Predictive Modeling of Tacrolimus Dose Requirement Based on High-Throughput Genetic Screening

Any biochemical reaction underlying drug metabolism depends on individual gene-drug interactions and on groups of genes interacting together. Based on a high-throughput genetic approach, we sought to identify a set of covariant single-nucleotide polymorphisms predictive of interindividual tacrolimus (Tac) dose requirement variability. Tac blood concentrations (Tac C₀ ) of 229 kidney transplant recipients were repeatedly monitored after transplantation over 3 mo. Given the high dimension of the genomic data in comparison to the low number of observations and the high multicolinearity among the variables (gene variants), we developed an original predictive approach that integrates an ensemble variable-selection strategy to reinforce the stability of the variable-selection process and multivariate modeling. Our predictive models explained up to 70% of total variability in Tac C₀ per dose with a maximum of 44 gene variants (p-value <0.001 with a permutation test). These models included molecular networks of drug metabolism with oxidoreductase activities and the multidrug-resistant ABCC8 transporter, which was found in the most stringent model. Finally, we identified an intronic variant of the gene encoding SLC28A3, a drug transporter, as a key gene involved in Tac metabolism, and we confirmed it in an independent validation cohort.

Interindividual variability in dabigatran and rivaroxaban exposure: contribution of ABCB1 genetic polymorphisms and interaction with clarithromycin

Essentials Rivaroxaban and dabigatran are substrates of the P-glycoprotein (P-gp) encoded by the ABCB1 gene. We tested the effect of ABCB1 polymorphisms and of a P-gp inhibitor on both drugs' pharmacokinetics. The ABCB1 genotype was not a clinically relevant determinant of both drugs' pharmacokinetics. Administration of P-gp inhibitors with dabigatran or rivaroxaban should be exercised with caution.

SUMMARY

Background The direct oral anticoagulants (DOACs) dabigatran and rivaroxaban are both substrates of the P-glycoprotein (P-gp) transporter, encoded by the ABCB1 gene. Rivaroxaban is metabolized by cytochrome P450 A4 (CYP3A4). Interindividual variability in DOAC exposure and frequent P-gp-associated drug-drug interactions have been described in patients. Objective To assess the influence of ABCB1 polymorphisms on the pharmacokinetics of dabigatran and rivaroxaban, associated or not with clarithromycin, a P-gp and CYP3A4 inhibitor. Methods Sixty healthy male volunteers, selected according to ABCB1 genotype (20 homozygous mutated, 20 heterozygous mutated, and 20 wild-type for haplotype 2677-3435), were included in this randomized, two-center, crossover study. All received sequentially a single dose of dabigatran etexilate (300 mg) and rivaroxaban (40 mg) associated or not with clarithromycin. Peak plasma concentration and area under the curve (AUC) were compared across the three ABCB1 genotypes. The effect of clarithromycin on dabigatran or rivaroxaban pharmacokinetics was assessed. Results Interindividual coefficients of variation for AUC were 77% for dabigatran and 51% for rivaroxaban. ABCB1 genotype did not significantly affect drug pharmacokinetics: AUC ratios between mutant-allele carriers and wild-type volunteers were 1.27 (95% confidence interval [CI] 0.84-1.92) and 1.20 (95% CI 0.96-1.51) for dabigatran and rivaroxaban, respectively. Clarithromycin coadministration led to a two-fold increase in both drugs' AUC, irrespective of ABCB1 genotype: ratios of geometric means were 2.0 (95% CI 1.15-3.60) and 1.94 (95% CI 1.42-2.63) for dabigatran and rivaroxaban, respectively. Conclusions ABCB1 genotype is not a significant determinant of interindividual variability in dabigatran and rivaroxaban pharmacokinetics. The levels of one drug did not predict the levels of the other. Coadministration of a P-gp/CYP3A4 inhibitor with dabigatran or rivaroxaban may warrant caution in patients at risk of overexposure.

Long-Term Clinical Impact of Adaptation of Initial Tacrolimus Dosing to CYP3A5 Genotype

Pretransplantation adaptation of the daily dose of tacrolimus to CYP3A5 genotype is associated with improved achievement of target trough concentration (C0 ), but whether this improvement affects clinical outcomes is unknown. In the present study, we have evaluated the long-term clinical impact of the adaptation of initial tacrolimus dosing according to CYP3A5 genotype: The transplantation outcomes of the 236 kidney transplant recipients included in the Tactique study were retrospectively investigated over a period of more than 5 years. In the Tactique study, patients were randomly assigned to receive tacrolimus at either a fixed dosage or a dosage determined by their genotype, and the primary efficacy end point was the proportion of patients for whom tacrolimus C0 was within target range (10-15 ng/mL) at day 10. Our results indicate that the incidence of biopsy-proven acute rejection and graft survival were similar between the control and the adapted tacrolimus dose groups, as well as between the patients who achieve the tacrolimus C0 target ranges earlier. Patients' death, cancer, cardiovascular events, and infections were also similar, and renal function did not change. We conclude that optimization of initial tacrolimus dose using pharmacogenetic testing does not improve clinical outcomes.

A double amino-acid change in the HLA-A peptide-binding groove is associated with response to psychotropic treatment in patients with schizophrenia

The choice of an efficient psychotropic treatment for patients with schizophrenia is a key issue to improve prognosis and quality of life and to decrease the related burden and costs. As for other complex disorders, response to drugs in schizophrenia is highly heterogeneous and the underlying molecular mechanisms of this diversity are still poorly understood. In a carefully followed-up cohort of schizophrenic patients prospectively treated with risperidone or olanzapine, we used a specially designed single-nucleotide polymorphism (SNP) array to perform a large-scale genomic analysis and identify genetic variants associated with response to psychotropic drugs. We found significant associations between response to treatment defined by the reduction in psychotic symptomatology 42 days after the beginning of treatment and SNPs located in the chromosome 6, which houses the human leukocyte antigen (HLA). After imputation of the conventional HLA class I and class II alleles, as well as the amino-acid variants, we observed a striking association between a better response to treatment and a double amino-acid variant at positions 62 and 66 of the peptide-binding groove of the HLA-A molecule. These results support the current notion that schizophrenia may have immune-inflammatory underpinnings and may contribute to pave the way for personalized treatments in schizophrenia.

