VALRUBICIN, A SEMISYNTHETIC ANALOG OF THE ANTHRA-CYCLINE DOXORUBICIN, IS A CYTOTOXIC AGENT. VALRUBICIN IS AN ANTHRACYCLINE THAT AFFECTS A VARIETY OF INTER-RELATED BIOLOGICAL FUNCTIONS, MOST OF WHICH INVOLVE NUCLEIC ACID METABOLISM. IT READILY PENETRATES INTO CELLS, WHERE IT INHIBITS THE INCORPORATION OF NUCLEOSIDES INTO NUCLEIC ACIDS, CAUSES EXTENSIVE CHROMOSOMAL DAMAGE, AND ARRESTS CELL CYCLE IN G2. ALTHOUGH VALRUBICIN DOES NOT BIND STRONGLY TO DNA, A PRINCIPAL MECHANISM OF ITS ACTION, MEDIATED BY VALRUBICIN METABOLITES, IS INTERFERENCE WITH THE NORMAL DNA BREAKING-RESEALING ACTION OF DNA TOPOISOMERASE II.