Morphine pharmacokinetics after pulmonary administration from a novel aerosol delivery system

BACKGROUND

Successful pharmacotherapy of pain often depends on the mode of drug delivery. A novel, unit dose, aqueous aerosol delivery system (AERx Pulmonary Drug Delivery System) was used to examine the feasibility of the pulmonary route for the noninvasive systemic administration of morphine.

METHODS

The study had two parts: (1) a dose-ranging study in four subjects with three consecutive aerosolized doses of 2.2, 4.4, and 8.8 mg (nominal) morphine sulfate pentahydrate at 40-minute intervals, and (2) a crossover study, on separate days, in six subjects with 4.4 mg (nominal) aerosolized morphine sulfate administered over 2.1 minutes on three occasions and intravenous infusions of 2 and 4 mg over 3 minutes. Subjects were healthy volunteers from 19 to 34 years old. Arterial blood was sampled for a total of 6 hours and plasma morphine concentrations were measured by gas chromatography-mass spectrometry.

RESULTS

In part 1, plasma morphine concentrations were proportional to dose. In part 2, the mean +/- SD peak plasma concentration (Cmax) occurred at 2.7 +/- 0.8 minutes after the aerosol dose, with mean values for Cmax of 109 +/- 85, 165 +/- 22, and 273 +/- 114 ng/ml for the aerosol and 2 and 4 mg intravenous doses, respectively. The bioavailability [AUC(0-360 min)] of aerosol-delivered morphine was approximately 100% relative to intravenous infusion, with similar intersubject variability in AUC for both routes (coefficient of variation < 30%).

CONCLUSION

The time courses of plasma morphine concentrations after pulmonary delivery by the AERx system and by intravenous infusions were similar. This shows the utility of the pulmonary route in providing a noninvasive method for the rapid and reproducible systemic administration of morphine if an appropriate aerosol drug delivery system is used.

The influence of age upon opioid analgesic use in the patient-controlled analgesia (PCA) environment

It is often asserted that older patients are more sensitive to opioid analgesics than younger patients but experimental evidence for this assertion remains sparse. Two studies were conducted investigating the relationship between age and opioid analgesic use in the patient-controlled analgesia environment. In study I, the relationship was analysed subsequent to our publication of a study investigating patients' responses to opioid use with patient-controlled analgesia. Fifty-five postoperative patients, stratified into 'older' and 'younger' patients by median age, received morphine or pethidine or fentanyl patient-controlled analgesia. A strong inverse relationship was found between age and fentanyl and morphine use but not between age and pethidine use. Study II was a retrospective study of the medical records of 199 patient-controlled analgesia patients who had received morphine or pethidine patient-controlled analgesia; there were insufficient patients who had used fentanyl for a reasonable sample. There was a difference in morphine use with the younger patients using significantly more morphine than the older patients (< 60 years). Findings were less clear for patients receiving pethidine but there was an inverse correlation between age and pethidine use as well. Overall, the findings of these two studies supported the common clinical belief that older patients require less opioids than younger patients.

Lack of effect of high doses of inhaled morphine on exercise endurance in chronic obstructive pulmonary disease

Systemic opiates may relieve dyspnoea and improve exercise tolerance in patients with chronic obstructive pulmonary disease (COPD). Small doses of inhaled opiates may have similar effects; however, recent studies have shown no benefit. We studied higher doses of inhaled morphine and measured systemic absorption to determine whether any beneficial actions are local or systemic. Twenty and 40 mg doses and 0.9% saline were nebulized in a randomized, double-blind study of 16 patients with stable COPD. Patients performed 6 min walk tests immediately after the nebulized test solution (Walk 1) and again 60 min later (Walk 2). Arterial oxygen saturation (Sa,O2), modified Borg dyspnoea score and cardiac frequency were recorded during each walk. There was no difference between placebo and either dose of nebulized morphine on these measurements. The higher dose of nebulized morphine achieved a higher plasma concentration. The highest plasma concentration was measured immediately after nebulization, and this decreased steadily in the hour thereafter (p<0.002). There was no correlation between the change in walk distance and the change in plasma morphine concentration after either dose of nebulized morphine. We conclude that higher doses of nebulized morphine do not improve exercise endurance or relieve dyspnoea in patients with chronic obstructive pulmonary disease, and that morphine is rapidly absorbed systemically after inhalation.

