Biomedical subjects

G M Pollack

Publications and source records attributed to G M Pollack.

At least 37 records · Page 2Linked to original sources

PubMed1999

Effects of a potent and specific P-glycoprotein inhibitor on the blood-brain barrier distribution and antinociceptive effect of morphine in the rat.

Previous data suggest that the analgesic effect of morphine may be modulated by P-glycoprotein (P-gp) inhibition. The effects of the P-gp inhibitor GF120918 on brain distribution and antinociceptive effects of morphine were examined in a rat cerebral microdialysis model. Pretreatment with GF120918 increased both the area under the concentration-time curve of unbound morphine in brain extracellular fluid (ECF) and morphine-associated antinociception. The area under the concentration-time curve ratio for unbound morphine in brain ECF versus unbound morphine in blood was significantly higher in GF120918-treated rats compared with control rats (1.21 +/- 0.34 versus 0.47 +/- 0.05, respectively; p <.05). Modulation of morphine brain-blood distribution was confirmed by quantitating brain tissue morphine in a separate group of rats; GF120918 increased the brain tissue:serum concentration ratio approximately 3-fold. The half-life of unbound morphine in brain ECF was approximately 3-fold longer in GF120918-treated rats compared with controls (p <.05). The fraction unbound of morphine in whole blood was not altered significantly in the presence of GF120918 (0.651 +/- 0.039) as compared with controls (0.662 +/- 0.035). Concentrations of unbound morphine-3-glucuronide in blood and brain ECF were increased in GF120918-treated rats versus controls. An integrated pharmacokinetic/pharmacodynamic model was developed to characterize the unbound blood and brain ECF morphine concentration profiles and concentration-effect relationships. The results of this study indicate that alteration of morphine antinociception by a potent P-gp inhibitor appears to be mediated at the level of the blood-brain barrier.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1998

Rationale for influx enhancement versus efflux blockade to increase drug exposure to the brain.

Various methods of optimizing brain exposure to polar compounds have been examined. However, direct comparisons of the potential efficacy of these methods have not been forthcoming. The present study utilized a mathematical approach to compare the efficacy of two pharmacologic methods of improving brain drug distribution: uptake enhancement and efflux blockade. In the present simulation study, the pharmacokinetics of a hypothetical marker and modifier were described by differential equations. The relationship between modifier effect on marker uptake into, or efflux from, the brain and modifier concentration in serum (uptake enhancement) or brain tissue (efflux blockade) was described by the Hill equation. Uptake enhancement increased both the rate and extent of marker penetration into the brain. Efflux blockade resulted in delayed attainment of maximum marker concentration, and prolonged marker residence, in brain tissue. Under all conditions and doses examined, uptake enhancement was more effective than efflux blockade in maximizing brain tissue exposure to the marker. Although development of agents that enhance uptake of polar compounds may be limited by the potential deleterious effects of blood-brain barrier disruption, use of this approach in theory could represent a significant advancement in the treatment of brain disorders.

Blood-Brain Barrier↗

PubMed1998

Evaluation of microdialysis sampling of aqueous humor for in vivo models of ocular absorption and disposition.

