Controlled drug delivery systems aim to deliver drugs at predetermined rates for extended periods of time. Sustained release systems specifically achieve slow drug release over an extended time frame. Controlled release drug delivery systems have several advantages including improving drug absorption and bioavailability, decreasing side effects, reducing dosing frequency, and allowing for better patient compliance. They operate via various mechanisms including diffusion, erosion, and ion exchange to provide release rates that are often zero order or first order.

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Controlled drug delivery is one which delivers the drug
at a predetermined rate, for locally or systemically,
for a specified period of time.
Sustained release are drug delivery system that achieve
slow release of drug over an extended period of time.

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Inert,
Biocompatible,
Mechanicallystrong,
Comfortableforthepatient,
Capableof achievinghighdrugloading,
Safefromaccidentalrelease,
Simpleto administerand remove,
Easyto fabricateandsterilize.

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Improves absorption, utilization and
thereby enhances bioavailability.
Decreases side effects and reduced GIT
irritation.
Reduced fluctuation in circulating drug
level.
Reduction in dosing frequency.
Better patient compliance.
Bio-availability of certain drugs can be
increased.
Eliminate over or under dosing.
Possibility of dose dumping due to food,
physiologic or formulation variables or chewing
or grinding of oral formulations by the patient
and thus, increased risk of toxicity.
Retrieval of drug is difficult in case of toxicity,
poisoning or hypersensitivity reactions.
Reduced potential for dosage adjustment of drugs
normally administered in varying strengths.
Stability problems.
Need for additional patient education and
counseling.
Decreased systemic availability in comparison to immediate release
conventionaldosageforms,whichmaybeduetoincompleterelease.

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Two different types of CRDDS have been described such as:
1. Single units (Capsules, Coated tablets, Osmotic pumps, Insoluble matrix tablets,
Soluble matrix, Degradable matrix, Ion-exchange resins)
II. Multiple units (Granules, Microcapsules, Beads, Ion-exchange resins)
These types are operated by various mechanisms:
 Diffusion through inert matrix
 Diffusion across hydrophilic matrix
 Diffusion-erosion.
 Diffusion-degradation
 Ion-exchange
 Osmosis- combinations
The release rates from such systems are usually by:-
 Zero order
 First-orders
 Square root of time

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The insets on the right hand side in Fig. symbolize
the compartment models..
A in Fig 3. is a one-compartment model,
B and C are two-compartment models.
Fig 3., Schematic blood concentration-time curves after
extra-vascularadministration.
For explanation see text.
The thickness of the arrows indicates the relative
magnitude of the corresponding rate of the process.
 In the case of CRDDS the absorption is not
completed instantly as with an I.V. push (Rapid
I.V. injection), it is pertinent to discuss here only
the concentration-time curves as they are observed
upon extra-vascular or infusion administration.
 Assuming first-order elimination, which is the
case for most drugs, and plotting the
concentration-time data semi-logarithmically, the
terminal slope is the elimination phase,
characterized by a straight line.
Compartment Model

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 In Below fig., the collapsing
of a two-compartment model
to an "apparent" one-
compartment model is shown
as a function of the absorption
rate constant ka.
 For all practicality, the one-
compartment open model is
quite suitable to design CRDDS
for most drugs.
Fig 4., Schematic Diagram to demonstrate collapsing of
A two compartment model to A one-compartment model.
Ka = Absorption Rate Constant;
K12 & K21 = Distribution Rate Constants;
K13= Elimination Rate Constant;
CC = Central Compartment;
PC =Peripheral Compartment
Peripheral Compartment
Central Compartment
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