Validation of a high-performance liquid chromatographic assay method for pharmacokinetic evaluation of busulfan

Development and Validation of a Novel LC-MS/MS Method for a TDM-Guided Personalization of HSCT Conditioning with High-Dose Busulfan in Children

Personalization of busulfan (Bu) exposure via therapeutic drug monitoring (TDM) is recommended for patients treated with high-dose conditioning regimens. Several laboratories' developed methods are available in the literature with a lack of standardization. The aim of this study is to develop a new standardized LC-MS/MS method and validate it according to the international ICH M10 (EMA) guidelines. Our method is based on rapid protein precipitation from 50 µL plasma followed by separation on a reversed-phase C-18 UHPLC column after the addition of deuterated internal standard and has been tested on real samples from pediatric patients treated with myeloablative conditioning regimens, including Bu, before autologous or allogeneic hematopoietic stem cell transplantation (HSCT). The validated LC-MS/MS method is linear over wide concentration ranges (125-2000 ng/mL), accurate, and reproducible in the absence of matrix effects, allowing for the specific and rapid quantification of Bu and allowing next-dose recommendations to be made in a timely fashion to answer clinicians' needs. Given the lack of data on the stability of Bu in real clinical samples, stability was assessed both on quality controls and on real samples to set up a robust protocol in real-life conditions. This novel LC-MS/MS method is suitable for application to the TDM-guided personalization of conditioning treatments with high-dose busulfan in pediatric patients undergoing HSCT.

Rapid and accurate method for quantifying busulfan in plasma samples by isocratic liquid chromatography-tandem mass spectrometry (LC-MS/MS)

Objectives

Administration of busulfan is extending rapidly as a part of a conditioning regimen in patients undergoing hematopoietic stem cell transplantation (HSCT). Monitoring blood plasma levels of busulfan is recommended for identifying the optimal dose in patients and for minimizing toxicity. The aim of this research was to validate a simple, rapid, and cost-effective analytical tool for measuring busulfan in human plasma that would be suitable for routine clinical use. This novel tool was based on liquid chromatography coupled to mass spectrometry.

Methods

Human plasma samples were prepared using a one-step protein precipitation protocol. These samples were then resolved by isocratic elution in a C18 column. The mobile phase consisted 2 mM ammonium acetate and 0.1% formic acid dissolved in a 30:70 ratio of methanol/water. Busulfan-d8 was used as the internal standard.

Results

The run time was optimized at 1.6 min. Standard curves were linear from 0.03 to 5 mg/L. The coefficient of variation (%CV) was less than 8%. The accuracy of this method had an acceptable bias that fell within 85-115% range. No interference between busulfan and the interfering compound hemoglobin, lipemia, or bilirubin not even at the highest concentrations of compound was tested. Neither carryover nor matrix effects were observed using this method. The area under the plasma drug concentration-time curves obtained for 15 pediatric patients who received busulfan therapy prior to HSCT were analyzed and correlated properly with the administered doses.

Conclusions

This method was successfully validated and was found to be robust enough for therapeutic drug monitoring in a clinical setting.

A Model-Based Workflow to Benchmark the Clinical Cholestasis Risk of Drugs

We present a generic workflow combining physiology-based computational modeling and in vitro data to assess the clinical cholestatic risk of different drugs systematically. Changes in expression levels of genes involved in the enterohepatic circulation of bile acids were obtained from an in vitro assay mimicking 14 days of repeated drug administration for 10 marketed drugs. These changes in gene expression over time were contextualized in a physiology-based bile acid model of glycochenodeoxycholic acid. The simulated drug-induced response in bile acid concentrations was then scaled with the applied drug doses to calculate the cholestatic potential for each compound. A ranking of the cholestatic potential correlated very well with the clinical cholestasis risk obtained from medical literature. The proposed workflow allows benchmarking the cholestatic risk of novel drug candidates. We expect the application of our workflow to significantly contribute to the stratification of the cholestatic potential of new drugs and to support animal-free testing in future drug development.

Is Salivary Busulfan the Cause of Oral Mucositis and the Changes in Salivary Antioxidant Enzymes After Hematopoietic Cell Transplantation?

BACKGROUND

To determine whether the busulfan (Bu) present in saliva during hematopoietic cell transplantation (HCT) conditioning correlates with oral mucositis and the changes in salivary antioxidant enzymes.

METHODS

Bu levels in the plasma and saliva of 19 patients who received HCTs were quantified. Salivary flow and salivary superoxide dismutase and catalase activities were measured during HCT. For the toxicity analysis of salivary Bu, an in vitro assay was conducted by exposing human keratinocytes to artificial saliva containing Bu.

