An osmotic oral drug delivery system is one which depends on the mechanics of osmotic pressure variations to regulate the delivery of the drug or active agent. This is a step forward in ensuring controlled, stable, and precise delivery of the drug. It makes it independent of gut factors, being regulated only by the nature of the formulation.
This, in turn, determines its solubility, the osmotic pressure exerted by the core components, the size of the orifice through which delivery occurs, and the type of semipermeable membrane used. These can be refined to deliver all kinds of drugs at desired rates both systemically as well as at organ-specific locations.
Rate-controlled delivery of drugs is usually preferred. It avoids repeated doses at regular intervals, as well as achieving the minimum effective concentration of the drug in a manner that is sustained over a preset and prolonged period of time, rather than dumping all the drug at the same time in an uncontrolled fashion. With conventional drug delivery systems, the drug bioavailability may change quite significantly depending on the properties of the drug, local gastrointestinal factors including the pH and the presence of various other food or non-food substances in the gut, and the presence of other non-drug substances in the formulation. Controlled delivery improves patient compliance and hence the effectiveness of the treatment.
Osmotic Pump Mechanism
The osmotic principle was first applied to this use by Rose and Nelson in 1955 using an implantable pump. Almost 20 years later, Theeuwes refined the concept to introduce the elementary osmotic pump, patenting a number of such products.
Further development continues to this day, when the osmotic pump tablet is available, consisting of a tablet at the core, with a semipermeable membrane having just one orifice. At first, the product was so irritating to the gastric and intestinal mucosa that it had to be withdrawn. But later, another system with controlled porosity was developed. At present, the controlled porosity osmotic pump (CPOP) and the push-pull osmotic pump (PPOP) are the most popular, being designed to avoid focused exposure to very irritating drugs and to enhance delivery of poorly soluble drugs, respectively.
Components of Osmotic Drug Systems
Osmotic drug delivery systems have become some of the most preferred novel pharmaceutical delivery systems. The components of such a system include:
- The core drug - Most drugs used in this type of system have a short half-life, are very potent, and require to be used over long durations.
- The osmotically active agent - The osmotic agents used for these drug delivery systems are ionic in nature, being composed of carbohydrates, salts of inorganic or organic acids, or hydrophilic polymers. These help to achieve very high osmotic pressures of 30-500 atm, resulting in high flow rates for water across the membrane.
- Excipients - These may contain wicking agents, solubilizing agents, and pore-forming agents, among others, to improve drug diffusion.
- A semipermeable membrane coating the formulation - Such membranes are usually composed of cellulose esters, especially cellulose acetate.
What is Osmosis?
An osmotic system is one in which the key substance moves down a solute concentration gradient across a semipermeable membrane. Such a membrane allows water to move through its pores freely, but not the solute. The water is drawn across the membrane by the difference in solute concentration on either side.
The efficacy of this type of system and its applicability to a wide variety of drugs has led to patents being issued for several hundred of such drug systems.
The osmotic drug delivery system has many benefits. The drug release follows zero order kinetics following the initial lag phase and the release does not vary with the drug concentration. It is released at constant levels maintaining safe levels in a sustained manner and the compliance with the therapeutic level achieves a reduction in adverse effects due to the drug.
Also, it is possible to delay delivery or to give the drug in pulsed doses and the drug is released independently of gut conditions including gut motility. The bioavailability is increased while the dose released remains predictable from patient to patient. The frequency of dosing is reduced and patient compliance is increased and very potent drugs are safer to administer in this manner because of the predictability of the released dose. Finally, the in vitro tests correspond well with the results of in vivo testing.
A highly effective novel drug delivery system will exhibit some negative aspects as well, of which the leading difficulties include the increased cost of such systems and the need for the meticulous design of the coating film to avoid any defects which would lead to the escape of the drug in uncontrolled doses. Also, some types require precise hole size in the membrane to achieve the desired effect and the drug release may be somewhat affected by food in the gut.