TRANSDERMAL AZIDOTHYMIDINE

Abstract: Pharmaceutical compositions and transdermal patches comprised of azidothymidine and a penetration enhancing compound selected from the group consisting of linalool, carvacrol, thymol, citral, menthol and t-anethole are disclosed for delivery of a therapeutically effective amount of azidothymidine.

Description: TRANSDERMAL AZIDOTHYMIDINE FIELD OF THE INVENTION The present invention is for a method of transdermal delivery of azidothymidine which uses a penetration enhancer to facilitate the percutaneous and transepidermal delivery of the azidothymidine. BACKGROUND OF THE INVENTION Azidothymidine (AZT) , also known as zidovudine and S'-azido-S'-deoxythymidine is an antiretroviral agent. It is used in the treatment of patients with symptomatic human immunodeficiency virus (HIV) infection which includes Acquired Immune Deficiency Syndrome (AIDS) and advanced AIDS-Related Complex (ARC) . Azidothymidine is currently administered orally and by intravenous infusion. The present invention results in a new method for administering azidothymidine by formulating it into a transdermal pharmaceutical composition which can be applied directly to the skin or via a transdermal patch. The pharmaceutical composition includes a penetration enhancer to achieve effective blood levels of azidothymidine for the treatment of AIDS and ARC. Transdermal delivery of azidothymidine offers advantages over other methods of delivery of azidothymidine. An advantage is the ease of application and removal of a transdermal formulation. This convenience of use offers the benefit of a lifestyle uninterrupted by hospital visits, which are needed when administering azidothymidine intravenously. A further advantage would be for pediatric use where oral and I.V. administration is difficult. Oral dosing of azidothymidine results in a spike concentration of AZT which may be toxic. Transdermal AZT offers continuous administration of AZT which would maintain effective amounts of AZT in the blood for longer periods and without spiking. Recognizing that the outer layer of the skin, the epidermis, protects the area under the skin from penetration of foreign chemicals, various enhancing agents have been used to deliver drugs transdermally. Substances that help promote drug diffusion through the stratum corneum and epidermis are referred to as skin- penetration enhancers, accelerants, adjuvants and sorption promoters. B. Idson, Cosmetics & Toiletries. 95, 59 (1980) states that the factors affecting drug penetration and consequently, in most cases, effectiveness, are complex and unpredictable. The vehicle that provides ideal penetration enhancement for one drug may prove unsatisfactory for another. Various penetration enhancers are known to be useful in transdermal drug delivery. U.S. 4,863,970, U.S. 4,722,941, U.S. 4,931,283 and EP 351,897 disclose some representative penetration enhancers used in transdermal compositions and for topical administration. SUMMARY OF THE INVENTION The present invention provides a method for transdermal delivery of azidothymidine using a penetration enhancer. The present inventors have determined that various compounds, selected from the group consisting of linalool, carvacrol, thymol, citral, menthol and t-anethole are effective penetration enhancers for delivery of azidothymidine transdermally. It is the primary object of this invention to provide a method for transdermal delivery of azidothymidine. The method is practiced by transdermally administering an effective amount of azidothymidine together with a penetration enhancer. A further object of this invention is to provide a transdermal composition or patch for effective transdermal delivery of azidothymidine. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a cross sectional view of a transdermal patch for placement on the skin for transdermal delivery of azidothymidine. FIG. 2 is a bottom plan schematic view of the embodiment illustrated in Fig. 1. FIG. 3 illustrates a template used for topical administration of a transdermal azidothymidine formulation. FIG. 4 illustrates a sectional schematic view of a second embodiment of a transdermal patch for the transdermal delivery of azidothymidine. FIG. 5 illustrates a sectional schematic view of a third embodiment of a transdermal patch. FIG. 6 is a graphical representation of the results of in vitro experiments using AZT compositions with various penetration enhancers. FIG. 7 is a graphical representation of the results of .in vitro experiments using AZT compositions with various