Mechanism for induction of hydroxyurea resistance and loss of latent EBV genome in hydroxyurea-treated Burkitt's lymphoma cell line Raji. 
Hydroxyurea (HU), as an inhibitor of ribonucleotide reductase (RR) through interaction with the R2 component, has been used in the treatment of malignancies.  Recently, therapeutic strategies in Epstein-Barr Virus (EBV)-targeted lymphoma have been reported.  In order to study the effect of HU on EBV, infected Burkitt's lymphoma (BL) Raji cells were passaged in medium containing 50 microM HU for more than 2 months.  EBV DNA was eliminated in about 40% of the cells in the HU-treated cultures.  The cells were cloned from such cultures, and only EBV-positive clones could be isolated in 102 examined clones.  No differences were observed in the EBV-latent state, EBV-gene expression, or cell growth between HU-untreated Raji cells and HU-treated clones.  However, relative to parental Raji cells, the HU-treated Raji clones were almost eight times resistant to growth inhibition by HU according to the ID50 value, and the expression of the R2 component of RR increased more than two to three times.  These results indicate that HU not only efficiently eliminates the EBV genome from Raji cells but also induces HU resistance.  HU resistance was accompanied by over-expression of the R2 component of RR.  However, the HU-resistant clones were sensitive to gemcitabine, another inhibitor of RR, and this seems highly relevant to chemotherapeutic combination in the use of these drugs.