Enhanced immunity in a mouse model of malignant glioma is mediated by a therapeutic ketogenic diet.

BACKGROUND: Glioblastoma multiforme is a highly aggressive brain tumor with a poor prognosis, and advances in treatment have led to only marginal increases in overall survival. We and others have shown previously that the therapeutic ketogenic diet (KD) prolongs survival in mouse models of glioma, explained by both direct tumor growth inhibition and suppression of pro-inflammatory microenvironment conditions. The aim of this study is to assess the effects of the KD on the glioma reactive immune response. METHODS: The GL261-Luc2 intracranial mouse model of glioma was used to investigate the effects of the KD on the tumor-specific immune response. Tumor-infiltrating CD8+ T cells, CD4+ T cells and natural killer (NK) cells were analyzed by flow cytometry. The expression of immune inhibitory receptors cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed death 1 (PD-1) on CD8+ T cells were also analyzed by flow cytometry. Analysis of intracellular cytokine production was used to determine production of IFN, IL-2 and IFN- in tumor-infiltrating CD8+ T and natural killer (NK) cells and IL-10 production by T regulatory cells. RESULTS: We demonstrate that mice fed the KD had increased tumor-reactive innate and adaptive immune responses, including increased cytokine production and cytolysis via tumor-reactive CD8+ T cells. Additionally, we saw that mice maintained on the KD had increased CD4 infiltration, while T regulatory cell numbers stayed consistent. Lastly, mice fed the KD had a significant reduction in immune inhibitory receptor expression as well as decreased inhibitory ligand expression on glioma cells. CONCLUSIONS: The KD may work in part as an immune adjuvant, boosting tumor-reactive immune responses in the microenvironment by alleviating immune suppression. This evidence suggests that the KD increases tumor-reactive immune responses, and may have implications in combinational treatment approaches.

The ketogenic diet for the treatment of malignant glioma.

Advances in our understanding of glioma biology has led to an increase in targeted therapies in preclinical and clinical trials; however, cellular heterogeneity often precludes the targeted molecules from being found on all glioma cells, thus reducing the efficacy of these treatments. In contrast, one trait shared by virtually all tumor cells is altered (dysregulated) metabolism. Tumor cells have an increased reliance on glucose, suggesting that treatments affecting cellular metabolism may be an effective method to improve current therapies. Indeed, metabolism has been a focus of cancer research in the last few years, as many pathways long associated with tumor growth have been found to intersect metabolic pathways in the cell. The ketogenic diet (high fat, low carbohydrate and protein), caloric restriction, and fasting all cause a metabolic change, specifically, a reduction in blood glucose and an increase in blood ketones. We, and others, have demonstrated that these metabolic changes improve survival in animal models of malignant gliomas and can potentiate the anti-tumor effect of chemotherapies and radiation treatment. In this review we discuss the use of metabolic alteration for the treatment of malignant brain tumors.

Bioequivalence of Alendronate and Vitamin D3 in a Combination Tablet Versus Corresponding-Dose Individual Tablets in Healthy Taiwanese Volunteers, Determined Using a Novel Plasma Alendronate Assay.

OBJECTIVE: This study was designed to demonstrate that alendronate (ALN)/vitamin D3 combination tablets (ALN/D5600) are bioequivalent to corresponding doses of ALN and vitamin D3 as individual tablets in healthy Taiwanese volunteers. METHODS: In this open-label, randomized, 2-period, crossover study, 68 volunteers were randomized to a single ALN/D5600 combination tablet or corresponding doses of 70 mg ALN + 5600 IU vitamin D3 (2 × 2800 IU), followed by a 12-day washout period and administration of the alternate formulation. Plasma ALN levels were measured using a newly developed assay. Geometric mean ratios of ALN AUC0-last, AUC0-∞, and Cmax, and unadjusted vitamin D3 AUC0-80h and Cmax were compared and considered bioequivalent if the 90% CI was within 0.8 to 1.25. RESULTS: The geometric mean ratios were: AUC0-last, 1.084 (90% CI, 0.937-1.253); AUC0-∞, 1.081 (90% CI, 0.935-1.249); and Cmax, 1.112 (90% CI, 0.959-1.289) for ALN, and AUC0-80h 0.953 (90% CI, 0.827-1.098) and Cmax, 0.982 (90% CI, 0.854-1.130) for vitamin D3 unadjusted for endogenous levels. CONCLUSIONS: The combination tablet was considered bioequivalent to coadministration based on ALN AUC0-∞ and unadjusted vitamin D3 parameters. Slight differences for ALN AUC0-last and Cmax (upper 90% CIs outside the bounds) were not considered clinically significant. The combination tablet was well tolerated. No serious adverse experiences were reported. © 2015. The Authors. Published by Elsevier Inc. All rights reserved.

