a plurality of potassium chloride crystals , preferably from about 20 mesh to about 70 mesh , more preferably from about 30 mesh to about 50 mesh , are coated with two distinct layers . the first layer applied to the crystals is composed of ethylcellulose , preferably with a viscosity of from about 90 to about 110 cp . such as ethocel 100 ( dow chemical ). utilization of this high viscosity ethylcellulose allows the crystals to retain their diffusion controlling characteristics even after compression into a tablet form . the ethylcellulose may be applied by any suitable technique known in the art , preferably by coacervation using polyethylene as a phase separator . if coacervation is used , trace amounts of the phase separator may be present in the first layer , preferably in an amount less than about one percent by weight of the ethylcellulose coated crystals . the ethylcellulose layer comprises from about 8 to about 19 . 5 percent , more preferably from about 11 to about 15 percent , of the total weight of the uncoated potassium chloride crystals . this first layer controls the release of the potassium chloride over time , total release time being proportionally dependent upon the thickness of ethylcellulose . after application of the ethylcellulose , a drying step should preferably be carried out for such a time period and at such temperatures so that the microencapsulated crystals do not adhere to other microcapsules . the resultant ethylcellulose encapsulated potassium chloride microcapsules are preferably of such a size that less than 5 % are greater than 20 mesh . a second , discrete layer of at least one hydrophilic polymer coating , preferable a hydroxypropylcellulose , such as klucel lf ( aqualon company ), is applied over the first layer of ethylcellulose . other hydrophilic polymer coatings include , but are not limited to , polyvinyl alcohol , polyvinyl pyrrolidone , and hydroxypropyl methylcellulose . the hydrophilic layer is applied by conventional techniques , such as from an aqueous solution using a fluidized bed coater , to the preformed layer of ethylcellulose . the hydrophilic polymer coating layer comprises from about 0 . 5 to about 4 percent , more preferably from about 0 . 5 to about 1 . 5 percent , of the weight of the ethylcellulose coated crystals . the hydrophilic polymer does not significantly diffuse into the ethylcellulose , but rather forms a distinct second layer . as this layer is soluble to gastric fluids , the hydrophilic polymer coating dissolves following ingestion of the resultant tablet . for all practical purposes , it does not contribute to the controlled release of potassium chloride . rather , the hydrophilic polymer coating is present primarily as a binder material so that a high dosage rate tablet can be formed with a minimal amount of conventional excipients and low compaction pressures to allow minimal disruption of the rate controlling ethylcellulose membrane . after the hydrophilic polymer coating layer is applied , the now double coated crystals are subjected to a final drying step the resultant coated potassium chloride microcapsules are preferably of such a size that less than 15 % are greater than 20 mesh . the coated crystals may then be formed into tablets by compression using conventional techniques . a minimal amount of excipients , no more than about 15 % more preferably no more than 12 %, and most preferably no more than 7 %, by weight of the final dosage tablet , is added to the coated crystals prior to compression . this is in contrast to the greater than 30 % excipients typically necessary for compression of potassium chloride microcapsules without a hydrophilic coating layer into a tablet form . in addition , this formulation allows the microencapsulated potassium chloride to be dispersed essentially intact over a wide area , reducing the risk of gastric irritation . the term &# 34 ; excipients &# 34 ;, as used herein , refers to any additional pharmaceutically acceptable ingredients which may be used in a tablet . these excipients include , but are not limited to , ingredients such as binders , disintegrants , wetting agents , and lubricating agents . binders include , but are not limited to , klucel ® lf ( hydroxypropylcellulose ) and avicel ® ( microcrystalline cellulose ). disintegrants include , but are not limited to , cornstrach , lactose , mannitol , sucrose , avicel ® ( microcrystalline cellulose ), primogel ® ( sodium carboxymethyl starch , emcompress ® ( dibasic calcium phosphate dihydrate ), crospovidone ® ( cross linked polyvinyl pyrrolidone ), and tricalcium phospate . wetting agents include , but are not limited to , sodium lauryl sulfate . lubricating agents include , but are not limited to stearates ( e . g . magnesium , calcium , and sodium ,) stearic acid , sterotex ®, talc , waxes , and stearowet ®. the final tablets will contain a pharmaceutically acceptable amount of potassium chloride to treat humans in need thereof , preferably from about 8 meq to about 20 meq . acceptable daily dosages may be found in the physicians &# 39 ; desk reference , 45th ed . ( 1991 ), e . g . 