a structured drug system that is useful for delivering a drug payload to a specific tissue or cell type is disclosed . the system is based on purified polymalic acid . this polymer isolated from natural sources is biocompatible , biodegradable and of very low toxicity . the polymer is extremely water soluble and contains a large number of free carboxyl groups which can used to attach a number of different active molecules . in the examples disclosed n - hydroxysuccinimide esters of the carboxyl groups are used to attach such molecules . the active molecules include monoclonal antibodies to promote specific cellular uptake and specific pro - drugs such as antisense nucleic acids designed to modify the cellular metabolism of a target cell . the pro - drugs are advantageously linked by a somewhat labile bond so that they will be released under specific conditions . in addition , the system contains amide - linked valine to encourage membrane disruption under lysosomal conditions . polyethylene glycol groups are attached to extend the drug system &# 39 ; s circulation half - life . in addition , fluorescent reported groups can be readily included to aid in visualizing and confirming drug system targeting . the drug system can deliver treatments for a wide range of diseases and is specially advantageous for treatment of neoplasms .