Influence of renal failure on the disposition of morphine, morphine-3-glucuronide and morphine-6-glucuronide in sheep during intravenous infusion with morphine

The influence of experimentally induced renal failure on the disposition of morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) was examined in seven sheep infused intravenously with morphine for 6 hr. Between 5 and 6 hr, blood was collected from the aorta, pulmonary artery, hepatic, hepatic portal and renal veins and posterior vena cava. Additional samples from the aorta and urine were collected up to 144 hr. Morphine, M3G and M6G were determined in plasma and urine by high-performance liquid chromatography. Constant concentrations of morphine, but not of M3G and M6G, were achieved in plasma between 5 and 6 hr. Significant (P < .001) extraction of morphine by the liver (0.72 +/- 0.05) and kidney (0.42 +/- 0.15) occurred. Compared with sheep with normal kidneys (Milne et al., 1995), renal failure did not alter (P = .11) the mean total clearance of morphine (1.5 +/- 0.3 liters/min); clearance by the kidney was less (P < .001). However, a paired comparison using sheep common to this study and from the study when their kidneys were normal revealed a significant reduction in mean total clearance of 25%. The renal extraction of M3G and M6G and urinary recovery of the dose as summed morphine, M3G and M6G were reduced by renal failure. The kidney metabolized morphine to M3G. The data suggest that nonrenal elimination of M3G becomes more important during renal failure.

Nausea and vomiting in the postoperative patient-controlled analgesia environment

Despite common clinical opinion that patient-controlled analgesia should be renamed 'patient-controlled nausea', there is little evidence in support of the notion that postoperative nausea and vomiting are exacerbated by the method. Indeed, data indicate that opioid-sparing techniques are not associated with less postoperative nausea and vomiting. Although some evidence suggests that certain opioids are less emetogenic than others, this too does not stand scrutiny when compared across patients, although research is still required to find whether individual patients are better treated with a particular opioid. Similarly, the emerging practice of combining anti-emetics with patient-controlled analgesia needs wider study before it can be supported.

Highly sensitive gas chromatographic-mass spectrometric method for morphine determination in plasma that is suitable for pharmacokinetic studies

A sensitive method was devised to determine morphine plasma concentrations by gas chromatography-mass spectrometry (GC-MS) using selected ion monitoring (SIM) with nalorphine as the internal standard. This method was rugged, reliable, selective and sensitive and was used to determine the morphine content of over 2000 samples. Sample preparation involved extraction of basified sample using n-butyl chloride-chloroform (5:1) and evaporation of the extract to dryness. The residue was derivatised with pentafluoropropionic anhydride, evaporated to dryness, reconstituted in 40 microl toluene and injected onto the GC-MS. For a sample size of 1 ml, the limit of quantitation was 0.75 ng/ml (S/N ratio 10:1) and the estimated limit of detection was calculated to be 0.2 ng/ml (S/N ratio 3:1), expressed as morphine base. Precision (n=5) was 4.9% at 0.75 ng/ml, 6.8% at 1.5 ng/ml, 3.0% at 37.5 ng/ml and 2.3% at 150 ng/ml. Standard curves for the range of 0-750 ng/ml morphine in plasma were linear with all r2 values greater than 0.99. No interfering peaks were seen for either morphine or internal standard in the blank samples. The method is suitable for pharmacokinetic studies after subclinical doses of morphine where it has been used to study morphine plasma concentrations for 6 h after a dose of only 2 mg.

Pharmacokinetics of thiopentone enantiomers following intravenous injection or prolonged infusion of rac-thiopentone

AIMS

Thiopentone is administered as a racemate (rac-thiopentone) for induction of anaesthesia as well as for neurological and neurosurgical emergencies. The pharmacokinetics and pharmacodynamics of rac-thiopentone have been extensively studied but the component R-(+)- and S-(-)- enantiomers, until very recently, have been largely ignored.

METHODS

The present study analyses the pharmacokinetics of R-(+)- and S-(-)-thiopentone in 12 patients given rac-thiopentone intravenously for induction of anaesthesia and five patients given a prolonged infusion of rac-thiopentone used for treatment of intracranial hypertension.

