Gene polymorphisms, pharmacokinetics, and hematological toxicity in advanced non-small-cell lung cancer patients receiving cisplatin/gemcitabine

A naïve pooled data approach for extrapolation of Phase 0 microdose trials to therapeutic dosing regimens

Microdosing is a strategy to obtain knowledge of human pharmacokinetics prior to Phase I clinical trials. The most frequently used method to extrapolate microdose (≤100 μg) pharmacokinetics to therapeutic doses is based on linear extrapolation from a noncompartmental analysis (NCA) with a two-fold acceptance criterion between pharmacokinetic metrics of the extrapolated microdose and the therapeutic dose. The major disadvantage of NCA is the assumption of linear extrapolation of NCA metrics. In this study, we used a naïve pooled data (NPD) modeling approach to extrapolate microdose pharmacokinetics to therapeutic pharmacokinetics. Gemcitabine and anastrozole were used as examples of intravenous and oral drugs, respectively. Data from microdose studies were used to build a parent-metabolite model for gemcitabine and its metabolite 2',2'-difluorodeoxyuridine (dFdU) and a model for anastrozole. The pharmacokinetic microdose models were extrapolated to therapeutic doses. Extrapolation of the microdose showed differences in pharmacokinetic shape for gemcitabine and dFdU between the simulated and observed therapeutic concentrations, whereas the observed therapeutic concentrations for anastrozole were captured by the extrapolation. This study demonstrated the possible use and feasibility of an NPD modeling approach for the evaluation and application of microdose studies in early drug development. Last, physiologically-based pharmacokinetic modeling might be an alternative for microdose extrapolation of drugs with complex pharmacokinetics such as gemcitabine.

Is age just a number? A population pharmacokinetic study of gemcitabine

PURPOSE

Pharmacokinetic exposure to gemcitabine and its metabolite, 2',2'-difluorodeoxyuridine (dFdU), might be altered in elderly compared to their younger counterparts. It is unknown if age-based dose adjustments are necessary to reduce the development of treatment-induced adverse events. The aim of this study was to assess the impact of age on the pharmacokinetics of gemcitabine and dFdU.

METHODS

Pharmacokinetic sampling following a flexible limited sampling strategy was performed in patients ≥ 70 years after gemcitabine infusion. The data were supplemented with pharmacokinetic data in patients included in four previously conducted clinical trials. Nonlinear mixed effects modelling was performed on the pooled dataset to assess the impact of age on the pharmacokinetics of gemcitabine and dFdU.

RESULTS

In total, pharmacokinetic data were available of 197 patients, of whom 83 patients were aged ≥ 70 years (42%). A two-compartment model for both gemcitabine and dFdU with linear clearances from the central compartments described the data best. Age, tested as continuous and categorical (< 70 years versus ≥ 70 years) covariate, did not statistically affect the pharmacokinetics of gemcitabine and dFdU.

CONCLUSION

Age was not of influence on the pharmacokinetics of gemcitabine or its metabolite, dFdU. Age-related dose adjustments for gemcitabine based on pharmacokinetic considerations are not recommended.

TRIAL REGISTRATION NUMBER

NL39647.048.12, registered on May 3rd 2012.

Gene-Gene Interactions of Gemcitabine Metabolizing-Enzyme Genes hCNT3 and WEE1 for Preventing Severe Gemcitabine-Induced Hematological Toxicity

