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Controlled release delivery system is used to show that the drug release kinetics is predictable as well as reproducible from one unit to another whether or not release profile follow zero order kinetics.
Classification of controlled release delivery system:
It can be classified on the basis of their release mechanism and the preparation as follows:
Physical system :
Diffusion controlled systems
Non constant activity
Ion exchange resin system
Osmotically control system
Hydradynamically balanced system
Other physical systems
Chemical systems :
Immobilization of the drugs
Biological system :
Polymer as a drug delivery carriers:
For controlled release drug delivery, polymers are known as structural backbones.
Polymers has following properties:
They may be swollen, non swollen, poriferous, non porous, erodible, bio adhesive etc.
Polymers utilized for the preparation of the controlled release drug delivery must fulfil following requirements:
2) Physical and mechanical properties
3) Pharmacokinetic properties
Polymer used in the controlled release drug delivery should be biocompatible. Harmful impurities must withdrawn before they included in controlled release drug delivery. Chemicals which are used in polymerisation process must be chosen carefully to meet regulatory requirement. E.g additives,stabilizers, plasticizers and catalyst.
Physical and mechanical properties :
Polymers should fulfil certain properties required for the controlled release drug delivery system design. These properties include elasticity, compactibility, resistance to tensile, swelling and shear stress as well as resistance to fatigue.
Pharmacokinetic properties :
Polymer should not chemically degraded. If chemical degradation of the polymer is there, degradation by product must be non toxic, non immunogenic and non carcinogenic.
Controlled release drug delivery
Controlled release drug delivery is defined as release of the drug in a predesigned manner.
The principal behind the controlled release delivery of drugs is to encourage therapeutic benefits at the same time reducing toxic effects.
Controlled release drug delivery is used to minimize unwanted fluctuation of drug levels.
MECHANISM OF CONTROLLED RELEASE
There are four principal mechanism behind the release of drug from controlled release delivery system are as follows:
Swelling followed by diffusion
Diffusion can takes place on a macroscopic scale through pores in the device matrix at the same time on a molecular level by passing between matrix molecules.
Diffusion takes as the drug passes from the polymer matrix into external surrounding.
As drug release increases, diffusion rate decreases because in controlled release system drug has to travel longer distance and needs a large diffusion time to release.
Diffusion is defined as a process of mass transfer of individual molecules of the substance brought about by random molecule motion and associated with a driving force such as concentration gradient.
Diffusion is a very significant phenomenon in pharmaceutical science.
e.g. diffusion of drug across a biological membrane needed for a drug to be absorbed into and excreted from the body.
Drug release involves multistep involves multistep that include diffusion, disintegration, deaggregation and dissolution.
Diffusion is a relatively slow process and as a result of random molecular motion.
Concentration gradient is the driving force.
Fickââ‚¬â„¢ laws of diffusion:
Fick realized that the mathematical equation of heat conduction originated by fourier is able to applied to mass transfer.
These fundamental relationships indicate the diffusion process in pharmaceutical systems.
Amount M, of the material passing through a unit cross section S, in a time interval t is known as flux J
J= dM/S.dt ................Ficks first law of diffusion equation.
Flux J is directly proportional to concentration gradients dc/dx
J = -D dc/dx
D = diffusion coefficient of a drug.
C = concentration (g/cm 2)
S = surface area (cm2)
t = time in seconds
J = g/cm2 second
Negative sign of the diffusion equation indicates the diffusion is apposite to increasing concentration. Diffusion is takes place apposite to that of increasing concentration.
D is known as diffusion coefficient and it is influenced by concentration, temperature, pressure ,solvent properties and chemical nature of the diffusant.
Dissolution is the process by which a solid phase (tablet or capsule) enters into a solution phase i.e. water.
Drug dissolution follows few steps:
Initially solid particles are separated
Solid particles integrate with liquid.
Then they look like a part of liquid solution.
Drug dissolution is the process of which drug molecules are released from a solid phase and goes into solution phase.
2) Swelling followed by diffusion:
Initially these system is dry and when the system is placed in body, absorbs water or other body fluid leads to swell.
Swelling increases the aqueous solvent capacity in the drug and polymer mesh size which leads to diffusion of drug through swollen network into external surrounding e.g hydrogels
Hydrogel are able to absorb a significant amount of the liquid upto 60 to 90 % water. In the hydrogel swelling can be activated by a change in environment surrounding the delivery system. These factor includes shifts in pH, temperature, ionic strength and other factor which lead to swelling or shrinking of the system. Swelling leads to release of the drug at the same time shrinking leads to obstruction to the drug release.
Biodegradable systems :
After the release of the active ingredient these systems degrade within the body as a result of natural biological process.
4) Active efflux :
Example: Elementary osmotic pump .It is also known as OROS.
When these sytem placed in the body, water passes through the semi permeable membrane of the system which leads to expansion of the osmotic pump. Osmotic pump pushes drug outside through a delivery orifice into gastro intestinal tract. Drug release of these osmotic pump is controlled by the composition of the membrane.
Advantage of the controlled release drug delivery:
In controlled release drug delivery reduction in dosing frequency as compared to conventional drug delivery.
Reduction in fluctuating drug levels
It helps to increase patient compliance
It provide more uniform effect
It help to avoid night time dosing
It also assist in reduction in dose related side effects
It helps to achieve more uniform pharmacological effect
Reduction in cost
It facilitate improvement in treatment efficiency
It assist to reduce local and systematic side effects
It helps to improve bioavailability of drugs
It also helpful in reduce loss of drug activity.
It minimizes drug accumulation
Disadvantage of the controlled release drug delivery:
Controlled release drug delivery is expensive, unpredictable
It has poor in vivo ââ‚¬"in vitro correlation
If drug undergoes significant first pass clearance, the drug bioavailability is reduced.
In controlled release drug delivery, there may be risk of accumulation of the drug in the body.
Some drug has narrow therapeutic index in this case we have to maintain serum drug level within narrow range such drugs are difficult to design as controlled release drug delivery.