Osmotically Driven Active Agent Dispenser

1. Field of the Invention

This invention relates to an osmotically driven active agent dispenser.

2. Description of the Prior Art

The invention is a modification of the osmotically driven active agent dispenser disclosed in U.S. Pat. No. 3,916,899. Briefly, the patented dispenser comprises: a core of an osmotically effective active agent composition; a water insoluble wall that is impermeable to the composition, has a controlled permeability to water and encloses the core; and one or more particularly sized passageways extending through the wall to the core. When placed in an aqueous environment, such as the gastrointestinal tract, eye, vagina, and uterus, water is imbibed from the environment by the composition through the wall causing the composition to be dissolved, a pressure gradient between the interior and exterior of the dispenser to be generated, and the dissolved composition to be pumped out the passageway(s) into the environment. The release of active agent from these dispensers is continuous, sustained, predetermined, and substantially constant over a major portion of the release period. The patent discloses that the passageways in the wall may be formed mechanically, such as by drilling, or in situ by erosion of a bioerodible component incorporated into the wall.

Commonly owned U.S. application Ser. No. 578,979 filed May 19, 1975 discloses a drug dispenser in the form of a unit body of a mixture of discrete particulate depots of an osmotically effective drug composition dispersed in a water insoluble, water permeable polymer. The depots imbibe water from the environment serially beginning with those nearest the exposed surface of the body until enough pressure is generated within the depots to cause them to burst open. As the depots burst open the drug composition within them is released.

U.S. Pat. No. 2,478,182 describes salt tablets comprised of compressed salt granules coated with cellulose acetate or cellulose nitrate. According to the patent the cellulose derivative penetrates the compressed granules to form a honeycomb structure. Upon ingestion of the tablets, water dialyzes into the compartments of the honeycomb and salt dialyzes out. The patent also says the honeycomb compartments burst when they become engorged with water.

U.S. Pat. No. 3,247,066 describes medicinal beads that burst to release medicine. The beads consist of a core of a water swellable colloid containig a drug that is coated with a polymer that is water permeable and inert to gastrointestinal fluids. When the beads are ingested, water diffuses inwardly through the coating to swell the colloid core. This swelling creates sufficient pressure to burst the coating, whereby the entire quantity of drug in the core is released.

Also, the agricultural chemical art literature describes various slow release formulations of fertilizer or pesticide and polymer. For instance, U.S. Pat. No. 3,708,276 describes grains of fertilizer that are coated with water insoluble resins containing foreign particles. The presence of such particles is said to affect the permeability and the rate at which the fertilizer is eluted.

The invention is an osmotically driven active agent dispenser for use in an aqueous environment comprising: a core of an osmotically effective active agent composition, and a substantially inexpandable wall enclosing the core, the wall having a controlled permeability to water and including means responsive to the pressure generated within the dispenser by imbibition of water from the environment by the core through the wall to form at least one passageway in situ in the wall through which the active agent composition in solution is pumped osmotically into the environment at a substantially predetermined, substantially constant rate that is dependent upon the rate at which water is being imbibed into the dispenser.

The drawing is a graph of the active agent release pattern of the dispensers of the example set forth below. The graph plots release rate versus time.

As indicated above, the dispenser of this invention is a unique modification of the dispenser of U.S. Pat. No. 3,916,899. The difference between the patented dispenser and the present dispenser is that the present dispenser does not have a preformed passageway or a bioerodible wall component that erodes in situ to form a passageway, but instead its wall includes means that is responsive to the osmotically generated pressure within the dispenser to form at least one and usually many passageways in situ in the wall. These passageways serve the same function as the preformed or in situ erosion formed passageway of the patented dispenser.

The terms "aqueous environment", "active agent", "drug", "osmotically effective", and "wall" used herein have generally the same meanings as in U.S. Pat. No. 3,916,899 and the respective descriptions of those terms that appear in the patent are incorporated herein by reference. Also the form and utility of the invention dispenser may be the same as that of the patented dispenser and the portions of the patent disclosure pertaining thereto are incorporated herein by reference. Likewise, the materials that are used to form the wall, the characteristics (except for the passageway forming means) of the wall, and the procedures used to form the core and enclose it within the wall may be the same as those of the patent and the disclosure of the patent that is relevant to these items is incorporated herein by reference.

As in the patented dispenser, the active agent composition that forms the core of the invention dispenser is an osmotically effective solute. In this regard the composition may be agent that itself is such a solute and that is neat or formulated with a carrier that may or may not also be such a solute, or agent that is not such a solute formulated with a carrier that is such a solute. In its osmotically effective solute role, the composition imbibes water from the environment inwardly through the wall of the dispenser. The imbibed water dissolves the composition and a hydrostatic pressure difference is established across the wall between the solution of active agent composition and the aqueous environment. The magnitude of and the rate of generation of this difference will depend upon the solubility of the composition in water, the number of ionic species it produces in solution, and the concentration gradient across the wall. The difference must be sufficient to activate the passageway forming means of the wall. Once the passageways are formed by the means, water is imbibed continuously into the dispenser in response to the osmotic pressure gradient across the wall. The imbibed water in turn continuously forces the solution of composition out the passageway(s) into the environment. Accordingly the solute should generate a significantly higher osmotic pressure in solution than the osmotic pressure of the aqueous environment in which the dispenser is used. For instance in embodiments that release drugs into body fluids solutes that exhibit a significantly higher osmotic pressure than such fluids (i.e. significantly higher than about 750 kPa) must be used. Solutes that exhibit osmotic pressures in the range of about 20,000 to about 40,000 kPa will normally be used in such drug dispensing embodiments. Osmotic pressure may be measured with a commercially available osmometer that measures the vapor pressure difference between pure water and the composition solution. The vapor pressure ratio may be converted into osmotic pressure difference by standard thermodynamic calculations.

