Wikipedia

Drug metabolism

(Redirected from Drug duration)

Drug metabolism is the biochemical modification and biotransformation of drugs by humans and other animals, usually through specialized enzymatic systems. It is a major aspect of pharmacokinetics and clinical pharmacology. Within this framework, it constitutes the metabolism component of ADME (absorption, distribution, metabolism, and excretion) and influences therapeutic therapeutic efficacy, toxicity, elimination, drug interactions, prodrug activation, and interindividual variation in drug response. [1] In the broader context of foreign-compound handling, drug metabolism is a clinically oriented subset of xenobiotic metabolism concerned with therapeutic agents and their effects on drug exposure, response, and toxicity, whereas the metabolism of environmental and other non-pharmaceutical compounds is covered in xenobiotic metabolism.[2]

Drug metabolism within ADME: from administration to clinical response

Many pharmaceutical drugs are lipophilic compounds that readily cross cell membranes and therefore require enzymatic conversion into more hydrophilic metabolites that can be eliminated through urine or bile.[3][4] In humans, most drug metabolism occurs in the liver, although the gastrointestinal tract, lungs, kidneys, and other tissues also contribute.[5] Drug metabolism is commonly described using a three-phase framework: Phase I reactions introduce or expose functional groups, Phase II reactions conjugate drugs or metabolites to endogenous polar molecules, and Phase III processes transport metabolites for excretion.[6][7][8] These pathways, especially those mediated by the cytochrome P450 enzyme family, are major determinants of drug disposition and phenomena such as the first pass effect.[5]

Drug metabolism varies substantially among individuals and is influenced by factors including age, disease, diet, gut microbiota, inherited genetic variation (pharmacogenetics), and exposure to enzyme inducers or inhibitors.[9][10][11][12] Expression of metabolic enzymes and transporters is further regulated by xenobiotic-sensing receptors such as the pregnane X receptor, constitutive androstane receptor, and aryl hydrocarbon receptor, contributing to changes in clearance and drug interaction profiles.[13][14] These sources of variability influence efficacy, toxicity, therapeutic index, multidrug resistance, and the development of personalized medicine approaches including pharmacogenetic testing.[15][16][17]

Drug metabolism may inactivate drugs, generate active metabolites, or bioactivate prodrugs and therefore underlies many clinically important drug and food interactions.[18][19][20] Integration of metabolic data with pharmacokinetic principles, including hepatic clearance models, physiologically based pharmacokinetic (PBPK) modeling, and in vitro–in vivo extrapolation, supports prediction of systemic exposure and optimization of drug development and clinical dosing.[21][22][23] The field emerged from nineteenth-century studies of chemical biotransformation and became established through the classification of metabolic reactions and the discovery of cytochrome P450 enzymes.[24][25][26]

Lipophilicity, permeability, and metabolic clearance

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Many pharmaceutical drugs are lipophilic compounds that diffuse across cell membranes and enter cells without requiring specific transport proteins. This property is important for drug absorption and distribution, but it also means that organisms cannot completely prevent exposure to membrane-permeable drugs and other foreign compounds. As a result, drug metabolism systems evolved to chemically modify and eliminate a wide range of structurally diverse compounds.[3]

Cell membranes act as hydrophobic permeability barriers that restrict the passive diffusion of most hydrophilic molecules. The uptake of endogenous metabolites and many therapeutic agents is therefore mediated by selective transport systems.[4] In contrast, many hydrophobic drugs can readily cross biological membranes and therefore require enzymatic conversion into more polar metabolites that can be excreted through urine or bile.

Drug-metabolizing enzymes consequently display broad substrate specificities and are capable of acting on many chemically unrelated compounds.[3] In humans, these systems are concentrated primarily in the liver and include enzymes such as the cytochrome P450 oxidases, which catalyse reactions that increase the polarity of drugs and facilitate their elimination.

