Metabolism of Drugs (Xenobiotics)

OVERVIEW.

Drugs and other xenobiotics typically contain lipophilic moieties, which promote gastrointestinal absorption, penetration of membranes, and retention by the body. The kidney does not readily excrete lipophilic substances for two reasons: (1) lipophilic molecules bind to plasma proteins, and therefore escape glomerular filtration, and (2) if filtered, their lipophilicity increases their reabsorption through the renal tubules. Biotransforming such molecules to hydrophilic metabolites hastens their elimination from the body. The lung, kidney, intestines, skin, and other tissues can all metabolize xenobiotics, but the liver is clearly the chief drug-metabolizing organ. Although biotransformation generally causes pharmacologic deactivation, metabolites are occasionally more active than parent compounds. The metabolism of xenobiotics occasionally produces reactive intermediates that cause liver damage directly or indirectly (by inducing immunopathologic events, as occurs with halothane hepatitis).^{[147] [148]}

PATHWAYS OF DRUG METABOLISM.

The enzymatic pathways that contribute to the hepatic clearance of drugs can be partitioned into three broad categories or phases. Phase 1 reactions typically work through cytochrome P450 (CYP) and increase the polarity of drugs. Phase 2 reactions are conjugations between the drug or a metabolite and an endogenous hydrophilic substance. Phase 3 reactions involve drug elimination via energy-dependent transporter systems. The hepatic clearance of a drug may occur in one or more of these phases.^{[147] [149] [150] [151]}

Phase 1 Reactions (Metabolism)

Phase 1 reactions alter the parent drug by inserting or unmasking a polar group (e.g., OH, NH₂ , SH). The major classes of reactions are oxidations, reductions, and hydrolysis. Compared with parent drugs, metabolites of phase 1 reactions are more hydrophilic and more readily excreted in the bile or urine. These metabolites may also be substrates for phase 2 conjugations.

Microsomal Oxidases and Cytochrome P-450

Microsomal drug oxidases catalyze more than 90% of drug biotransformation reactions. Most of these reactions involve hemoproteins of the cytochrome P-450 (CYP) gene superfamily. The human liver has more than 20 different CYP enzymes^{[147] [149] [150] [151] [152]} ; many of these isozymes contribute to the oxidation of drugs, environmental toxins, steroid hormones, lipids, and bile acids. Hepatocytes of zone 3 have the highest content of CYP proteins. The locations of specific CYP proteins are often responsible for the patterns of liver damage that are characteristic of certain drugs and toxins.

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In the CYP reaction cycle, oxygen binds to heme iron. Oxygen becomes activated after receiving an electron from a flavoprotein reductase—specifically, NADPH:hemoprotein oxidoreductase (CYP).^{[149] [150] [151] [152]} The incorporation of activated oxygen into lipophilic molecules produces substrates for mixed function oxidases. These oxidases take an oxygen atom from O₂ and transfer it to a substrate (e.g., parent drug), and get a second substrate (e.g., NADPH) to supply electrons to reduce the remaining oxygen atom (from O₂ ) to water. Intermediates of mixed function oxidase reactions include free radicals and various reduced oxygen species (ROS); these are highly reactive molecules that induce oxidative stress and can cause hepatocellular injury.^[153]

A multitude of chemicals—including drugs, insecticides, organic solvents, carcinogens, and environmental contaminants—stimulate microsomal drug metabolism.^[154] Some compounds, such as phenobarbital and phenytoin, increase the expression of several different CYP proteins,^{[149] [150] [151] [152]} whereas others selectively induce a specific CYP. Examples of the latter include smoke from cigarettes or cannabis, which induces CYP1A2,^[155] and isoniazid, which is a powerful inducer of CYP2E1. Hypericum, the active ingredient of St. John's wort, and rifampicin are potent inducers of CYP3A4.^[156] Alcohol induces both CYP2E1 and CYP3A4.^{[147] [157]} Inducers of CYP not only affect their own metabolism, but also influence the metabolism and biologic actions of many other substances. Furthermore, CYP inducers, by activating nuclear orphan receptor transcriptional regulators, can increase the expressions of alkaline phosphatase and γ-glutamyl transpeptidase (GGTP), which are part of the "hepatic adaptation" to chronic drug administration.^[147]

