Abstract C112: Debrisoquine as a model substrate of the organic cation transporter OCT1

Organic cation transporters are involved in the cellular uptake of chemotherapeutic drugs like cisplatin, oxaliplatin and imatinib. We have also shown that genetic polymorphisms in the liver-specific organic cation transporter OCT1 may affect pharmacokinetics and efficacy of supportive care drugs like the antiemetics ondansetron and tropisetron or the opioid tramadol. Here we tested whether debrisoquine, a model drug used to phenotype CYP2D6 activity, is a substrate of the liver-specific organic cation transporter OCT1 and whether genetic polymorphisms in OCT1 affect debrisoquine cellular uptake.

We measured the inhibition of OCT1 by debrisoquine and OCT1-mediated uptake of debrisoquine using HEK293 cells genetically engineered to overexpress wild type or loss-of-function OCT1 isoforms. We overexpressed OCT1 isoforms carrying the common loss-of-function amino acids substitutions in Caucasians Arg61Cys, Cys88Arg, Gly401Ser, Gly465Arg or deletion of Met420. Debrisoquine inhibited the uptake of the model OCT1 substrate MPP+ with IC50 below 10 μM. Debrisoquine uptake was significantly increased both in time and in concentration dependent manner in cells overexpressing wild-type OCT1 (KM of 5.9 ×1.5 μM and vMAX of 41.9 ×4.5 pmol/min/mg protein). OCT1-mediated uptake of debrisoquine was reduced or missing in cells expressing the common loss-of-function isoforms. The deletion of Met420 and the substitutions of Arg61Cys and Gly401Ser reduced vMAX by 48, 63 and 91%, respectively, but did not affect the Km of the OCT1-mediated debrisoquine uptake. The OCT1 isoforms carrying Cys88Arg or Gly465Arg substitutions completely lacked OCT1-mediated debrisoquine uptake.

In conclusion, debrisoquine is a substrate of OCT1 and common loss-of-function OCT1 polymorphisms affect debrisoquine uptake. Debrisoquine has a potential to be used as phenotyping marker for OCT1 activity. Moreover, known in vivo variations in debrisoquine pharmacokinetic and their relation to development of lung and bladder cancer may be related to variation not only in CYP2D6, but also in OCT1 activity.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C112.

Association of metabolic gene polymorphisms with tobacco consumption in healthy controls

Polymorphisms in genes that encode for metabolic enzymes have been associated with variations in enzyme activity between individuals. Such variations could be associated with differences in individual exposure to carcinogens that are metabolized by these genes. In this study, we examine the association between polymorphisms in several metabolic genes and the consumption of tobacco in a large sample of healthy individuals. The database of the International Collaborative Study on Genetic Susceptibility to Environmental Carcinogens was used. All the individuals who were controls from the case-control studies included in the data set with information on smoking habits and on genetic polymorphisms were selected (n = 20938). Sufficient information was available on the following genes that are involved in the metabolism of tobacco smoke constituents: CYP1A1, GSTM1, GSTT1, NAT2 and GSTP1. None of the tested genes was clearly associated with smoking behavior. Information on smoking dose, available for a subset of subjects, showed no effect of metabolic gene polymorphisms on the amount of smoking. No association between polymorphisms in the genes studied and tobacco consumption was observed; therefore, no effect of these genes on smoking behavior should be expected.

Variations in the 5-hydroxytryptamine type 3B receptor gene as predictors of the efficacy of antiemetic treatment in cancer patients

PURPOSE

Serotonin (5-hydroxytryptamine type 3 [5-HT3]) receptor antagonists have substantially reduced but not eliminated nausea and vomiting in patients undergoing cancer chemotherapy. They act through specific binding to the 5-HT3A, 5-HT3B receptor complex. The 5-HT3B subunit seems to be most important for its functionality. We hypothesized that patients with genetic variations in the 5-HT3B receptor gene might respond differently to antiemetic treatment.

PATIENTS AND METHODS

We included 242 cancer patients on their first day of chemotherapy. Nausea and vomiting were documented before and twice during the chemotherapy using standardized interviews and visual analog scales. We sequenced the entire 5-HT3B receptor gene, including the 5' flanking region and at least a 20-base pair intronic sequence of each intron-exon splice site of all patients.

RESULTS

Approximately 30% of all patients suffered from nausea or vomiting. Sequencing of the 5-HT3B receptor gene revealed 13 polymorphisms: two of them were amino acid exchanges (Tyr129Ser, Ala223Thr) and two were deletion variants. In both observation periods, patients homozygous for the -100_-102delAAG deletion variant of the promotor region experienced vomiting more frequently than did all the other patients.

