Insights, challenges, and future directions in irinogenetics

Irinotecan is widely used in the treatment of metastatic colorectal cancer and extensive small-cell lung cancer. Its use is limited by severe toxicities such as neutropenia and delayed-type diarrhea. Irinotecan is converted to its active metabolite SN-38. SN-38 is further metabolized to SN-38G by various hepatic and extrahepatic UGT1A isozymes, mainly UGT1A1. Impaired glucuronidation activity of the UGT1A1 enzyme has been linked with elevated levels of SN-38, leading to toxicities. UGT1A1*28 involves an extra TA repeat in the UGT1A1 promoter region and is the variant most frequently contributing to interpatient variability in irinotecan pharmacokinetics and toxicities. This information led to the revision of the irinotecan label by the US Food and Drug Administration. Recently, UGT1A1*6 seems to contribute to the risk of toxicity of irinotecan in Asian patients. The pharmacogenetics of irinotecan (irinogenetics) is one of few promising examples of the application of pharmacogenetics to individualized drug therapy. This review summarizes ongoing studies and unanswered questions on irinogenetics.     

The role of SN-38 exposure, UGT1A1*28 polymorphism, and baseline bilirubin level in predicting severe irinotecan toxicity

Irinotecan, an anticancer drug, is associated with severe and potentially fatal diarrhea and neutropenia. The objective of this analysis was to evaluate the role of SN-38 exposure, the active metabolite of irinotecan, UGT1A1 genotypes, and baseline bilirubin on the maximum decrease (nadir) in absolute neutrophil counts following irinotecan. This analysis extended the work of a previous study that examined the effect of UGT1A1 genotypes on the incidence of severe neutropenia in 86 advanced cancer patients following irinotecan treatment. Regression analysis showed that the absolute neutrophil count nadir depended on SN-38 exposure (AUC) and UGT1A1*28 homozygous 7/7 genotype. An increased SN-38 AUC and the 7/7 genotype were significantly associated with a lower absolute neutrophil count nadir (R2 = .49). An alternate model suggested that higher baseline bilirubin and the 7/7 genotype were also significantly associated with a lower absolute neutrophil count nadir, although with a lower coefficient of determination (R2 = .31). Based on these findings and other reports, the irinotecan label was modified to indicate the role of UGT1A1*28 polymorphism in the metabolism of irinotecan and the associated increased risk of severe neutropenia. The label modifications also included recommendations for lower starting doses of irinotecan in patients homozygous for the UGT1A1*28 (7/7) polymorphism.     

Polymorphisms of UDP-glucuronosyltransferase and pharmacokinetics of irinotecan.

Irinotecan is a prodrug that is hydrolyzed by carboxylesterase in vivo to form an active metabolite SN-38. SN-38 is further conjugated and detoxified by UDP-glucuronosyltransferase (UGT) to yield its beta-glucuronide (SN-38G). Although irinotecan is widely used, the drug causes unpredictably severe, occasionally fatal, toxicity of leukopenia or diarrhea. Interindividual variation of sensitivity to irinotecan is related to large variations of biotransformation of the active metabolite SN-38, some of which would be caused by genetic polymorphism of UGT1A1, an isozyme responsible for the SN-38 glucuronidation. As a surrogate for the UGT activity, the polymorphic frequency distribution of the area under the concentration-time curve (AUC) ratios of SN-38 to SN-38G (AUC(SN-38)/AUC(SN-38G)) using pooled pharmacokinetic data from four independent study groups in Japan was explored. The data from 100 cancer patients was analyzed, including 14 who were genotyped for UGT1A1 gene in the previous studies. The median ratios of AUC(SN-38)/AUC(SN-38G) was 0.40 (interquartile range, 0.30 to 0.55; range, 0.09 to 2.32). Frequency distribution of the AUC (SN-38)/AUC(SN-38G) was skewed to the right without bimodality and the patient population could not be segregated into discrete subgroups that differ in the UGT activity by the AUC ratios. The 4 subjects carrying UGT1A1*28 allele had values of the AUC(SN-38)/AUC(SN-38G) above the 75th percentile of the total population, suggesting a potential pharmacogenetic/pharmacokinetic relationship. Ordinary values with a median of 0.41 (interquartile range, 0.33 to 0.49) were obtained for the UGT1A1*6 heterozygous patient and the 9 UGT1A1*1 homozygous patients (the reference sequence). The large variation in the UGT activity being related to the genetic status would warrant pharmacogenetic-guided dosing of irinotecan.     

