Excretion of phenobarbitone in urine after intake of large doses

Prophylactic phenobarbitone in young children with severe falciparum malaria: pharmacokinetics and clinical effects

1. A method is described for the measurement of phenobarbitone (PB) by reversed phase high performance liquid chromatography (h.p.l.c.) from small samples of whole blood dried onto filter paper strips. 2. The disposition of PB given prophylactically to young children with severe malaria on parenteral quinine is contrasted with that in aparasitaemic Kenyan children on no antimalarial drugs. There were no differences in the disposition of PB between the two groups. 3. Peak blood PB concentrations were equal to or greater than 15 mg l-1 in 27% of the patients on quinine and 23% of those not on quinine; a concentration of 10 mg l-1 was achieved or exceeded by 100% and 92% of each group, respectively, and was maintained for 39 +/- 24 h (mean +/- s.d.), and 33 +/- 21 h, respectively. 4. In an open, dose-finding study, the progress of young children with cerebral malaria given prophylactic PB (10 mg kg-1), was contrasted with that of controls given no seizure prophylaxis. 5. The drug had no apparent effect on depth or duration of coma, but neither was the incidence of seizures reduced. 6. A controlled trial of prophylactic PB in young children with cerebral malaria is needed, but a larger dose than 10 mg kg-1 should be studied.

The clinical utility and evaluation of drug screening techniques

A better understanding of the methods of drug detection, their sensitivities, and limitations increases the efficiency of medical decision making for better patient care. Many of the sociomedical and medicolegal questions are yet to be answered through court challenge. The physician must remember his or her role as a clinician, and practice the art in a fashion that best serves the interest of his or her teenage patient.

Studies with stable isotopes II: Phenobarbital pharmacokinetics during monotherapy

Six healthy adults receiving no other medications were given tracer doses of 90 mg of stable isotope-labeled phenobarbital (PB) intravenously before, and four weeks after, and 12 weeks after beginning therapy. Serum samples were collected for 96 hours after each injection, and the concentration of stable isotope-labeled PB in each sample was determined by gas chromatographic mass spectrometry. The volume of distribution, elimination half-life, and total clearance of PB did not differ significantly on any of the three occasions measured. Phenobarbital clearance did not correlate significantly with total PB serum concentration. Clearances determined from single-dose studies before beginning PB therapy accurately predicted steady-state PB serum concentrations. Therefore, it is not necessary to adjust PB dosage for time-dependent or dose-dependent changes in clearance during monotherapy. In addition, clearance or serum concentration determined at one dosing rate directly predicts serum concentration at another dosing rate.

The fate of phenobarbitone in children in hypothermia and at normal body temperature

Four critically injured children receiving large doses of phenobarbitone were studied during hypothermia (30 degrees - 31 degrees C) and at normal body temperature. The volume of distribution of phenobarbitone varied from 0.79 to 1.01 litres per kg and the serum t 1/2 ranged from 36.8 +/- 9.4 to 86.2 +/- 10.5 hrs. The percentage of dose recovered in urine in 16 days ranged from 40.5 to 65.5 per cent: 2.7 to 12.4 per cent as hydroxyphenobarbitone, 1.7 to 19.7 per cent as conjugated hydroxyphenobarbitone, 6.0 to 22.4 per cent as phenobarbitone-N-glucoside and 17.8 to 23.1 per cent as unchanged drug. After the body temperature was allowed to return to normal the rate of excretion of metabolites increased substantially and the rate of excretion of the unchanged drug decreased markedly. It is concluded that reduction in body temperature influences the volume of distribution, rate of metabolism and excretion of phenobarbitone.

Phenobarbital metabolism in adults and in newborn infants

Two adult volunteers and four newborn infants were given a single dose of phenobarbital. The output in the urine o f unchanged phenobartital and of the two main metabolites p-hydroxy phenobarbital and conjugated p-hydroxy phenobarbital was followed during 8 days in the newborns and during 2 or 4 weeks in the adults. The plasma levels were also determined and some pharmacokinetic constants calculated. It was found that the newborn patients excreted unchanged phenobartibal and p-hydroxy phenobarbital in the same proportions relative to dose as did the adult volunteers, 2.e. 16--17% unchanged drug and 9--10% of the metabolite during the first 8 days after administration. On the other hand, there was a clear-cut age difference in output of conjugated metabolite where the newborns excreted only 5% of the given dose during the 8-day observation period. The corresponding value for the adults was 15%. It is concluded that a poor conjugating capacity in the newborn may not have any serious consequences with a drug like phenobarbital where major alternative routes of excretion exist (unchanged drug and unconjugated metabolite). The clinical significance of immature drug metabolites and of unchanged drug is taken into consideration.

The use of gas chromatographic-mass spectrometric-computer systems in pharmacokinetic studies

Pharmacokinetic studies involving plasma, urine, breast milk, saliva and liver homogenates have been carried out by selective ion detection with a gas chromatographic-mass spectrometric-computer system operated in the chemical ionization mode. Stable isotope labeled drugs were used as internal standards for quantification. The half-lives, the concentration at zero time, the slope (regression coefficient), the maximum velocity of the reaction and the apparent Michaelis constant of the reaction were determined by regression analysis, and also by graphic means.