20 - 200 meq / day . the coated potassium chloride microcapsules of the present invention may be produced by coacervation as follows : ______________________________________ingredient amount______________________________________potassium chloride , usp 567 kgcyclohexane 875 galethylcellulose , nf 83 . 5 kgpolyethylene 58 kghydroxypropyl cellulose , nf 6 . 9 kgpurified water , usp 35 kg______________________________________ the potassium chloride , cyclohexane , ethylcellulose , and polyethylene are mixed together with controlled agitation and heating until the polymers are in solution . the solution is then cooled to achieve encapsulation of the potassium chloride with the ethylcellulose . the resulting microcapsule slurry is next filtered using a rotary vacuum filter and the resulting wet microcapsules are dried in a fluidized bed drier . the dry microcapsules are sized , preferably through a 14 mesh sieve to produce potassium chloride microcapsules ready for coating . a solution of hydroxypropylcellulose and purified water is prepared . the potassium chloride microcapsules are coated with this solution using a fluidized bed coater . the resulting coated microcapsules are then dried and sized through a 12 mesh sieve . by the foregoing technique , other hydrophilic polymers which provide a binding effect may be used , i . e . polyvinyl alcohol , polyvinyl pyrrolidone , and hydroxypropyl methylcellullose , which have found acceptance in coating pharmaceuticals . the release rate of the microcapsules of example 1 were measured using the usp basket method , 900 ml purified water , 100 rpm at 37 degrees c . ______________________________________release rate (%/ hour ) time ( hours ) trial 1 trial 2 trial 3______________________________________1 21 . 8 17 . 6 15 . 04 71 . 3 57 . 6 54 . 78 99 . 5 92 . 1 89 . 6______________________________________ the microcapsules of example 1 are used to prepare 20 meq potassium chloride tablets using conventional tabletting techniques , e . g . compressed on an instrumented rotary tablet press , and conventional tabletting excipients , e . g . binders , disintegrants , wetting agents , and lubricating agents . the release rate of the potassium chloride tablets were measured using the usp basket method , 900 ml purified water , 100 rpm at 37 degrees c . ______________________________________release rate (%/ hour ) time ( hours ) trial 1 trial 2 trial 3______________________________________1 21 . 4 22 . 0 17 . 02 40 . 0 36 . 9 33 . 24 71 . 6 65 . 1 60 . 86 90 . 2 87 . 3 84 . 58 99 . 4 101 . 3 97 . 4______________________________________ the coated potassium chloride microcapsules of example 1 were made into tablets using conventional tabletting techniques and the following formulation : ______________________________________ingredient percent______________________________________coated kcl microcapsules 88microcystalline cellulose 10cross - linked polyvinyl pyrrolidone 2______________________________________ the potassium chloride microcapsules of example 1 were also prepared without the hydroxypropylcellulose coating or any other hydrophilic polymer coating . these uncoated microcapsules could not be made into tablets using conventional tabletting techniques and the above formulation . the amount of kcl microcapsules in the formulation had to be decreased to about 68 % in order that they might be tabletted using conventional tabletting techniques . the above description is for the purpose of teaching the person of ordinary skill in the art how to practice the present invention and is not intended to detail all those obvious modifications and variations of it which will become apparent to the skilled worker upon reading the description or to limit the invention in any way . it is intended , however , that all such obvious modifications and variations be included within the scope of the present invention and by the following claims .
a pharmaceutical dosage form is prepared from a multiplicity of coated potassium chloride crystals coated with two distinct layers , the first of ethylcellulose and the second of a hydrophilic coating polymer , preferably hydroxypropylcellulose , resulting in microcapsules . these microcapsules are capable of being compressed into tablets of suitable hardness and friability with minimum quantities of excipients . the resultant controlled release tablets are useful for treatment of potassium deficiencies in humans .