1. Time to peak concentration (Tmax)

At Tmax, peak drug absorption occurs. Drug absorption still continues after Tmax but with a slower rate. At Tmax, rate of absorption is equal to rate of elimination.

Tmax is an approximate of indication of drug absorption rate when comparing different products. Smaller Tmax reflects less time to reach Cmax as the absorption rate for the drug becomes more rapid.

2. Maximum concentration (Cmax)

Cmax is an indication that the drug is sufficiently systematically absorbed to provide a therapeutic response. It can be used as a warning of possibly toxic levels of drug.

It should be noted that when designing the sampling scheme, the first sample should not be Cmax. This is referred to as first point Cmax. First point Cmax questions of measurement of “true” Cmax as there are no other concentrations before Cmax to ensure if this is indeed at maximum. Sufficient early sampling times are needed to avoid this phenomenon. Early time point between 5 and 15 minutes after dosing is usually needed.

3. Area under the concentration time curve (AUC)

AUC reflects measurement of the extent of drug bioavailability and total amount of active drug that reaches the systemic circulation (Equation C5Eq4).

(C5Eq4)

AUC is independent of the route of administration and the process of drug elimination as long as the elimination processes do not change. For many drugs, the AUC is directly proportional to dose.

In some cases, AUC is not directly proportional to the administered dose for all dosage levels (e.g. as the dosage of drug increased, one of the pathways of drug elimination may become saturated). In these cases, bioavailability of the drug is difficult to evaluate because drug kinetics may be dose dependent.

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Modified: 2016-06-12

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