An increasing body of evidence suggests that USP28 could be a target for cancer treatment.To identify its inhibitors, the authors screened a 100-thousand synthetic compound library and found 3 lead compounds, CT1001-1003, that showed significant inhibitory activity.The optimization of these compounds led to CT1018, a much stronger inhibitor.CT1018 is almost the same as CT1002 except that it is semi-methylated at the 3-amino group. Interestingly, fully methylating the 3-amino group (CT1038) almost killed the inhibitory activity, and replacing the Me groupwith an Et group (CT1047) also led to a weaker inhibitor.These results indicate that the semi-methylation of the 3-amino group is critical for the inhibitory activity.However, CT1018 was inactive in cell-based assays.Continued optimization resulted in CT1073 and CT1113.These two compounds are potent against USP28 as well as the closely-related USP25.The importance of the semi-methylation at the 3-amino group is also true for CT1073, as its unmethylated version, CT1008, is a much weaker inhibitor.Furthermore, we measured the interaction kinetics between the inhibitors and USP25/28 with an SPR instrument.Next, we subjected USP25/28 inhibitors to various cell-based assays.Both CT1073 and CT1113 were able to reduce c-MYC levels quite dramatically in a diverse set of cancer cell lines and the reduction occurred within 1-2 h of the treatment.As expected, the half-lives of LSD1 and TNKS were all shortened by CT1073 treatment and the destabilization of c-MYC, LSD1, and TNKS caused by CT1073 or CT113 treatment could be reversed by the proteasome inhibitor MG132.These data suggest that the anti-proliferative effect of CT1113 is largely an on-target effect, although the contributions from potential off-target effects could not be completely excluded.In vivo results demonstrate that USP25/28 inhibitor CT1113 is a strong anti-tumor agent.In summary, we identified a class of potent USP25/28 inhibitors that shows broad anti-tumor activity.