Pharmacogenetics of uridine diphosphoglucuronosyltransferase (UGT) 1A family members and its role in patient response to irinotecan

Glucuronidation, catalyzed by the glucuronosyltransferase (UGT) superfamily, is a major biotransformation pathway for several drugs, including irinotecan. Irinotecan is commonly used in colorectal cancer chemotherapy. Irinotecan undergoes metabolism in humans and is converted to its active metabolite SN-38, a topoisomerase I inhibitor. SN-38 is inactivated via glucuronidation catalyzed by various hepatic and extrahepatic UGT1A isozymes. Although the role of the UGT1A1 *28 genetic variant has received much attention in altered toxicity upon irinotecan treatment, other UGT1A enzymes also play an important role. This review summarizes pharmacokinetic, toxicologic, and pharmacogenetic studies carried out to date in irinotecan and SN-38 disposition.     

Pharmacogenetics of Uridine Diphosphoglucuronosyltransferase (UGT) 1A Family Members and its Role in Patient Response to Irinotecan

Glucuronidation, catalyzed by the glucuronosyltransferase (UGT) superfamily, is a major biotransformation pathway for several drugs, including irinotecan. Irinotecan is commonly used in colorectal cancer chemotherapy. Irinotecan undergoes metabolism in humans and is converted to its active metabolite SN-38, a topoisomerase I inhibitor. SN-38 is inactivated via glucuronidation catalyzed by various hepatic and extrahepatic UGT1A isozymes. Although the role of the UGT1A1 *28 genetic variant has received much attention in altered toxicity upon irinotecan treatment, other UGT1A enzymes also play an important role. This review summarizes pharmacokinetic, toxicologic, and pharmacogenetic studies carried out to date in irinotecan and SN-38 disposition.     

Clinical pharmacogenetics of irinotecan (CPT-11)

Irinotecan (CPT-11) is now widely used, especially for colorectal and lung cancers, whereas the drug causes severe adverse drug reactions (ADR), such as leukopenia/neutropenia or diarrhea. Irinotecan undergoes drug metabolism to form an active SN-38, which is further converted to its beta-glucuronide by UDP-glucuronosyltransferase (UGT) 1A1. A variant in the promoter of UGT1A1 gene, UGT1A1*28 allele, has been extensively studied, and pharmacogenetic relationships between the variant and ADR to irinotecan have been reported. A case-control study of Japanese cancer patients demonstrated that the patients having UGT1A1*28 were at significantly increased risk of severe ADR to irinotecan. To date, genetic variations of the UGT1A1 gene is the most important hereditary factor to predict severe ADR to irinotecan. The UGT1A1*28 is the only one variant that has multiple lines of clinical evidence in multiple races, whereas genetic variations of other UGT isoforms, drug-metabolizing enzymes and drug transporters need more confirmations of its clinical significance in multiple patient groups. At present, irinotecan chemotherapy based on a patient's UGT1A1 genetic status is scientifically reasonable.     

Common human UGT1A polymorphisms and the altered metabolism of irinotecan active metabolite 7-ethyl-10-hydroxycamptothecin (SN-38)

