Acute and chronic rapamycin use in patients with Fibrodysplasia Ossificans Progressiva: A report of two cases

Fibrodysplasia Ossificans Progressiva (FOP) is an ultrarare genetic disorder of progressive, disabling heterotopic ossification for which there is presently no definitive treatment. Several recent studies in genetic mouse models of FOP support involvement of the mechanistic target of rapamycin complex 1 (mTORC1) pathway in the pathophysiology of FOP and propose the repurposed use of rapamycin, an inhibitor of mTORC1 signaling in clinical trials for the management of FOP. Here we report two patients with the classic FOP mutation who received rapamycin-one for four months on a compassionate basis for treatment of acute flare-ups of the neck and back that were refractory to corticosteroid therapy-and the other for 18 years for chronic immunosuppression following liver transplantation for intercurrent cytomegalovirus infection. In both patients, FOP progressed despite the use of rapamycin. This report highlights the real-world use of rapamycin in two FOP patients and provides insight into the use of rapamycin in clinical trials for the management of FOP.