Quantitation of anacetrapib in human and animal adipose by liquid chromatography with mass spectrometric detection.

Cholesteryl ester transfer protein (CETP) inhibitor is a target for both lowering low-density lipoproteins and raising high-density lipoproteins. Anacetrapib was the lead compound in our cholesteryl ester transfer protein inhibitor program. Preclinical studies were initiated to support the safety of anacetrapib deposition in adipose tissue, followed by a clinical trial to evaluate the effects of anacetrapib in people with vascular disease. An ultra-high performance liquid chromatography/tandem mass spectrometry method was developed to determine tissue anacetrapib concentrations in the adipose of three animal species and humans. The assays were validated in the concentration ranges of 5-5000 ng/ml and 0.1-100 µg/ml. The anacetrapib concentrations in adipose tissue from preclinical and clinical studies were determined.

[Box: see text].

Cloud security in a bioanalytical world: considerations for use of third-party cloud services for bioanalysis.

While using the cloud environment for various functions has become commonplace, relatively little attention has been given to considerations for the use of third-party cloud services for regulated bioanalytical workflow and data management. Little guidance has been provided as to how to utilize the cloud to support bioanalytical activities. It can be intimidating when considering how to go about using cloud services for data acquisition, but there are some general ideas to keep in mind when evaluating ways to accommodate regulated bioanalysis online. Determining how to incorporate the use of cloud storage with data that are generated from regulated bioanalytical analysis is an important step in maintaining the security of the data.

Ultrasensitive liquid chromatography-tandem mass spectrometric methodologies for quantification of five HIV-1 integrase inhibitors in plasma for a microdose clinical trial.

HIV-1 integrase strand transfer inhibitors are an important class of compounds targeted for the treatment of HIV-1 infection. Microdosing has emerged as an attractive tool to assist in drug candidate screening for clinical development, but necessitates extremely sensitive bioanalytical assays, typically in the pg/mL concentration range. Currently, accelerator mass spectrometry is the predominant tool for microdosing support, which requires a specialized facility and synthesis of radiolabeled compounds. There have been few studies attempted to comprehensively assess a liquid chromatography-tandem mass spectrometry (LC-MS/MS) approach in the context of microdosing applications. Herein, we describe the development of automated LC-MS/MS methods to quantify five integrase inhibitors in plasma with the limits of quantification at 1 pg/mL for raltegravir and 2 pg/mL for four proprietary compounds. The assays involved double extractions followed by UPLC coupled with negative ion electrospray MS/MS analysis. All methods were fully validated to the rigor of regulated bioanalysis requirements, with intraday precision between 1.20 and 14.1% and accuracy between 93.8 and 107% at the standard curve concentration range. These methods were successfully applied to a human microdose study and demonstrated to be accurate, reproducible, and cost-effective. Results of the study indicate that raltegravir displayed linear pharmacokinetics between a microdose and a pharmacologically active dose.

Evaluation of dried blood spot (DBS) technology versus plasma analysis for the determination of MK-1775 by HILIC-MS/MS in support of clinical studies.

The collection of human blood samples as dried blood spots (DBS) for the pharmacokinetic assessment of investigational drugs in clinical trials offers a number of advantages over conventional plasma sampling, namely, small sample volume, simplified sample handling, and cost-effective shipping and storage. The use of DBS coupled with liquid chromatography-tandem mass spectrometry analysis was evaluated for the quantification of MK-1775, a Wee-1 inhibitor under development as a chemo/radio-sensitizer for the treatment of cancer. The DBS method exhibited an assay performance comparable to that of the existing plasma assay, which is currently used in support of clinical studies. Both assays used the same linear dynamic range of 2-1,000 ng/mL, with a lower limit of quantification of 2 ng/mL. Based on the intra-day assay validation results, the accuracy of the DBS method ranged from 94.0 to 105.0%, with a coefficient of variation of <4.8%. The blood-to-plasma ratio calculated from the DBS data (blood concentrations) and the plasma data (plasma concentrations) was in good agreement with the one obtained from the in vitro assessment using conventional methodology. No significant hematocrit impact on the assay was observed as hematocrit ranged from 16 to 85%. The correlation between the measured MK-1775 concentrations in plasma and that determined in dried blood spots from oncology patients during the ongoing clinical study was discussed.