Most patients experience severe hematological toxicity during treatment with gemcitabine; thus, preventing such toxicity would improve the treatment effects and patient quality of life. We analyzed 13 polymorphisms in the transporters, metabolizing enzymes, targets, and genes involved in DNA damage and the folate pathway among 132 patients treated with gemcitabine and studied their association with the severity of the hematological toxicities. Single-locus analysis showed that the single-nucleotide polymorphisms (SNPs) RRM1 rs12806698 and rs11031918 and DCTD rs7663494 were significantly associated with severe neutropenia, hENT1 rs760370 and hCNT3 rs7867504 and rs4877831 were associated with severe leukopenia, CDA rs2072671, DCTD rs7663494, and WEE1 rs3910384 were associated with severe anemia, and MTHFR rs1801133 was associated with severe thrombocytopenia after stringent Bonferroni correction (P < .0038). The gene-gene interaction analysis identified the overall best models, including a 2-way interaction model (hCNT3 rs7867504 and dCK rs12648166) for severe leukopenia (P = .0022) and a 3-locus model (CDA rs207671, DCTD rs7663494, and WEE1 rs3910384) for severe anemia with a strong synergistic effect (P = .0001). The association with hematological toxicity was further strengthened by the results of a haplotype analysis, in which the homozygous genotype combination of rs3910384 CC, rs2072671 AA, rs12648166 GG, rs7867504 CC, and rs7663494 TT conferred high genetic susceptibility to severe thrombocytopenia. Our results suggest that the gene-gene interaction of gemcitabine metabolic pathway genes and WEE1 contributes to susceptibility to gemcitabine-induced hematological toxicity. Moreover, we propose a promising data-mining analysis approach (generalized multifactor dimensionality reduction) to detect and characterize gene-gene interactions.

An initial genetic analysis of gemcitabine-induced high-grade neutropenia in pancreatic cancer patients in CALGB 80303 (Alliance)

OBJECTIVES

One of the standard of care regimens for advanced pancreatic cancer is gemcitabine-based chemotherapy. The efficacy of gemcitabine is limited by dose-limiting hematologic toxicities especially neutropenia. Uncovering the variability of these toxicities attributed to germline DNA variation is of great importance.

PATIENTS AND METHODS

CALGB 80303 was a randomized study in advanced pancreatic cancer patients treated with gemcitabine with or without bevacizumab. The study protocol included genotyping of genes of gemcitabine disposition (CDA, DCTD, SLC29A1, SLC28A1, and SLC29A2), as well as a genome-wide analysis. The clinical phenotype was time to early high-grade neutropenia event accounting for progression or death or other treatment-terminating adverse events as competing for informative events. The inference was carried out on the basis of the association between genotype and cause-specific hazard of a neutropenic event.

RESULTS

The primary analyses were carried out on the basis of 294 genetically estimated European pancreatic cancer patients. For CDA rs2072671 (A>C), AC and CC patients had a lower risk of neutropenia than AA patients (P=0.01, hazard ratio: 0.61, 95% confidence interval: 0.41-0.89). For SLC28A1 rs3825876 (G>A), AA patients have a higher risk of neutropenia than GA and GG patients (P=0.02, hazard ratio: 1.51, 95% confidence interval: 1.06-2.16). CDA rs2072671 was associated with increased mRNA expression in whole blood in three studies (P=2.7e-14, 6.61e-62, and 9.70e-65). In the genome-wide analysis, variants in TGFB2 were among the top hits (lowest P=1.62e-06) but had no effect in luciferase assays.

CONCLUSION

This is the first genetic analysis of gemcitabine-induced neutropenia using a competing risk model in a prospective randomized clinical study has proposed a potentially novel mechanism of the protective effect of the CDA rs2072671 variant. Further confirmation is needed.

Role of transportome in the pharmacogenomics of hepatocellular carcinoma and hepatobiliary cancer

An important factor determining the pharmacological response to antitumor drugs is their concentrations in cancer cells, which accounts for the net interaction with their intracellular molecular targets. Accordingly, mechanisms leading to reduced intracellular levels of active agents play a crucial role in cancer chemoresistance. These include impaired drug uptake through solute carrier (SLC) proteins and efficient drug export by ATP-dependent pumps belonging to the ATP-binding cassette (ABC) superfamily of proteins. Since the net movement of drugs in-and-out the cells depends on the overall expression of carrier proteins, defining the so-called transportome, special attention has been devoted to the study of transcriptome regarding these proteins. Nevertheless, genetic variants affecting SLC and ABC genes may markedly affect the bioavailability and, hence, the efficacy of anticancer drugs.