7-Ethyl-10-hydroxycamptothecin (SN-38) is the pharmacologically active metabolite of irinotecan, in addition to being responsible for severe toxicity. Glucuronidation is the main metabolic pathway of SN-38 and has been shown to protect against irinotecan-induced gastrointestinal toxicity. The purpose of this study was to determine whether common polymorphic UDP-glucuronosyltransferase (UGT) affects SN-38 glucuronidation. First, kinetic characterization of SN-38-glucuronide (SN-38-G) formation was assessed for all known human UGT1A and UGT2B overexpressed in human embryonic kidney 293 cells. To assess the relative activity of UGT isoenzymes for SN-38, rates of formation of SN-38-G were monitored by liquid chromatography/mass spectrometry analysis and normalized by level of UGT cellular expression. Determination of intrinsic clearances predicts that hepatic UGT1A1 and UGT1A9 and the extrahepatic UGT1A7 are major components in SN-38-G formation, whereas a minor role is suggested for UGT1A6, UGT1A8, and UGT1A10. In support of the involvement of UGT1A9, a strong coefficient of correlation was observed in the glucuronidation of SN-38 and a substrate, mainly glucuronidate, by UGT1A9 (flavopiridol) by human liver microsomes (coefficient of correlation, 0.905; p = 0.002). In vitro functional experiments revealed a negative impact of the UGT1A1 allelic variants. Residual activities of 49, 7, 8, and 11% were observed for UGT1A1*6 (G(71)R), UGT1A1*27 (P(229)Q), UGT1A1*35 (L(233)R), and UGT1A1*7 (Y(486)D), respectively. Common variants of UGT1A7, UGT1A7*3 (N(129)K;R(131)K;W(208)R), and UGT1A7*4 (W(208)R), displayed residual activities of 41 and 28% compared with the UGT1A7*1 allele. Taken together, these data provide the evidence that molecular determinants of irinotecan response may include the UGT1A polymorphisms studied herein and common genetic variants of the hepatic UGT1A9 isoenzyme yet to be described.     

Role of organic anion transporter OATP1B1 (OATP-C) in hepatic uptake of irinotecan and its active metabolite, 7-ethyl-10-hydroxycamptothecin: in vitro evidence and effect of single nucleotide polymorphisms.

Irinotecan hydrochloride (CPT-11) is a potent anticancer drug that is converted to its active metabolite, 7-ethyl-10-hydroxycamptothecin (SN-38), and other metabolites in liver. The disposition and gastrointestinal toxicity of irinotecan exhibit a wide interpatient variability. Here, we examined the contribution of an organic anion-transporting polypeptide, OATP1B1 (OATP-C), which transports a variety of drugs and their metabolites from blood to liver in humans, to the hepatic disposition of irinotecan, SN-38, and its glucuronide conjugate (SN-38G) by using HEK293 cells stably transfected with SLCO1B1*1a (OATP-C*1a) coding wild-type OATP1B1. We further examined the effect of single nucleotide polymorphisms in OATP1B1 by measuring uptake activity in Xenopus oocytes expressing OATP1B1*1a and three common variants. In all cases, transport activity for SN-38 was observed, whereas irinotecan and SN-38G were not transported. Moreover, SN-38 exhibited a significant inhibitory effect on OATP1B1-mediated uptake of [(3)H]estrone-3-sulfate. Among the variants examined, OATP1B1*15 (N130D and V174A; reported allele frequency 10-15%) exhibited decreased transport activities for SN-38 as well as pravastatin, estrone-3-sulfate, and estradiol-17beta-glucuronide. This study is the first to yield evidence that OATP1B1 is involved in the hepatic disposition of SN-38 and that genetic polymorphisms of OATP1B1 may contribute to the known interpatient variability in disposition of irinotecan.     

Clinical Pharmacogenetics of Irinotecan (CPT-11)

Irinotecan (CPT-11) is now widely used, especially for colorectal and lung cancers, whereas the drug causes severe adverse drug reactions (ADR), such as leukopenia/neutropenia or diarrhea. Irinotecan undergoes drug metabolism to form an active SN-38, which is further converted to its β-glucuronide by UDP-glucuronosyltransferase (UGT) 1A1. A variant in the promoter of UGT1A1 gene, UGT1A1*28 allele, has been extensively studied, and pharmacogenetic relationships between the variant and ADR to irinotecan have been reported. A case-control study of Japanese cancer patients demonstrated that the patients having UGT1A1*28 were at significantly increased risk of severe ADR to irinotecan. To date, genetic variations of the UGT1A1 gene is the most important hereditary factor to predict severe ADR to irinotecan. The UGT1A1*28 is the only one variant that has multiple lines of clinical evidence in multiple races, whereas genetic variations of other UGT isoforms, drug-metabolizing enzymes and drug transporters need more confirmations of its clinical significance in multiple patient groups. At present, irinotecan chemotherapy based on a patient's UGT1A1 genetic status is scientifically reasonable.     