In addition to metabolizing xenobiotic drugs, cells also contain specialized systems that detoxify reactive by-products generated during normal metabolism. Examples include the glyoxalase system, which removes the reactive aldehyde methylglyoxal,[27] and antioxidant systems that eliminate reactive oxygen species.[28]

Phases

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Phase I and Phase II metabolism of a lipophilic drug.

Drug metabolism in humans and other animals is commonly described using a three-phase framework derived from the broader principles of xenobiotic metabolism. Phase I reactions introduce or expose functional groups, phase II reactions conjugate drugs or their metabolites to endogenous polar molecules, and phase III processes export these species from cells into bile or urine. Together, these steps convert lipophilic drugs into more polar metabolites and define overall clearance, bioavailability, and the potential for drug–drug interactions.[6][7][8]

Sites in the body

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Quantitatively, the smooth endoplasmic reticulum of the liver cell is the principal organ of drug metabolism, although every biological tissue has some ability to metabolize drugs. Factors responsible for the liver's contribution to drug metabolism include that it is a large organ, that it is the first organ perfused by chemicals absorbed in the gut, and that there are very high concentrations of most drug-metabolizing enzyme systems relative to other organs. Drugs absorbed in the gastrointestinal tract enters the liver through the hepatic portal system. The drug is said to show the first pass effect if it becomes significantly metabolized before reaching systemic circulation.

Other sites of extrahepatic drug metabolism include epithelial cells of the GI tract, lungs, kidneys, and skin. These sites are usually responsible for localized toxicity reactions.

Modifying factors

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The duration and intensity of the pharmacological action of many lipophilic drugs are determined by the rate at which they are metabolized to inactive products. The Cytochrome P450 monooxygenase system (CYP) is particularly important in this process. In general, factors that increase metabolic activity (enzyme induction) decrease drug exposure, whereas inhibition increases drug concentrations and prolongs drug action. However, for prodrugs that require metabolic activation, enzyme induction may increase toxicity rather than reduce it.[medical citation needed][29]

Drug interactions

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Many clinically important drug interactions result from induction or inhibition of drug-metabolizing enzymes. For example, the prodrugs cyclophosphamide and ifosfamide are activated primarily by CYP2B6 and CYP3A4 into cytotoxic metabolites such as phosphoramide mustard and chloroacetaldehyde. Co-administration of strong CYP inducers such as phenytoin or rifampicin can accelerate bioactivation and increase the risk of toxicity, including severe myelosuppression and hemorrhagic cystitis.[30][31][32]

Drug interactions are commonly evaluated relative to theoretical models of noninteraction, including Loewe additivity and Bliss independence.[33][34][35] Interactions may be synergistic, producing greater-than-expected effects or toxicity, or antagonistic, reducing therapeutic efficacy.

Physiological factors

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Physiological factors affecting drug metabolism include age, sex, diet, enterohepatic circulation, gut microbiota, and inherited genetic variation (pharmacogenetics).[9][10]

Age

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Drug metabolism is generally slower in fetal, neonatal, and elderly individuals than in healthy adults.

Gut microbiota

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Gut microorganisms can alter drug metabolism through chemical modification of drugs, thereby affecting efficacy and toxicity. For example, Eggerthella lenta can inactivate digoxin.[10]

Pharmacogenetics

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Genetic polymorphisms in drug-metabolizing enzymes contribute substantially to interindividual variability in drug response.[10] One well-known example is the alcohol flush reaction, caused by variants in ALDH2, which reduce aldehyde dehydrogenase activity and impair acetaldehyde metabolism.[36][37][38]

Polymorphisms in enzymes such as N-acetyltransferase, CYP2D6, CYP3A4, DPYD, and UGT1A1 can alter drug clearance and toxicity risk. Slow acetylators with reduced NAT2 activity are more susceptible to adverse effects from drugs such as isoniazid, hydralazine, procainamide, and phenelzine.[39][40]

Genotyping for some drug-metabolizing enzymes is now recommended prior to therapy. For example, DPYD testing is used before treatment with 5-fluorouracil or capecitabine, and UGT1A1 testing before irinotecan administration, in order to reduce the risk of severe toxicity.[41]

