The purpose of this study is to determine whether maintenance therapy with oral AP23573 (ridaforolimus), by preventing and controlling tumor growth for a prolonged period of time in patients with metastatic soft-tissue or bone sarcomas responding to chemotherapy, will result in clinically significant improvement in progression-free survival as compared to oral placebo.

Inclusion Criteria:

• Confirmed diagnosis of metastatic soft-tissue or bone sarcoma
• Ongoing complete response, partial response, or stable disease (RECIST) after a minimum of 4 cycles (and maximum of 12 months) of any one first, second, or third line of prior cytotoxic chemotherapy for metastatic disease
• Eastern Cooperative Oncology Group performance status of 0 or 1
• Adequate organ and bone marrow function
• Completed prior chemotherapy with last dose received at least 3 and up to 12 weeks prior to randomization

Exclusion Criteria:

• Prior therapy with rapamycin or rapamycin analogs
• Ongoing toxicity associated with prior anticancer therapy
• Another primary malignancy within the past three years
• Concomitant medications that induce or inhibit CYP3A
• Significant, uncontrolled cardiovascular disease