Prodrug Targeting Based Upon the NAALADase Activity to Prostate Specific Membrane Antigen by Prostate Cancer Cells
Final rept. 1 Oct 1998-31 Mar 2001
JOHNS HOPKINS UNIV BALTIMORE MD SCHOOLOF MEDICINE
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The majority of our present chemotherapeutic agents only kill cells effectively when they are proliferating, this may explain why these agents have been of such limited success in patients. In contrast to these ineffective agents, we have chemically modified a plant toxin, Thapsigargin TG, to produce primary amine-containing analogs that are potent, cell proliferation independent, inducers of apoptosis in prostate cancer cells. These TG-analogs, however, are not prostate cancer-specific cytotoxins. The original hypothesis was that these TG analogs could be converted to inactive prodrugs that can be efficiently converted back to active killing drugs only by the enzymatic activity of Prostate Specific Membrane Antigen PSMA. Since PSMA is expressed in high levels only by prostate cancer cells and not by normal cells, this should allow specific targeting of the TG-analogs killing ability to prostate cancer cells. The objectives of the study were to first define a peptide substrate that was specifically hydrolyzed by PSMA. The second objective was to identify a cytotoxic TG analog that is inactivated upon coupling to this peptide. To accomplish the first objective, a larger number of peptide substrates for PSMA activity and specificity had to be rapidly screened. We began by synthesizing a series of analogs based on methotrexate consisting of the pteridine ring-para-aminobenzoic acid APA portion of methotrexate coupled to a variety of peptides. Using this approach a series of methotrexate analogs consisting of the APA moiety coupled to peptide substrates were screened for PSMA hydrolysis and serum stability. The preferred substrate had the sequence APA-Asp-GluGluAsp-Glu where hyphen equals alpha linkage and star equals gamma. Cytotoxic glutamate and aspartate containing analogs of TG that can be coupled to this peptide carrier were also identified.
- Medicine and Medical Research