Inclusion Criteria:

• Documented histological or cytological diagnosis confirming the presence of malignant solid tumor
• Tumor progression after conventional therapy for cancer or a malignant solid tumor for which no conventional therapy exists
• Subjects enrolled in the MTD confirmation cohort should have a documented histologic and/or cytologic diagnosis of metastatic breast cancer or non-small cell lung cancer (NSCLC) and should not have received more than 2 prior chemotherapy regimens for metastatic disease (adjuvant and neoadjuvant chemotherapy will not be included in the maximum of 2 prior regimens described above)

Exclusion Criteria:

• Subjects with symptomatic or clinically active central nervous system (CNS) metastases. Subjects who have had prior treatment with radiotherapy or surgical resection for CNS metastases will be permitted if CNS metastases have remained stable and have not required any treatment for at least 3 months prior to first dose of test article
• History of any other primary malignancy with less than 5 years documentation of a disease-free state (Subjects with a history of basal cell or squamous cell carcinomas of the skin or cervical carcinoma in situ, which have been successfully treated, are not excluded)
• Recent major surgery (within 14 days before the first dose) or chemotherapy within 28 days before the first dose of MST-997 (42 days if the previous chemotherapy included nitrosoureas or mitomycin C)