Association Study Among Candidate Genetic Polymorphisms and Chemotherapy-Related Severe Toxicity in Testicular Cancer Patients

Testicular cancer is one of the most commonly occurring malignant tumors in young men with fourfold higher rate of incidence and threefold higher mortality rates in Chile than the average global rates. Surgery is the initial line of treatment for testicular cancers, and is generally followed by chemotherapy, usually with combinations of bleomycin, etoposide, and cisplatin (BEP). However, the adverse effects of chemotherapy vary significantly among individuals; therefore, the present study explored the association of functionally significant allelic variations in genes related to the pharmacokinetics/pharmacodynamics of BEP and DNA repair enzymes with chemotherapy-induced toxicity in BEP-treated testicular cancer patients. We prospectively recruited 119 patients diagnosed with testicular cancer from 2010 to 2017. Genetic polymorphisms were analyzed using PCR and/or qPCR with TaqMan^®probes. Toxicity was evaluated based on the Common Terminology Criteria for Adverse Events, v4.03. After univariate analyses to define more relevant genetic variants (p < 0.2) and clinical conditions in relation to severe (III–IV) adverse drug reactions (ADRs), stepwise forward multivariate logistic regression analyses were performed. As expected, the main severe ADRs associated with the non-genetic variables were hematological (neutropenia and leukopenia). Univariate statistical analyses revealed that patients with ERCC2 rs13181 T/G and/or CYP3A4 rs2740574 A/G genotypes are more likely to develop alopecia; patients with ERCC2 rs238406 C/C genotype may develop leukopenia, and patients with GSTT1-null genotype could develop lymphocytopenia (III–IV). Patients with ERCC2 rs1799793 A/A were at risk of developing severe anemia. The BLMH rs1050565 G/G genotype was found to be associated with pain, and the GSTP1 G/G genotype was linked infection (p < 0.05). Multivariate analysis showed an association between specific ERCC1/2 genotypes and cumulative dose of BEP drugs with the appearance of severe leukopenia and/or febrile neutropenia. Grades III–IV vomiting, nausea, and alopecia could be partly explained by the presence of specific ERCC1/2, MDR1, GSTP1, and BLMH genotypes (p < 0.05). Hence, we provide evidence for the usefulness of pharmacogenetics as a tool for predicting severe ADRs in testicular cancer patients treated with BEP chemotherapy.

Predictive Value of Single Nucleotide Polymorphisms of ERCC1, XPA, XPC, XPD and XPG Genes, Involved in NER Mechanism in Patients with Advanced NSCLC Treated with Cisplatin and Gemcitabine

The combination of cisplatin and gemcitabine is still one of the most frequently used first-line chemotherapy scheme in patients with advanced non-small cell lung cancer (NSCLC), in which tyrosine kinase inhibitors (TKIs) cannot be administered. Unfortunately, more than half of the patients have no benefit from chemotherapy but are still exposed to its toxic effects. Therefore, single nucleotide polymorphisms (SNPs) in the genes involved in nucleotide excision repair (NER) mechanism may be a potential predictive factor of efficiency of cytostatic based chemotherapy. The aim of the study was to evaluate the correlation between SNPs of the genes involved in NER mechanism and the effectiveness of chemotherapy based on cisplatin and gemcitabine in patients with advanced NSCLC. The study group included 91 NSCLC patients treated with first-line chemotherapy using cisplatin and gemcitabine. Genotyping was carried out using a mini-sequencing technique (SNaPshot™ PCR). The median progression-free survival (PFS) was significantly shorter in carriers of CC genotype of the XPD/ERCC2 (2251A > C) gene compared to patients with AA/AC genotypes (2 vs. 4.5 months; p = 0.0444; HR = 3.19, 95%CI:1.03–9.91). Rare CC genotype of XPD/ERCC2 gene, may be considered as an unfavorable predictive factor for chemotherapy based on cisplatin and gemcitabine in patients with advanced NSCLC.

