â– LECTURE OVERVIEW: Pharmacokinetics defines how the body handles a xenobiotic, focusing on the concentration-dependent kinetics of drug elimination.
â– KINETIC MECHANISMS:
1. First-Order Kinetics (Normal/Dominant):
- Definition: The rate of drug elimination is directly proportional to the plasma concentration of the drug.
- Half-Life Model: The drug exhibits a constant half-life (t1/2), meaning a constant fraction of the drug is eliminated per unit of time.
- Clearence Stability: Clearance is constant. Most clinical therapeutic agents obey first-order elimination across standard therapeutic ranges.
2. Zero-Order Kinetics (Saturation Kinetics):
- Definition: The rate of drug elimination is completely independent of plasma drug concentration. A constant amount of drug is eliminated per unit of time.
- Carrier Saturation: Occurs when the drug concentration exceeds the saturation point of metabolic enzymes or active transport clearance channels.
- Toxicity Risk: The half-life is variable (longer at higher concentrations). Continued administration easily leads to rapid, toxic drug accumulation.
â– PROFESSOR'S ADVANCED PATHOPHYSIOLOGY:
The cellular cascade undergoes active remodeling in response to sustained stressors. Intracellular signalling involves key phosphorylation tracks and secondary lipid messengers, culminating in altered gene transcription and structural adaptations in target tissues.
â– GERIATRIC PHYSIOLOGIC ADJUSTMENTS:
Older patients display reduced physiological reserves, altered muscle-to-fat distributions, and distinct renal filtration profiles.
[HY-BOARD-1121]
🌟 Dynamic Clinical Key:
Three classic substances exhibit saturated, zero-order kinetics at high or therapeutic doses: Ethanol, Phenytoin (epilepsy management), and High-dose Aspirin (salicylates). In these cases, minor dosage changes can lead to disproportionately massive spikes in serum levels and severe toxicity. Assess patient clearance profiles (creatinine clearance and LFTs) before starting multi-drug regimens to avoid severe toxic accumulation. Always adjust therapeutic doses based on age-related glomerular filtration clearance.