Small-molecule inhibitors (nibs) have revolutionized cancer therapy with the 
emergence of clinically efficacious treatment for advanced-stage malignancies. 
Fibroblast growth factor receptor (FGFR) inhibitors have shown therapeutic 
efficacy in malignancies with molecular-genetic alterations in the 
FGFR/fibroblast growth factor pathway. In a phase 1 clinical trial, erdafitinib, 
a pan FGFR inhibitor, was well tolerated with a manageable toxicity profile. 
Hyperphosphatemia was a frequent adverse event in patients treated with 
erdafitinib; however, no serious complications were observed with this therapy. 
Here, we report the development of calcinosis cutis dermatologic toxicity in a 
patient with hyperphosphatemia while treated with a novel selective FGFR 
inhibitor, INCB 54828-101. Awareness of this form of dermatologic toxicity from 
an FGFR inhibitor will be important for close monitoring of serum levels of 
phosphate, FGF23, vitamin D, and calcitriol, the management of adverse serum 
chemistry with chelators, and treatment decisions to either reduce dose or 
withhold FGFR inhibitor.