INTRODUCTION:

Besides the drug itself, the right dosage over time is crucial for an effective therapy. Rate-controlled release systems allow maintaining the drug concentration within the body at an optimum level. This minimizes the risk of side effects, poor therapeutic activity, or even adverse effects. Over the years, a multitude of different technological approaches addressing this goal have been developed. However, only few of them succeeded in becoming cutting edge technologies applied to versatile therapeutic applications. A very successful approach for rate controlled drug delivery is represented by osmotic micropumps. This might be related to the bionic concept applying one of the most fundamental principles of biology, osmosis, in a technical device.

Osmotic pumps belong to the class of rate-controlled systems providing continuous delivery and offer a set of distinct advantages. Firstly, this includes the simple principle of operation requiring no electric energy resulting in increased robustness along with a high potential for miniaturization. Within osmotic pumps, drugs can be stored in liquid or solid form.

In the latter, the drug is efficiently stored in concentrated manner requiring a minimum of space. It is then dissolved by water used as solvent and delivered as a liquid solution. Hence, osmotic systems can be considered to be one of the most space-saving drug delivery technologies.

Considering that water is available in all body fluids, extremely miniaturized implantable devices for use in body parts that are not accessible in other ways can be developed. Furthermore, the efficient drug storage enables implantable devices providing constant drug release over a prolonged duration. Orally administered drugs often encounter poor pharmacokinetics, e.g. too slow or too fast absorption in the gastrointestinal tract. Osmotic technologies can be used to improve the pharmacokinetic properties of drugs by better adjustment of the release rate with respect to conventional tablets or pills. Therefore, in the past a lot of effort was dedicated to the development of osmotic pumps for oral drug delivery. The historical evolvement of these devices was summarized in multiple literature reviews [1–6].

Milestones in ODD’s:

In 1748, first report of osmosis was reported(7). Later in 1877 quantitative measurement of osmotic pressure was reported by Pfeffer(8). First osmotic pump (Rose-Nelson pump) for pharmacological research was developed in 1955 (9,10). Higuchi and Leeper introduced a variation of Rose-Nelson pump in 1973 (11). First patent was granted for the design of ALZET^® osmotic pumps in 1976 (12). A patent was issued for an osmotic system in 1982, which consists of a layer of fluid swellable hydrogel to deliver insoluble to very soluble agents(13). Controlled porosity osmotic pump was developed in 1985(14,15). A patent was issued in 1986 for a delivery system for controlled administration of drug to ruminants(16).

Procardia-XL^®, a billion dollar product was released in market in 1989 using push-pull osmotic pump by Pfizer (17, 18). In 1995, a patent was issued to an osmotic dosage form for liquid drug delivery (19). First implantable osmotic pump DUROS^® for human use was approved in 2000 (18). In 2001, patent was granted for a dosage form comprising liquid drug formulation which can self emulsify to enhance the solubility (20).

Principle of ODD systems:

Osmosis is one of the fundamental phenomena in biologiocal systems. An osmotic flow is described as a physical process in which any solvent moves into a higher solute concentration, without input of energy, across a semipermeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations., as illustrated in Fig. A below. As a consequence net movement of solvent is from the less concentrated (hypotonic) to the more concentrated (hypertonic) solution, which tends to reduce the difference in concentrations. This effect can be countered by increasing the pressure of the hypertonic solution, with respect to the hypotonic leading to the equilibrium as showed in below Fig. A. In equilibrium, there will be no net movement of solvent. Osmotic pressure depends on the molar concentration of the solute but not on its identity.

Basic Components of Osmotically Controlled Drug Delivery System (Osmotic Pumps):

a. Drug

b. Osmotic agent

c. Semi permeable membrane

d. Plasticizers

a. Drugs: Characteristics of drug candidate for osmotically controlled drug delivery are: it should possess short half-life (2-6h); the drug should be highly potent; and it needs to required for prolonged treatment e.g. Nifedipine, Glipizide, Verapamil.

b. Osmotic agents: Osmotic agents used for fabrication of osmotic dispensing device are inorganic or organic in nature. A water soluble drug by itself can serve the purpose of an osmogent.