RESULTS

The mean total body clearance (CLT) and apparent volume of distribution at steady-state (Vss) showed trends towards higher values for R-(+)- than for S-(-)-thiopentone in both patient groups; CLT and Vss of unbound fractions of R-(+)- and S-(-)-thiopentone, however, did not show these trends. The time courses of R-(+)- and S-(-)- thiopentone serum concentrations were so similar that EEG effect could not be attributed to one or other enantiomer. Serum protein binding for S-(-)-thiopentone was greater than for R-(+)-thiopentone (P = 0.02) and 24 h urinary excretion of R-(+)-thiopentone was greater than for S-(-)-thiopentone (P = 0.03). In one patient, concomitant measurement of CSF and serum thiopentone concentrations found that serum: CSF equilibration of unbound fractions of both enantiomers was essentially complete.

CONCLUSIONS

The study was unable to determine any pharmacokinetic difference of clinical significance between the R-(+)- and S-(-)-thiopentone enantiomers and concludes that minor differences in CLT and Vss could be explained by enantioselective difference found in serum protein binding.

Morphine blood concentrations in elderly postoperative patients following administration via an indwelling subcutaneous cannula

The pharmacokinetics of morphine in venous blood after a 5 mg bolus dose via an indwelling subcutaneous cannula were characterised in 22 elderly patients undergoing elective major surgery. In a subgroup of seven patients, the kinetics were also characterised after a second 5 mg dose of morphine administered 180 min after the first dose. Blood morphine concentrations following the single dose were highly variable--the coefficients of variation of Cmax, Tmax and the AUC up to 180 min (AUC180) were 54, 37 and 39%, respectively, with mean values of 86.6 ng.ml-1, 15.9 min and 3954 ng.ml-1, respectively. These mean values for the second dose were not statistically different to those of the first dose but were more variable. It was concluded that the injection of morphine via an indwelling subcutaneous cannula results in blood concentrations that are comparable to, and as variable as, those arising from intramuscular injection.

Determination of subnanogram concentrations of fentanyl in plasma by gas chromatography-mass spectrometry: comparison with standard radioimmunoassay

A method was devised to determine fentanyl plasma concentrations by GC-MS using selected-ion monitoring (SIM) with sufentanil as internal standard. This was compared with a commonly used commercial radioimmunoassay (RIA). Sample preparation for GC-MS involved basification of plasma then extraction using n-butyl chloride followed by concentration to dryness and reconstitution in toluene for chromatography. Using 1-ml plasma samples, the estimated limit of detection of fentanyl was 20 pg/ml. Blood samples for pharmacokinetic studies were split and assayed by GC-MS and RIA which had a limit of detection of 200 pg/ml. Pearson's r (r - 0.80, p < 0.0001) indicated the methods were highly correlated at all plasma concentrations. Owing to the greater sensitivity of the method, GC-MS is recommended over RIA for subnanogram determination of fentanyl in plasma.

Urineless estimation of glomerular filtration rate and renal plasma flow in the rat

A simple method to perform serial renal clearance studies without urine collection in rats is described. This was applied to nonradiolabelled para-aminohippurate sodium (PAH) and iothalamate sodium (IOT) which were used respectively to estimate renal plasma flow (RPF) and glomerular filtration rate (GFR). Under isoflurane-anesthesia the jugular vein and carotid artery of Fischer 344 rats were cannulated, and a loading dose followed by a continuous infusion of PAH and IOT was administered. Steady state was reached after 30 min, and four arterial blood samples were collected over the next 30 min for analysis by HPLC (day 0). The clearances of PAH and IOT were calculated according to Fick's principle as estimates of RPF and GFR from the ratio of the infusion rates to the median solute concentrations. Three days later the femoral artery and vein were cannulated, and the same study was repeated (day 3). There was no significant difference in renal clearances of PAH and IOT between days 0 and 3. The described method gives values that compare well with others in the literature based on other methods and presents an accurate and simple way of detecting changes in renal function before and after a potentially nephrotoxic treatment regimen.