Molecular Genetic Basis of CYP Induction

Studies on CYP3A4—the predominant CYP isoform in human liver—provide insight into the molecular genetics of CYP induction.^[158] Inducers of CYP3A4 (rifampin, hypericum) interact with and activate the pregnane x-receptor (PXR). PXR is a transcriptional regulator in the orphan nuclear receptor family.^[159] When activated, PXR and the analogous constitutive androstane receptor (CAR) bind to cognate nucleotide sequences located upstream to the CYP3A4 structural gene, within a "xenobiotic responsive enhancer module" (XREM).^{[158] [160]} The binding of PXR and CAR activates the CYP3A4 promoter (downstream) and induces the synthesis of the mRNA for CYP3A4 protein. Other CYP pathways are similarly regulated, including those involved in bile acid synthesis, where the nuclear receptors include the FXR.^{[158] [160]}

Phase 2 Reactions (Metabolism)

Phase 2 reactions create conjugates of parent compounds (or their metabolites) and endogenous, hydrophilic substrates, such as glucuronic acid, acetate, sulfates, amino acids, and glutathione.^{[147] [161]} These conjugations often involve glucuronic acid and UDP-glucuronosyltransferase in the endoplasmic reticulum. Other enzymes that catalyze phase 2 reactions include sulfatases, glutathione S-transferases, acetyl N-transferases, and amino acid N-transferases. In comparison with their precursors, conjugated metabolites are typically less efficacious, less toxic, more hydrophilic, and more readily excreted in bile or urine.

Phase 3 Reactions (Elimination)

Phase 3 elimination reactions involve ATP-binding cassette (ABC) transport proteins. These proteins use the energy of ATP hydrolysis to drive molecular transport, and are essential for excreting many endogenous and exogenous substances. Included among the ABC proteins are the cystic fibrosis transmembrane conductance regulator (CFTR), canalicular and intestinal copper transporters, multidrug resistance protein (MDR), and multidrug resistance related protein (MRP). MDRs (or MDR-1, formerly termed p-glycoprotein), which are on apical (canalicular) hepatocellular surfaces, play an important role in the transport of cationic compounds (e.g., many anticancer drugs) into the bile.^{[162] [163]}

Another family of ABC proteins (MRP) excretes conjugated molecules. MRP-1, which is on the lateral hepato-cellular surfaces, transports drug conjugates into the sinusoids. MRP-2 (formerly termed canalicular multispecific organic anion transporter [cMOAT]), on canalicular membranes, pumps drug conjugates and endogenous substances (bilirubin diglucuronide, leukotriene-glutathionyl conjugates) into bile. Dysfunction of ABC transport proteins hinders the flow of bile, predisposing to drug accumulation in the body, and cholestatic liver injury.^{[164] [165]}

Determinants of Drug Metabolism

Genetic and environmental influences (including drugs) are the most important of the variables affecting drug metabolism. The genetic makeup controls the expression of CYP enzymes and is chiefly responsible for the more than fourfold differences in rates of drug metabolism among healthy subjects. Many different drugs and chemicals can either stimulate or inhibit drug biotransformation.^{[147] [149] [150] [151]} Nutritional status and disease states influence CYP proteins.^[166] Both obesity and fasting increase CYP2E1,^{[147] [149] [150] [151]} whereas CYP may be inhibited by infusions of nitrogen-free^[167] or nitrogen-rich solutions,^[168] systemic inflammatory disorders,^[169] or fever.^[170] Diseases that affect specific CYP isozymes include diabetes mellitus (increases CYP2E1), hypothyroidism (decreases CYP1A), and hypopituitarism (decreases CYP3A4). ^{[149] [150] [151]} Advanced cirrhosis lowers both total CYP and hepatic perfusion and results in significantly reduced clearances of many substances.^{[149] [150] [151]}

In hepatic drug metabolism, sex and age are relatively unimportant variables. Women express CYP3A4 and CYP2E1 to a greater extent than men, but drug metabolism by these CYP proteins (erythromycin, chlordiazepoxide, midazolam) is just slightly greater in women than men.^[147] It is only at the extremes of the life cycle that age has an impact on biotransformation reactions. In newborns, the hepatic content of certain CYP proteins is only 25% of the adult level; therefore clearances of drugs with a low extraction ratio may be much lower than in adults.

