Pharmacokinetic and Pharmacodynamic Characteristics of the Drug

By: | Views: 21423 | Date: 24-Apr-2011

A detailed knowledge of the ADME characteristics of a drug is essential in the design of a controlled release product. An optimum range of a given pharmacokinetics parameter of a drug is necessary beyond which controlled delivery is difficult or impossible.

Pharmacokinetic Characteristics of the Drug
A detailed knowledge of the ADME characteristics of a drug is essential in the design of a controlled release product. An optimum range of a given pharmacokinetics parameter of a drug is necessary beyond which controlled delivery is difficult or impossible.
1. ABSORPTION RATE:
For a drug to be administered as controlled release formulation, its absorption must be efficient since the desired rate limiting step of drug release Kr i.e. Kr << Ka. A drug with slow absorption is a poorly candidate for such dosage forms since continuous release will result in a pool of unabsorbed drug e.g. iron. Aqueous soluble but poorly absorbed potent drug like decamethonium are also unsuitable candidates since a slight variation in the absorption may precipitate potential toxicity.

2.    ELIMINATION HALF-LIFE:
Smaller the t1/2 larger the amount of drug to be incorporated in the controlled release dosage form, For drugs with t1/2 less than 2 hour, a very large dose may be required to maintain the high release rate. Drugs with half' life in the range 2 to 4 hours make good candidates for such a system e.g. amlodipine. For some drugs e.g. MAO inhibitor, the duration of action is longer than that predicted by their half-lives. A candidate drug must have t1/2 that can be correlated with its pharmacological response .In terms of MRT , a drug administered as controlled release dosage form should have MRT significantly longer than that from conventional dosage form.
3.    RATE OF METABOLISM:
A drug which is extensively metabolized is suitable for controlled release system as long as the rate of metabolism is not too rapid. The extend of metabolism should be identical and predictable when the drug is administered by different routes .A drug capable of inducing or inhibiting metabolism is a poor candidate for such a product since steady state blood levels would be difficult to maintain
4.    DOSAGE FORM INDEX (di):
It is defined as the ratio of Css.max to Css.min. Since the goal of controlled release formulation is to improved therapy by reducing the dosage form index while maintaining the plasma drug levels within the therapeutic window, ideally its value should as close to one as possible.

Pharmacodynamic Characteristics of the Drug:

1. THERAPEUTIC RANGE:
A candidate drug for controlled delivery system should have a therapeutic rage wide enough such that variations in the rate do not result in a concentration beyond this level.

2. THERAPEUTIC INDEX (TL):
The release rate of a drug with narrow therapeutic index should be such that plasma concentration attained is with in the therapeutically sale and effective range. This is necessary because such drug s have toxic concentration nearer to their therapeutic range. Precise control of release rate of a potent drug with narrow margin of safety is difficult. A drug with short half-life and narrow therapeutic index should he administered more frequently than twice day .one must also consider the activity of drug metabolites since controlled delivery system controls only the release of parent drug but not its metabolism.

3. PLASMA CONCENTRATION-RESPONSE RELATIONSHIP:
Drug such as reserpine whose pharmacological activity is independent of its concentration are poor candidates poor controlled system.

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