Models of
oral drug delivery
Session Objectives
By the end of this session, students will be able to:
• Explain different models of oral drug
delivery
• Comment the mechanism of drug release in
various oral delivery systems
• Apply the concepts of diffusion, dissolution,
diffusion & dissolution in dosage form
development
Diffusion controlled release
systems
Reservoir Devices
• These
systems will offer zero order type of drug release
• Drug
release are varied by the types of polymer used
• They
should be removed physically from the implant site after treatment
• Incorporation
of high molecular weight compound is difficult
• It’s
expensive
dm/dt = ADK
∆c/L
Where,
A- Area
D- Diffusion coefficient
K- Partition coefficient
L- Thickness of coat
∆c- concentration difference across membrane
Advantages
v Zero-order
delivery is possible
v Release
rate variable with polymer type
Disadvantages
Ø Removal
of system from implants
Ø Bad
for high-molecular weight compounds
Ø Cost
Ø Potential
toxicity if system fails
Reservoir Diffusional Products
PRODUCT
Nico-400, Nitro-Bid, Nitrospan
MATRIX DEVICES
• Consists
of drug dispersed homogeneously throughout a polymer matrix
• Drug
in the outside layer is exposed to the bathing solution is dissolved and
diffuses out of the matrix
• This
process continues with the interface between bathing solution and the solid
drug moving toward the interior
Higuchi Equation
Q =
DE/T(2A.E Cs)Cs.t)1/2
Where,
Q=amount of drug release per unit surface area at time
t.
D=diffusion coefficient of drug in the release medium.
E=porosity of matrix.
Cs=solubility of drug in release medium.
T=tortuosity of matrix.
A=concentration of drug present in matrix per unit volume.
• In
this systems drug is dispersed in an insoluble matrix , the drug release depends on diffusion process but not on the
solid dissolution process
• Easy
to prepare when compared to the reservoir devices
• Drug
of high molecular weight can be used
• Dose
dumping is less likely to happen
• Zero
order release is not possible
• Removal
of residual matrix is necessary for implanted systems
• Hydrophobic
matrices : PVC , EC ,
• Lipid
matrices : carnauba wax with stearic acid
• Hydrophilic
matrices : carbopol , sodium alginate
Advantages
1) Easier
to produce than reservoir devices
2) Can
deliver high molecular-weight compounds
Disadvantages
1)
Cannot obtain zero-order release
2)
Removal of remaining matrix is necessary for
implanted systems
MATRIX DIFFUSIONAL
PRODUCTS
Procan SR, Desoxyn-Gradumet,
Choledyl SA
Combination of Dissolution and Diffusion System
• Therapeutic
systems will never be dependent on dissolution only or diffusion only
• Bioerodibile
devices, however, constitute a group of systems for which mathematical descriptions of release is
complex
• Complexity
of the system arises from the fact that, as the polymer dissolves, the diffusion
path length for the drug may change. this
usually results in a moving-boundary diffusion system
• Zero-order
release can occur only if surface erosion occurs and surface area does not
change with time
Dissolution & Diffusion controlled release system
• Drug
encased in a partially soluble membrane.
• Pores
are created due to dissolution of parts of membrane.
• It
permits entry of aqueous medium into core & drug dissolution.
• Diffusion
of dissolved drug out of system.
• Ex-
Ethyl cellulose & PVP mixture dissolves in water & create pores of
insoluble ethyl cellulose membrane.
• The
main feature is that the drug core is enclosed with a partially soluble membrane
• Dissolution
of part of the membrane allows for diffusion of the contained drug through pores in the polymer coat
• Which
permit entry of aqueous medium into the core and hence drug dissolution, and allow diffusion of dissolved
drug out of the system
Release profile of drug described by the following
equation:
Release
rate = AD(C1 – C2) / l
Where,
A= surface area
D= diffusion coefficient of the drug through the pore
l= diffusion pathlength
C1 = concentration of the drug in the core
C2 = concentration of the drug in the
dissolution medium
• Example
of obtaining such a coating is using a mixture of ethyl cellulose with PVP or methyl cellulose , later dissolve in
water and creates pores in the insoluble
ethyl cellulose membrane
• The
drug release from the system depends on:
a) Solubility
of drug exposed to media
b) Solubility
of hydrophilic polymer
c) Degree
of pore formation
d) Intactness
of insoluble polymer
e) Permeability
& diffusivity of polymer
Summary
- Different
types of controlled release systems are:
Dissolution system, Diffusion system, Dissolution &
diffusion system, Osmotic regulated
system, pH regulated system, Ion exchange
controlled systems and Hydrodnynamically
balanced systems - Types
of dissolution models are controlled and pulsed delivery systems. The drug
release from dissolution controlled delivery systems are dependent upon porosity
,wettability ,particle size & hydrophobic additives in the formulation - Drug
release from diffusion controlled can offer zero order drug release kinetics - Drug
release from diffusion and dissolution controlled system is depended on the: - Solubility
of drug exposed to media - Solubility
of hydrophilic polymer - Degree
of pore formation - Intactness
of insoluble polymer - Permeability
& diffusivity of polymer
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