EXTRACHROMOSOMAL DNA--A NEW TARGET FOR ANTITUMOR DRUGS

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Extrachromosomal DNA (episomes) can be extraordinally sensitive to the actions of DNA inter-active antitumor drugs including topoisomerase II targeted agents. Furthermore, episomal elements associated with gene amplification in neoplastic cells and tumors have been found to be precursors of double minute chromosomes and homogeneously staining regions. Such DNAs often contain genes associated with oncogenesis (e.g. c-myc) and drug resistance (e.g. p-glycoprotein). The goal of this program is to develop chemotherapeutic strategies based upon episomes as new targets for evaluation of DNA reactive drugs. A mouse fibroblast tissue culture cell line (935.11) neoplastically transformed by an episomal element is used to evaluate drug mediated damage and its repair on a define extrachromosomal DNA target in vitro. These same cells can also be grown as a sub-cutaneous (s.c.) tumor in mice which allows for in vivo evaluation of drug-induced damage to the episome. The extent of damage to 935.11 cell episomal sequences in the in vivo assay correlates with therapeutic efficacy. Based upon an understanding of the mechanisms of action of DNA reactive drugs on episomal elements. We shall explore the biological consequences of preferential drug induced damage to episomes. SPECIFIC AIMS: 1) To characterize drug-induced lesions on episomal elements and evaluate the potential for targeting extrachromosomal DNA as a means for improving drug treatment strategies. Antitumor drug interaction with episomes will be investigated using the 935.1 in vitro model system. This study includes quantitation of DNA lesions in episomes based on topological forms conversion of the circular DNA as well as elucidation of factors contributing to preferential damage of extrachromosomal versus genomic DNA. Comparisons will be made between damage to episomes and cytotoxic activity. 2) To assess relationships between therapeutic efficacy and selective targeting of episomal DNA that confer neoplastic phenotypes. Drug-induced damage of episomes and corresponding therapeutic efficacy will be characterized in mice bearing the 935.1 cells as a solid s.c. tumor. Quantitative and qualitative damage to the episomes will be the basis for selecting and scheduling drugs with improved therapeutic value. 3) To evaluate as a basis for therapeutic strategies, enhanced targeting of antitumor drugs toward spontaneously occurring episomes which harbor drug resistance and oncogenic factors. Anti-tumor drug will be investigated for their abilities to damage episomal precursors of double minutes and homogenous staining regions containing amplified sequences of drug resistance and oncogenic factors. Relationships will be sought between drug effects on these DNAs and biological responses that lead to cell death or phenotypic changes.