Irinotecan (IRI) is an anti-proliferative cytotoxic agent used to treat metastatic colorectal cancer Articles: 16489087, 16895999. Colorectal cancer is currently the most common cancer seen in the United States and is the second leading cause of cancer-related fatality Article: 23516488. Irinotecan is a prodrug so it must be metabolized, requiring metabolism in the liver to activate the Topoisomerase I inhibiting component, SN-38. This metabolite contributes to the dose limiting toxicities: such as myelotoxicity, neutropenia, and most prominently, diarrhea Articles: 23386248, 18797458. IRI is used in combination therapy; thus, drug-drug interactions contribute to variance in toxicity, in addition to pharmacogenomics and differences in patient intestinal microflora Article: 16489087.
IRI uptake and transport into the liver is facilitated by: OATP1B1 (SLCO1B1), ABCB1, MRP1 (ABCC1), MRP2 (ABCC2), and MXR (ABCG2). Specifically, ABCB1 is present on the bile membrane and is responsible for the secretion of IRI and its metabolites into the liver Articles: 16895999, 18221820. IRI is metabolized in the liver and converted to SN-38, the active metabolite and Topoisomerase I inhibitor, by carboxylesterases (CES) mediated hydrolysis. CES1 and CES2 have a low affinity for IRI, resulting in a small percentage of IRI converted to active SN-38 (<3%) Articles: 23386248, 23516488. SN-38 targets Topoisomerase I, inhibiting DNA replication in cancer cells and leading to cell death. SN-38 is then glucoronized to SN-38 glucoronic acid and detoxified in the liver via conjugation by the UGT1A1 family, which releases SN-38G into the intestines for elimination Articles: 23516488, 18221820, 16489087. Approximately 70% of SN-38 becomes SN-38G, which has 1/100 of the antitumor activity and is virtually inactive Article: 8706020. In the intestinal lumen, bacterial beta-glucoronidases can reverse the reaction and transform inactive SN-38G back into the active form SN-38. This is a factor contributing to varied toxicity, specifically dose limiting diarrhea Articles: 16489087, 19293816.
Simultaneously, and competing with the activation and detoxification pathways of IRI, the SN-38 oxidation pathway is mediated by the P450 CYP3A genes. Oxidation of IRI results in the inactive metabolites APC (7-ethyl-10-[4-N-(5-aminopentanoic acid)-1-piperidino] carbonyloxycamptothecin) and NPC (7-ethyl-10-[4-(1-piperidino)-1-amino] carbonyloxycamptothecin). NPC can further be metabolized into SN-38 by CES1 and CES2. Both CYP3A4 and UGT1A1 are responsible for elimination of IRI, and do so by limiting the amount and timing of the active product SN-38 Articles: 20068078, 18221820. All of IRI's metabolites are pH sensitive and thus are at risk of transforming from inactive to active products, and vice versa Article: 18797458.
Metabolism of IRI results in three main dose limiting toxicities: myelosuppresion, diarrhea, and neutropenia. All three of these can vary in response to UGT1A1 gene variation since UGT1A1 mediates the detoxification and conjugation of the active component SN-38 Article: 23386248. The cytotoxicity of SN-38 is significantly higher than IRI itself Article: 10815927.
Neutropenia is directly related to the concentration of SN-38 in plasma with higher rates of SN-38 secretion resulting in higher rates of neutropenia. SN-38 also plays a central role in late onset diarrhea, as it is caused by excessive biliary secretion of SN-38 in the lumen of the intestines. It is estimated that 70% of patients receiving IRI treatment suffer from diarrhea, which makes an increased dosing schedule difficult. Approximately 55% of these patients receive a lower dose than scheduled after 1 month Article: 16489087. Acute diarrhea is caused by the anti-cholinesterase activity of IRI, which destroys the secretory and absorptive functions and properties of the intestinal mucosa. SN-38 has mitotic inhibitory properties, which also contributes to structural and functional defects in the intestinal lumen Article: 8706020.
