Biopharmaceutics Characteristics of the Drug

By: Pharma Tips | Views: 5155 | Date: 24-Apr-2011

The performance of a drug presented as a controlled release system depends upon its;a. Release from the formulationb. Movement within the body during its passage to the site of action.

The performance of a drug presented as a controlled release system depends upon its;
a.    Release from the formulation
b.    Movement within the body during its passage to the site of action.

       The former depends on fabrication of the formulation and the physicochemical properties of the drug while the latter element is dependent upon pharmacokinetics of drug. In comparison to conventional dosage form where the rate-limiting step in drug availability is usually absorption through the biomembrane, the rate-determining step in the availability of the drug from controlled delivery system is the rate of release of drug from which is much smaller than the intrinsic absorption rate for the drug.

       The type of delivery system and the route of administration so the drug presented in controlled release dosage form depends upon the physicochemical properties of the drug and its biopharmaceutic characteristics. The desired biopharmaceutic properties of a drug to be used in a controlled delivery system are discussed below.

1. MOLECULAR WEIGHT OF THE DRUG:
The lower the molecular weight, the faster and more complete the absorption. For drugs absorbed by pore transport mechanism, the molecular size threshold is 150 daltons for spherical compounds and 400 daltons for linear compounds. However, more than 95% of drugs are absorbed by passive diffusion. Diffusivity, defined as the ability of a drug to diffuse through the membranes, is inversely related to molecular size. The upper limit of drug molecular size for passive diffusion is 600 daltons. Drugs with large molecular size are poor candidates for oral controlled release systems. Eg. Peptides and proteins.

2.    AQUEOUS SOLUBILITY OF THE DRUG:
A drug with good aqueous solubility, especially if pH-independent, serves as a good candidate for controlled released dosage forms e.g. Pentoxifylline. Drugs with pH-dependent aqueous solubility e.g. Phenytoin, or drugs with solubility in nonaqueous solvents e.g. Steroids are suitable for parenteral (e.g. I.m. depots) controlled released dosage forms; the drug precipitates at the injection site and thus, its release is slowed down due to change in pH or contact with aqueous body fluids. Absorption of poorly soluble drugs is dissolution rate-limited which means that the controlled release device does not control the absorption process; hence, they are poor candidates for such systems.

3.    DRUG APPARENT PARTITION COEFFICIENT:
Greater the apparent coefficient of a drug, greater is its rate and extent of absorption. Such drugs have increased tendency to cross even the more selective barriers like BBB. The apparent volumes of distribution of such drugs also increases due to increased partitioning into the fatty tissues and since the blood flow rate to such tissues is always lower than that to an aqueous tissue like liver, they may exhibit characteristics of models having two or more compartments. The parameter is also important in determining the release rate of the drug form lipophilic matrix or device.

4.    DRUG pKa AND IONIZATION AT PHYSIOLOGIC pH:
The pKa range for acidic drugs whose ionization is pH-sensitive is 3.0 to 7.5 and that for basic drugs is 7.0 to 11.0. For optimum passive absorption, the drugs should be nonionized at that site at least to an extent 0.1 to 5%. Drugs existing largely in ionized forms are poor candidates for controlled delivery e.g. Hexamethonium.

5.    DRUG STABILITY:
Drugs unstable in GI environment cannot be administered as oral controlled releases formulation because of bioavailability problems. E.g. Nitroglycerine. A different route of administration should then be selected such as the transdermal route.


6.MECHANISM AND SITE OF ABSORPTION:
Drugs absorbed by carrier-mediated transport processes and those absorbed through a window are poor candidates for controlled release systems e.g. Several B vitamins.

7.BIOPHARMACEUTIC ASPECTS OF ROUTE OF DRUG ADMINISTRATION:

Oral and parenteral routes are the most popular folled by transdermal application.

INTRAMASCULAR/SUBCUTANEOUS ROUTE.

This route is suitable when the duration of action is to be prolonged from 24 hours to 12 months. Only small amount of drug, about 2ml or 2 gms, can he administrated by the route. Factor important in drug release by such as route are ;
1.    Solubility of drug in the surrounding tissues,
2.    Molecular weight.
3.    Partition coefficient,
4.    pKa of the drug and
5.    Contact surface between the drug and surrounding tissues.
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