Oral tablets for ingestion:Modified release tablet
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The main aim behind formulation of this dosage form is to release the medicament slowly
for long time duration after administration of a single tablet.More over, these type of formulations are generally used to target
the site specific releases.
FIGURE.5. GRAPHICAL COMPARISON OF BLOOD CONCENTRATION V/S TIME
A widespread use of this type of tablet is seen in present scenario, as well as many researchers have concentrated their attention in this direction. This is mainly because of improvement in patient’s compliance as the dosage frequency is reduced, patient can take an undisturbed sleep at night, it’s also beneficial for psychiatric patients who forget to take their tablets regularly and the dose related side effects and toxicities are reduced. Any adjuvant that can alter water uptake rate, swelling and gelling characteristics of Matrixing agents can alter the release rate of API e.g., electrolytes in HPMC matrix tablet. It’s also possible to achieve pulsed drug release. Weakly basic drugs exhibit good solubility at low pH while less soluble at high pH conditions, which can result in incomplete drug release for sustained release formulations. The drug release can be modified by providing suitable micro environmental pH in the tablet e.g., acidic polymer, succinic acid, etc. Similarly, inclusion of alkaline polymers results in desirable drug release of acidic drugs. On the other hand, formulation of this type of dosage form presents challenge for the formulator: increases the cost of manufacturing, chances of burst drug release and drop in drug release rate in terminal phase and thus incomplete release on API. In case of accidental poisoning, the doctor has to deal with special treatment problems. Due to large size, patient may feel difficulties in swallowing as the matrixing agent to drug ratio is high. Classic approaches are usually based on adaptation of either film coated or multiparticulate technologies or those involving slow release matrices.
Coating technology (6)
It combines semi permeable coatings and osmotic tablet cores to produce “zero order release” technology. Attention is also focused to trigger drug release at critical time point e.g., to achieve drug release 1 -2 hours before the patient awakens. Alza’s prolific research activities have yielded a technology called “Ringcap” which is based on a tablet, preferentially film coated, partially coated with a series of rings whose respective thickness provides the means of moderating the rate at which the drug is released from final dosage form.
FIGURE.6. RINGCAP (COATED) TABLET
Matrix technology
Classically matrix products exhibit first order (or perhaps square-root-of-time) drug release characteristics. In order to achieve zero order release characteristics, it’s necessary to employ specially designed materials or strategies that seek to manipulate tablet structure or geometry. Combination of conventional HPMC matrix technology with upper and lower layer. This helps to moderate drug release by increase in surface area with concomitant reduction in drug concentration within the device.
FIGURE.7. MATRIX TABLET
Release of medicament can follow various mechanisms (2)
i)Diffusion is rate limiting
Diffusion is driving force where the movement of drug molecules occurs from high concentration in the tablet to lower concentration in gastro intestinal fluids. This movement depends on surface area exposed to gastric fluid, diffusion pathway, drug concentration gradient and diffusion coefficient of the system.
FIGURE.8. DIFFUSION RELEASE PATTERN
In practice, we can follow either of the two methods,
1.The drug is formulated in an insoluble matrix; the gastric fluid penetrates the dosage form and dissolves the medicament and release the drug through diffusion.
2.The drug particles are coated with polymer of defined thickness so as the portion of drug slowly diffuse through the polymer to maintain constant drug level in blood.
ii)Dissolution is rate limiting
The drugs with poor water solubility (BCS class 2 and 4) are inherently sustained release forms. While for water soluble drugs, it’s possible to incorporate a water insoluble carrier to reduce dissolution of the drug particles are coated with this type of materials e.g. Polyethylene Glycol. One may skip the use of disintegrating agent to promote delayed release.
iii)Osmotic pressure is rate limiting
Osmosis is a phenomenon in which the flow of liquid occurs from lower concentration to higher concentration through a semi permeable membrane which allows transfer of liquid only. The whole drug is coated with a semi permeable membrane with a hole on one end of tablet made by a laser beam. The gastric fluid penetrates through the membrane, solubilizes the drug and increases the internal pressure which pumps the drug solution out of the aperture and releases the drug in gastric environment. The delivery rate is constant provided that the excess of drug present inside the tablet. But, it declines to zero once the concentration drops below saturation.
FIGURE.9. OSMOTIC RELEASE PATTERN
iv) Release is controlled by ion exchange
Ion exchangers are water insoluble resinous materials containing salt forming anionic or cationic groups. While manufacturing, the drug solution is mixed with resin and dried to form beads which are tableted. The drug release depends upon high concentration of charged ions in gastro intestinal tract where, the drug molecules are exchanged and diffused out of the resin into the surrounding fluid. This mechanism relies upon the ionic environment of resin and not pH or enzyme on absorption site.
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