Problem 3 - Brenner and Stevens’ Pharmacology

Question

A woman with chronic myeloid leukemia responds to imatinib therapy as indicated by a cytologic remission. Which enzyme is inhibited by this drug? (A). DNA polymerase (B). dihydrofolate reductase (C). BCR-ABL tyrosine kinase (D). thymidylate synthetase (E). hypoxanthine guanine phosphoribosyltransferase

Step-by-Step Solution

Answer

The enzyme inhibited by imatinib is BCR-ABL tyrosine kinase (option C).

Step 1: Understand the context

The exercise involves identifying which enzyme is inhibited by imatinib, a drug used to treat chronic myeloid leukemia (CML). It is helpful to know that CML is associated with a specific genetic abnormality known as the Philadelphia chromosome, which involves a fusion protein that is crucial for the pathogenesis of this leukemia.

Step 2: Recall the mechanism of imatinib

Imatinib is a targeted cancer therapy drug that specifically inhibits a particular enzyme involved in the pathogenesis of CML. Knowing the specific target of imatinib is key to identifying the correct answer.

Step 3: List of possible enzyme targets

The option list includes different enzymes: DNA polymerase, dihydrofolate reductase, BCR-ABL tyrosine kinase, thymidylate synthetase, and hypoxanthine guanine phosphoribosyltransferase. One of these is the known target of imatinib.

Step 4: Determine the correct enzyme target

Imatinib inhibits the BCR-ABL tyrosine kinase, which is the fusion protein created by the Philadelphia chromosome. The inhibition of this enzyme prevents the proliferation of cancer cells, leading to remission in chronic myeloid leukemia.

Key Concepts

Understanding Chronic Myeloid Leukemia

Chronic Myeloid Leukemia (CML) is a type of cancer that affects white blood cells and tends to progress slowly over time. It stems from a genetic mutation known as the Philadelphia chromosome. This flaw creates an abnormal fusion protein causing unregulated cell division in bone marrow.
This new protein, named BCR-ABL, results from pieces of chromosome 9 and chromosome 22 swapping places. This mutation isn't inherited but occurs later in life due to unknown reasons. The main challenge patients with CML face is a chronic over-production of white blood cells, leading to various symptoms like fatigue, fever, and tenderness below ribs.

Management of CML often involves targeting this specific mutation to control cancer growth and help induce remission. Understanding this biological anomaly is pivotal in tackling the leukemia from its roots.

Imatinib Mechanism of Action

Imatinib is a revolutionary drug in the treatment of Chronic Myeloid Leukemia. It is a type of targeted therapy designed to act where the problem begins. Imatinib functions primarily by inhibiting a crucial enzyme, the BCR-ABL tyrosine kinase, which drives the development of CML. This method contrasts traditional chemotherapy that affects both cancerous and normal cells.

• Imatinib is highly specific, focusing on the fusion protein BCR-ABL, responsible for continuous cell division.
• By blocking this enzyme's activity, imatinib effectively stops the growth of leukemia cells and leads to their gradual death.
• This results in cytologic remission, which means a reduction in cancer cells to manageable levels.

This approach minimizes side effects compared to regular chemotherapy because it targets cancer cells more selectively, offering patients an improved quality of life while under treatment.

The Role of BCR-ABL Tyrosine Kinase

BCR-ABL Tyrosine Kinase plays a central role in the pathogenesis of Chronic Myeloid Leukemia. This abnormal kinase results from a fusion between two proteins due to the Philadelphia chromosome genetic swap.
Normally, tyrosine kinases are enzymes that transfer a phosphate group from ATP to a tyrosine residue in a protein. However, in CML, the BCR-ABL fusion protein is always "on," allowing continuous signaling for cell growth without any external control.

• This relentless signal causes increased white blood cell proliferation, contributing to the trademark features of CML.
• By understanding the behavior of BCR-ABL, researchers were able to design drugs like imatinib to inhibit its activity specifically.

Blocking this kinase is a cornerstone of current CML treatment strategies and represents a significant shift to more personalized medicine in oncology.