a closely related isomer is called isoforskolin and it has the following structure isoforskolin also has been reported to have many similar pharmacological properties as forskolin . we have used these six commercially available cyclodextrins , namely , α -, β -, γ - cyclodextrins as well as their derivatized products such as ramebcd , hpbcd , hpgcd to solubilize the rather sparingly water soluble forskolin . to solubilize forskolin using cyclodextrins , the chosen cyclodextrin and forskolin are mixed in water in specific proportions . the aqueous solution is filtered to remove any undissolved particles to obtain a clear aqueous solution of forskolin in water . alternatively , the cyclodextrin and forskolin in certain proportions are dissolved in a suitable solvent such as ethanol or acetone or ethyl acetate . the solvent is removed to leave behind a white powder . such powder freely dissolves in water as the examples will illustrate . further , additives to the aqueous solution of forskolin can also be added . these additives are usually used for maintaining sterility , ph maintenance , maintenance of osmolarity etc . a wide variety of choice exists in the selection of such additives . while benzalkonium chloride is used in the illustrative example for preservative , one could equally choose from many others such as benzethonium chloride , chlorobutanol , methyl paraben , propyl paraben , thimerosal etc . an antioxidant such as the disodium salt of edta is used to stabilize the preparation ; other antioxidants such as sodium bisulfite , sodium metabisulfite , thiourea could be used also among others . especially for ophthalmic solutions , viscosity desired for an ophthalmic solution is in the range 25 and 50 cps . viscosity enhancers such as polyvinyl alcohol , polyvinylpyrrolidone , methyl cellulose , hydroxypropylmethyl cellulose , hydroxyethyl cellulose could be used . the examples that are described below serve their purpose only as illustrative examples and do not limit in any way the broad scope of this invention . determination of the aqueous solubility of forskolin . forskolin ( 300 mg ) was dried at 105 ° c . for 6 hours . dried forskolin 200 mg was stirred with 100 ml water for 48 hours for the determination of intrinsic solubility at ambient temperature . the resulting solution was filtered through 0 . 45 μm nylon filter and analyzed for the content of forskolin by hplc . the content of forskolin by hplc was 0 . 01 mg / ml or 0 . 001 % w / v . in other words forskolin has a solubility of ˜ 0 . 001 % w / v in water . forskolin ( 98 . 5 % assay , 25 mg ) was added to 1 ml water containing in the dissolved state 500 mg hydroxy propyl - β - cyclodextrin , hpbcd , (˜ 50 %). the suspension was agitated at 75 rpm in an isothermal shaker for 60 hours at temperature ˜ 30 ° c . the resulting solution was filtered through 0 . 45 μm nylon filter and analyzed for the content of forskolin by hplc 1 . 33 mg / ml or 0 . 133 % w / v . forskolin ( 98 . 5 % assay , 50 mg ) was added to 1 ml water containing 500 mg hydroxy - γ - cyclodextrin in the dissolved state . ( hpgcd ) (˜ 50 %). the suspension was agitated at 75 rpm in an isothermal shaker for 60 hours at temperature ˜ 30 ° c . the resulting solution was filtered through 0 . 45 μm nylon filter and analyzed for the content of forskolin by hplc 1 . 52 mg / ml or 0 . 152 % w / v . experiments were performed by “ changing ” the crystallinity of forskolin by recrystallizing from methylene chloride and from ethyl acetate . the resulting “ amorphous ” forskolin was used for complexation with hydroxyropyl γ - cyclodextrin hpgcd . forskolin ( 29 . 3 mg ) recrystallized with methylene dichloride ( forskolin assay 99 . 0 %) was added to 3 ml water containing 1 . 5 gram hydroxy propyl γ - cyclodextrin , hpgcd (˜ 50 %). the suspension was agitated at 75 rpm in an isothermal shaker for 160 hour at temperature 30 ° c . the resulting solution was filtered through 0 . 45 μm nylon filter and analyzed for the content of forskolin by hplc 1 . 74 mg / ml or 0 . 174 % w / v . forskolin ( 30 . 3 mg ) recrystallized with ethyl acetate ( forskolin assay 98 . 8 %) was added to 3 ml water containing 1 . 5 gram hydroxy propyl γ - cyclodextrin , hpgcd (˜ 50 %). the suspension was agitated at 75 rpm in an isothermal shaker for 160 hour at temperature 30 ° c . the resulting solution was filtered through 0 . 45 μm nylon filter and analyzed for the content of forskolin by hplc 3 . 38 mg / ml or 0 . 338 % w / v . forskolin ( 98 . 5 % assay , 330 mg ) was added to 10 ml water containing 4 g of ramebcd (˜ 40 %). the suspension was agitated at 75 rpm in an isothermal shaker for 40 hours at temperature 30 ° c . the resulting solution was filtered through 0 . 45 μm nylon filter and analyzed for the content of forskolin by hplc 20 . 46 mg / ml or 2 . 046 % w / v . solubility of forskolin in water was determined at the different concentrations of ramebcd ranging from 5 to 66 %. the relationship is nearly linear and indicates that the solubility of forskolin is increased by increasing the concentration of ramebcd . s . n . % concentration of ramebcd % forskolin w / v 1 5 % ramebcd 0 . 09 2 10 % ramebcd 0 . 272 3 15 % ramebcd 0 . 767 4 20 % ramebcd 1 . 15 5 40 % ramebcd 2 . 