The duration of drug therapy ranges from a single dose of a drug taken for relieving an acute condition such as headache to drugs taken life-long for chronic conditions such as hypertension, diabetes, asthma or epilepsy.
Pharmacokinetics:
Basic Considerations
The duration of drug therapy ranges from a single
dose of a drug taken for relieving an acute condition such as headache to drugs
taken life-long for chronic conditions such as hypertension, diabetes, asthma
or epilepsy. The frequency of
administration of a drug in a
particular dose is called as dosage
regimen. Depending upon the therapeutic objective to be attained, the duration of drug therapy and the dosage
regimen are decided.
Rational and optimal therapy with a drug depends
upon –
1. Choice of a suitable drug, and
2. A balance between the
therapeutic and the toxic effects.
Both, the therapeutic and the toxic effects, depend
upon the concentration of drug at the site of action which is difficult to
measure. However, it corresponds to a specific concentration of drug in plasma
which can be measured with accuracy. The drug fails to elicit a therapeutic
response when the concentration is below the effective level and precipitates
adverse reactions when above the toxic level. The plasma drug concentration
between these two limits is called as the therapeutic
concentration range or therapeutic
window (the ratio of maximum safe
concentration to minimum effective concentration of the drug is called as the therapeutic index). Thus, in order to
achieve therapeutic success, plasma
concentration of the drug should be maintained within the therapeutic window.
For this, knowledge is needed not only of the mechanisms of drug absorption,
distribution, metabolism and excretion, but also of the kinetics of these
processes i.e. pharmacokinetics. Pharmacokinetics
is defined as the kinetics of drug absorption, distribution, metabolism and
excretion (KADME) and their relationship with the pharmacological, therapeutic
or toxicological response in man and animals. There are two aspects of pharmacokinetic studies –
1. Theoretical aspect – which involves development of
pharmacokinetic models to predict
drug disposition after its administration. Statistical methods are commonly
applied to interpret data and assess various parameters.
2. Experimental aspect – which involves development of biological
sampling techniques, analytical
methods for measurement of drug (and metabolites) concentration in biological
samples and data collection and evaluation.
Several relevant terms can now be defined –
·
Clinical Pharmacokinetics is defined as the application of pharmacokinetic principles in the safe and effective management of
individual patient.
·
Population Pharmacokinetics is defined as the study of pharmacokinetic differences of drugs in various population groups.
·
Toxicokinetics is defined as the application of pharmacokinetic principles to the design, conduct and interpretation of drug
safety evaluation studies.
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