all documents cited are , in relevant part , incorporated herein by reference ; the citation of any document is not to be construed as an admission that it is prior art with respect to the present invention . the active core of the dosage form of the present invention may be comprised of an inert particle or an acidic or alkaline buffer crystal , which is coated with a drug - containing film - forming formulation and preferably a water - soluble film forming composition to form a water - soluble / dispersible particle . alternatively , the active may be prepared by granulating and milling and / or by extrusion and spheronization of a polymer composition containing the drug substance . the amount of drug in the core will depend on the dose that is required , and typically varies from about 5 to 60 weight %. generally , the polymeric coating on the active core will be from about 4 to 20 % based on the weight of the coated particle , depending on the type of release profile required and / or the polymers and coating solvents chosen . those skilled in the art will be able to select an appropriate amount of drug for coating onto or incorporating into the core to achieve the desired dosage . in one embodiment , the inactive core may be a sugar sphere or a buffer crystal or an encapsulated buffer crystal such as calcium carbonate , sodium bicarbonate , fumaric acid , tartaric acid , etc . which alters the microenvironment of the drug to facilitate its release . the drug - containing particle may be coated with an extended release ( er ) coating comprising a water insoluble polymer or a combination of a water insoluble polymer and a water soluble polymer to provide er beads . in accordance with certain embodiments , the water insoluble polymer and the water soluble polymer may be present at a weight ratio of from 100 / 0 to 65 / 35 , more particularly from about 95 / 5 to 70 / 30 , and still more particularly at a ratio of from about 85 / 15 to 75 / 25 . the extended release coating is applied in an amount necessary to provide the desired release profile . the extended release coating typically comprises from about 1 % to 15 %, more particularly from about 7 % to 12 %, by weight of the coated beads . the present invention also provides a method of making a modified release dosage form including a mixture of two bead populations . in accordance with one embodiment , the method includes the steps of : 1 . preparing a drug - containing core by coating an inert particle such as a non - pareil seed , an acidic buffer crystal or an alkaline buffer crystal with a drug and a polymeric binder or by granulation and milling or by extrusion / spheronization to form an immediate release ( ir ) bead ; 2 . coating the ir bead with a plasticized water - insoluble polymer alone such as ethylcellulose or in combination with a water soluble polymer such as hydroxypropylmethylcellulose to form an extended release ( er ) bead ; 3 . filling into hard gelatin capsules er beads alone or in combination with ir beads at a proper ratio to produce mr ( modified release ) capsules providing the desired release profile . ir beads when tested in accordance with the following procedure release at least about 70 %, more specifically at least about 90 % of the active within 30 minutes . dissolution apparatus : usp apparatus 2 ( paddles at 50 rpm ), dissolution medium : 900 ml 0 . 1 n hcl ( or a suitable dissolution medium ) at 37 ° c . and drug release determination by hplc ). an aqueous or a pharmaceutically acceptable solvent medium may be used for preparing drug - containing core particles . the type of film forming binder that is used to bind the drug to the inert sugar sphere is not critical but usually water soluble , alcohol soluble or acetone / water soluble binders are used . binders such as polyvinylpyrrolidone ( pvp ), polyethylene oxide , hydroxypropyl methylcellulose ( hpmc ), hydroxypropylcellulose ( hpc ), polysaccharides such as dextran , corn starch may be used at concentrations from about 0 . 5 to 5 weight %, although other concentrations may be useful . the drug substance may be present in this coating formulation in the solution form or may be dispersed at a solid content up to about 35 weight % depending on the viscosity of the coating formulation . in accordance with certain embodiments , the drug substance , optionally a binder such as pvp , a dissolution rate controlling polymer ( if used ), and optionally other pharmaceutically acceptable excipients are blended together in a planetary mixer or a high shear granulator such as fielder and granulated by adding / spraying a granulating fluid such as water or alcohol . the wet mass can be extruded and spheronized to produce spherical particles ( beads ) using an extruder / marumerizer . in these embodiments , the drug load could be as high as 90 % by weight based on the total weight of the extruded / spheronized core . representative muscle relaxants include cyclobenzaprine , dantrolene sodium , methocarbamol , metaxalone , carisoprodol , diazepam and pharmaceutically acceptable salts or derivatives thereof . cyclobenzaprine hydrochloride is a particularly useful muscle relaxant . as used herein , the useful muscle relaxants include the base , pharmaceutically acceptable salts thereof such as hydrochloride , stereoisomers thereof and mixtures thereof . representative examples of water insoluble polymers useful in the er coating include ethylcellulose powder or an aqueous dispersion ( such as aquacoat ® ecd - 30 ), cellulose acetate , polyvinyl acetate ( kollicoat sr # 30d from basf ), neutral copolymers based on ethyl acrylate and methylmethacrylate , copolymers of acrylic and methacrylic acid esters with quaternary ammonium groups such as eudragit ne , rs and rs30d , rl or