She was administered St John’s wort for her depression.
  3. She was prescribed antifungals (itraconazole).
  4. Patients with depression are likely to not take their medications.
  5. She has a CYP2D6 variant that makes her a poor metabolizer.

3. A 40-year-old man is being treated with triazolam for severe insomnia. Triazolam is primarily metabolized by CYP3A4 and produces pharmacologically inactive metabolites. Which of the following can be safely given to this patient?

  1. Ketoconazole
  2. Fluoxetine
  3. Grapefruit juice
  4. Itraconazole
  5. Erythromycin
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