Cloning and initial characterization of the human DPYD gene promoter

Variants of carboxylesterase 1 have no impact on capecitabine pharmacokinetics and toxicity in capecitabine plus oxaliplatin treated-colorectal cancer patients

PURPOSE

Capecitabine is a prodrug that undergoes metabolism in three steps to form an active 5-fluorouracil (5-FU). The first step is primarily catalyzed by liver carboxylesterases (CES) 1. Here, we examined the effects of CES1 variants on pharmacokinetics and toxicity of capecitabine.

METHODS

We enrolled postoperative colorectal cancer (CRC) patients administered with adjuvant capecitabine plus oxaliplatin (CapeOX) and metastatic CRC patients receiving CapeOX. The pharmacokinetic analysis of the first capecitabine dose (1000 mg/m²) was done on day 1, and oxaliplatin administration was shifted to day 2. Plasma concentrations of capecitabine, 5'-deoxy-5-fluorocytidine, 5'-deoxy-5-fluorouridine (5'-DFUR), and 5-FU were analyzed by high-performance liquid chromatography. CES1 polymorphisms (rs3217164, rs2244614, rs2244613, rs7187684, and rs11861118) and the functional CES1 genes (1A1, var1A1, 1A2, and pseudo 1A3) in their diplotype configurations were analyzed by direct sequencing.

RESULTS

Thirty-seven patients were enrolled from September 2017 to February 2020. Patients with a higher area under the plasma concentration-time curve to capecitabine dose ratio (AUC/dose) of 5'-DFUR than its mean showed a higher frequency of overall ≥ grade 3 toxicity and lower relative dose intensity (RDI) of capecitabine than those with a lower ratio. Higher CES1 activity expressed as a metabolic ratio (AUC of capecitabine/sum of three AUCs of each metabolite) lower than its mean was associated with higher 5'-DFUR AUC/dose and lower RDI, indicating essential roles of CES1 in capecitabine activation to produce 5'-DFUR. However, the association between CES1 variants and capecitabine pharmacokinetics and toxicity was not significant.

CONCLUSION

CES1 variants are not associated with capecitabine pharmacokinetics and toxicity.

A rapid HPLC-ESI-MS/MS method for determination of dihydrouracil/uracil ratio in plasma: evaluation of toxicity to 5-flurouracil in patients with gastrointestinal cancer

BACKGROUND

A liquid chromatography-tandem mass spectrometry method for the simultaneous quantitation of endogenous uracil (U) and dihydrouracil (UH2) was developed and tested in a Brazilian population of patients with gastrointestinal cancer previously exposed to 5-fluorouracil (5FU).

METHODS

The analytes were extracted by a liquid-liquid method using 5-clorouracil as internal standard. The separation was performed on a reversed-phase XTerra C18 column with a mobile phase composed of methanol and aqueous 0.1% ammonium hydroxide (15:85). Mass spectrometry detection was carried out using negative electrospray ionization and selected reaction monitoring. Bovine serum albumin was employed as an alternative matrix to prepare the calibration standards, aiming to avoid the measurement of physiologic U and UH2. Calibration curves were constructed over the range of 5-200 ng/mL for U and 10-500 ng/mL for UH2.

RESULTS

The mean RSD values in the intrarun precision were 6.5% and 10.0% and in the interrun precision were 7.8% and 9.0% for U and UH2, respectively. The mean accuracy values were within the range of 90%-110% for both analytes. The analytes were stable in plasma under different conditions of temperature and time. The validated method was successfully applied to determine the plasma concentrations of U and UH2 in patients with gastrointestinal cancer (n = 32) previously treated with 5FU and for whom clinical toxicity was well documented. U concentrations varied from 21.8 to 56.6 ng/mL, whereas UH2 concentrations varied from 57.7 to 271.5 ng/mL. UH2/U ratio ranged from 1.56 to 6.18.

CONCLUSIONS

The method has proved to provide a quick, reliable, and reproducible quantitation of the plasma concentrations of U and its metabolite UH2. The UH2/U ratios did not discriminate patients previously exposed to 5FU with and without severe toxicities, possibly due to the small sample. Further studies in a larger population are desirable.

