BGJ398 for the Treatment of Tumor-Induced Osteomalacia

Background: People with tumor-induced osteomalacia (TIO) have small tumors that may cause low blood phosphorus, weak muscles, bone pain, and broken bones. The tumors may be so small they are hard to find or impossible to remove. Researchers want to test a drug that may help treat TIO. Objective: To see how the drug BGJ398 affects people with tumor-induced osteomalacia. Eligibility: People ages 18-85 who are in NIH protocol 01-D-0184 and have TIO that cannot be found or easily removed Design: At every study visit, participants will have: * Medical history * Physical exam * Blood and urine tests * Questions about their health and fatigue At the screening visit, participants will also have a heart and eye tests. They may have other tests to find their tumor. The baseline visit will be a 1-week stay in the clinic. Participants will have the regular study tests, plus: * Their first dose of the study drug capsules * Blood and urine collected every 2-4 hours for 24 hours. A thin plastic tube will be inserted in a vein to collect blood. * Heart and kidney ultrasounds * Activities that test strength * 6-minute walk test Participants will take the study drug for six 1-month cycles. In each cycle, participants will: * Take the study drug every day for 4 weeks. * Have 1 visit. Participants will collect their urine for 24 hours and have their blood drawn. Participants will have the regular study tests and repeat some baseline tests. * Have blood and urine tests at their local lab. Participants will have 1 visit at the end of the last cycle and another 3 months later....