a benefit of the present invention is that the composition is stable , which means that it is not prone to supersaturation or crystallisation during its pharmaceutical shelf life . this may be contrasted with transdermal patches in which crystallisation of the active agent has presented a problem in the past . thus the composition of the present invention can be held in a primary container during the shelf life without encountering shelf - life problems of the prior art transdermal patches . the composition of the present invention may contain from about 0 . 1 % to about 10 % of the physiologically active agent , and from about 85 % to about 99 . 8 % of the volatile solvent . optionally , the vehicle may have additional pharmaceutical excipients , for example gelling agents , such as carbopol and cellulose derivatives . the invention will now be described with reference to the following examples . it is to be understood that the examples are provided by way of illustration of the invention and that they are in no way limiting to the scope of the invention . the formation of a metastable deposit of drug within the stratum corneum can be expected to increase the penetration of a drug across the skin relative to a saturated solution of a drug or a simple dispersion of the solid drug on the surface of the skin as shown in fig1 ( levels not detected ). melting point reduction for various active agents mixed with volatile solvent the ability to form a metastable deposit of the drug after evaporation of a volatile solvent can be determined by the reduction in melting point achieved 24 hours after solvent evaporation , as shown in fig2 and 3 . fentanyl buspirone testosterone ethinyl estradiol ( ee2 ) 95 % etoh isopropyl alcohol ( ipa ) chloroform acetone the active agents chosen represent a diverse range of physicochemical properties as shown in the following table : mw daltons logp mp fentanyl 336 . 48 3 . 79 84 . 0 ° c . buspirone 385 . 51 2 . 63 103 . 5 ° c . testosterone 288 . 4 3 . 01 156 . 0 ° c . ee 2 296 . 4 3 . 91 180 . 8 ° c . various % w / v mixtures of active agent and volatile solvent were prepared . a 10 microlitre aluminium micro - dsc pan was then placed in a 50 microlitre aluminium dsc pan , and 5 microlitre aliquots of each formulation were pipetted into the 10 microlitre dsc pan . the volatile solvent was allowed to evaporate and further aliquots were re - applied until sufficient residue remained . the pans were maintained at ambient temperature and 33 % relative humidity for 24 hours . after this time the pans were covered and hermetically sealed . dsc was then performed under a stream of nitrogen and at 10 ° c ./ minute and within a temperature range that depended on the drug . evaporation by each solvent resulted in melting point modulation ( fig2 ). dsc analysis of buspirone after solvent evaporation demonstrated the steadier melting point reduction ( fig3 ). in contrast , fentanyl , testosterone and ee2 showed very solvent dependent melting point reductions . in each case , isopropyl alcohol demonstrated the most significant difference in melting point change when compared to the compound base . melting point depression is , we have found , an indication that the composition will have an increase in percutaneous absorption . in addition , the stratum corneum may prevent crystallisation , resulting in a solvent deposited metastable solid within the stratum corneum . the choice of solvent used in a composition can be selected on the basis of the desired transdermal delivery profile as measured by percutaneous penetration ( an example of which is shown in fig4 ) in order to achieve the desired pharmacological effect . the aforementioned examples are not meant to be limiting and it is envisaged that combinations of volatile solvents could also be used to obtain the desired pharmacological effect ; for example on a weight basis . diffusion studies were conducted to determine the delivery profile of metastable compositions across skin . all formulations were prepared by accurately weighing the appropriate amount of physiological active into a volumetric flask and made up to volume with appropriate volatile solvent . in vitro diffusion experiments were performed using stainless steel flow - through diffusion cells , using human epidermis maintained at 32 ° c . the receptor solution consisted of either 10 % etoh in 0 . 002 % nan3 or 20 % etoh in 0 . 002 % nan3 . eight cells for each condition were treated with 5 μl of appropriate donor phase , each a finite dose that was formulation dependent . samples were collected at appropriate time points and analysed by high performance liquid chromatography ( hplc ). fentanyl diffusion profile , through human epidermis , demonstrates a change in delivery profile which is solvent dependent ( fig5 ). after evaporation from ethanol , fentanyl showed a sigmoidal , first order diffusion profile indicating that the initial burst across the skin is limited . however , after evaporation from isopropyl alcohol , fentanyl shows a zero order release rate profile . therefore , the leaving tendency may be modified to suit the desired delivery rate by altering the volatile solvent vehicle .
the present invention provides a pharmaceutical composition for transdermal delivery comprising ; one or more physiologically active agents ; and a volatile pharmaceutically acceptable solvent ; and wherein the physiological active agent forms a metastable deposit upon evaporation of the volatile solvent .