Thymidine kinase (TK) and dihydrofolate reductase (DHFR) are two enzymes that play major roles in the salvage- and de novo pathways of DNA synthesis in many eukaryotes, prokaryotes, and viruses. Both enzymes have been exploited extensively as molecular targets for the therapy and imaging of cancer and infectious diseases. This thesis deals with three different research projects that are related to these two enzymes.
The first project focuses on the development of novel computational techniques for the structure-based design of carborane-containing therapeutics. These new methods were developed at the example of DHFR-targeting carboranyl antifolates using Autodock, Glide, FlexX, and Surflex as software platforms. Applying these new techniques, AutoDock and Glide were found to be superior to Surflex and FlexX in docking of closo-carboranyl antifolates into the active site of hDHFR. In the case nido-carboranyl antifolates, Autodock, Glide, and Surflex produced more accurate docking results than FlexX.
The second project describes the design and synthesis of a novel type of tumor imaging agents. These are 3-carboranyl thymidine analogues (3CTAs) that are labeled with iodine-125 at the carborane cluster. 3CTAs are substrates of human TK1 (hTK1) and are selectively entrapped in tumor cells through phosphorylation by this enzyme. The advantage of radiolabeling at the carborane cluster in these compounds is that boron-iodine bonds are apparently far less susceptible to cleavage than carbon-iodine bonds in conventional imaging agents, which may reduce the undesired accumulation of radioactive iodine in the thyroid and stomach The key step in the synthesis of a 125I-labelled 3CTA designated as N5-125I was a palladium catalyzed isotope exchange of cold iodine-127 with hot iodine-125 using commercially available Na125I as the radiohalogen source. Two different methods were developed for the synthesis of 127I-labelled N5 (N5-127I),the starting material in this reaction.
The third project deals with the design and synthesis of novel agents for a “combination therapy” of malignancies that are associated with the Epstein-Barr Virus (EBV). These include Burkitt’s lymphoma, Hodgkin’s disease, nasopharyngeal carcinoma, T/NK-cell lymphoma, and post-transplant lymphoproliferative disease. The agents were designed to selectively target EBV thymidine kinase (EBVTK) following induction of the lytic cycle of the virus within the proliferating cells. The EBV lytic cycle can be induced e.g. by DNA methyltransferase inhibitors, histone deacetylase (HDAC) inhibitors, and various chemotherapeutic agents. A small focused library of 24 compounds was synthesized for future evaluation in enzyme assay with hTK1 and EBVTK.