RESULTS

Plasma and salivary Bu concentrations were very similar (rho = 0.92, P < 0.001). Salivary Bu concentration correlated with the degree of oral mucositis severity (rho = 0.391, P = 0.029) and was inversely proportional to salivary superoxide dismutase and catalase activities (rho = -0.458, P = 0.036; rho = -0.424, P = 0.043, respectively). Cells exposed to Bu-containing saliva had fewer viable cells (P < 0.01) and more apoptotic cells (P = 0.001) than those exposed to non-Bu-containing saliva.

CONCLUSIONS

Bu found in saliva during HCT conditioning was correlated with severe oral mucositis and the reduction in salivary antioxidative activity. Furthermore, Bu can be toxic to keratinocytes.

Dispersive micro solid phase extraction of busulfan from plasma samples using novel mesoporous sorbent prior to determination by HPLC-MS/MS

Determination of busulfan concentration in patients undergoing bone marrow transplantation is necessary in order to reduce toxic effects and/or graft rejection due to unadjusted dose exposure. A new extraction method namely dispersive micro solid phase extraction (DMSPE) based on mesoporous sorbent was used for cleaning-up the plasma samples. DMSPE coupling with liquid chromatography with tandem mass spectrometry (LC-MS/MS) was implemented for the determination of busulfan dosage in plasma samples. The linear range was found from 10 to 2000 ng/ml. The precision and accuracy were found better than 15% according to Food and drug Administration (FDA) guideline. This method was successfully used to determine the busulfan in patients administrated busulfan as part of the preparative regimen for bone marrow transplantation.

Comparison of LC-MS Assay and HPLC Assay of Busulfan in Clinical Pharmacokinetics Studies

Busulfan is used in preparative regimens for bone marrow transplantation and timely busulfan plasma concentration reporting is critical for subsequent dose adjustment. We compared two sensitive methods for pharmacokinetics studies including LC-MS assay and HPLC precolumn derivatization assay. Chromatographic separation was performed on a Gemini C18 column. Liquid-liquid extraction with ethyl acetate was used for plasma sample preparation. Busulfan and internal standard ([2H8]-busulfan) were detected as ammonium adducts at m/z 264.2 and 272.2 for LC-MS assay. For HPLC assay, the extraction from plasma was derivatized with 2-naphathalenethiol using synthesized internal standard (1,6-(methanesulfonyloxy)octane). The Ex and Em wavelength was 255 nm and 370 nm. The limit of detection was 15.6 ng/mL and 7.8 ng/mL for HPLC and LC-MS assay and good linearity ranging from 31.25–1000 ng/mL for HPLC and 15.6-1000 ng/mL for LC-MS assay. The intra and interday assay precision were less than 9.2% and 12.0% for LC-MS and HPLC assay. The pharmacokinetic parameters were estimated using noncompartmental pharmacokinetic model with WinNonlin. Busulfan AUClast showed an average difference of 0.7% between the two methods. The LC-MS method is accurate, reproducible, and requires less specimen, sample preparation and analysis time over the HPLC assay, making busulfan monitoring faster and easier in clinical practice.

High busulfan exposure is associated with worse outcomes in a daily i.v. busulfan and fludarabine allogeneic transplant regimen

Low plasma busulfan (Bu) area under the concentration-time curve (AUC) is associated with graft failure and relapsed leukemias, and high AUC with toxicities when Bu is used orally or i.v. 4 times daily combined with cyclophosphamide in myeloablative hematopoietic stem cell transplantation (SCT) conditioning regimens. We report Bu AUC and its association with clinical outcomes in 130 patients with hematologic malignancies given a once-daily i.v. Bu (3.2 mg/kg days -5 to -2) and fludarabine (Flu, 50 mg/m(2) days -6 to -2) regimen. Total-body irradiation (TBI) 200 cGy x 2 was added for 51 patients with acute leukemias. Plasma AUC varied 3.6-fold (2184-7794 microM.min, median 4699 microM.min). Patients with an AUC >6000 microM.min had lower overall survival (OS) than those with AUC < or =6000 microM.min at 12 months (38% versus 74%) and 36 months (23% versus 68%, P < .001). This effect was apparent in patients with standard-risk and high-risk disease, and persisted when potential confounders were considered (hazard ratio 3.2, 95% confidence interval 1.7-6.3). Nonrelapse mortality (NRM) at 100 days (6% versus 19%) and progression free survival (PFS; 58% versus 16%) at 3 years were better with AUC < or =6000 microM.min. These data support a role for therapeutic dose monitoring and dose adjustment with daily i.v. busulfan.