Dose limiting diarrhea is a result of damaged intestinal mucosa caused by SN-38's biliary secretory mechanism Article: 16489807. If the rate of SN-38 glucoronidation in the intestines is low, active SN-38 is exposed to the epithelial cells for a longer length of time, resulting in severe diarrhea. Thus, glucoronidation of SN-38 is imperative for limited gastrointestinal toxicity Articles: 1689599, 18221820, 12181437. Once SN-38G is excreted in the feces, it risks re-activation by bacterial beta-glucoronidases to SN-38. The active SN-38 further destroys the intestinal epithelial cells leading to delayed severe diarrhea.
Variations in patient microflora may play two roles in toxicity: septicemia by displaced bacteria, and/or contribution to mucosal damage. In a rat model, it was estimated that 10% of the bacterial microflora in the cecum and feces have beta-glucoronidase activity. Patients with neutropenia are at risk for septicemia, as the muscosal damage by SN-38 may lead to bacterial translocation Article: 16489087. In addition, both IRI and SN-38 do not have any antibacterial mechanics, so unless treated with antibiotics or alkylates, these beta-glucoronidases are not limited Article: 8706020. It is advised to co-administer oral alkalinization to raise the pH of the intestinal lumen making it harder for the beta-glucoronidases to deconjugate inactive SN-38G Article: 16489087.
Variation in dosing of IRI and its toxic affects are not only dependent on environmental factors but on genetic variance in the UGT1A, CYP3A, and ABC gene families.
The UGT1A family is responsible for conjugation of the active SN-38 to inactive SN-38G. The UGT1A1 gene locus has 13 exons, which are alternatively spliced resulting into several mRNA isoforms that encode a varied spectrum of active enzymes specifically in the liver that are responsible for drug metabolism. The UGT1A1*28 (rs8175347) allele is characterized by having a 7th dinucleotide repeat in the TATA box of the promoter region, as opposed to the UGT1A1*1 allele with 6 repeats. This increased number of repeats results in decreased rates of transcription, initiation, expression, and enzyme activity. With a decrease in these rates, there is a decrease in SN-38 detoxification, and the exposure time of active SN-38 in the intestines is prolonged. Thus, patients homozygous or heterozygous for the UGT1A1*28 commonly develop dose limiting severe neutropenia and late diarrhea Articles: 23386248, 23516488, 18797458. UGT1A1*28 is prevalent within the Caucasian and African American populations with frequencies of 0.26-0.31 and 0.42-0.56 respectively (Articles: 10591539, 9653159). Conversely, the frequency of this allele in the East Asian populations is only 0.09-0.16, but, the UGT1A1*6 (rs4148323) variant has a frequency of 0.23 in the Korean and Chinese populations (Article: 9784835). UGT1A1*6 leads to decreased expression and increased toxicity similar to UGT1A1*28. Articles: 23303296, 23236239. Like the UGT1A1*28 polymorphism, the UGT1A1*6 allele can cause decreased enzyme activity in the heterozygous or homozygous form. Patients homozygous for UGT1A1*6 allele are at great risk for grade 4 neutropenia Articles: 23386248, 23303296, 18221820. Not only do the UGT1A1*6 and UGT1A1*28 alleles affect toxicity, but they also affect the metabolic rate. These polymorphisms are directly related to glucoronidation rates as the concentration of hepatic SN-38G is based upon UGT1A1 expression Articles: 23236239, 12181437. Individuals heterozygous for the UGT1A1*6 or UGT1A1*28 allele typically suffer from increased toxicity rates as these alleles can have up to ½ of the glucuronidating enzyme activity versus the UGT1A1*1 Article: 12181437.
