Phase I Reactions

The CYP enzymes likely evolved as a mechanism by which the host was able to protect itself from toxins ingested from the environment. Most enzymes involved in hepatic drug metabolism are categorized in three distinct families: CYP1, CYP2, and CYP3. Each family is further divided into subfamilies that are designated with capital letters and numbered in the order in which they were discovered. The CYP enzymes are generally conserved across species, but their regulation and catalytic activity vary among species, which highlights the challenges associated with laboratory analysis of drug metabolism.³

Genetic and nongenetic factors contribute to the variability in the enzymatic activity seen across all CYP enzymes.⁴ Approximately 5% of Caucasian populations lack CYP2D6 activity, which is associated with altered metabolism of some drugs.⁵ For example, a lack of CYP2D6 activity enhances the effect of drugs such as haloperidol and metoprolol that require the enzyme for efficient metabolism, whereas codeine, which is metabolized to morphine by CYP2D6, provides little analgesia in a child with CYP2D6 deficiency.⁶

Nongenetic factors that influence CYP activity include concomitant disease states, malnutrition,⁷ and exposure to a host of pharmacologic and naturally occurring compounds. Many drugs can inhibit or stimulate the enzyme system (Table 28-1). Inhibition of the CYPs occurs when drugs compete for the same enzyme. The degree to which this competition becomes clinically significant depends on five factors: the relative amount of the specific CYP, the concentrations of each drug, the degree of pharmacologically active metabolite generated through this system, the importance of the enzyme in elimination of the drugs, and the therapeutic index of the drug.³

Enhanced CYP expression occurs after amplified transcription of the specific gene that is induced by a variety of compounds. For example, rifampin and phenytoin induce CYP3A4 by binding the cytosolic human pregnenolone-X receptor (hPXR) or the steroid xenobiotic receptor (SXR).⁸ The activated receptor translocates into the nucleus, where it binds the regulator elements of the CYP3A4 gene and promotes increased transcription of CYP3A4, which can lead to toxic levels of intermediate compounds, as is the case with erythromycin, or to subtherapeutic levels of cyclosporine in transplant recipients.⁹

Cytochrome P-450 Activity

The superfamily of CYP enzymes is divided into subfamilies based on sequence homology and on demonstration of broad substrate specificities. An important example is the CYP3A subfamily, which is the most abundant group of cytochromes involved in the metabolism of xenobiotics. The three identified isoforms are CYP3A4, CYP3A5, and CYP3A7. CYP3A4 is the most abundant single enzyme in the human liver, accounting for the metabolism of approximately 50% of clinically used pharmaceuticals.¹⁰ CYP3A5 is more commonly found in the kidneys and lungs and to a lesser degree in the liver. CYP3A7 is the predominant isoform in the neonatal liver but is replaced after birth by CYP3A4. Given its critical involvement in hepatic biotransformation of xenobiotics, the CYP3A family of enzymes is used to study and estimate hepatic drug clearance in various age and gender groups.

Changes in the distribution and activity of the CYP enzyme systems occur with hepatic growth and maturation (Fig. 28-2). The CYP3A family is homogeneously distributed across the liver parenchyma in the fetal liver and shortly after birth. However, during postnatal growth, expression of the CYP3A protein shifts toward the centrilobular region of the acinus (i.e., Rappaport zone 1). By adulthood, expression of CYP2A becomes increasingly limited to the zone 1 and zone 2 hepatocytes, with sparse expression occurring in zone 3.¹¹ Other examples of developmental changes in the CYP system include the CYP2C and CYP3A3/4 subfamilies, which have negligible expression in the fetus but have increased expression in the first few weeks of life.^12,13 CYP2D6 reaches adult activity levels within 1 month chronological age; variability thereafter is determined primarily by genetic polymorphisms.^13a

FIGURE 28-2 Changes in metabolic capacity versus percentage of adult activity.

(Modified from Kearns G, Abdel-Rahman SM, Alander SW, et al. Developmental pharmacology—drug disposition, action, and therapy in infants and children. N Engl J Med 2003;349:1157-67.)