Inorganic water-soluble osmogent:

Magnesium sulphate, Sodium chloride, Sodium sulphate, Potassium chloride, Sodium bicarbonate.

Organic polymer osmogents:

Sodium carboxymethyl cellulose, Hydroxypropylmethyl cellulose, Hydroxyethylmethylcellulose, Methylcellulose, Polyethylene oxide, polyvinyl pyrollidine.

c. Semi Permeable Membrane:

The semi permeable membrane should be stable both to the outer and inner environment of the device. The membrane must be sufficiently rigid so as to retain its dimensional integrity during the operational lifetime of the device. The membrane should also be relatively impermeable to the contents of dispenser so that osmogent is not lost by diffusion across the membrane, finally the membrane must be biocompatible.

Ideal Properties of Semi Permeable Membrane:

The Semi Permeable Membrane must meet some performance criteria:

· The material must possess sufficient wet strength (-105) and wet modulus so as to retain its dimensional integrity during the operational lifetime of the device.

· The membrane exhibit sufficient water permeability so as to retain water flux rate in the desired range. The water vapor transmission rates can be used to estimate water flux rates

· The reflection coefficient and leakiness of the osmotic agent should approach the limiting value of unity. Unfortunately, polymer membranes that are more permeable to water are also, in general more permeable to the osmotic agent.

· The membrane should also be biocompatible

· Rigid and non-swelling

· Should be sufficient thick to withstand the pressure within the device.

d. Plasticizers:

Different types and amount of plasticizers used in coating membrane also have a significant importance in the formulation of osmotic systems. They can change visco-elastic behavior of polymers and these changes may affect the permeability of the polymeric films. Some of the plasticizers used are:

· Polyethylene glycols

· Ethylene glycol monoacetate; and diacetate- for low permeability

· Tri ethyl citrate

· Diethyl tartarate or Diacetin- for more permeable films

Commercially available osmotic pumps:

Osmotic pumps for experimental research:

ALZET^® - originally discovered by Alza corp., later in 2000, acquired by Durect Corp. Miniature implantable osmotic pumps for animals.

OSMET^® - design similar to above pumps which can be used for human pharmacological studies.

Osmotic pumps for human use:

For oral use:

OROS^® - first developed by Alza. Different modifications available such as elementary, push-pull, delayed push-pull osmotic pump etc.

SCOT^TM – single composition osmotic system developed by Andrx Corp.

EnSoTrol^® - developed by Shire labs

OSMODEX – developed by Osmotica Corp.

Controlled porosity osmotic pump developed by Merck

Implantable:

DUROS^® - originally developed by Alza, Durect Corp licensed in 1998 for therapeutic use such as treatment of chronic moderate-to-severe pain.

Veterinary use:

VlTS system - Veterinary Implantable Therapeutic System, developed by Alza Corp.

RUTS - Ruminal Therapeutic System, developed by Alza Corp.

IntelliDrug:

IntelliDrug is a highly integrated osmotic microdosage system designed to operate in the oral cavity and delivering the drug to the buccal mucosa. The system has the size of two mandibular molar teeth and was developed to circumvent drug absorption by the stomach and the associated disadvantages [21]. In particular, poor patient compliance, e.g. pills not taken as prescribed, poor bioavailability by hepatic first pass metabolism, and fluctuating drug plasma levels have been addressed. Drug administration to the buccal mucosa is an advantageous route to the blood stream. A microsystem designed as a dental implant can easily be refilled by a physician without the need for surgery. The main challenges for such a system are the limited space and the harsh environment in the oral cavity including mechanical loads of up to 250 N. Moreover, the device has to withstand varying pH values, temperatures, saliva secretion, and the bacterial flora of the oral cavity. The IntelliDrug system is shown in Fig. B below.