Kidney transplant recipients carrying the CYP3A4*22 allelic variant have reduced tacrolimus clearance and often reach supratherapeutic tacrolimus concentrations

CYP3A4*22 is an allelic variant of the cytochrome P450 3A4 associated with a decreased activity. Carriers of this polymorphism may require reduced tacrolimus (Tac) doses to reach the target residual concentrations (Co). We tested this hypothesis in a population of kidney transplant recipients extracted from a multicenter, prospective and randomized study. Among the 186 kidney transplant recipients included, 9.3% (18 patients) were heterozygous for the CYP3A4*22 genotype and none were homozygous (allele frequency of 4.8%). Ten days after transplantation (3 days after starting treatment with Tac), 11% of the CYP3A4*22 carriers were within the target range of Tac Co (10-15 ng/mL), whereas among the CYP3A4*1/*1 carriers, 40% were within the target range (p = 0.02, OR = 0.19 [0.03; 0.69]). The mean Tac Co at day 10 in the CYP3A4*1/*22 group was 23.5 ng/mL (16.6-30.9) compared with 15.1 ng/mL (14-16.3) in the CYP3A4*1/*1 group, p < 0.001. The Tac Co/dose significantly depended on the CYP3A4 genotype during the follow-up (random effects model, p < 0.001) with the corresponding equivalent dose for patients heterozygous for CYP3A4*22 being 0.67 [0.54; 0.84] times the dose for CYP3A4*1/*1 carriers. In conclusion, the CYP3A4*22 allelic variant is associated with a significantly altered Tac metabolism and carriers of this polymorphism often reach supratherapeutic concentrations.

Influence of genetics and non-genetic factors on acenocoumarol maintenance dose requirement in Moroccan patients

WHAT IS KNOWN AND OBJECTIVE

Coumarin derivatives such as acenocoumarol represent the therapy of choice for the long-term treatment and prevention of thromboembolic diseases. Many genetic, clinical and demographic factors have been shown to influence the anticoagulant dosage. Our aim was to investigate the contribution of genetic and non-genetic factors to variability in response to acenocoumarol in Moroccan patients.

METHODS

Our study included 114 adult Moroccan patients, receiving long-term acenocoumarol therapy for various indications. Tests for VKORC1 -1639G>A promoter polymorphism (rs9923231), CYP2C9*2 rs1799853, CYP2C9*3 rs1057910, and CYP4F2 rs2108622 alleles were undertaken using Taq Man(®) Pre-Developed Assay Reagents for allelic discrimination. The statistical analysis was performed using the SAS V9 statistical package.

RESULTS AND DISCUSSION

Genotyping showed that the allele frequencies for the SNPs studied were no different to those found in Caucasians population. A significant association was observed between the weekly maintenance dose and the VKORC1 (P = 0·0027) and CYP2C9 variant genotypes (P = 0·0082). A final multivariate regression model that included the target International Normalized Ratio, VKORC1 and CYP2C9 genotypes explained 36·2% of the overall interindividual variability in acenocoumarol dose requirement.

WHAT IS NEW AND CONCLUSION

Our study shows large interindividual variability in acenocoumarol maintenance dose requirement in our population. VKORC1 and CYP2C9 variants significantly affected acenocoumarol dose, in-line with results in other populations. For the Moroccan population, the SNPs that have the largest effect on acecoumarol dose are CYP2C9 rs1799853, CYP2C9 rs1057910 and VKORC1 rs9923231.

Review article: the benefits of pharmacogenetics for improving thiopurine therapy in inflammatory bowel disease

BACKGROUND

Thiopurines represent an effective and widely prescribed therapy in inflammatory bowel disease (IBD). Concerns about toxicity, mainly resulting from a wide inter-individual variability in thiopurine metabolism, restrict their use. Optimal thiopurine dosing is challenging for preventing adverse drug reactions and improving clinical response.

AIM

To review efficacy and toxicity of thiopurines in IBD. To provide pharmacogenetic-based therapeutic recommendations.

METHODS

We conducted a query on PubMed database using 'inflammatory bowel disease', 'thiopurine', 'azathioprine', '6-mercaptopurine', 'TPMT', 'pharmacogenetics', 'TDM', and selected relevant articles, especially clinical studies.

RESULTS

Thiopurine metabolism - key enzyme: thiopurine S-methyltransferase (TPMT) - modulates clinical response, as it results in production of the pharmacologically active and toxic metabolites, the thioguanine nucleotides (6-TGN). Adjusting dosage according to TPMT status and/or metabolite blood levels is recommended for optimising thiopurine therapy (e.g. improving response rate up to 30% or decreasing haematological adverse events of 25%). Other enzymes or transporters of interest, as inosine triphosphatase (ITPase), glutathione S-transferase (GST), xanthine oxidase (XO), aldehyde oxidase (AOX), methylene tetrahydrofolate reductase (MTHFR) and ATP-binding cassette sub-family C member 4 (ABCC4) are reviewed and discussed for clinical relevance.

CONCLUSIONS

Based on the literature data, we provide a therapeutic algorithm for thiopurines therapy with starting dose recommendations depending on TPMT status and thereafter dose adjustments according to five metabolite profiles identified with therapeutic drug monitoring (TDM). This algorithm allows a dosage individualisation to optimise the management of patients under thiopurine. Furthermore, identification of new pharmacogenetic biomarkers is promising for ensuring maximal therapeutic response to thiopurines with a minimisation of the risk for adverse events.

Metabolic stress promotes renal tubular inflammation by triggering the unfolded protein response

The renal epithelium contributes to the development of inflammation during ischemic injury. Ischemia induces endoplasmic reticulum (ER) stress and activates the unfolded protein response (UPR). Ischemic tissues generate distress signals and inflammation that activates fibrogenesis and may promote adaptive immunity. Interestingly, the UPR may activate inflammation pathways. Our aim was to test whether the UPR is activated during metabolic stress and mediates a tubular inflammatory response. Glucose deprivation, not hypoxia and amino acids deprivation, activated the UPR in human renal cortical tubular cells in culture. This stress activated NF-κB and promoted the transcription of proinflammatory cytokines and chemokines, including IL-6, IL-8, TNF-α, RANTES and MCP-1. The protein kinase RNA (PKR)-like ER kinase signaling pathway was not required for the induction of inflammation but amplified cytokine. Inositol-requiring enzyme 1 activated NF-κB signaling and was required for the transcription of proinflammatory cytokines and chemokines following metabolic stress. Moreover, acute ischemia activated ER stress and inflammation in rat kidneys. Finally, the ER stress marker GRP78 and NF-κB p65/RelA were coexpressed in human kidney transplants biopsies performed before implantation, suggesting that ER stress activates tubular inflammation in human renal allografts. In conclusion, this study establishes a link between ischemic stress, the activation of the UPR and the generation of a tubular inflammatory response.

