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 injectable liquid which may have self-gelling properties, or of a solid implant. The invention further provides a pharmaceutical kit comprising the composition, methods for preparing the composition, and the pharmaceutical uses relating thereto.