pharmaceutical carriers. FIG. 8 is a graphical representation of the results of in vitro experiments using AZT compositions with varying amounts of AZT. FIG. 9 is a graphical representation of the results of in vivo experiments using various AZT formulations. DETAILED DESCRIPTION OF THE INVENTION This invention encompasses transdermal compositions of azidothymidine as previously described. Azidothymidine has the structural formula I The term ■■ composition" as used herein means a product which results from the mixing or combining of more than one element or ingredient. The term "pharmaceutically-acceptable carrier" as used herein means a pharmaceutically acceptable material, composition or vehicle, such as a liquid or solid filler, diluent, excipient, or solvent involved in carrying or transporting a chemical agent from one organ or portion of the body to another organ or portion of the body. The term "transdermal delivery" as used herein means administration of the pharmaceutical composition topically to the skin wherein the active ingredient, azidothymidine, will be percutaneously delivered in a therapeutically effective amount. The term "transdermal patch" as used herein means a skin patch to be applied to the patient's skin containing the pharmaceutical composition. The technology for constructing transdermal patches is well known in the pharmaceutical art. The terms "backing layer" and "reservoir" as used herein are components of the transdermal patch. Suitable materials and designs are well known in the transdermal drug delivery art. See for example D. Hsieh, "Multiple Lamination for Transdermal Patches," Controlled Release Systems Fabrication. Technology. Vol. 1, pp. 167-188, 1988. The term "penetration enhancers" as used herein means compounds which enhance the percutaneous absorption of drugs and selected from the group consisting of linalool, carvacrol, thymol, citral, menthol and t-anethole. Selection of an effective penetration enhancer for a particular drug must be experimentally deduced. A penetration enhancer which works for one drug will not necessarily work for every other drug. By virtue of its activity as an antiviral azidothymidine is useful in the treatment of AIDS and ARC (HIV-infection) . In the present invention azidothymidine is administered transdermally by topical application of an azidothymidine composition to the skin. More preferably azidothymidine can be administered in the form of a gel and covered with an occlusive bandage or it can be administered using a transdermal patch in which the azidothymidine is incorporated. The pharmaceutical compositions and patches of the present invention can administer azidothymidine in admixture with a penetration enhancer and suitable pharmaceutical diluents, carriers and excipients such as gelling agents, emollients, preservatives. Preferably the azidothymidine is administered in admixture with the penetration enhancer thymol. The practicality of administering a given drug percutaneously on a continuous basis depends upon the concentration of drug in the blood that is required to provide the desired pharmacologic effect, the degree to which the skin is permeable to the drug, and the amount of skin surface area that is available for drug administration. The skin surface area which is available for drug administration, while theoretically being unlimited, is, for practical reasons, typically confined to a range of from about 1 square centimeter to about 100 square centimeters. With the available skin surface area fixed within this range, the matter then narrows as to whether sufficient drug will pass through the defined skin surface area to provide the desired therapy. If it will, then it is possible to effectively administer the drug percutaneously. If, however, the inherent permeability of the skin to the drug is so high or so low that too much or too little drug will pass through that area of skin, then the rate of administration of the drug to the skin must be controlled, or the permeability of the skin to the drug must be increased, as the case may be, to make percutaneous administration practical. The present invention involves a drug delivery system in which the percutaneous administration of the drug is enhanced by the presence of a penetration enhancer. In order for a compound to be useful as a percutaneous penetration enhancer the compound must meet a number of different requirements. Firstly, the compound must be a dermatologically acceptable compound which when used topically on the skin does not cause adverse side effects. Secondly, the