rl30d and the like . representative examples of water soluble polymers useful herein include low molecular weight hydroxypropyl methylcellulose ( hpmc ), methylcellulose , hydroxypropylcellulose , polyvinylpyrrolidone , polyethylene glycol ( peg of molecular weight & gt ; 3000 ) and mixtures thereof . the extended release coating will typically be applied at a thickness ranging from about 1 weight % up to 15 weight % depending on the solubility of the active in water and the solvent or latex suspension based coating formulation used . the coating compositions used in forming the membranes are usually plasticized . representative examples of plasticizers that may be used to plasticize the membranes include triacetin , tributyl citrate , triethyl citrate , acetyl tri - n - butyl citrate diethyl phthalate , polyethylene glycol , polypropylene glycol , castor oil , dibutyl sebacate , acetylated monoglycerides and the like or mixtures thereof . the plasticizer may comprise about 3 to 30 wt . % and more typically about 10 to 25 wt . % based on the polymer . the type of plasticizer and its content depends on the polymer or polymers , nature of the coating system ( e . g ., aqueous or solvent based , solution or dispersion based and the total solids ). in general , it is desirable to prime the surface of the particle before applying an extended release membrane coating or to separate the different membrane layers by applying a thin hydroxypropyl methylcellulose ( hpmc ) ( opadry ® clear ) film . while hpmc is typically used , other primers such as hydroxypropylcellulose ( hpc ) can also be used . the membrane coatings can be applied to the core using any of the coating techniques commonly used in the pharmaceutical industry , but fluid bed coating is particularly useful . the present invention is applied to multi - dose forms , i . e ., drug products in the form of multi - particulate dosage forms ( pellets , beads , granules or mini - tablets ) or in other forms suitable for oral administration . as used herein , these terms are used interchangeably to refer to multi - particulate dosage forms . the invention also provides a method of making an extended release dosage form which includes a mixture of two or more bead populations . in accordance with one aspect of the present invention , the method includes the steps of : ( a ) coating an inert particle such as a non - pareil seed , an acidic buffer crystal or an alkaline buffer crystal with a drug and polymeric binder to form an active drug particle ( ir beads ), which may be present in the unit dosage form to act as a bolus dose ; ( b ) coating the active drug particle with a solution or suspension of a water insoluble polymer or a mixture of water soluble and water insoluble polymers to form an extended release coated drug particle ( er beads ); ( c ) filling into a hard gelatin capsule er beads alone and optionally , in combination with ir beads at a proper ratio ranging from 95 / 5 to 70 / 30 ( er beads / ir beads ) to produce a mr ( modified release ) capsule exhibiting a target drug release profile . the following non - limiting examples illustrate the capsule dosage forms manufactured in accordance with the invention using cyclobenzaprine hydrochloride as a test case , which exhibit in vitro drug release profiles , similar to that predicted by performing modeling exercises . such dosage forms when orally administered , would enable maintaining drug plasma concentrations at therapeutically effective levels over extended periods of time , thereby significantly improving patient compliance . cyclobenzaprine is well absorbed after oral administration , but there is a large intersubject variation in plasma levels . it is eliminated quite slowly with a half - life as long as one to three days . the present treatment regimen of 10 mg three times daily is an issue of patient compliance , especially the elderly . hence , a modified release dosage form ( capsule ) was designed with a release profile shown in fig1 . to determine if this is the proper release profile , the pharmacokinetics data of cyclobenzaprine following a single dose of 10 mg flexeril ® tablets administered 3 times a day was taken from the literature . a pharmacokinetic model was developed from this data using winnonlin ™ version 1 . 5 . theoretical plasma levels were simulated using the pharmacokinetic model given above and the target release rate given in fig1 . fig2 shows the simulated plasma levels for day one following dosing of 1 × 10 mg flexeril ® tablet given 3 times a day and the proposed cyclobenzaprine hcl mr capsule , 30 mg given once a day . cyclobenzaprine hydrochloride ( 1 , 200 g ) was slowly added to an aqueous solution of polyvinylpyrrolidone such as povidone usp ( k - 29 / 32 , 80 g ) and mixed well . # 25 - 30 mesh sugar spheres ( 2 , 640 g ) were coated with the drug solution in a glatt fluid bed coater , equipped with a 9 ″ bottom spray wurster insert to provide ir beads with a coating weight of about 9 %. the drug containing particles were dried , and a seal coat of opadry ® clear ( 2 % w / w ) was first applied and dried in the glatt fluid bed unit as a precautionary measure to drive off excessive surface moisture . the composition and batch quantities of the ir beads were given in 5 to 10 kg . following the second coating process the ir beads were passed through 14 and 25 mesh screens . beads remaining on the 14 - mesh screen were discarded as oversized beads and beads passing through the 25 - mesh screen were discarded as undersized beads . the next step in the process was to apply an extended release polymer membrane by spraying aquacoat ® ecd 30 , an aqueous dispersion of ethylcellulose with dibutyl sebacate ( 76 : 24 ), onto the ir beads for a weight gain of approximately 10 %. the same fluid bed equipment was used to produce er ( extended release ) beads by further coating the aquacoat ® coated beads with opadry ® clear for a weight gain of 2 % w / w prior to curing at 60 ° c . in a conventional oven for a period of 24 hours . the batch size was 5 to 10 kg . the drug release profiles are shown in fig3 . the figure also shows the drug release profiles from er beads stored in induction sealed hdpe bottles at 25 ° c ./ 60 % rh for 6 months . cyclobenzaprine hydrochloride ( 2 . 