Regulation by xenobiotic receptors

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Expression of drug‑metabolizing enzymes and transporters is dynamically regulated by ligand‑activated xenobiotic‑sensing nuclear receptors, including the pregnane X receptor (PXR), constitutive androstane receptor (CAR), and aryl hydrocarbon receptor (AhR). When activated by therapeutic drugs and other xenobiotics, these receptors upregulate overlapping sets of Phase I and Phase II enzymes and efflux transporters, leading to enzyme induction, altered drug interaction profiles, and changes in clearance and exposure that can necessitate dose adjustment or alternative therapy.[13][14] The broader roles of these receptors in coordinating xenobiotic defense across different organisms and chemical classes are described in xenobiotic metabolism#Xenobiotic sensing and regulation.

Other factors

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Dose, frequency, route of administration, tissue distribution, and protein binding can also influence drug metabolism.[42]

Disease

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Pathological conditions including liver, kidney, and heart disease may substantially alter drug metabolism and clearance.[11][43][12]

Clinical significance

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Drug metabolism has major clinical implications because it determines drug exposure, therapeutic response, and the risk of adverse effects in individual patients.[15][44][45] Variability in metabolic capacity due to genetic polymorphisms, comorbid disease, age, and drug–drug interactions can lead to underexposure with loss of efficacy or overexposure with toxicity at standard doses.[15][16] For these reasons, characterization of metabolic pathways is an essential component of dose selection, clinical trial design, and rational management of combination drug therapy.[45][16]

First-pass metabolism

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First-pass metabolism (first-pass effect) refers to presystemic biotransformation, primarily in the gut wall and liver, that occurs after oral administration and reduces the fraction of active drug reaching the systemic circulation.[15][44][46] The extent of first-pass metabolism influences oral bioavailability and may necessitate higher oral doses, alternative routes of administration, or specific formulation strategies.[46][47]

Prodrugs

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A prodrug is a pharmacologically inactive or less active compound that is designed to undergo enzymatic or chemical conversion in the body to release an active drug.[15][48] Prodrug strategies exploit predictable metabolic pathways, including first-pass metabolism, to improve oral bioavailability, solubility, tissue targeting, or tolerability of therapeutic agents.[48]

Active metabolites

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Active metabolites are biotransformation products that retain pharmacological activity and can contribute substantially to both therapeutic and adverse effects of a parent drug.[15][49] Recognition of clinically important active metabolites is critical for interpreting dose–response relationships, understanding prolonged or delayed effects, and anticipating toxicity in patients with impaired clearance or altered metabolism.[49][50]

Drug toxicity

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Drug metabolism can decrease toxicity through detoxification of xenobiotics but can also generate reactive or toxic metabolites that cause organ injury.[15][51][52] Bioactivation to reactive intermediates, often mediated by cytochrome P450 enzymes, is an important mechanism underlying dose‑related and idiosyncratic hepatotoxicity, hypersensitivity reactions, and other forms of drug‑induced tissue damage.[52]

Therapeutic index

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The therapeutic index (TI) is commonly defined as the ratio of the median toxic dose (TD50) to the median effective dose (ED50).[53] Metabolic clearance is a key determinant of the therapeutic index because it influences systemic exposure to parent drug and metabolites at a given dose.[44][16] Alterations in drug metabolism caused by enzyme induction, enzyme inhibition, or genetic variation can therefore significantly change the therapeutic index of a drug, particularly for drugs with narrow safety margins. Reduced metabolism may increase drug concentrations and toxicity, whereas accelerated metabolism may reduce efficacy. Interindividual differences and drug–drug interactions that alter metabolic clearance can narrow or widen a drug’s effective and safe concentration range, sometimes necessitating dose adjustment or therapeutic drug monitoring for medicines with a narrow therapeutic index.[16]

Personalized medicine

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Variability in drug‑metabolizing enzymes provides a major rationale for personalized medicine approaches such as pharmacogenetic testing and individualized dosing regimes.[15][54] Genotype‑guided selection and dosing of drugs metabolized by polymorphic enzymes, for example CYP2D6, CYP2C19 or thiopurine S‑methyltransferase, can improve efficacy and reduce the risk of serious adverse drug reactions.[17]

