The plasma drug concentration-time curve is a crucial tool in pharmacokinetics, representing the drug's concentration in plasma at different time intervals post-administration. This curve illustrates the drug's journey from absorption into the systemic circulation, distribution to body tissues, and eventual elimination through excretion or biotransformation.
Two pivotal parameters are the minimum effective concentration (MEC) and the minimum toxic concentration (MTC). The MEC is the lowest drug concentration needed to produce a therapeutic effect, assuming equilibrium between plasma and tissue concentrations. Meanwhile, the MTC is the concentration threshold beyond which toxic effects occur. The time taken for the drug to reach MEC is called the onset time.
The intensity of the pharmacological effect is proportional to the number of occupied drug receptors, suggesting that higher plasma drug concentrations yield a greater response. The duration of drug action is the time span between the onset time and when the drug concentration reverts to the MEC.
The therapeutic window, defined as the range between MEC and MTC, indicates a drug's safety margin. A wider therapeutic window signifies a safer drug profile.
Pharmacokinetics also considers parameters like peak plasma level (Cmax), time for peak plasma level (Tmax), and the area under the curve (AUC). Cmax is linked to the dose, absorption rate constant, and the drug's elimination constant, while Tmax roughly indicates the average rate of drug absorption. Lastly, the AUC corresponds to the total amount of systemically absorbed drug.