Pharmacogenomic research for avoiding adverse reactions by anti-cancer drugs

Anti-cancer drugs have relatively low effective rates and high frequencies of adverse reactions, occasionally leading to cessation of their treatments. Use of pharmacogenomic (PGx) information could be able to select the patients with high-response and less-adverse reactions, resulting in increase of patients' QOL and proper use of drugs. We have been collaborating with National Cancer Center for PGx analysis of anti-cancer drugs including irinotecan and gemcitabine in Japanese cancer patients. Irinotecan, now used for treatments of many cancers, is metabolically activated to SN-38 and then inactivated to SN-38 glucuronide by a UDP-glucuronosyltransferase UGT1A1. In the UGT1A1 gene, two representative genetic polymorphisms, *28 and *6, were detected at 0.138 and 0.167, respectively in 177 Japanese cancer patients. When the patients were homozygotes of *28 or *6, or compound heterozygotes of them, statistically significant decreases were observed in the SN-38 glucuronidation activity and increases in the rate of severe neutropenia, compared to those in the patients without *28 or *6. Our results and papers were cited in the Japanese package inserts of irinotecan. Gemcitabine was inactivated by cytidine deaminase (CDA) into 2'-2'-difluorodeoxyuridine. A CDA polymorphism 208G>A (Ala70Thr) was detected at 0.037 frequency in 256 Japanese cancer patients and associated with reduced gemcitabine clearance as well as increased frequency of severe neutropenia. In the 4 patients suffered from very severe bone marrow toxicities, 3 patients were homozygous CDA*3, suggesting that this polymorphism is exquisite for predicting severe adverse reactions by gemcitabine in Japanese.     

Pharmacogenetics of irinotecan

Pharmacogenetics focuses on intersubjects variation in therapeutic drug effects and toxicity depending on genetic polymorphisms. This is particularly interesting in oncology since anticancer drugs usually have a narrow margin of safety. Irinotecan [7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin] is used in cancer chemotherapy as a topoisomerase I inhibitor and it is characterised by a sometimes unpredictable severe toxicity. It is mostly intestinal with nausea, vomit and diarrhoea or haematologic with leuko-thrombocytopenia. Its complex metabolism involves many proteins. Human carboxylesterase isoforms 1 and 2 (hCE1, hCE2) activate irinotecan to its metabolite SN-38 (7-ethyl-10-hydroxycamptothecin); cytochrome P450 isoforms 3A4 and 3A5 (CYP3A4, CYP3A5) mediate the oxidation of the parental compound to irinotecan; uridino-glucuronosil transferase isoform 1A1 (UGT1A1) catalyses glucuronidation of SN-38; the multi-resistance protein isoform 2 (MRP2) allows the cellular excretion of the SN-38 glucuronide (SN-38G) and the multi-drug resistance gene (MDR1), encoding for P-glycoprotein, is responsible for the excretion of irinotecan from the cell. Polymorphic structures in the genes encoding for all these proteins have been described. In particular, the UGT1A1*28 allele has been associated with an increased toxicity after irinotecan chemotherapy. Classical parameters used in the clinic, such as body-surface area, have no longer a meaningful correlation with clinical outcome. Hence it emerges the importance of studying the individual genotype to predict the toxicity and efficacy of irinotecan and to individualise therapy. In this review, we summarise the new developments on the study of the pharmacogenetics of irinotecan, stressing its importance in drug cytotoxic effect.     

Pharmacogenetics of irinotecan toxicity

Irinotecan is an anticancer drug approved in combination therapy for advanced colorectal cancer. Severe, life-threatening toxicities can occur from irinotecan treatment. Although multiple genes may play a role in irinotecan activity, the UDP glycuronosyltransferase 1 family, polypeptide A1 (UGT1A1) enzyme has been strongly associated with toxicity. A common dinucleotide repeat polymorphism in the UGT1A1 promoter region (UGT1A1*28) has been correlated with severe toxicity in cancer patients receiving irinotecan-containing therapy. Prospective screening of patients prior to chemotherapy selection may reduce the frequency of severe toxicities by allowing alternate therapy selections for patients carrying the UGT1A1*28 polymorphism.     

