Background: Major resistant mechanisms of temozolomide (TMZ) include the expression of the DNA repair protein (O6-methylguanine-DNA methyltransferase: MGMT). The effect of TMZ on cancer physiology and drug resistance was investigated here. Methods: TMZ was orally administered (100 mg/kg/day × 2 doses/week) for 5 weeks to mice bearing orthotopic MCF-7 human breast carcinoma (MCF-7wt) or MCF-7 adriamycin resistant phenotype (MCF-7adr), and tumor volumes were measured. During TMZ chemotherapy, tumor vascular volume (VV) and permeability surface area product (PS) in each mouse were non-invasively measured by MRI. Using the identical time-line, tumors were resected, and TMZ concentration was measured using LC/MS/MS. MGMT and VEGF expressions were analyzed using real-time RT-PCR and ELISA, respectively. Experimental methods used in this study are listed in Table 1. Results/Discussion: TMZ demonstrated a tumor growth inhibitory effect against MCF-7wt xenografts in mice, but not for MCF-7adr. MCF-7wt xenografts initially responded to TMZ, but then the tumor volume gradually increased. Table 1 summarizes the effect of TMZ on MCF-7 cell lines and xenografts. In drug resistant tumors, increase in VV and PS caused by TMZ treatment did not result in the sensitization of the tumor to the therapy probably due to intrinsic MGMT expression. No significant changes in MGMT and VEGF expression were detected in vivo in MCF-7wt xenografts treated with TMZ possibly due to low drug concentrations in the tumor. Therefore, the eventual resistance of MCF-7wt tumors from TMZ therapy may be attributed to the suppression of the tumor vasculature with the corresponding reduction of drug delivery to the tumor. Acknowledgment: Supported by NIH grant R01 CA097310. Special thanks to Dr. Zaver M. Bhujwalla for her helpful discussions, and Mr. William Yutzy, who assisted us in using a real-time RT-PCR instrument.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]