Enhancement of the loss of multiple drug resistance by hydroxyurea

Gene amplification is one mechanism whereby tumor cells can become resistant to antineoplastic agents. Unstably amplified genes occur either on submicroscopic circular pieces of extrachromosomal DNA called episomes or on small acentric chromosomes called double minutes. Double minutes are frequently associated with cells containing amplified drug-resistance genes. Human epidermoid carcinoma KBV1 cells contain unstably amplified mdr1 genes and overexpress P glycoprotein, resulting in decreased intracellular drug accumulation. In this cell line, a nonlethal concentration of hydroxyurea accelerated the rate of loss of vinblastine resistance once vinblastine had been removed from the culture medium. After removal of vinblastine, KBV1 cells exposed to hydroxyurea for the time required to complete 12 cell doublings accumulated more vinblastine than control cells grown in the absence of hydroxyurea. In contrast, hydroxyurea had no effect on vinblastine sensitivity and accumulation in parental drug-sensitive KB-3-1 cells. Hydroxyurea also had no effect on sensitivity to cisplatin in cisplatin-resistant human ovarian carcinoma 2008/C13* cells, indicating that hydroxyurea's effect on drug sensitivity was specific for a drug-resistance phenotype associated with unstably amplified drug-resistance genes. These results indicate that, after removing selective pressure, hydroxyurea accelerates loss of resistance to vinblastine and increases accumulation of vinblastine in KBV1 cells, presumably by accelerating loss of amplified mdr1 genes and thus P glycoprotein.