Influence of glutathione S-transferase A1 polymorphism on the pharmacokinetics of busulfan

BACKGROUND

High-dose oral busulfan is used for myeloablative chemotherapy before hematopoietic stem-cell transplantation. Fatal adverse effects or relapse may occur with excess or insufficient busulfan exposure. Glutathione S-transferase (GST) A1, whose genetic polymorphism in its promoter region has been reported, is responsible for busulfan metabolism. We investigated the polymorphism of GSTA1 on busulfan pharmacokinetics.

METHODS

Blood samples (6 or 7 points) were taken from patients receiving high-dose oral busulfan (approximately 1 mg/kg every 6 h) on Doses 1 and 5. Pharmacokinetic parameters were calculated from plasma busulfan concentration.

RESULTS

Twelve patients were enrolled in this study. Nine patients were genotyped as wildtype (GSTA1*A/*A), and 3 as heterozygous variants (GSTA1*A/*B). At Dose 5, the heterozygous group had significantly lower elimination constant (0.176+/-0.038 vs. 0.315+/-0.021 h-1; P=0.008) and clearance corrected by bioavailability (0.118+/-0.013 vs. 0.196+/-0.011 l/h/kg; P=0.004), and significantly higher mean plasma busulfan concentration (1344+/-158 vs. 854+/-44 ng/ml; P=0.001) than the wildtype.

CONCLUSIONS

This is the first report on the significant influence of GSTA1 polymorphism on busulfan elimination. This may account for the large inter-individual variance in busulfan pharmacokinetics, and with more information confirming our study, busulfan high-dose therapy may be optimized by GSTA1 genotyping in advance.

Tandem Mass Spectrometry Method for the Quantification of Serum Busulfan

Busulfan, an alkylating agent, is most commonly used as a component of bone marrow transplantation preoperative regimens. Significant interpatient and intrapatient variations in pharmacokinetics require individualizing the dosage based on area under the time-versus-concentration curve. Timely result reporting is critical to dose adjustment to reduce morbidity and mortality associated with the regimen. The authors developed a rapid, accurate, and sensitive method for the quantification of serum busulfan using direct inject tandem mass spectrometry. Plasma samples (50 microL) are extracted in 1 mL of methanol containing 1,6-bis-(methanesulfonyloxy)hexane as an internal standard. The supernatant is dried under nitrogen (40 degrees C, 30 minutes) and then dissolved in 200 microL methanol and transferred into a clean glass vial suitable for LC/MS/MS analysis. The sample is delivered using an HPLC pump that delivers 0.2 mL of methanol per minute, and 20 microL of sample is injected into a turbo ion spray-equipped tandem mass spectrometer. Total analysis time is 5 minutes. The Q1/Q3 transition for busulfan (BU) is monitored at 269/55 and 297.1/55.1 for the internal standard. The assay is linear to 10 micromol/L and sensitive to at least 0.5 micromol/L. The interassay reproducibility at 1, 2.2, and 4.4 micromol/L were 4.2%, 5.6%, and 6.3%, respectively. Within-run precision using 3 different control samples was 3.9%, 3.9%, and 6.9%. Mean recovery of 4 different BU concentrations spiked into 10 different BU free plasma samples was 98%. Correlation with an established HPLC-UV method revealed a slope of 0.98, an intercept of 0.1, and r = 0.95 (n = 48). No significant interfering substances or ion suppression was identified. This method is a significant improvement over the existing HPLC-UV method for BU determination. The method is highly accurate, reproducible, and requires less specimen, sample preparation, and analysis time.

Development of a rapid and specific assay for detection of busulfan in human plasma by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry

A sensitive and specific assay for detection of busulfan in human plasma was developed. The assay is based on rapid isolation of busulfan by liquid-liquid extraction with ethyl acetate, and detection by high-performance liquid chromatography with electrospray ionization and tandem mass spectrometry. 1,6-Bis(methanesulfonyloxy)hexane, a synthesized analogue of busulfan, was used as the internal standard (IS). The acquisition was performed in the multiple reaction monitoring mode; busulfan and the IS were detected with no interferences from plasma matrix. The method was linear over the range 5-2500 ng mL(-1), with r2 > 0.99 and a run time of only 3.5 min. The intra- and inter-assay precisions were in the ranges 2.1-11.9% and 3.2-10.1%, respectively, and the intra- and inter-assay accuracies were 92.2-107.6% and 94.7-104.1%, respectively. The absolute recoveries were 82.0% (20 ng mL(-1)), 90.6% (1000 ng mL(-1)) and 80.0% (2000 ng mL(-1)) for busulfan, and 89.1% for the IS (1000 ng mL(-1)). The limits of detection and quantification were 2 and 5 ng mL(-1), respectively. The validated method was successfully applied to analyze plasma samples obtained from six adults receiving doses of 1 mg kg(-1) in a conditioning regimen prior to bone marrow transplantation. A marked intra-patient variation in busulfan concentrations during the steady state was observed, which limits the application of pharmacokinetic modeling and suggests that continuous therapeutic monitoring is necessary for adequate individualized dosing. In this regard, the present assay brings important advantages relative to other methods described in the literature, i.e., it is highly specific and simple to perform, with a rapid chromatographic run time (3.5 min), and the whole procedure can be completed in 4-5 h, which would permit dose corrections after the third dose allowing earlier and better dosing adjustments towards the target level of busulfan.

