An active agent and a non-polymeric polyanionic compound are allowed to be associated with a microparticle containing an anionic macromolecule and an anionic polymer in the presence of a crosslink activator. In a non-limiting example, the non-polymeric polyanionic compound is a polyanionic amino acid such as aspartic acid or glutamic acid. The microparticle can be used for modified release of the active agent in vitro and/or in vivo.