Robert J. Perchalski

Steady-state kinetics of valproic acid in epileptic patients.

Pharmacokinetic evaluation and prediction were carried out in 20 epileptic patients. Using conventional pharmacokinetic techniques and a one‐compartment model, predicted and observed valproic acid plasma concentrations were compared. Valproic acid assay was performed by gas‐liquid chromatography. There was good agreement between predicted and observed plasma concentrations. Most patients had predicted half‐lives (t½s) of 6 to 8 hr, independent of the plasma concentration of valproic acid. Five patients had predicted t½s of 12 hr. The correlation between dose and plasma level was poor. Most patients had valproic acid plasma levels between 55 and 100 µg/ml. Administration ofvalproic acid three times a day with determination of individual plasma concentrations offers a reliable method of monitoring. Constant levels are maintained in individual patients, but there is substantial intersubject variation.

Carbamazepine Metabolism in Humans: Effect of Concurrent Anticonvulsant Therapy

Free and total carbamazepine (CBZ) and carbamazepine-epoxide (CBZ-EP) plasma levels were obtained on 113 patients with epilepsy (18–61 years old) controlled on either monotherapy or coadministration with either phenobarbital (PB), phenytoin (PHT), valproic acid (VPA), or all three. A subset of patients were administered tetradeuterium labeled CBZ to evaluate the effects of autoinduction and coadministration of VPA on the kinetics of CBZ and its metabolite CBZ-EP. Polytherapy had variable effect on free and total CBZ plasma levels compared to monotherapy. Coadministered PHT (co-PHT), or all three anticonvulsants together (PHT, PB, and VPA: co-AEDs) decreased free and total CBZ plasma levels. No change was noted for coadministered VPA (co-VPA). Compared to monotherapy the free and total CBZ-EP levels increased with co-VPA, less with coadministered PB (co-PB), and no change with co-PHT or co-AEDs. Protein binding of CBZ and CBZ-EP was not affected by any antiepileptic drugs studied. The free and total CBZ-EP/CBZ ratio was tripled with co-VPA or co-AEDs, and doubled with co-PHT or co-PB. Isotope labeling did not demonstrate any differences in half-life (t 1/2), plasma clearance (Cl), or volume of distribution (Vd). Compared to naive controls, monotherapy and co-VPA decreased CBZ t 1/2 by 50%, and more than doubled the CBZ Cl without a significant change in the Vd. Autoinduction is one explanation for these changes with chronic CBZ therapy. With the addition of VPA to chronic CBZ therapy no change was noted on the t 1/2, Cl, Vd, or mean CBZ plasma level, but the CBZ-EP plasma level more than quadrupled. This suggests a decrease in CBZ-EP clearance. These data support the autoinduction phenomena for chronic CBZ therapy and the concept of VPA inhibiting carbamazepine-epoxide metabolism.

A dihydropyridine conjugate which generates high and sustained levels of the corresponding pyridinium salt in the brain does not exhibit neurotoxicity in cynomolgus monkeys

Many drugs can be selectively delivered to the brain by using a dihydropyridine in equilibrium pyridinium salt chemical delivery system (CDS). The interaction of these systems with central dopaminergic function was examined in this communication. Castrate female Sprague-Dawley rats when treated with a CDS for estradiol (i.e. 3-hydroxy-17 beta-[( (1-methyl-1,4-dihydropyridin-3-yl)carbonyl]oxy) estra-1,3,5(10)-triene or E2CDS) exhibit sustained and profound suppression of serum levels of leuteinizing hormone (LH). Treatment of rats with pargyline (80 mg/kg) prior to E2CDS (2 mg/kg) did not mitigate the biological effectiveness of this estrogen indicating at least indirectly that monoamine oxidate (MAO) was not involved in the CDS activation. In a more direct examination, cynomolgus monkeys treated with various repeated doses of E2CDS (cumulative doses of 0.2-40.0 mg/kg) demonstrated neither impaired motor function nor depleted striatal dopamine concentrations. The latter parameter was measured using liquid chromatographic-electrochemical analysis. These experiments support the contention that the CDS is not neurotoxic and further strengthens the strict structure-activity requirements for MPTP-induced neurotoxicity.

Simultaneous determination of the anticonvulsants, cinromide (3-bromo-N-ethylcinnamamide), 3-bromocinnamamide, and carbamazepine in plasma by high-performance liquid chromatography

A high-performance liquid chromatographic method is described for monitoring plasma concentrations of cinromide (3-bromo-N-ethylcinnamamide) and its de-ethylated metabolite. Carbamazepine levels can be easily measured by the same technique. The N-isopropyl analogue of cinromide is used as internal standard, and all compounds are easily separated on a reversed-phase column operated at 55 degrees with a small-diameter pre-column maintained at the same temperature. The extraction is rapid and generally applicable to plasma and urine samples that are to be analyzed by reversed-phase chromatography. Short- and long-term reproducibility studies show less than 4% relative standard deviation for replicate determinations for all drugs. Limits of quantitation are 10-20 ng/ml with an internal standard concentration of 3 micrograms/ml. Another metabolite of cinromide, 3-bromocinnamic acid, which may have some anticonvulsant effect, can be analyzed simultaneously by buffering the mobile phase and adding an ion-pairing reagent.

