For drugs producing a quantal response, onset occurs when plasma concentration reaches a minimum effective level (Cmin). The drug's action duration depends on how long the plasma concentration remains above Cmin.
Two primary factors influence this duration: dose size and the rate of drug removal from the action site. Both depend on the drug's redistribution to poorly perfused tissues and elimination processes. A larger dose promotes rapid onset and prolongs the effect's duration.
Consider a rapidly distributing drug administered as an intravenous bolus dose. Eventually, the plasma concentration falls below which the drug elicits no response. This time defines the drug's duration of effect. This duration is also a function of the drug's half-life; in the elimination phase, each dose doubling increases the duration by approximately one half-life, provided the concentration remains well above Cmin. However, this approach can be harmful for drugs with a narrow therapeutic index. An alternative is administering the same dose when the drug level drops to Cmin, resulting in a minimal increase in duration.
These observations provide valuable insights into optimizing drug dosing strategies, particularly for drugs with narrow therapeutic indices, thereby improving patient outcomes.
Administered drugs begin to act once the minimum effective plasma level, Cmin, is attained. They remain effective as long as the concentration stays above Cmin.
The duration of drug action depends on the dose size, the drug removal rate, tissue binding, and route of administration.
With an increase in dose, Cmin is achieved rapidly, leading to the drug’s quicker onset and longer duration of action.
As per one-compartment kinetics, the following equation provides plasma drug concentration after an IV bolus. If plasma concentration is Cmin, then t becomes td or duration of the drug effect. Rearranging the equation yields td, with the resulting plot between td and log dose being linear.
As a result, doubling the dose increases the effect duration by one half-life.
However, for drugs having a narrow therapeutic index, doubling the dose may cause toxic effects. Instead, the same dose can be repeated when the drug levels fall.
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