Multidrug resistance

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Multidrug resistance is the ability of tumour cells, microorganisms, or other pathological cells to resist the effects of multiple structurally and functionally distinct drugs, often through mechanisms that reduce intracellular concentrations of active drug.[55][56] Altered expression or function of drug-metabolizing enzymes and associated transporters can contribute to multidrug resistance by lowering intracellular concentrations of active drug.[15][56] In oncology and infectious diseases, induction of metabolic pathways or efflux transporters in tumour or microbial cells, as well as host variability in drug metabolism, can reduce exposure to active drug and compromise treatment outcomes.[55][56]

Clinical examples

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The following examples illustrate how differences in drug-metabolizing enzymes and transport pathways translate into clinically important variability in efficacy, toxicity, and drug–drug interactions. They highlight common mechanisms, including prodrug activation, enzyme inhibition or induction, and formation of reactive metabolites, and show how these mechanisms guide prescribing decisions, drug monitoring, and guideline recommendations.

Prodrug activation: clopidogrel

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Clopidogrel is a thienopyridine antiplatelet prodrug that requires oxidative bioactivation by hepatic cytochrome P450 enzymes, particularly CYP2C19, to form an active thiol metabolite that irreversibly inhibits the P2Y12 receptor on platelets.[57][58] Reduced CYP2C19 function, due to loss-of-function genetic polymorphisms or concomitant use of CYP2C19 inhibitors, decreases the formation of the active metabolite and attenuates the antiplatelet effect.[57][58] Such variability in clopidogrel metabolism has been associated with increased rates of thrombotic events, particularly stent thrombosis and myocardial infarction, and has prompted consideration of alternative P2Y12 inhibitors or genotype-guided therapy in some clinical settings.[58][18]

Prodrug activation: codeine

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Codeine is a prodrug that requires O-demethylation by cytochrome P450 2D6 (CYP2D6) to form morphine, which contributes substantially to its analgesic effect.[59][60] Individuals who are CYP2D6 poor metabolizers may obtain little or no analgesia from standard codeine doses, whereas ultrarapid metabolizers can produce higher morphine concentrations and are at increased risk of opioid toxicity, including serious respiratory depression.[59][61][60] Concerns about variable metabolism and reports of fatal toxicity in children, particularly in CYP2D6 fast metabolizers, have led regulatory agencies and professional societies to recommend limiting or avoiding the use of codeine in paediatric patients and to favor alternative analgesic strategies.[62][63]

Food–drug interaction: grapefruit juice

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Grapefruit juice contains furanocoumarins and related compounds that inhibit cytochrome P450 3A4 (CYP3A4) in the intestinal wall, leading to reduced first-pass metabolism of susceptible drugs.[64] For oral medications that undergo extensive intestinal CYP3A4-mediated presystemic metabolism, co-administration with grapefruit juice can markedly increase systemic exposure and thereby heighten the risk of concentration-dependent adverse effects.[64] This interaction illustrates the clinical importance of enzyme inhibition and first-pass metabolism in determining the bioavailability and safety of many drugs.[64]

Enzyme induction: rifampicin

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Rifampicin is a rifamycin antibiotic that acts as a potent inducer of several drug-metabolizing enzymes and transporters, including cytochrome P450 3A4 (CYP3A4) and P-glycoprotein, predominantly through activation of the pregnane X receptor (PXR).[19][65][66] Induction by rifampicin can increase the clearance and reduce the plasma concentrations of many co-administered drugs, including oral contraceptives, anticoagulants, and certain antiretroviral agents, and has been associated with loss of efficacy and therapeutic failure.[19] This interaction illustrates the clinical relevance of enzyme and transporter induction in altering drug exposure and may necessitate dose adjustment or the selection of alternative therapy.[19][65]

