Metabolism — Pharmacokinetics

Key Points

• Metabolism is the third ADME stage: the body chemically alters drugs (primarily in the liver) to produce metabolites that can be excreted.
• Phase I (modification) renders drugs less active or converts prodrugs to active forms. Phase II (conjugation) produces water-soluble, pharmacologically inert compounds ready for excretion. Phase III involves additional excretion of conjugates from cells.
• Key metabolizing enzymes are monoamine oxidase (MAO) and cytochrome P450 (CYP450), responsible for metabolizing dozens of drugs.
• Factors that reduce metabolism include depot binding, enzyme inhibition, competitive enzyme pathways, and hepatic disease — all result in higher drug levels and toxicity risk.
• Enzyme induction (repeated drug use) accelerates metabolism, causing tolerance and requiring dose escalation.
• Half-life is the time required to eliminate 50% of a drug; liver disease prolongs half-life and increases toxicity risk at standard doses.
• Life span differences: neonates have immature liver enzymes (↓metabolism → ↑drug levels); older adults have declining hepatic function and reduced first-pass metabolism (↑free drug → ↑toxicity risk).

Pathophysiology

Metabolism is the third stage of pharmacokinetics (ADME). After a drug is absorbed and distributed, it undergoes chemical alteration — primarily in the liver — to produce compounds that are more easily excreted. These chemical alterations are called biotransformations. The liver’s role in this process makes it the body’s principal detoxifying organ.

Classification

Phase I Biotransformations (Modification)

Phase I reactions alter the chemical structure of the drug. Most Phase I metabolites are less pharmacologically active than the original compound (detoxification). However, some Phase I reactions produce metabolites that retain or equal the activity of the original drug:

• Example: Diazepam (anxiolytic) → desmethyldiazepam → oxazepam. Both metabolites produce similar physiological and psychological effects as diazepam.

In some cases, Phase I converts an inactive prodrug into its active form:

• Prodrug example: Sulfasalazine (for rheumatoid arthritis) is pharmacologically inert as administered; Phase I modification activates it. Prodrugs may also be designed to bypass certain side effects or toxicities.

Phase II Biotransformations (Conjugation)

Phase II reactions attach the drug molecule to another molecule (conjugation), typically rendering the compound:

1. Pharmacologically inert
2. Water-soluble and easily excreted

Conjugation can occur in the liver, kidney, lungs, intestines, and other organs.

• Example: Oxazepam (active Phase I metabolite of diazepam) is conjugated with glucuronide → becomes physiologically inactive → excreted without further modification.

Phase III Biotransformations

Phase III involves the excretion of conjugates and metabolites from cells after Phase II processing.

Key Metabolizing Enzymes

The two most clinically important metabolizing enzymes are:

1. Monoamine oxidase (MAO): Breaks down catecholamines (dopamine, serotonin, norepinephrine). Inhibited by MAO inhibitors (MAOIs).
2. Cytochrome P450 (CYP450): A superfamily of liver enzymes responsible for metabolizing the majority of drugs in clinical use. The CYP3A4 isoenzyme alone metabolizes nearly 50% of all drugs.

Factors Affecting Metabolism

Depot Binding

Depot binding occurs when drug molecules couple with inactive sites in the body (not receptors), removing the drug from systemic circulation and making it inaccessible for metabolism:

• The drug is sequestered and unavailable for therapeutic action or metabolic breakdown.
• Depot binding slows metabolism and prolongs the drug’s duration of action.
• Example: Tetrahydrocannabinol (THC) is highly lipid-soluble and depot-binds in adipose tissue; its metabolism is drastically slowed, so THC metabolites can be detected in urine weeks after the last use.

Enzyme Induction (Tolerance)

Repeated exposure to the same drug triggers the body to increase production of the enzyme responsible for that drug’s metabolism:

• More enzyme → faster metabolism → lower drug blood levels → reduced effect at same dose.
• This produces tolerance, requiring ever-increasing doses to achieve the original therapeutic effect.
• Example: Patients on long-term opioid analgesics notice progressive reduction in effectiveness over time.

Enzyme Inhibition

Some drugs inhibit metabolizing enzymes, reducing the body’s ability to break down other drugs processed by the same enzyme:

• Inhibition → reduced metabolism → higher drug concentrations → enhanced (or toxic) effects of affected drugs.
• Example: Monoamine oxidase inhibitors (MAOIs) block MAO, increasing concentrations of serotonin and dopamine in the CNS. Concurrent use of other serotonergic drugs (e.g., dextromethorphan in cough syrup) can precipitate serotonin syndrome.

