Princípios de Farmacocinética - Parte 3: Curva Farmacocinética

Understanding Pharmacokinetics: Blood Concentration Over Time

Overview of Pharmacokinetic Curves

• The pharmacokinetic curve illustrates the relationship between plasma concentration and time following drug administration, starting from zero at time zero.

• The curve initially rises as absorption occurs faster than elimination, leading to an increase in plasma concentration over time.

• A peak concentration is reached when the rate of absorption equals the rate of elimination, marking a transition to the elimination phase.

Elimination Phase Dynamics

• During elimination, factors such as renal clearance and tissue distribution contribute to decreasing blood concentration levels.

• As drugs distribute into tissues, they may return to circulation until equilibrium is achieved between distribution and redistribution rates.

Multiple Dosing Effects

• In multiple dosing scenarios, initial drug distribution occurs rapidly within the central compartment (blood), followed by gradual tissue distribution until balance is reached.

• Immediate intravenous administration results in 100% bioavailability with no absorption phase; maximum concentration is observed right after administration.

Steady-State Concentrations

• With repeated doses administered every two hours, cumulative effects are noted as new doses are absorbed while existing concentrations decline.

• Achieving steady-state depends on drug characteristics rather than just the number of doses; it typically occurs after approximately four half-lives.

Therapeutic Window Considerations

• At steady state, absorption and elimination rates equalize; this balance ensures therapeutic effectiveness without cumulative toxicity risks.

• Each drug has its own therapeutic window where efficacy is maximized without reaching toxic levels; exceeding this range can lead to adverse effects.

Practical Implications for Drug Administration

• For example, antipyretics may begin showing effects around 40–45 minutes post-administration once within the therapeutic window.

• Careful dose management is crucial; overshooting can lead to dangerous concentrations that exceed safe limits within the therapeutic window.