The systemic and cerebral kinetics of thiopental in sheep: enantiomeric analysis

Thiopental is used as a racemate (rac-thiopental). Enantiomeric pharmacokinetic differences could therefore influence the onset and duration of anesthesia of rac-thiopental. We studied the systemic and cerebral kinetics of R(+)- and S(-)-thiopental in five adult ewes after a 2-min intravenous infusion of 500 mg rac-thiopental sodium. Systemic kinetic values were determined from the time course of concentrations in arterial plasma; cerebral kinetic values were deduced from the time course of the concentration differences between arterial and superior sagittal sinus blood plasma. Enantiomeric differences were found in both sites, with the (R:S) ratio of thiopental enantiomer blood concentrations initially being > or = 1 then decreasing to < 1 after approximately 60 min. This is consistent with the finding of the mean total body clearance of R(+)-thiopental being 17% (SD 12%) greater than that of S(-)-thiopental (P = .04). Sagittal sinus plasma concentrations of both enantiomers followed closely behind those in arterial plasma and this is consistent with facile bidirectional exchange of thiopental between plasma and brain. No significant differences were found between enantiomers in the rates of brain influx or efflux. Onset and regression of anesthesia occurred while the enantiomer blood concentrations were similar. Hence published pharmacokinetic-pharmacodynamic models of the onset of thiopental effects probably are not significantly compromised by neglecting the enantiomeric duality of thiopental, but models based on its elimination kinetics could be compromised if enantiomeric differences are neglected.

Comparison of the respiratory and systemic kinetics of nitrous oxide in the sheep

BACKGROUND

To determine whether discrepancies in views on the kinetics of nitrous oxide (N2O) may have a methodological basis, we compared its kinetics, simultaneously, in the respiratory system and systemic circulation.

METHODS

Six merino ewes (40-50 Kg) were previously prepared with catheters in the pulmonary artery and aorta. The animals were anaesthetised with thiopentone then ventilated on a mixture of 70% N2O, 1% halothane in oxygen for 4 h. Simultaneous serial arterial and pulmonary arterial blood samples were assayed for N2O by gas chromatography and respiratory gases were monitored continuously by mass spectrometry.

RESULTS

Marked differences were observed between the respiratory and systemic kinetics of N2O uptake. While the expired/inspired N2O concentration ratio rose within 30 min to a value close to unity, the pulmonary arterial/arterial blood N2O concentration ratio did not reach unity during the 4 h of each study, but approached a constant rate of uptake shown by the mean ratio of 0.94 (SD 0.01) from about 2 h onward.

CONCLUSIONS

Discrepancies in fluid flow between respiratory gas and the cardiovascular systems, a concentration effect of N2O in the lungs, the relative solubility of N2O in blood and tissues, and ventilation/perfusion inequalities all may contribute to the observed differences. The ongoing uptake is consistent with persistent extrapulmonary losses. There remains a need for experimental data on the pharmacokinetics of N2O. Unequivocal studies on the disposition of N2O can be undertaken only by using direct measurement of fluxes of N2O across relevant organs or tissues.

High-performance liquid chromatographic determination of thiopentone enantiomers in sheep plasma

An HPLC method was developed to determine the plasma concentrations of R(+)- and S(-)-thiopentone for pharmacokinetic studies in sheep. The method required separation of the thiopentone enantiomers from the corresponding pentobarbitone enantiomers which are usually present as metabolites of thiopentone. Phenylbutazone was used as an internal standard. After acidification, the plasma sample were extracted with a mixture of ether and hexane (2:8). The solvent was evaporated to dryness and the residues were reconstituted with sodium hydroxide solution (pH 10). The samples were chromatographed on a 100 mm x 4 mm I.D. Chiral AGP-CSP column. The mobile phase was 4.5% 2-propanol in 0.1 M phosphate buffer (pH 6.2) with a flow-rate of 0.9 ml/min. This gave k' values of 1.92, 2.92, 5.71, 9.30 and 11.98 for R(+)-pentobarbitone, S(-)-pentobarbitone, R(+)-thiopentone, S(-)-thiopentone, and phenylbutazone, respectively. At detection wavelength of 287 nm, the limit of quantitation was 5 ng/ml for R(+)-thiopentone and 6 ng/ml for S(-)-thiopentone. The inter-day coefficients of variation at concentrations of 0.02, 0.1 and 8 micrograms/ml were, respectively, 4.8, 4.4 and 3.5% for R(+)-thiopentone and, respectively, 5.0, 4.3 and 3.9% for S(-)-thiopentone (n = 6 each enantiomer). At the same concentrations, the intra-day coefficients of variation from six sets of replicates (measured over six days) were, respectively, 8.0, 8.0 and 8.8% for R(+)-thiopentone and 8.8, 7.4 and 9.6% for S(-)-thiopentone. Linearity over the standard range, 0.01-40 micrograms/ml, was shown by correlation coefficients > 0.998. This method has proven suitable for pharmacokinetic studies of thiopentone enantiomers after administration of rac-thiopentone in human plasma also and would be suitable for pharmacokinetic studies of the pentobarbitone enantiomers.