The dynamics of beta-adrenergic-associated reductions in aqueous humor production for treatment of elevated intraocular pressure are not well understood. In particular, the relationship between ocular pharmacokinetics and pharmacodynamics has yet to be established. This study was undertaken to develop a procedure for examining the ocular absorption and disposition of topically administered ophthalmic beta-adrenergic antagonists in individual animals. Dogs were anesthetized with isoflurane and a microdialysis probe was implanted in the anterior chamber of one eye and perfused with 0.9% saline at a rate of 2 microliters min-1. 3H-propranolol was administered by intracameral injection or topically. Each dog received intracameral and topical propranolol, in alternate eyes on separate days, in a randomized cross-over fashion. Microdialysis probe effluent was collected every 5 min for > or = 2.5 h; concentrations of propranolol were determined by liquid scintillation spectroscopy and were corrected for probe recovery of the substrate as determined by in vivo retrodialysis (approximately 46%) to estimate aqueous humor concentrations. In separate experiments in rabbits, microdialysis probes were implanted in each eye. 3H-propranolol was administered topically to one eye; the contralateral eye received intracameral 3H-propranolol. Model-independent pharmacokinetic parameters for each treatment phase were calculated. The mean +/- S.D. times to peak concentration of propranolol in aqueous humor were 86.6 +/- 47.6 min in the dog and 54.1 +/- 20.4 min in the rabbit. The terminal rate constant was 0.0189 +/- 0.00429 min-1 in the dog vs. 0.00983 +/- 0.00546 min-1 in the rabbit. Intraocular tissue availability of propranolol differed markedly between the dog (n = 3) and rabbit (n = 3) (approximately 0.056 in the dog vs. approximately 0.55 in the rabbit). These results demonstrate the utility of microdialysis sampling for examination of ocular pharmacokinetics.

Absorption↗

PubMed1998

Effect of GF120918, a potent P-glycoprotein inhibitor, on morphine pharmacokinetics and pharmacodynamics in the rat.

PURPOSE: The objective of this study was to evaluate the effect of a potent P-gp inhibitor, GF120918, on the systemic pharmacokinetics and antinociceptive pharmacodynamics of a single intravenous dose of morphine in rats. METHODS: Male Sprague-Dawley rats received either 500 mg base/kg/d GF120918 or vehicle for 4 days by gavage, or no pretreatment. On day 4, morphine was administered as a 1- or 2-mg/kg i.v. bolus. Antinociception, expressed as percent of maximum possible response (%MPR), was evaluated over 300 min after morphine administration. Serial blood samples were collected and analyzed for morphine and morphine-3-glucuronide (M3G) by HPLC. RESULTS: Morphine clearance and distribution volume were not altered significantly by GF120918. M3G AUC in the GF120918-treated rats was approximately 2-fold higher than in vehicle-treated rats. For both morphine doses, %MPR and the area under the effect-time curve at 300 min were significantly higher in the GF120918-treated rats. A pharmacokinetic/pharmacodynamic effect model accurately described the effect-concentration data for the rats that received 1-mg/kg morphine; ke0 was significantly smaller for GF120918- vs. vehicle-treated and control rats (0.060 +/- 0.028 vs. 0.228 +/- 0.101 vs. 0.274 +/- 0.026 min-1, p = 0.0023). EC50 and gamma were similar between treatment groups. CONCLUSIONS: Pretreatment with GF120918 enhanced morphine antinociception, as assessed by the hot-lamp tail-flick assay, and elevated systemic M3G concentrations in rats. The differential pharmacologic response to morphine in the GF120918-treated animals could not be attributed to alterations in systemic morphine pharmacokinetics.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1998

Comparison of two cyclosporine formulations in healthy volunteers: bioequivalence of the new Sang-35 formulation and Neoral.

This study was conducted to establish bioequivalence between a newly developed oral cyclosporine formulation, Sang-35 (SangStat Medical Corp., Menlo Park, CA), and the microemulsion formulation Neoral (Novartis Pharmaceuticals, East Hanover, NJ). In a randomized, open-label, two-way crossover study, 36 fasted, healthy male volunteers received a single 500-mg cyclosporine dose formulated either as Sang-35 or Neoral. Mean are under the concentration-time curve to infinity (AUC0-infinity) for Sang-35 was 13,900 microg x hr/L compared with 14,000 microg x hr/L for Neoral, with a 90% confidence interval (CI) of 96% to 103% for the geometric mean ratio of the two formulations. Mean maximum concentration (Cmax) was 1,690 microg/L for Sang-35 and 1,700 microg/L for Neoral, with a 90% CI of 96% to 103%. Geometric mean ratios for both AUC0-infinity and Cmax were within the acceptance criteria for bioequivalence (80-125%). Additional studies showed no differences between Sang-35 and Neoral after high-fat meals (n = 19), in female volunteers (n = 25) and in black volunteers (n = 7). It is concluded that single doses of the oral cyclosporine formulations Sang-35 and Neoral are bioequivalent in healthy fasted subjects, after high-fat meals, in women, and in blacks.

