Design of Modified/Controlled Release Dosage Forms Flashcards
Aims of modified release
Improved bioavailability
Designed to release drug slowly after ingestion leading to maintained blood levels
Reduced side-effects related to reduced plasma concentrations and reduced dose
Reduced irritancy
More constant blood levels can bring improved efficacy
Economic savings
More convenient form with the need for less frequent dosing
Repeat action systems designed to give patients two full doses
Limitations of modified release
Physiological factors GI transit time Lodging of dosage form Not all drugs appropriate (decreased half life drugs hazardous) Tolerance Physical size Unit cost Activity must be aligned closely to unit concentration
What is the benefit of controlled release?
A dosage form that is able to provide some actual therapeutic control
There is often a desire to achieve zero order release
Temporal control- release over time
Spatial control- only deliver in certain area e.g. colon
In practice achieving constant blood levels is very difficult:
Maintenance dose must be released at a precise controlled rate that is in mass balance with drug elimination and at required therapeutic concentration
Physiological conditions often variable
Inter-patient variability in drug absorption
What you need to know to make controlled release
The rate limiting step for drug action The optimal blood concentration curve Elimination Dose size Metabolism- drugs that induce, inhibit or have a high first pass are not suitable
Enteric coating
Stops the tablet disintegrating at acid pH
They are used to prevent an unstable active being destroyed, prevent the stomach irritation, and facilitate drug absorption in the GI later in transit to improve bioavailability
Delay relates to gastric residence time
Polymers used for enteric coating
Film coating of tablets, granules, hard and soft gelatin capsules
The COOH means they are insoluble in aqueous media at low pH (1-3)
As pH rises, there is a sharp increase in solubility, ideally > pH 5
Delay and significant variability in onset of activity (note gastric emptying can range from 30 minutes to several hours)
Enteric coated granules or pellets
Entrapment in a rapidly dissolving hard gelatin capsule or rapidly disintegrating tablet
Eliminates dependence on all or none gastric emptying and dose dumping onto intestinal mucosa
<1mm pellets or granules empty from the stomach with liquids via closed pyloric sphincter
Reservoir type- diffusion controlled
Tablet or multi-particulate pellet containing drug
Drug does not diffuse in the solid state
Membrane does not swell
Liquid diffusing in forms a continuous phase
Reservoir type- diffusion controlled single unit
Usually compressed tablets
Difference from immediate release form is that the core is not designed to disintegrate, but to dissolve
e.g. lactose, microcrystalline cellulose, dextrose
Soluble fillers are chosen to minimise osmotic effects
Reservoir type- diffusion controlled multiple units
Usually coated pellets, ~1mm in a hard gelatin capsule or compressed tablet
No issue with gastric emptying
To achieve pulsatile release, change thickness of coating on half of reservoirs
Advantages of diffusion controlled
GI transit is less variable
Less likely to show total dose dumping
Release can be more easily optimised
Disadvantages of diffusion controlled
Dose dumping can occur from a single-unit system
Multiunit systems can be difficult to retain in higher GI
Control of polymer membrane coating and its characterisation can be difficult
Filling multi-unit systems into capsules can be a problem because of static build up
Soluble and hydrophilic colloid matrices
Monolithic matrix system
Drugs particles dispersed in a soluble matrix
Drug becomes available as the matrix dissolves, swells or swells and dissolves
Lipid and insoluble polymer matrices
Monolithic matrix system
Drug particles dispersed in an insoluble matrix
Drug becomes available as the solvent enters the matrix and dissolves the drug particles
Soluble matrices
Monolithic matrix system
Drug particles dispersed in a soluble matrix
Excipients used to make a soluble matrix include polyanhydrides and polyorthoesters
Surface erosion with little bulk erosion as drug is solubilised from the degrading surface
Bulk erosion- degrades from interior creating more channels for diffusion of drug out
Dissolution of polymer
Lipid and insoluble polymer matrices
Monolithic matrix system
Drug particles dispersed in an insoluble matrix
Fluid dissolves drug particles- not suitable for poorly soluble drugs
Chanelling agents- can be used to leach from the formulation leaving tortuous channels
Hydrophilic colloid matrix
Swellable matrices swell and sometimes following gel formation dissolve
Tortuosity and microviscosity control diffusion rate
Diffusion of drug through hydrated polymer fibrils controls release rate
Advantages of hydrophilic colloid matrix
Comparatively simple
Cheap and easy to manufacture
Different types of release profile
Can contain a high drug loading
Disadvantages of hydrophilic colloid matrix
Release of drug is dependent on two diffusion processes- water in and drug out
Erosion of outer layer complicates release profile
Scale-up can be a problem and need good batch reproducibility of rate controlling excipients
Use of nan-sized formulations
Becoming popular for poorly soluble new CNS drugs
Can reduce inter-patient variability
Ca reduce the effect of a fatty diet on increasing oral absorption
Can ensure rapid early absorption, can be very important e.g. in pain control
Using a poorly soluble drug or use of a specific drug salt form it is possible to tailor a modified release form
Dissolution controlled systems: using coatings
Drug release is sustained according to the different dissolution rates of the coating around the active ingredient
Rate depends on saturation solubility, concentration of solute in bulk solution, surface area and thickness of the coating
Repeat-action dosage form
Repeat action tablet or gelatin capsule
Usually contains 2 or 3 doses of drug- the first to give a rapid onset of action
The second dose is often delayed by use of an enteric coat
Pulsatile/repeat release systems
Varying coat thickness or its solubility
Use a number of different size releasing units to prepare tablets
