Distribution — Pharmacokinetics

Key Points

• Distribution is the second ADME stage: drug disperses from systemic circulation into tissues where drug-receptor interactions produce effects.
• Distribution depends on blood flow, tissue permeability, and protein-binding — only free (unbound) drug is pharmacologically active.
• Protein-bound drug is inactive while bound but acts as a reservoir, releasing drug slowly for prolonged effect. Albumin is the primary binding protein.
• The blood-brain barrier (tight capillary junctions) limits CNS drug entry to lipid-soluble or carrier-assisted drugs.
• The placental barrier is permeable to some drugs — always check drug safety before administering to pregnant clients.
• Life span differences alter distribution: neonates have less body fat and reduced protein-binding; older adults have more body fat and decreased albumin, requiring lower doses of many medications.

Pathophysiology

Distribution is the second stage of pharmacokinetics (ADME). After a drug enters systemic circulation via absorption or direct IV administration, it disperses from vascular spaces into body tissues. At the tissue level, drug-receptor interactions produce the intended (and sometimes unintended) pharmacologic effects.

Drugs are designed to bind most strongly to a specific receptor site, producing a primary effect. However, when a drug binds to additional sites, side effects or adverse effects occur — ranging from tolerable (for example stomach irritation from ibuprofen) to life-threatening.

Distribution is determined by four key factors: blood flow, tissue differences, protein-binding, and physiologic barriers.

Classification

Blood Flow

The circulatory system transports drug throughout the body. Any condition that impairs blood flow reduces drug delivery to target tissues:

• Dehydration → decreased circulating volume → reduced drug delivery
• Atherosclerosis → blocked vessels → impaired drug delivery to distal tissues (for example antibiotic in a diabetic patient with infected toe and atherosclerotic vessels)
• Uncontrolled hypertension → vasoconstriction → reduced tissue perfusion
• Heart failure → decreased cardiac output → reduced drug transport

Tissue Differences

• Tissues with high vascularity (lungs, kidneys, liver, brain) receive drug most rapidly.
• Tissues with low vascularity (adipose tissue) receive drug more slowly.
• Lipophilic drugs (lipid-soluble) preferentially accumulate in adipose tissue, particularly in obese patients.
• Capillary permeability varies by tissue: liver and kidney capillaries are porous (allowing greater permeability); CNS capillaries have tight junctions (blood-brain barrier limits entry).

Protein-Binding

After entering the bloodstream, drug exists in two forms:

• Free (unbound) drug: dissolved in plasma water; pharmacologically active; crosses from blood into tissues to produce drug-receptor effects.
• Protein-bound drug: bound to plasma proteins (primarily albumin); pharmacologically inactive while bound; acts as a drug reservoir — releases drug gradually, producing prolonged action.

For many drugs, protein-bound forms account for 95–98% of the total drug in circulation. Only the free fraction produces therapeutic effects (and toxicity).

Key clinical implications:

• Low albumin (from malnutrition or liver disease) reduces protein-binding capacity → more free drug → enhanced effects and toxicity risk at standard doses.
• Drug competition: Drugs that compete for the same protein-binding sites displace each other, increasing the free fraction of both drugs. For example, aspirin and warfarin compete for the same albumin binding site — administering both simultaneously increases unbound warfarin, raising bleeding risk.

Blood-Brain Barrier (BBB)

The BBB is a nearly impenetrable mesh of tightly joined capillary endothelial cells that protects the CNS from potentially harmful substances. Only two drug types cross effectively:

1. Lipid-soluble drugs — dissolve in the lipid cell membranes of capillary walls.
2. Carrier-assisted drugs — use transport proteins to cross.

Clinical examples:

• Sinemet® (carbidopa/levodopa): Carbidopa acts as a carrier, conveying levodopa across the BBB, where levodopa converts to dopamine for Parkinson’s disease treatment.
• Diphenhydramine (Benadryl): Inadvertently crosses the BBB, depresses CNS, and causes drowsiness as a side effect. (Intentionally exploited in sleep aids.)

Placental Barrier

The placenta regulates transfer of molecules between maternal and fetal circulation. Drug transporters govern whether drugs cross into fetal circulation. Key points:

• The placenta is permeable to some medications; some drugs cause significant fetal harm.
• Many medications have not been studied specifically in pregnant patients and carry unknown fetal risk.
• Nurses must always consult a current, evidence-based drug reference before administering any medication to a pregnant or potentially pregnant patient and must notify the prescriber of any potential safety concerns.

Nursing Assessment

NCLEX Focus

Know the clinical consequences of altered protein-binding (low albumin, drug competition), what types of drugs cross the BBB, and why pregnant patients require special medication review.

