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Introduction to Clinical Pharmacology: General Issues & Fundamentals of Rational Pharmacotherapy

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1. What Is Clinical Pharmacology?

1. Describe the conditions under which drug actions vary among human subjects.
2. Determine the mechanisms underlying this variability, to improve therapy with existing drugs and to identify new therapeutic targets.

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2. Key Domains of Clinical Pharmacology

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3. Pharmacokinetics (PK) — What the Body Does to the Drug

3.1 ADME Framework

3.2 Core PK Parameters

• Half-life (t½): Time for plasma concentration to fall by 50%. Determines dosing interval. t½ = 0.693 × Vd / Clearance
• Clearance (CL): Volume of plasma cleared of drug per unit time. The most important determinant of maintenance dose.
• Volume of distribution (Vd): Apparent volume drug occupies. Large Vd = extensive tissue distribution (e.g., amiodarone).
• Bioavailability (F): Fraction of administered dose reaching systemic circulation (IV = 100%).
• Steady state: Achieved after ~4–5 half-lives. Plasma concentration at steady state is determined by dose and clearance.
• Loading dose: Used when rapid steady state is needed: LD = Vd × Target concentration / F

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4. Pharmacodynamics (PD) — What the Drug Does to the Body

4.1 Mechanisms of Drug Action

• Receptor agonism/antagonism: Most drugs act via specific receptors (GPCRs, ion channels, nuclear receptors, enzyme-linked receptors).
• Enzyme inhibition: e.g., ACE inhibitors, statins (HMG-CoA reductase), COX inhibitors.
• Ion channel blockade: e.g., local anesthetics, Ca²⁺ channel blockers, Na⁺ channel blockers.
• Carrier/transporter inhibition: e.g., SSRIs (serotonin reuptake), thiazides (NaCl cotransporter).
• Direct physicochemical action: e.g., antacids, osmotic diuretics.

4.2 Dose-Response Relationships

• Graded response: Continuous response increasing with dose; described by the sigmoid E_max model.
• Key parameters:
  • E_max: Maximum possible effect
  • EC₅₀: Concentration producing 50% of E_max — measures potency
  • Efficacy (intrinsic activity): Maximum effect achievable (full vs. partial agonist)
• Therapeutic index (TI): TI = TD₅₀ / ED₅₀. Drugs with narrow TI (e.g., digoxin, warfarin, lithium, aminoglycosides) require therapeutic drug monitoring (TDM).

4.3 Tolerance and Tachyphylaxis

• Tolerance: Diminished response with repeated drug administration (e.g., opioids, nitrates).
• Tachyphylaxis: Rapid tolerance after a few doses (e.g., decongestant nasal sprays).
• Mechanisms: receptor downregulation, desensitization, increased metabolism, physiological adaptation.

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5. Variability in Drug Response

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6. Principles of Rational Pharmacotherapy

"The desired goal of therapy with any drug is to maximize the likelihood of a beneficial effect while minimizing the risk of adverse drug reactions (ADRs)."

6.1 The WHO Rational Use Framework (5-Step Process)

1. Define the patient's problem — accurate diagnosis is prerequisite.
2. Specify the therapeutic objective — what outcome are you trying to achieve?
3. Select the appropriate drug — based on efficacy, safety, suitability, and cost (WHO's "P-drug" concept).
4. Start the treatment — appropriate dose, dosage form, duration.
5. Monitor the result — assess efficacy and safety; adjust as needed.

6.2 Dose Selection Principles (Harrison's p. 1900)

• Use prior PK/PD data (from clinical trials or postmarketing experience) to guide initial dosing.
• Titrate to therapeutic endpoint or target plasma concentration (TDM for narrow TI drugs).
• Adjust for:
  • Renal impairment (reduce dose or extend interval based on GFR/CrCl)
  • Hepatic impairment (reduce dose for drugs with high hepatic extraction)
  • Age extremes
  • Drug interactions

6.3 Criteria for Drug Selection

6.4 Avoiding Adverse Drug Reactions (ADRs)

• Type A (Augmented): Predictable, dose-dependent (e.g., bleeding with anticoagulants). Managed by dose reduction.
• Type B (Bizarre): Unpredictable, not dose-dependent, often immunologic (e.g., anaphylaxis to penicillin). Require drug withdrawal.
• Type C: Chronic use effects (e.g., adrenal suppression with long-term steroids).
• Type D: Delayed effects (e.g., carcinogenesis, tardive dyskinesia).
• Type E: End-of-treatment effects (withdrawal reactions, e.g., benzodiazepine withdrawal).

6.5 Drug Interactions

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7. Therapeutic Drug Monitoring (TDM)

• The drug has a narrow therapeutic index.
• There is a clear concentration-effect relationship.
• Response is difficult to assess clinically.

• Sample at trough (just before next dose) for most drugs — reflects minimum steady-state concentration.
• Interpret concentrations in clinical context — a "therapeutic" level in a toxic patient may still indicate toxicity.

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8. Pharmacogenomics in Clinical Practice

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9. Special Populations

Elderly Patients

• Decreased renal clearance (use CrCl, not serum creatinine alone)
• Reduced hepatic first-pass metabolism
• Increased fat:lean ratio → larger Vd for lipophilic drugs
• Polypharmacy risk — use Beers Criteria to identify potentially inappropriate medications

Renal Impairment

• Calculate GFR/CrCl (Cockcroft-Gault)
• Reduce dose or extend interval for renally cleared drugs
• Avoid nephrotoxic drugs or monitor closely (aminoglycosides, NSAIDs, contrast)

Hepatic Impairment

• Use Child-Pugh or MELD score to guide dosing
• Avoid drugs with extensive first-pass metabolism at standard doses
• Watch for hepatotoxic combinations

Pregnancy

• FDA (now PLLR) categories replaced by narrative labeling
• Consider teratogenicity, fetal distribution, placental transfer
• Physiological changes in pregnancy alter PK (increased Vd, increased renal clearance, altered protein binding)

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10. Summary: Pillars of Rational Pharmacotherapy

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