The invention discloses a pharmaceutical composition for the controlled release of relatively toxic active compounds, in particular for bioactive proteins from the class of interferons. The composition comprises a biodegradable block copolymer constructed from poly(ethylene glycol) terephthalate (PEGT) and poly(butylene terephthalate) (PBT). The composition is provided in the form of injectable microparticles, of an in-jectable liquid which may have self-gelling properties, or of a solid implant. The inven-tion further provides a pharmaceutical kit comprising the composition, methods for pre-paring the composition, and the pharmaceutical uses relating thereto.