On the lingual side, water from saliva enters the system through a water-permeable membrane. The solid drug pill stored in the reservoir is dissolved and a hydrostatic pressure is built up by compression of a fluidic capacity implemented as a compressible polymer balloon filled with air. The pressurized drug solution can be released by opening a microvalve that is (i) designed to be normally-closed for medical safety reasons and (ii) based on an ionic electroactive polymer polypyrrole (PPy) actuator keeping the energy consumption as well as the actuation voltage on a minimum level [22,23]. Downstream of the microvalve, a flow sensor with integrated impedance measurement [24] is implemented to allow the metering of both, the flow rate and the concentration of the released drug solution. Multiplying the flow rate with the concentration of the solution allows to determine the drug release rate on the buccal side as well as the moment when the solid drug pill is fully dissolved and depletion starts. The system was supposed to find application with Alzheimer's disease [25,26] and drug addiction [27,28] administering galantamine and naltrexone hydrochloride, respectively. However, the extreme system complexity delayed the development and did not allow demonstrating fully functional prototypes. Nevertheless, a first human in vivo trial was done for treating drug addiction. A simplified system without microvalve was mounted on a partial removable prosthesis and resulted in a 25-times increased bioavailability compared to the same drug load administered per-oral [29].

Fig. B. working principle of the IntelliDrug (intraoral drug delivery system).

The IntelliDrug system requires sufficient saliva secretion flow to work properly. Because of this, the system might be contraindicated in individuals suffering from xerostomia or dry mouth. Hence, the patient's salivary gland function should be considered.

Some commercially marketed products:

In 1983, Novartis introduced Acutrim (phenyl propanolamine) as an appetite suppressant. Later Calan SR (verapamil) was introduced for hypertension by GD Searle & Co. Ditropan XL (oxybutynin chloride) for overactive bladder was introduced by Alza Corp & UCB pharma Inc in 1998. Novartis introduced Efidac 24 in 1992, 94 and 96 with varying formulations to treat cold medication (pseudoephedrine), allergies (chlorphenaramine) and cold and allergies (pseudoephedrine/bromphenaramine) respectively. In 1989 and 94, Pfizer introduced Procardia

XL (nifedipine), Minipress XL (prazosin), and Glucotrol XL (glipizide) to treat non-insulin dependent diabetes. Teczem (enalapril and diltiazem) and Tiamate (diltiazem) were introduced by Hoechst in 1996 as a second line therapy for hypertension. Glaxo Wellcome Inc introduced Volmax (albuterol) for bronchospasm in 1989.

CONCLUSION:

Osmotic micropumps for drug delivery are applied to a broad range of different applications addressing multiple routes of administration. In general, three different operating principles of osmotic pumps having distinct advantages and disadvantages can be distinguished. Requiring solvents for operation, osmotic pumps are predominantly used in liquid environments, e.g. body fluids. In order to be able to operate osmotic pumps in any environment, multi-compartment architectures with an additional reservoir for solvents were developed. Although these increases further the device complexity, new fields of application such as the use as extracorporeal drug delivery systems can be addressed. However, until now only few approaches exist here. In general, osmotic drug delivery systems provide distinct advantages compared to the standard therapies. Osmotic devices are especially beneficial for long-term applications eliminating the need for frequent intake of single doses as it is the case for tablets and injections. This can result in better patient compliance and adherence as well as a less strict therapy

plan. Moreover, this makes them suitable for patients with substantial therapy adherence problems. Furthermore, by applying microengineering and new MEMS fabrication technologies, further miniaturization of the devices was enabled and resulted in new devices for body regions that were difficult to access so far. Potential drawbacks of osmotic pumps include the temperature dependency of the principle. While this is mostly not an issue for intra-corporeal use, it could be of relevance for extracorporeal devices subjected to changing environmental conditions. Although the release rate of osmotic pumps is constant, the rate cannot be modified during operation with few exceptions. This requires knowledge on the optimum delivery rate already before operation. Additionally, refilling of the pumps is mostly not possible or complicated making osmotic pumps predominantly single-use devices. With respect to implantation, this resulted in a trend towards biodegradable materials requiring no explantation anymore.

By transforming ordinary drugs into better acting drugs, osmotic pumps offer pharmaceutical companies an accessible technology that can also manage the life cycle of drugs with closely expiring patents. However, many of the shown pumps which arrive now at the end of the technical development pipeline have still to prove their competitiveness on the medical markets.

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Received on 09.10.2012 Modified on 20.10.2012

Research J. Pharm. and Tech. 6(1): Jan. 2013; Page 12-16