Condensed-phase microparticle composition and method

A method of achieving controlled release of a cationic therapeutic compound from microparticles made from anionic polymers, said microparticles having the property that they undergo transition from a swelled, decondensed state to a condensed state in the presence of Ca.sup.+2 ions, and rapid transition back to the decondensed state when exposed to monovalent cations, said method comprising

• mixing said cationic therapeutic compound with a suspension of microparticles (i) having average sizes in a selected size range between 0.05 and 5.0 microns, and (ii) composed of a condensed-phase matrix of crosslinked polyionic polymer filaments capable of expanding to a decondensed phase in the presence of monovalent counterions, said mixing being carried out with the particles in their decondensed state, such that said cationic therapeutic compound becomes incorporated into the matrix of the microparticles, and
• condensing the microparticles by addition of polycations containing at least three charged groups, said polycations being effective to delay the release of the compound from the microparticles when the microparticles are exposed to monovalent cations.