UGT1A1基因多态性与伊立替康所致不良反应的相关性分析

目的：研究本院肿瘤患者UGT1A1基因多态性分布,探讨UGT1A1基因多态性与伊立替康所致不良反应的相关性。方法：回顾性分析2016~2018年就诊于本院并接受伊立替康治疗的肿瘤患者UGT1A1基因多态性分布,分析用药后不良反应发生情况,比较不同基因型之间的差异。结果：共计76例肿瘤患者被纳入分析。76例患者均行UGT1A1*28基因检测（突变比率23.68%）,其中有45例患者同时行UGT1A1*6基因检测（突变比率24.44%）。UGT1A1*28基因突变患者发生Ⅲ~Ⅳ度白细胞减少的风险显著高于野生型（OR=10.79,95% CI：1.24~93.86, P=0.016）。伊立替康化疗引起的其他不良反应,包括血小板减少、中性粒细胞减少、腹泻、肝损伤在本研究中未显示出与UGT1A1基因型的显著相关性（P>0.05）。结论：UGT1A1*28基因多态性与伊立替康引起的严重白细胞减少相关。     

Glucuronidation of 7-ethyl-10-hydroxycamptothecin (SN-38), an active metabolite of irinotecan (CPT-11), by human UGT1A1 variants, G71R, P229Q, and Y486D.

7-Ethyl-10-hydroxycamptothecin (SN-38), an active metabolite of antitumor agent irinotecan (CPT-11), is conjugated and detoxified to SN-38-glucuronide by UDP-glucuronosyltransferase (UGT) 1A1. Genetic polymorphisms in UGT1A1 are thought to contribute to severe diarrhea and/or leukopenia caused by CPT-11. In this regard, it has been reported that polymorphisms in the promoter region could affect the CPT-11 pharmacokinetics and interindividual variation of toxicity. However, little information is available on the influence of UGT1A1 polymorphisms in the coding region on the SN-38 glucuronidation activity. In the present study, wild-type (WT) and three variant (G71R, P229Q, and Y486D) cDNAs of human UGT1A1s were transiently expressed in COS-1 cells, and the kinetic parameters of these UGT1A1s were determined for SN-38 glucuronidation. A partially reduced UGT1A1 protein expression was observed in COS-1 cells for G71R and Y486D. WT UGT1A1 catalyzed SN-38 glucuronidation with an apparent K(m) value of 11.5 microM, whereas those of G71R, P229Q, and Y486D were 14.0, 18.0, and 63.5 microM, respectively. The SN-38 glucuronidation efficiency ratio (V(max)/K(m)) normalized for the level of expression was 1.4, 0.66 (47% of WT), 0.73 (52%), and 0.07 (5%) microl/min/mg of protein for WT, G71R, P229Q, and Y486D, respectively. Thus, the SN-38 glucuronidation activity of Y486D was drastically reduced, whereas the reduction in the G71R and P229Q activities was fractional. The decreased SN-38 glucuronidation efficiency ratio of G71R and P229Q could be critical in combination with other polymorphisms in the UGT1A1 gene.     

Identification and characterization of a functional TATA box polymorphism of the UDP glucuronosyltransferase 1A7 gene

UDP glucuronosyltransferases (UGT) detoxify bilirubin and therapeutic drugs, a process influenced by single nucleotide polymorphisms (SNPs) in their structural genes and promoter elements. UGT1A1*28 is a functional UGT promoter polymorphism associated with Gilbert's disease and severe irinotecan toxicity, which also occurs in the absence of UGT1A1*28. The aim of this study was to identify and characterize UGT promoter variants relevant for irinotecan detoxification. Recombinant UGT1A proteins were analyzed for irinotecan metabolite glucuronidation by UGT activity assays. In 427 healthy blood donors and 71 homozygous UGT1A1*28 carriers, the 5'-untranslated region of the UGT1A7 gene locus was studied. An SNP was detected by allelic discrimination and characterized by reporter gene experiments. A novel -57 T--> G SNP with a gene frequency of 0.39 in healthy blood donors was identified in the putative TATA box of the UGT1A7 gene, reducing promoter activity to 30%. It is in linkage dysequilibrium with a variant of the UGT1A7 first exon that is present in the reduced-activity UGT1A7*3 and UGT1A7*4 alleles. Homozygous UGT1A1*28 carriers simultaneously carried this variant in 97%. We identified a novel reduced-function TATA box SNP of the UGT1A7 gene that catalyzes irinotecan metabolite detoxification. Its association with variants of the UGT1A1 promoter and UGT1A7 gene may influence irinotecan metabolism. Our finding emphasizes the importance of combinations of structural and regulatory gene polymorphisms that may be useful as markers of drug toxicity.     

