Studies on drug metabolism. IV. Effects of high dose administration of pentobarbital and phenylbutazone on the plasma biologic half lives in various species

Improving cardiac function with new-generation plasma volume expanders

BACKGROUND

Plasma expander (PE) based on polyethylene glycol (PEG) conjugated to albumin has shown positive results maintaining blood volume during hemodilution and restoring blood volume during resuscitation from hemorrhagic shock. Polyethylene glycol conjugation to human serum albumin (HSA), PEG-HSA, increases size, weight, and colloidal osmotic pressure, with minor effects on solution viscosity.

METHODS

This study was designed to test the hypothesis that PEG-HSA (2 g/dL) produced by direct PEGylation chemistry improves cardiac function during 2 experimental models, (i) moderate hemodilution and (ii) resuscitation from hemorrhagic shock, compared with a conventional colloidal PE (Dextran 70 kd [Dx70], 6 g/dL). Cardiac function was studied using a miniaturized pressure volume conductance catheter implanted in the left ventricle and evaluated in terms of cardiac indices derived from the pressure volume measurements.

RESULTS

Polyethylene glycol-HSA increased cardiac output, stroke volume, and stroke work and decreased systemic vascular resistance compared with Dx70 in both experimental models. The improvements induced by PEG-HSA in cardiac function were sustained over the observation time. Polyethylene glycol-HSA cardiac mechanoenergetics changes are the result of increased energy transferred per stroke and decreased resistance of the vasculature connecting the heart. In summary, PEG-HSA decreased left ventricle ejection impedance.

CONCLUSION

Ejection of blood diluted with PEG-HSA presented a reduced load to the heart, increased contractile function, and lowered the energy consumed per unit volume compared with Dx70. Our results emphasize the importance of heart function as a parameter to be included in the evaluation changes induced by new PEs.

Cardiac systolic function recovery after hemorrhage determines survivability during shock

BACKGROUND

Small animal model has not been available to study cardiac pathophysiology during hemorrhagic shock. The main purpose of this study, therefore, was to establish earlier differences in left ventricle functional disturbances during hypovolemia comparable in survival and nonsurvival animals. Ventricular pressure-volume relationships have become well established as the most rigorous and comprehensive venue to assess intact heart function.

METHODS

Studies were performed in anesthetized hamsters subjected to a 40% of blood volume hemorrhage to induce the hypovolemic shock. A miniaturized conductance catheter was used to measure left ventricular pressure and volume. Derived from the pressure-volume measurements, cardiac performance was evaluated using systolic and diastolic function indices.

RESULTS

Thirteen animals were included; all animals survived the hemorrhage. Survival rate after 30 minutes of hypovolemic shock was 61.5%. End-systolic pressure was improved at the late stage of shock in the survival group, whereas no change of this index was found in the nonsurvival group. No significant differences in end-diastolic pressure and relaxation time constant were found between the nonsurvival and the survival groups. Fifteen minutes after the hemorrhage, the stroke work per stroke volume ratio significantly improved in the survival compared with nonsurvival, which also restored blood pressure.

CONCLUSION

The unique advantage of the pressure-volume methodology over all other available approaches to measure cardiac function is that it enables more specific measurement of the left ventricle performance independently from loading conditions and heart rate. Our findings demonstrated that failure to recover cardiac systolic function after hemorrhage, is a major determinant of mortality during hypovolemic shock.

Dose-dependent pharmacokinetics and hepatobiliary transport of bromophenol blue in the beagle

The pharmacokinetic profile of bromophenol blue (I) in the plasma, urine, and bile of beagle dogs was determined after intravenous administration of 5-, 20-, and 30-mg/kg doses. In addition, two competitors, probenecid and phenylbutazone, were interacted with I in vivo and with I and rat liver cytoplasmic protein fractions Y and Z in vitro as a means of elucidating the mechanism of intrahepatic transport of I. Compound I was determined spectrophotometrically at 587 nm. In plasma, I displayed apparent first-order dose-dependent kinetics. The percentage of I bound to plasma proteins was approximately 92.5% over the dose range studied. Consecutive injections of equal doses of I produced statistically different terminal half-lives (p less than 0.05), suggesting the possibility of a saturable uptake process. In the presence of each competitor, the disposition of I was altered significantly (p less than 0.05): phenylbutazone displaced I from plasma protein, while probenecid decreased the binding of I to liver proteins in the Z-fraction. The Z-fraction bound a larger amount of I than the Y-fraction, suggesting a larger binding capacity. Under no circumstances was the binding of I to the Y-fraction altered. Cumulative biliary excretion data showed that the elimination of I in bile accounted for 92-99% of the dose delivered. The biliary excretion sigma- plots displayed no dose dependency, suggesting that the dose-dependent plasma half-life is due to a dose-dependent liver uptake (as opposed to elimination) process.

Alteration of pharmacokinetic parameters for pentobarbital by ischemic stroke and reversion to normal by dexamethasone treatment

The values of the pharmacokinetic parameters for pentobarbital were determined in 18 cats, 12 of which were subjected to acute ischemic stroke by ligation of the left middle cerebral artery (LMCA). All 18 ats received 50 mg/kg sodium pentobarbital during operation. The following three experimental groups were formed: control (sham-operated); ischemic stroke plus administration of 4 mg/kg dexamethasone; and ischemic stroke without dexamethasone administration. Ischemic stroke significantly prolonged the plasma half-life of pentobarbital, but concurrent administration of dexamethasone prevented this effect. Ischemic stroke significantly reduced the plasma clearance of pentobarbital, but dexamethasone prevented this reduction. Ischemic stroke significantly increased the area under the plasma pentobarbital concentration-time curve, but dexamethasone prevented this increase. Ischemic stroke significantly reduced the volume of distribution, but dexamethasone did not prevent this reduction. The alterations of the value of these pharmacokinetic parameters for pentobarbital by ischemic stroke and reversion to normal by dexamethasone treatment are discussed in the light of certain known circulatory changes which occur secondary to ischemic stroke and dexamethasone treatment.

The influence of allopurinol and size of dose on the metabolism of phenylbutazone in patients with gout

Administration of allopurinol 300 mg/day produced minimal changes in the disappearance of phenylbutazone in each of five subjects following single doses (6 mg/kg) in clinical range and caused some prolongation (21%, 52%) of drug half-lives in two of six subjects after single small doses (0.5 mg/kg); mean half-life was not significantly altered by allopurinol at either dose level (means of 52 versus 48 at 0.5 mg/kg and 68 versus 70 h at 6 mg/kg). Size of dose altered half-life when phenylbutazone was used alone; three subjects showed considerably longer half-lives at the higher dose (86 versus 47, 91 versus 41, 65 versus 38 h). Allopurinol caused a greater than 50% prolongation of half-lives in two of five subjects who received single 0.5 g doses of probenecid. These preliminary data do not indicate a need to change the dose of phenylbutazone when subjects are receiving allopurinol.