The comparative bioavailability of Lanoxin tablets and Lanoxicaps with and without sorbitol

Comparison of steady-state serum concentrations of digoxin in tablets (Lanoxin) and capsules (Lanoxicaps) in the elderly

The bioavailability of digoxin in solution (Lanoxicaps, 90-100 percent) is superior to that of Lanoxin tablets (60-80 percent) in young healthy volunteers. The alleged bioequivalence of digoxin in tablets (Lanoxin 0.25, 0.125 mg) and capsules (Lanoxicaps 0.2, 0.1 mg) was studied in 20 elderly inpatients (14 women, 6 men, aged 84 +/- 5 years), treated with digoxin for atrial fibrillation. In a crossover design, steady-state digoxin concentrations were measured in 16 patients after once-daily administration of Lanoxin 0.125 mg or Lanoxicaps 0.1 mg, each for at least ten days. Four other patients took Lanoxin 0.25 mg and Lanoxicaps 0.2 mg successively. The steady-state digoxin concentrations were statistically significantly different, being 1.3 +/- 0.5 nmol/L for Lanoxin and 0.9 +/- 0.4 nmol/L for Lanoxicaps (p = 0.003). Whereas in young healthy volunteers the bioavailability of digoxin from Lanoxicaps (90-100 percent) exceeds that from Lanoxin (80 percent), in elderly patients the absorption appears to be similar. This suggests a lower than expected bioavailability of Lanoxicaps, which may result in subtherapeutic serum concentrations.

A routine method for the determination of digoxin in urine by radioimmunoassay

We describe a radioimmunoassay for the routine determination of digoxin in urine employing readily available commercial reagents. The standard curve was found to exhibit very good linearity represented by a composite correlation coefficient of 0.9993 and a standard error of estimate which was less than 1% of the digoxin concentration range of interest. Between-assay precision was excellent as reflected by the coefficient of variation (CV) which averaged 5.29%. Studies assessing the recovery of the present method showed satisfactory results of 106% at a low level of digoxin excretion and 102% at a higher excretion level. Our laboratory carried out a study of digoxin bioavailability in which a standard digoxin dose of 500 micrograms was given to 16 volunteers and the present radioimmunoassay method was employed to measure digoxin excretion. An amount equal to 33.4 +/- 6% of the drug given was eliminated 48 h following digoxin dosing.

A standard approach to compiling clinical pharmacokinetic data

A standard format for a Clinical Pharmacokinetic Summary is proposed. It consists of a heading, tables, notes, and references for each drug reviewed. The table presents a unified and logical set of clinically useful population pharmacokinetic parameters. They concern four major areas: absorption, distribution, elimination, and the relationship of concentration to effect. Within each major group, parameters dealing with extents and rates of processes are given. Each such parameter is really two: a population mea value (for example, average volume of distribution) and the standard deviation of individual values about this mean. The first value allows individual predictions of dosage or drug level to be made; the second allows computation of the likely proximity of subsequently observed quantities to those predictions. The table presents single consensus values for each population parameter, rather than a list of values. A procedure for computing these consensus values, and for revising them in the light of new data, or reinterpreted old data, is given. Examples of Summaries are given. The method appears applicable to a variety of drugs. We suggest our approach as a standard one for preparing Clinical Pharmacokinetic Summaries, and urge our colleagues to consider it for that purpose.