PK/PD of Vitamin D3 in Adults With CF

Despite the extensive literature on adverse clinical outcomes associated with vitamin D deficiency, there are currently no proven treatment strategy that effectively achieves and maintains optimal serum vitamin D status in cystic fibrosis (CF) patients. For the treatment of vitamin D deficiency, CF Foundation currently recommends 2,000 IU daily. However, because achieving adequate serum 25(OH)D levels is a challenge in CF, higher doses of vitamin D may be necessary to reach and maintain vitamin D sufficiency. Poor oral bioavailability of ergocalciferol has been demonstrated in CF patients, which may potentially also be an issue with cholecalciferol. In order to optimize the treatment of vitamin D deficiency in CF, the kinetic disposition must be well understood. However, there are very few data currently available describing the kinetics of both vitamin D and 25-hydroxyvitamin D, and to the investigator's knowledge, no studies have yet characterized the pharmacokinetic disposition of vitamin D and its metabolites in cystic fibrosis. Addressing this issue is crucial in effectively and safely correcting vitamin D deficiency in CF.