Sharon Marsh

Use of Pharmacogenetic and Clinical Factors to Predict the Therapeutic Dose of Warfarin

Initiation of warfarin therapy using trial‐and‐error dosing is problematic. Our goal was to develop and validate a pharmacogenetic algorithm. In the derivation cohort of 1,015 participants, the independent predictors of therapeutic dose were: VKORC1 polymorphism −1639/3673 G>A (−28% per allele), body surface area (BSA) (+11% per 0.25 m2), CYP2C9*3 (−33% per allele), CYP2C9*2 (−19% per allele), age (−7% per decade), target international normalized ratio (INR) (+11% per 0.5 unit increase), amiodarone use (−22%), smoker status (+10%), race (−9%), and current thrombosis (+7%). This pharmacogenetic equation explained 53–54% of the variability in the warfarin dose in the derivation and validation (N= 292) cohorts. For comparison, a clinical equation explained only 17–22% of the dose variability (P < 0.001). In the validation cohort, we prospectively used the pharmacogenetic‐dosing algorithm in patients initiating warfarin therapy, two of whom had a major hemorrhage. To facilitate use of these pharmacogenetic and clinical algorithms, we developed a nonprofit website, http://www.WarfarinDosing.org.

Doxorubicin pathways: pharmacodynamics and adverse effects.

The goal of this study is to give a brief background on the literature supporting the PharmGKB pathway about doxorubicin action, and provides a summary of this active area of research. The reader is referred to recent in-depth reviews [1–4] for more detailed discussion of this important and complex pathway. Doxorubicin is an anthracyline drug first extracted from Streptomyces peucetius var. caesius in the 1970’s and routinely used in the treatment of several cancers including breast, lung, gastric, ovarian, thyroid, non-Hodgkin’s and Hodgkin’s lymphoma, multiple myeloma, sarcoma, and pediatric cancers [5–7]. A major limitation for the use of doxorubicin is cardiotoxicity, with the total cumulative dose being the only criteria currently used to predict the toxicity [4,8]. As there is evidence that the mechanisms of anticancer action and of cardiotoxicity occur through different pathways there is hope for the development of anthracycline drugs with equal efficacy but reduced toxicity [4]. Knowledge of the pharmacogenomics of these pathways may eventually allow for future selection of patients more likely to achieve efficacy at lower doses or able to withstand higher doses with lesser toxicity. We present here graphical representations of the candidate genes for the pharmacogenomics of doxorubicin action in a stylized cancer cell (Fig. 1) and toxicity in cardiomyocytes (Fig. 2), and a table describing the key variants examined so far. Open in a separate window Fig. 1 Graphical representation of the candidate genes involved in the pharmacodynamics of doxorubicin in a stylized cancer cell. A fully interactive version of this pathway is available online at PharmGKB at http://www.pharmgkb.org/do/serve?objId=PA165292163o ROS, reactive oxygen species.

Diflomotecan pharmacokinetics in relation to ABCG2 421C>A genotype

The adenosine triphosphate–binding cassette transporter ABCG2 (breast cancer resistance protein [BCRP]) functions as an efflux transporter for many drugs, including the topoisomerase I inhibitor diflomotecan, and is expressed at high levels in the intestine and liver. We performed an exploratory analysis to evaluate the effects of the natural allelic variant ABCG2 421C>A on the pharmacokinetics of diflomotecan.

Pharmacogenetic Assessment of Toxicity and Outcome After Platinum Plus Taxane Chemotherapy in Ovarian Cancer: The Scottish Randomised Trial in Ovarian Cancer

PURPOSE Standard therapy for advanced ovarian cancer consists of a platinum agent in combination with a taxane, which has a 5-year survival rate of approximately 45%. The large individual variability for ovarian cancer patients in both outcome and toxicity risk from chemotherapy makes the identification of pharmacogenetic markers that can be used to screen patients before therapy selection an attractive prospect. PATIENTS AND METHODS We assessed 27 selected polymorphisms based on previously described associations or putative functional effects in 16 key genes from pathways that may influence cellular sensitivity to taxanes (ABCB1, ABCC1, ABCC2, ABCG2, CDKN1A, CYP1B1, CYP2C8, CYP3A4, CYP3A5, MAPT, and TP53) and platinum (ABCC2, ABCG2, ERCC1, ERCC2, GSTP1, MPO, and XRCC1) using polymerase chain reaction and Pyrosequencing in 914 ovarian cancer patients from the Scottish Randomised Trial in Ovarian Cancer phase III trial who were treated at presentation with carboplatin and taxane regimens after cytoreductive surgery. RESULTS No reproducible significant associations between genotype and outcome or toxicity were found for any of the genes analyzed. Previously reported genotype associations could not be replicated in this large study of a well-defined patient population within one specific clinical trial. CONCLUSION There are no clear candidates for taxane/platinum pharmacogenetic markers. This study highlights the need for validation of putative genetic markers in large, well-defined clinical sample sets.

