Introduction: Inadequate tumor vascularization and/or anti-vascular effects of cancer chemotherapy contribute to anticancer drugs’ failure. We pursued physiological drug resistance of cancer chemotherapy in mouse breast cancer models using magnetic resonance imaging (MRI) and spectroscopic imaging (MRSI). Methods: All animal experiments were conducted according to the institutional guidelines. Anesthetized SCID mice with orthotopic MCF-7 breast cancer were immobilized in the probe and maintained under gas anesthesia (Isoflurane 0.5%, 1 l/min). 13C-Labeled-temozolomide ([13C]TMZ) or temozolomide (TMZ: Temodar® capsule), alkylating agent, was used as a model drug. Two hundred microliter of [13C]TMZ (50 mM)/dimethyl-β-cyclodextrin (100 mM) inclusion complex was infused into the mouse via an i.p. catheter. Three-dimensional indirect 1H/13C MRSI was performed with an 8x8x8 matrix for a 16 mm isotropic field of view. During 3D map acquisition, a total of 400 μl of inclusion complex was injected. After completing 13C studies, a GdDTPA bolus was administered and 3D isotropic gradient echo imaging (TE/TR = 15/200 ms) was performed. Tumor growth inhibitory effect of TMZ was evaluated in vivo after oral administration. Plasma and intratumoral concentration of TMZ was measured using high performance liquid chromatography (HPLC). In vitro P-glycoprotein expression and vascular endotherial growth factors (VEGF) expression of MCF-7 cells were also examined by the exposure of TMZ to the cell culture, and detected by flow cytometry and enzyme-linked immunosorbent assay (ELISA), respectively. Effect of TMZ chemotherapy on tumor vasculature was evaluated by dynamic MRI with macromolecular contrast agent. Vascular volume (VV) and permeability surface area (PS) of tumor were defined for control and treated tumors. Results & Discussion: TMZ showed growth inhibitory effect at the primary stage of chemotherapy, but cancer volume was gradually increased at the later stage. VV and PS of cancer were decreased during TMZ chemotherapy for 5 weeks (2 mg/kg/day × 2 doses/week). The concentration of TMZ in the tumor was decreased at the latter stages of chemotherapy, but was not significantly different (p > 0.05). Flow cytometry showed that MCF-7 cells did not express P-glycoproteins during TMZ chemotherapy in this dosing regime. VEGF expression of MCF-7 was suppressed by TMZ at a dose of 50 μg/ml, suggesting one possible mechanism of the anti-vascular TMZ effect within the tumor. Conclusion: This functional MRI study demonstrated that TMZ chemotherapy decreased VV and PS of tumor, suggesting chemotherapy may induce physiological drug resistance by restricting drug delivery to the tumor. HPLC, flow cytometry, and VEGF quantification studies supported the findings from the functional MRI. Acknowledgements: We acknowledge support from NIH RO1 CA097310.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]