ACTIMMUNE in Intermediate Osteopetrosis

Osteopetrosis is a rare inherited metabolic bone disease characterized by impaired osteoclast function resulting in defective bone resorption and generalized high bone mass and mineral density (BMD). In patients with severe disease, this high bone mass compromises bone marrow space leading to marrow failure and frequent infections, along with hepatosplenomegaly from extramedullary hematopoiesis. Currently, the only treatment for individuals with severe forms of osteopetrosis is hematopoietic cell transplantation (HCT), however survival in patients with osteopetrosis treated with HCT is only around 55%. Therefore, this treatment is only indicated in select individuals with life-threatening complications of their disease. Thus additional treatments for osteopetrosis are needed both for individuals who are not candidates for HCT and to prolong the time until HCT is needed. Interferon gamma (IFN-γ) is a naturally occurring cytokine that has been shown to have anti-microbial and anti-viral immunomodulatory effects, and is a potent stimulator of superoxide anion production which in turn promotes the formation and activation of osteoclasts. Two previous studies of IFN-γ1b in a small group of individuals with osteopetrosis found a decrease in trabecular bone area, an increase in marrow space, a decrease in the number of severe infections requiring antibiotic therapy, and an increase in superoxide generation by granulocyte-macrophage colonies. Therefore, the investigators will conduct an early phase 2, multi-center, open-label, 12-month clinical trial of ACTIMMUNE (IFN-γ1b) treatment of patients with intermediate osteopetrosis to determine the following: 1. The feasibility and tolerability of interferon gamma-1b treatment for 1 year in patients with intermediate osteopetrosis. Specifically, i) the ability to enroll patients, and ii) continued treatment throughout the 1-year observational period. 2. Change in immunologic and hematologic function, bone mineral density and osteoclast function, physical function and quality of life.