Controlled release hydroxybutyrate polymer microspheres

Details Controlled release hydroxybutyrate polymer microspheres Controlled release hydroxybutyrate polymer microspheres

1989-01-27

1991-06-11

Page, Thurman

Drug, bio-affecting and body treating compositions

Preparations characterized by special physical form

Biocides; animal or insect repellents or attractants

424408, 424409, 424417, 424490, 424497, A61K 950

Patent

active

050230816

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the microencapsulation of active substances and to microspheres thus prepared.
The microencapsulation of active substances, that is, substances which have a chemical or biological effect in a suitable environment, is a technique which is well known to the art. Typical examples of active substances which have been encapsulated are drugs and fungicides. The possible reasons for microencapsulation are numerous, for example, to permit the administration of an active substance which could not otherwise be administered, to preserve the active substance in a hostile environment until it can be released in the correct environment, or to extend the release of an active substance over a period of time so as to extend correspondingly the effect of the active substance. The two last-named categories are often described as "controlled release" applications. Controlled release is an especially useful property in fields such as pharmaceuticals where use has been made of gelatin capsules. A more recent development is the use of microspheres, especially those prepared by solvent evaporation. In this case, the active substance and the microsphere- forming material are dissolved or dispersed in a liquid, commonly an organic liquid, which is then dispersed in a liquid in which the organic liquid is immiscible (water is commonly used) and the organic liquid removed by evaporation to leave microspheres.
The procedure is described by Bissery and co-workers in a paper given at the 3rd Exposition-Congress of the International Technology of Pharmacy (Assoc. Pharm. Galenique Ind. 1983), Vol. 3, pp. 233-9. This paper describes the application of the technique to a number of polymers, including poly(.beta.-hydroxybutyrate). This polymer is described in a further publication by Bissery (chapter 4 of "Microspheres and Drug Therapy : Pharmaceutical, Immunological and Medical Aspects", ed. Davis, Illum, McVie and Tomlinson, Elsevier 1984, pp 217-227) in connection with the anticancer agent CCNU.
Bissery has observed that the surface of a poly(.beta.-hydroxybutyrate) (hereinafter referred to as "PHB"; microsphere contains many macroscopic surface pores. This undoubtedly contributes to the fact that PHB microspheres release the active substances encapsulated therein very quickly; the release rates given by Bissery in the second publication referred to hereinabove are very high. However, this speed of release is not suitable for the whole range of active substances and this limits the usefulness of PHB.
It has now been found that it is possible to prepare microspheres comprising PHB wherein the release rate may be altered in a predictable manner. There is therefore provided, according to the present invention, controlled release hydroxybutyrate polymer microspheres comprising at least one active substance, the polymer comprising a copolymer of 3-hydroxybutyric acid and 3-hydroxyvaleric acid, and the microspheres comprising a continuous skin covering the surface thereof.
PHB, sometimes known in the literature as poly(3-hydroxybutyric acid), is a thermoplastic polymer which is accumulated by many organisms. European Patent Specifications 15669 and 46344 describe typical processes for making PHB. The production of hydroxybutyrate/hydroxyvalerate copolymers is described by, for example, Wallen and co-workers in "Environmental Science and Technology" 8 (1974), 576-9.
It is a special feature of the microspheres of this invention that, in contrast to the microspheres of the known art, they have a skin, that is, a thin surface layer different in structure to that of the bulk of the microspheres. This skin is continuous over essentially the entire surface of the microspheres. It is, however, permissible that it may have pores therein which give rise to openings at the surface of the skin. The skin can in fact vary between essentially completely pore-less and quite highly porous (pore ends covering up to about 50% of the total skin surface area). It is the combination of the nature of the skin and that of the interior of the

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