The present invention provides methods of forming a solid, biodegradable implant in-situ in a body by administering a liquid pharmaceutical composition comprising an effective amount of a biocompatible, water-insoluble, biodegradable polymer and an effective amount of a therapeutic peptide covalently modified with one or more lipophilic or amphiphilic moieties, which are dissolved or dispersed in a biocompatible, water-soluble organic solvent. This invention also provides related compositions and methods.