Drug Overview
Gefitinib (brand name Iressa) is an oral targeted therapy belonging to the class of epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors. It is designed to interfere with cell signaling pathways that promote cancer cell proliferation.
Gefitinib is primarily indicated for the treatment of non–small cell lung cancer (NSCLC) in patients whose tumors have activating EGFR mutations. It helps to slow or stop tumor growth and can improve progression-free survival.
Mechanistically, gefitinib binds to the ATP‐binding site of the EGFR tyrosine kinase domain, blocking downstream signaling cascades involved in cell division and survival.
Relevant Genes and Their Roles
CYP2D6 is a liver enzyme that metabolizes many drugs, including gefitinib, through oxidative reactions. It helps convert gefitinib into more water-soluble forms for elimination. Genetic variants in CYP2D6 can alter the enzyme’s metabolic activity.
When CYP2D6 function is reduced, the body clears gefitinib more slowly, leading to higher drug levels. Conversely, increased CYP2D6 activity can lower drug exposure. Understanding these variations can help predict response and risk of side effects.
Impact of Genetics on Drug Response
Individuals with different CYP2D6 metabolizer phenotypes—ultrarapid, normal, intermediate or poor—can have significantly different gefitinib plasma levels. Poor metabolizers may accumulate higher drug concentrations, increasing toxicity risk, while ultrarapid metabolizers may clear the drug too quickly, potentially reducing efficacy.
Want to know what your genetics say about how you'll respond to Gefitinib?
A Gene2Rx report reads your own DNA to show how it may affect your response to Gefitinib and your other medications.
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Expected Clinical Effects of Genetic Variation
Ultrarapid Metabolizer
- Effect on drug levels: Lower than average exposure due to rapid clearance
- Clinical consequence: Potential reduced efficacy of gefitinib
- Side effects: Generally fewer and milder adverse effects; infrequent occurrence
Normal Metabolizer
- Effect on drug levels: Expected standard exposure
- Clinical consequence: Typical therapeutic response
- Side effects: As reported in clinical trials; moderate frequency and severity
Intermediate Metabolizer
- Effect on drug levels: Slightly increased exposure compared to normal
- Clinical consequence: Possible mild increase in efficacy and toxicity
- Side effects: Mild adverse reactions; monitor frequency
Poor Metabolizer
- Effect on drug levels: Higher than average exposure due to reduced clearance
- Clinical consequence: Increased risk of toxicity and adverse reactions
- Side effects: More frequent and severe side effects; close monitoring required
Indeterminate/Not Available
- Effect on drug levels: Unknown
- Clinical consequence: No specific guidance; follow standard dosing with clinical monitoring
- Side effects: Unknown; monitor patient as per usual clinical practice
Dosing Guidelines
The following dosing guidelines are based on the available guidelines for gefitinib metabolism by CYP2D6.
CYP2D6 Dosing Guideline
| Phenotype |
Clinical Consequence |
Guideline Recommendation |
| Ultrarapid Metabolizer |
Likely lower drug exposure; potential reduced efficacy |
Use standard gefitinib dosing; no adjustment needed |
| Normal Metabolizer |
Expected standard exposure and response |
Use standard gefitinib dosing |
| Intermediate Metabolizer |
Slightly increased exposure; monitor for toxicity |
Use standard dosing; monitor for adverse reactions |
| Poor Metabolizer |
Increased exposure and risk of toxicity |
Monitor closely for adverse reactions; consider dose reduction if toxicity occurs |
| Indeterminate / Not available |
Unknown impact |
Initiate standard starting dose |
Alternative Treatment Options
Examples of alternative EGFR-targeted therapies include erlotinib (Tarceva), afatinib (Gilotrif), and osimertinib (Tagrisso). These are provided as examples and do not constitute medical advice.
Genes that affect Gefitinib
See the full list of drugs affected by each gene:
Brand names containing Gefitinib
See how your genetics affect each product Gefitinib shows up in:
Disclaimer: This document is for informational purposes only and is not a substitute for medical advice. Clinical decisions should be made by a qualified healthcare professional.
Interested in learning more about how your genetics may affect your response to medication? Get started with Gene2Rx today.
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