Dihydropyrimidine dehydrogenase gene variation and severe 5-fluorouracil toxicity: a haplotype assessment

AIMS

The importance of polymorphisms in the dihydropyrimidine dehydrogenase gene (DPYD) for the prediction of severe toxicity in 5-fluorouracil (5-FU)-based chemotherapy is still unclear. This study aims to assess the predictive value of DPYD variation with respect to previously described DPYD variants for 5-FU toxicity. It represents the first analysis of the gene at the haplotype level, also capturing potentially important genetic variation located outside the coding regions of DPYD.

MATERIALS & METHODS

The entire coding sequence and exon-flanking intronic regions of DPYD were sequenced in 111 cancer patients receiving fluoropyrimidine-based chemotherapy. DPYD haplotypes were inferred and their associations with severe 5-FU toxicity were assessed.

RESULTS

None of the previously described deleterious variants (IVS14+1G>A, c.2846A>T and c.1679T>G) were detected in 24 patients who experienced severe 5-FU toxicity. A potential association was observed between a haplotype containing three novel intronic polymorphisms (IVS5+18G>A, IVS6+139G>A and IVS9-51T>G) and a synonymous mutation (c.1236G>A), which was observed five- out of eight-times in patients with severe adverse effects.

CONCLUSION

The association of a haplotype containing no nonsynonymous or splice-site polymorphisms indicates that additional important genetic variation may be located in noncoding gene regions. Furthermore, a comparison with other studies suggests that the relative importance of particular DPYD mutations (IVS14+1G>A and c.2846A>T) for predicting severe 5-FU toxicity differs geographically across Europe.

Clinical relevance of different dihydropyrimidine dehydrogenase gene single nucleotide polymorphisms on 5-fluorouracil tolerance

PURPOSE

Although single nucleotide polymorphisms (SNP) of the dihydropyrimidine dehydrogenase gene (DPYD) have been reported, which affect enzyme activity and the severity of 5-fluorouracil (5-FU) toxicity, no pretherapeutic detection has thus far been developed. We investigated 22 DPYD gene SNPs, their respective incidence, their link with grade 3 to 4 toxic side effects, and their management in practice: 9 were looked for in 487 patients, whereas 13 others were investigated in 171 patients.

PATIENTS AND METHODS

SNPs were detected before 5-FU-based treatment in WBC using a Pyrosequencing method. Close clinical and biological follow-up was done.

RESULTS

Five different SNPs were found in 187 patients (IVS14 + 1G>A, 2846A>T, 1679T>G, 85T>C, -1590T>C). Three hundred patients had no SNP. Forty-four patients had grade 3 to 4 toxic side effects in either the first or second cycle. Sixty percent of patients with either IVS14 + 1G>A or 2846A>T SNPs and the only patient with 1679T>G SNP experienced early grade 3 to 4 toxicity, compared with 0%, 5.5%, and 15% of those with either -1590T>C, 85T>C SNP, or no SNP, respectively. In cases with grade 3 to 4 toxicity, treatment either had to be quickly stopped, or could be safely continued with an individual dose adjustment. Sensitivity, specificity, and positive and negative predictive values of the detection of these three major SNPs as toxicity predictive factors were 0.31, 0.98, and 0.62 and 0.94, respectively.

CONCLUSION

Pretreatment detection of three DPYD SNPs could help to avoid severe toxic side effects. This approach is suitable for clinical practice and should be compared or combined with pharmacologic approaches. In the case of dihydropyrimidine dehydrogenase deficiency, 5-FU administration often can be safely continued with an individual dose adjustment.

Identification of a novel mutation in the dihydropyrimidine dehydrogenase gene in a patient with a lethal outcome following 5-fluorouracil administration and the determination of its frequency in a population of 500 patients with colorectal carcinoma

OBJECTIVES

Life-threatening toxic side-effects following 5-FU exposure have been related to deficiency of dihydropyrimidine dehydrogenase (DPD), the rate-limiting enzyme in its catabolism. We presently report a new DPYD gene SNP in a Spanish woman who died from multivisceral 5-FU-induced toxicity.

DESIGN AND METHODS

We looked for 22 known SNPs by Pyrosequecing. Then, we sequenced the whole 23 exons of DPYD, along with adjacent intronic sequences. PCR was carried out to determine whether or not exons were deleted in the DPYD. To determine whether the predicted stop codon indeed resulted in a truncated protein, a bacterial expression vector was employed to generate the predicted protein. 500 patients were genotyped to determine allele frequency.

