PHARMACOKINETICS & ITS MODEL FOR DRUG DESIGN
INTRODUCTION
It is a disciplinary that concerns with the study & characterization of time course of change in concentration of drug and it’s metabolites in the body fluid.
Pharmacokinetics describes what the body does to a drug i.e. The movement of drug into, through and out of the body.
The main goal of pharmacokinetics is to quantity drug’s absorption, distribution, metabolism and excretion in living organism and to use these informations to predict the effect of alteration of the drug’s dose, dosage form, route of administration, and physiological effects of drug on ADME.
IMPORTANCE
Near about 40% drug fails in it’s clinical trial due to poor ADME properties. These later stage failure contribute significant effect to the cost of new drug development. The ability to detect problems in the early stage can dramatically reduce the wastage of amount of time and resources.
Accurate prediction of ADME properties, prior to expensive procedures can eliminate unnecessary testing of compounds that will ultimately fail. ADME prediction can also be used to focus on lead optimization efforts to enhance the desired properties of a given compound.
Finally ADME prediction as a part of drug development process can generate compounds that are more likely to exhibit satisfactory ADME performance during clinical trial.
The increased speed of computer as well as their storage capacity has led to development of numerous software programs that now allow rapid solution of complicated pharmacokinetic process i.e. In-silico pharmacokinetics study.
E.g. Of In-silico pharmacokinetics softwares are:
Gastroplus
Volsurf
Metrabase
S-plus
Trial simulator etc.
APPLICATION OF PK SOFTWARE IN DRUG MODELING
1. Discovery
2. Candidate selection
3. Characterization of safety & pharmacokinetics of new chemical entity
4. Trail in patients to assess the efficiency
5. Dose determination
VARIOUS MATHEMATICAL MODEL
1. Zero order model: Drug dissolution from dosage form that do not disaggregate and release the drug slowly can be representing by equation:-
Where Qo = Initial conc. Of drug in the dosage form
Qt = amount of drug in dosage form after time t
Ko = zero order release constant
Application: Used to study & design release of coated form of drug, transdermal etc.
2. First order model: This model is used to describe absorption or elimination of some drugs which release in concentration basis is expressed in form of expression:-
Where Qt = amount of drug release at time t
Qo = initial conc. Of drug
K₁ = first order rate constant
Application: Used to study & design water soluble drug in porous matrix.
3. Higuchi model: It's the first mathematical model aimed to describe drug release from matrix system proposed by Higuchi in 1961. The expression for the model is:-
Where Q = amount of drug released at time t
C = initial conc. Of drug
Cs =drug solubility in matrix media
D = diffusibility of drug from matrix surface
Application: Used to study & design low soluble drugs incorporated in semisolid/ solid polymer matrix.
4. Weibull model: It describes different dissolution process by equation:-
Where M = amount of drug dissolved as a function of time t
Mo = total amount of drug being released
T = lag time
a = scale parameter describes time dependence
b=shape of the dissolution curve progression
Application: Used for comparing release profile of matrix type drug delivery to select best formulation.
5. Hixson-crowell model: This model states that the drug release volume by dissolution changes with change in surface area or size of particle or tablet. Thus the equation is:-
Where Wo = initial conc. Of drug in dosage form
Wt = remaining amount of drug in dosage form at time t
Ks = constant representing surface-volume relationship
Application: Used to study & design dosage form like tablets where the dissolution occurs in plane that are parallel to drug's surface where the dimension diminishes but the geometric form remains constant.
6. Korsemeyer-Peppas model: This relationship helps to study drug release from polymeric system. The equation for this model is:-
Where Mt/Mκ = fraction of drug release at time t
K = release rate constant
n = release exponent
Application: Used to study drug release from several modified dosage form, thus helpful in designing modified dosage form.
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