Quantitation of digitoxin and the bis- and monodigitoxosides of digitoxigenin in serum

Measurement of serum digitoxin in patients by radioimmunoassay using specific antiserum

INTRODUCTION

Digitoxin is used to treat patients with heart failure.

METHODS

A radioimmunoassay procedure for the specific determination for digitoxin in serum was developed using the antiserum (antiserum (A)) raised against digitoxin 3'-hemisuccinate-BSA conjugate.

RESULTS

The intra- and interassay variability were <10% in the range of 5-100 ng/ml. The specificities of antiserum (A) and the commercial anti-digitoxin antiserum (antiserum (B)) were assessed by cross-reactivity studies with various related compounds. Antiserum (A) was highly specific for digitoxin. Mean digitoxin concentrations in serum samples (n=34) from digitalized patients by RIA using these antisera were 10.0 and 12.4 ng/ml, respectively.

CONCLUSION

This RIA using antiserum (A) measure unmetabolized digitoxin and may be applicable for pharmacokinetic studies.

Chromatography of cardiac glycosides

Most of the recently reported methods for the quantitation of cardiac glycosides have been for digoxin and its metabolites. Recent procedures using high-performance liquid chromatography-radioimmunoassay (HPLC-RIA) and HPLC following derivatization show appreciable improvements in accuracy and specificity for quantitating digoxin in the low nanogram range. Gas chromatographic procedures have been explored to a very limited extent and further advances in the quantitation of cardiac glycosides are anticipated to arise from the use of laser desorption-Fourier transform mass spectrometry. However, currently, HPLC with derivatization and HPLC-RIA techniques remain the techniques of choice for quantitation of digoxin and/or its metabolites based on considerations of ease of use, sensitivity, specificity, accuracy and reproducibility.

Drug level monitoring: cardiovascular drugs

Methods for the determination of cardiovascular drugs in blood and plasma are critically reviewed with emphasis on gas and liquid chromatographic techniques. The importance of the various procedures is discussed, in particular sample work-up where the conditions for isolation and derivatization of the compounds are decisive for the accuracy and precision of the methods. Compared with other assay techniques chromatographic methods are generally to be preferred owing to their better selectivity. In the review the following groups are discussed: digitalis glycosides, antiarrhythmic agents, beta-adrenoceptor antagonists, vasodilating agents, antihypertensive compounds, and diuretics.

Assessment of the potential pharmacokinetic interaction between digoxin and ethmozine

The potential for a pharmacokinetic interaction between the investigational antiarrhythmic drug ethmozine (moricizine HCl, the generic name that is infrequently used in existing literature) and digoxin was evaluated in nine healthy male adults. Serum and urinary digoxin concentrations were measured by radioimmunoassay following intravenous digoxin administration before and during steady-state ethmozine dosing. Plasma ethmozine levels following a single oral dose were measured before and after a single intravenous dose of digoxin. A mean elimination half-life of 45.6 hours was determined for digoxin alone, compared to 43.1 hours in combination with ethmozine. Average values for digoxin systemic clearance, apparent volume of distribution, and renal clearance were 2.87 mL/min/kg, 11.3 L/kg, and 2.44 mL/min/kg, respectively for digoxin alone, compared to 3.01 mL/min/kg, 11.3 L/kg, and 2.64 mL/min/kg, respectively for digoxin with ethmozine. A mean half-life of 2.0 hours was determined for ethmozine alone, compared with 1.8 hours following a single intravenous dose of digoxin. No change was observed in the oral pharmacokinetics of ethmozine following a single intravenous dose of digoxin, as indicated by the area under the plasma concentration versus time curve, Cmax or Tmax. These findings suggest that no pharmacokinetic interaction occurs when single intravenous doses of digoxin are co-administered with multiple oral doses of ethmozine.

Bioavailability of the bis- and monodigitoxosides of digitoxigenin

The pharmacokinetic behavior of digitoxigenin is affected by the number of glycosides present. This study was conducted to compare the bioavailabilities of the bis- and monodigitoxosides of digitoxigenin in man. Intravenous and oral doses of the two drugs were administered to six normal volunteers. Blood samples were collected up to 28 days after each dose, and assayed for the specific drug administered and for total radioassayable drug. Both drugs were virtually completely absorbed, based on serum concentrations of administered drug plus metabolites. However, the mean bioavailability of unchanged bisdigitoxoside was only 56.3% indicating that substantial metabolism occurred prior to entry into the systemic circulation. Monodigitoxoside was virtually completely metabolized prior to entry into the systemic circulation.