Targeting BCL-xL in Myeloid Malignancies: From Inhibitors to PROTAC

Abstract

Restoring apoptosis in malignant cells represents a central goal of anticancer therapy. Tumour cells often escape cell death by overexpressing anti-apoptotic members of the BCL-2 protein family, particularly BCL-2, BCL-xL, and MCL1. These proteins inhibit the intrinsic mitochondrial apoptotic pathway through intricate interactions with pro-apoptotic partners and direct modulation of the mitochondrial outer membrane. Their pivotal role in cell survival has established them as attractive therapeutic targets. Over the past two decades, significant efforts have been devoted to developing selective small-molecule inhibitors capable of neutralising these proteins and reactivating apoptosis. A first milestone was the discovery of ABT-263 (navitoclax), a dual BCL-2/BCL-xL inhibitor. Building on this achievement, the development of venetoclax, a highly selective BCL-2 inhibitor, marked a major breakthrough, demonstrating potent pro-apoptotic activity and clinical efficacy in several leukaemia subtypes. Despite these advances, the design of inhibitors of BCL-2 family members remains challenging, largely due to the structural characteristics of the BH3-binding groove, which is both shallow and hydrophobic, complicating the identification of molecules with optimal binding affinity and selectivity. PROTACs targeting BCL-xL may represent a promising future strategy, potentially overcoming the intrinsic limitations of small molecule inhibitors.