Pharmacokinetics and pharmacogenetics of Gemcitabine as a mainstay in adult and pediatric oncology: an EORTC-PAMM perspective

Gemcitabine is an antimetabolite ranking among the most prescribed anticancer drugs worldwide. This nucleoside analog exerts its antiproliferative action after tumoral conversion into active triphosphorylated nucleotides interfering with DNA synthesis and targeting ribonucleotide reductase. Gemcitabine is a mainstay for treating pancreatic and lung cancers, alone or in combination with several cytotoxic drugs (nab-paclitaxel, cisplatin and oxaliplatin), and is an option in a variety of other solid or hematological cancers. Several determinants of response have been identified with gemcitabine, i.e., membrane transporters, activating and inactivating enzymes at the tumor level, or Hedgehog signaling pathway. More recent studies have investigated how germinal genetic polymorphisms affecting cytidine deaminase, the enzyme responsible for the liver disposition of gemcitabine, could act as well as a marker for clinical outcome (i.e., toxicity, efficacy) at the bedside. Besides, constant efforts have been made to develop alternative chemical derivatives or encapsulated forms of gemcitabine, as an attempt to improve its metabolism and pharmacokinetics profile. Overall, gemcitabine is a drug paradigmatic for constant searches of the scientific community to improve its administration through the development of personalized medicine in oncology.

Pharmacogenomics in the treatment of lung cancer: an update

Significant advances have been made in the analysis of the human genome in the first decades of the 21st century and understanding of tumor biology has matured greatly. The identification of tumor-associated mutations and the pathways involved has led to the development of targeted anticancer therapies. However, the challenge now in using chemotherapy to treat nonsmall-cell lung cancer is to identify more molecular markers predictive of drug sensitivity and determine the optimal drug sequences in order to tailor treatment to each patient. This approach could permit selection of patients who could benefit most from a specific type of chemotherapy by matching their tumor and individual genetic profile. Nevertheless, this potential has been limited so far by reliance on the single biomarker approach, though this is now on the way to being overcome through whole genome studies.

Predictive value of STMN1 gene promoter polymorphism (-2166T>C) in patients with advanced NSCLC treated with the combination of platinum compounds and vinorelbine

Purpose

The combination of platinum compounds and vinorelbine is often used as a first-line treatment in patients with locally advanced or metastatic non-small cell lung cancer (NSCLC), without activating EGFR mutations and ALK rearrangement. Unfortunately, less than half of the patients benefit from chemotherapy. Moreover, majority of patients are exposed to a number of side effects of chemotherapy. Stathmin-1 (STMN1, oncoprotein 18) affects significantly microtubule dynamics and formation of the mitotic spindle. Therefore, the change in the STMN1 gene may be a potential predictive factor of response to treatment regimens containing a cytostatics-disrupting microtubule dynamics (vinca alkaloids and taxoids). The aim of the study was to determine the relationship between a single nucleotide polymorphism (SNP) of the promoter of STMN1 gene −2166T>C) and the effectiveness of chemotherapy based on platinum compounds and vinorelbine in patients with NSCLC.

Methods

The investigated population consisted of 110 locally advanced or metastatic NSCLC patients treated with first-line chemotherapy, based on platinum compounds and vinorelbine. SNP was determined by SNaPshot™ PCR in DNA isolated from peripheral blood leukocytes.

Results

The median progression-free survival (PFS) was significantly shorter in carriers of TT genotype of the STMN1 gene compared with patients with CC or CT genotypes (2.75 vs. 6.5 months; p = 0.0033; HR 5.91, 95 % CI 1.81–19.33). Evaluated SNP did not significantly affect the response to treatment and the overall survival of the patients.

Conclusion

Rare TT genotype of STMN1 gene may be an unfavorable predictive factor of chemotherapy based on platinum compounds and vinorelbine, in patients with NSCLC.

