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4. Dosage Form Design: Biopharmaceutic and Pharmacokinetic Considerations. General principles of drug absorption Dissolution and drug absorption Bioavailability and bioequivalence Routes of drug administration Fate of drug after absorption Pharmacokinetic principles. Contents.
Body membranes are generally classified as three main types:
1) by passive diffusion
2) through specialized transport mechanisms
in which C1 and C2 refer to the drug concentrations on each side of the membrane and P is a permeability coefficient or constant.
For practical purposes the value of C1-C2 may be taken simply as that of C1 and the equation written in the standard form for a first order rate equation:
The magnitude of the permeability constant, depends on
For an acid:
pH=pKa+logionized conc.(salt)/unionized conc.(acid)
For a base:
/ ionized conc.(salt)
in which dc/dt is the rate of dissolution
k is the dissolution rate constant
S is the surface area of the dissolving solid,
Cs is the saturation concentration of drug in the diffusion layer
Ct is the concentration of the drug in the dissolution medium at time t
The amorphous or crystalline character of a drug substance may be of considerable importance to
Certain medical agents may be produced to exist in either a crystalline or an amorphous state.
In practice, it would be rare for a drug to be completely absorbed into the circulation following oral administration.
f=fraction of drug absorbedfraction escaping first-pass metabolism
Consequently, the bioavailability of those drugs that undergo a first-pass effect then would be expected to increase, e.g. cirrhotic patients.
The oral route is considered the most natural, uncomplicated, convenient, and safe means of administering drugs.
Oral or sublingual
Drugs are administered topically, or applied to the skin, for their action at the site of application or for systemic drug effects.
Other metabolic processes, including methylation, and acylation conjugation reactions, occur with certain drugs to foster elimination.
The simplest pharmacokinetic model is the single compartment open-model system.
Kelt/2.303=log C0 -logCp=log C0 /Cp
If it assumed that Cp is equal to one-half of C0p, the equation will become:
Kelt/2.303= log C0/0.5C0=log2
The three main mechanisms by which a drug is removed or cleared from the body include
There are two approaches to the development of dosage regimens:
1) The empirical approach, which involves the administration of a drug in a certain quantity, noting the therapeutic response and then modifying the dosage of drug and the dosing interval accordingly.
4) A typical factors may influence the dosage regimen.
1．What are general principles of drug absorption？
2．Write Henderson-Hasselbalch equation and explain it.
3．What is Noyes-Whitney equation?
4．What factors could affect drug absorption?
5．Describe the routes of drug administration and their characteristics？
6．What is biotransformation？What are the biochemical mechanisms of biotransformation?
7．Explain shortly about one compartment model and two compartment model？
8．How to develop dosage regimens？