In addition, UGT1A7 and UGT1A9 haplotypes have variants involved in inter-patient toxicity differences. The UGT1A7 gene is primarily involved in extra-hepatic metabolism, while the UGT1A9 gene is necessary for conjugation of SN-38 to SN-38G in the liver Article: 23386248. Homozygous UGT1A7*2 haplotype patients display higher enzyme activity as compared to the UGT1A7*3 haplotype and UGT1A7*1 allele carriers. UGT1A7 is located in the intestine and is responsible for detoxifying SN-38, but the UGT1A7*3 and UGT1A7*4 haplotypes display less activity and inhibit SN-38 conjugation Articles: 23303296, 12181437. UGT1A9 expressed in the liver has the highest affinity for SN-38, therefore, it is the principal enzyme involved in catalyzing the glucoronidation of SN-38 to inactive SN-38G. Patients homozygous for UGT1A9*1 results in more severe diarrhea than patient carriers of UGT1A9*9 or UGT1A9*22 (rs3832043). UGT1A9*22 is found predominantly in Eastern Asian populations, and leads to higher enzyme expression and glucuronidation rates. Articles: 23303296, 23236239, 18221820.
CYP3A4 & CYP3A5
The CYP3A4 and CYP3A5 genes contribute to varied metabolic rates. Both of these genes are crucial for oxidative metabolism of IRI to inactive APC and NPC. The CYP3A4* 16 (rs12721627) 661 G>C variant has shown decreased activity in vitro and consequently could result in a decreased APC product. This is attributed to the higher Km value associated with the allele. Similarly, the CYP3A4*18 (rs28371759) allele leads to a reduction in the catalysis of this pathway due to a larger Vmax value. The CYP3A5 gene is important for metabolism of IRI in the liver, and the CYP3A5*3 (rs776746) allele undergoes alternative splicing that eliminates CYP3A5 expression and severely decreases the rates of oxidative metabolism, thus less APC and NPC metabolite production.Articles: 20847137, 18221820.
The ABC gene class isotype variance affects the distribution of hepatic mRNA. The ABCC2 gene is an important transporter facilitating secretion of IRI and its metabolites from hepatocytes. The ABCC2 * 2 haplotype is beneficial in that it is associated with lower rates of IRI induced diarrhea, but the ABCC2 3972 C>T (rs3740066) variant is a predictor of grade 3 diarrhea Articles: 16895999, 18221820. The ABCG2 gene is found in the intestine and colon and actively transports IRI. Similar to 3972 >2, the ABCG:34G>A (rs2231137) variant is associated with grade 3 diarrhea. In vitro, it has been demonstrated in LLC-PKI cells with the 34G> A variant has a diminished efflux activity, and there is a lack of apical membrane localization, which results in prolonged exposure of SN-38 and IRI to the intestines leading to late onset diarrhea. Patients with at least one ABCG2 A allele have a greater risk of developing this grade 3 diarrhea as compared to the G allele Articles: 16895999, 18221820.
M. Whirl-Carrillo, E.M. McDonagh, J. M. Hebert, L. Gong, K. Sangkuhl, C.F. Thorn, R.B. Altman and T.E. Klein. "Pharmacogenomics Knowledge for Personalized Medicine" Clinical Pharmacology & Therapeutics (2012) 92(4): 414-417. Full text
If you would like to reproduce this PharmGKB pathway diagram:
Entities in the Pathway
Drugs/Drug Classes (2)
Relationships in the Pathway
|Arrow From||Arrow To||Controllers||PMID|
|irinotecan||M4||CYP3A4, CYP3A5||10815927, 11602529|
|irinotecan||NPC||CYP3A4, CYP3A5||10815927, 11602529|
|irinotecan||SN-38||BCHE, CES1, CES2||10197614|
|SN-38||SN-38G||UGT1A1, UGT1A9||11990381, 12181437, 12730278, 9466980|
|SN-38||SN-38||ABCB1, ABCC2, ABCG2||12810652|
Download data in TSV format. Other formats are available on the Downloads/LinkOuts tab.