Reactive metabolite toxicity: acetaminophen

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At therapeutic doses, acetaminophen (paracetamol) is primarily cleared by hepatic conjugation pathways, including glucuronidation and sulfation, which produce non-toxic, water-soluble metabolites.[20][67] A small fraction is oxidized by cytochrome P450 enzymes, particularly CYP2E1, to form N-acetyl-p-benzoquinone imine (NAPQI), a highly reactive intermediate that is normally detoxified by conjugation with glutathione.[20][67] In overdose or in conditions that deplete glutathione, the accumulation of NAPQI leads to covalent binding to cellular proteins, mitochondrial oxidative stress, and centrilobular hepatocellular injury that can progress to acute liver failure.[20][67] This example illustrates how reactive metabolites generated during drug metabolism can be central mediators of toxicity.[67]

Pharmacokinetic integration

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The integration of drug metabolism data with pharmacokinetic principles enables quantitative prediction of systemic drug exposure and supports clinical dose optimization across diverse patient populations.[21][22]

Hepatic extraction and clearance

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Drug metabolism is a primary determinant of systemic drug exposure, quantified through pharmacokinetic parameters including area-under-the-curve (AUC), clearance, and elimination half-life. The hepatic extraction ratio (EH) describes the fraction of drug irreversibly removed during a single pass through the liver and ranges from 0 (no extraction) to 1 (complete extraction). This ratio is determined by the relationship between intrinsic clearance (CLint, the liver's enzymatic capacity to metabolize drug) and hepatic blood flow (QH, approximately 90 L/hour in humans).[68]

The hepatic extraction ratio can be defined experimentally as: where is the hepatic extraction ratio, is the drug concentration entering the liver, and is the drug concentration leaving the liver.

According to the well-stirred model of hepatic clearance, hepatic extraction ratio can be predicted from physiological and drug-specific parameters as: where is the unbound fraction of drug in blood, is intrinsic hepatic metabolic clearance, and is hepatic blood flow.

For drugs with high extraction ratios (EH > 0.7), hepatic clearance becomes blood flow-limited and largely independent of enzyme activity or protein binding; conversely, drugs with low extraction ratios (EH < 0.3) exhibit clearance limited by intrinsic metabolic capacity rather than perfusion. This distinction has critical implications for first-pass metabolism following oral administration, where hepatic bioavailability may be approximated as 1 − EH for drugs that are completely absorbed and undergo negligible intestinal first-pass metabolism.[69]

Physiologically-based pharmacokinetic modeling

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Physiologically based pharmacokinetic modelling (PBPK) modeling integrates drug-specific properties (molecular weight, lipophilicity, protein binding) with physiological parameters (organ volumes, blood flows, metabolic enzyme abundances) to mechanistically predict drug disposition in diverse patient populations.[70] By incorporating tissue-specific expression of drug-metabolizing enzymes (cytochrome P450s, conjugating enzymes) and transporter proteins, PBPK models simulate how metabolic pathways influence systemic exposure, tissue distribution, and elimination across different patient populations.[71][72] These models enable prediction of untested clinical scenarios including metabolism-mediated drug-drug interactions, effects of hepatic impairment on metabolic clearance, and age-related changes in enzyme activity affecting pediatric dosing without conducting dedicated clinical trials.[73]

PBPK models commonly represent organs as interconnected compartments governed by mass-balance equations describing drug transport and partitioning between blood and tissues:[74]

where is tissue volume, is tissue drug concentration, is tissue blood flow, is arterial drug concentration, and is venous drug concentration exiting the tissue.

In vitro to in vivo extrapolation

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In vitro to in vivo extrapolation (IVIVE) scales metabolic stability data from human liver microsomes or hepatocytes to predict human clearance. Intrinsic clearance measured in vitro is scaled using physiological factors (microsomal protein per gram liver, liver weight) and empirical correction factors to account for differences between simplified in vitro systems and intact liver function. While microsomal data with physiological scaling factors consistently underpredict in vivo clearance, empirical scaling factors (typically 5- to 10-fold corrections) substantially improve prediction accuracy.[75][76][77]

Intrinsic clearance measured in vitro can be scaled to whole-liver intrinsic clearance as:[77]

is whole-liver intrinsic clearance, is intrinsic clearance measured in vitro, is microsomal protein per gram liver, and is liver weight.