compound must be compatible with the active ingredient in the transdermal delivery system. Thirdly, it is preferable for the compound to be safe for use in humans, preferably approved or approvable by the Food and Drug Administration of the United States. Further, the compound must increase the transdermal rate of delivery of the pharmaceutically active drug. It has been found herein that for transdermal delivery of azidothymidine a penetration enhancer is desirable. Suitable penetration enhancers for azidothymidine include any compound selected from the group consisting of linalool, carvacrol, thymol, citral, menthol and t-anethole. In treatment of AIDS or ARC, use of a non-toxic but therapeutically effective amount of azidothymidine is employed. The dosage regimen for treating AIDS or ARC with azidothymidine is selected in accordance with a variety of factors including the type, age, weight, sex and medical condition of the patient. A physician of ordinary skill can readily determine and prescribe an effective amount of the drug required to treat the condition. The minimum inhibitory concentration of AZT in humans is reported to be 0.27 mcg/ml. The clearance of AZT in humans is about 1.3 1/hr/kg. With a delivery rate of 1 mg/cm 2 /h, it would take about a 20 cm 2 patch to give the minimum inhibitory concentration of AZT in humans. Therapeutic levels are maintained with 5 mg/kg given as a 1 hr infusion every 4 hr. or 10 mg/kg given orally every 4 hr. Assuming a typical body weight of 55 kg, this equals 1650 mg/day, I.V., or 3300 mg/day orally. Based on the minimum inhibitory concentration of AZT (0.27 mcg/ml) for transdermal delivery of AZT a minimum of 460 mg/day is required to maintain the blood levels at minimum inhibitory concentration. In order to administer the dosage set out above the concentration of azidothymidine in the composition can be from about 3 to about 30% w/v. The concentration of azidothymidine, is however, preferably from about 5 to about 25%, and more preferably from 15 to 25%. The concentration of the penetration enhancer in the composition can be from about 0.5% to about 20% w/v. The concentration of the penetration enhancer is, however, preferably from about 1 to about 10% and more preferably from about 2 to about 5%. While concentrations of azidothymidine and penetration enhancers are given herein in ranges .with a maximum and a minimum concentration, amounts greater than or less than such limits can be used without varying from the invention as is understood by one having skill in the art. The amount of azidothymidine and penetration enhancer incorporated in the reservoir of the transdermal patch of the invention to obtain the desired therapeutic effect can vary depending upon the desired dosage of the azidothymidine, the length of time the patch is to remain on the skin of the patient and the area and thickness of the patch. Patient serum concentrations of the azidothymidine can thus be adjusted by varying the concentration of the azidothymidine in the patch, the length of time the patch is to remain on the skin of the patient or the patch size. The effective rate of release of the azidothymidine from the patch to the skin of a patient can be in the range of from about 10 to about 70 milligrams of active agent per square centimeter of skin per day (mg.cm^.day *1 ) . A more preferred range is from about 24 to about 50 milligrams of active agent per square centimeter of skin per day. The exact amount depends on the desired dosage of the active agent and whether the treatment is in an HIV asymptomatic patient or in an HIV symptomatic patient. Those skilled in the art can readily determine the rate of permeation of active drug ingredient through a particular matrix material, and through selected combinations of matrix materials, to be employed in a patch for transdermal delivery of the azidothymidine. Standard techniques employed for making such rate of permeation determinations are described in the Encvclopedia of Polymer Science and Technology, Volumes 5 and 9, Pages 65 to 85 and 795 to 807 (1968) , and the references cited therein. Other excipients and pharmaceutical carriers can be present in the compositions and patches of the present application. An example of an excipient useful as a gelling agent for preparing a topical gel is hydroxypropyl cellulose (HPC) . Pharmaceutical carriers which can be used include for example mixtures of isopropanol (IPA) and water or propylene glycol (PG) and water. The parabens can be suitable preservatives for use in transdermal azidothymidine