Optimization of initial tacrolimus dose using pharmacogenetic testing

Retrospective studies have demonstrated that patients who are expressors of cytochrome P4503A5 (CYP3A5) require a higher tacrolimus dose to achieve a therapeutic trough concentration (C(0)). The aim of this study was to evaluate this effect prospectively by pretransplantation adaptation. We randomly assigned 280 renal transplant recipients to receive tacrolimus either according to CYP3A5 genotype or according to the standard daily regimen. The primary end point was the proportion of patients within the targeted C(0). Secondary end points included the number of dose modifications and the delay in achieving the targeted C(0). In the group receiving the adapted dose, a higher proportion of patients had values within the targeted C(0) at day 3 after initiation of tacrolimus (43.2% vs. 29.1%; P = 0.03); they required fewer dose modifications, and the targeted C(0) was achieved by 75% of these patients more rapidly. The clinical end points were similar in the two groups. Pharmacogenetic adaptation of the daily dose of tacrolimus is associated with improved achievement of the target C(0). Whether this improvement will affect clinical outcomes requires further evaluation.

Cyclosporine-induced endoplasmic reticulum stress triggers tubular phenotypic changes and death

The molecular mechanisms by which cyclosporine induces chronic nephrotoxicity remain poorly understood. A previous transcriptomic study suggested that cyclosporine might induce endoplasmic reticulum (ER) stress in human tubular cells. The aim of the present study was to characterize the features of tubular ER stress induced by cyclosporine and to investigate its effects on cell differentiation and viability. Using primary cultures of human tubular cells, we confirmed that cyclosporine is responsible for ER stress in vitro. This was also confirmed in vivo in the rat. In vitro, cyclosporine and other ER stress inducers were responsible for epithelial phenotypic changes leading to the generation of protomyofibroblasts, independent of transforming growth factor-beta signaling. RNA interference directed against cyclophilin A supported the role of its inhibition in triggering ER stress as well as epithelial phenotypic changes induced by cyclosporine. Salubrinal, which is known to protect cells from ER stress, significantly reduced epithelial phenotypic changes and cytotoxicity induced by cyclosporine in vitro. Salubrinal also reduced cyclosporine nephrotoxicity in rat kidneys. Thus, we describe a novel mechanism that initiates dedifferentiation and tubular cell death upon cyclosporine treatment. These results provide an interesting framework for further nephroprotective therapies by targeting ER stress.

La vitamine K époxyde réductase: du sang neuf dans les traitements anticoagulants oraux

INTRODUCTION

Vitamin K epoxide reductase complex subunit I (VKORC1) is a key enzyme in the vitamin K cycle, cofactor required for the activation of vitamin K-dependent clotting factors.

EXEGESIS

VKORC1 recycles vitamin K 2,3 epoxide back to active vitamin K hydroquinone, an important factor for the carboxylation step of clotting factors. VKORC1 is the target enzyme of inhibition by oral anticoagulants or anti-vitamin K (warfarin, acenocoumarol).

CONCLUSION

We show here the clinical consequences of genetic variations of VKORC1 during VKA therapy.

Comparison of different cellular models measuring in vitro the whole human serum cholesterol efflux capacity

BACKGROUND

Fu5AH rat hepatoma cells and cAMP (cyclic AMP)-pretreated J774 mouse macrophages are commonly used as models for SR-BI (scavenger receptor class B type I) and ABCA1 (ATP binding cassette transporter 1)-mediated free cholesterol efflux to whole serum, respectively. However, the responsiveness of Fu5AH, control or cAMP pretreated J774 cells to the various lipids and HDL (high-density lipoprotein)-parameters from both normo- and dyslipidaemic subjects has never been compared within the same study.

MATERIALS AND METHODS

Fifty-eight men were classified into four groups: type IIa hypercholesterolaemic (n = 12), type IIb dyslipidaemic (n = 13), type IV hypertriglyceridaemic (n = 18) and normolipidaemic (n = 15) were recruited. A complete lipid profile including prebeta-HDL was performed. Cholesterol efflux from Fu5AH cells as well as from control or cAMP pretreated J774 cells were measured; the difference between these two latter values being taken as the ABCA1-mediated efflux.

RESULTS

The Fu5AH and the control J774 cells delivered cholesterol to mature HDLs, especially to phospholipid (PL)-rich HDL. Using cAMP pretreated cells, the ABCA1-dependent efflux was highly sensitive to prebeta-HDL, which appeared to be a factor in determining the efflux. Consistent with the dependence of the SR-BI-mediated efflux on HDL-PL levels, which are not different between groups, all sera displayed similar efflux capacities from the Fu5AH cells. Conversely, in accordance with their high prebeta-HDL levels, the ABCA1-dependent efflux highlighted the efficiency of type IV sera.

CONCLUSION

Two complementary cellular models providing SR-BI and ABCA1-dependent efflux should be used to measure the capacity of a biological fluid which contains a wide variety of components to promote cholesterol efflux.

Role of P-glycoprotein in cyclosporine cytotoxicity in the cyclosporine-sirolimus interaction

Cyclosporine nephrotoxicity remains a major side effect in solid organ transplantation, and can be exacerbated by concomitant administration of sirolimus. Cyclosporine and sirolimus are P-glycoprotein (Pgp) substrates. We hypothesized that the Pgp activity level may affect cyclosporine cytotoxicity by interfering with the ability of Pgp to remove cyclosporine from within tubular cells, and that an interaction between cyclosporine and sirolimus on Pgp function may explain the enhancement of cyclosporine nephrotoxicity by sirolimus. Cyclosporine cytotoxicity was evaluated in primary cultures of normal human renal epithelial cells (HRECs) by cell viability and cytotoxicity assays. Verapamil, quinine, PSC833, and PGP-4008 were used as Pgp inhibitors. Rhodamine-123 (R-123), a fluorescent substrate of Pgp, was used to assess Pgp-mediated transport. Cellular cyclosporine concentration was measured by high-performance liquid chromatography coupled to tandem mass spectrometry. Pgp expression and function were confirmed in HRECs and cyclosporine and sirolimus were shown to be Pgp inhibitors in this model. Verapamil-induced inhibition of Pgp led to a significant increase in cellular concentration of cyclosporine (P<0.05). Cyclosporine exerted a concentration-dependent cytotoxic effect on HRECs that was significantly increased by inhibition of Pgp activity. Sirolimus exerted an inhibitory effect on R-123 efflux in HRECs and increased cellular cyclosporine concentrations in a dose-dependent manner. These data demonstrate that Pgp plays a critical role in protecting renal epithelial cells from cyclosporine toxicity. The inhibitory effect of sirolimus on Pgp-mediated efflux and the cellular concentration of cyclosporine could explain the exacerbation of cyclosporine nephrotoxicity observed clinically.

