It is well known that the chemotherapeutic regimen of irinotecan (CPT-11, Camptosar) in combination with 5-fluorouracil and leucovorin is an effective treatment for one of the most common forms of cancer in the western world, metastatic colorectal carcinoma.(1) Like many chemotherapeutic agents, irinotecan is known to cause a number of adverse drug events (ADE).  Of greatest concern is the development of severe myelosuppression (in particular neutropenia) that may be life-threatening.   Patients at greatest risk for this are those over the age of 65, those having previously received pelvic/abdominal irradiation, patients with low performance status, and patients heterozygous (TA6/TA7) or homozygous (TA7/TA7) for UGT1A1*28 allele.(1,2)  

What does heterozygous and homozygous mean? 
Since all humans have 2 copies of a gene coding sequence (or allele), a person is heterozygous if they carry 1 copy of the normal gene and 1 copy of the mutant gene and are homozygous if they have two identical copies of the mutant gene (or gene variation).(3)  It is this genetic polymorphism (or variation) or risk factor that is the focus of this newsletter issue.  It is estimated that approximately 10% of the North American population is homozygous for this allele.(1) 

What role does UGT1A1*28 polymorphism play in causing irinotecan-induced myelosuppression?  
When cells replicate, each strand of their DNA is copied.  In order to prevent "knotting" of the unwinding DNA, topoisomerase I is present to relieve torsional strain on the negative supercoiled DNA and to help reconnect the broken DNA strands.(3)  Without topoisomerase I, DNA replication and cell division would cease and the cell would be rendered dysfunctional and die.  Thus, functional topoisomerase I is highly desirable for unregulated, rapidly replicating cancer cells.  Irinotecan and its main active metabolite, SN-38, bind to topoisomerase I-DNA complex impairing division of cancer cells.(4)  Normally irinotecan is metabolized to its active and lipophilic metabolite, SN-38, which is known to be about 1,000 more potent at inhibiting topoisomerase I than its parent drug.(1,4)  As such, anything that prevents the metabolism or elimination of SN-38 can have a profound impact on cell division and lead to pronounced toxicities.  SN-38 is metabolized via glucuronidation to SN-38G by uridine diphosphate glucuronosyltransferase (UGT) to a more water soluble metabolite that is readily available for renal elimination.  UGT1A1 is the enzyme primarily responsible for the conversion of SN-38 to SN-38G.  In addition, UGT1 is responsible for conjugating bilirubin.    Unfortunately, UGT1A1 is subject to genetic polymorphisms that can directly impact its gene expression resulting in both a decreased availability for metabolizing irinotecan and familial unconjugated hyperbilirubinemia diseases such as Gilbert's syndrome and Crigler-Najjar syndrome types I and II.(5).  It appears that UGT1A1*28 polymorphism results in the promoter region of a gene coding sequence that has a greater number (TA7/TA7 or TA6/TA7) of TA (thymine and adenine) repeats than normal (TA6/TA6) wild-type patients.7This directly affects the ability of RNA polymerase binding to the promoter region just prior to the gene coding sequence of interest and thereby prevents the gene from being transcribed and produced.(3)  Therefore, patients with UGT1A1*28 make less UGT1A1 than normal patients and thus cannot efficiently metabolize the potent irinotecan metabolite, SN-38.(1,2)  As a result, a more profound inhibition of DNA replication occurs.  Since the bone marrow is a place for both continual white blood cell replication and turnover, these cells (in addition to the cancer cells) are significantly impacted.(1,2)  The prevalence of developing grade 4 neutropenia (neutrophil count < 1000 cells/microliter) has been reported to be 50% in TA7/TA7 patients and 12.5% in TA6/TA7 patients for UGT1A1*28.(2)  It is apparent that other risk factors or variables also contribute to the risk of developing neutropenia since the prevalence is not 100%. 

Fortunately, there are now genetic tests that can be done to determine a patient's genotype (genetic makeup or profile) for various genes to determine if there are variations compared to the norm.  One example of an FDA-approved test is the Invader® UGT1A1 Molecular Assay by Third Wave Technologies.(8)  This test is recommended by the manufacturer and can be used in clinical practice to determine if the patient will require a dose reduction for irinotecan.  Lastly, the National Comprehensive Cancer Network clinical guidelines for the treatment of colon cancer state, "Irinotecan should be used with caution and with decreased doses in patients with Gilbert's disease or elevated serum bilirubin.  There is a commercially available test for UGT1A1.  Guidelines for use in clinical practice have not been established."(6)

References:

1. Irinotecan (Camptosar) product package insert.  Pfizer and Pharmacia & Upjohn Co.  New York, NY. July 2008.  Last accessed on 1-27-09.
2. Innocent F, Undevia SD, Iyer L et al.  Genetic variants in the UDP-glucuronosyltransferase 1A1 gene predict risk of severe neutropenia of irinotecan.  J Clin Oncol  2004;22:1382-8. 
3. Leiberman M, Marks AD, eds.  Mark's Basic Medical Biochemistry A Clinical Approach.  3^rd Ed.  Philadelphia, PA: Lippincott Williams & Wilkins; 2009:479-566.
4. Kawato Y, Aonuma M, Hirota Y et al.  Intracellular roles of SN-38, a metabolite of the camptothecin derivative CPT-11, in the antitumor effect of CPT-11.  Cancer Res  1991;51:4187.  \
5. Sampietro M, Iolascon A.  Molecular pathology of Crigler-Najjar type I and II and Gilbert's syndrome.  Haematologica  1999;84:150-7.  
6. National Comprehensive Cancer Network (NCCN).  Guidelines for the treatment of colon cancer.  
7. Beutler E, Gelbart T, Demina A.  Racial variability in the UDP-glucuronosyltrasnferase1 (UGT1A1) promoter: a balanced polymorphism for regulation of bilirubin metabolism.  Proc Natl Acad Sci USA  1998;95:8170-4. 
8. Food and Drug Administration.  FDA News. August 22, 2005.  Last accessed on 1-27-09.

UGT1A1*28, Irinotecan, CPT-11, Camptosar, Neutropenia, BMS, Bone Marrow Suppression