• Assess serum albumin levels in patients with malnutrition, liver disease, or chronic illness — low albumin increases free drug and toxicity risk.
• Assess for concurrent medications that may compete for protein-binding sites (for example aspirin + warfarin).
• Assess cardiovascular status, peripheral circulation, and adequacy of blood flow to target tissues.
• Assess patient’s body composition — obesity increases distribution volume for lipophilic drugs and prolongs their duration.
• Assess pregnancy status before administering any medication; consult evidence-based references for fetal safety.
• Assess for CNS side effects of drugs not intended to act centrally (inadvertent BBB crossing — for example sedation from antihistamines).

Nursing Interventions

• Monitor drug effects closely in patients with low albumin; anticipate enhanced effects at standard doses and report toxicity signs promptly.
• Review all concurrent medications for protein-binding interactions before administering drugs with narrow therapeutic windows (for example warfarin, phenytoin, digoxin).
• Educate patients receiving lipophilic medications that drug effects may persist longer than expected if they have significant adipose tissue.
• Verify pregnancy status and consult current drug references before medication administration; escalate safety concerns to the prescriber.
• Educate patients that some medications (for example antihistamines) may cause CNS effects (drowsiness) because they cross the blood-brain barrier.

Low Albumin and Drug Toxicity

Patients with liver disease or malnutrition have reduced albumin levels. Standard doses of highly protein-bound drugs (for example warfarin, phenytoin) will have a greater free fraction in these patients, significantly increasing the risk of toxicity at doses that would be safe in patients with normal albumin.

Life Span Considerations

Neonate and Pediatric

• Decreased body fat (greater proportion of total body water) reduces distribution volume for lipophilic drugs.
• Decreased protein-binding capacity results in more free drug in circulation at standard doses.
• Developing blood-brain barrier allows more drugs to enter the CNS, increasing risk of CNS side effects compared to adults.

Older Adult

• Increased body fat (even at the same BMI) causes lipophilic drugs to accumulate in adipose tissue, prolonging their duration of action.
• Decreased serum albumin reduces protein-binding, increasing the free fraction of highly protein-bound drugs — lower doses are often required.
• These factors combined mean many older adult patients require lower medication doses to achieve therapeutic effects without toxicity.

Pharmacology

Distribution Factor	Clinical Example	Nursing Implication
Blood flow impairment	Atherosclerosis → antibiotic fails to reach infected diabetic toe	Assess peripheral perfusion; report inadequate treatment response
Protein-binding competition	Aspirin + warfarin → increased free warfarin → bleeding risk	Monitor INR closely; review all concurrent medications
Low albumin	Liver disease → excess free phenytoin → toxicity at standard dose	Monitor drug levels; anticipate dose reduction needs
Lipophilic accumulation	Diazepam accumulates in adipose → prolonged sedation in obese patients	Monitor for extended effects; use caution with repeat dosing
Blood-brain barrier	Diphenhydramine → inadvertent CNS depression → drowsiness	Educate patients; caution when operating machinery
Placental barrier	Warfarin crosses placenta → fetal hemorrhage risk	Always verify safety before administering to pregnant patients

Clinical Judgment Application

Clinical Scenario

An 80-year-old patient with cirrhosis is prescribed phenytoin (an antiepileptic) at a standard adult dose. Two days later, the patient develops nystagmus, ataxia, and confusion.

• Recognize Cues: Signs of phenytoin toxicity (nystagmus, ataxia, confusion) in an older adult with liver disease on a standard dose.
• Analyze Cues: Cirrhosis reduces albumin production → less protein-binding → increased free phenytoin fraction. Older age further reduces albumin. Combined effect: toxicity at standard dose.
• Prioritize Hypotheses: Phenytoin toxicity secondary to altered distribution from low albumin is the priority concern.
• Generate Solutions: Hold phenytoin, obtain serum phenytoin level and albumin, notify prescriber for dose adjustment.
• Take Action: Withhold next dose, document neurological assessment, draw stat drug level, and notify provider immediately.
• Evaluate Outcomes: Phenytoin level confirms supratherapeutic range; dose is adjusted; neurological symptoms resolve after levels normalize.

Related Concepts

• pharmacokinetics-and-pharmacodynamics — Distribution is the second of four ADME stages.
• absorption-pharmacokinetics — Absorption precedes distribution; drug must reach systemic circulation before distribution occurs.
• metabolism-pharmacokinetics — After distribution to tissues, drug undergoes hepatic metabolism.
• dopaminergic-therapy — Carbidopa/levodopa as a clinical example of carrier-assisted BBB crossing.
• anticoagulants — Warfarin’s high protein-binding and competition interactions are a key clinical application of distribution principles.

Self-Check

1. Why is only the free (unbound) fraction of a drug pharmacologically active?
2. A patient with cirrhosis is started on a drug that is 98% protein-bound. What distribution change occurs, and what is the nursing concern?
3. What two properties allow a drug to cross the blood-brain barrier?