## BCL-2 Inhibition in CLL **Key Point:** Venetoclax is a selective BCL-2 inhibitor (BH3-mimetic) that directly binds to the BCL-2 protein and displaces pro-apoptotic proteins (BAX/BAK), restoring the intrinsic apoptotic pathway. ### Mechanism of Action Venetoclax mimics the BH3 domain of pro-apoptotic proteins, competitively binding to the hydrophobic groove of BCL-2. This releases sequestered BAX and BAK, allowing them to oligomerize on the mitochondrial outer membrane and trigger cytochrome c release, activating the caspase cascade. ### Clinical Context in CLL - **High-Yield:** Venetoclax is FDA-approved and guideline-recommended for relapsed/refractory CLL and treatment-naïve CLL (especially with TP53 mutations or del(17p)). - BCL-2 overexpression is a hallmark of CLL pathogenesis, making it an ideal therapeutic target. - Venetoclax + rituximab (anti-CD20 monoclonal antibody) is a standard combination regimen. ### Comparison with Alternatives | Drug | Mechanism | Target | Role in CLL | |------|-----------|--------|-------------| | Venetoclax | BH3-mimetic, BCL-2 inhibitor | Intrinsic pathway | First-line for high-risk disease | | Imatinib | Tyrosine kinase inhibitor | BCR-ABL (CML), KIT, PDGFR | Not used in CLL; for CML | | Rituximab | Anti-CD20 monoclonal antibody | Extrinsic pathway (ADCC/CDC) | Adjunct; not monotherapy for CLL | | Fludarabine | Nucleoside analog | DNA synthesis inhibitor | Older chemotherapy; less effective than venetoclax | **Clinical Pearl:** Venetoclax requires careful monitoring for tumor lysis syndrome (TLS) due to rapid apoptosis of leukemic cells, especially in high disease burden patients. **Warning:** ~~Imatinib~~ is the drug of choice for chronic myeloid leukemia (CML), not CLL. Do not confuse BCR-ABL-driven leukemias with BCL-2-driven lymphomas/leukemias.
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