Administration, Oral↗

PubMed1998

Altered disposition and antinociception of [D-penicillamine(2,5)] enkephalin in mdr1a-gene-deficient mice.

This study was undertaken to test the hypothesis that P-glycoprotein (P-gp) modulates opioid peptide pharmacodynamics. [D-Penicillamine2, 5]enkephalin (DPDPE) (10 mg/kg i.v.) was administered to mdr1a(-/-) and wild-type mice to assess systemic disposition and antinociception. A subsequent dose-response experiment examined the impact of P-gp on DPDPE antinociception. In addition, the time course of antinociception was determined after a 0.9-mg/kg [mdr1a(-/-) mice] or 24-mg/kg (FVB mice) i.v. dose. Data were fit with a series of pharmacokinetic-pharmacodynamic models to compare the disposition and action of DPDPE in the two mouse strains. A 10-mg/kg dose produced >80% maximum possible response at all time points in mdr1a(-/-) mice; peak antinociception was <20% maximum possible response in FVB mice. DPDPE systemic disposition did not differ between the two mouse strains. Although brain tissue concentrations were 2- to 4-fold higher in mdr1a(-/-) compared to FVB mice, the dose required to elicit comparable antinociception was nearly 30-fold lower in mdr1a(-/-) mice; brain tissue EC50 differed by an order of magnitude in the two mouse strains. Pharmacokinetic-pharmacodynamic modeling indicated that the difference in antinociception between mdr1a(-/-) and FVB mice was a function of DPDPE distribution within brain, as well as between blood and brain, and not due to differences in intrinsic response. The results of this study suggest that DPDPE is a substrate of P-gp, and that P-gp is responsible, in part, for the low penetration of DPDPE into brain. The substantial difference in brain tissue EC50 in the absence vs. presence of P-gp suggests that P-gp modulates DPDPE-associated antinociception at sites other than the blood-brain interface.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1997

Sensitive analysis of [D-Pen2,5]enkephalin in rat serum by capillary electrophoresis and laser-induced fluorescence detection.

A highly sensitive analytical method based on capillary zone electrophoresis (CZE) coupled with a laser-induced fluorescence (LIF) detector was explored for the analysis of [D-Pen2,5]enkephalin (DPDPE) in rat serum. DPDPE and the internal standard Phe-Leu-Glu-Glu-Ile (P9396) were extracted from serum samples with C18 solid-phase extraction disk cartridges, followed by derivatization with tetramethylrhodamine-5-isothiocyanate (TRITC) isomer G before introduction onto the capillary column. Complete resolution of DPDPE and the internal standard from other serum components was achieved within 20 min on a 140 cm x 50 microns I.D. capillary column with borate buffer (25 mM. pH 8.3). With the current method, it is possible to detect 1.3E-18 mol of DPDPE on column. The results suggest that CZE-LIF is a promising method for the sensitive and specific quantitation of therapeutic peptides in biological matrices.

Animals↗

PubMed1997

Effect of prior morphine-3-glucuronide exposure on morphine disposition and antinociception.

