The invention relates to a delivery system for the sustained and controlled delivery of a group of bioactive agents. More particularly, the invention relates to a delivery system and a method for delivery of a bioactive agent containing a nucleophilic functional group by means of a biodegradable, sustained-release implant. The implant may be a preformed implant, microparticles or an in situ formed implant. The implant includes a biodegradable thermoplastic polymer, the bioactive agent having a nucleophilic group substituent and a stabilizing associate as well as other optional components. The combination of the stabilizing associate with the bioactive agent prevents and/or minimizes and/or lessens degradation of the thermoplastic polymer by the bioactive agent.