RESULTS

A novel mutation 464 T>A was identified in DPYD gene exon 5, resulting in the replacement of leucine 155 by a stop codon in the protein. We confirmed this mutation by Pyrosequencing and its involvement by a protein truncation test. We genotyped the patient's family and the allele frequency was 0.2%.

CONCLUSION

The involvement of this variant in 5-FU life-threatening toxicity supports its inclusion in pretherapeutic genetic screening.

5-Fluorouracil-related severe toxicity: a comparison of different methods for the pretherapeutic detection of dihydropyrimidine dehydrogenase deficiency

5-Fluorouracil (5-FU)-related early toxicity, due to a metabolic deficiency, is rare but is potentially severe and even lethal (0.1%). It is due to dihydropyrimidine dehydrogenase (DPYD) gene polymorphism or some epigenetic factors. The detection of metabolic change could prevent severe toxicity, but until now it has not been carried out in clinical practice.

PURPOSE

To find the simplest and most accurate pretherapeutic test to predict DPD deficiency in patients treated with 5-FU by comparing different approaches.

RESULTS

Two hundred and fifty two French Caucasian patients treated by 5-FU infusion were studied. A two-step strategy, combining firstly SNP detection and uracil plasma measurement, followed, in cases where metabolic deficiency was suspected, by dihydrouracil/uracil ratio determination to confirm deficiency and to determine the optimum 5-FU dosage, appeared the best approach, with 83% and 82% sensitivity and specificity, respectively.

CONCLUSION

These data support the future use of this approach, suitable to clinical practice, as screening test to identify DPD deficiency before 5-FU-based therapy.

Dihydropyrimidine Dehydrogenase Activity in 150 Healthy Japanese Volunteers and Identification of Novel Mutations

PURPOSE

Dihydropyrimidine dehydrogenase (DPD) is the initial and rate-limiting enzyme catalyzing the metabolic degradation of the anticancer drug 5-fluorouracil (5-FU). Population studies of DPD activity in peripheral blood mononuclear cells (PBMC) were reported in healthy volunteers and cancer patients. Although these studies were done in mainly Caucasian and African American populations, only a little information is available for a Japanese population.

EXPERIMENTAL DESIGN

One hundred fifty healthy Japanese volunteers were screened for a population distribution of PBMC-DPD activity. Genetic analysis of a volunteer with very low DPD activity was carried out by reverse transcriptase-PCR and genomic sequencing. Bacterially expressed recombinant mutant DPD proteins were purified and characterized.

RESULTS

Mean and median values of PBMC-DPD activity for 5-FU reduction in the study population were 0.173 and 0.166 nmol/min/mg protein, respectively. A 57-year-old female volunteer (proband in this study) had very low DPD activity (0.014 nmol/min/mg protein) with a very low level of expression of DPD protein. Two novel nucleotide substitutions, at nucleotide positions 1097 (1097G > C) and 2303 (2303C > A), resulting in amino acid substitutions at positions 366 (G366A) and 768 (T768K), respectively, were identified. The G366A mutation caused not only a marked decrease in the affinity of the enzyme to cofactor NADPH but also reduced Vmax for 5-FU-reducing activity to approximately 0.5. T768K mutant lost its activity much faster than did wild DPD.

CONCLUSIONS

We found one healthy volunteer (0.7% of the population) with very low PBMC-DPD activity due to heterozygosity for a mutant allele of the DPYD gene in a population of 150 Japanese.

Aberrant methylation of DPYD promoter, DPYD expression, and cellular sensitivity to 5-fluorouracil in cancer cells

PURPOSE

Dihydropyrimidine dehydrogenase (DPD), the initial rate-limiting enzyme in the degradation of 5-fluorouracil (5-FU), is known to be a principal factor in clinical responses to the anticancer agent 5-FU, and various reports have clearly demonstrated that DPD activity is closely correlated to mRNA levels. However, the regulatory mechanisms of DPD gene (DPYD) expression remain unclear. In this study, the regulatory mechanisms have been intensively studied.

EXPERIMENTAL DESIGN AND RESULTS

A subcloned 3.0-kb fragment of the 5' region of DPYD contains a total of 60 CpG sites, suggesting that methylation status may affect the repression of DPYD. The clone showed various promoter activities that were largely correlated with mRNA levels in most cell lines, except HSC3 and HepG2. Bisulfite sequencing analysis revealed that various CpG sites around the transcription start site were abnormally methylated in cells with low DPYD expression: Reversal of hypermethylation by 5-azacytidine treatment significantly increased DPYD expression in HSC3 and HepG2 cells that showed strong promoter activity. In HepG2, in vitro methylation of the DPYD promoter directly decreased promoter activity, and 5-azacytidine treatment restored higher DPYD expression in a dose- and time-dependent manner, along with decreased sensitivity to 5-FU.