The wall of the invention dispenser is substantially inexpandable as is the wall of the patented dispenser. This means that there is no substantial increase in the volume of the core before or during the dispensing period despite imbibition of water.

The passageway forming means of the wall may be an inherent property of the wall, such as the brittleness of the polymer forming the wall, or it may be sites of structural weakness in the wall such as might be caused by: incorporating foreign materials, e.g., fillers, in the polymer forming the wall; using a blend of incompatible polymers to make the wall; or, depending on the wall material, post-treating the wall thermally, by solvent crazing, or by irradiation. Inherent properties of the wall material that constitute such means may be a result of the composition of the wall (such as in the case of a polymer its crystallinity, molecular weight, degree and nature of cross-linking, and the presence and structure of side groups on the polymer backbone) and/or the manner in which the wall is formed.

The passageways formed by the means will generally be irregularly shaped, sized, and positioned and may be in the form of discrete holes or a network of cracks or fissures extending over part or all of the dispenser surface. Individually the passageways are normally minute, usually microscopic, and they constitute an outlet for the solution of active agent that falls within the maximum and minimum size limitations described in columns 20-21 of U.S. Pat. No. 3,916,899.

In the dispensers of the invention, the thickness of the wall may affect the operability of the passageway forming means and in such instances wall thickness will be correlated with the particular passageway forming means to insure that one or more passageways meeting the abovementioned minimum/maximum size limitations are formed timely. Normally the thickness of the wall will be in the range of about 100 to about 500 microns.

Active agent is released from the invention dispensers as follows. Upon placement of the dispenser in an aqueous environment, water is imbibed through the wall by the core of active agent composition. Since the wall is substantially inexpandable the imbibed water will cause a build up of pressure within the dispenser. During this build up there is no release of composition by osmotic pumping. The pressure builds up until it is sufficient to activate the passageway forming means of the wall. For instance if the brittleness of the wall is the means, the pressure has built up to the point at which brittle failure is initiated and the wall cracks. Alternatively, in instances in which there are sites of structural weakness in the wall the pressure has built up to the point at which stress fracture occurs at such a site or sites. The magnitude of this threshold pressure will vary with the particular means involved and the thickness of the wall. Desirably it will be relatively low, that is in the range of about 50 to about 5000 kPa. Once the passageway(s) is/are formed the pressure is relieved and osmotic pumping of the solution begins. After a short period of time, steady state conditions are reached and the solution of active agent composition is being pumped out of the in situ-formed passageway(s) at a substantially constant rate. The magnitude of the constant rate is dependent primarily upon the rate of water imbibition into the dispenser and is independent of the size of the passageway(s), provided such size is within the above-described maximum/minimum size limitations. This substantially constant rate prevails over the major portion of the dispensing period. After the active agent composition has been depleted to the extent that a saturated solution of composition is not maintained within the dispenser, the rate of release begins to tail off, decreasing continuously as the concentration of active agent in solution within the dispenser decreases.

The following example illustrates an embodiment the invention that may be used to dispense the drug potassium chloride. This example is not intended to limit the invention in any manner. Unless indicated otherwise, percentages are by weight.

Cores of potassium chloride containing 5% FD&C Blue No. 1 water soluble dye in the form of compressed tablets were made using a Manesty rotary tableting machine with 0.95 cm diameter standard concave punches set to a hardness of 8 kg. A mixture of cellulose acetate (sold by Eastman Kodak under the designation E-320) of 32% acetyl content and the water insoluble aluminum lake dye FD&C Yellow No. 5 in a 99:1 weight ratio was made up as a 5% solution in acetone/water (88.5%/11.5%). This solution was deposited on the potassium chloride cores using a Wurster air suspension apparatus to form a 125 micron thick wall or membrane about each core.

The theoretical steady state potassium chloride release rate from the coated tablets was calculated to be 45 mg/hr, based on a wall surface area of 2.3 cm.sup.2, a KCl solubility of 360 mg/ml, and a wall water transmission of 0.27 cm.sup.3 mil/cm.sup.2 hr. The formulas appearing in col 18 et seg. of U.S. Pat. No. 3,916,899 were used to make these calculations.

Five of the coated tablets were placed in known quantities of water at 37.degree. C. After a short time a passageway/passageways was/were formed in each tablet at a site/sites of structural weakness in the tablet wall caused by the presence of the yellow dye, as evidenced by the appearance of a blue spot/spots on the walls of the tablets. Aliquots of the water were taken at one hour intervals beginning 1/2 hour after the tablets were placed in the water. The KCl content of each aliquot was determined by standard KCl analysis techniques. From these determinations KCl rates from the tablets were calculated. The average release rate from the five tablets plotted against time is shown in the accompanying drawing. As illustrated in the drawing the in vitro release rate between hours 1 and 9 was substantially constant and corresponded approximately with the predicted, theoretical rate.

Modifications of the invention dispenser that are obvious to those skilled in the pharmaceutical and/or chemical arts are intended to be within the scope of the following claims.