CpG Island Methylation of DNA Damage Response Genes in Advanced Ovarian Cancer

We have determined the methylation frequencies of 24 CpG islands of genes associated with DNA damage responses or with ovarian cancer in 106 stage III/IV epithelial ovarian tumors. We have analyzed this data for whether there is evidence of a CpG island methylator phenotype or associations of CpG island methylation with response to chemotherapy in advanced ovarian cancer. Frequent methylation was observed for OPCML, DCR1, RASSF1A, HIC1, BRCA1, and MINT25 (33.3%, 30.7%, 26.4%, 17.3%, 12.3%, and 12.0%, respectively), whereas no methylation was observed for APAF-1, DAPK, FANCF, FAS, P14, P21, P73, SOCS-3, and SURVIVIN. The remaining genes showed only a low frequency of methylation, <10%. Unsupervised gene shaving identified a nonrandom pattern of methylation for OPCML, DCR1, RASSF1A, MINT25, HIC1, and SFRP1, supporting the concept of concordant methylation of these genes in ovarian cancer. Methylation of at least one of the group of genes involved in DNA repair/drug detoxification (BRCA1, GSTP1, and MGMT) was associated with improved response to chemotherapy (P = 0.013). We have examined the frequency of a polymorphism in the DNA methyltransferase gene DNMT3b6, which has been previously reported to affect gene transcription and cancer risk. The genetic polymorphism in the DNMT3b6 gene promoter (at position -149) is not significantly associated with the concordant methylation observed, but is weakly associated with the overall frequency of methylation at the genes examined (P = 0.04, n = 56). This supports the hypothesis that genetic factors affecting function of DNMT genes may underlie the propensity of tumors to acquire aberrant CpG island methylation.

ABCG2 Pharmacogenetics: Ethnic Differences in Allele Frequency and Assessment of Influence on Irinotecan Disposition

Purpose: The ATP-binding cassette transporter ABCG2 (breast cancer resistance protein) is an efflux protein that plays a role in host detoxification of various xenobiotic substrates, including the irinotecan metabolite 7- ethyl-10-hydroxycamptothecin (SN-38). The ABCG2 421C>A polymorphism has been associated with reduced protein expression and altered function in vitro. The aim of this study was to evaluate the ethnic distribution and potential functional consequence of the ABCG2 421C>A genotype in cancer patients treated with irinotecan. Experimental Design: ABCG2 genotyping was performed using Pyrosequencing on DNA from 88 American Caucasians, 94 African Americans, 938 Africans, and 95 Han Chinese, as well as in 84 European Caucasian patients treated with irinotecan undergoing additional blood sampling for pharmacokinetic studies. Results: Significant differences in allele frequencies were observed between the given world populations (P < 0.001), the variant allele being most common in the Han Chinese population with a frequency as high as 34%. The mean area under the curve of irinotecan and SN-38 were 19,851 and 639 ng × hour/mL, respectively. The frequency of the variant allele (10.7%) was in line with results in American Caucasians. No significant changes in irinotecan pharmacokinetics were observed in relation to the ABCG2 421C>A genotype, although one of two homozygous variant allele carriers showed extensive accumulation of SN-38 and SN-38 glucuronide. Conclusions: The ABCG2 421C>A polymorphism appears to play a limited role in the disposition of irinotecan in European Caucasians. It is likely that the contribution of this genetic variant is obscured by a functional role of other polymorphic proteins.

Association of CYP2C8, CYP3A4, CYP3A5, and ABCB1 Polymorphisms with the Pharmacokinetics of Paclitaxel

Purpose: To retrospectively evaluate the effects of six known allelic variants in the CYP2C8, CYP3A4, CYP3A5, and ABCB1 genes on the pharmacokinetics of the anticancer agent paclitaxel (Taxol). Experimental Design: A cohort of 97 Caucasian patients with cancer (median age, 57 years) received paclitaxel as an i.v. infusion (dose range, 80-225 mg/m2). Genomic DNA was analyzed using PCR RFLP or using Pyrosequencing. Pharmacokinetic variables for unbound paclitaxel were estimated using nonlinear mixed effect modeling. The effects of genotypes on typical value of clearance were evaluated with the likelihood ratio test within NONMEM. In addition, relations between genotype and individual pharmacokinetic variable estimates were evaluated with one-way ANOVA. Results: The allele frequencies for the CYP2C8*2, CYP2C8*3, CYP2C8*4, CYP3A4*3, CYP3A5*3C, and ABCB1 3435C>T variants were 0.7%, 9.2%, 2.1%, 0.5%, 93.2%, and 47.1%, respectively, and all were in Hardy-Weinberg equilibrium. The population typical value of clearance of unbound paclitaxel was 301 L/h (individual clearance range, 83.7-1055 L/h). The CYP2C8 or CYP3A4/5 genotypes were not statistically significantly associated with unbound clearance of paclitaxel. Likewise, no statistically significant association was observed between the ABCB1 3435C>T variant and any of the studied pharmacokinetic variables. Conclusions: This study indicates that the presently evaluated variant alleles in the CYP2C8, CYP3A4, CYP3A5, and ABCB1 genes do not explain the substantial interindividual variability in paclitaxel pharmacokinetics.