Pharmacokinetics of Sugammadex: An Open-Label, 3-Period, Fixed-Sequence, 3-Single-Doses Study in Healthy Chinese Subjects.

The main purpose of this study was to assess pharmacokinetic parameters (area under the plasma concentration-time curve from time 0 to infinity, area under the plasma concentration-time curve from time 0 to the last measurable concentration, maximum concentration, and apparent terminal half-life) after administration of 3 single intravenous (IV) doses of sugammadex to evaluate the safety and tolerability in healthy nonanesthetized subjects. This was an open-label, 3-period, nonrandomized, single-rising-dose study in 12 healthy Chinese subjects 18 to 45 years of age. In each period, every subject received a single IV dose of sugammadex in a fixed sequence (1, 2, and 4 mg/kg) under fasting conditions. There was a 3-day washout phase between dosing in each treatment period. In this study, a total of 11 (6 men and 5 women) Chinese subjects (with mean age of 25 ± 4.0 years; body weight, 64 ± 6 kg; height, 167 ± 6 cm; and body mass index, 22.9 ±1.2 kg/m2 ) received single doses of sugammadex 1, 2, and 4 mg/kg sequentially, with a 3-day washout phase between consecutive doses, and completed the study. The geometric mean maximum concentrations were 16.81, 30.99, and 60.55 µg/mL for sugammadex 1, 2, and 4 mg/kg, respectively. The median time to maximum concentration was 0.03 hour after bolus administration, and plasma sugammadex concentrations declined with a mean apparent terminal half-life of ≈1.7 hours across all 3 doses. Single-dose IV administration of sugammadex 1, 2, and 4 mg/kg was generally safe and well tolerated in healthy Chinese male and female subjects.

A Model-Based Approach to Bridging Plasma and Dried Blood Spot Concentration Data for Phase 3 Verubecestat Trials.

In-clinic venous dried blood spot (DBS) pharmacokinetic (PK) sampling was incorporated into two phase 3 studies of verubecestat for Alzheimer's disease (EPOCH [NCT01739348] and APECS [NCT01953601]), as a potential alternative to plasma PK sampling. Initially, plasma and DBS PK samples were collected concurrently to better understand the DBS-plasma verubecestat concentration relationship, with the intention of discontinuing DBS or plasma sampling following interim analysis. Following initial analyses and comparison of results with prespecified selection criteria, plasma PK sampling was discontinued; however, a stability issue resulting in generally lower DBS verubecestat concentrations with longer collection-to-assay times was subsequently discovered (associated with non-compliance in DBS sample handling), prompting reintroduction of plasma sampling. To enable inclusion of DBS data in population PK analyses, a conversion algorithm for calculating plasma-equivalent concentrations (accounting for DBS sample instability) was developed using paired (time-matched) plasma and DBS data from the EPOCH study. Verubecestat population PK models developed from pooled phase 1/1b and EPOCH data using either (1) plasma-only data or (2) plasma and plasma-equivalent concentrations (calculated from non-paired DBS samples) yielded similar results. The algorithm robustness was demonstrated using DBS data from paired samples from the APECS study and comparison between plasma and plasma-equivalent concentrations. The population PK model was updated with APECS data (both plasma and, if no plasma sample available, plasma equivalents). The results demonstrated similar PK in the two phase 3 populations and exposures consistent with expectations from phase 1 data. This case study illustrates challenges with employing new sampling techniques in large, global trials and describes lessons learned.

An Investigation of Instability in Dried Blood Spot Samples for Pharmacokinetic Sampling in Phase 3 Trials of Verubecestat.