compositions. The azidothymidine compositions herein can be incorporated in a transdermal patch for delivery of the azidothymidine percutaneously. Methodology and design of transdermal patches for drug delivery are well known in the pharmaceutical art. Fig. 1 illustrates a transdermal patch (10) that is designed to be placed on a patient's skin. Transdermal patch (10) comprises a backing layer (12) ; a drug reservoir (18) ; an adhesive layer (16) ; and a removable protective undercoating lamina (19) . The backing layer (12) defines the top (outward surface) of the patch. The backing layer can be made from a material or a combination of materials that is substantially impermeable to the components of the reservoir layer. The backing layer serves as a protective cover for the transdermal patch and keeps the components of the reservoir layer from escaping. Various occlusive and non-occlusive, flexible and non-flexible backing members can be used as backing layers in the transdermal patches of the present invention. Examples of suitable backing layers for use in the transdermal patches herein include cellophane, cellulose acetate, ethylcellulose, plasticized vinylacetate, vinylchloride copoly ers, polyethylene terephthalate, nylon, polyethylene, polypropylene, polyvinylidenechloride, paper, cloth, foam and aluminum foil. The drug reservoir (18) is immediately below the backing layer (12) and contains the transdermal azidothymidine composition. The amount of azidothymidine incorporated in the reservoir of the patch of the invention to obtain the desired therapeutic effect can vary depending upon the desired dosage of the azidothymidine, the length of time the patch is to remain on the skin of the patient and the area and thickness of the patch. Patient serum concentrations of azidothymidine can be adjusted by varying the concentration of the azidothymidine in the patch, the length of time the patch is to remain on the skin of the patient or the patch size. The adhesive layer (16) is affixed to the backing layer surrounding the drug reservoir. This is better illustrated by a view of the patch from the bottom (patient side) as illustrated in Fig. 2 with the undercoating lamina removed. The adhesive layer is the means by which the transdermal patch is affixed to the skin. Contact adhesive compositions can be selected from pressure sensitive adhesives many of which are commonly used on transdermal patches. The transdermal patch (10) can include a removable protective undercoating lamina (19) which covers the entire patient side of the transdermal patch. Prior to use, the undercoating lamina (19) is pulled away from the adhesive layer (16) and discarded to expose the adhesive layer and drug reservoir. Fig. 3 and Fig. 4 depict another embodiment of a transdermal patch for the azidothymidine composition which is formed on the patient. Fig 3. depicts a template (20) applied to an area of the patient's skin. The inner circumference of the template (20) defines an area (22) of skin to which the transdermal azidothymidine composition is to be delivered. The area of skin can be determined by the age, sex, and weight of the patient, whether treatment is prophylactic or remedial, and the blood level of - azidothymidine to be achieved, discussed supra. A sealable gel tube or syringe containing the transdermal azidothymidine composition in a predetermined dosage is opened. The dosage is determined by the factors discussed above. The transdermal azidothymidine composition is applied within the inner circumference of the template and then the template is removed from the skin. Fig 4. depicts an occlusive covering (36) which is placed over the azidothymidine composition and the same area of the patient's skin after the template has been removed. The occlusive covering can include a backing layer (42) and an adhesive layer (40) which have the same properties and are chosen for the same reasons as stated above for the transdermal patch. The adhesive layer can be the same width and circumference as the template defining the area to which the azidothymidine is applied to the skin, i.e. it forms a perimeter or surrounding barrier to such area. The occlusive covering keeps the transdermal azidothymidine composition in a predefined space or reservoir (38) on the skin (44) . Alternatively, the dosage of the transdermal azidothymidine composition to be administered can be applied from a dispenser to the occlusive covering (36) within the boundaries of the adhesive layer (38) depicted in Fig. 3. The then-formed transdermal patch can be