Pharmacokinetics: Dosage Regimens

Key Concepts in Pharmacokinetics

• Dosage Regimen Design and Optimization
  • Critical for determining:
    • Dose: Amount of drug to administer.
    • Dosing Interval: Frequency of administration (e.g., once a day, twice a day).
    • Route of Administration: Method of delivery (e.g., oral vs. IV).

Continuous Infusion

• Easiest way to achieve therapeutic concentrations.
• Graph Dynamics:
  • When drug concentration increases, the drug undergoes metabolism in the liver and excretion via kidneys.
  • Steady State: Achieved when drug dosing rate equals elimination rate, maintaining equilibrium.
  • Steady state concentration is independent of half-life but depends on the infusion rate.

Steady State Concentration

• Definition: Concentration where dosing rate equals elimination rate.
• Time to Reach Steady State:
  • Approximately 4-5 half-lives required to reach steady state.
  • Same time required to eliminate 95% of the drug.

Maintenance and Loading Doses

• Maintenance Dose:
  • Required to maintain steady state concentration.
  • Calculated as: Target Plasma Concentration × Clearance × Dosing Interval / Bioavailability.
  • Clearance Impact:
    • Decreased clearance (e.g., due to renal or hepatic dysfunction) necessitates lowered maintenance dose.
• Loading Dose:
  • Used to quickly achieve steady state in critical conditions.
  • Calculated as: Target Plasma Concentration × Volume of Distribution / Bioavailability.
  • Volume of Distribution affects loading dose size.
    • High volume of distribution requires a higher loading dose.

Practical Application

• Case Study: Female patient with diabetes
  • Calculate oral loading dose given volume of distribution, bioavailability, and target plasma concentration.

Important Relationships

• Loading Dose & Volume of Distribution
  • High volume = High loading dose needed.
• Maintenance Dose & Clearance
  • Decreased clearance = Decreased maintenance dose.

Conclusion

• Understanding dosage regimens is crucial for effective treatment.
• Key formulas and relationships must be memorized and understood for accurate dosage calculations.

These notes summarize the key points from the lecture on pharmacokinetics, focusing on dosage regimens, steady state concentrations, and the importance of loading and maintenance doses. Make sure to review the formulas provided for dosage calculations as they are pivotal for both exams and practical applications.