Cytidine deaminase polymorphism predicts toxicity of gemcitabine-based chemotherapy

BACKGROUND

The aim of this study was to ascertain whether single nucleotide polymorphisms of cytidine deaminase (CDA), a key enzyme in the metabolism pathway of gemcitabine, could predict clinical outcomes of cancer patients with gemcitabine-based chemotherapy.

METHODS

We searched MEDLINE and EMBASE up to January 2013 to identify eligible studies. A rigorous quality assessment of eligible studies was conducted according to the Newcastle-Ottawa Quality Assessment Scale. For each included study, the overall survival (OS), overall response rate (ORR) and toxicities were extracted and pooled using random-effects model.

RESULTS

In total, data from 13 studies were included. CDA 208A>G and CDA 435C>T were not included in quantified synthesis due to limited data. CDA 79A>C polymorphism was not significantly associated with OS; however, patients carrying the variant CDA 79C allele were likely to have a poor survival, hazard ratio (HR)=1.03, 95% CI 0.957-1.27 (AC+CC vs. AA). CDA 79A>C polymorphism did not correlated with ORR, odds ratio (OR)=0.719, 95% CI 0.363-1.425 (AC+CC vs. AA). However, patients with the variant CDA 79C allele would experience more grade ≥ 3 leucopenia (OR=2.933, 95% CI 1.357-6.605) and tended to have more severe neutropenia (OR=1.313, 95% CI 0.157-10.981).

CONCLUSIONS

These results suggest that CDA 79A>C polymorphisms is a potential biomarker for toxicity of gemcitabine-based chemotherapy and a CDA testing before gemcitabine administration is preferred.

The Impact of CDA A79C Gene Polymorphisms on the Response and Hematologic Toxicity in Gemcitabine-Treated Patients: A Meta-Analysis

Purpose

To investigate the impact of the cytidine deaminase (CDA) A79C polymorphism on both the response to gemcitabine in non-small cell lung cancer (NSCLC) patients and the risk of hematologic toxicities in patients bearing any kind of cancer taking gemcitabine.

Methods

The PubMed and Embase databases were searched from the first available article to January 2013. Eligible studies included clinical trials that contained the keywords “gemcitabine” or “cytidine deaminase” and information about response rate of NSCLC patients or hematologic toxicities in patients with any kind of cancer. Relative risk (RR) of different genotypes and 95% confidence intervals (CI) were calculated.

Results

A total of 7 articles (623 patients from 6 studies) were included. The results showed that patients with wild type CDA (AA and AC) had a significantly lower rate of severe anemia than the homozygote mutant type CC (RR=0.308; 95%CI, 0.113-0.021, p=0.021). However, the rate of severe neutropenia, thrombocytopenia, and the response rate were identical between different CDA genotypes.

Conclusion

The A79C CDA polymorphism did not show a significant impact on the response rate to gemcitabine in NSCLC patients, while the wild type CDA genotype was indeed correlated to a lower rate of incidence of severe anemia in patients taking gemcitabine.

SLC28A3 genotype and gemcitabine rate of infusion affect dFdCTP metabolite disposition in patients with solid tumours

Background:

Gemcitabine is used for the treatment of several solid tumours and exhibits high inter-individual pharmacokinetic variability. In this study, we explore possible predictive covariates on drug and metabolite disposition.

Methods:

Forty patients were enrolled. Gemcitabine and dFdU concentrations in the plasma and dFdCTP concentrations in peripheral blood mononuclear cell were measured to 72 h post infusion, and pharmacokinetic parameters were estimated by nonlinear mixed-effects modelling. Patient-specific covariates were tested in model development.

Results:

The pharmacokinetics of gemcitabine was best described by a two-compartment model with body surface area, age and NT5C2 genotype as significant covariates. The pharmacokinetics of dFdU and dFdCTP were adequately described by three-compartment models. Creatinine clearance and cytidine deaminase genotype were significant covariates for dFdU pharmacokinetics. Rate of infusion of <25 mg m⁻² min⁻¹ and the presence of homozygous major allele for SLC28A3 (CC genotype) were each associated with an almost two-fold increase in the formation clearance of dFdCTP.

