Drug Interactions in Special Populations

Special Populations Overview - Not One Size Fits All

• Special Populations: Groups (e.g., elderly, pediatrics, pregnant, organ impairment) with distinct physiological or pathological traits altering drug response.
• Pharmacokinetic (PK) Variations: Altered Absorption, Distribution, Metabolism (e.g., ↓ CYP450 activity), and Excretion (e.g., ↓ GFR).
• Pharmacodynamic (PD) Variations: Changes in receptor sensitivity, number, or post-receptor events.
• Individualized Therapy: Crucial to tailor dosage, ensuring efficacy and minimizing adverse drug reactions (ADRs) due to ↑ variability.

⭐ Key factors influencing inter-individual variability in drug response include genetics (pharmacogenomics), age, organ function (renal/hepatic), disease state, and concomitant drug use (drug-drug interactions).

Pediatric Pharmacology - Tiny Patients, Big Risks

• Pharmacokinetic (PK) Changes:
  • Absorption: ↑ Gastric pH (neonate), variable emptying.
  • Distribution: ↑ Total Body Water, ↓ Plasma protein binding (esp. neonates).
  • Metabolism: Immature enzymes (e.g., glucuronidation ↓ in neonates).
  • Excretion: ↓ Glomerular Filtration Rate (GFR) (doubles by 1 week, adult levels by 6-12 months).
• Pharmacodynamic (PD) Differences: Altered receptor sensitivity, paradoxical reactions (e.g., benzodiazepines).
• Key Problematic Drugs & Risks: 📌 C.A.S.T.F.
  • Chloramphenicol: Gray Baby Syndrome.
  • Aspirin: Reye's Syndrome.
  • Sulfonamides: Kernicterus (neonates).
  • Tetracyclines: Teeth/bone issues.
  • Fluoroquinolones: Cartilage damage.
• Dosing Considerations: Typically $mg/kg$ or Body Surface Area ($BSA$).

⭐ Gray Baby Syndrome: Caused by chloramphenicol accumulation due to deficient glucuronidation in neonates. Presents with vomiting, flaccidity, hypothermia, gray color, shock, and cardiovascular collapse.

Geriatric Pharmacology - Elderly & Polypharmacy Perils

• PK Changes: ↓Renal clearance (Est. by Cockcroft-Gault: $CrCl = \frac{(140 - age) \times Wt (kg)}{72 \times SCr (mg/dL)} (\times 0.85 \text{ if female})$), ↓hepatic metabolism (Phase I), ↑Vd (lipid-soluble drugs due to ↑body fat), ↓Vd (water-soluble drugs due to ↓body water), ↓serum albumin.
• PD Changes: ↑Sensitivity to drugs (e.g., benzodiazepines, opioids, antipsychotics), impaired homeostatic mechanisms (e.g., ↑risk of orthostatic hypotension, altered thermoregulation).
• Polypharmacy: Concurrent use of ≥5 drugs. Risks: ↑Adverse Drug Reactions (ADRs), drug interactions, non-adherence, prescribing cascade.
• Beers Criteria / STOPP-START: Tools to identify Potentially Inappropriate Medications (PIMs) & guide safer prescribing.
  • Key PIMs: Strong anticholinergics, long-acting benzodiazepines, certain NSAIDs, sliding scale insulin.
  • 📌 Anticholinergic side effects: "Can't see, can't pee, can't spit, can't shit."
• Common Problematic Interactions:
  • Warfarin + NSAIDs (↑bleeding risk).
  • ACE Inhibitors + K-sparing diuretics (↑hyperkalemia risk).

⭐ Benzodiazepines and anticholinergic drugs are frequently implicated in preventable adverse drug events and hospitalizations in the elderly population due to increased sensitivity and accumulation.

Pregnancy & Lactation - Maternal Meds, Safe Steps

• PK Changes (Pregnancy): ↑Vd, ↑GFR, ↑metabolism (some drugs), ↓plasma proteins. N/V → altered absorption.
• Placental Transfer: Factors: lipid solubility, MW, protein binding, transporters.
• Teratogenicity: Fetal harm. Critical period: organogenesis (1st trimester).
  • Major Teratogens (📌 Mnemonic: TWA VIM):
    • Thalidomide: Phocomelia
    • Valproate: NTDs
    • Warfarin: Nasal hypoplasia
    • ACEi: Renal dysgenesis
    • Isotretinoin: CNS/craniofacial defects
    • Methotrexate: CNS/limb defects
  • 
• Labeling: Old FDA (A,B,C,D,X). New: PLLR (descriptive risk summary).
• Lactation: Excretion factors: M/P ratio, pKa, lipid solubility. Resource: LactMed.
  • Avoid/caution: Lithium, amiodarone, cytotoxic drugs.

⭐ Warfarin exposure during 6th-9th week gestation can cause nasal hypoplasia & chondrodysplasia punctata.

Organ Impairment - System SOS, Dose Adjust

• Renal Impairment: Impacts drug excretion. Adjust dose (↓dose, ↑interval) based on CrCl/eGFR.
  • Dose adjust: Aminoglycosides, Digoxin, Lithium, Gabapentin.
  • Nephrotoxic (avoid/monitor): NSAIDs, Contrast media, Amphotericin B.
• Hepatic Impairment: Impacts drug metabolism. Assess with Child-Pugh score (A, B, C).
  • Caution: Paracetamol (high doses), Statins (active/decompensated disease).

⭐ Metformin is contraindicated if eGFR < 30 mL/min/1.73m².

High‑Yield Points - ⚡ Biggest Takeaways

• Pediatrics: Dose by weight/BSA; immature organ function (hepatic/renal) alters drug PK.
• Geriatrics: High risk from polypharmacy & ↓ renal clearance; increased drug sensitivity.
• Pregnancy: Avoid teratogens; drugs cross placenta; physiological PK changes occur.
• Lactation: Assess drug excretion in breast milk & potential infant risk.
• Renal Impairment: Dose adjustment crucial for drugs with significant renal excretion.
• Hepatic Impairment: Reduced drug metabolism & altered protein binding; monitor closely.
• Genetic Polymorphisms: Impact drug metabolizing enzymes (e.g., CYPs), causing varied responses.