CONCLUSIONS

We found that DPD activity was controlled, at least in part, at the transcription level of DPYD and that aberrant methylation of the DPYD promoter region acted as one of the repressors of DPYD expression and affected sensitivity to 5-FU in cancer cells. Our new results could lead to a more precise understanding of the molecular basis of 5-FU response.

Activator Protein Accelerates Dihydropyrimidine Dehydrogenase Gene Transcription in Cancer Cells

Dihydropyrimidine dehydrogenase is the most extensively investigated predictive marker for individual response to 5-fluorouracil. Clinical responses to the anticancer agent, along with various reports, have clearly shown that dihydropyrimidine dehydrogenase activity is closely correlated to its mRNA levels, but the regulatory mechanisms of its expression have remained unclear. We attempted to clarify the mechanisms and found that activator protein (AP-1) is probably one of the key factors in the transcriptional regulation of DPYD in cancer cells, and that phorbol 12-myristate 13-acetate (PMA) plus ionomycin treatment enhances transcription of DPYD via AP-1 activation. In this study, we characterized our previously subcloned 5′ region of human DPYD, an ∼3.0-kb fragment (accession no. AB162145). Luciferase reporter assay showed that the clone showed strong promoter activities in 293T and HSC42 cells, and comparative analysis using 5′ deletion mutants suggested the existence of several positive and negative regulatory regions, including putative binding sites for AP-1, SP-1, and nuclear factor-κB. PMA/ionomycin treatment increased the mRNA level of DPYD in HSC42 cells, and electrophoretic gel mobility shift assay showed that the complex on the putative AP-1 binding site was drastically induced by PMA/ionomycin treatment. The complexes formed were competed out by preincubation with the cold-consensus AP-1 binding site, and the DNA binding complex formed on the site contained c-Jun and c-Fos, which are components of AP-1 transcription factor. We further identified the functional AP-1 binding site (nucleotide positions from −290 to −280), whose nucleotide mutations abolished PMA/ionomycin-induced DPYD promoter activation.

Mutations in exon 14 of dihydropyrimidine dehydrogenase and 5-Fluorouracil toxicity in Portuguese colorectal cancer patients

PURPOSE

Dihydropyrimidine dehydrogenase is a critical enzyme in the catabolism of 5-Fluorouracil, a drug frequently used in cancer therapy. Patients with deficient dihydropyrimidine dehydrogenase activity are at risk of developing severe 5-Fluorouracil-associated toxicity. Genetic analysis of the gene coding for dihydropyrimidine dehydrogenase has shown that mutations in exon 14, especially the splice-site mutation IVS14+1G-->A, were associated with dihydropyrimidine dehydrogenase enzymatic deficiency.

METHODS

We evaluated the frequency of mutations in exon 14 of dihydropyrimidine dehydrogenase (DPYD) gene in 73 unselected colorectal cancer patients treated with 5-Fluorouracil after surgery at a Portuguese Cancer Institute.

RESULTS

Sequencing the entire exon 14 allowed the detection of mutations in two of the 73 patients (2.7%), namely two of the eight (25%) patients who presented grade 3-4 toxicity after 5-Fluorouracil chemotherapy. One patient was heterozygous for the splice-site mutation IVS14+1G-->A, whereas the second patient was heterozygous for a novel missense mutation 1845G-->T (E615D) in exon 14 of DPYD gene.

CONCLUSION

We conclude that mutations in exon 14 of DPYD gene are responsible for a significant proportion of life-threatening toxicity to 5-Fluorouracil, and should therefore be excluded before its administration to cancer patients.