Conclusion:

Prolonged dFdCTP systemic exposures (⩾72 h) were commonly observed. Infusion rate <25 mg m⁻² min⁻¹ and carriers for SLC28A3 variant were each associated with about two-fold higher dFdCTP formation clearance. The impacts of these covariates on treatment-related toxicity in more selected patient populations (that is, first-line treatment, single disease state and so on) are not yet clear.

Safety and pharmacology of gemcitabine and capecitabine in patients with advanced pancreatico-biliary cancer and hepatic dysfunction

PURPOSE

We assessed the impact of hepatic dysfunction on the safety and pharmacology of gemcitabine/capecitabine in patients with advanced pancreatico-biliary cancer.

METHODS

We included 12 patients receiving 3 weekly gemcitabine 1,000 mg/m(2) day 1, 8 and oral capecitabine 650 mg/m(2) b.i.d. over 2 weeks until disease progression or intolerable toxicity. Patients were included into one normal hepatic function cohort [total bilirubin (TB) ≤15 μmol/L] and 3 cohorts with increasing TB (16-39, 40-80, >80 μmol/L). Three patients with a creatinine clearance <60 ml/min were also included. Patients were sampled for gemcitabine, difluoro-deoxy uridine, intracellular gemcitabine triphosphates, capecitabine, 5'-deoxy-5-fluorocytidine, 5'-deoxy-5-fluorouridine and 5-fluorouracil up to 4 h after initiation of chemotherapy on day 1, and up to 90 min on day 8. All compounds were analyzed using validated liquid chromatography-tandem mass spectrometry. Nonlinear mixed-effect modeling was used for population analysis.

RESULTS

Hepatic dysfunction was caused by intrahepatic cholestasis in 4 out of 8 patients (50 %) and extrahepatic cholestasis in another 4 patients (50 %). Dose-limiting toxicity was increasing hyperbilirubinemia and severe neutropenia in 2 patients each. Hepatic dysfunction was not associated with dose-limiting toxicity or severe hematological or non-hematological toxicity. However, hepatic dysfunction was associated with low clearance of both gemcitabine (p = 10(-3)) and capecitabine (p = 10(-5)), and low intracellular gemcitabine triphosphate concentrations (p = 10(-3)).

CONCLUSIONS

Gemcitabine/capecitabine can be given at the standard dose in patients with severe hyperbilirubinemia, though the present data suggest that gemcitabine's activity may be limited due to poor intracellular activation. In patients with severe hyperbilirubinemia, initial monotherapy with capecitabine should be considered, followed by the addition of gemcitabine with improving hyperbilirubinemia.

Predicting toxicity in advanced lung cancer patients treated with platinum-based chemotherapy

SUMMARY Platinum-based chemotherapy is currently the state-of-the-art first-line cytotoxic treatment of advanced-stage small-cell lung cancers and non-small-cell lung cancers, but at the cost of potentially severe toxicity. As platinum-related adverse events may impede the success of palliative chemotherapy, while negatively affecting the patients’ quality of life, treatment strategies should aim at optimizing the toxicity:benefit ratio. The identification and application of clinical and laboratory biomarkers, including pharamacogenetic tools, for reliably predicting toxicity in this setting would significantly contribute to such a customized approach. This is a comprehensive overview of the toxicity-prediction factors with the most potential, including ethnicity, age, gender, performance status, comorbidity and nutrition, as well as molecular biomarkers, such as germline genetic polymorphisms.

Pharmacogenetics of conventional chemotherapy in non-small-cell lung cancer: a changing landscape?

