Time Course of Drug Effect

The concentration-time course is determined by the drug's half-life and is influenced by pharmacokinetic factors including absorption, distribution, metabolism, and elimination. For example, a drug with a 3-hour half-life reaches peak plasma concentration of 64 ng/ml at 3 hours, then drops below 1% of peak concentration by 24 hours, demonstrating how half-life controls drug levels over time.

Drug effect is calculated using the maximum drug effect and the C50 value, which is the plasma concentration producing 50% of maximal effect. For a drug with C50 of 1 ng/ml, when plasma concentration is half the C50 value at 24 hours, the drug achieves 33% ACE inhibition, showing the direct relationship between concentration and pharmacological response.

When drug concentration exceeds the C80 value, the effect curve becomes nearly flat, meaning further increases in concentration produce negligible changes in drug effect. This plateau region indicates the drug has reached maximum therapeutic response and additional concentration increases do not enhance the pharmacological outcome.

Between C80 and C20 concentrations, the effect curve closely resembles a straight line, indicating a linear relationship between drug concentration changes and drug effect. Within this therapeutic window, changes in plasma concentration directly correlate with proportional changes in pharmacological response, making dose adjustments predictable.

When drug concentration drops below the C20 value, the effect curves become nearly parallel, indicating that further decreases in concentration have minimal impact on drug effect. This suggests the drug's pharmacological action is largely exhausted at low concentrations, and additional dose reductions produce little additional change in response.

Half-life directly controls how quickly drug plasma concentration declines, which in turn determines the duration and intensity of drug effect over time. A shorter half-life means faster concentration decline and shorter effect duration, while a longer half-life maintains therapeutic concentrations and effects for extended periods after dosing.

Understanding the concentration-effect relationship allows clinicians to optimize dosing by targeting the linear therapeutic range between C80 and C20, where dose changes produce predictable effect changes. This knowledge helps establish rational dosage regimen maintenance dose and loading dose strategies that maximize therapeutic benefit while minimizing toxicity risk.