The Synergistic Effect of Selumetinib/Docetaxel Combination Therapy Monitored by [18 F]FDG/[18 F]FLT PET and Diffusion-Weighted Magnetic Resonance Imaging in a Colorectal Tumor Xenograft Model

Positron emission tomography (PET) and diffusion-weighted MRI (DW-MRI) were used to characterize the treatment effects of the MEK1/2 inhibitor selumetinib (AZD6244), docetaxel, and their combination in HCT116 tumor-bearing mice on the molecular level. Mice were treated with vehicle, selumetinib (25 mg/kg), docetaxel (15 mg/kg), or a combination of both drugs for 7 days and imaged at four time points with 2-deoxy-2-[(18) F]fluoro-D-glucose ([(18) F]FDG) or 3'-deoxy-3'-[(18) F]fluorothymidine ([(18) F]FLT) followed by DW-MRI to calculate the apparent diffusion coefficient (ADC). Data was cross-validated using the Pearson correlation coefficient (PCC) and compared to histology (IHC). Each drug led to tumor growth inhibition but their combination resulted in regression. Separate analysis of PET or ADC could not provide significant differences between groups. Only PCC combined with IHC analysis revealed the highest therapeutic impact for combination therapy. Combination treatment of selumetinib/docetaxel was superior to the respective mono-therapies shown by PCC of PET and ADC in conjunction with histology.