In-clinic dried blood spot (DBS) pharmacokinetic (PK) sampling was incorporated into two phase 3 studies of verubecestat for Alzheimer's disease (EPOCH [NCT01739348] and APECS [NCT01953601]), as a potential alternative to plasma PK sampling for improved logistical feasibility and decreased blood volume burden. However, an interim PK analysis revealed verubecestat concentrations in DBS samples declined with time to assay in both trials. An investigation revealed wide variation in implementation practices for DBS sample handling procedures resulting in insufficient desiccation which caused verubecestat instability. High-resolution mass spectrometry evaluations of stressed and aged verubecestat DBS samples revealed the presence of two hydrolysis degradants. To minimize instability, new DBS handling procedures were implemented that provided additional desiccant and minimized the time to analysis. Both verubecestat hydrolysis products were previously discovered and synthesized during active pharmaceutical ingredient stability characterization. A liquid chromatography-mass spectrometry assay to quantitate the dominant verubecestat degradant in DBS samples was developed and validated. The application of this method to stressed and aged verubecestat DBS samples confirmed that degradant concentrations accounted for the observed decreases in the verubecestat concentration. Furthermore, after increasing desiccant amounts, degradant concentrations accounted for approximately 7% of the verubecestat concentration in DBS clinical samples, indicating that issues with sample handling were minimized with new storage and shipping conditions. This case study illustrates the challenges with employing new sampling techniques in large, global trials, and the importance of anticipating and mitigating implementation risks.

Incorporating protein precipitation to resolve hybrid IP-LC-MS assay interference for ultrasensitive quantification of intact therapeutic insulin dimer in human plasma.

For pharmacokinetics characterization of a therapeutic insulin dimer, an ultrasensitive plasma method was required due to the expected low circulating levels in humans. A bioanalytical strategy combining immunoprecipitation enrichment with liquid chromatography - tandem mass spectrometry (LC-MS/MS) analysis of the intact protein offers the opportunity to resolve the analyte from endogenous and exogenous insulin and insulin analogs. Nonetheless, interference from complex background matrix was observed limiting reliable measurements at the low concentration range. A sample preparation approach incorporating protein precipitation and immunoprecipitation was developed and optimized to further reduce sample complexity prior to LC-MS/MS analysis. This approach enabled a deeper level of selectivity and presented a cleaner mass spectrometric detection that may otherwise be confounded. Sample preparation was automated to allow high throughput analysis. The method reached a limit of quantitation at 0.3 ng/mL (25 pM), and a linear dynamic range from 0.3 to 300 ng/mL. Results were highly reproducible, with intra-day and inter-day precision and bias below 11%. Furthermore, the organic solvent treatment involved in protein precipitation is expected to improve assay resistance to the bias introduced by endogenous protein binding such as that exerted by anti-drug antibodies. The method was successfully applied to support clinical pharmacokinetics studies. This approach may potentially be adapted to bioanalysis of low abundance proteins.

Volumetric absorptive microsampling (VAMS®) in therapeutic protein quantification by LC-MS/MS: Investigation of anticoagulant impact on assay performance and recommendations for best practices in method development.

Microsampling techniques have been employed as an alternative to traditional serum/plasma sampling because of their inherently proven and desirable advantages across the pharmaceutical industry. These include reduced animal usage in pre-clinical studies, as well as, permitting the collection of samples that would otherwise be inaccessible in clinical studies. The application of volumetric absorptive microsampling (VAMS®) technology, a second-generation dried microsampling method, coupled with LC-MS, has been extensively explored for small molecule drugs at various drug development stages. However, the potential of using VAMS technology and LC-MS analysis for biological therapeutic development has yet to be well-established. In this work, we describe the method development, validation, and a proof-of-concept non-human primate study of a LC-MS/MS method for VAMS utilized to obtain pharmacokinetic (PK) data for a therapeutic monoclonal antibody. A good correlation between VAMS data and data from conventional serum samples was established in rhesus monkeys and indicated the possibility of using of this novel sampling technology in clinical studies. However, during the initial clinical study, a significant difference in internal standard (IS) response between the patient fingerstick samples and the standard/QC samples was observed, which posed a question on the accuracy of the clinical results. A comprehensive investigation confirmed that the EDTA anticoagulant used in the standard/QC samples was the root cause of the observed anomalous IS responses. Special considerations and corresponding best practices during method development and validation are proposed to ensure early detection of potential issues and appropriate implementation of VAMS technology in clinical studies in the future.

Cloud solutions for GxP laboratories: considerations for data storage.

Challenges for data storage during drug development have become increasingly complex as the pharmaceutical industry expands in an environment that requires on-demand availability of data and resources for users across the globe. While the efficiency and relative low cost of cloud services have become increasingly attractive, hesitancy toward the use of cloud services has decreased and there has been a significant shift toward real-world implementation. Within GxP laboratories, the considerations for cloud storage of data include data integrity and security, as well as access control and usage for users around the globe. In this review, challenges and considerations when using cloud storage options for the storage of laboratory-based GxP data are discussed and best practices are defined.