Improved assay for determination of busulfan by liquid chromatography using postcolumn photolysis

A highly sensitive and time-reduced HPLC assay for the quantitative analysis of busulfan in plasma and aqueous samples is described. The assay is based on a precolumn derivatization of busulfan to 1,4-diiodobutane and UV-detection of iodide ions generated by a postcolumn photochemical dissociation of the derivative. The extraction and derivatization were carried out in a one-pot reaction without any solid phase extraction and is therefore suitable for high throughput analysis. Quantification was performed by using 1,5-pentanediol-bis-(methanesulfonate), a homologue of busulfan, as an internal standard. Linearity was demonstrated for concentrations from 50 to 10,000ng/ml. The limit of detection was found at 10ng/ml. Precision is indicated by an intra-day variety of 2.81% and by an inter-day variety of 6.61% for aqueous samples, 2.93 and 5.76% for plasma samples, respectively. The recovery of busulfan in plasma was more than 95%. No coelution with metabolites of busulfan or other drugs used in cancer therapy was found. The method was generated for measurements of busulfan in aqueous or plasma samples and applied in therapeutic drug monitoring of busulfan.

Separation methods for alkylating antineoplastic compounds

The separating method for alkylating neoplastic compounds were reviewed based on the classification of the Merck Index (12th Edition). Each section, whenever available or relevant, was subdivided according to the following approach: stability studies, extraction methods, gas chromatography, high-performance liquid chromatography and capillary electrophoresis. At the end of each chapter a separate table summarizing the main characteristics of the separating method were established. In particular LODs and/or LOQs were expressed as quantity to facilitate comparison between methods. This review highlights the problems to measure trace levels of these compounds into biological fluids with respect to their instability, adsorption to glass and plastic or derivatization requirements. Over the last decades, HPLC seems to be more popular than GC for separating the alkylating agents. The development of narrow- or microbore LC coupled to MS is certainly the way to further improve both separation and sensitivity obtained in the different papers surveyed for this review.

Quantification of busulfan in plasma by liquid chromatography-ion spray mass spectrometry. Application to pharmacokinetic studies in children

Optimisation of busulfan dosage in patients undergoing bone marrow transplantation is recommended in order to reduce toxic effects associated with high drug exposure. A new method was developed coupling liquid chromatography with mass spectrometry (LC-MS) and was validated for the determination of busulfan concentrations in plasma. Recovery was 86.7%, the limit of detection was 2.5 ng/ml and linearity ranged from 5 to 2500 ng/ml. The correlation between the busulfan concentrations measured by our previously published HPLC-UV method and the new HPLC-MS method was highly significant (P<0.0001). Sample volume was reduced and the method was rapid, sensitive and less expensive than the methods previously used in our laboratory. This method was used to determine the pharmacokinetic parameters of busulfan after the first administration of 1 mg/kg orally, in 13 children receiving the drug as part of the preparative regimen for bone marrow transplantation. Our results were similar to previously reported data. They showed that the apparent oral clearance of busulfan was 0.299+/-0.08 l/h/kg, and that it was significantly higher (P=0.02) in patients below the age of 5 years than in older children.

Sensitive and Rapid Quantification of Busulfan in Small Plasma Volumes by Liquid Chromatography–Electrospray Mass Spectrometry

Background: High-dose busulfan is widely used in conditioning regimens before hematopoietic stem cell transplantation in both adults and children. Large interindividual variability in pharmacokinetics after oral administration has been reported; therefore, therapeutic drug monitoring of busulfan may decrease the incidence of drug-related toxicity (for example, hepatic venoocclusive disease) and may also improve therapeutic efficacy.