Predicted hepatic clearance can then be estimated using the well-stirred model:[75]

is hepatic clearance, is hepatic blood flow, is the unbound fraction of drug in blood, and is whole-liver intrinsic clearance.

Regulatory applications

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PBPK models incorporating drug metabolism parameters are increasingly accepted by regulatory agencies including the FDA for supporting clinical pharmacology decisions, justifying clinical study designs, and waiving studies in special populations when adequately validated.[23][78][79] Common metabolism-related regulatory applications include predicting drug-drug interactions involving metabolic enzyme inhibition or induction, assessing the impact of genetic polymorphisms in drug-metabolizing enzymes on exposure, and evaluating dose adjustments needed in patients with hepatic impairment affecting metabolic capacity.[23][79] The FDA issued guidance in September 2018 outlining format and content expectations for PBPK analyses submitted to support drug applications.[23] PBPK model predictions can be used to support decisions on whether, when, and how to conduct certain clinical pharmacology studies, particularly those examining metabolic pathways and their variability, and to support dosing recommendations in product labeling, with acceptance determined on a case-by-case basis considering the quality, relevance, and reliability of the metabolic and pharmacokinetic analyses.[23][80]

History

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Drug metabolism emerged from broader nineteenth- and early twentieth-century investigations into the biotransformation and detoxification of foreign compounds, including medicinal agents, aromatic compounds, and other xenobiotics (see Xenobiotic metabolism § History).[24][81] Early work demonstrated that administered drugs could undergo chemical transformation in the body through reactions such as oxidation and amino acid conjugation, altering pharmacological activity and facilitating elimination.[24] By the early twentieth century, additional pathways including oxidation–reduction, methylation, acetylation, and sulfonation had been recognized in the metabolism of therapeutic agents.[24][81]

Drug metabolism became a distinct discipline during the mid-twentieth century alongside the development of modern pharmacology and pharmacokinetics. Richard Tecwyn Williams helped formalize the conceptual framework for metabolic biotransformation by classifying reactions into phase I and phase II processes in his 1947 monograph Detoxication Mechanisms.[25][24] Subsequent research identified microsomal cytochrome P450 enzymes as the principal catalysts of phase I drug oxidation and glutathione S-transferases as major phase II enzymes.[82][83][84] These discoveries established the biochemical basis of drug clearance, drug interactions, metabolic bioactivation, and interindividual variability in therapeutic response.[82][83][84]

Since the late twentieth century, drug metabolism research has advanced through improved characterization of enzyme systems and regulatory mechanisms, with particular emphasis on interindividual variability and prediction of human clearance.[85][26] Major developments included quantitative proteomic methods for measuring enzyme abundance in human tissues, enabling more accurate physiologically-based pharmacokinetic modeling and in vitro-in vivo extrapolation of clearance predictions.[86] Recognition of membrane transporter proteins—particularly organic anion-transporting polypeptides (OATPs), P-glycoprotein, and breast cancer resistance protein (BCRP)—as critical determinants of drug disposition led to regulatory guidance for assessing transporter-mediated drug-drug interactions alongside metabolic pathways.[85][86] Non-cytochrome P450 enzymes including UDP-glucuronosyltransferases, aldehyde oxidase, and carboxylesterases received increased attention as significant contributors to drug clearance, though predicting their in vivo activity from in vitro systems remains challenging.[86]

See also

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References

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Further reading

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  • Ioannides C (2001). Enzyme Systems That Metabolise Drugs and Other Xenobiotics. John Wiley and Sons. ISBN 0-471-89466-4.
  • Ioannides C (1996). Cytochromes P450: Metabolic and Toxicological Aspects. CRC Press Inc. ISBN 0-8493-9224-1.
  • Awasthi YC (2006). Toxicology of Glutathionine S-transferses. CRC Press Inc. ISBN 0-8493-2983-3.
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