The effectiveness of quality control samples in pharmaceutical bioanalysis.

The use of quality control (QC) samples in bioanalysis is well established and consistent with regulatory guidance. However, a systematic evaluation of whether QC samples serve the intended purpose of improving data quality has not been undertaken. The Translational and ADME Sciences Leadership Group (TALG) of the International Consortium for Innovation and Quality in Pharmaceutical Development (IQ) conducted an evaluation to assess whether closer agreement is observed when comparing pharmacokinetic data from two passed runs, than when comparing data from failed and passed (retest) runs. Analysis of data collected across organizations, molecular types and analytical platforms, revealed that bioanalytical methods are very reproducible; and that QC samples improve the overall quality of pharmacokinetic concentration data and justifies their continued use.

Immunoaffinity purification using anti-PEG antibody followed by two-dimensional liquid chromatography/tandem mass spectrometry for the quantification of a PEGylated therapeutic peptide in human plasma.

Quantification of a PEGylated peptide in human plasma using LC-MS/MS to support clinical studies presented challenges in terms of assay sensitivity, selectivity, and ruggedness. To ensure specific recognition of PEGylated species, an immunoaffinity purification method (IAP) using anti-PEG antibody followed by two-dimensional (2D) LC-MS/MS was developed for MK-2662, an investigational peptide containing 38 amino acids with a 40 kDa branched PEG [poly(ethylene glycol)] at C-terminus. Biotinylated anti-PEG antibody, bound to streptavidin-coated magnetic beads, was used to capture MK-2662 and its stable-isotope-labeled internal standard from human plasma. After on-bead digestion with trypsin, the supernatant was injected on a 2D high-performance liquid chromatography (HPLC) system constructed with strong cation-exchange and reversed-phase columns, followed by MS/MS detection of the surrogate N(1-12)-mer of MK-2662 on an API5000. The assay ruggedness was improved by optimizing the trypsin digestion and sample storage conditions. The intraday validation, conducted in parallel with protein precipitation (PPT) assay, demonstrated 94.8-105.8% accuracy with <9.76% coefficient of variation (CV) for IAP, and 99.0-101.0% accuracy with <3.43% CV for PPT, over a dynamic range of 2-200 nM and 1-1000 nM, respectively. A cross comparison, performed using clinical samples, showed that the values obtained from IAP assay were about 15-30% lower than those from PPT method, which supports more specific PEG recognition provided by IAP.

A Tiered Approach for Characterization to Ensure Quality, Reproducibility, and Long-Term Stability of Critical Reagents in Regulated Bioanalysis to Support PK/ADA/NAb Assays for Biologics and Vaccines Programs.

The robustness of good laboratory practice and clinical data is reliant upon a clear understanding of the bioanalytical assays. One of the most important components of ligand-binding based assays is critical reagents used to directly or indirectly measure biologic markers or signals. High quality, reproducible, sustainable critical reagents through the development lifecycle could avoid unnecessary rework, multiple validations, cross-validations, and ensure consistency of the data. Numerous analytical methods (UPLC-size exclusion chromatography, cation exchange chromatography, biacore/octet, and high-resolution mass spectrometry) have been evaluated by using current critical reagents. A comprehensive analytical toolbox of biochemical and biophysical methods has been employed to evaluate the quality of critical reagents and explore potential issues if there are any. Moving forward, this "tiered approach" of critical reagents characterization will be used not only to establish critical quality attributes for new reagents but also to evaluate stability in support of reagents recertification.

Clinical application of volumetric absorptive microsampling to the gefapixant development program.

In this paper we show the application of the Tasso OnDemand™, a novel automated sample collection device, in conjunction with volumetric absorptive microsampling (VAMS) for the development of gefapixant, a P2X3 receptor antagonist currently under clinical development for the treatment of refractory and unexplained chronic cough and endometriosis-related pain. A LC-MS/MS bioanalytical method was developed and validated using VAMS to support this development program. This method was utilized in a drug-drug interaction study to establish a mathematical bridging relationship with data obtained from a validated plasma assay used to support the program. The VAMS bioanalytical method and the predictability of the mathematical relationship is reported and discussed here.

Provision of rapid and specific ex vivo diagnosis of central nervous system lymphoma from rodent xenograft biopsies by a fluorescent aptamer.

