IDO1 and neoplasm: MSNM@CY1-4 increased the anti-tumor activity of CY1-4, and the superior anti-tumor performance of 20 mg/kg MSNM@CY1-4 can be attributed to the improvement of bioavailability of CY1-4 on the one hand, and on the other hand, it may be related to CY1-4-mediated IDO inhibition and related immune responses.