Enprofylline disposition in the presence and absence of amoxycillin or erythromycin

Species-specific uncertainty factors for compounds eliminated principally by renal excretion in humans

An uncertainty factor of 100 is used to derive health-based guidance values for human intakes of chemicals based on data from studies in animals. The 100-fold factor comprises 10-fold factors for species differences and for interindividual differences in response. Each 10-fold factor can be subdivided into toxicokinetic and toxicodynamic aspects with a 4.0-fold factor to allow for kinetic differences between test species and humans. The current work determined the extent of interspecies differences in the internal dose (toxicokinetics) of compounds which are eliminated primarily by renal excretion in humans. An analysis of the published data showed that renal excretion was also the main route of elimination in the test species for most of the identified probe substrates. Interspecies differences were apparent for both the mechanism of renal excretion (glomerular filtration, tubular secretion and/or reabsorption) and the extent of plasma protein binding, both of which may affect renal clearance and therefore the magnitude of species differences in the internal dose. For compounds which are eliminated unchanged by both humans and the test species, the average differences in the internal doses between humans and animals were 1.6 for dogs, 3.3 for rabbits, 5.2 for rats and 13 for mice. This suggests that for renal excretion, the differences between humans and the rat and especially the mouse may exceed the 4.0-fold default factor for toxicokinetics.

Influence of prolonged adenosine receptor blockade on fetal sleep and breathing patterns

Chronically prepared fetal sheep were subjected to 48 h infusions of theophylline, an adenosine antagonist, enprofylline, a xanthine without adenosine antagonism, or saline. Theophylline increased mean (+/- SD) incidence of REM sleep from 49.3 +/- 8.3% to 57.3 +/- 6.7% (p < 0.02) and wakefulness from 1.3 +/- 1.4% to 8.1 +/- 7.1% (p < 0.01). On the first day of theophylline infusion incidence of fetal breathing (FB) increased from 37.9 +/- 8.1% to 53.7 +/- 11.6% of total time (p < 0.002) and from 76.4 +/- 10.2% to 87.6 +/- 10.3% of REM sleep (p < 0.02). Diaphragmatic EMG/min increased from 6.9 +/- 4.0 to 17.3 +/- 13 arbitrary units (p < 0.02). By the second day of infusion, FB had returned to baseline value. Enprofylline and saline had no effect. 125 micrograms phenyl isopropyl adenosine (PIA) i.v. caused fetal apnea that was reduced from 143 +/- 45.5 min on the control day to 39.8 +/- 34.7 min (p < 0.001) during theophylline infusion. Enprofylline and saline had no effect, suggesting that the observed theophylline effect was due to its adenosine antagonism rather than to non-specific xanthine action. We conclude that endogenous adenosine suppresses FB, but since theophylline did not alter the basic relationship between FB and REM sleep it is not primarily responsible for apnea during NREM sleep.

Slow release theophylline disposition and effect in elderly patients with chronic obstructive lung disease: influence of dose formulation and institutionalization

We studied the steady-state disposition of slow release theophylline tablets and granules in 12 institutionalized (I) and 12 community-dwelling (C) elderly patients with fixed chronic obstructive lung disease. Design was open label with random order crossover; each formulation was given 5 min before food every 12 h for 7 days. Age (median 70 y, range 55-88), sex, smoking status, and baseline lung function off drug were similar. Though plasma concentration (Cp) was higher with the tablets as was the area under the Cp vs time curve: 134 (74-252) vs 121 (75-197) mg h l-1; p = 0.028. The standard deviation of Cp over one dose interval was lower with the granules. FEV 1.0 was slightly improved over baseline. Dose required to reach target Cp was higher in the institutionalized group (12.6 vs 8.6 mg kg-1 day-1; p = 0.003) as was apparent clearance; I:94 (43-148) ml hr-1 kg-1 vs C:68 (34-163); p = 0.003. Although bioavailability was slightly reduced for the granules, fluctuations of Cp was less, and we failed to find a food effect that was clinically important in geriatric subjects.

Pharmacokinetics of enprofylline administered intravenously and as a sustained-release tablet at steady state in children with asthma

The pharmacokinetics of enprofylline (3-propylxanthine) were studied in 10 children with asthma (mean age 7.9 years), after enprofylline 1 mg/kg given intravenously and after enprofylline 7.5 +/- 1.3 mg/kg given as a sustained-release tablet after 8 days of oral dosing twice daily. The mean +/- SD enprofylline serum elimination half-life was 1.06 +/- 0.20 hours, considerably shorter than the half-life reported in adults. The mean steady-state volume of distribution was 0.55 +/- 0.05 L/kg. The mean clearance rate was 0.44 +/- 0.06 L/hr/kg. The mean enprofylline serum concentration at steady state was 1.7 +/- 0.5 mg/L. The mean peak to trough ratio was 3.02 +/- 1.31. On the first and ninth study days, 87% +/- 8% and 90% +/- 16%, respectively, of the dose of enprofylline was recovered as unchanged drug in the urine. Enprofylline has a short half-life in children, but the sustained-release formulation provides stable serum concentrations and satisfactory relief of asthma throughout the 12-hour dosing interval.