Changes in activity of CYP enzyme families and subfamilies have correlated with drug clearance. For example, midazolam clearance correlates with changes in CYP3A4 activity, with decreased clearance in fetal and neonatal livers and with adult clearance rates achieved by 3 months of age.^14,15 In contrast, CYP3A7 activity peaks at about 1 week postpartum and steadily diminishes during the first year of life, reaching approximately 10% of fetal liver activity by adulthood.

Phase II Reactions

Conjugation of lipophilic compounds decreases their lipid solubility and facilitates renal excretion.^16,17 Conjugation reactions (i.e., glucuronidation, sulfation, glutathione conjugation, acetylation, and methylation) are decreased in infants compared with adults.¹⁸

Glucuronidation is catalyzed by uridine 5′-diphosphate (UDP)–glucuronosyltransferase (UGT) family of enzymes, which are derived from assimilation of proteins from separate genes or alternate splicing from single-gene transcripts.¹⁹ UGT enzymes are responsible for the metabolism of several drugs, including phenols, estrogens, and opioids (see Chapter 6). As with the CYP enzymes, individual UGT enzymes demonstrate substrate specificity and can act in concert to metabolize single compounds. Glucuronidation is not fully active in infants and can place this population at risk for toxic drug accumulation.²⁰ Hepatic UGT enzyme concentrations are reduced during fetal and early postnatal development. At 3 months of age, the levels of many UGTs are 25% of those in adults.²¹

UGT1A enzyme activity, which is involved in the conjugation of bilirubin and ethinylestradiol, is decreased in the fetus, but it increases to adult levels within 3 to 6 months after a term delivery.²² UGT1A6, which conjugates acetaminophen and naproxen, has 10% of adult activity in the fetus and neonate, and it achieves only 50% of adult activity by 6 months of age.¹⁸ UGT2B7 is active in the metabolism of the nonsteroidal antiinflammatory drugs naloxone, codeine, and lorazepam. Fetal activity of this enzyme approaches 10% to 20% of adult levels, with a rapid increase to adult levels by 2 months of age.²³

Sulfation is accomplished by sulfotransferases, a family of cytosolic enzymes that are divided into two categories: catechol and phenol sulfotransferase. These enzymes conjugate inorganic sulfate from 3′-phosphoadenosine-5′-sulfophosphate (PAPS) with compounds containing functional hydroxyl groups.¹⁶ The catechol transferases develop earlier in fetal life than the phenol counterparts and appear to exhibit decreased activity in the developing neonate. Although specific sulfotransferase substrates require identification, the activity of these enzymes is increased in fetuses and neonates and theorized to be an efficient conjugation pathway in this age group.

Glutathione S-transferases (GSTs) conjugate glutathione with a broad spectrum of lipophilic and electrophilic compounds. The family of GSTs is composed of up to five different groups in various classes designated µ, α, θ, and π, which are derived from at least three genetic loci.²⁴ Tissue-specific expression of these enzymes has been demonstrated, with the liver expressing the greatest amount of protein. Variable time-dependent expression has also been shown, with α- and π-class GSTs having enhanced expression between 16 and 24 weeks gestation, whereas only the α-class enzymes predominate in the neonate and adult liver.²⁵ The hepatic π-class enzymes disappear from their hepatocellular location by 6 months of age and can be found only in the epithelial cells of the biliary canaliculi. Variations in the developmental expression of this class of enzymes have made it challenging to fully appreciate what is likely a multitude of clinical interactions.²⁵