Methods: Busulfan concentrations were quantified using 200 μL of plasma and liquid–liquid extraction with diethyl ether after the addition of [2H8]busulfan as the internal standard. Separation and detection of busulfan and [2H8]busulfan were achieved with a LUNA C8 column (5 μm; 150 × 2 mm i.d.) at 30 °C, a HP 1100 liquid chromatography system, and a HP 1100 single-quadrupole mass spectrometer. Busulfan and [2H8]busulfan were detected as ammonium adducts in selected-ion monitoring mode at m/z 264.2 and 272.2, respectively.

Results: The calibration curve was linear at 5–2000 μg/L busulfan. Intra- and interassay imprecision (CV) and bias were both &lt;11%. The limits of detection and quantification were 2 and 5 μg/L, respectively. Extraction recovery of busulfan was &gt;87%. Analysis of pharmacokinetics in four patients receiving high-dose busulfan indicated that minimum busulfan concentrations before the next dose were 405–603 μg/L, with no interference observed.

Conclusions: The new rapid and sensitive liquid chromatographic–mass spectrometric assay is an appropriate method for quantification of busulfan in human plasma, making therapeutic drug monitoring of busulfan faster and easier in clinical practice.

Rapid and sensitive high-performance liquid chromatographic method for busulfan assay in plasma

A reversed-phase liquid chromatographic method with ultraviolet detection has been developed to determine busulfan concentrations in plasma of children undergoing bone marrow transplantation. Plasma samples (200 microl) containing busulfan and 1,6-bis(methanesulfonyloxy)hexane as an internal standard were prepared by a simple derivatization method with diethyldithiocarbamate followed by extraction with ethyl acetate and solid-phase purification on C8 columns conditioned with methanol and water and eluted with acetonitrile (recovery 99%). Chromatography was accomplished using a Hypersil octadecylsilyl column (10 cm x 4.6 mm I.D.) and a mobile phase of acetonitrile, tetrahydrofuran and distilled water (65:5:30, v/v). The limit of detection was 25 ng/ml (signal-to-noise ratio of 5). Calibration curves were linear up to 25,000 ng/ml. Intra-day and inter-day coefficients of variation of the assay were < or =5%. This method was used to analyse busulfan plasma concentrations after oral administration within the framework of therapeutic drug monitoring and pharmacokinetic studies in children.

High-performance liquid chromatographic method for quantification of busulfan in plasma after derivatization by tetrafluorothiophenol

A high-performance liquid chromatographic (HPLC) method was developed and validated for the determination of busulfan in plasma. Busulfan was extracted in toluene, derivatized by 2,3,5,6-tetrafluorothiophenol to obtain di-TFTP-butane, the derivatization product was then re-extracted in toluene and injected into the HPLC system with ultraviolet detection (wavelength: 275 nm). Recovery from extraction was 80%, the limit of quantification was 50 ng/ml and linearity ranged from 50 to 2000 ng/ml. In addition, forty-two samples obtained from pediatric patients treated with busulfan were analyzed by the HPLC and GC-MS assays based on the same derivatization procedure. The correlation between the di-TFTP-butane concentrations was highly significant (p<0.0001), demonstrating that the two methods were in good agreement.

Routine analysis of plasma busulfan by gas chromatography–mass fragmentography

Busulfan (BU) is a widely used alkylating agent for antineoplastic therapy and marrow ablation in preparation for bone marrow transplantation (BMT). High-dose BU often leads to successful preparation and low relapse but is associated with veno-occlusive disease of liver. We established a protocol to determine postdosage plasma BU concentrations by gas chromatography–mass fragmentography in an attempt to relate clinical outcome to plasma BU concentrations. We used nonisotopic pusulfan as the internal standard. After extraction into ethyl acetate, BU and pusulfan were iodinated into 1,4-diiodobutane and 1,5-diiodopentane, respectively. Gas chromatography–mass spectrometry (GC–MS) analysis was carried out on an Hewlett–Packard (HP) 5890II gas chromatograph with a 30-m 100% methyl silicon narrow bore, fused-silica capillary column interfaced with an HP 5970A mass spectrometer. Helium was the carrier gas. The sample molecules were identified by total ion monitoring and quantified by selective ion monitoring of m/z 183 and 197. The calibration curve was linear to 4 mg/L. The limit of quantification was 0.04 mg/L, and the analytical recovery was ∼97%. The within-day and between-day imprecision (CV) was &lt;6% and 9%, respectively. In a preliminary study of 12 children, the BU areas under the BU-time curve were 616–949 μmol · min/L after the first dose and 793-1143 μmol · min/L after the fifth dose. We conclude that the GC–MS procedure is suitable for routine analysis of plasma BU.