OBJECTIVE: Differentiating central nervous system (CNS) lymphoma from other intracranial malignancies remains a clinical challenge in surgical neuro-oncology. Advances in clinical fluorescence imaging contrast agents and devices may mitigate this challenge. Aptamers are a class of nanomolecules engineered to bind cellular targets with antibody-like specificity in a fraction of the staining time. Here, the authors determine if immediate ex vivo fluorescence imaging with a lymphoma-specific aptamer can rapidly and specifically diagnose xenografted orthotopic human CNS lymphoma at the time of biopsy. METHODS: The authors synthesized a fluorescent CNS lymphoma-specific aptamer by conjugating a lymphoma-specific aptamer with Alexa Fluor 488 (TD05-488). They modified human U251 glioma cells and Ramos lymphoma cells with a lentivirus for constitutive expression of red fluorescent protein and implanted them intracranially into athymic nude mice. Three to 4 weeks postimplantation, acute slices (biopsies, n = 28) from the xenografts were collected, placed in aptamer solution, and imaged with a Zeiss fluorescence microscope. Three aptamer staining concentrations (0.3, 1.0, and 3.0 µM) and three staining times (5, 10, and 20 minutes) followed by a 1-minute wash were tested. A file of randomly selected images was distributed to neurosurgeons and neuropathologists, and their ability to distinguish CNS lymphoma from negative controls was assessed. RESULTS: The three staining times and concentrations of TD05-488 were tested to determine the diagnostic accuracy of CNS lymphoma within a frozen section time frame. An 11-minute staining protocol with 1.0-µM TD05-488 was most efficient, labeling 77% of positive control lymphoma cells and less than 1% of negative control glioma cells (p < 0.001). This protocol permitted clinicians to positively identify all positive control lymphoma images without misdiagnosing negative control images from astrocytoma and normal brain. CONCLUSIONS: Ex vivo fluorescence imaging is an emerging technique for generating rapid histopathological diagnoses. Ex vivo imaging with a novel aptamer-based fluorescent nanomolecule could provide an intraoperative tumor-specific diagnosis of CNS lymphoma within 11 minutes of biopsy. Neurosurgeons and neuropathologists interpreted images generated with this molecular probe with high sensitivity and specificity. Clinical application of TD05-488 may permit specific intraoperative diagnosis of CNS lymphoma in a fraction of the time required for antibody staining.

Hydrophilic interaction chromatography/tandem mass spectrometry for the simultaneous determination of three polar non-structurally related compounds, imipenem, cilastatin and an investigational beta-lactamase inhibitor, MK-4698, in biological matrices.

A method coupling hydrophilic interaction chromatography (HILIC) with tandem mass spectrometry (MS/MS) has been developed for the simultaneous determination of three polar non-structurally related compounds--a carbapenem antibiotic, imipenem (IMP), a renal dehydropeptidase inhibitor, cilastatin (CIL), and an investigational beta-lactamase inhibitor, MK-4698 (BLI), in rat plasma, monkey plasma and mouse blood. The analytes were extracted through protein precipitation, chromatographed on a Waters Atlantis HILIC column, and detected on a Sciex API4000 mass spectrometer using a Turbo-Ion Spray ion source in positive ionization mode following multiple-reaction monitoring (MRM). The assay dynamic range was 0.1-100 microg/mL for IMP, CIL and BLI, respectively, using a total of 20-25 microL biologic samples, and the total HPLC/MS/MS run time was 4 min/injection. The assay was found to be sensitive, selective and reproducible. The challenges, namely, sample stability, blood sample processing, matrix effect in monkey study samples, and dilution re-assays for the limited mouse blood samples, are resolved and discussed. This technique allowed rapid analysis of polar compounds in biologic matrixes with satisfactory chromatographic retention and increased throughput.

Learning how to interpret 'dangerous' internal standard behaviors.

Internal standard (IS) response has been an active topic of discussion within the bioanalytical community. Initial discussions focused on developing criteria for anomalous responses. Recently, understanding the cause and potential impact of variable IS response has been emphasized. Following a review of recommendations from industry discussions regarding variable IS responses, case studies where interferences with IS response resulted in quantitation inaccuracy are presented. The examples illustrate that variable IS response cannot always be attributed to compensation of matrix effects. Anomalous IS responses, even for stable label internal standards should be investigated and the root cause for the anomalous behavior should, if possible, be determined.