This pilot study will evaluate use of IGF-Methotrexate conjugate (765IGF-MTX) in patients with advanced, previously treated MDS, CMML and O-AML. 765IGF-MTX at a dose of 0.20 to 2.5 µequivalents per kg is administered as an IV infusion over 1.5 hours on days 1, 8 and 15 of a 28 day cycle. Treatment continues until disease progression, as assessed after 2 cycles, unacceptable toxicity, or patient refusal. Assessment of response will be confirmed by bone marrow studies performed at the end of cycles 2, 4, and 6 (each +/- 3 days).