1. The pharmacokinetics of single intravenous doses of antipyrine were determined in 96 volunteers using multiple (12 or more) plasma antipyrine concentrations measured by high-pressure liquid chromatography during 24-48 h after dosage. These kinetic estimates were compared with those based on: A, the 4 h and 12 h points only; B, the 4 h through 12 h points; C, the 8 h and 24 h points only. 2. Mean clearance values for the complete study (48.0 ml min-1) were nearly identical to abbreviated approaches A, B, and C (49.1, 49.3, and 46.4 ml min-1), and were highly correlated (r = 0.99). 3. Coefficients of variation (CV) between individual clearance values for complete vs abbreviated studies averaged 5.5%, 5.8% and 2.9%, and CVs were less than 15% in 95.8%, 93.7% and 98.9% of subjects, respectively, for methods A, B, and C. 4. Overall mean values of elimination half-life (11.9, 12.1, 12.0 and 12.5 h) and volume of distribution (43.7, 45.1, 45.2, and 44.71) were likewise very similar for complete A, B and C analyses respectively. 5. The best correlation with the complete study was observed for the 8 and 24 h sampling scheme, for which clearance values were within 5% of the reference method in 84% of subjects, and within 10% in 97% of subjects. 6. Antipyrine pharmacokinetic parameters can be estimated with reasonable precision using a simplified two-point blood sampling procedure following a single intravenous dose. Estimates of elimination half-life, volume of distribution and clearance based on 8 h and 24 h data points correlated best with complete pharmacokinetic studies.
Microdose (1 mg) captopril therapy is commonly used for the initial dose titration in patients with congestive heart failure. Since this dosage form is not commercially available, it has to be extemporaneously compounded. Quality control of each batch is commonly evaluated using the weight variation technique described in the USP. Despite careful titration with microdose, captopril capsules variability in patient's response has been observed. In order to explain this fluctuation, the actual content of extemporaneously compounded microdose captopril capsules was evaluated using a high pressure liquid chromatographic assay. Microdose captopril capsules were prepared by triturating 25 mg tablets with lactose in a mortar using standard geometric dilution technique and a Sharpe-Dohme hand-operated capsule filling machine. Forty-eight microdose captopril capsules were randomly selected from the compounded batch and were individually assayed for captopril amount. The mean +/- standard deviation (SD) amount of captopril in each capsule was 1.27 g +/- 0.31 mg with a range of 0.84 g to 1.96 mg. The coefficient of variation was 24.5%. Ten captopril capsules were randomly selected from the compounded batch and were individually weighed. The capsules had a mean weight +/- SD of 198.3 g +/- 21.2 mg and a coefficient of variation of 7.3%. Even though the extemporaneously prepared microdose captopril capsules were within acceptable limits for weight variation described in the USP, the actual dose administered to the patients could vary by as much as 24.5%.
Six healthy volunteers received a single i.v. dose of 'low dose' lorazepam (0.0225 mg/kg), 'high dose' lorazepam (0.045 mg/kg) and placebo by 1-min infusion in a double-blind three-way crossover study. Plasma concentrations were measured 24 hr after dosage, and the EEG power spectrum was simultaneously computed by fast-Fourier transform to determine the percentage of total EEG amplitude occurring in the 13-30-Hz range. Low and high dose lorazepam did not differ significantly in distribution volume (1.89 versus 1.81 l/kg) or elimination half-life (11.5 versus 12.2 hr); clearance was slightly although significantly reduced at the higher dose (2.08 versus 1.88 ml/min/kg, P less than .005). EEG effects were of relatively slow onset, reaching their maximum change over baseline 30 min after infusion. The duration of action was prolonged, with the fraction of EEG activity in the 13-30-Hz range still significantly above baseline 8 hr after the 0.045 mg/kg dose. Five of these subjects received 0.15 mg/kg of i.v. diazepam in a companion study of identical design. EEG effects of diazepam were shorter than those of lorazepam, probably because of the more rapid and extensive decline in plasma diazepam concentrations in the postinfusion distribution phase. In addition, the onset of diazepam's effect was immediate. In male CD-1 mice that received i.v. diazepam (8.3 mg/kg) or lorazepam (3.3 mg/kg), the brain:plasma concentration ratio was maximal 2.5 min after dosage for diazepam, but equilibration was delayed at least 30 min after dosage for lorazepam. Thus the slow onset of action of lorazepam is probably attributable to slow entry into brain.
We compared fenoprofen calcium, 200 mg; fenoprofen calcium, 400 mg; aspirin, 650 mg; and a placebo in 85 women for the relief of primary dysmenorrhea in a double-blind, clinical trial. The usefulness of these drugs was judged from data obtained over four consecutive menstrual periods on: restriction of daily activity, pain intensity scores, need for rescue analgesics, withdrawal due to lack of efficacy, and adverse events. Both fenoprofen, 200 mg, and fenoprofen, 400 mg, offered significant (P less than .01) pain relief when compared to placebo and aspirin. Analyses of data on 1, 2 and 3 indicated that aspirin was not significantly different from placebo. The aspirin-treated group reported the greatest number of adverse reactions, but the differences between the four groups were not statistically significant. Our study lends support to the concept of a "plateau analgesic effect" of nonsteroidal antiinflammatory drugs (NSAIDs): fenoprofen, 200 mg, appears to be as effective as fenoprofen, 400 mg. When this type of drug fails to provide relief for a woman suffering from primary dysmenorrhea, switching to another NSAID may be more appropriate than increasing the dosage and the probability of dosage-related side effects.
Changes in body composition with age may alter tissue drug uptake and result in altered pharmacokinetics and pharmacodynamics. Four young, 4 middle-aged and 4 old Fischer-344 male rats were given a single intraperitoneal dose of alprazolam (2.5 mg/kg), diazepam (5 mg/kg) and triazolam (1.25 mg/kg) and sacrificed after 1 h. Diazepam, desmethyldiazepam, oxazepam, temazepam, alprazolam, and triazolam concentrations were determined in brain, kidney, liver, spleen, lung, heart, adrenal, muscle, fat and plasma by gas chromatography. Free fraction in plasma was determined by equilibrium dialysis. Drug uptake varied widely among tissues. Highest uptake ratios relative to free (unbound) drug in plasma were in adrenal (56-135), liver (35-116) and kidney (19-50). Free fraction in plasma varied from 0.13 for desmethyldiazepam to 0.30 for triazolam, and was unrelated to age. Tissue drug uptake relative to muscle, total plasma or free plasma concentration showed no significant variation with age or body habitus. In vivo fat uptake was highly correlated (R = 0.95) with in vitro octanol/buffer partition ratio. Muscle and fat were the largest quantitative drug storage sites, with total uptake explained by lipophilicity. Thus, age-related changes in body habitus and clearance do not alter tissue binding of benzodiazepines at distribution equilibrium.
