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Current Topics in Therapeutic Drug Monitoring

Friday, October 22, 2010

Abstract: Lamotrigine (LTG) is metabolized by UGT1A4 but UGT2B7 also contributes to its glucuronidation. The aim of this study was to determine whether UGT2B7_− 161C>T and UGT2B7_372A>G polymorphisms contribute to the intersubject variability in LTG concentration-to-dose ratio (LTG-CDR) in epileptic patients. Fifty-three white epileptic patients attending the Neuropediatric and Neurology Services at the Marqués de Valdecilla University Hospital, in whom LTG serum concentration was to be measured for pharmacokinetic monitoring, were selected according to predefined criteria for LTG-CDR evaluation. All patients had at least one steady-state LTG serum concentration obtained before the first dose in the morning. Patients were classified in 3 groups of comedication: (1) LTG in combination with metabolism-inducer anticonvulsants (n = 22), (2) LTG in combination with valproate (n = 13), and (3) LTG as monotherapy (n = 16) or in combination with valproate and inducers (n = 2). Genotypes were determined by Applied Biosystems Genotyping Assays with TaqMan probes. A significant association was found between LTG-CDR and UGT2B7_−161C>T polymorphism (P = 0.021) when patient age and concomitant antiepileptic drugs were taken into account. Comedication explained 70% of the LTG-CDR variability, patient age 24%, and UGT2B7_−161C>T 12%. In contrast, a significant association between LTG-CDR and this polymorphism was not found in the bivariate study when age and comedication groups were not considered. A significant association between UGT2B7_372A>G and LTG-CDR was not found in the bivariate or the multivariate studies. UGT2B7_−161C>T polymorphism is significantly associated with LTG-CDR when comedication with other antiepileptic drugs and patient age are taken into account in a multivariate analysis.

The full article is available on the Therapeutic Drug Monitoring website.

Thursday, October 7, 2010

Abstract: The objective of this study was to evaluate the analytical performance of the Abbott ARCHITECT Cyclosporine (CsA) immunoassay in 7 clinical laboratories in comparison to liquid chromatography/tandem mass spectrometry (LC/MS/MS), Abbott TDx, Cobas Integra 800, and the Dade Dimension Xpand immunoassay. The ARCHITECT assay uses a whole blood specimen, a pretreatment step with organic reagents to precipitate proteins and extract the drug, followed by a 2-step automated immunoassay with magnetic microparticles coated with anti-CsA antibody and an acridinium-CsA tracer. Imprecision testing at the 7 evaluation sites gave a range of total % coefficient of variations of 7.5%-12.2% at 87.5 ng/mL, 6.6%-14.3% at 411 ng/mL, and 5.2%-10.7% at 916 ng/mL. The lower limit of quantification ranged from 12 to 20 ng/mL. Purified CsA metabolites AM1, AM1c, AM4N, AM9, and AM19 were tested in whole blood by the ARCHITECT assay and showed minimal cross-reactivity at all 7 sites. In particular, AM1 and AM9 cross-reactivity in the ARCHITECT assay, ranged from −2.5% to 0.2% and −0.8% to 2.2%, respectively, and was significantly lower than for the TDx assay, in which the values were 3.2% and 16.1%, respectively. Comparable testing of metabolites in the Dade Dimension Xpand assay at 2 evaluation sites showed cross-reactivity to AM4N (6.4% and 6.8%) and AM9 (2.6% and 3.6%) and testing on the Roche Integra 800 showed cross-reactivity to AM1c (2.4%), AM9 (10.7%), and AM19 (2.8%). Cyclosporine International Proficiency Testing Scheme samples, consisting of both pooled specimens from patients receiving CsA therapy as well as whole-blood specimens supplemented with CsA, were tested by the ARCHITECT assay at 6 sites and showed an average bias of −24 to −58 ng/mL versus LC/MSMS CsA and −2 to −37 ng/mL versus AxSYM CsA. Studies were performed with the ARCHITECT CsA assay on patient specimens with the following results: ARCHITECT CsA assay versus LC/MSMS, average bias of 31 ng/mL; ARCHITECT versus the Dade Dimension assay (4 sites), average biases of −7 to −228 ng/mL; ARCHITECT versus AxSYM and TDx, average biases of −4 and −53 ng/mL, respectively. Spearman correlation coefficients were ≥0.89. The ARCHITECT CsA assay has significantly reduced CsA metabolite interference relative to other immunoassays and is a convenient and sensitive semiautomated method to measure CsA in whole blood.
The full article is available on the Therapeutic Drug Monitoring website.