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Anesthetic agents and techniques that decrease hepatic blood flow should be expected to reduce clearances of substances with a high extraction ratio (perfusion-dependent clearance). During halothane anesthesia, decreases in liver blood flow account for the reduced clearances of agents such as fentanyl, verapamil, and propranolol.^[175] However, some anesthetics may also affect hepatic clearance by altering CYP and glucuronosyltransferase activity. ^[176] Ketamine induces its own metabolism, which may contribute to the tolerance that develops to this agent.^[177] Diazepam can increase or decrease its own metabolism.^[178] Halothane-induced decreases of metabolism may be responsible for reduced clearances of certain drugs, such as phenytoin,^[179] warfarin, ^[180] and ketamine.^[181] Both halothane and enflurane cause concentration-dependent decreases in the metabolism of acetaminophen, antipyrine, and sulfanilamide in isolated rat hepatocytes. Halothane inhibits enflurane metabolism, possibly by competing for drug-metabolizing sites on CYP.^[182] In contrast to effects of brief treatments, persistent exposure to halothane can induce CYP (e.g., cytochrome c reductase) and stimulate drug metabolism^[183] ; this may explain the 29% increase in antipyrine metabolism in people exposed to trace amounts of halothane for prolonged periods.^[184]

Pharmacokinetics

Perfusion models of drug elimination incorporate the major determinants of hepatic drug disposition, namely hepatic blood flow, intrinsic hepatic clearance, and protein binding. The extraction ratio (ER) is a measure of the relative efficiency with which the liver extracts or eliminates a given drug (ER = intrinsic hepatic clearance of the drug / hepatic blood flow).^{[185] [186] [187] [188] [189]} Table 19-1 presents a compilation of drugs with high and low extraction ratios. Generally speaking, the liver efficiently extracts calcium channel blockers, β-adrenoceptor antagonists (except atenolol), opioid analgesics, tricyclic antidepressants, and organic nitrates. Poorly extracted compounds include warfarin, aspirin, alcohol, and many anticonvulsants. Changes in intrinsic hepatic clearance or protein binding affect the elimination of drugs with a low ER (i.e., capacity-limited elimination), whereas changes in hepatic blood flow are inconsequential. Conversely, only changes in hepatic blood flow affect the elimination of drugs with

TABLE 19-1 -- Drugs that are efficiently and poorly extracted from blood flowing through the liver

Efficiently Extracted Drugs	Poorly Extracted Drugs
Amitriptyline	Acetaminophen
Desipramine	Amobarbital
Imipramine	Antipyrine
Labetalol	Aspirin
Lidocaine	Clindamycin
Meperidine	Diazepam
Metoprolol	Digitoxin
Morphine	Ethanol
Nortriptyline	Hexobarbital
Pentazocine	Phenobarbital
Propoxyphene	Phenytoin
Propranolol	Tolbutamide
Ranitidine	Valproic acid
Verapamil	Warfarin
Zidovudine

a high intrinsic hepatic clearance (i.e., flow-dependent elimination); changes in protein binding or drug metabolizing enzymes do not ( Table 19-2 ).

Using pharmacokinetic data as the primary basis for clinical decision-making has important limits. For example, laboratory data show that changes in protein binding do not affect the hepatic elimination of highly extracted drugs. Such changes, however, can alter pharmacologic activity by changing the free fraction (active form), volume of distribution, and elimination of drugs. Furthermore, various diseases alter the pharmacokinetics and pharmacodynamics of many drugs. The pathophysiologic changes of advanced liver disease (e.g., hepatic encephalopathy) or renal failure (e.g., uremic encephalopathy) markedly alter the relation between plasma concentrations and pharmacologic responses to certain drugs. In such settings, ensuring efficacy and minimizing toxicity involves adjusting standard drug dosages, based on an analysis of all relevant pharmacologic information.