the purpose of this study was to evaluate the release of recombinant human growth hormone ( rhgh ) from a non - polymeric sucrose acetate isobutyrate sustained release system . the system comprised sucrose acetate isobutyrate ( saib ) and a solvent . two spray freeze dried formulations of rhgh were evaluated , rhgh in sodium bicarbonate and rhgh complexed with zinc . the rhgh powders were homogenized with various systems at two different protein loads ( 5 and 15 % w / v ). the release rate and protein stability was monitored by reverse phase - hplc , size exclusion chromatography and bca for 28 days . the effect of zinc and surface area on release rate and protein stability was also investigated . the in vitro results for the zinc complexed rhgh indicated a very low burst from 0 . 1 ( saib : ethanol ) to 2 . 2 % ( saib : miglyol ) followed by protein release over 28 days . the release rates and total protein released by the different preparations varied widely . the high protein load ( 15 %) and the low protein load ( 5 %) released approximately the same amount of protein indicating that the surface area of the sucrose acetate isobutyrate : solvent / protein mix proved to be an important factor in the initial burst and the release rate . in vitro experiments that increased the surface area of the sucrose acetate isobutyrate : solvent / protein in contact with the release medium resulted in increased bursts of 1 to 4 % with a higher total percentage of released protein . the bicarbonate rhgh suspension had a higher initial burst ( 7 to 14 %) and released more protein in 28 days when compared to the zinc complexed rhgh suspension . changing the solvent polarity , the ratio of solvent to saib , and the addition of zinc can modify the release rate of the rhgh from sucrose acetate isobutyrate : solvent systems . these results demonstrate that the sucrose acetate isobutyrate : solvent delivery system is capable of providing sustained release of intact rhgh in vitro . sucrose acetate isobutyrate extended release systems are described in u . s . pat . no . 5 , 747 , 058 , for example , the disclosure of which is specifically incorporated herein by reference . the growth hormone ( gh ) is preferably human growth hormone ( hgh ), preferably biologically active non - aggregated hgh . according to the present invention the gh is complexed with at least one type of multivalent metal cation , preferably having a valence of + 2 or more , preferably from a metal cation component of the formulation . suitable multivalent metal cations include biocompatible and non - toxic metal cations . a preferred metal cation component for gh is zn + 2 . typically , the molar ratio of metal cation component to gh is between 1 : 1 and 100 : 1 , preferably , between 1 : 1 and 20 : 1 and preferably between 1 : 1 and 10 : 1 . the following examples are offered by way of illustration and not by way of limitation . the disclosures of all citations in the specification are expressly incorporated herein by reference . preparation of zinc complexed rhgh : a 20 mg / ml rhgh solution in 25 mm sodium bicarbonate was complexed with zinc at a rhgh : zinc ratio of 10 : 1 . the rhgh / zinc suspension was spray freeze dried to create a fine powder that is approximately 70 % rhgh by weight . preparation of bicarbonate rhgh : a solution of approximately 5 mg / ml rhgh in 10 mm ammonium bicarbonate was lyophilized to produce an excipient free powder . saib / rhgh suspension preparation : the rhgh saber suspensions were prepared by mixing rhgh powders with saber formulations using a shear homogenizer . release rate determination : 0 . 2 ml of each rhgh / saib suspension was added to eppendorf tubes in duplicate , then 0 . 5 ml of release medium ( 50 mm hepes , 10 mm kcl , 0 . 1 % nan3 , ph 7 . 2 ) was added above the suspension . the eppendorf tubes were incubated at 37 deg . c . and sampled at various time points . at each time point , 0 . 5 ml of release medium was removed and 0 . 5 ml of fresh release medium added . collected samples were stored at − 70 deg . c . prior to analysis . the release samples were analyzed for protein concentration and protein quality . bca assay : the bca assay in a microtiter plate format was used to determine the protein concentration of the release samples . rhgh protein standards were prepared in release medium at 0 , 0 . 005 , 0 . 01 , 0 . 02 , 0 . 05 , 0 . 2 , 0 . 5 g / ml . 0 . 02 ml of each blank , standards , and release samples were mixed with 0 . 2 ml of the bca working reagent in a microtiter plate . the microtiter plate was incubated at 37 deg . c . for 1 hr and the absorbance determined at 562 nm using a microtiter plate reader . the protein concentrations of the release samples were determined from the standard curve using a four parameter non - linear curve fit . the amount of oxidized variants in the rhgh release samples was determined by rp - hplc . this assay was run using a 4 . 6 × 15 cm , 8 mm , 300 angstrom plrps column held at room temperature . the mobile phase a contained 50 mm nah2po4 , ph 7 . 0 and mobile phase b contained 20 % propanol in acetonitrile . the separation was isocratic at 49 % ( b ) and the eluent was monitored for absorbance at 214 nm . size exclusion chromatography was used to determine amount of monomer present in the release samples . this assay was run using a 7 . 8 × 300 mm tsk 2000swxl column held at room temperature . the mobile phase used was 50 mm nah2po4 , 150 mm nacl ph 7 . 2 with a flow rate of 1 . 0 ml / min and a run time of 20 min . 10 g protein was injected and the eluent monitored for absorbance at 214 nm . in vivo pharmacokinetics of rhgh were determined in after sc injection of rhgh saber formulations ( saib : benzyl alcohol ; 85 : 15 w / w and saib : benzyl benzoate ; 70 : 30 w / w ) in sprague dawley ( sd ) rats . serum rhgh levels were determined by elisa ( genentech ) with an assay detection limit of 0 . 