Clinicopathological significance of mitochondrial D-Loop mutations in head and neck carcinoma

Mitochondrial DNA mutations have been reported in several types of tumours, including head and neck squamous cell carcinoma (HNSCC). The noncoding region of the Displacement-Loop (D-Loop) has emerged as a mutational hotspot and we recently found that they were associated with prognosis and response to 5 fluorouracil (5FU) in colon cancers. In order to evaluate the frequence of D-Loop mutations in a large series of HNSCC and establish correlations with clinicopathologic parameters, we sequenced the D-Loop of 109 HNSCC before a treatment by neoadjuvant 5FU-cisplatin-based chemotherapy and surgery. Then, we correlated these mutations with prognosis and response to chemotherapy. A D-Loop mutation was identified in 21% of the tumors, the majority of them were located in a C-tract (D310). The prevalence of D310 mutations increased significantly with the number of cytosines in the matched normal tissue sequence (P=0.02). Hypopharyngeal cancer was significantly more frequent (P=0.03) and tobacco consumption more important (P=0.01) in the group of patients with D-Loop mutation. The presence of D-Loop mutation was not associated with prognosis or with response to neoadjuvant chemotherapy. These results suggest that D-Loop mutations should be considered as a cancer biomarker that may be useful for the early detection of HNSCC in individuals at risk of this cancer.

[Rapamycine and mTOR inhibitors: from bench to bedside]

Rapamycin is a macrocyclic lactone with antifungal and antibiotic properties isolated from Streptomyces hygroscopicus during the 70's. Studies of rapamycin properties in yeast led to the discovery of TOR (Target Of Rapamycin) and its mammalian analogue, mTOR. mTOR is a central regulator of cell growth and proliferation in response to environmental stimuli such as growth factors or nutrients. There are two proteins that have been shown to be regulated by mTOR in response to a broad range of mitogenic stimuli. The translation regulation induced by mTOR is mediated by the p70 S6 kinase activation and the 4E-BP1 inhibition. Both proteins participate in the regulation of translation process and growth in cells stimulated by either mitogens or hormones. Antiproliferative effects of rapamycin and analogues have been demonstrated on numerous cell types, explaining the development of these drugs in clinical practice: as immunosuppressive drugs in solid organ transplantation, in oncology for the treatment of various types of cancer, and for the prevention of restenosis after coronary angioplasty. Rapamycin is a potent immunosuppressive drug used in solid organ transplantation for the prevention of acute rejection. In oncology these antiproliferative effects are evaluated in several types of cancers. Rapamycin is now widely used for coating stents to reduce post-stenting restenosis phenomenon after coronary angioplasty. Finally, rapamycin is now evaluated in various diseases characterized by proliferative disorders.

TEGDMA modulates glutathione transferase P1 activity in gingival fibroblasts

Dental resinous materials can contain large amounts (from 30 to 50%) of triethylene-glycol-dimethacrylate (TEGDMA). This compound leaches into aqueous media and is toxic to dental pulp, as well as to gingival fibroblasts in vitro. To elucidate the mechanism of TEGDMA toxicity, we investigated the effects on glutathione (GSH) level and glutathione transferase P1 (GSTP1) activity in cultured human gingival fibroblasts. TEGDMA cytotoxic concentrations (from 0.5 to 2 mM) induced a depletion of GSH without formation of oxidized GSH (GSSG). In fibroblasts expressing the wild-type GSTP1, TEGDMA both inhibited and potentiated GSTP1 activity at high (IC50 = 1.1 mM) and low concentrations, respectively. In contrast, cells expressing the GSTP1 *A/*B variant showed a weak inhibition of GST activity only, associated with greater sensitivity to drug toxicity. Biochemical analysis of GSTP1 inhibition revealed that TEGDMA is a non-competitive antagonist with respect to GSH and substrate. Thus, TEGDMA interference with GSH and GSTP1 activity may contribute to dental-resin-induced adverse effects.

[A new, rapid and robust genotyping method for CYP2C9 and MDR1]

Single nucleotide polymorphisms (SNPs) can significantly affect human phenotypes. Detection of allelic variant carriers has become a major goal for clinical pharmacologists in order to study phenotype-genotype relationships. However, there is a crucial need for rapid, and validated pharmacogenetic tests. The aim of the study was to validate a new fluorescence PCR strategy for cytochrome P450 2C9 (CYP2C9) and multidrug resistance gene (MDR1) genotyping. Results of CYP2C9 and MDR1 genotypes determined with reference techniques were compared to those obtained by allelic discrimination assays employing fluorescent TaqMan probes. Sixteen subjects carrying CYP2C9*2 and CYP2C9*3 allelic variants (heterozygous and homozygous) previously identified by sequencing and 55 subjects previously genotyped for MDR1 exon 26 (C3435T) SNP by conventional PCR-RFLP were genotyped with fluorescent PCR. Fluorescent PCR gave 100 % accuracy with the results obtained with reference genotyping strategies for each of the 3 SNPs. Genotyping results with fluorescent PCR repeated on three consecutive occasions remained constant over time for each of the 3 SNPs. Allelic discrimination assays based on fluorescent PCR gave entire satisfaction for CYP2C9 and MDR1 genotyping. This reliable genotyping strategy can be easily used in clinical practice and should be further developed for additional SNPs identification.

Genetic and environmental risk factors for oral anticoagulant overdose

BACKGROUND

Cytochrome P450 2C9 (CYP2C9) allelic variant carriers have been shown to experience hyper-responsiveness to small doses of oral anticoagulants (OAs) (warfarin or acenocoumarol) and a higher bleeding rate.

OBJECTIVES

To determine the relative frequencies of different risk factors for OA overdose including diet, concomitant diseases, drug interactions, recent increment of OA dose and CYP2C9 genetic polymorphism among hospitalised patients.

MATERIALS AND METHODS

Frequencies of the different risk factors for OA overdose were determined in a prospective case-control study. Seventy-five consecutive patients with an International normalised ratio (INR) greater than 4 were matched with seventy-five control patients with an INR greater than 2 but less than 3.5 with respect to age, prescribed OA and daily dose. Genotyping of CYP2C9*2 and CYP2C9*3 allelic variants was detected by the TaqMan allelic discrimination assay.