746 6 53 . 28 % ramebcd 4 . 165 7 66 . 6 % ramebcd 5 . 029 a typical aqueous formulation of forskolin with a cyclodextrin is prepared as follows , ramebcd , being used as the example of cyclodextrin ramebcd ( 100 g ) is taken in a one litre flask with mechanical or magnetic stirring facility . forskolin ( 5 . 5 g ) was charged into the flask . water ( 400 ml ) is charged to the flask and the contents were agitated at room temperature . a clear solution is obtained . if any undissolved forskolin particles are seen , they are resuspended and stirred . benzalkonium chloride ( 50 mg ) and disodium edta ( 500 mg ) are added and dissolved in the flask . the ph of the contents could be adjusted to the desired range with the help of 0 . 1n sodium hydroxide . ( usually ph range 3 . 5 to 7 . 5 ). a calculated amount of sodium chloride solution is added to maintain the osmolarity of the solution equivalent to that of 0 . 9 % sodium chloride . the total volume of the solution is made up to 500 ml after sterile filtration . a solution thus prepared has approximately 1 % of forskolin in the dissolved state . other cyclodextrins also could be used and depending on the cyclodextrin used , the dissolved content of forskolin in water differed . forskolin ( 50 mg ) was dissolved in 5 ml acetone , and 1 gram of ramebcd was dissolved in 5 ml acetone separately . both the solutions were mixed together and solvent acetone was evaporated under reduced pressure . the residue was dried and dissolved in 5 ml water . this residue dissolved very easily within 1 hour of stirring forming a clear colorless solution . isoforskolin also could be used in place of forskolin . in one preparation , isoforskolin ( 50 mg ) was suspended in water containing a suitable amount of a cyclodextrin , for example , r amebcd ( 20 g ) in about 100 ml water . after agitation at room temperature , the solution was filtered and the resulting solution was analyzed by hplc which showed the presence of isoforskolin approximately 0 . 5 %. the amount of dissolved isoforskolin could be altered by changing the amount of ramebcd . an illustrative example of the biological activity of the preparation is presented . the anti - glaucoma activity of the forskolin composition was studied in albino rabbits . a 1 % solution of forskolin in water as described in example 8 was used for the experiments . study design : animal model : albino rabbit number of groups : 4 number of animals in each group : 6 in treatment group and 2 in control group . materials and methods : six albino rabbits of new zealand strain , of both sexes , weighing 1 . 0 - 1 . 5 lb were chosen . the rabbits were housed in clean and well - ventilated open space . each rabbit was fed with standard diet daily and water was administered ad libitum throughout the study . ocular hypertension was induced by the method reported by bonomi et al ( invest ophthalmol . 1976 september ; 15 ( 9 ): 781 - 4 .) the rabbits were given 0 . 3 ml subconjunctival injection of betnesol containing betamethasone sodium 4 mg / ml , every day to each eye for a period of three weeks ( the intraocular pressure ( iop ) at third week was maximum as per literature ). local anesthetic propracaine eye drops were used prior to subconjunctival injections . in each rabbit the left eye was kept as control for glaucoma and the right eye was treated for glaucoma using forskolin , timolol , and the placebo . for each treatment , the iop readings were measured at intervals of 30 minutes up to 210 minutes using the non - contact tonometer ( nct ). iop readings * ( mmhg ) time timolol ( mins ) placebo right eye ( treated ) forskolin 0 14 13 14 30 13 8 7 60 12 7 6 . 5 90 11 . 5 5 4 . 5 120 11 4 5 150 10 . 5 3 . 5 6 180 10 4 . 5 7 210 9 6 7 . 5 * average of 6 determinations iop of left eye ( control ) ranged between 12 - 13 mmhg . iop of control group animals ranged between 4 - 4 . 5 mmhg . statistical analysis : the iop readings of the placebo , forskolin and timolol were subjected to anova ( one way ). the p value was 0 . 0022 which is very significant , indicating that the variation in column means is not by chance . the iop readings of the placebo and forskolin were subjected to “ t ” test to determine whether the medians of forskolin and the placebo differ significantly . the p value was found to be 0 . 0177 which is considered significant . similarly , the iop readings of the placebo and timolol had a “ p ” value of 0 . 0087 , which is again significant . the iop readings of forskolin and timolol were also subjected to “ t ” test . the p value was found to be 0 . 3999 , which is not considered significant , implying that the activity of forskolin preparation is not significantly different from timolol .
aqueous solutions of diterpenes such as forskolin , its congeners , analogs and derivatives , up to approximately 6 % concentration , are prepared using suitably substituted cyclodextrin as a solubilizing agents . in the absence of cyclodextrin , some diterpenes such as forskolin are soluble in water only to concentrations of about 0 . 001 %. such aqueous solutions find applications in topical and systemic use , as pharmaceutical , cosmeceutical , nutraceutical preparations containing diterpenes such as forskolin and congeners .