Pharmacogenetics might be used to select patients who may benefit from specific chemotherapy that best matches the individual and tumor genetic profile, thus allowing maximum activity and minimal toxicity. Even if most studies in non-small-cell lung cancer yielded contradictory results, several potential biomarkers for sensitivity/resistance to platinum compounds, gemcitabine, taxanes and pemetrexed have been proposed. However, these markers need to be validated within larger prospective randomized trials of customized chemotherapy in homogeneous populations. Other critical points include the optimization/standardization of technical procedures, and further studies to unravel the extremely complex regulation of gene function. From this perspective, the evaluation of key factors influencing genotype-phenotype relationships, such as miRNAs, and functional studies to clarify pharmacokinetic/pharmacodynamic interactions, are fundamental for the pharmacogenetic optimization of cancer chemotherapy. Finally, limitation of the traditional pharmacogenetic approach relying only on candidate genes suspected of affecting drug response is now being overcome by the use of novel genome-wide studies.

Anticancer metallodrug research analytically painting the "omics" picture--current developments and future trends

Anticancer metallodrug development has for a long time been characterised by the similarity of new drug candidates to cisplatin and DNA as the primary target. Recent advances in bioanalytical techniques with high sensitivity and selectivity have revealed that metal-based drugs can undergo a wide range of biomolecular interactions beyond DNA and have generated interest in proteins as possible targets for metallodrugs. In fact, implementation of metallomics approaches that are able to reveal the fate of the compounds in biological systems can help to move drug development towards more targeted and rational design of novel metallodrugs. Additionally, proteomic screening and gene expression analysis can provide insight into physiological response to drug treatment and identify the reasons for drug resistance. Herein, we review selected applications which led to a better understanding of the mode of action of clinically established metal-based anticancer agents and novel metallodrug candidates.

High incidence of severe neutropenia after gemcitabine-based chemotherapy in Chinese cancer patients with CDA 79A>C mutation

Cytidine deaminase (CDA) is a crucial enzyme in gemcitabine inactivation. Mutations in CDA gene have been found to influence the activity of CDA enzyme, which might lead to altered pharmacokinetics profile and clinical outcome of gemcitabine. In this study, the screening for the presence of CDA 79A>C and CDA 208G>A was performed by allele-specific PCR and restriction fragment length polymorphism, respectively. No difference in CDA allele frequencies was found between Chinese cancer patients and healthy volunteers. The frequencies for CDA 79A>C and 208G>A in the studied population were 12.2% and 1.0%, respectively. While a high incidence of grade 3-4 neutropenia was noted as 5 out of 8 (62.5%) patients heterozygous or homozygous for CDA 79A>C mutation developed it, in patients homozygous for the wild-type allele, the incidence was only 17.2% (5 out of 29) (p=0.021). Our study suggests that CDA 79A>C mutation might be a potential risk factor of gemcitabine-induced neutropenia toxicity.

Covariate pharmacokinetic model building in oncology and its potential clinical relevance

When modeling pharmacokinetic (PK) data, identifying covariates is important in explaining interindividual variability, and thus increasing the predictive value of the model. Nonlinear mixed-effects modeling with stepwise covariate modeling is frequently used to build structural covariate models, and the most commonly used software-NONMEM-provides estimations for the fixed-effect parameters (e.g., drug clearance), interindividual and residual unidentified random effects. The aim of covariate modeling is not only to find covariates that significantly influence the population PK parameters, but also to provide dosing recommendations for a certain drug under different conditions, e.g., organ dysfunction, combination chemotherapy. A true covariate is usually seen as one that carries unique information on a structural model parameter. Covariate models have improved our understanding of the pharmacology of many anticancer drugs, including busulfan or melphalan that are part of high-dose pretransplant treatments, the antifolate methotrexate whose elimination is strongly dependent on GFR and comedication, the taxanes and tyrosine kinase inhibitors, the latter being subject of cytochrome p450 3A4 (CYP3A4) associated metabolism. The purpose of this review article is to provide a tool to help understand population covariate analysis and their potential implications for the clinic. Accordingly, several population covariate models are listed, and their clinical relevance is discussed. The target audience of this article are clinical oncologists with a special interest in clinical and mathematical pharmacology.