RESULTS

Drug interactions and a recent increment of OA dose were the only significant independent risk factors identified in the first analysis with odds ratio 2.13 (95% CI: 1.06-4.28) and 3.38 (95%CI: 1.51-7.57), respectively. A recent increment of OA dose was the only significant independent risk factor identified among the patients treated with coumarin derivatives (acenocoumarol or warfarin), excluding those treated with fluindione; the odds ratio was 4.3 (95% CI: 1.5-12.3). CYP2C9 genetic polymorphism did not significantly predict the increased risk of OA overanticoagulation in this study. However three homozygous CYP2C9*3/CYP2C9*3 genotype patients were found among the cases, whereas no such patients could be identified among controls.

CONCLUSION

This is the first observational study investigating the role of CYP2C9 genetic polymorphism together with other environmental OA overdose risk factors. Our results support the view that although the CYP2C9*3/CYP2C9*3 genotype is associated soon after the introduction of OA with dramatic overanticoagulation, OA overdose is mostly related to environmental factors.

Role of specific cytochrome P450 enzymes in the N-oxidation of the antiarrhythmic agent mexiletine

1. Mexiletine is extensively metabolized in man by C- and N-oxidation and the aim of the present study was to characterize major cytochrome P450 enzyme(s) involved in the formation of N-hydroxymexiletine. 2. Incubations with genetically engineered microsomes indicated that the formation rate of N-hydroxymexiletine was highest in the presence of microsomes expressing high levels of either CYP1A2 or CYP2E1 and the formation of N-hydroxymexiletine by human liver microsomes was inhibited about 40% by antibodies directed against CYP1A1/1A2 or CYP2E1. Additional incubations demonstrated that formation of N-hydroxymexiletine was decreased 47 and 51% by furafylline, 40 microm and 120 microm, respectively, and decreased 55 and 67% by alpha-naphthoflavone, 1 microm and 3 microm, respectively (all p < 0.05 versus control). 3. The formation rate of N-hydroxymexiletine in human liver microsomes was highly correlated with CYP2B6 (RS-mexiletine, r = 0.7827; R-(-)-enantiomer, r = 0.7034; S-(+)-enantiomer, r = 0.7495), CYP2E1 (S-(+)-enantiomer, r = 0.7057) and CYP1A2 (RS-mexiletine, r = 0.5334; S-(+)-enantiomer, r = 0.6035). 4. In conclusion, we have demonstrated that CYP1A2 is a major human cytochrome P450 enzyme involved in the formation of N-hydroxymexiletine. However, other cytochrome P450 enzymes (CYP2E1 and CYP2B6) also appear to play a role in the N-oxidation of this drug.

Polymorphisms of human aryl hydrocarbon receptor (AhR) gene in a French population: relationship with CYP1A1 inducibility and lung cancer

The Ah receptor (AhR) is a ligand-dependent transcription factor that positively regulates the expression of the CYP1A1 gene. We investigated the genetic polymorphisms of the AhR gene including the promoter, and examined the link between these polymorphisms, CYP1A1 inducibility and the lung cancer incidence. The AhR promoter region and the 11 exons of 30 subjects were screened. Among the three polymorphisms found, two [(2417)(A/G) ((157)G/A)] have never been described previously. The (1721)(G/A) and (2417)(A/G) are localized in exon 10 and lead to Arg(554)Lys and Met(786)Val substitutions, respectively. The other polymorphism was found in the 5'-untranslated region, resulting in the substitution of a G by an A at position 157 (157)(G/A). To evaluate the frequency of this allelic variant found, a DNA library of a case-control study of lung cancer (162 controls and 177 patients) was studied. There is no significant association between (1721)(G/A), (157)(G/A) and lung cancer: (1721)(G/A) and (157)(G/A) were detected at the same allele frequency of 0.086 and 0.25, respectively in both controls and patients. (2417)(A/G) was found in only one control of 100 (allele frequency 0.005). Statistical analysis did not show any relationship between both (1721)(G/A) and (157)(G/A) polymorphisms found and CYP1A1 inducibility. Considering the rareness of the (2417)(A/G) allelic variant we were not able to evaluate its association with inducibility. In conclusion, none of the polymorphisms were found to play a key role in the CYP1A1 inducibility or in the susceptibility to develop lung cancer.

Early acenocoumarol overanticoagulation among cytochrome P450 2C9 poor metabolizers

Cytochrome P450 2C9 (CYP2C9) is the enzyme that terminates the anticoagulant effect of warfarin. The heterozygous carriers of the two allelic variants CYP2C9*2 and CYP2C9*3 have been associated with impaired warfarin metabolism and a higher risk of haemorrhage. Only three CYP2C9 poor metabolizers (CYP2C9*3/CYP2C9*3) initiating warfarin treatment have so far been identified, all of them with a dramatic overdose occurring a few days after treatment initiation. Acenocoumarol, another coumarinic anticoagulant, has recently been shown to be metabolized by CYP2C9. We report, for the first time, two cases of dramatic overanticoagulation occurring in patients starting acenocoumarol treatment while taking recommended doses (4 mg/day). In both cases, the overdose was discovered at the first INR control with values above 9. Genotyping revealed that the two patients were homozygous for the CYP2C9*3 allele. Our report highlights the need for CYP2C9 genotyping before starting oral anticoagulants in order to prevent early overanticoagulation episodes.

Glutathione-associated enzymes in head and neck squamous cell carcinoma and response to cisplatin-based neoadjuvant chemotherapy

Glutathione S-transferases (GSTs) are metabolic phase II enzymes that promote reactive metabolite elimination by conjugating them to glutathione (GSH). Because of their important role in xenobiotic metabolism and detoxification, they have been implicated in carcinogenesis processes, especially epithelium transformation. Moreover, their influence on response to chemotherapy in cancer patients has been demonstrated. Genetic polymorphisms for GSTM1, GSTT1 and GSTP1 have been found in human populations and have been shown to have phenotypic consequences. To investigate the role of GST enzymes in carcinogenesis and in response to chemotherapy in patients with head and neck squamous cell carcinoma (HNSCC), GSTP1, GSTM1 and GSTT1 were studied prospectively in a large series of HNSCC patients. Correlations between GST alterations, p53 mutation status and clinical response to chemotherapy were investigated. We showed that the risk of developing laryngeal cancer was increased by 2.6-fold [95% CI 1.6--6.1] in patients with the GSTM1 null genotype and by 2.8-fold [95% CI 0.9--8.1] in patients with the homozygous GSTP1 val105 genotype. Furthermore, individuals with this latter genotype were over-represented in the p53 mutation group (p = 0.05). After storage duration and hemolysis adjustment, a significantly lower plasmatic GSTP1 level was observed in complete responders compared with partial and non-responders (mean: 4.4 +/- 0.06 microg/l, 4.7 +/- 0.06 microg/l and 4.7 +/- 0.07 microg/l; p = 0.05), respectively. The prevalence of p53-mutated tumors was significantly higher in the group of non-responders (81%) compared with partial (60%) and complete responders (64%) (p = 0.05). Two types of multivariate analysis were performed including parameters that have been shown to influence response to chemotherapy significantly in univariate analysis. p53 mutations and high tumor stage are independent factors of non-response to chemotherapy, whereas plasmatic GSTP1 levels and low tumor stage are independent factors of complete response. Our data suggest that GST enzymes are associated with larynx cancer and that their use as predictive factors and treatment targets should be further explored.

