We have demonstrated a highly significant relationship between incorporation of 5-fluorouracil (5-FU, FUra) in total cellular RNA and loss of clonogenic survival of the human MCF-7 breast carcinoma cell line. Identical results have also been obtained in the presence of thymidine (dThd) to bypass the block of thymidylate synthetase and reverse inhibition of DNA synthesis. These studies suggest that the incorporation of 5-FU in RNA ia a major mechanism of cytotoxicity. Further, we have extended these studies by demonstrating that the enhancement of (5-FU)RNA formation with N-phosphonacetyl-L-aspartate (PALA), 6-methylmercaptopurine riboside (MMPR) and methotrexate (MTX) is associated with increased cell lethality. We have also demonstrated that 5-FU incorporates in DNA fo MCF-7 breast carcinoma cells and that there is excision of these residues. Finally, we have shown that the incorporation of 5-FU into eukaryotic DNA also results in lethal cellular events. These observations have demonstrated a new mechanism of action for this cytotoxic and mutagenic agent. These findings provide major new insights into the mechanism of action of an effective agent in the treatment of human solid tumors. These findings also provide the basis for exploring further the molecular effects of 5-FU on RNA processing and DNA repair. We will attempt to correlate these RNA and DNa effects of 5-FU with loss of MCF-7 clonogenic survival. A post-labeling assay will also be develped that would be applicable in monitoring incorporation of 5-FU in RNA and DNA obtained from clinical samples. Thus, the proposed studies will attempt to provide insight into the effects of 5-FU on: 1) synthesis and processing of ribosomal RNA; 2) synthesis and processing of messenger RNA; 3) excision and repair of 5-FU incorporation in DNA: and 4) to develop a post-labeling assay that monitors formation of (5-FU)RNA and (5-FU)DNA.