Pharmaceutical and clinical analysis by tandem mass spectrometry

Tandem mass spectrometry (MS/MS) is a promising technique for trace determination of compounds in complex mixtures. The application of triple-quadrupole MS/MS to clinical and pharmacological studies has been investigated with major emphasis on rapid screening and determination of drugs and biomolecules in physiological fluids and tissues. These techniques have been applied to a range of problems, including the determination of chlorinated compounds in humans, subpicogram analysis of neurochemicals and the detection of illegal drugs in racing animals. A new technique for determining the structures of all the metabolites of a particular drug has also been developed. It is possible to identify in a single sample all molecular ions which contain substructures characteristic of the parent drug. The structure of each metabolite can then be determined by obtaining the MS/MS spectrum of the molecular ion.

Neuropharmacology of zonisamide, a new antiepileptic drug

Zonisamide readily crosses the blood-brain barrier and is readily absorbed after oral administration with a Tmax of about 3 hr. The half-life of ZNA in epileptic patients is about 28 hr. Zonisamide has a broader therapeutic range than other antiepileptic drugs. Neurotoxic, hemapoietic, renal, and liver effects have been minimal in patients participating in controlled clinical studies. It is effective in several experimental models of epilepsy and in initial clinical trials has been shown to be effective in generalized tonic-clonic, simple, and complex partial seizures.

Biotransformation and excretion: metabolite identification: other mass spectrometric methods.

M ass spectrometry is the principal technique for structural elucidation of drug metabolites. The focus of this report is on a relatively new development in the field, tandem mass spectrometry (MS-MS), in which two or more mass spectrometers are linked in series. In particular, we will cover the application of MS-MS to the detection and identification of drug metabolites. Multistage mass spectrometers provide not only the capability of mass spectrometric identification of components in complex mixtures, but also a means of separating and selecting the components of interest. This latter function has typically been handled by some chromatographic method. Tandem mass spectrometry can, in many cases, eliminate the need for chromatography and can provide additional capabilities not obtainable with the relatively slow chromatographic techniques in which sample components must necessarily elute one at a time. These capabilities, inherent in the special operating modes of the tandem mass spectrometer, make the system particularly well suited to the detection and identification of drug metabolites.1

Chapter 30. Tandem Mass Spectrometry for the Identification of Drug Metabolites

Publisher Summary The combination of gas chromatography and mass spectrometry (GC/MS) has become a principal technique for the separation and structural elucidation of drug metabolites. Drug metabolite identification methods are based on the incorporation of radioactive or stable isotopes into the drug molecule, and tracking the tracer by some form of chromatography in physiological samples from test animals or humans. Selected fractions are then identified by mass spectrometry. Tandem mass spectrometry (MS/MS) offers such a method. MS/MS can eliminate the need for chromatography and provide additional capabilities not obtainable with the relatively slow chromatographic techniques in which sample components elute one at a time. MS/MS is capable of rapidly (2–3 hrs.) identifying metabolites in physiological samples with minimal sample preparation, and can be used to postulate new metabolite structures without the use of reference standards. This chapter provides an overview of the application of MS/MS to detection and identification of drug metabolites, and illustrates the use of tandem quadrupole MS/MS (TQMS). Described here is the development of MS/MS for mixture analysis, and the unique characteristics and special operating modes of a tandem mass spectrometer that make the system particularly well suited to the rapid detection and identification of drug metabolites. Characteristics of a tandem quadrupole mass spectrometer have been reviewed and its operational modes discussed. Premise of the MS/MS method have been delved upon and a summary of the MS/MS procedure provided. Metabolite identification by MS/MS involving both old and new metabolites and conjugated drug metabolites with details on major metabolites of the antiepileptic drugs primidone, cinromide, and phenytoin identified in urine and plasma by MS/MS, normal and daughter spectra of pure zonisamide. and neutral loss, and parent spectra of enzyme- hydrolyzed urine extract.

Tandem mass spectrometry for studies of drug action and metabolism

Abstract We have investigated the application of triple quadrupole MS/MS to toxicological, pharmacokinetic, and pharmacological studies. Techniques have been developed for rapid screening and quantitation of drugs and metabolities in physiological fluids and tissues, including the detection of illegal drugs in racing animals. Studies on antiepileptic drugs have centered on determination of the regional distribution of drugs and metabolites in target organs and on monitoring the time course of drug uptake and release. A new method for rapidly determining all the metabolites of a particular drug has also been developed.

Gas-liquid chromatographic determination of carbamazepine and phenylethylmalonamide in plasma after reaction with dimethylformamide dimethylacetal

A previously published procedure for the gas chromatographic analysis of carbamazepine has been modified and expanded to allow simultaneous determination of phenylethylmalonamide, a metabolite of primidone. Internal standards that closely resemble each compound are used, and derivatives are made by reaction with dimethylformamide dimethylacetal. This change of internal standard for carbamazepine and the use of a commercial, pretested column-packing material eliminate the major pitfalls of the original method.