Natalizumab in Recurrent, Refractory or Progressive Pulmonary Metastatic Osteosarcoma

Inclusion Criteria: * Subjects may be male or female and must be equal to or greater than 5 years of age but less than or equal to 30 years of age at the time of enrollment. No large studies have evaluated the use of Natalizumab in younger pediatric patients, and Natalizumab is currently only FDA approved for adult use; for this reason, children younger than 5 years of age are excluded from this study. * Subjects must have histologic verification of pOS. * Subjects must have measurable pulmonary disease or pleural disease per RECIST 1.1 documented by clinical, radiographic and histologic criteria, and have progressed, relapsed or become refractory to conventional therapy. \-- Subjects despite having peripheral diseases elsewhere outside of pulmonary disease or pleural disease, may be eligible: * if these diseases have failed upfront standard therapy AND * one or two salvage therapies * Subjects must have recovered from the acute toxic effects with ≤ Grade 1 as defined by the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE) Version 5.0 of all prior chemotherapy and immunotherapy with the exception of alopecia, anorexia, bone pain, and tumor pain prior to entering this study. * Myelosuppressive chemotherapy: Must have adequate recovery of counts from previous treatment prior to entry onto this study. * Monoclonal antibodies: At least 3 half-lives must have elapsed since prior therapy that included a monoclonal antibody. * Subjects mus…