Determination of microsatellite instability, p53 and K-RAS mutations in hepatic metastases from patients with colorectal cancer: relationship with response to 5-fluorouracil and survival

In vitro and clinical studies have suggested that high-frequency microsatellite instability (MSI-H) phenotype, p53 and K-ras mutations might influence the response to chemotherapy in a variety of tumors, including primary colorectal cancers (CRC). Unresectable hepatic metastases from CRC are commonly treated with 5-fluorouracil (5FU) and folinic acid. Since several new active drugs are now used for treating CRC, molecular determinants predictive to response to 5FU would thus be crucial for optimizing indications of chemotherapy to those patients. MSI-H phenotype, p53 and K-ras status were characterized in a prospective study of 56 patients with CRC metastatic to the liver and treated with 5FU-based chemotherapy. The objective response rate after a 3-month treatment was 32.1%. The prevalence of p53 mutations, K-ras mutations and MSI-H phenotype was 62.5%, 30.3% and 1.8%, respectively. No significant association was found between response to chemotherapy and p53 mutations (78% mutated tumors in responders vs. 55% in nonresponders; p = 0.10) and K-ras mutations (39% mutated tumors in responders vs. 26% in nonresponders; p = 0.34). Survival was longer for patients with p53-mutated metastases than for patients with unresected wild-type p53 metastases (median survival 15 months vs. 17 months; p = 0.06). The determination of the MSI-H phenotype, p53 and K-ras status in hepatic metastases from CRC does not discriminate a group of patients that should preferentially benefit from 5FU-based chemotherapy. The prognosis of patients with treated liver metastases is better when p53 is mutated.

Determination of microsatellite instability, p53 and K-RAS mutations in hepatic metastases from patients with colorectal cancer: relationship with response to 5-fluorouracil and survival

In vitro and clinical studies have suggested that high-frequency microsatellite instability (MSI-H) phenotype, p53 and K-ras mutations might influence the response to chemotherapy in a variety of tumors, including primary colorectal cancers (CRC). Unresectable hepatic metastases from CRC are commonly treated with 5-fluorouracil (5FU) and folinic acid. Since several new active drugs are now used for treating CRC, molecular determinants predictive to response to 5FU would thus be crucial for optimizing indications of chemotherapy to those patients. MSI-H phenotype, p53 and K-ras status were characterized in a prospective study of 56 patients with CRC metastatic to the liver and treated with 5FU-based chemotherapy. The objective response rate after a 3-month treatment was 32.1%. The prevalence of p53 mutations, K-ras mutations and MSI-H phenotype was 62.5%, 30.3% and 1.8%, respectively. No significant association was found between response to chemotherapy and p53 mutations (78% mutated tumors in responders vs. 55% in nonresponders; p = 0.10) and K-ras mutations (39% mutated tumors in responders vs. 26% in nonresponders; p = 0.34). Survival was longer for patients with p53-mutated metastases than for patients with unresected wild-type p53 metastases (median survival 15 months vs. 17 months; p = 0.06). The determination of the MSI-H phenotype, p53 and K-ras status in hepatic metastases from CRC does not discriminate a group of patients that should preferentially benefit from 5FU-based chemotherapy. The prognosis of patients with treated liver metastases is better when p53 is mutated.

Genetic polymorphisms of cytochrome P450 2A6 in a case-control study on lung cancer in a French population

Cytochrome P450 2A6 (CYP2A6) is involved in the C-oxidation of nicotine and in the metabolic activation of tobacco nitrosamines. Recent data have suggested that CYP2A6 genetic polymorphisms might play a role in tobacco dependence and consumption as well as in lung cancer risk. However, the previously published studies were based on a genotyping method that overestimated the frequencies of deficient alleles, leading to misclassification for the CYP2A6 genotype. In this study, we genotyped DNA from 244 lung cancer patients and from 250 control subjects for CYP2A6 (wild-type allele CYP2A6*1, and two deficient alleles: CYP2A6*2, and CYP2A6*4, the latter corresponding to a deletion of the gene) using a more specific procedure. In this Caucasian population, we found neither a relation between genetically impaired nicotine metabolism and cigarette consumption, nor any modification of lung cancer risk related to the presence of defective CYP2A6 alleles (odds ratio = 1.1, 95% confidence interval = 0.7-1.9).

Differential sensitivity of rat hepatocyte CYP isoforms to self-generated nitric oxide

Early loss of P450 in rat hepatocyte cultures appears directly related to nitric oxide (NO) overproduction. This study investigates the influence of endogenously generated NO (or NO-derived species) on the relative expression of cytochrome P450 (CYP) isoforms in rat hepatocytes. Our results support the view that loss of P450 holoenzyme in culture is the ultimate consequence of a NO driven process, activated during the common hepatocyte isolation procedure, that leads to an accelerated and selective degradation of specific CYP apoproteins. Under conditions in which NO and peroxynitrite formation is operative, changes in the level of specific CYP isoforms result in a significant alteration of the CYP apoprotein profile that after 24 h of culture is quite different from that found in the liver of uninduced rats. This process is reverted by the early and efficient inhibition of NO synthesis, which allows for (1) maintenance of total P450 holoenzyme content, (2) preservation of the initial constitutive CYP pattern in culture and (3) the early expression of the normal inducibility in response to model inducers.

