Lothar Schmidt

Pharmacokinetics of etoposide: correlation of pharmacokinetic parameters with clinical conditions

SummaryThe pharmacokinetic parameters of etoposide were established in 35 patients receiving the drug parenterally within the framework of different polychemotherapy protocols. A total of 62 data for 24-h kinetics were analysed. After sample extraction and high-performance liquid chromatography (HPLC) or thin-layer cromatographic (TLC) separation, etoposide was measured by means of [252Cf]-plasma desorption mass spectrometry (PDMS). This highly specific detection system proved to be very practicable and reproducible. The present study comprised two parts that were absolutely comparable in terms of clinical and pharmacokinetic parameters. In part II of the study, sensitivity was improved by modifying the analytical technique. After the exclusion of patients who had previously been given cisplatin or who exhibited renal impairment and of one patient who showed extremely high levels of alkaline phosphatase, γ-GT and SGPT, the mean values calculated for the pharmacokinetic parameters evaluated were: beta-elimination half-life (t1/2β), 4.9±1.2 h; mean residence time (MRT), 6.7±1.4 h; area under the concentration-time curve (AUC), 5.43±1.74 mg min ml−1; volume of distribution at steady state (Vdss), 6.8±2.7 l/m2; and clearance (Cl), 18.8±5.3 ml min−1 m−2. The pharmacokinetic parameters were correlated with 12 different demographic or biochemical conditions. Impaired renal function, previous application of cisplatin and the age of patients were found to influence etoposide disposition to a statistically significant extent. We suggest that the dose of etoposide should be reduced in elderly patients and/or in individuals with impaired renal function, especially in those exhibiting general risk factors such as reduced liver function with regard to the polychemotherapy.

A 252Cf fission fragment-induced desorption mass spectrometer: Design, operation and performance

This time-of-flight mass spectrometer utilizes the 252Cf fission fragment-induced ionization of organic solids and electrostatic desorption. It is designed for the high sensitivity needed in the quantitative analysis of pharmaceuticals and can be combined with a high-performance liquid chromatograph. The solute emerges into a rough vacuum stage sampling up to twelve fractions in a vacuum-drying process. Samples with low vapor pressure are introduced periodically into the ion source by means of an interfacing disk and analysed successively. The sensitivity of ion detection employing conversion electrons and microchannel plates is investigated. The performance of the mass spectrometer is discussed in terms of mass resolution, efficiency and reproducibility.

Pharmacokinetics of high-dose etoposide after short-term infusion

SummaryThe pharmacokinetics of high-dose etoposide (total dose, 2100 mg/m2 divided into three doses given as 30-min infusions on 3 consecutive days) were studied in ten patients receiving high-dose combination chemotherapy followed by autologous bone marrow transplantation. In addition to etoposide, all subjects received 2×60 mg/kg cyclophosphamide and either 6×1,000 mg/m2 cytosine arabinoside (ara-C), 300 mg/m2 carmustine (BCNU), or 1,200 mg/m2 carboplatin. Plasma etoposide concentrations were determined by252Cf plasma desorption mass spectrometry. In all, 27 measurements of kinetics in 10 patients were analyzed. According to graphic analysis, the plasma concentration versus time data for all postinfusion plasma ctoposide values were fitted to a biexponential equation. The mean values for the calculated pharmacokinetic parameters were:t1/2β, 256±38 min; mean residence time (MRT), 346±47 min; AUC, 4,972±629μg min ml−1 (normalized to a dose of 100 mg/m2); volume of distribution at steady state (Vdss), 6.6±1.2l/m2; and clearance (CL), 20.4±2.4 ml min−1 m−2. A comparison of these values with standard-dose etoposide pharmacokinetics revealed that the distribution and elimination processes were not influenced by the dose over the range tested (70–700 mg/m2). Also, the coadministration of carboplatin did not lead to significant pharmacokinetic alterations. Although plasma etoposide concentrations at the time of bone marrow reinfusion (generally at 30 h after the last etoposide infusion) ranged between 0.57 and 2.39 μg/ml, all patients exhibited undelayed hematopoietic reconstitution.

Drug monitoring of etoposide (VP16-213)

SummaryDrug monitoring is performed by means of sample extraction, sample purification by high-performance liquid chromatography (HPLC), and sample detection by time-of-flight mass spectrometry. This mass spectrometry utilizing 252Cf fission fragment-induced ionization and desorption of nonvolatile compounds is suitable as a universal, nondestructive detector in HPLC. Liquid chromatography and mass spectrometry are combined, so that mass analysis can be operated online and offline to the fractional sampling of the effluent and the samples can still be recovered.As an alternative to HPLC separation, samples can be purified by thin-layer chromatography (TLC), resulting an offline TLC + MS combination. Preliminary pharmacokinetic data for etoposide (VP16-213) together with calibration data are presented, and are discussed with reference to the sensitivity and detection limit of the new experimental methods.

