Targeted drug delivery represents a novel approach for cancer therapy that promises to avoid unwanted side effects of cytostatic drugs. Several delivery systems are currently investigated, including various dendrimeric and polymer molecules that mostly use monoclonal antibodies as the "homing device".
We set to prepare hydrophilic polymers using synthetic small molecule inhibitors of a tumor marker (prostate specific membrane antigen, PSMA) as the molecular address for the recognition, imaging and potential therapy of human tumors. PSMA, also known as glutamate carboxypeptidase II (GCPII) is a transmembrane metallopeptidase, abundantly expressed in prostate carcinoma cells and in the neovasculature of most solid tumors. Using structure-based molecular design, we developed and characterized novel low molecular inhibitors of GCPII and evaluated their binding affinity to the pure recombinant GCP/II as well as to GCPII-expressing cells. For the cell-based assays, we established a mammalian cell line U373MG with doxycycline-controlled GCPII expression (Tet-Off system). We also prepared conjugates of these compounds with synthetic biocompatible polymer, N-(2-hydroxypropyl)methacrylamide (HPMA), and analyzed their binding to the target enzyme using enzymatic assay and surface plasmon resonance. The HPMA copolymers, decorated by GCPII inhibitors, a fluorescent label and/or a cytotoxic drug (doxorubicin), were successfully used for specific imaging of GCPII-expressing cells in tissue cultures as well as for the imaging of GCPII-positive tumors in xenografts of human cancer cells in mice. The copolymers decorated by doxorubicin selectively bind to GCPII-expressing cells and their subsequent internalization results in the cell death. Since GCPII on endothelial cells in tumor-associated neovasculature is accessible to active drug targeting, the highly selective and tight-binding polymer conjugates might represent a promising tool for tumor imaging as well as for anti-angiogenic cancer therapy.