Morphine-3-glucuronide (M3G), the primary metabolite of morphine in humans and rats, has been reported to antagonize morphine-induced pharmacologic effects. The present experiment was conducted to evaluate the effect of prior systemic M3G exposure on morphine disposition and antinociceptive response in male Sprague-Dawley rats. Saline (N = 6), low dose M3G (0.15 mg/hr, N = 7), or high dose M3G (0.30 mg/hr, N = 6) was infused for 720 min prior to the administration of morphine by i.v. bolus (2 mg/kg). Tail-flick latencies in response to hot water (50 degrees) were assessed prior to and for 180 min after the morphine test dose. M3G exposure had no significant effect on morphine pharmacokinetics, although a disproportionate increase in M3G concentrations was observed following the morphine i.v. bolus dose in rats infused with high dose M3G. Morphine-induced antinociception, expressed as the percent of maximum response (%MPR), was maximum 15 min after morphine administration and returned to baseline by 180 min. A pharmacokinetic-pharmacodynamic model was constructed to relate tail-flick latencies to morphine serum concentrations. In saline-exposed rats, the antinociceptive response to morphine was characterized by a sigmoidal Emax model, with an EC50 of 328 ng/mL, a Hill coefficient (gamma) of 4.5, and a half-life for the offset of pharmacologic effect of 11 min. No statistically significant differences in the intensity or duration of morphine-induced response were detected between saline- and M3G-exposed animals. These results suggest that systemic formation of M3G is unlikely to contribute significantly to the development of tolerance to morphine antinociception.

Animals↗

PubMed1997

A modified residual method to estimate the zero-order absorption rate constant in a one-compartment model.

The objective of this work was to develop a simple residual method to estimate the rate constant for actual or apparent zero-order absorption into a one-compartment model. The method is based on the fact that, in theory, a plot of residuals versus e-Kt is linear for a zero-order absorption process, where K represents the elimination rate constant governing the terminal phase of the concentration-time profile. The apparent absorption rate constant (K0) can be calculated from the slope and intercept of the residual plot. Simulated concentration-time data with superimposed random error (CV = 5, 10, 15%, n = 8), as well as data sets from the literature for hydroflumethiazide and theophylline were analyzed with the proposed method of residuals. Parameters derived with the new technique were compared to both the nonlinear least-squares regression and the Wagner-Nelson method, all of which yield comparable K0 estimates. These results indicate that the proposed method of residuals represents a simple approach for estimating the apparent zero-order absorption rate constant analogous to classic residual analysis for first-order absorption.

Administration, Oral↗

PubMed1997

Pharmacokinetics and effect of food on the bioavailability of orally administered venlafaxine.

Venlafaxine is a unique antidepressant currently under evaluation for treatment of various affective disorders. The pharmacokinetics and relative bioavailability of venlafaxine were evaluated in healthy volunteers after oral administration. The bioavailability of 50 mg of venlafaxine as a tablet relative to a solution was determined in a two-period randomized crossover study. The rate of absorption from the gastrointestinal tract was assessed by the time to peak plasma concentration (tmax), a model-dependent calculation of the first-order absorption rate constant, and a model-independent calculation of mean residence time. The extent of absorption was assessed by peak plasma concentration (Cmax) and area under the concentration-time curve (AUC). No statistically significant differences were observed between the two formulations for either the rate or extent of absorption. Similarly, systemic concentrations of the active O-demethylated metabolite did not significantly differ after administration of the two venlafaxine formulations. AUC ratios indicated that the relative bioavailabilities of the parent drug, and formulation of metabolite were approximately 98% and 92%, respectively, for the tablet versus the solution. A separate study was conducted to examine the influence of food on venlafaxine absorption from the 50-mg tablet. A standard, medium-fat breakfast eaten immediately before drug administration delayed the tmax of venlafaxine but did not affect Cmax or AUC. Therefore the tablet formulation of venlafaxine is bioequivalent to the oral solution, and the presence of food appears to decrease the rate but not the extent of absorption of venlafaxine from the tablet formulation.

Administration, Oral↗

PubMed1997

Development of a physiologically based pharmacokinetic model to describe the disposition of methanol in pregnant rats and mice.

