Excipient osmotic agents

Polysaccharides such as starch and vegetable gums have important pharmaceutical uses as excipients, binding agents for tablets, emulsifying agents and demulcents. Dextran finds employment as a plasma extender to maintain the osmotic pressure of blood. 

Tonicity agents are added to injectable preparations to prevent osmotic shock at the site of injection upon administration, and thereby reduce local irritation. Typical excipients used for tonicity adjustment include saline, glycerin, mannitol, dextrose, and trehalose. Tonicity is a colligative property that depends primarily on the number of dissolved particles in solution. Hence, the amount of tonicity agent to be added depends on the specific formulation. Typically, osmolality of 280 to 320mOsm is considered iso-osmotic. 

Delivery systems that use a multicompartment core can theoretically deliver drugs of any solubility [48,49], A basic Push-Pull System consists of two layers the Lrst contains the drug, osmotically active hydrophilic polymer(s), and other pharmaceutical excipients the second layer, often called the push layer, contains a hydrophilic expansion polymer, other osmotically active agents, and the excipients, as shown in Figure 22.6. Poorly water-soluble compounds can be delivered using an ORO Push-Pull tlelivery system by incorporating drug as a micronized form, or as a hot-melt material suspended in a polymer matrix. 

In a preparation, the active substance (if present in dissolved form) and the excipients (e.g. buffering agents, preservatives, antioxidants, and disodium edetate) aU contribute to the osmotic value of a preparation. 

From Equation 28 it may be observed that the rate of drug release from an osmotically controlled system is directly proportional to the osmotic pressure within the tablet. As a result the osmotic pressure is an important design consideration for these systems. Osmotic pressure is a colligative property and is therefore dependent on the number of ions and molecules in solution. If the solubility of the drug is low, the inherent osmotic pressure within the tablet will be low and therefore the rate of drug release will be low. Under these conditions the inclusion of excipients, e.g., mannitol, sodium chloride, potassium chloride or hydrophilic polymers, is required within the tablet core. Upon dissolution within the tablet the osmotic pressure will increase thereby enhancing the rate of release of the therapeutic agent (a.47, a. 167). 

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