Competitive Enzyme Pathways (CYP450 Competition)

When two drugs share the same metabolic pathway, they compete for the same enzyme binding sites, decreasing the efficiency of metabolism of both:

• The enzyme can only process a limited number of drug molecules at once.
• Example: Alcohol and sedatives are both metabolized by CYP450. If a sedative is taken after alcohol consumption, most CYP450 enzymes are occupied by alcohol — the sedative is poorly metabolized, resulting in accumulation of both drugs and dangerously enhanced CNS depression. Co-administration of alcohol and sedatives can be fatal.

Grapefruit Juice Effect (CYP3A4 Inhibition)

Grapefruit juice and sour (Seville) orange juice contain dihydroxybergamottin, which inhibits the CYP3A4 isoenzyme in the intestinal wall:

• CYP3A4 inhibition → reduced first-pass metabolism → higher-than-intended drug blood levels (as if the patient received a larger dose).
• Affected drug classes include medications for allergies, cardiac conditions (e.g., calcium channel blockers such as felodipine), and certain infections.
• Regular orange juice does not have this effect.
• Patients should avoid grapefruit and sour orange juice when taking drugs metabolized by CYP3A4.

Hepatic Disease

Liver disease (e.g., cirrhosis, hepatitis) reduces the number and activity of liver enzymes, impairing metabolism:

• Slower metabolism → prolonged half-life → drug accumulation → toxicity at standard doses.
• Drug dosing, frequency, and selection must be adjusted based on the patient’s liver function.

Half-Life

Half-life is the time required for the plasma concentration of a drug to decrease by 50%. Key clinical points:

• Short half-life drugs (a few hours) require multiple daily doses to maintain therapeutic levels.
• Long half-life drugs (≥12 hours) can often be dosed once daily.
• Most drugs are eliminated at a rate proportional to their plasma concentration (first-order kinetics).
• Some drugs (e.g., ethanol) are eliminated at a constant rate independent of plasma concentration (zero-order kinetics): ethanol is metabolized at approximately 15 mL/hour regardless of blood alcohol level.
• Liver disease prolongs half-life — the liver can no longer metabolize the drug at its normal rate, causing accumulation.

Nursing Assessment

NCLEX Focus

Know how liver disease, enzyme induction (tolerance), and enzyme inhibition (MAOIs, grapefruit) alter drug metabolism and the resulting clinical consequences. Understand half-life and why it matters for dosing intervals.

• Assess liver function (LFTs, clinical signs of hepatic impairment: jaundice, ascites, encephalopathy) for all patients on hepatically metabolized drugs.
• Assess renal function in concert with hepatic function — both organs participate in drug elimination.
• Assess all concurrent medications for CYP450 interactions (enzyme induction or inhibition).
• Assess for tolerance development in patients on long-term opioids, benzodiazepines, or other enzyme-inducible drugs.
• Assess dietary history for grapefruit or sour orange juice consumption in patients prescribed CYP3A4-metabolized drugs.
• Assess for signs of drug toxicity (especially in patients with liver disease or on multiple interacting drugs).
• Assess life span factors that alter metabolism (see Life Span Considerations).

Nursing Interventions

• Monitor drug levels and clinical response closely in patients with hepatic impairment; anticipate dose reduction and longer dosing intervals.
• Educate patients on MAOIs to avoid serotonergic drugs and foods high in tyramine (which is normally metabolized by MAO — inhibition causes dangerous accumulation).
• Educate patients taking CYP3A4-metabolized medications to avoid grapefruit and sour orange juice; provide a written list of affected drugs.
• Monitor for tolerance in patients on long-term opioids or sedatives; collaborate with prescribers on dose reassessment and consider non-pharmacological adjuncts.
• Counsel patients and families about the danger of mixing sedatives with alcohol (CYP450 competition → potentially fatal CNS depression).
• Consult pharmacists or evidence-based drug references for half-life data and dosing interval recommendations, particularly for drugs with narrow therapeutic indices.
• Monitor for signs of drug accumulation (sedation, confusion, respiratory depression, arrhythmias) when metabolism is impaired.

Hepatic Impairment and Drug Toxicity

Patients with cirrhosis or other liver disease have reduced enzyme activity, prolonged drug half-lives, and decreased first-pass metabolism. Standard doses of hepatically metabolized drugs (e.g., opioids, benzodiazepines, warfarin) can rapidly accumulate to toxic levels. Always verify liver function before prescribing or administering these agents, and anticipate the need for dose reduction.

