phosphatidylcholine is used as a carrier for the topical drug delivery of macromolecules in the practice of this invention . phosphatidylcholine is a basic component of cell membrane bilayers and the main phospholipid circulating in the plasma . phosphatidylcholine is highly absorbable and supplies choline which is needed to facilitate movement of fats and oils across and maintain cell membranes in animals . phosphatidylcholine compositions ( herein abbreviated “ pc compositions ”) of the present invention are formulated to contain macromolecules soluble in pc , which are then applied to skin for transdermal delivery of the macromolecule . pc compositions of the invention are efficacious in the delivery of macromolecular drugs that are conventionally administered intramuscularly , intravenously or orally , including , but not limited to polypeptides such as insulin and somatropin , prostaglandins , glucocorticoids , estrogens , androgens , and the like . it is an advantage of the invention that topical delivery is easier and pleasanter as an administration route than injections , particularly for drugs such as insulin that must be given to patients over a period of time , or for a lifetime . furthermore , unlike oral administration where a substantial amount of the drug can be destroyed in the digestive process , the drugs in a topical application are not wasted . topical application allows a steady diffusion of the drug to the desired target area without the cyclic dosages typical of orally or parenterally administered drugs . typical phosphatidylcholine compositions of the present invention are nonpolar and contain about 85 % phosphatidylcholine . by “ phosphatidylcholine ” is meant a mixture of stearic , palmitic , and oleic acid diglycerides linked to the choline ester of phosphoric acid , commonly called lecithin . many commercial lecithin products are available , such as , for example , lecithol ®, vitellin ®, kelecin ®, and granulestin ® because lecithin is widely used in the food industry . compositions of the invention can contain synthetic or natural lecithin , or mixtures thereof . natural preparations are preferred because they exhibit desirable physical characteristics and are both economical and nontoxic . the macromolecular drugs are mixed with the pc composition under conditions to become entrapped in a phosphatidylcholine bilayer . phosphatidylcholine forms a bilayer entrapping the macromolecular drug , which may be a polypeptide , contributing to the stability of the active molecule and enhancing penetration . the pc composition therein comprises a carrier - drug combination to he applied topically . while not wishing to be bound by any particular theory , it is believed that the following mechanism illustrates how the pc composition acts to efficiently transport the drug across the epidermis , maximizing penetration of the drug . the pc composition , in liquid crystal phase , is loosely arranged in multilamellar fashion , with the drug being bonded and entrapped within the lipid bilayers formed by the pc composition . this forms a loosely arranged , yet stable . pc composition carrier - drug complex . when placed on the epidermis , the carrier - drug complex begins to diffuse through the epidermis . the phosphatidylcholine molecular chain remains loosely linked with the drug molecular chain and the diffusing phosphatidylcholine molecules “ drag ” the drug molecules along as they pass through the skin layers . moreover , the phosphatidylcholine molecules may begin to separate from the loosely arranged carrier - drug complex and become integrated into the dermis . as the phosphatidylcholine molecules separate from the crystallized phophoslipid bilayer structure of the carrier - drug complex the drug molecules are released . as these drug molecules are released , they are now within into the dermis and may enter the dermal vasculature so they may act accordingly in the bloodstream . drug molecules which were once too large to diffuse , by themselves , into the pores of the epidermis , have instead been forced through the epidermis by phosphatidylcholine carriers which naturally enter and integrate into lipid bilayer structures within the cells of the epidermis and / or dermis and resultantly are required to release their bonds to the drug molecules and set them free within the dermis . preferred pc compositions comprise phosphatidylcholine in crystal phase to increase fluidity of the lipid bilayer formed . by reducing rigidity and loosening the phospholipid bilayer of the pc composition , larger molecules may embed therein and penetration of the carrier - drug composition by the cell membrane is facilitated . the skin is more permeable to the fluid , less structured lipid bilayer of the pc / carrier - drug composition applied thereon than to the drug by itself , or entrapped in an organized , arranged vesicle such as a liposome . the loosely packed lipid bilayer of the crystallized carrier - drug composition integrates into the cell membrane , and as a result , has transported the drug so it can enter the bloodstream