Opioids: a pharmacologist's delight!

1. Opioids, in one form or another, have been used for their pain-relieving properties from prehistoric times: they are still the first line medication for the treatment of severe nociceptive pain and are likely to remain so for the foreseeable future. 2. The therapeutic index of opioids used for pain management is low: opioid side effects are essentially extensions of therapeutic effects and no available agent has a marked advantage over the others. When used for opioid 'anaesthesia', differences in therapeutic index are more obvious due to differences in non-opioid effects. 3. Opioid receptors in brain and spinal cord periphery are the main 'therapeutic targets' and clinical dosage strategies have been derived using a variety of systemic (indirect or blood-borne) methods as well as intraspinal and intracerebroventricular (direct) methods: no method, however, is without potential side effects. Peripheral opioid effects are now being exploited with intra-articular injection. 4. Opioid pharmacokinetics and pharmacodynamics are characterized by high inter-subject variability: accordingly, patient-controlled dosage strategies are found to be more successful for pain control than deterministic recipes.

1994 John J. Bonica Lecture. The clinical effects of morphine pharmacology

BACKGROUND AND OBJECTIVES

The literature is replete with studies of morphine. It is not surprising that some viewpoints expressed tend to be often repeated without critical analysis. The goal of this lecture is to probe some commonly held viewpoints about the pharmacology of morphine. This involves retracing some of its history, re-examining its physicochemical nature, re-evaluating its physiologic disposition and the pharmacologic activity of its metabolites.

METHODS

The background material selected for inclusion consists of historic pharmacologic secondary sources, selections from the recent primary literature on morphine pharmacokinetics and metabolism, and vignettes from the author's personal research in progress with colleagues in Australia.

RESULTS

The author's perspective is that of a chemical pharmacologist with more than a passing interest in pharmacokinetics. The synthesis evaluates the therapeutic consequences of the pharmacologic aspects chosen.

CONCLUSIONS

Morphine in various forms has been used successfully in the alleviation of pain for at least several millenia but it is only relatively recently that a rationale for its action has been found. Morphine is a relatively poorly lipophilic opioid but equilibrates with tissue of the brain relatively easily. While the metabolism of morphine occurs in the liver, it is metabolized in the kidneys and brain as well. The clearance of morphine is not notably decreased by renal dysfunction, but clearance of its glucuronide metabolites is decreased markedly. While the metabolites produced may be shown to be pharmacologically active in particular experimental preparations, it is not yet certain that they contribute pharmacologically after systemic administration of morphine in experimental animals or humans with generally normal physiology.

Comparative disposition of morphine-3-glucuronide during separate intravenous infusions of morphine and morphine-3-glucuronide in sheep. Importance of the kidney

The disposition of morphine-3-glucuronide (M3G) in sheep was compared during separate constant infusions of morphine and M3G. Five ewes received a 15-min loading dose, followed by a constant infusion of morphine sulfate (10 mg/hr) or M3G (4 mg/hr for 4 sheep, 7.5 mg/hr for 1 sheep) for a further 5.75 hr. During the 5th-6th hr of infusion, blood was collected simultaneously from the aorta, pulmonary artery, hepatic vein, hepatic portal vein, renal vein, and posterior vena cava. Additional samples were collected from the aorta from 0 to 5 hr and from 6 to 48 hr. Urine was collected via an indwelling catheter from 0 to 6 hr, with further free-flowing urine up to 48 hr. An HPLC assay was used to determine simultaneously morphine, M3G, and morphine-6-glucuronide (M6G) in plasma and urine. Constant concentrations of morphine, M3G, and M6G in plasma were achieved during the 5- to 6-hr period of infusion with morphine, as were the concentrations of M3G while M3G was infused. Regional net extraction ratios and total and regional clearances were calculated during the 5- to 6-hr period. After the infusions were ceased, there was prolonged elimination of M3G formed in situ from morphine compared to when infused as M3G. No morphine or M6G was detected in the plasma during and after infusion with M3G, nor were they found in urine collected up to 6 hr.(ABSTRACT TRUNCATED AT 250 WORDS)