Life Span Considerations

Neonate and Pediatric

• Neonates and infants: Hepatic enzymes are immature, resulting in decreased capacity to metabolize medications. Drug half-lives are prolonged; blood drug levels are higher than expected for weight-adjusted doses. Lower or less frequent dosing is often required.
• Older children: Once hepatic enzymes fully mature, children may actually metabolize drugs faster than adults and may require proportionally higher weight-based doses to achieve therapeutic blood levels.

Older Adult

• Declining hepatic mass and blood flow with aging reduce the liver’s metabolic capacity — drug half-lives are prolonged and blood levels accumulate.
• Decreased first-pass metabolism means oral drugs that normally undergo extensive hepatic extraction reach systemic circulation at higher concentrations than in younger adults.
• Combined effect: older adults are at significantly higher risk of drug toxicity at standard adult doses — lower doses and/or less frequent dosing are often required.

Pharmacology

Metabolic Factor	Mechanism	Clinical Example	Nursing Implication
Enzyme induction	Repeated drug use → ↑enzyme production → ↑metabolism	Long-term opioid use → tolerance → escalating doses needed	Monitor for tolerance; reassess analgesic regimen
Enzyme inhibition (MAOI)	MAO blocked → ↑serotonin/dopamine → enhanced CNS effects	MAOI + dextromethorphan → serotonin syndrome risk	Review all concurrent medications for serotonergic activity
CYP450 competition	Shared metabolic pathway → reduced metabolism of both drugs	Alcohol + sedative → CYP450 saturated → fatal CNS depression	Educate on alcohol-drug interactions; assess alcohol use
Grapefruit juice (CYP3A4)	Dihydroxybergamottin inhibits CYP3A4 → ↑drug bioavailability	Felodipine + grapefruit juice → elevated blood levels	Educate patients to avoid grapefruit/sour orange juice
Depot binding	Drug sequestered in adipose tissue → slow release/metabolism	THC detectable in urine weeks after last use	Aware that lipophilic drug effects may persist far longer
Hepatic impairment	↓Liver enzymes → prolonged half-life → drug accumulation	Cirrhosis → warfarin toxicity at standard dose	Monitor LFTs; anticipate dose reduction; assess for toxicity signs
Prodrug activation (Phase I)	Phase I converts inactive prodrug to active metabolite	Sulfasalazine → active form after hepatic Phase I	Confirm adequate hepatic function for prodrug activation

Clinical Judgment Application

Clinical Scenario

A 72-year-old patient with known cirrhosis is prescribed an opioid analgesic for chronic pain and reports taking an antibiotic that his neighbor said was safe. Three days after starting the opioid, he develops excessive sedation, confusion, and slow respirations.

• Recognize Cues: Signs of opioid toxicity (sedation, confusion, respiratory depression) in an older adult with cirrhosis on a new opioid.
• Analyze Cues: Cirrhosis reduces hepatic enzyme activity → prolonged opioid half-life → accumulation over days. Age further reduces hepatic metabolism and first-pass effect. Unknown antibiotic may also compete for CYP450 pathways, further impairing opioid metabolism.
• Prioritize Hypotheses: Opioid toxicity due to impaired hepatic metabolism (compounded by age-related decline and possible drug interaction) is the priority.
• Generate Solutions: Hold next opioid dose; obtain medication history including the antibiotic; assess respiratory rate and oxygen saturation; notify prescriber; prepare for naloxone administration if respiratory depression worsens.
• Take Action: Withhold opioid, apply continuous pulse oximetry, notify provider stat, document neurological and respiratory assessment, retrieve antibiotic name to check CYP450 interactions.
• Evaluate Outcomes: Prescriber reduces opioid dose and extends dosing interval; sedation resolves; patient and family educated on reporting confusion or slowed breathing immediately.

Related Concepts

• pharmacokinetics-and-pharmacodynamics — Metabolism is the third of four ADME stages.
• distribution-pharmacokinetics — After distribution to tissues, drug returns to circulation and undergoes hepatic metabolism.
• excretion-pharmacokinetics — After hepatic metabolism, water-soluble metabolites are excreted primarily by the kidneys.
• anticoagulants — Warfarin is highly dependent on CYP450 metabolism; interactions and hepatic impairment dramatically affect INR.
• antidepressants — MAOIs illustrate enzyme inhibition with life-threatening drug interaction potential.

Self-Check

1. What is the difference between a Phase I biotransformation and a Phase II biotransformation? Give one example of each.
2. A patient with cirrhosis is started on a standard opioid dose. Why are they at higher risk for toxicity, and what two metabolism-related mechanisms explain this?
3. A patient asks whether she can drink grapefruit juice while taking her new calcium channel blocker. What is the nurse’s evidence-based response?