Acetylation reactions are catalyzed by N-acetyltransferases, which transfer an acetyl group from acetyl coenzyme A to a variety of substrates (e.g., p-aminobenzoic acid, p-aminosalicylic acid, procainamide). Two genes, NAT1 and NAT2, are responsible for yielding two specific enzymes with different allelic forms. Despite having 87% sequence homology, these enzymes exhibit different substrate specificities.²⁶ Both are cytosolic enzymes involved in the biotransformation of several drugs and the bioactivation of several human carcinogens. NAT1 is present in multiple fetal and postnatal tissues and accounts for the most N-acetyltransferase substrate metabolism in children younger than 1 year of age. NAT2 is located primarily in the liver and becomes the dominant acetylator after 1 year of age. NAT2 has polymorphisms with enzyme kinetics that differentiate patients with slow or rapid acetylation capabilities. Infants younger than 1 year of age usually are slow acetylators; subsequent age-dependent alterations lead an individual’s targeted acetylator status.²⁷ Individuals who are genetically destined for rapid acetylation manifest this feature by 2 to 4 years of age.

Inhalational Anesthetic Metabolism

Inhalational anesthetics are poorly metabolized, with 15% to 20% of halothane undergoing biotransformation. Although both oxidative and reductive pathways are involved, the primary pathway of metabolism of halothane is through oxidation to a reactive intermediate, trifluoroacetyl chloride,²⁸ which then undergoes glutathione conjugation.²⁹

Isoflurane is metabolized by CYP2E1 to a limited extent (0.2%).³⁰ Isoflurane is excreted as inorganic fluoride and trifluoroacetic acid after oxidative metabolism in the liver.³¹ Desflurane is the least metabolized (0.02%) of the volatile anesthetics, which is approximately 10% of the rate of isoflurane.³² Both are metabolized in the liver along similar paths because the urinary metabolite for desflurane, trifluoroacetic acid, is the same as for isoflurane.

Only 2% to 5% of inhaled sevoflurane is metabolized in humans by means of the hepatic CYP2E1 enzyme, as occurs for the other ether anesthetics.³³ Oxidation of sevoflurane generates the intermediate formyl fluoride, a highly reactive species thought to generate liver protein adducts. Carbon dioxide and inorganic fluoride are released through this oxidative mechanism, and the final product is hexafluoroisopropanol, which undergoes glucuronide conjugation and is further excreted in the urine (see Chapter 6).³⁴

Neuromuscular Blocking Drugs

Neuromuscular blockade is achieved by depolarizing or nondepolarizing neuromuscular blocking agents (see Chapter 6). The only depolarizing agent still in use is short-acting suxamethonium (i.e., succinylcholine).³⁵ Succinylcholine is hydrolyzed completely by plasma cholinesterases, which are synthesized by the liver.³⁶ Enzyme activity varies with age, but it is decreased in liver disease and has been used as a metric of liver prognosis.³⁷

Nondepolarizing muscle relaxants are divided into aminosteroids (i.e., pipecuronium, pancuronium, vecuronium, and rocuronium) and benzylisoquinolinium diesters (i.e., doxacurium, atracurium, and cisatracurium besylate). Renal and hepatic diseases affect their safety and efficacy.³⁸ Hepatic elimination depends on protein binding, hepatic blood flow, and drug extraction. The volume of drug distribution is increased in hepatic disease, leading to a slower onset and prolonged effect. In children with cholestatic liver disease, such as biliary atresia, uptake of these compounds by the liver is decreased, which decreases plasma clearance and prolongs their effects.^39,40 Approximately 75% of the administered dose is bound to plasma proteins, with most bound to albumin. Despite these problems, children with liver disease and a correspondingly low albumin concentration are at minimal risk for the adverse effects from this group of drugs.⁴¹

Rocuronium is an analogue of vecuronium with a more rapid onset of action. Unlike other aminosteroids, which largely undergo renal excretion, only 12% to 22% of rocuronium is cleared through the kidney.⁴² In patients with hepatic disease, the volume of distribution of rocuronium increased by 33% compared with healthy controls.⁴³ In patients undergoing liver transplantation, the clearance of rocuronium was only slightly reduced by the diseased native liver compared with the functioning allograft and healthy individuals.⁴⁴ Reduced infusion requirements of rocuronium during liver transplantation may indicate graft dysfunction.⁴⁵