IMPDH1 and neoplasm: Then, we pharmacologically inhibited the IMPDH1 activity of HNSCC cells using a pan IMPDH inhibitor (MPA) and its pro-drug (MMF) to assess their anti-tumor effects.14,15 CCK8 assay showed that treatment with MPA or MMF dramatically decreased HNSCC cell viability (Fig. 6c, d).