Bupivacaine enantiomer pharmacokinetics after intercostal neural blockade in liver transplantation patients

Bupivacaine, being a racemic local anesthetic, exists as an equal mixture of its component enantiomers R(+)- and S(-)-bupivacaine, which behave pharmacokinetically as independent drugs after injection into the body. Intercostal neural blockade using bupivacaine was performed for postoperative analgesia in 12 patients after orthotopic liver transplantation. Arterial blood, sampled serially, was assayed by enantioselective high-performance liquid chromatography for R(+)- and S(-)-bupivacaine. The average of the simultaneous R(+):S(-) ratios of blood bupivacaine concentrations in the 12 patients was 0.74 (SD 0.11); however, the use of a population mean value or a mean value for any patient denies the time-dependence of this entity. The blood enantiomer concentration difference was reflected in the maximum measured concentrations which, after the first dose, were, respectively, 0.38 (SD 0.19) and 0.52 (SD 0.28) mg.L-1.100 mg-1 RS-bupivacaine administered (P = 0.0003). The difference in blood concentrations between the enantiomers, reflected by the R(+):S(-) ratio being less than unity, could be explained by a greater mean total body clearance and a larger apparent volume distribution of R(+)-bupivacaine. Elimination of both enantiomers was prolonged in these patients after liver transplantation compared to data from the literature, but there was no tendency for either enantiomer to accumulate selectively, even upon repeated dosing. We conclude that this demonstration of differences in pharmacokinetics (and, in laboratory studies, also in pharmacodynamics) between the bupivacaine enantiomers points to the need for future studies to recognize the enantiomeric duality of this local anesthetic.

Pharmacokinetics of bupivacaine enantiomers in sheep: influence of dosage regimen and study design

Bupivacaine is used as a racemate. In previous studies the mean total body clearance of R(+)-bupivacaine was found to be greater than S(-)-bupivacaine by 65% after iv bolus dose of separate enantiomers and by 20% after iv infusion to steady state of racemate. The present studies were performed to determine whether different study designs using different iv dosage regimens could influence the pharmacokinetic parameters determined for either bupivacaine enantiomer. rac-Bupivacaine.HCl was administered iv to 6 adult Merino ewes by bolus, brief infusion, and prolonged infusion. Arterial blood concentrations of R(+)- and S(-)-bupivacaine were measured by enantioselective HPLC. These regimens consistently produced lower arterial blood concentrations of R(+)-bupivacaine than S(-)-bupivacaine due to R(+)-bupivacaine having a greater initial dilution volume by 16 (95% CI = 3-29)%, volume of distribution at steady state equilibrium by 32 (95% CI = 17-32)% and mean total body clearance by 28 (95% CI = 21-35)%. The slow half-life of R(+)-bupivacaine, however, was found to be 15 (95% CI = 0-31)% longer than that of S(-)-bupivacaine. The difference between enantiomers in mean total body clearance thus was similar to the previous study based upon infusion to steady state of rac-bupivacaine. Differences in pharmacokinetics attributable to the dosage regimen consisted of a greater mean total body clearance for R(+)-bupivacaine along with a smaller terminal half life with the bolus regimen and a longer half-life of S(-)-bupivacaine after prolonged infusion. Differences in pharmacokinetics between the bupivacaine enantiomers occurred consistently in both distribution and clearance but the magnitude of the effect was less than 50% in each case. Systematic differences in pharmacokinetics associated with the dosage regimen were found mainly in terminal half-life. Dosage regimen, thus, was found to influence the pharmacokinetic results found experimentally and is therefore a significant variable in its own right.

