Bcl-2 and Bcl-2 inhibitors

Bcl-2 (B-cell lymphoma 2), encoded in humans by the BCL2 gene, is the founding member of the Bcl-2 family of regulator proteins that regulate cell death (apoptosis), by either inducing (pro-apoptotic) or inhibiting (anti-apoptotic) apoptosis. Bcl-2 is specifically considered an important anti-apoptotic protein and is thus classified as an oncogene.

BCL-2 is localized to the outer membrane of mitochondria, where it plays an important role in promoting cellular survival and inhibiting the actions of pro-apoptotic proteins. The pro-apoptotic proteins in the BCL-2 family, including Bax and Bak, normally act on the mitochondrial membrane to promote permeabilization and release of cytochrome C and ROS, that are important signals in the apoptosis cascade. These pro-apoptotic proteins are in turn activated by BH3-only proteins, and are inhibited by the function of BCL-2 and its relative BCL-Xl..

Damage to the Bcl-2 gene has been identified as a cause of a number of cancers, including melanoma, breast, prostate, chronic lymphocytic leukemia, and lung cancer, and a possible cause of schizophrenia and autoimmunity. It is also a cause of resistance to cancer treatments.

Bcl-2 Inhibitors

Genasense -An antisense oligonucleotide drug Genasense (G3139) was developed by Genta Incorporated to target Bcl-2. An antisense DNA or RNA strand is non-coding and complementary to the coding strand (which is the template for producing respectively RNA or protein). An antisense drug is a short sequence of RNA that hybridises with and inactivates mRNA, preventing the protein from being formed.

ABT-737 and ABT-263
In the mid-2000s, Abbott Laboratories developed a novel inhibitor of Bcl-2, Bcl-xL and Bcl-w, known as ABT-737. This compound is part of a group of BH3 mimetic small molecule inhibitors (SMI) that target these Bcl-2 family proteins, but not A1 or Mcl-1. ABT-737 is superior to previous BCL-2 inhibitors given its higher affinity for Bcl-2, Bcl-xL and Bcl-w. In vitro studies showed that primary cells from patients with B-cell malignancies are sensitive to ABT-737. ABT-737 does not directly induce apoptosis; it enhances the effects of apoptotic signals and causes single-agent-mechanism-based killing of cells in small-cell lung carcinoma and lymphoma lines.

Clinical trials studied the effects of venetoclax (ABT-199), a BH3-mimetic drug designed to block the function of the Bcl-2 protein, on patients with chronic lymphocytic leukemia (CLL). Good responses have been reported. A phase 3 trial started in Dec 2015. It was approved by the US FDA in April 2016 for CLL associated with 17-p deletion. This is the first FDA approval of a BCL-2 inhibitor.


Readings and References

1-ABT-199 (venetoclax) and BCL-2 inhibitors in clinical development

2-FDA Approves AbbVie's BCL-2 Targeting Drug for CLL

3-Bcl-2 family proteins and cancer

4-The Bcl-2 family: roles in cell survival and oncogenesis - Review