Modified Release tablets
The main purpose of this dosage form is to release the medicament slowly for long time duration after administration of a single tablet.

Matrix technology
Matrix products exhibit first order drug release characteristics. These are the type of controlled drug delivery systems, which release the drug in continuous manner. These release the drug by both dissolution controlled as well as diffusion controlled mechanisms. To control the release of the drugs, which are having different solubility properties, the drug is dispersed in swellable hydrophilic substances, an insoluble matrix of rigid nonswellable hydrophobic materials or plastic materials.

Drug Release from Matrix systems
Drug in the outside layer exposed to the bathing solution is dissolved first and then diffuses out of the matrix. This process continues with the interface between the bathing solution and the solid drug moving toward the interior. It follows that for this system to be diffusion controlled, the rate of dissolution of drug particles within the matrix must be much faster than the diffusion rate of dissolved drug leaving the matrix.

Combination of conventional HPMC matrix technology with upper and lower layer has moderate drug release by increase in surface area with concomitant reduction in drug concentration within the device.

Matrix Tablet

Rate limiting for matrix odified release drug are:
  1. Diffusion
  2. Dissolution
  3. Osmotic pressure
  4. Release is controlled by ion exchange

Diffusion is rate limiting
Diffusion is movement of drug molecules from high concentration in the tablet to lower concentration in gastro intestinal fluids. This movement influence by :
  1. surface area exposed to gastric fluid
  2. diffusion pathway
  3. drug concentration gradient
  4. diffusion coefficient of the system.

Diffusion Release Pattern

There are two methods in diffusion release drug :
  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.

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.

Osmotic pressure is rate limiting
Osmosis is flow of liquid 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.

Osmotic Release Pattern

OROS is a trademarked name owned by Alza Corporation. OROS medications: Invega (paliperidone), Adalat Oros/XL (Nifedipine), Jurnista (Hydromorphone), (Venlafaxine) ER and Concerta (methylphenidate).

Release is controlled by ion exchange
An ion-exchange resin or ion-exchange polymer is an insoluble matrix (or support structure) normally in the form of small (1–2 mm diameter) beads, usually white or yellowish, fabricated from an organic polymer. The material has highly developed structure of pores on the surface of which are sites with easily trapped and released ions. The trapping of ions takes place only with simultaneous releasing of other ions; thus the process is called ion exchange.

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.

Three ion-exchange resins sodium polystyrene sulfonate, colestipol, and cholestyramine, are used as active ingredients. Sodium polystyrene sulfonate is a strongly acidic ion-exchange resin and is used to treat hyperkalemia. Colestipol is a weakly basic ion-exchange resin and is used to treat hypercholesterolemia. Cholestyramine is a strongly basic ion-exchange resin and is also used to treat hypercholesterolemia. Colestipol and cholestyramine are known as bile acid.
Ion-exchange resins used as excipients in tablets, capsules, and suspensions has function for taste-masking, extended release, tablet disintegration, and improving the chemical stability of the active ingredients.

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