Comparison of Information-Dependent Acquisition on a Tandem Quadrupole TOF vs a Triple Quadrupole Linear Ion Trap Mass Spectrometer for Broad-Spectrum Drug Screening.

BACKGROUND: Liquid chromatography high-resolution mass spectrometry (LC-HRMS) with untargeted data collection is especially attractive for general unknown drug screening owing to its ability to identify unexpected compounds. LC-HRMS offers several advantages over traditional selected reaction monitoring (SRM) techniques and could be an ideal screening platform as long as its analytical performance is comparable to that of SRM-based methods. METHODS: We developed a broad-spectrum drug screen on a high-resolution mass spectrometer [tandem quadrupole time-of-flight (QqTOF)] that collected data in an untargeted manner and compared its performance to a nominal mass instrument [triple quadrupole linear ion trap (QqLIT)] that collected data in a targeted manner. Both methods used information-dependent acquisition of product ion spectra. We evaluated the lower limits of detection and matrix effects for each method and compared their ability to identify drugs in 100 routine clinical urine samples. Additional information (patient prescription history, drug screening results, etc.) was used to confirm discordant results. RESULTS: QqLIT was slightly more analytically sensitive than QqTOF; however, this difference did not significantly affect compound identification in patient samples. QqLIT identified 596 drugs in the urine samples, of which 531 (89%) were confirmed. QqTOF identified 515 drugs, of which 500 (97%) were confirmed. There were 562 instances of a confirmed drug (68 unique drugs) in the 100 urine samples; the methods were concordant in 469 of these instances. CONCLUSIONS: Overall, QqTOF performed similarly to QqLIT and could serve as an alternative method for general unknown screening.

Measuring the overall genetic component of nevirapine pharmacokinetics and the role of selected polymorphisms: towards addressing the missing heritability in pharmacogenetic phenotypes?

OBJECTIVE: Nevirapine is an important component of highly active antiretroviral therapy used in the treatment of HIV infection. There is a considerable variation in the pharmacokinetics of nevirapine and this variation can impact the efficacy and toxicity of nevirapine. Although some of this variation can be attributed to environmental factors, the degree to which heritability influences nevirapine pharmacokinetics is unknown. This study aims to estimate how much variation in nevirapine pharmacokinetics is due to genetic factors and to investigate the contribution of selected polymorphisms to this variability. METHODS: Two doses of immediate-release nevirapine were administered to European (n=11) and African American (n=6) participants recruited from the Research in Access to Care in the Homeless cohort. A repeated drug administration method was then used to determine the relative genetic contribution (r(GC)) to variability in nevirapine AUC(0-6 h). Nevirapine plasma levels were quantified using LC/MS/MS. Patients were also genotyped for selected polymorphisms in candidate genes that may influence nevirapine pharmacokinetics. RESULTS: A significant r(GC) for nevirapine AUC(0-6 h) was found in Europeans (P=0.02) and African Americans (P=0.01). A trend toward higher nevirapine AUC(0-6 h) for the CYP2B6 516TT (rs3745274; Q172H) genotype was observed in European Americans (P=0.19). CONCLUSION: This study demonstrates that there is a significant genetic component to variability in nevirapine pharmacokinetics. Although genetic variants such as CYP2B6 polymorphisms attributed to some of this variation, these data suggest that there may be additional genetic factors that influence nevirapine pharmacokinetics.

Pervasive Gabapentin Interference in the LC-MS/MS Analysis of Amphetamine.

BACKGROUND: An LC-MS/MS urine confirmation assay was developed using a "dilute and shoot" sample preparation method that was subject to interference arising from gabapentin column overload in approximately 4% of patient samples, leading to interference in amphetamine analysis. METHODS: The initial analysis method used dilute and shoot sample preparation followed by LC-MS/MS analysis. The improved assay used solid-phase extraction sample preparation followed by LC-MS/MS analysis. RESULTS: The improved assay using solid-phase extraction and alternative chromatographic conditions resolved the gabapentin interference in amphetamine analysis. CONCLUSIONS: This experience illustrates the importance of thorough knowledge of likely comedications in a patient population and these drugs' elimination mechanisms. Laboratorians should be aware of the phenomenon of mass effect LC-MS/MS interference, in addition to the more common LC-MS/MS interferences caused by matrix effects and isobaric compounds.

Elucidating the effect of final-day dosing of rifampin in induction studies on hepatic drug disposition and metabolism.

Because rifampin (RIF) induces hepatic enzymes and inhibits uptake transporters, dosing a drug that is a dual substrate of enzymes and uptake transporters on the final day of an inducing regimen should exhibit less inductive effect than dosing on the following day in the absence of RIF, since RIF decreases drug uptake into liver. In vitro and in vivo rat studies were conducted using digoxin as a model substrate. Digoxin was administered to an uninduced control group to obtain baseline values. The second group (induced with dexamethasone) received digoxin alone, mimicking administration of a test drug 1 day following completion of an induction regimen, whereas the third group (induced) received digoxin with RIF mimicking the concomitant dosing on the final day of an induction regimen. Results from hepatocyte concentration-time course studies showed that compared with uninduced control (26.9 +/- 1.3 microM . min/mg), digoxin area under the time-concentration curve (AUC) in induced cells when no RIF is present decreased significantly (13.7 +/- 0.9 microM . min/mg; p < 0.01), suggesting induction of Cyp3a. However, digoxin AUC for induced cells in the presence of RIF (27.3 +/- 0.9 microM . min/mg) matched the control. Rat pharmacokinetic studies showed that compared with digoxin clearance in uninduced controls (7.08 +/- 1.57 ml/min/kg), digoxin clearance in induced rats increased 2-fold (15.6 +/- 3.7 ml/min/kg; p < 0.001), but when RIF was coadministered in the induced rats, digoxin clearance (7.14 +/- 1.24 ml/min/kg) overlapped with control. That is, concomitant dosing of RIF and digoxin masked the inductive effect. To observe full inductive effects, test drugs should be administered 1 day after final dosing of RIF to minimize potential organic anion transporting polypeptide inhibition effects.