Abstract 4337: Tumor T1 is a non-invasive imaging marker of proliferation and response to the mTOR inhibitor everolimus

Abstract

Abstract Everolimus (RAD001) is an oral mTOR inhibitor recently approved for the treatment of 2nd-line renal cell carcinoma. Further development of everolimus will be aided by identification of suitable biomarkers for stratification and response. We have recently shown that a non-invasive imaging marker (T1) is a rapid and robust marker of tumor response for five compounds with different mechanisms of action (Clinical Cancer Res, in the press). Here we have correlated everolimus-induced changes in T1 (ΔT1) with other biomarkers in a murine tumor model. Cultured RIF-1 tumor cells (5 × 106) were injected s.c. in the flank of C3H mice (n=14) and after 2 weeks were treated daily with everolimus (10 mg/kg p.o.) or vehicle. Magnetic resonance imaging and 1H-spectroscopy (MRI and MRS) were performed at baseline (day-0) and after 5 days treatment on a 2 mm central slice of the tumor. MRI used an IR-TrueFISP method to quantify the spin-lattice relaxation time of water protons (T1) and MRS the total levels of choline as the Cho/H20 ratio. On day-5, the tumors were ablated, paraffin-embedded and stained for the percentage of cells positive for cleaved caspase-3 (apoptosis) and the proliferation marker Ki67 using immunohistochemistry (IHC). Effects are summarized as mean T/C (ratio of treated divided by vehicle-control); relationships between variables were assessed using Pearson's correlation, with significance set at p<0.05. Everolimus strongly inhibited tumor growth (T/C=0.05, p<0.001) without affecting body-weight. MRS signals for the proliferation marker choline were clearly detectable in all tumors at both baseline and day-5, and in 6/7 tumors everolimus reduced choline (T/C=0.73, p=0.1). Basal T1 was 2.3 sec and this was always reduced by everolimus (T/C=0.89, p<0.01). The ΔT1 correlated with changes in tumor volume, dTVol (r=0.81, p<0.001), and choline (r=0.59, p<0.05). Basal T1 did not correlate with dTVol, i.e. did not predict response to everolimus, but choline did (r= −0.81, p<0.05), so that higher levels of choline predicted a better response. IHC showed that the %-caspase and %-necrosis were unchanged by everolimus but the %Ki67 positive cells was reduced (T/C=0.55, p<0.001) and there was a significant correlation between %Ki67 and ΔT1 (r=0.61, p<0.05). These results show that everolimus-induced changes in T1 correlate with known markers of proliferation. The speed and robustness of the method should facilitate its use in clinical trials as an early-response marker. In this model, basal levels of choline were a stratifier for response to everolimus. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4337.