applied to the skin of the patient to be treated. The methods of administration described with regard to Figs. 3 and 4 are particularly advantageous because they permit dose titration to meet individual patient needs. Fig. 5 illustrates another embodiment of a transdermal patch of the invention, generally designated as 26. The transdermal patch (26) includes a backing layer (28) , a transdermal composition layer (30) and an adhesive layer (34) . The backing layer and transdermal composition layer can have the same properties as the backing layer and the transdermal azidothymidine composition respectively discussed above and are chosen for the reasons stated above. The adhesive layer can have the same properties as the adhesive layer discussed above and is chosen for the same reasons as above. Additionally, the adhesive layer (34) must be permeable to the transdermal composition and can not constitute a significant barrier to its passage. The transdermal patch (26) can also include a protective undercoating lamina (not shown) . Prior to use the undercoating lamina is pulled away from the adhesive layer (34) and discarded to expose the adhesive layer. The undercoating lamina can be made from materials that are substantially impermeable to the azidothymidine, the penetration enhancer and any other components of the transdermal composition layer and adhesive layer (34) . The transdermal patch of the invention can be applied to the skin of patients. The transdermal patch is positioned in firm contact with the patient's skin, preferably forming a tight seal therewith. The azidothymidine in the patch migrates from the patch to the patient's skin by diffusion. When the azidothymidine is in contact with the patient's skin, azidothymidine molecules along the outer surface of the transdermal patch migrate through, and are absorbed by, the skin, entering the patient's circulation through the capillary network. The transdermal patch can be applied to any area of the patient's skin. The following examples describe and illustrate pharmaceutical formulations used to deliver azido¬ thymidine transdermally. The results achieved by administering the formulation are also discussed in further detail. Both an explanation of, and the actual procedures for, the various aspects of the present invention are described where appropriate. These- examples are intended to be merely illustrative of the present invention, and not limiting thereof in either scope or spirit. Those of skill in the art will readily understand that known variations of the conditions and processes of the preparative procedures described in these examples can be used to prepare the transdermal compositions and patches of the present invention. Experimental In vitro Experiments: The in vitro studies described herein measured the rate of azidothymidine transport across skin excised from a hairless (NU/NU) , male mouse. The skin was placed between the donor and receptor compartment of the Franz cells. The skin (1.77 cm 2 ) partitions a donor compartment containing an azidothymidine formulation to be tested in contact with the outside of the skin and a receptor compartment containing a saline medium in contact with the inside of the skin. A constant temperature of 37°C is maintained. The receptor aqueous medium was changed at the following time points during the assay: 4 hr, 8 hr and 24 hr. The concentrations of azidothymidine were determined using HPLC. Most formulations in the in vitro studies were tested in triplicate. Examples 1-12 Compositions 1-12 were prepared by the following method except for variations in the amounts of ingredients. AZT was weighed and added to a vial. The enhancer was then weighed and added to the AZT. IPA/water and/or PG/water was weighed into each vial until all particulate was dissolved. TABLE I COMPOSITION OP THE FORMULATIONS USED IN THE AZT IN VITRO STUDIES Table II shows the results of AZT transport in excised hairless (NU/NU) , male mouse skin using compositions 1-7. These results show the effect that varying the penetration enhancer has on AZT transport in vitro. Fig. 6 is a graphical representation of the results of this study. TABLE II AZT TRANSPORT IN EXCISED HAIRLESS (NU/NU) MICE SKIN AT 37°C 5% ENHANCER CONCENTRATION EXAMPLE CUMULATIVE AZT RATE OF TRANSFER 1 ENHANCEMENT 2 (mg/cm--/24h) (mg/cnr/h) RATIO mean (S.D. ) (1 0.55 (0.31) 0.03 1 (2 20.96 (0.98) 0.92 38.0 (3 18.65 (1.57) 0.81 33.8 (4 12.82 (9.36) 0.69 24.6 (5 14.16 (1.84) 0.57 26.7 (6 16.40 (1.02) 0.78 29.7 (7 18.73 (1.71) 0.82 33.9 The values were obtained from the amount