5 kg ) was dissolved in 50 / 50 acetone / purified water . 25 - 30 mesh sugar spheres , ( 7 . 3 kg ) were coated with the drug solution in a glatt fluid bed coater , equipped with a 9 ″ bottom spray wurster insert . the drug containing particles were dried , and a seal coat of opadry ® clear ( 2 % w / w ) was first applied and dried in the glatt fluid bed unit as a precautionary measure to drive off excessive surface moisture . 910 g of ethylcellulose ( ethocel premium standard 10 cps ) and 90 g of diethyl phthalate were dissolved in 98 / 02 acetone / purified water and applied onto the ir beads ( 9 kg ) in the glatt gpcg 5 in accordance with the present invention . the release rates of the er beads will vary depending upon the film weight of the er coating . one batch of ir beads was coated for a final weight gain of 7 % based on the weight of coated beads wherein samples of the er beads were removed during the er coating process to yield beads with increasing coating weights . another batch was coated for 10 % weight gain and all the coated bead batches were cured at 60 ° c . for 4 hours in a conventional oven . fig4 shows the relationship between the er coating weights and the release rate of the finished er coated beads . a batch was coated with a 7 % er coating and cured at 60 ° c . for 4 hours . no changes were noted in the release rates , assay values or impurity levels after storage in hdpe bottles at 40 ° c ./ 75 % rh for a period of 6 months . the release rates for the samples are shown in fig5 . the drug layering , seal coating , and er coating processes were scaled - up to glatt gpcg 120 equipped with an 18 ″ bottom spray wurster insert ( batch size : 80 kg for ir beads and 85 kg for er beads ). the process parameters of each of the processes were optimized . the drug layering solution ( 9 % weight gain ), seal coating solution , and the er coating solution ( 9 % weight gain ) were sprayed onto the sugar spheres or ir beads while maintaining the product temperature between narrow limits . following the seal or er coating the beads were passed through 14 and 25 mesh screens discarding any beads remaining on the 14 mesh screen . the er beads were also cured at 60 ° c . for a period of 4 hours . the extended release beads were then filled into size 4 capsules to produce cyclobenzaprine hcl mr capsules , 15 and 30 mg . the drug release profiles of 30 mg capsules of one pivotal clinical and three registration stability batches are presented in fig6 a randomized double - blind two - period crossover study to assess the safety and bioavailability of cyclobenzaprine hcl modified - release ( cmr ) 15 mg and 30 mg in healthy male and female volunteers ( n = 14 or 15 ) was performed . each subject received one 15 mg or 30 mg capsule of cmr in the morning , separated by a 14 - day washout period between doses . the results are presented in table 1 and fig7 wherein auc 0 - 168 refers to the area under the plasma concentration - time curve to the last measurable time point ( 168 hrs ) calculated by the linear trapezoidal rule , auc 0 -∞ refers to area under the concentration - time curve to infinity , c max refers to the maximum blood plasma concentration and t max refers to the time to maximum plasma levels of cyclobenzaprine . the treatments were significantly different from each other as values for aucs and c max were higher for cmr 30 mg than those for cmr 15 mg . the bioavailability of cmr 30 mg was approximately twice that of cmr 15 mg as shown by the aucs . the adjusted mean ratio of cmr 30 mg to cmr 15 mg was greater than about 2 for each of the aucs and c max , specifically the calculated values were 2 . 42 for auc 0 - 168 ( p & lt ; 0 . 001 ), 2 . 286 for auc 0 -∞ ( p & lt ; 0 . 001 ), and 2 . 424 for c max ( p & lt ; 0 . 001 ). overall , both cmr 15 mg and 30 mg were well tolerated during the study . accordingly , one aspect of the invention relates to a dosage form containing cyclobenzaprine hydrochloride as a skeletal muscle relaxant wherein the pharmaceutical dosage form provides a maximum blood plasma concentration ( c max ) within the range of about 80 % to 125 % of about 20 ng / ml of cyclobenzaprine hcl , an auc 0 - 168 within the range of about 80 % to 125 % of about 740 ng · hr / ml and a t max within the range of about 80 % to 125 % of about 7 hours following oral administration of a single 30 mg cyclobenzaprine hcl mr capsule . while the invention has been described in detail and with reference to specific examples thereof , it will be apparent to one skilled in the art that various changes and modifications can be made without departing from the spirit and scope thereof .
a unit dosage form , such as a capsule or the like , for delivering a skeletal muscle relaxant , such as cyclobenzaprine hydrochloride , into the body in an extended or sustained release fashion comprising one or more populations of drug - containing particles is disclosed . at least one bead population exhibits a pre - designed sustained release profile . such a drug delivery system is designed for once - daily oral administration to maintain an adequate plasma concentration — time profile , thereby providing relief of muscle spasm associated with painful musculoskeletal conditions over a 24 hour period .