We would like to propose a novel parameter that helps predict the chemosensitivity of cancer patients to FGFR inhibitors (FGFRi). Although FGFRi were introduced into the clinical trials on a variety of cancer types, they were found to be particularly efficacious on urothelial cancer and on cholangiocarcinoma for which the FDA had approved several compounds based on clinical studies. The enrollment criteria in such clinical trials were genomic changes in the FGFR genes including amplifications, translocations that caused gene fusions, and point mutations. By scrutinizing the published data on these reports, we have noticed an unresolved issue. Namely, most of these reports showed waterfall plots where tumors of some good fractions of enrolled patients responded well, demonstrating efficacy of the FGFRi. Importantly, however, there were sizable subsets in which tumors did not respond to the FGFRi despite that the patients’ tumors carried the genomic changes that met the enrollment criteria. As already established, both FGFR and EGFR share the downstream signaling pathway of MAPK activation as well as other pathways. Accordingly, it is conceivable that chemotherapeutic inhibition of FGFR alone may leave the MAPK signaling unaffected when the signaling through EGFR is relatively strong. To test this hypothesis, we calculated the FGFR to EGFR mRNA ratio (F/E for short) based on the RNA sequencing data from the Broad Institute Cancer Cell Line Encyclopedia (https://portals.broadinstitute.org/ccle) of cholangiocarcinoma and urothelial cancer cell lines for which FGFRi chemosensitivity had been published in preclinical studies. Because human FGFR has four paralogs FGFR1–4, we first summed up the expression levels of FGFR1–4 mRNAs by adding the read numbers per MB of FGFR paralogs (sumFGFR). To take EGFR expression into consideration, we then divided this number with that of EGFR; F/E. In six biliary tract cancer cell lines tested, two responsive cell lines had higher F/E ratios of 9.5 and 9.0, whereas four non-responsive lines had substantially lower ratios of 0.1–1.8. In 22 urothelial cancer cell lines, four of the five responsive lines showed F/E of 2.8–4.9, whereas 17 non-responsive lines had 0.01–2.7. To obtain further insights into the F/E ratio, we looked into our patient-derived colorectal cancer (CRC)-stem cell lines. Seven (28%) of 25 RAS/RAF-wild type CRC-stem cell lines responded to the pan-FGFRi erdafitinib singly in culture whereas 21 (84%) did respond to erdafitinib in combination with an EGFR inhibitor (erlotinib). The seven singly responsive cell lines showed relatively high F/E ranging 11–207 with the mean of 46, whereas the 18 non-responsive lines had the ratio of 3.5–37 with the mean at 12. These results suggest that F/E can be a useful biomarker also for colorectal cancer chemosensitivity where EGFR expression level can vary widely. Taken together, F/E is another strong predictor of responses to FGFRi that is as useful as the current genomic criteria that are based on the FGFR genomic changes.
Overall design: RNA sequencing data of 25 colorectal cancer spheroid lines, three colorectal cancer whole tissues, and three normal colorectal epithelial spheroid lines
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