Dihydropyrimidine dehydrogenase circadian rhythm in mouse liver: comparison between enzyme activity and gene expression

Dihydropyrimidine dehydrogenase (DPD) is the rate-limiting enzyme of 5-fluorouracil (FU) catabolism. The relevance of the measurement of DPD activity for identifying DPD-deficient patients is lessened by circadian variability in DPD activity. Our purpose was to determine whether or not DPD mRNA is sustained by a circadian rhythm. Synchronised mice (male B6D2F1) were sacrificed at 3, 7, 11, 15, 19 or 23 Hours After Light Onset (HALO; eight mice per time-point). Liver DPD activity was determined by a radio-enzymatic assay and liver DPD expression by a reverse transcriptase-polymerase chain reaction (RT-PCR) enzyme-linked immunosorbent assay (ELISA) method. Mice synchronisation was controlled by leucocyte and neutrophil counts. Individual DPD activity ranged from 555 to 1575 pmol/min/mg prot; mean DPD activity was highest at 3 HALO (mean+/-standard error of the mean (S.E.M.); 1105+/-70) and lowest at 15 HALO (889+/-71). Individual liver DPD expression varied from 761 to 3481 units (DPD/beta actin ratio); the mean was lowest at 3 HALO (1406+/-112) and highest at 15 HALO (2067+/-214). Cosinor analysis indicated that respective double amplitudes of DPD activity and expression were 21 and 30% of the 24-h mean. The acrophases for activity and expression were 6:40 and 14:10 HALO, respectively, meaning that maximum activity occurred 16 h after the maximum observed expression. These results, revealing the existence of a circadian rhythm in DPD expression, should stimulate further studies to enhance our understanding of the molecular mechanisms involved in the circadian regulation of the DPD enzyme.

Novel disease-causing mutations in the dihydropyrimidine dehydrogenase gene interpreted by analysis of the three-dimensional protein structure

Dihydropyrimidine dehydrogenase (DPD) deficiency is an autosomal recessive disease characterized by thymine-uraciluria in homozygous deficient patients. Cancer patients with a partial deficiency of DPD are at risk of developing severe life-threatening toxicities after the administration of 5-fluorouracil. Thus, identification of novel disease-causing mutations is of the utmost importance to allow screening of patients at risk. In eight patients presenting with a complete DPD deficiency, a considerable variation in the clinical presentation was noted. Whereas motor retardation was observed in all patients, no patients presented with convulsive disorders. In this group of patients, nine novel mutations were identified including one deletion of two nucleotides [1039-1042delTG] and eight missense mutations. Analysis of the crystal structure of pig DPD suggested that five out of eight amino acid exchanges present in these patients with a complete DPD deficiency, Pro86Leu, Ser201Arg, Ser492Leu, Asp949Val and His978Arg, interfered directly or indirectly with cofactor binding or electron transport. Furthermore, the mutations Ile560Ser and Tyr211Cys most likely affected the structural integrity of the DPD protein. Only the effect of the Ile370Val and a previously identified Cys29Arg mutation could not be readily explained by analysis of the three-dimensional structure of the DPD enzyme, suggesting that at least the latter might be a common polymorphism. Our data demonstrate for the first time the possible consequences of missense mutations in the DPD gene on the function and stability of the DPD enzyme.

CPT-11 alters the circadian rhythm of dihydropyrimidine dehydrogenase mRNA in mouse liver

Combination chemotherapy consisting of 5‐fluorouracil (5‐FU) and 7‐ethyl‐10‐[4‐(l‐piperidino)‐l‐piperidino]carboxycamptothecin (CPT‐11) is a promising regimen for gastrointestinal cancer. The circadian‐dependent efficacy and toxicity of 5‐FU are related to the circadian variation in the activity of dihydropyrimidine dehydrogenase (DPD), which is a rate‐limiting enzyme in the pyrimidine catabolic pathway. To optimize the schedule of the CPT‐11 plus 5‐FU combination, we investigated the effect of CPT‐11 on the circadian rhythm of DPD in vivo. In control mice, the DPD mRNA level in the liver was significantly higher at 14:00 than that at 02:00. After intravenous administration of CPT‐11 (30 mg/kg) at 20:00, the circadian rhythm of the DPD mRNA level in the liver was no longer observed 18 h later (14:00), but it was unaffected 6 and 18 h later (at 14:00 and 02:00) when CPT‐11 was given at 08:00. In addition, a dose‐dependent lengthening of the period of the circadian rhythm of DPD was observed for 42 h after intravenous injection of CPT‐11 at 20:00. The levels of DPD protein and activity at 21 h after administration of CPT‐11 (at 17:00) were significantly higher than at 9 h (at 05:00). These results suggest that CPT‐11 may influence the circadian rhythm of DPD at the transcriptional level. Modulation of the circadian rhythm of DPD by CPT‐11 may be a factor in optimizing the combination of 5‐FU and CPT‐11.

