Hepatic Drug Metabolism in Pediatrics

Overview

Hepatic drug metabolism is essential for determining drug efficacy, clearance, and toxicity in pediatric patients. It involves enzymatic transformation of drugs to facilitate excretion and reduce adverse effects. Enzyme activity varies with age, making pediatric pharmacokinetics distinct from adults.

Phase I: Activation

Goal: Convert lipophilic drugs into more polar, reactive metabolites.

Primary Enzymes: Cytochrome P450 mono-oxygenases
Key Reactions: Hydroxylation, dealkylation, dehalogenation
Mechanism: Insertion of molecular oxygen into substrates
Characteristics:
- Overlapping substrate specificity
- Inducible by drugs, diet, and environment
Important Subfamilies: CYP1A, CYP2B, CYP2C, CYP2D, CYP2E, CYP3A

Phase II: Detoxification

Goal: Conjugate Phase I metabolites to enhance solubility and promote excretion.

Primary Enzymes:
- Glutathione S-transferases (GSTs)
- Uridine diphosphate-glucuronosyltransferases (UGTs)
- Epoxide hydrolases
- Sulfotransferases (SULTs)
- N-acetyltransferases (NATs)
- Glycine conjugation enzymes
Note: Many Phase II enzymes mature postnatally, affecting drug clearance in neonates and infants.

Phase III: Transport and Excretion

Goal: Actively transport drug metabolites out of hepatocytes into bile or blood for excretion.

Primary Transporters:
- P-glycoprotein (ABCB1)
- Multidrug resistance-associated proteins (MRPs)
- Breast cancer resistance protein (BCRP)
Mechanism: ATP-dependent efflux pumps move conjugated metabolites into bile (for fecal excretion) or into blood (for renal excretion).
Pediatric Consideration: Transporter expression and activity are developmentally regulated and may influence drug bioavailability and toxicity.

Genetic Polymorphisms

Polymorphic variants in drug-metabolizing enzymes influence individual responses to medications:

Poor Metabolizers: Increased drug levels, higher risk of toxicity
Rapid Metabolizers: Reduced drug efficacy due to fast clearance
Clinical Impact: Important for predicting drug-induced liver injury (DILI) and tailoring therapy

Pediatric Considerations

Ontogeny: Enzyme expression evolves from fetal to neonatal to adolescent stages
CYP3A7: Dominant in fetal liver; replaced by CYP3A4 postnatally
Clinical Relevance: Pediatric dosing must consider developmental pharmacokinetics
Monitoring: Liver function and drug levels should be closely tracked in children

Drug Metabolism Phases

Phase	Main Reactions	Primary Location	Key Enzymes	Typical Substrates
Phase I	Oxidation, Reduction, Hydrolysis	Liver (Smooth ER)	Cytochrome P450, Flavin Monooxygenases (FMOs), Esterases	Lipophilic drugs, xenobiotics
Phase II	Conjugation (Glucuronidation, Sulfation, Acetylation)	Liver (Cytosol)	Transferases (UGTs, SULTs, NATs, GSTs)	Phase I metabolites, some parent drugs
Phase III	Transport and Excretion	Liver, Kidney, Intestines	Transport Proteins (P-glycoprotein, MRP, BCRP)	Conjugated drug metabolites

Mnemonic: "One Clever Liver Transforms Drugs Swiftly"

One → Phase I (Oxidation, Reduction, Hydrolysis)
Clever → Cytochrome P450 enzymes
Liver → Main site of metabolism
Transforms → Phase II (Conjugation reactions)
Drugs → Substrates (lipophilic drugs)
Swiftly → Phase III (Transport and excretion)