Pharmacokinetic Parameters
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The three important pharmacokinetic parameters that describe the plasma level-time curve and useful in assessing the bioavailability of a drug from its formulation are
Pharmacokinetic Parameters
The three important pharmacokinetic parameters that
describe the plasma level-time curve
and useful in assessing the bioavailability of a drug from its formulation are
–
1. Peak Plasma Concentration (Cmax)
The point of maximum concentration of drug in plasma is called as the peak and the concentration of drug at
peak is known as peak plasma concentration. It is
also called as peak height concentration and maximum
drug concentration. Cmax is expressed in mcg/ml. The peak plasma
level depends upon –
·
The administered dose
·
Rate of absorption, and
·
Rate of elimination.
The peak represents the point of time when
absorption rate equals elimination rate of drug. The portion of curve to the
left of peak represents absorption phase
i.e. when the rate of absorption is
greater than the rate of elimination. The section of curve to the right of
peak generally represents elimination
phase i.e. when the rate of
elimination exceeds rate of
absorption. Peak concentration is often related to the intensity of
pharmacological response and should ideally be above minimum effective
concentration (MEC) but less than the maximum safe concentration (MSC).
2. Time of Peak Concentration (tmax)
The time for drug to reach peak concentration in plasma (after
extravascular administration) is called as the time of peak concentration. It is expressed in hours and is
useful in estimating the rate of absorption. Onset time and onset of action are
dependent upon tmax. This parameter is of particular importance in
assessing the efficacy of drugs used to treat acute conditions like pain and
insomnia which can be treated by a single dose.
3. Area Under the Curve (AUC)
It represents the total integrated area under the plasma level-time
profile and expresses the total amount of drug that comes into the systemic
circulation after its administration. AUC is
expressed in mcg/ml X hours. It is the most important parameter in evaluating the bioavailability of a
drug from its dosage form as it represents the extent of absorption. AUC is
also important for drugs that are administered repetitively for the treatment
of chronic conditions like asthma or epilepsy.
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