The pharmacokinetics of meperidine in the myocardium of conscious sheep

The pharmacokinetics of meperidine in blood and myocardium were studied in five conscious sheep. After an intravenous (i.v.) bolus of 100, 200, or 300 mg meperidine HCl, the maximum arterial blood concentrations (mean +/- SD) were 27.8 +/- 4.6, 66.8 +/- 13.3, and 114.5 +/- 23.1 micrograms/mL, respectively, and coronary sinus blood concentrations were 2.7 +/- 1.0, 7 +/- 1.5, and 13.7 +/- 2.5 micrograms/mL, respectively. These were linearly related to dose. Net uptake of meperidine into the myocardium occurred during the first minute and the maximum rates of uptake (influxes) were 5.1 +/- 2.6, 15.1 +/- 4.6, and 27.5 +/- 8.7 mg/min. Myocardial meperidine concentrations, calculated using mass balance principles for the 100-, 200-, and 300-mg doses, respectively, were 9.0 +/- 1.8, 20.5 +/- 8.6, and 34.1 +/- 7.4 micrograms/g at peak and had decreased to 57% +/- 11% of peak 5 min after injection. No pseudoequilibrium between blood and myocardial meperidine concentrations had been reached within the 15-min study period. Myocardial perfusion and blood-myocardial concentration gradients were both important determinants of meperidine myocardial pharmacokinetics. The fast uptake and brief sojourn of meperidine in the myocardium agreed with its rapid but transient negative inotropic effect reported previously.

Determination of ketorolac enantiomers in plasma using enantioselective liquid chromatography on an alpha 1-acid glycoprotein chiral stationary phase and ultraviolet detection

A chirally selective high-performance liquid chromatographic assay was developed to measure the R(+) and S(-) enantiomers of ketorolac in plasma for pharmacokinetic studies. Naproxen sodium [S(+) enantiomer] (10 micrograms) was used as an internal standard. Plasma samples (0.5 ml) were acidified (50 microliters of 4 M H3PO4 to pH 1.5), extracted into 0.4 ml of 10% pentan-2-ol in hexane and back-extracted into 0.15 ml of base (20 mM NaOH pH to 7-8), of which samples (5 microliters) were chromatographed on a 100 x 4 mm I.D. column packed with an HPLC chiral stationary phase based upon immobilized alpha 1-acid glycoprotein (Chiral AGP-CSP) with 4% propan-2-ol in 0.1 M NaH2PO4 pH 5.5, at 0.9 ml/min. Detection was at 325 nm and run time was 10 min. Retention times of R- and S-ketorolac and of S(+)-naproxen were 3.3, 4.8 and 6.4 min, respectively. The metabolite p-hydroxyketorolac was not resolved enantiomerically and had a retention time of 2.2 min. The assay was linear over the range 0.5-10 mg/l, with precisions < 5% C.V. Good separations (alpha > 1.35) and resolutions (Rs > 3.23) between peaks were achieved. The sensitivity could be extended to 35 micrograms/l with less precision by increasing the injection volume to 100 microliters.

The hemodynamic effects of intravenous bolus doses of meperidine in conscious sheep

The hemodynamic effects of 100, 200, and 300 mg of meperidine injected intravenously were studied in five chronically instrumented adult ewes. The maximum rate of increase of left ventricular pressure was decreased, respectively, by 27.4% +/- 3.9%, 37.5% +/- 5.6%, and 31.9% +/- 13.0%, and recovery occurred by 5, 8, and 0.5 min, respectively. Mild central nervous system stimulatory effects (agitation) were observed in three of five sheep at 200 mg and moderate effects (rigor and jumping movements) were observed in four of five sheep at 300 mg. These doses also produced increases in heart rate (43%-64%) and mean arterial blood pressure (17%-27%). At these doses, cardiac output was increased for 0.5 min by approximately 25% without changes in stroke volume and left ventricular stroke work. Coronary blood flow was increased by 44%-81% for 0.5 min. We conclude that, in unpremedicated sheep, meperidine has a brief direct negative inotropic effect on the myocardium, but that at larger doses this is overridden by stimulatory central nervous system (CNS) and indirect hemodynamic effects.

