3′-Deoxy-3′-¹⁸F-fluorothymidine PET predicts response to V600EBRAF-targeted therapy in preclinical models of colorectal cancer

Selective inhibition of oncogenic targets and associated signaling pathways forms the basis of personalized cancer medicine. The clinical success of V600EBRAF inhibition in melanoma, coupled with the emergence of acquired resistance, underscores the importance of rigorously validating quantitative biomarkers of treatment response in this and similar settings. Because constitutive activation of BRAF leads to proliferation in tumors, we explored 39′-deoxy-39′-¹⁸F-fluorothymidine (¹⁸F-FLT) PET to noninvasively quantify changes in tumor proliferation that are associated with pharmacologic inhibition of V600EBRAF downstream effectors and that precede changes in tumor volume. Methods: Human colorectal cancer (CRC) cell lines expressing V600EBRAF were used to explore relationships between upregulation of p27 and phosphorylation of BRAF downstream effectors on smallmolecule V600EBRAF inhibitor exposure. Athymic nude mice bearing V600EBRAF-expressing human CRC cell line xenografts were treated with a small-molecule V600EBRAF inhibitor (or vehicle) daily for 10 d. Predictive ¹⁸F-FLT PET was conducted before changes in tumor volume occurred. Correlations were evaluated among PET, inhibition of phosphorylated MEK (p-MEK) and phosphorylated-ERK (p-ERK) by Western blot, tumor proliferation by histology, and small-molecule exposure by matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS). Results: Treatment of CRC cell lines with PLX4720 reduced proliferation associated with target inhibition and upregulation of p27. In vivo, PLX4720 treatment reduced ¹⁸F-FLT uptake, but not ¹⁸F-FDG uptake, in Lim2405 xenografts before quantifiable differences in xenograft volume. Reduced ¹⁸F-FLT PET reflected a modest, yet significant, reduction of Ki67 immunoreactivity, inhibition of p-MEK and p- ERK, and elevated tumor cell p27 protein levels. Both ¹⁸F-FLT PET and ¹⁸F-FDG PET accurately reflected a lack of response in HT-29 xenografts, which MALDI imaging mass spectrometry suggested may have stemmed from limited PLX4720 exposure. Conclusion: We used preclinical models of CRC to demonstrate ¹⁸F-FLT PET as a sensitive predictor of response to V600EBRAF inhibitors. Because ¹⁸F-FLT PET predicted reduced proliferation associated with attenuation of BRAF downstream effectors, yet ¹⁸F-FDG PET did not, these data suggest that 18FFLT PET may represent an alternative to ¹⁸F-FDG PET for quantifying clinical responses to BRAF inhibitors. COPYRIGHT