Irinotecan-induced diarrhea: Functional significance of the polymorphic ABCC2 transporter protein

Interindividual pharmacokinetic variability of the anticancer agent irinotecan is high. Life‐threatening diarrhea is observed in up to 25% of patients receiving irinotecan and has been related with irinotecan pharmacokinetics and UGT1A1 genotype status. Here, we explore the association of ABCC2 (MRP2) polymorphisms and haplotypes with irinotecan disposition and diarrhea. A cohort of 167 Caucasian cancer patients who were previously assessed for irinotecan pharmacokinetics (90‐min infusion given every 21 days), toxicity, and UGT1A1*28 genotype were genotyped for polymorphisms in ABCC2 using Pyrosequencing. Fifteen ABCC2 haplotypes were identified in the studied patients. The haplotype ABCC2*2 was associated with lower irinotecan clearance (28.3 versus 31.6 l/h; P=0.020). In patients who did not carry a UGT1A1*28 allele, a significant reduction of severe diarrhea was noted in patients with the ABCC2*2 haplotype (10 versus 44% odds ratio, 0.15; 95% confidence interval, 0.04–0.61; P=0.005). This effect was not observed in patients with at least one UGT1A1*28 allele (32 versus 20% odds ratio, 1.87; 95% confidence interval, 0.49–7.05; P=0.354). This study suggests that the presence of the ABCC2*2 haplotype is associated with less irinotecan‐related diarrhea, maybe as a consequence of reduced hepatobiliary secretion of irinotecan. As the association was seen in patients not genetically predisposed at risk for diarrhea due to UGT1A1*28, confirmatory studies of the relationships of ABCC2 genotypes and irinotecan disposition and toxicity are warranted.

Pharmacogenetic Predictors of Adverse Events and Response to Chemotherapy in Metastatic Colorectal Cancer: Results From North American Gastrointestinal Intergroup Trial N9741

PURPOSE With three available chemotherapy drugs for advanced colorectal cancer (CRC), response rate (RR) and survival outcomes have improved with associated morbidity, accentuating the need for tools to select optimal individualized treatment. Pharmacogenetics identifies the likelihood of adverse events or response based on variants in genes involved in drug transport, metabolism, and cellular targets. PATIENTS AND METHODS Germline DNA was extracted from 520 patients on the North American Gastrointestinal Intergroup N9741 study. Three study arms were evaluated: IFL (fluorouracil [FU] + irinotecan [IRN]), FOLFOX (FU + oxaliplatin), and IROX (IRN + oxaliplatin). Information on adverse events, response, and disease-free survival was available. Thirty-four variants in 15 candidate genes for analysis based on previous associations with adverse events or outcome were assessed. Genotyping was performed using pyrosequencing. RESULTS All variants were polymorphic. The homozygous UGT1A1*28 allele observed in 9% of patients was associated with risk of grade 4 neutropenia in patients on IROX (55% v 15%; P = .002). Deletion in GSTM1 was associated with grade 4 neutropenia after FOLFOX (28% v 16%; P = .02). Patients with a homozygous variant genotype for GSTP1 were more likely to discontinue FOLFOX because of neurotoxicity (24% v 10%; P = .01). The presence of a CYP3A5 variant was significantly associated with RR on IFL (29% v 60%; P = .0074). Most previously published genotype-toxicity or -efficacy relationships were not validated in this study. CONCLUSION This study provides a platform to evaluate pharmacogenetic predictors of response or severe adverse events in advanced CRC. Pharmacogenetic studies can be conducted in multicenter trials, and our findings demonstrate that with continued research, clinical application is practical.

Pharmacogenetic pathway analysis of docetaxel elimination

The purpose of this study was to evaluate the affinity of docetaxel for 14 transporter proteins and assess the functional significance of 17 variants in five genes involved in drug elimination. Among the transfected models investigated, OATP1B3 (SLCO1B3) was identified as the most efficient influx transporter for docetaxel. None of the observed genotypes (SLCO1B3, ABCB1, and ABCC2) was related with docetaxel clearance in 92 white patients (P > 0.17). However, the simultaneous presence of the CYP3A4*1B and CYP3A5*1A alleles was associated with a 64% increase in docetaxel clearance (P = 0.0015), independent of both sex and CYP3A activity (as determined using the erythromycin breath test). This haplotype was also associated with increased midazolam clearance in another population (P = 0.0198). An analysis of the CYP3A locus among CEPH‐HapMap samples revealed that CYP3A4*1B is present exclusively among a subset of CYP3A5 expressors. Therefore, future studies should first stratify the population on the basis of CYP3A5 genotype and then compare CYP3A activity between individuals with and without the CYP3A4*1B allele.