The dramatic biochemical and structural response to this treatment confirms the role of FGFR1 signaling in phosphaturic mesenchymal tumor growth and FGF23 production. medical trial (ClinicalTrials.gov quantity, “type”:”clinical-trial”,”attrs”:”text”:”NCT02160041″,”term_id”:”NCT02160041″NCT02160041). The immediate response was dramatic. Within 24 hours, the FGF23 level decreased from 15,500 relative devices (RU) per milliliter to 1765 RU per milliliter (normal value, 180); after 2 weeks, the FGF23 level was normal and the phosphate level elevated (Fig. 1A). Positron-emission tomographic imaging with 18F-fluorodeoxyglucose in combination with computed tomography indicated the metastatic lesions experienced regressed (Fig. 1C). With treatment, progressive calcification of several soft-tissue lesions was observed. Pretreatment and post-treatment biopsies of one lesion exposed that a previously sarcomatous tumor had been replaced with lamellar bone, a finding consistent with metaplastic transformation (Fig. 1B). Open in a separate window Number 1. Response to Infigratinib Treatment.Panel A shows FCGR1A the marked decreases in FGF23 levels (black curve) induced by infigratinib (gray bars indicate doses of the drug), which reversed when the patient was not receiving the drug. The timing of the imaging studies are indicated by arrows labeled a through e, which correspond to the images shown in Panel C. Panel B shows images of the lesions, acquired by computed tomography (CT) and microscopy. Calcification of the metastatic lesion on the scapula was seen in pretreatment and post-treatment noncontrast CT images (arrows). Biopsy of the GNF-7 pretreatment lesion exposed features consistent with a sarcomatous phosphaturic mesenchymal tumor. Biopsy of the calcified lesion after treatment exposed mature, lamellar bone, a finding consistent with infigratinib-induced metaplastic ossification. Panel C shows serial whole-body positron-emission tomographic imaging with 18F-fluorodeoxyglucose in combination with CT (18F-FDG-PETCCT); the scans show the response of metastatic tumors to infigratinib before treatment (a), during treatment (b), after discontinuation (c), after reinitiation (d), and during disease progression (e). Despite dose modifications, tyrosine kinase inhibitorCrelated side effects led to infigratinib becoming discontinued after 18 months of treatment. Immunotherapy with nivolumab and ipilimumab, given in the context ofa medical trial (“type”:”clinical-trial”,”attrs”:”text”:”NCT02834013″,”term_id”:”NCT02834013″NCT02834013), was ineffective. The FGF23 level continued to increase to 78,800 GNF-7 RU per milliliter, and common metastases were mentioned (Fig. 1A and ?and1C).1C). Severe anemia developed and was treated having a transfusion; granulocyte-colony stimulating factor-mediated paraneoplastic neutrophilia also developed (Fig. S7 in the Supplementary Appendix, available with the full text of this letter at NEJM.org). Treatment with infigratinib was reinitiated 28 weeks after the earlier discontinuation. Again, the FGF23 level decreased rapidly, from 78,800 RU per milliliter to 670 RU per milliliter over a period of 100 days (Fig. 1A). Imaging showed designated improvement (Fig. 1C). The white-cell count normalized in parallel with the FGF23 levels. Blood transfusions, which experienced previously been given GNF-7 every 2 to 3 3 weeks, were avoided for 7 weeks. Ultimately, the side effects necessitated intermittent dose interruptions. The individuals disease progressed, and his practical status deteriorated. Ten weeks after resuming therapy, and 5 years since the initiation of infigratinib, the patient died. Additional details are provided in the Supplementary Appendix. This case enabled us to identify a much-needed treatment for malignant tumorCinduced osteomalacia; in addition, it shows how an approach based on customized medicine, made possible by GNF-7 molecular diagnostics and mechanistically directed therapy, prolonged this individuals life. The dramatic biochemical and structural response to this treatment confirms the part of FGFR1 signaling in phosphaturic mesenchymal tumor.