Drug combination studies of isoquinolinone AM12 with curcumin or quercetin: a new combination strategy to synergistically inhibit 20S proteasome

In the eukaryotic cells, the ubiquitin-proteasome system (UPS) plays a crucial role in the intra-cellular protein turnover. It is involved in several cellular functions as the control of the regular cell cycle progression, the immune surveillance, and the homeostasis. Within 20S proteasome barrel-like structure, the catalytic subunits, β1, β2, and β5, are responsible for different proteolytic activities: caspase-like (C-L), trypsin-like (T-L) and chymotrypsin-like (ChT-L), respectively. The β5 subunit is particularly targeted for its role in antitumor activity: the synthesis of β5 subunit inhibitors could be a promising strategy for the treatment of solid and hematologic tumors. In the present work, we performed two combination study of AM12, a recently developed synthetic proteasome inhibitor, with curcumin and quercetin, two nutraceuticals endowed of many pharmacological properties. We measured the Combination Index (CI), applying the Chou and Talalay method, comparing the two studies, from 50% to 90% of proteasome inhibition. In the case of the combination AM12 + curcumin an increasing synergism was observed from 50% to 90% of pro-teasome inhibition, while in the case of the combination AM12 + quercetin an additive effect was observed only from 50% to 70% of β5 subunit inhibition. These results suggest that combining AM12 with curcumin is a more promising strategy than combining it with quercetin for potential therapeutic applications, especially in treating tumors