Monitoring Drug Therapy

MONITORING DRUG THERAPY

Management of drug therapy in individual patient often requires evaluation of response of the patient to the recommended dosage regimen. This is known as monitoring of drug therapy. It is necessary to ensure that the therapeutic objective is being attained and failure to do so requires readjustment of dosage regimen.

Depending upon the drug and the disease to be treated, management of drug therapy in individual patient can be accomplished by:

1. Monitoring therapeutic effects — therapeutic monitoring

2. Monitoring pharmacological actions — pharmacodynamic monitoring

3. Monitoring plasma drug concentration — pharmacokinetic monitoring.

Therapeutic Monitoring

In this approach, the management plan involves monitoring the incidence and intensity of both the desired therapeutic effects and the undesired adverse effects. A direct measure of the desired effect is considered as a therapeutic endpoint e.g. prevention of an anticipated attack of angina or shortening of duration of pain when attack occurs through the use of sublingual glyceryl trinitrate. In certain cases, definition of therapeutic endpoint may not be clear and onset of toxicity is used as a dosing guide i.e. dosage regimen is titrated to a toxic  endpoint e.g. excessive dryness of mouth with atropine when used as an antispasmodic agent.

Pharmacodynamic Monitoring

In some instances, the pharmacological actions of a drug can be measured and used as a guide to therapeutic process. The response observed may or may not correlate exactly with the therapeutic effect e.g. blood glucose lowering with insulin, lowering of blood pressure with antihypertensives, enhancement of haemoglobin levels with haematinics, etc.

Pharmacokinetic Monitoring

This approach involves monitoring the plasma drug concentration within a target concentration range (called as target concentration strategy) and based on the principle that free drug at the site of action is in equilibrium with the drug in plasma. The strategy is particularly useful when:

1. Therapeutic endpoints are difficult to define or are lacking e.g. control of seizures with phenytoin.

2. The objective is to maintain the therapeutic effect in order to obtain optimum drug use.

3. The probability of therapeutic failure is high as with drugs having low therapeutic indices, erratic absorption, pharmacokinetic variability or when the drug is used in multiple drug therapy.

Successful application of this approach requires complete knowledge of pharmacokinetic parameters of the drug, the situation in which these parameters are likely to be altered and the extent to which they could be altered, and a sensitive, specific and accurate analytical method for determination of drug concentration. Examples of drugs monitored by this method include digoxin, phenytoin, gentamicin, theophylline, etc. The strategy is very useful in individualizing therapy in patients with hepatic or renal impairment when dose adjustments are necessary.