Prospective evaluation of detoxification pathways as markers of cutaneous adverse reactions to sulphonamides in AIDS

The use of sulphonamides is complicated by a high rate of cutaneous reactions in AIDS. Metabolic risk factors have been suspected for these reactions. We conducted a prospective study to evaluate whether glutathione S-transferase M1 null genotype, glutathione deficiency and acetylator status as risk factors. To explain the high frequency of slow acetylator phenotype in AIDS patients, we compared N-acetyltransferase-2 phenotype and genotype in this population. AIDS patients treated with sulphonamides for Pneumocystis carinii pneumonia or toxoplasmosis were followed up for cutaneous reactions. Glutathione S-transferase genotyping, glutathione level determination, N-acetyltransferase-2 genotyping and phenotyping were performed. One hundred and thirty-six AIDS patients were studied. Glutathione S-transferase M1 and T1 null genotypes, intracellular glutathione level, slow acetylator genotype and phenotype were not risk factors for cutaneous sulphonamides reactions. The association of glutathione S-transferase M1 null genotype and the slow acetylator one was a risk factor [Fisher's exact test, odds ratio (OR) = 2.6, 95% confidence interval (CI) = 1.2-5.9; P = 0.02]. A discordance between acetylator genotype and phenotype was found in 35% of patients. This frequency was significantly higher than the 6-7% expected (Fisher's exact test: OR = 7.5, 95% CI = 4.2-13.4; P < 0.0001). Suspected metabolic risk factors for sulphonamides cutaneous reactions were not confirmed prospectively. However, the association of glutathione S-transferase M1 null genotype and the slow acetylator one appeared to increase the risk of reactions. We clearly showed that the acetylation phenotype measured by caffeine probe could be modified by the disease.

Structural characterization of a new variant of the CYP2A6 gene (CYP2A6*1B) apparently diagnosed as heterozygotes of CYP2A6*1A and CYP2A6*4C

During the course of investigating the frequency of a CYP2A6 whole deletion-type polymorphism (CYP2A6*4C) in Japanese, an unexpectedly large population of heterozygotes for CYP2A6*4C and the wild-type (CYP2A6*1A) was found. Cloning of a cDNA encoding CYP2A6 from the liver of individuals judged as heterozygotes for CYP2A6*4C and the CYP2A6*1A was carried out to identify the causal allele(s) responsible for a possible overestimation. A clone isolated from the liver cDNA library possessed 58 bp sequences in the 3'-untranslated region, which was replaced with the corresponding region of the CYP2A7 gene. The same gene conversion existed in the genomic DNA, indicating that the replacement was not a cloning artifact. Based on the gene structure of the allele (CYP2A6*1B), this variant was thought to be one of the causal alleles responsible for overestimation of heterozygotes for CYP2A6*4C and CYP2A6* A. To investigate this further, we developed a genotyping method which could distinguish the CYP2A6*A, CYP2A6*1B and CYP2A6*4C alleles from each other. The results clearly showed that CYP2A6*1B was the sole allele responsible for the overestimation. We conclude that the new genotyping method allows determination of six genotypes of the CYP2A6 gene, simultaneously and precisely, in both Oriental and Caucasian populations.

Relation between inducibility of CYP1A1, GSTM1 and lung cancer in a French population

Smoking is the principal cause of lung cancer. However, not all smokers will develop this disease. Individual susceptibility to chemically induced cancer may be explained in part by genetic differences in the activation and detoxification of procarcinogens. The activation phase of polycyclic aromatic hydrocarbon (PAH) metabolism is governed by the enzyme CYP1A1, induced by PAH when it enters the body. The extent to which PAH induces CYP1A1 activity varies greatly from one subject to another. CYP1A1 inducibility has long been associated, although inconsistently, with an increased risk of lung cancer. In 1982, Kouri corroborated Kellerman's results with a new method for measuring inducibility, but few studies have reported using this method. The glutathione S-transferases (GSTs) are involved in the detoxification phase of PAH, and the allelic deletion of GSTM1 has been also associated with an increased risk of lung cancer. We conducted a case-control study to examine the risk of lung cancer related, separately and together, to CYP1A1 inducibility, GSTM1 polymorphism and cigarette smoking in a French population. The 611 subjects were 310 incident lung cancer cases and 301 hospital control subjects. We were able to constitute a DNA bank for 552 subjects (89.5%) and gather detailed information on smoking history for all of them. Inducibility could be measured for 195 cases and 183 control subjects. Results for GSTM1 polymorphism concern 247 cases and 254 control subjects. GSTM1 polymorphism and inducibility could both be assessed for 179 cases and 166 control subjects. The odds ratio related to inducibility was 1.7 [1.0-3.0] for medium and 3.1 (1.3-7.4) for hyper inducers. The association with GSTM1 was 1.6 (1.0-2.6). With a reference category of subjects who were both low inducers and GSTM1(+), we found an odds ratio for lung cancer of 8.1 (2-31) for the subjects with both risk factors [i.e. GSTM1(-) and hyper inducers]. Our data did not reveal evidence of interaction between smoking and inducibility. On the other hand, we found an interaction of 3.6 (0.6-21) between inducibility and GSTM1.

Human microsomal epoxide hydrolase is the target of germander-induced autoantibodies on the surface of human hepatocytes

Germander, a plant used in folk medicine, caused an epidemic of cytolytic hepatitis in France. In about half of these patients, a rechallenge caused early recurrence, suggesting an immunoallergic type of hepatitis. Teucrin A (TA) was found responsible for the hepatotoxicity via metabolic activation by CYP3A. In this study, we describe the presence of anti-microsomal epoxide hydrolase (EH) autoantibodies in the sera of patients who drank germander teas for a long period of time. By Western blotting and immunocytochemistry, human microsomal EH was shown to be present in purified plasma membranes of both human hepatocytes and transformed spheroplasts and to be exposed on the cell surface where affinity-purified germander autoantibodies recognized it as their autoantigen. Immunoprecipitation of EH activity by germander-induced autoantibodies confirmed this finding. These autoantibodies were not immunoinhibitory. The plasma membrane-located EH was catalytically competent and may act as target for reactive metabolites from TA. To test this hypothesis CYP3A4 and EH were expressed with human cytochrome P450 reductase and cytochrome b(5) in a "humanized" yeast strain. In the absence of EH only one metabolite was formed. In the presence of EH, two additional metabolites were formed, and a time-dependent inactivation of EH was detected, suggesting that a reactive oxide derived from TA could alkylate the enzyme and trigger an immune response. Antibodies were found to recognize TA-alkylated EH. Recognition of EH present at the surface of human hepatocytes could suggest an (auto)antibody participation in an immune cell destruction.