浙江台州地区使用伊立替康人群UGT1A1基因多态性相关性研究

目的 研究浙江台州地区使用伊立替康（CPT-11）人群UGT1A1基因多态性和不良反应的相关性。方法 以使用含CPT-11化疗的132例台州地区汉族肿瘤患者为研究对象,取其外周血提取基因组DNA,进行UGT1A1基因多态性检测。结果 132例以CPT-11为基础化疗方案的台州地区肿瘤患者中UGT1A1*28 TA（6/6）野生型93例（70.45%）,TA（6/7）杂合突变型共36例（27.27%）,TA（7/7）纯合突变型仅3例（2.27%）;UGT1A1*6 G/G野生型共97例（73.48%）,G/A杂合突变型共35例（26.52%）,未找到A/A纯合突变型。UGT1A1*28非野生型（6/7+7/7）患者发生腹泻的概率显著高于野生型患者（P=0.040）。而粒细胞减少、血小板减少、血红蛋白减少与UGT1A1*28基因多态性无显著性差异。迟发性腹泻、粒细胞减少、血小板减少、血红蛋白减少水平与UGT1A1*6基因多态性无显著性差异。结论 浙江台州地区UGT1A1基因突变频率较高,TA（6/6）野生型人群相比TA（6/7）和TA（7/7）,CPT-11使用后的腹泻风险增加。建议浙江台州地区肿瘤患者使用CPT-11化疗前进行UGT1A1基因多态性检测,以预测患者对CPT-11的耐受性,保证化疗的顺利进行。     

Genetic polymorphisms of SLCO1B1 for drug pharmacokinetics and its clinical implications

Various drug transporters are selectively expressed in single or multiple tissues, such as the intestine, liver and kidney, where these transporters play various roles in drug absorption, distribution and excretion. Genetic polymorphisms in drug transporters as well as drug-metabolizing enzymes are associated with interindividual differences in drug disposition, efficacy and toxicity. Organic anion transporting polypeptide 1B1 (OATP1B1, gene SLCO1B1) is expressed on the basolateral membrane of hepatocytes and can facilitate hepatic uptake of certain clinically relevant drugs such as statins except for fluvastatin, angiotensin converting enzyme inhibitors, angiotensin II receptor antagonists, antidiabetic drug (repaglinide) and anticancer drugs (SN-38 and methotrexate). Some single nucleotide polymorphisms or haplotypes of the SLCO1B1 gene have been identified and demonstrated to have functional significance for transporter activity. For examples, the SLCO1B1*15 haplotype (or 521T>C genotype) results in decreased uptake activity of SN-38 from systemic circulation, leading to increased plasma concentration of SN-38 and an enhanced risk of neutropenia. This review focuses on the impact of genetic polymorphisms of the SLCO1B1 gene on transport activity, and implications for the clinical efficacy and toxicity of clinically useful drugs.     

Pharmacogenetics and irinotecan therapy.

PURPOSE: Irinotecan metabolism, irinotecan pharmacogenetic research, and the role of genetic testing before administration of the drug are reviewed. SUMMARY: Irinotecan is approved worldwide for the treatment of metastatic colorectal cancer but causes dose-limiting neutropenia and diarrhea. When severe, these can lead to dehydration, infection, patient discomfort, additional medication requirements, hospitalization, and death. The identification of predictive markers in irinotecan therapy has been a significant goal of pharmacogenetic research. The labeling of irinotecan was recently changed and now includes a warning of greater neutropenia risk in patients with reduced activity in the drug-metabolizing enzyme uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1). A known marker of reduced UGT1A1 activity is the genetic variant UGT1A1*28. Numerous studies have demonstrated the effects of genetic factors, especially UGT1A1*28, that contribute to interpatient variability in irinotecan pharmacokinetics and toxicity. Irinotecan's new labeling recommends that clinicians consider reducing the dosage of irinotecan in patients homozygous for UGT1A1*28. CONCLUSION: At least part of the interpatient variability of irinotecan toxicity can be explained by the UGT1A1*28 polymorphism. Patients who are homozygous for the UGT1A1*28 allele have an increased risk of developing severe neutropenia when receiving irinotecan, especially the 300-350- mg/m2 regimen. A molecular assay is now available to identify the at-risk subgroup and should be used by health care professionals to help guide irinotecan-treatment decisions.