Genetic variability in susceptibility and response to toxicants

Xenobiotic metabolism is carried out by phase I and phase II enzymes which are to a large extent polymorphic. The majority of cytochrome P450 (CYP) enzymes involved in xenobiotic metabolism are polymorphic and inducible, resulting in abolished, quantitatively or qualitatively altered or enhanced drug metabolising activity. Stable duplication, multiduplication or amplification of active genes have been described. In mouse models it is apparent that inactivation of specific enzymes active in xenobiotic metabolism can affect the risk for cancer development in relation to specific xenobiotic exposure, whereas the situation in humans is far more complex. The polymorphism of CYP enzymes is expected to influence individual sensitivity and toxicity for different environmental agents, although there is as yet no real consensus in the literature about specific firm relationships in this regard. The incidence of serious and fatal adverse drug reactions (ADRs) has been found to be very high among hospitalised patients, the cost of ADRs to society is large and they are responsible for 5-10% of all hospital admissions. It is likely that predictive genotyping could avoid 10-20% of ADRs. In the present contribution an overview is presented regarding our present knowledge about the polymorphism of phase I enzymes, with emphasis on xenobiotic metabolising CYPs and the importance for metabolic activation of xenobiotics.

Molecular cloning and characterization of the human dihydropyrimidine dehydrogenase promoter

Several studies have demonstrated that dihydropyrimidine dehydrogenase (EC 1.3.1.2) has a critical role in the pharmacokinetics of the anticancer agent 5-fluorouracil. We previously reported the structural organization of the human DPYD gene. In this article, we describe the molecular cloning and functional characterization of 1.2 kb of the 5' flanking region of the DPYD gene. Sequence analysis demonstrated that this region of the DPYD gene lacks the typical TATA or CCAAT boxes with several GC-rich regions containing potential cis-regulatory elements. Progressive 5' deletions of the 5' flanking region were fused to the luciferase reporter gene and transient expression measured following transfection into HeLa and 293 cells. Comparative analysis of luciferase activity revealed that a 208 bp region of the DPYD gene (-121/+86) contained equivalent transcriptional activity to the complete 1.2 kb 5' flanking region of the DPYD gene. Site-directed mutagenesis of the luciferase reporter constructs demonstrated that the -72/-23 sequence contained two regulatory regions (designated elements I and II) essential for promoter activity. Gel shift experiments demonstrated that both regulatory elements specifically bind with protein(s) from nuclear extracts of 293 cells. Competitive binding experiments with 293 nuclear extracts and radiolabeled oligonucleotides (corresponding to elements I and II) suggest that the same protein(s) bind to both regulatory elements. We conclude that constitutive expression of the DPYD gene involves a limited GC-rich region of the 5' flanking sequence of the DPYD gene which contains two regulatory elements.

Pharmacogenetics: a tool for individualizing antineoplastic therapy

This article reviews the clinical relevance of pharmacogenetics in cancer chemotherapy, with emphasis on drugs for which genetic differences in enzyme metabolism have been demonstrated to affect patient outcome. About 10% of children with leukaemia are intolerant to mercaptopurine (6-mercaptopurine) because of genetic defects in mercaptopurine inactivation by thiopurine S-methyltransferase. However, mercaptopurine dose intensity, a critical factor for outcome in patients deficient in thiopurine S-methyltransferase, can be maintained by means of thiopurine S-methyltransferase phenotyping or genotyping. Patients with reduced fluorouracil (5-fluorouracil) catabolism are more likely to be exposed to severe toxicity. The measurement of dihydropyrimidine dehydrogenase activity in patients cannot be considered fully predictive, and the role of dihydropyrimidine dehydrogenase gene variants in this syndrome has yet to be clarified. With regard to irinotecan, patients with Gilbert's syndrome phenotype have reduced inactivation of the active topoisomerase I inhibitor 7-ethyl-10-hydroxycamptothecin (SN-38) caused by a mutation in the UDP-glucuronosyltransferase 1A1 gene promoter. This subset of patients is more likely to be exposed to irinotecan toxicity and could be identified by genotyping for gene promoter variants. Finally, the experience with amonafide represents a model for dose individualization approaches that use simple phenotypic probes.