An ¹⁹F magnetic resonance-based in vivo assay of solid tumor methotrexate resistance: Proof of principle

Purpose: Studies in oncology have implicated multiple molecular mechanisms as contributors to intrinsic and acquired tumor resistance to antifolate therapy. Here we show the utility of an ¹⁹F-labeled methotrexate (FMTX) with ¹⁹F magnetic resonance to differentiate between sensitive and resistant tumors in vivo and thus predict therapeutic response. Experimental Design: Human sarcoma xenografts in nude mice were used in this study. The sarcoma cell lines chosen for this study (HT-1080, HS-16, and M-805) are well characterized in terms of their methotrexate sensitivity and molecular mechanisms of resistance. The pharmacokinetics of tumor uptake/washout of FMTX were monitored via in vivo ¹⁹F magnetic resonance spectroscopy (pulse/acquire with surface coil localization) following an i.v. bolus injection. Response post-therapy, following leucovorin rescue, was monitored via tumor growth. Results: The three tumor models show differences in both the peak concentrations of tumor FMTX and the dynamics of uptake/retention. These differences are most pronounced for time points late in the magnetic resonance observation period (225-279 minutes post-injection). A statistically significant linear correlation between tumor tissue concentrations of FMTX at these late time points and therapeutic response in the days/weeks post-treatment is shown (R = 0.81, F = 9.27, P < 0.001). Interestingly, a 400 mg/kg i.v. bolus injection of FMTX is a more potent cytotoxic agent in vivo against methotrexate-sensitive tumors than is the parent compound (P = 0.011). Conclusions: In principle, the assay method described herein could be implemented in the clinic as a diagnostic tool to make decisions regarding therapeutic protocol for the treatment of osteosarcoma on a case-by-case basis.