Vehicle for Topical Application of Pharmaceuticals

Prostaglandins are a group of cyclic fatty acids that possess diverse and potent biologic activities affecting cellular function in every organ system. The parent compound, prostanoic acid, contains a 20 carbon chain with a cyclopentane ring. ##STR1## Variations in the number and position of the double bonds and hydroxyl groups determine the physiologic activities of the various prostaglandins.

Conventionally, prostaglandins are divided into types E, F, A, B, C and D based on functions in the cyclopentane ring. Numerical subscripts refer to the number of unsaturations in the side chains and, .alpha. or .beta. subscripts refer to the configuration of the substituents in the ring. The naturally occurring prostaglandins are types E, F, A and B. All naturally ocurring prostaglandins have a trans 13,14 position bond and an hydroxyl group at C.sub.15.

The E- and F-type prostaglandins possess an additional hydroxyl at C.sub.11. At C.sub.9, E-type prostaglandins have a carbonyl function while F-type prostaglandins have an hydroxyl. In general, A- and B-type prostaglandins may be regarded as dehydration products of E-type prostaglandins; i.e., the removal of the C.sub.11 hydroxyl and the formation of a double bond in the cyclopentane ring.

The known biologic activities of prostaglandins of the E-type include activities as hypotensive agents, brochodilators, and gastric acid secretion inhibition agents. [Bergstrom, et al., PHARMACOL., REV., 20: 1 (1968)]. However, pharmaceutical use of E-type prostaglandins has been impeded by their instability. E-type prostaglandins generally decompose slowly at room temperature and above, which decomposition is accelerated in the presence of small amounts of acid or base. Accordingly, E-type prostaglandins are unstable in pharmaceutical formulations containing water or hydroxylic compounds. Even in neutral, aqueous solution or in neat state there is a gradual decomposition of E-type prostaglandins to A- and B-type prostaglandins.

Good stability of the E-type prostaglandins has been observed in some solutions and in pure form at temperatures of -20.degree. C. or lower. However, storage at such temperatures is impractical. Some success at stabilization at room temperature has been reported when non-alcoholic compounds such as ethyl acetate and chloroform are employed as solvents for E-type prostaglandins. Such solvents, however, are unsuitable for pharmaceutical dosage applications.

More recently, good stability of E-type prostaglandins was reported with use of triethyl citrate as a solvent (U.S. Pat. No. 4,211,793) and with use of hydroxylated derivatives of fatty acids (U.S. Pat. No. 4,431,833).

It has been discovered that a formulation of: (1) polydimethylsiloxane (20 to 1,000,000 centistokes viscosity); (2) C.sub.12 -C.sub.15 alcohols benzoate, isopropyl palmitate, isopropyl myristate, diisopropyl adipate, octyl hydroxystearate, PPG-2 myristyl ether propionate, almond oil or mixtures thereof; and (3) volatile silicones such as octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, hexamethyldisiloxane and other low molecular weight polydimethylsiloxanes and/or pharmaceutically acceptable chlorofluorocarbons. It has also been determined that such formulation provides a vehicle for stable-storage of E-type prostaglandins.

In its broadest sense the formulation of the present invention may be described as:

In a specific sense, the composition according to the present invention may be described by the following formula:

The following description details the preparation of a typical formulation encompassed by this invention.

The polydimethylsiloxane was placed in a suitable container. One half of the total quantity of cyclomethicone was added to the container and the contents were stirred until homogenous.

The C.sub.12 -C.sub.15 alcohols benzoate was added to the polydimethylsiloxane-cyclomethicone mixture. The resulting mixture was then stirred until homogeneous.

The remaining portion of cyclomethicone was added and this final solution was stirred until homogeneous. The resulting Formulation A can then be placed into suitable containers.

Additionally, the formulation of the present invention has been determined to be useful as a pharmaceutical vehicle for topical application of E-type prostaglandin. As more fully described hereinabove, a principle problem encountered with pharmacological utilization of prostaglandins resides in their relatively unstable nature in pharmaceutical formulations. The few successful attempts at providing stable formulations have resulted in oily liquid compositions such as the triethyl citrate formulation of U.S. Pat. No. 4,211,793. Such formulation is unsatisfactory for topical application, as such results in greasy build-up on the skin. Formulation A, it has been determined, presents a suitable vehicle for topical application of E-type prostaglandins. In particular, it presents a vehicle for topical application of viprostol, methyl (.+-.)-[11.alpha., 5Z(5E), 13 E, 16R and 16S]-16 ethenyl-11,16-dihydroxy-9-oxoprosta-5, 13-dienloate, an E-type prostaglandin. Viprostol is active as an anti-hypertensive agent, inter alia, and can be used for such purpose with topical application. Formulation A and other formulations according to the present invention provide a non-greasy vehicle for topical application of viprostol. Additionally, viprostol may be dissolved in such vehicle without significant loss of its potency over extended periods of time at room temperature.