Fractional sampling interface for combined liquid chromatography–mass spectrometry with 252CF fission fragment-induced ionization

Abstract Time-of-flight mass spectrometry (MS) utilizing p252Cf fission fragment-induced ionization and desorption of non-volatile compounds is suitable as a universal detector in high-performance liquid chromatography (HPLC). The LC-MS combination is achieved by nebulizing the unsplit effluent (ca. 0.5 cmp3minp-1) into a vacuum chamber (ca. 10p-4 bar) and vacuum-drying the non-volatile compounds on collector foils. The permanent storage of the separated compounds on twelve discrete sample foils decouples the LC and MS operations. Stepping the disk interface periodically, each sample is transported into the ion source for the non-destructive MS analysis. It can be repeated afterwards and samples may be recovered for other analyses.

Fragmentation of molecules sliding along surfaces in the speed range above thermal and below Bohr velocity.

A theoretical model and experimental time-of-flight mass spectrometric data for the fragmentation of molecules grazing along surfaces at velocities v = 105–106 cm/s are presented. The effect of enhanced surface-induced dissociation at grazing incidence (GI-SID) is shown for hexadecylpyridine ions. The velocity dependence of the GI-SID fragmentation probability is studied in experiments with adduct ions of cyclodextrin derivatives. Surfaces used in the various collision experiments are aluminum oxide, gold, and a liquid film of perfluorinated polyether. In the theoretical model of the GI-SID effect we consider polyatomic molecules with substructures consisting of chains of identical biatomic dipoles. Because of the interaction with the periodic Coulomb field of the surface, collective vibrational excitations (excimols) are induced in these chains. Energy accumulation of several excimols and a subsequent energy transfer to a trap bond can induce its dissociation. An analytical expression for the velocity dependent GI-SID fragmentation probability is given, which is in good agreement with the experimental data.

Fragmentation of polyatomic molecules by grazing incidence surface-induced dissociation (GI-SID).

The grazing incidence surface-induced dissociation (GI-SID) of n-hexadecylpyridinium and verapamil ions generated by fission fragment desorption was studied. These molecules show the effect of enhanced surface-induced dissociation at grazing incidence as it was observed in former experiments with metal organic ions. A liquid film of perfluorinated polyether is used as collision surface. Small hydrocarbon fragment ions predominate in the GI-SID spectra. Pyridine ions appear as specific fragment ions in the GI-SID spectrum of n-hexadecylpyridinium. The GI-SID conversion efficiency varies in the range 40-70%. The experimental results are discussed within the scope of a quantum mechanical model which is based on the accumulation of internal molecular energy by resonant excitation of collective vibrational states and energy transfer to a trap bond due to dipole-dipole interactions. In this context the GI-SID spectra of n-hexadecylpyridinium and verapamil ions are compared with the fragmentation occurring in regular (252)Cf plasma desorption mass spectrometry.

Quantitative matrix assisted plasma desorption mass spectrometry

Abstract The development of optimized sample preparation methods accompanied the history of successful applications of 252Cf-PDMS. Studying the pharmacokinetics of the antineoplastic agent etoposide serum samples from cancer patients were labelled with the homologeous compounds teniposide as internal standard for the quantitative PDMS analysis. Sample purification by chloroform extraction and by thin layer chromatography turned out to be insufficient to guarantee a satisfying final PDMS result. Embedding the purified sample into a matrix of suitable substances on the target reduced the negative influence of impurities, raised the signal-to-noise ratio of molecular ions and improved the reproducibility of calibration. This preparation method was again successfully employed for the quantitative analysis of the cytostatic drug doxorubicin. The application of a different matrix optimized for the preparation of this anthracycline and its homologeous compound daunorubicin, improved the sensitivity, linearity and detection limit.

Grazing incidence surface‐induced dissociation of protonated peptides generated by matrix‐assisted laser desorption/ionization

The grazing incidence surface-induced dissociation (GI-SID) of various protonated peptides with typical kinetic energies of 350 eV was investigated. Peptide ions were generated by matrix-assisted laser desorption/ionization (MALDI) using delayed extraction. The collision target surfaces used were aluminum and a liquid film of perfluorinated hydrocarbons. All peptides studied in these experiments showed enhanced fragment ion yields at grazing incidence (GI-SID effect) as observed in our former experiments with other precursor ion types. In general the GI-SID spectra exhibit N-terminal a(1)-type fragment ions, immonium ions and side-chain fragment ions in the low mass-to-charge region. Fragment ion series of the peptide backbone were not observed, which are typical and abundant in the spectra of established fragmentation techniques like collision-induced dissociation, MALDI post-source decay or surface-induced dissociation at steeper angles. The potential of the GI-SID process to yield useful information for primary structure determination of peptides is indicated by the observed differences in the GI-SID spectra of the isomeric dipeptides LR and IR.

Liquid chromatography/mass spectrometry with 252Cf fission fragment induced ionization

Abstract A time-of-flight desorption mass spectrometer utilizing 252Cf fission fragment induced ionization analyses samples of nonvolatile organic solids nondestructively. Using a 7 μCi 252Cf-source and thin samples (0.1 - 10 μg cm−2) a complete mass spectrum (m/z 1 −1000) is obtained in approximately 1 minute. The spectrometer is combined with a high-performance liquid chromatograph via an interfacing disk samplin up to 12 fractions of involatile compounds in the effluent (⋍ 0.5 cm3 min−1 MeOH-H2O) under rough vacuum conditions (⋍ 1 mbar) in a vacuum-drying process. This combined LC/MS method has been applied for the quantitative analysis of antitumor drugs in human serum.