Targeting the Ubiquitin-Proteasome System for Cancer Therapeutics by Small-Molecule Inhibitors

The ubiquitin-proteasome system (UPS) is a critical regulator of cellular protein levels and activity. It is, therefore, not surprising that its dysregulation is implicated in numerous human diseases, including many types of cancer. Moreover, since cancer cells exhibit increased rates of protein turnover, their heightened dependence on the UPS makes it an attractive target for inhibition via targeted therapeutics. Indeed, the clinical application of proteasome inhibitors in treatment of multiple myeloma has been very successful, stimulating the development of small-molecule inhibitors targeting other UPS components. On the other hand, while the discovery of potent and selective chemical compounds can be both challenging and time consuming, the area of targeted protein degradation through utilization of the UPS machinery has seen promising developments in recent years. The repertoire of proteolysis-targeting chimeras (PROTACs), which employ E3 ligases for the degradation of cancer-related proteins via the proteasome, continues to grow. In this review, we will provide a thorough overview of small-molecule UPS inhibitors and highlight advancements in the development of targeted protein degradation strategies for cancer therapeutics.

XIAP antisense therapy with AEG 35156 in acute myeloid leukemia

INTRODUCTION

AEG 35156 is an antisense oligonucleotide to X-linked inhibitor of apoptosis protein (XIAP). Overexpression of XIAP is common in acute myeloid leukemia (AML) and other cancers and is thought to cause resistance to cancer therapy. Effective treatment options for patients with relapsed or refractory AML are limited and survival continues to be poor. Targeting resistance mechanisms is expected to improve results in relapsed as well as front-line settings.

AREAS COVERED

Role of XIAP in apoptosis pathways, structure of AEG 35156, mechanism of action, pharmacokinetics and pharmacodynamics, clinical efficacy and review of clinical trials in AML.

EXPERT OPINION

AEG 35156 in combination with standard chemotherapy was generally very well-tolerated and had shown some evidence of anti-leukemic activity in AML. The target knock down was transient and has not always correlated with response. Future studies may be done with variations in dose scheduling and with more emphasis on comprehensive pharmacodynamic studies simultaneously analyzing other inhibitor of apoptosis proteins (IAPs) and various XIAP regulators. Use of small molecule mimetics of second mitochondria derived activator of caspases (Smac) simultaneously targeting other IAPs appears to be an attractive option.