Hyperphosphatemia Secondary to the Selective Fibroblast Growth Factor Receptor 1–3 Inhibitor Infigratinib (BGJ398) Is Associated with Antitumor Efficacy in Fibroblast Growth Factor Receptor 3–altered Advanced/Metastatic Urothelial Carcinoma

Background Infigratinib (BGJ398) is a potent, selective fibroblast growth factor receptor (FGFR) 1–3 inhibitor with significant activity in metastatic urothelial carcinoma (mUC) bearing FGFR3 alterations. It can cause hyperphosphatemia due to the “on-target” class effect of FGFR1 inhibition. Objective To investigate the relationship between hyperphosphatemia and treatment response in patients with mUC. Intervention Oral infigratinib 125 mg/d for 21 d every 28 d. Design, setting, and participants Data from patients treated with infigratinib in a phase I trial with platinum-refractory mUC and activating FGFR3 alterations were retrospectively analyzed for clinical efficacy in relation to serum hyperphosphatemia. The relationship between plasma infigratinib concentration and phosphorous levels was also assessed. Outcome measurements and statistical analysis Clinical outcomes were compared in groups with/without hyperphosphatemia. Results and limitations Of the 67 patients enrolled, 48 (71.6%) had hyperphosphatemia on one or more laboratory tests. Findings in patients with versus without hyperphosphatemia were the following: overall response rate 33.3% (95% confidence interval [CI] 20.4–48.4) versus 5.3% (95% CI 0.1–26.0); disease control rate 75.0% (95% CI 60.4–86.4) versus 36.8% (95% CI 16.3–61.6). This trend was maintained in a 1-mo landmark analysis. Pharmacokinetic/pharmacodynamic analysis showed that serum phosphorus levels and physiologic infigratinib concentrations were correlated positively. Key limitations include retrospective design, lack of comparator, and limited sample size. Conclusions This is the first published study to suggest that hyperphosphatemia caused by FGFR inhibitors, such as infigratinib, can be a surrogate biomarker for treatment response. These findings are consistent with other reported observations and will need to be validated further in a larger prospective trial. Patient summary Targeted therapy is a new paradigm in treating bladder cancer. In a study using infigratinib, a drug that targets mutations in a gene called fibroblast growth factor receptor 3 (FGFR3), we found that elevated levels of phosphorous were associated with greater clinical benefit. In the future, these data may help inform treatment strategies.