New trends in synthetic drugs and natural products targeting 20S proteasomes in cancers

Cancer is a global health challenge that remains to be a field of extensive research aiming to find new anticancer therapeutics. The 20S proteasome complex is one of the targets of anticancer drugs, as it is correlated with several cancer types. Herein, we aim to discuss the 20S proteasome subunits and investigate the currently studied proteasome inhibitors targeting the catalytically active proteasome subunits. In this review, we summarize the protein degradation mechanism of the 20S proteasome complex and compare it with the 26S proteasome complex. Afterwards, the localization of the 20S proteasome is summarized as well as its use as a diagnostic and prognostic marker. The FDA-approved proteasome inhibitors (PIs) under clinical trials are summarized and their current limited use in solid tumors is also reviewed in addition to the expression of the β5 subunit in different cell lines. The review discusses in-silico analysis of the active subunit of the 20S proteasome complex. For development of new proteasome inhibitor drugs, the natural products inhibiting the 20S proteasome are summarized, as well as novel methodologies and challenges for the natural product discovery and current information about the biosynthetic gene clusters encoding them. We herein briefly summarize some resistance mechanisms to the proteasome inhibitors. Additionally, we focus on the three main classes of proteasome inhibitors: 1] boronic acid, 2] beta-lactone and 3] epoxide inhibitor classes, as well as other PI classes, and their IC 50 values and their structure–activity relationship (SAR). Lastly, we summarize several future prospects of developing new proteasome inhibitors towards the treatment of tumors, especially solid tumors.