Disposition of morphine and its 3- and 6-glucuronide metabolites during morphine infusion in the sheep

A sheep preparation was used to examine the regional formation and extraction of morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), relative to the regional extraction of morphine, at four morphine dose rates. On separate occasions, four ewes received a 15-min loading infusion of morphine sulfate, followed by a constant infusion at 2.5, 5, 10, or 20 mg/hr for an additional 5.75 hr. During the 5th to 6th hr of infusion, blood samples were collected simultaneously from the aorta, pulmonary artery, hepatic vein, hepatic portal and renal veins, posterior vena cava, and coronary and sagittal sinuses. Urine was collected for 48 hr. Morphine, M3G, and M6G in plasma and urine were determined simultaneously by HPLC. The blood/plasma concentration ratio (lambda) for morphine, M3G, and M6G was determined in spiked "blank" blood. Steady-state plasma concentrations were achieved during the sampling period, and dose-normalized concentrations were independent of the infusion rate. There was significant (p < 0.05) extraction (mean +/- SD) of morphine by the liver (0.676 +/- 0.014) and kidney (0.602 +/- 0.039), net extraction of M3G (0.106 +/- 0.046) and M6G (0.104 +/- 0.030) by the kidney, and net formation of M3G (-0.057 +/- 0.017) by the gut. The mean lambda for morphine, M3G, and M6G was 1.25 +/- 0.17, 0.80 +/- 0.03, and 0.82 +/- 0.09, respectively. The mean total body clearance of morphine with respect to blood was 1.58 +/- 0.27 liters/min. Mean (+/-SD) percentage urinary recoveries as morphine, M3G, and M6G were 14.7 +/- 8.5, 75.4 +/- 11.1, and 0.49 +/- 0.39, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Patient-controlled analgesia. Pharmacokinetic and therapeutic considerations

The opioid analgesic agents exhibit relatively large pharmacokinetic differences between drugs, and there is substantial pharmacokinetic and pharmacodynamic variability across subjects or patients with each agent. The advent of patient-controlled analgesic administration techniques and their widespread use in contemporary pain management, especially in postsurgical and cancer patients, has decreased the unfortunate impact of interpatient variability on achieving the optimal balance between pain relief and opioid adverse effect intensity. The improvements in pain management provided by patient-controlled analgesia do not, however, decrease the importance of knowledge of opioid pharmacokinetics towards enlightened use of these drugs and attainment of maximal benefits from them in any patient. Future improvements in patient-controlled analgesia technology will probably be based on the pharmacokinetic behaviour of different opioid analgesic agents in specific receptor-containing regions. Finally, physicochemical and pharmacokinetic characteristics of these agents are important determinants of the speed of onset of effects, duration of action and spinal selectivity of epidurally and intrathecally administered analgesics. Thus, effective patient-controlled analgesia depends on an understanding of the differential pharmacokinetics of opioids self-administered by a variety of possible modes.

Tissue distribution of bupivacaine enantiomers in sheep

rac-Bupivacaine HCl was infused intravenously to constant arterial blood drug concentrations in sheep using a regimen of 4 mg/min for 15 min followed by 1 mg/min to 24 h. At 24 h, arterial blood was sampled, the animal was killed with a bolus of KCl solution, then rapidly dissected and samples were obtained from heart, brain, lung, kidney, liver, muscle, fat, gut, and rumen. Tissue:blood distribution coefficients for (+)-(R)-bupivacaine exceeded those of (-)-(S)-bupivacaine (P < 0.05) for heart, brain, lung, fat, gut, and rumen by an overall mean of 43%. Blood:plasma distribution coefficients of (-)-(S)-bupivacaine exceeded those of (+)-(R)-bupivacaine by a mean of 29% and this offset the tissue:blood distribution coefficients so that the previously significant enantioselective differences disappeared. It is concluded that although enantioselectivity of bupivacaine distribution is shown by the measured tissue:blood distribution coefficients, it is not shown when tissue:plasma water distribution coefficients are calculated, suggesting that there is no intrinsic difference between the bupivacaine enantiomers in tissue affinity. Sheep given fatal intravenous bolus doses of rac-bupivacaine had significantly greater concentrations of (+)-(R)-bupivacaine than (-)-(S)-bupivacaine in brain (P = 0.028) and ventricle (P = 0.036); these could augment the greater myocardial toxicity of this enantiomer found in vitro.