1 ng / ml . the effect of the saib / solvent ratio on protein released was examined by plotting the cumulative release for rhgh in saib : ethanol ratios , 85 : 15 , 75 : 25 , and 50 : 50 ( w / w ). this plot is shown in fig2 a . the 85 : 15 , 75 : 25 , and 50 : 50 w / w ratio resulted in a 10 %, 13 %, and 26 % release of the protein at 28 days . the saib / solvent ratio is a factor in release rate , but it does not effect the initial burst for the saib : ethanol formulations . the effect of solvent on the rate of release from saber is shown in fig3 . all saib / solvent preparations show a low initial burst of rhgh in the first day and protein release out to 28 days . the rhgh / saib : miglyol suspension was the only sample with a poor release curve . the total amount of protein released over the 28 days for all samples was no higher than 13 % of the total protein load . this result was expected due to the lack of enzymatic degradation in these in vitro experiments . the release results for all saib / solvent preparations and both protein loads are detailed in fig2 b – c . ideally a one month sustained release system should have an initial burst of approximately & lt ; 10 % and an average daily release of 3 %. the results for the saber with rhgh show a burst from 0 . 1 to 2 . 2 %, with an average daily release over 28 days from 0 . 1 to 0 . 9 %. these values are extremely low but expected due to the lack of in vitro degradation of saber . the effect of zinc on rhgh release from saber was evaluated by comparing release rates of zinc complexed rhgh and lyophilized rhgh in bicarbonate from saber . 5 % w / v suspensions were prepared using two saib / solvent preparations , benzyl benzoate , and ethanol . the release curves are shown in fig4 . the bicarbonate rhgh produces a higher initial burst than the zinc complexed rhgh for both saber preparations . the initial burst for the bicarbonate rhgh from saib : ethanol is 6 . 53 % compared to 0 . 53 % for the zinc complexed rhgh . the initial burst from saib : benzyl benzoate is 14 . 64 % for the bicarbonate rhgh compared to 1 . 06 % for the zinc complexed rhgh . the daily release and the overall total protein released is also much higher for the bicarbonate rhgh . these results indicate that excipients such as zinc can affect protein release from saber . this effect may be due to differences in particle morphology or more likely differences in protein solubility . zinc complexed rhgh has lower solubility than the bicarbonate formulation . the integrity of the released protein was determined by rp - hplc and sec . the results indicate a decrease in native protein over time ( fig5 ). this decrease was most pronounced in protein released from saber formulations containing benzyl benzoate and ethanol . protein released from the 5 % load formulations was less native than protein released from the 15 % load formulations . this may be due to a decrease in the protein : solvent ratio in the 5 % load formulations , leading to higher solvent exposure in the release medium . during the course of these experiments several grades of benzyl benzoate were used ( reagent grade and usp grade ). samples from experiments using these solvent grades were tested for oxidation ( rp - hplc ) and aggregation ( sec ). the results show protein released from the saber formulations containing usp grade benzyl benzoate were less degraded than protein released from reagent grade benzyl benzoate ( fig6 ) after 21 days the amount of rhgh monomer remaining was over 90 % for the usp grade benzyl benzoate formulation compared to 75 % for the reagent grade formulation . the reversed phase results also show an improvement in protein quality with the usp grade benzyl benzoate . at 21 days 80 % of the main peak remained compared to 60 % seen with the reagent grade solvent . the purity of solvent used in saber formulations has a direct effect on protein quality and thus should be monitored . to determine the effect zinc had on the protein release rate , zinc complexed gh and bicarbonate rhgh were mixed with two saber formulations containing ethanol and benzyl benzoate as solvents . in vitro release experiments were carried out using an edta containing release medium ( 50 mm hepes , 10 mm kcl , 50 mm edta , 0 . 1 % nan3 , ph 7 . 2 ). these results are summarized in fig7 . the presence of edta in the release medium increased both the initial burst and the overall release for both rhgh saber formulations . exposed solvent accessible surface area and saber : buffer ratio appeared to influence release of rhgh from saber formulations ( fig8 ). when a larger surface area and lower saber : buffer ratio (& gt ; buffer volume ) was used more rhgh was released . this result indicates that both exposed surface area and saber : buffer ratio should be controlled during in vitro experiments . in vivo pharmacokinetics show saber formulations are able to deliver rhgh for prolonged periods of time with a fairly low initial burst ( fig9 ). however , saber solvent properties play a large role in the release mechanism . the saber formulation containing benzyl benzoate released & gt ; 80 % of available loaded material in the first 48 hrs while the benzyl alcohol formulation delivered target ( 10 ng / ml ) levels of rhgh for the duration of these studies . when compared to control microspheres the benzyl alcohol formulation had a significantly lower initial burst yet maintained similar serum levels for 7 days . in vitro release kinetics are dependent on saib / solvent type , saib / solvent ratio , excipients , release medium , and surface area . the quality of the released protein is dependent upon the type of solvent and purity of solvent used in the saber preparation . the rhgh saber formulations can provide a low burst , sustained release system for delivery of rhgh . however in vivo kinetics could depend on protein formulation and saber solvent choice .
a composition for sustained release comprises a carrier material containing a non - polymeric , non - water soluble liquid material having a viscosity of at least 5 , 000 cp at 37 ° c . that does not crytallize neat under ambient physiological conditions , a multivalent metal cation , and growth hormone .