to act upon the body , the pc composition may be a multilamellar liquid crystal phase or a liquid crystal phase suspension in water which may be converted to multilamellar liquid lipid vesicles . in preferred embodiments , nonpolar preparations of phosphatidylcholine are formulated to contain adjunct ingredients , e . g ., lipoic acid and ascorbyl palmitate , in addition to the macromolecular drug . the adjunct ingredients act synergistically to help to minimize degradation and thus preserve the integrity of the insulin polypeptide chains , and to enhance transdermal penetration of active insulin so that it can be absorbed by the dermal vasculature . preferred pc compositions of the invention contain some polyenylphosphatidylcholine ( herein abbreviated “ ppc ”) to enhance epidermal penetration . by “ polyenylphosphatidylcholine ” is meant any phosphatidylcholine bearing two fatty acid substituents , wherein at least one is an unsaturated fatty acid with at least two double bonds such as linoleic acid . preferred ppcs contain a mixture of substituents such as those found in natural products such as soybean lecithin , which contains 11 . 7 % palmitic , 4 . 0 % stearic , 8 . 6 % palmitoleic , 9 . 8 % oleic , 55 . 0 % linoleic , and 4 . 0 % linolenic acid substituents and is a by - product of soybean oil manufacture . certain types of soybean lecithin , for example , contain higher levels of polyenylphosphatidylcholine , with dilinoleoylphosphatidylcholine ( 18 : 2 - 18 : 2 phosphatidylcholine ) as the most abundant phosphatidylcholine species , than conventional food grade lecithin , and are useful in formulating phosphatidylcholine insulin compositions of the invention . alternatively , conventional soybean lecithin is enriched with ppc by adding soybean extracts containing high levels of ppc . as used herein , this type of phosphatidylcholine is called “ ppc - enriched ” phosphatidylcholine , even where the term encompasses lecithin obtained from natural sources exhibiting ppc levels higher than ordinary soybean varieties , these products are commercially available from american lecithin , rhône - poulenc and other lecithin vendors . american lecithin markets its products with a “ u ” designation , indicating high levels of unsaturation ; rhône - poulenc &# 39 ; s product is a soybean extract containing about 42 % dilinoleoylphosphatidylcholine and about 24 % palmitoyllinoleylphosphatidylcholine ( 16 : 0 - 18 : 2 pc ) as the major pc components . pc compositions are used for transdermal polypeptide delivery in some preferred embodiments . polypeptide drugs that are delivered transdermally using formulations can be small , e . g ., ocytocin and vasopressin nonapeptides or large , e . g ., insulin , gonadotropin , and somatropin . pc compositions of the invention deliver drugs including , but are not limited to , oxytocin , vasopressin , somatotropin , calcitonin , chorionic gonadotropin , menotropins , follitropins , somatostatins , progestins , and combinations of any of these . these drugs are readily available from a variety of commercial sources . insulin , for example , is marketed under the tradenames humulin ®, novolin ®, humalog ®, and inutral ®. somatotropin is marketed under the tradenames gentropin ®, humatrope ®, nutropin ®, and serostim ®. some of these products and other polypeptides contain porcine sequences , preferable compositions of the invention are preferably formulated with recombinant human polypeptides . it is an advantage of the invention that pc insulin compositions are formulated with commercially available ingredients . one , non - limiting , example of an insulin topical preparation was formulated by combining 0 . 75 % methyl paraben with a commercial phosphatidylcholine preparation marketed as a solution denoted nat - 8729 ( containing peg - 400 at 40 % and p . g . at 5 %) by mixing for an hour or more to emulsify . to this is slowly added dow corning fluid 200 - 5 or 10 cst ( 1 % by weight ), the formulation is mixed , and then dow corning fluid 190 ( 1 % by weight ) is slowly added , and the formulation is further mixed to provide a stock insulin carrier . prior to topical administration , insulin is added at a level of about 3 . 8 mg / ml to provide about 100 insulin units per ml . another , non - limiting , example of a pituitary growth hormone ( somatotropin ) composition was formulated with 85 % phosphatidylcholine to which lipoic acid and ascorbyl palmitate was added as antioxidants . somatotropin readily dispersed in phosphatidylcholine and remained stable in it . growth hormone appeared to penetrate the skin well when the composition was topically applied . it is appreciated that the foregoing is illustrative and not limiting of the invention , and that various changes and modifications to the preferred embodiments described above will be apparent to those skilled in the art , such changes and modifications can be made without departing from the spirit and scope of the present invention , and it is therefore intended that such changes and modification be covered by the following claims .