Expression of CYP1A1, CYP1B1 and CYP3A, and polycyclic aromatic hydrocarbon-DNA adduct formation in bronchoalveolar macrophages of smokers and non-smokers

Variability in the expression of enzymes metabolizing carcinogens derived from cigarette smoke may contribute to individual susceptibility to pulmonary carcinogenesis. This study was designed to determine the effects of smoking and 3 major cytochrome P450 (CYP) enzymes, i.e., CYP1A1, CYP1B1 and CYP3A, which metabolize polycyclic aromatic hydrocarbons (PAH) on PAH-DNA adduct formation in the bronchoalveolar macrophages (BAM) of 31 smokers and 16 non-smokers. CYP protein levels were determined by immunoblotting and PAH-DNA adduct levels by the nuclease P1 enhanced (32)P-postlabeling method. The expression of specific CYP forms was confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR) from 10 additional samples. CYP3A protein, CYP3A5 by RT-PCR, was detected in the majority of samples from smokers and non-smokers. The levels of CYP3A appeared to be lower in active smokers than in ex-smokers (p = 0.10) or never smokers (p = 0.02). CYP1A1 was not detectable by either immunoblotting or RT-PCR. The expression of CYP1B1 was low or undetectable in most samples. The PAH-DNA adduct levels were higher (mean 1.57/10(8) nucleotides) in samples from smokers compared with non-smokers (mean 0.42/10(8) nucleotides, p < 0.001) and the number of adducts correlated with the number of cigarettes smoked daily (regression analysis, p < 0. 001). Higher levels of adducts were detected in samples from smokers with a high level of CYP3A compared with those with a low level (regression analysis, p = 0.002). As CYP3A5 is abundant in both lung epithelial cells and BAM, its association with adduct formation suggests that this CYP form may be important in the activation of cigarette smoke procarcinogens.

Combined glutathione-S-transferase M1 and T1 genetic polymorphism and tacrine hepatotoxicity

BACKGROUND

Glutathione conjugation of tacrine reactive metabolites depends in part on the activity of glutathione-S-transferases (GST), of which two isozymes (GST M1 and GST T1) are polymorphically expressed.

OBJECTIVE AND METHODS

To determine whether GST M1, GST T1, and the combined GST M1 and GST T1 null genotypes predict individual susceptibility to tacrine hepatotoxicity, 141 patients with mild to moderate Alzheimer's disease treated with tacrine were genotyped.

RESULTS

During the treatment period, 52 patients had elevated alanine aminotransferase (ALT) levels at least three times the upper limit of normal, whereas 89 patients had normal ALT values (< or = upper limit of normal). Both groups were comparable in demographic and clinical characteristics. Twenty-eight patients were found to be GST T1-negative (20%; with a 95% confidence interval [95% CI] from 13% to 27%), and 68 patients (48%; 95% CI from 40% to 57%) were GST M1-negative. The combined GST M1-T1 null genotype was observed in 18 patients (13%; 95% CI from 7% to 18%) of whom 13 had an elevated plasma ALT at least three times the upper limit of normal during the study period. Although the cumulative percentage of elevated plasma ALT tended to be higher in the GST M1 null genotype, neither GST M1 nor GST T1 alone could predict individual susceptibility to tacrine hepatotoxicity. Multivariate Cox hazards model showed that the association of the GST M1-T1 null genotype was an independent risk factor of hepatotoxicity.

CONCLUSIONS

The presence of combined alleles M1 and T1 deficiencies in glutathione-S-transferase genes increases the susceptibility to tacrine hepatotoxicity.

p53 alterations predict tumor response to neoadjuvant chemotherapy in head and neck squamous cell carcinoma: a prospective series

PURPOSE

The tumor suppressor gene p53 plays a crucial role in cell cycle control and apoptosis in response to DNA damages. p53 gene mutations and allelic losses at 17p are one of the most common genetic alterations in primary head and neck squamous cell carcinoma (HNSCC). Alterations of the p53 gene have been shown to contribute to carcinogenesis and drug resistance.

PATIENTS AND METHODS

In this prospective series, patients with HNSCC were treated with cisplatin-fluorouracil neoadjuvant chemotherapy. p53 status was characterized in 106 patients with HNSCC (p53 mutations, allelic losses at p53 locus, and plasma anti-p53 antibodies) to determine the existence of a relationship between p53 gene status and response to neoadjuvant chemotherapy.

RESULTS

Exons 4 to 9 of the p53 gene were analyzed, and mutations were found in 72 of 106 patients with HNSCC. p53 mutations were associated with loss of heterozygosity at chromosome 17p (P <.001). The prevalence of p53-mutated tumors was higher in the group of patients with nonresponse to neoadjuvant chemotherapy than in the group of responders (81% v 61%, respectively; P <.04). When compiling p53 mutations and anti-p53 antibodies in plasma, the correlation between p53 status and response to chemotherapy was significant (87% v 57%, respectively; P =.003). A multivariate analysis showed that p53 status is an independent predictive factor of response to chemotherapy.

CONCLUSION

This prospective study suggests that p53 status may be a useful indicator of response to neoadjuvant chemotherapy in HNSCC.

Detection of plasma tumor DNA in head and neck squamous cell carcinoma by microsatellite typing and p53 mutation analysis

Recent arguments have suggested that tumor DNA in cancer patients could be found in plasma, but different points remain unclear. Using a series of 117 head and neck squamous cell carcinoma tumors, our goals for this study were: (a) to quantify the amount of plasma DNA; (b) to evaluate the presence of plasma tumor DNA; and (c) to analyze the clinical relevance of tests based on plasma DNA analyses. Low levels of plasma DNA were found in most samples, but all were successfully amplified. Two different methods were used to detect tumor-specific genetic alterations: (a) microsatellite instability at UT5085 with an established sensitivity of 1:500; and (b) p53 mutation screening. Of the 117 tumors typed at UT5085, 65 demonstrated bandshifts (55%). Plasma and tumor DNA a showed similar alteration in only one case among these samples, and the prevalence of tumor DNA in plasma was estimated to be <2% using microsatellite analysis. Tumor DNA was detected in plasma at a higher prevalence (2 of 11 cases) when using p53 mutant allele-specific amplification. These results showed that in plasma, tumor DNA is largely diluted by normal DNA. By comparison with previously published studies, the prevalence of microsatellite alterations in plasma in this series of head and neck squamous cell carcinomas is very low, despite the fact that a large series of tumors was analyzed. To explain this discrepancy, we analyzed the possibility of PCR artifacts as suspected by the presence of loss of heterozygosity in two plasma DNA samples without a similar tumor DNA alteration. When DNA concentrations were under the threshold of detection (<100 ng/ml), we demonstrated that PCR artifacts could occur at random, and, if misinterpreted, these false genetic alterations could artificially enhance the frequency of plasma DNA alterations. This may have been suspected in previously published series, but it has never been discussed before. Microsatellite analysis on plasma DNA is difficult to interpret and can frequently be misleading. Plasma DNA should be analyzed with very sensitive and specific methods such as mutant allele-specific amplification, which excludes artifacts but requires specific optimization that is probably not compatible with routine and clinical use.