Simultaneous protection of G156A methylguanine DNA methyltransferase gene-transduced hematopoietic progenitors and sensitization of tumor cells usingO-6-benzylguanine and temozolomide

O-6-Benzylguanine (BG) potentiates temozolomide (TMZ) cytotoxicity in tumor s by inactivating O-6-alkylguanine DNA alkyltransferase but also increases toxicity in hematopoietic cells. To improve the hematopoietic cell toleranc e to alkylating agents, we retrovirally transduced the BG-resistant mutant G156A methylguanine DNA methytransferase gene (Delta MGMT) into hematopoiet ic progenitors and evaluated whether Delta MGMT expression in hematopoietic colony-forming units would result in greater drug resistance to TMZ. Delta MGMT expression in human and mouse colony-forming units followed by BG tre atment resulted in a >7.7-fold increase in the TMZ 90% inhibitory concentra tion (IC90) and a 5.6-fold increase in the 1,3-bis(2-chloroethyl)-1-nitroso urea (BCNU) IC90 relative to untransduced cells. This degree of protection enabled Delta MGMT-transduced CD33 cells to become much more resistant to B G and TMZ than SW480 cells, which express high O-6-alkylguanine DNA alkyltr ansferase and are normally resistant to TMZ or BCNU alone. Delta MGMT-trans duced long-term culture initiating cells were also resistant to the BG and TMZ combination, as were untransduced long-term culture initiating cells, s uggesting that noncycling early progenitors may be partially protected from TMZ, These data indicate that retroviral transduction of Delta MGMT into h ematopoietic progenitors followed by BG and TMZ treatment may selectively p rotect hematopoietic cells more efficiently than BCNU, allowing dose-intens ive and repetitive therapy without the risk of cumulative myelosuppression.