Thursday, September 23, 2010

Abstract: 6-Thioguanine nucleotides are the sum of 6-thioguanosine 5′-monophosphate (TGMP), -diphosphate (TGDP), and -triphosphate (TGTP) representing essential metabolites involved in drug action of thiopurines. Elevated levels of TGDP have been associated with poor response to azathioprine therapy in patients with inflammatory bowel disease. The conversion of TGDP to TGTP is supposed to be catalyzed by nucleoside diphosphate kinase (NDPK). The aim of this work was to investigate simultaneously individual 6-thioguanosine phosphate levels and NDPK activity in red blood cells (RBCs) of patients on azathioprine therapy. Ion-pair high-performance liquid chromatography methods with fluorescence and ultraviolet detection were applied to quantify individual levels of 6-thioguanosine 5′-phosphates and NDPK activity, respectively, in RBCs. Recombinantly expressed NDPK isoforms A and B were unequivocally identified to catalyze the formation of TGTP (30.6 ± 3.88 nmol·min−1·mg−1 for NDPK A versus 41.2 ± 1.05 nmol·min−1·mg−1 for NDPK B). Comprehensive analyses on the stability of TGMP, TGDP, and TGTP and the reproducibility of NDPK activity in RBCs were performed to provide a reliable sampling protocol for clinical practice. Of note, isolation of RBCs within 6 hours followed by immediate storage at -80°C is crucial for prevention of degradation of 5′-phosphates. In a clinical study of 37 patients on azathioprine, TGTP was the predominant 6-thioguanosine phosphate in RBCs. In contrast, three patients showed TGTP/(TGDP + TGTP) ratios of 57.2%, 64.3%, and 66% corresponding to elevated TGDP levels. NDPK activity ranged from 4.1 to 11.3 nmol·min−1·mg−1 hemoglobin. No correlation between NDPK activity and the 6-thioguanosine phosphate levels was found. The question whether interindividual variability of NDPK activity may explain differences in 6-thioguanosine 5′-phosphates levels has to be investigated in a prospective large-scale study.
The full article is available on the Therapeutic Drug Monitoring website.

Thursday, September 9, 2010

Abstract: The detection of ethyl glucuronide (EtG) and ethyl sulfate (EtS) in meconium has been investigated recently as an alternative to meconium fatty acid ethyl esters (FAEEs) measurement as an objective estimate of prenatal alcohol exposure, independent of maternal self-reporting. We report the results of the first study conducted to investigate the concentrations of EtG and EtS in meconium from newborns with and without intrauterine exposure to ethanol, defined by questionnaire and meconium FAEEs concentration. FAEEs, EtG, and EtS were quantified by liquid chromatography tandem mass spectrometry in meconium samples obtained from the Arcispedale Santa Maria Nuova, Reggio Emilia, Italy (n = 80) and from the Hospital del Mar in Barcelona, Spain (n = 105). Median EtG and EtS values in meconium from newborns without intrauterine exposure to ethanol varied between 0.100 and 0.140 nmol/g and 0.010 and 0.020 nmol/g in Reggio Emilia and Barcelona samples, respectively. In meconium from newborns with uncertain prenatal ethanol exposure, the EtG median value was 0.160 nmol/g in the Italian cohort and 0.250 nmol/g in the Spanish one. The median EtS concentration was 0.020 in both cohorts. EtG and EtS median values in 5 meconium samples from newborns of heavily drinking mothers were 7.240 nmol/g and 0.033 nmol/g, respectively. A positive cutoff of 2.0 nmol/g for EtG yielded the best sensitivity and specificity (100%) to discriminate for true prenatal exposure to ethanol. It was not possible to establish a proper cutoff for EtS because of the low number of positive samples. Based on our results, meconium EtG can be proposed as an alternate biomarker for intrauterine alcohol exposure. In contrast to the 7 FAEEs, EtG is just one molecule that could be screened in meconium samples from all newborns by a simple, low-cost, easy-to-perform immunoassay, which can be routinely applied in neonatology wards for the early diagnosis of prenatal exposure to ethanol.
The full article is available on the Therapeutic Drug Monitoring website.

Thursday, August 26, 2010

Abstract: Warfarin remains a difficult drug to manage due to a narrow therapeutic range and wide interindividual variability in dose requirements. The relationship between warfarin sensitivity and CYP2C9 and VKORC1 variants is strong, and is the basis for several proposed dosing algorithms. Here a gene-based dosing algorithm was compared with standard of care dosing in patients receiving warfarin to prevent venous thromboembolism after joint replacement surgery. Participants (n = 229) were adults (≥18 years) undergoing elective total hip or knee arthroplasty and receiving warfarin under the direction of a dedicated anticoagulation services team. Patients were assigned to genotype-based or standard of care dosing arms in an alternating fashion. Initial dose for patients was determined by validated algorithms from Sconce 2005 and Pendleton 2008. Management was based on INR, but dose was adjusted less aggressively for patients with CYP2C9 variants. The primary endpoint was reduction in the incidence of adverse events; additional endpoints included time to first therapeutic INR (1.8-2.9), time to first supratherapeutic INR, and percent of INR determinations that fell below, within, and above the therapeutic range. Endpoints did not achieve statistical significance, possibly due to the management of this study by a dedicated and experienced anticoagulation services team. Trends in the data suggest that patients with genetic variants progressed to a therapeutic INR faster than patients in whom genetic variants were not detected, and there were fewer adverse events in the genotype-based dosing arm. In addition, the results of this study confirm those of others demonstrating clear relationship of genotype for CYP2C9 and VKORC1 with warfarin dose requirements; as the number of variants in these genes increases, the dose requirement decreases. Of note, the gene-based algorithm utilized here significantly underpredicted the dose requirement for participants with no variants, indicating that patients with no variants should be managed with a different algorithm than patients who inherit genetic variants in CYP2C9 and/or VKORC1. In conclusion, gene-based dosing did not improve warfarin management as defined by INR dose response, using the described protocols for implementation. Findings suggest alternative strategies for dosing based on the presence or absence of genetic variants is needed.
The full article is available on the Therapeutic Drug Monitoring website.