transported between 8 and 24 hours. The values were obtained from the ratio of the cumulative AZT transported for the control (#1) and enhancer containing formulations. Fig. 7 shows the graphical results of the in vitro studies using composition 1 and 8-10. These results show the effect that varying the pharmaceutical carrier has on AZT transfer in vitro. Fig. 8 is the graphical representation of the results of in vitro studies using compositions 5, 11 and 12. These results show the effect that varying the concentration of AZT has on AZT transfer in vitro. In vivo Experiments: In vivo transdermal AZT experiments were carried out in 300-600g, male Sprague Dawley rats which were fasted overnight. One day prior to the experiment, an area approximately 5 cm x 5 cm was shaven on the dorsal side with a pet shaver. A Hill Top Chamber with a surface area of 2.54 cm 2 was filled with 0.5 ml formulation and attached with adhesive tape and cyanoacrylate glue to the shaven area of each rat. Animals were allowed to drink throughout the experiment and eat after 8 hours. Systemic blood was collected at predetermined time points through cardiac puncture. Animals were under general anesthesia with nitrous oxide/isofluorene during sample collection. Plasma was stored at -20°C until analysis. Formulations were made with 150 mg/ml AZT, 5% t-anethole, menthol, carvacrol or thymol as enhancers, with and without PG. In the formulations 3% HPC was added at 60°C and then allowed to cool to form a gel. At the end of each experiment the Hill Top Chamber was removed and skin was observed for any changes. Exa ples 13-16 Gel formulations (Examples 13-16) were prepared in the same manner as Examples 1-12 except after all the particulate was dissolved, the mixture was heated to 60'C and 3% HPC was weighed and added to the vial. After thorough mixing the mixture was allowed to cool and gel. Table III Composition of the Formulations Used in the AZT in vivo Studies EXAMPLE # INGREDIENTS 13 14 15 16 (in grams) AZT 0.60 0.60 0.75 0.75 IPA/H 2 0 2.19 1.61 3.58 3.58 (60/40, v/v) PG/H 2 0 1.01 1.62 (60/40, v/v) t-Anethole 0.20 Menthol 0.20 Carvacrol 0.25 Thymol 0.25 HPC 0.12 0.12 0.15 0.15 Fig 9. is a graphical representation of the results of the in vivo experiments using these compositions. The absolute bioavailability values are given in Table I.V. Table IV PERCENT ABSOLUTE BIOAVAILABILITY OF AZT TRANSDERMALLY IN THE RAT *The bioavailability is adjusted for the weight of each rat. The transdermal compositions and transdermal patches according to the present invention are advantageous in that they offer ease of application and they offer continuous administration of AZT which maintains the required amount of AZT in the blood. After 24 hrs. no observable changes were seen in the skin under the gel containing chamber compared to before gel application.

Claims: What is claimed is: 1. A method for transdermal delivery of azidothymidine comprising: administering topically a therapeutically effective amount of azidothymidine together with a penetration enhancing amount of a compound selected from the group consisting of linalool, carvacrol, thymol, citral, menthol and t-anethole for delivering a therapeutically effective amount of azidothymidine through the skin. 2. A method according to Claim 1 wherein the therapeutically effective amount of azidothymidine is from about 5 to about 20 mg/kg. 3. A method according to Claim 8 wherein the penetration enhancing amount of the compound is from about 0.5 to about 20%. 4. In a patch for transdermal delivery of a pharmaceutically active agent having a backing, an adhesive layer and a drug reservoir containing the active agent, an improvement comprising a pharmaceutical composition comprising the active agent azidothymidine and a penetration enhancing compound selected from the group consisting of linalool, carvacrol, thymol, citral, menthol and t-anethole. 5. A pharmaceutical composition for transdermal delivery of azidothymidine comprising a therapeutically effective amount of azidothymidine and a penetration enhancing amount of a compound selected from the group consisting of linalool, carvacrol, thymol, citral, menthol and t-anethole.

Inventors: GRIFFIN MARTIN JOHN, KARARLI TAGRUL T, KIRCHHOFF CAROL F, PENZOTTI STANLEY C JR, SCHMIDT CYNTHIA MARIE, GRIFFIN, MARTIN, JOHN, KARARLI, TAGRUL, T., KIRCHHOFF, CAROL, F., PENZOTTI, STANLEY, C., JR., SCHMIDT, CYNTHIA, MARIE
Applicants: SEARLE & CO, G.D. SEARLE & CO.
Application year: 1992
Main classification: A61K47
IPC codes: A61K9, A61K31, A61K47, A61P31, C07H19