CONCLUSION

A more efficient antiemetic treatment with 5-HT3 receptor antagonists might be possible on a pharmacogenetic basis. However, only a small fraction of the therapeutic failure is explained by the -AAG deletion variant of the 5-HT3B receptor gene. Additional clinical and biochemical studies are needed to confirm the association.

Polymorphisms in CYP1A1, GSTM1, GSTT1 and lung cancer below the age of 45 years

BACKGROUND

A genetic component of early-onset lung cancer has been suggested. The role of metabolic gene polymorphisms has never been studied in young lung cancer cases. Phase 1 and Phase 2 gene polymorphisms are involved in tobacco carcinogens' metabolism and therefore in lung cancer risk.

METHODS

The effect of metabolic gene polymorphisms on lung cancer at young ages was studied by pooling data from the Genetic Susceptibility to Environmental Carcinogens (GSEC) database. All primary lung cancer cases of both sexes who were Caucasian and </=45 years of age at diagnosis, and the corresponding controls were selected. We obtained 261 cases and 1452 controls.

RESULTS

There was a marginally significant association between lung cancer and GSTT1 null genotype (OR=1.2; 95% CI:1.0-1.6), and a significant association between lung cancer and the homozygous CYP1A1 Msp1 variant allele (CYP1A1*2A and *2B) genotype (OR=4.7 95% CI:1.2-19.0). When data were stratified by smoking status, the association between CYP1A1 genotype and lung cancer was confined to never smokers.

CONCLUSIONS

These results suggest that metabolic genetic factors play a role in lung cancer developing at young ages.

Metabolic gene polymorphism frequencies in control populations

Using the International Project on Genetic Susceptibility to Environmental Carcinogens (GSEC) database containing information on over 15,000 control (noncancer) subjects, the allele and genotype frequencies for many of the more commonly studied metabolic genes (CYP1A1, CYP2E1, CYP2D6, GSTM1, GSTT1, NAT2, GSTP, and EPHX) in the human population were determined. Major and significant differences in these frequencies were observed between Caucasians (n = 12,525), Asians (n = 2,136), and Africans and African Americans (n = 996), and some, but much less, heterogeneity was observed within Caucasian populations from different countries. No differences in allele frequencies were seen by age, sex, or type of controls (hospital patients versus population controls). No examples of linkage disequilibrium between the different loci were detected based on comparison of observed and expected frequencies for combinations of specific alleles.

Current smoking, occupation, N-acetyltransferase-2 and bladder cancer: a pooled analysis of genotype-based studies

The aim of this study was to investigate the association of NAT2 gene polymorphism with bladder cancer using the data derived from the International Project on Genetic Susceptibility to Environmental Carcinogens. Four case control studies conducted in four European countries, plus two case series, one from England and one from Germany, for a total of 1530 cases and 731 controls (all Caucasian) were included. The interaction between NAT2 and bladder cancer considering smoking habits and occupational exposure was studied. There was a significant association between NAT2 and bladder cancer (odds ratio: 1.42, 95% confidence interval: 1.14-1.77), with a slightly significant heterogeneity among studies. However, heterogeneity disappeared when smokers were divided into current and ex-smokers. The risk of cancer was elevated in smokers and occupationally exposed subjects, with the highest risk among slow acetylators. The increase in risk was limited, in fact, to current smokers (odds ratio = 1.74, 95% confidence interval: 0.96-3.15). This analysis confirms that the NAT2 genotype is a risk factor for bladder cancer by interacting with smoking or occupational exposures. Our observation suggests that NAT2 is not a risk factors per se but modulates the effect of carcinogens contained in tobacco smoke (probably arylamines) or associated with occupational exposures.

Identification and functional characterization of eight CYP3A4 protein variants

The genetic component of the inter-individual variability in CYP3A4 activity has been estimated to be between 60% and 90%, but the underlying genetic factors remain largely unknown. A study of 213 Middle and Western European DNA samples resulted in the identification of 18 new CYP3A4 variants, including eight protein variants. A total of 7.5% of the population studied was found to be heterozygous for one of these variants. In a bacterial heterologous expression system, two mutants, R130Q and P416L, did not result in detectable P450 holoprotein. One mutant, T363M, expressed at significantly lower levels than wild-type CYP3A4. G56D, V170I, D174H and M445T were not significantly different when compared with wild-type CYP3A4 in expression or steroid hydroxylase activity. L373F displayed a significantly altered testosterone metabolite profile and a four-fold increase in the Km value for 1'-OH midazolam formation. The results suggest a limited contribution of CYP3A4 protein variants to the inter-individual variability of CYP3A4 activity in Caucasians. Some variants may, however, play a role in the atypical response to drugs or altered sensitivity to carcinogens.