Structural optimization and preliminary structure–activity relationship studies of a series of N- substituted maleimide fused-pyrazole analogues with Cdc25B inhibitory activity, starting from a high-throughput screening hit, are illustrated. A simplified 3, 5-diacyl pyrazole analogue was obtained as the most potent compound (118, IC50= 0.12 μM) with a 270-fold increase in potency.