Physiologically based pharmacokinetic (PBPK) models have been developed in recent years to describe the disposition of xenobiotics during gestation. These models can account for the dynamics of physiologic changes associated with pregnancy and represent a significant advantage in quantitatively assessing potential exposure of the conceptus. The PBPK approach was used to develop a model of methanol disposition during gestation in rats and mice. To validate this model, concentrations of methanol in the dam and the conceptus were determined after methanol exposure of rats on Gestational Day (gd) 14 and 20 and of mice on gd 18. At the developmental stages examined, the model provided a good description of methanol disposition in the maternal circulation and the conceptus of both species. Furthermore, the model was capable of providing good fits to methanol concentration-time data from the literature. In pregnant animals, conceptal/maternal AUC and Cmax ratios decreased with increasing dose at both gd 14 and gd 20 in the rat and at gd 18 in the mouse. Additionally, the conceptal/maternal diffusion constant ratio consistently decreased with increasing dose in pregnant rats and mice. These results are consistent with earlier observations that methanol limits its own delivery to the conceptus. Further experimentation is required to continue the process of developing a generalized PBPK model to describe the disposition of xenobiotics in pregnancy, to examine specific mechanisms of nonlinear conceptal methanol disposition, and to expand the model to extrapolate to low-dose human exposures.

Animals↗

PubMed1997

Dosimetry of chlorinated quinone metabolites of pentachlorophenol in the livers of rats and mice based upon measurement of protein adducts.

The dosimetry of chlorinated quinones arising from metabolism of pentachlorophenol (PCP), in the livers of male Sprague-Dawley rats and B6C3F1 mice was investigated via measurements of cysteinyl protein adducts and estimates of the second-order reaction rate constants between the quinones and the proteins. We had previously shown that adducts of tetrachloro-1,4-benzoquinone (Cl4-1,4-BQ) and tetrachloro-1,2-benzosemiquinone (Cl4-1,2-SQ) were observed at the highest levels in the livers of Sprague-Dawley rats to which PCP had been administered by gavage (5-40 mg/kg body wt) (Biomarkers 1, 232-243, 1996). In the current study we observed that adducts of Cl4-1,4-BQ and tetrachloro-1,2-benzoquinone (CL4-1,2-BQ) were predominant in the livers of B6C3F1 mice receiving 20 mg PCP/kg body wt. The second-order rate constants, representing in vitro reactions between Cl4-1,2-BQ and Cl4-1,4-BQ and various cysteine residues of hepatic proteins of liver cytosol and liver nuclei, were estimated to be 0.012-1.96 L(g protein)(-1) hr(-1) in rats and 0.082-1.67 L(g protein)(-1) hr(-1) in mice. The estimated tissue doses of the quinones to liver cytosol decreased in the order rat Cl4-1,4-BQ > mouse Cl4-1,4-BQ > mouse Cl4-1,2-BQ and to liver nuclei in the order mouse Cl4-1,2-BQ > mouse Cl4-1,4-BQ > rat Cl4-1,4-BQ. The corresponding doses of Cl4-1,2-SQ could not be inferred due to our inability to estimate the second-order rate constants. After aggregating the estimated contributions of all quinone species, mice had a fourfold greater dose to liver nuclei than rats, whereas rats had a threefold greater dose to liver cytosol. The increased nuclear dose to mouse liver compared to that of the rat suggests that the mouse is at greater risk to hepatic DNA damage from PCP-derived quinones. Investigation of the time course of levels of unconjugated tetrachlorohydroquinone (Cl4HQ) in the livers indicated that about 0.4% of Cl4HQ was oxidized to Cl4-1,4-BQ in both rats and mice.

Animals↗

PubMed1997

Extensive biliary excretion of the model opioid peptide [D-PEN2,5] enkephalin in rats.

PURPOSE: This study was designed to test the hypothesis that the enzymatically stable opioid peptide, [D-pen2,5] enkephalin (DPDPE), is excreted extensively into bile. METHODS: Following an i.v. bolus dose of DPDPE (10 mg/kg) to rats, concentrations of DPDPE in serum, bile, liver homogenate and urine were measured by a novel capillary zone electrophoresis method. Data were analyzed to recover the fundamental pharmacokinetic parameters (volumes of distribution; distribution and elimination rate constants governing DPDPE systemic and biliary disposition). Parallel in vitro experiments were performed to evaluate the partitioning of DPDPE between erythrocytes and plasma, as well as to assess the degree of binding of DPDPE to serum proteins. RESULTS: The majority of the administered dose (approximately 80%) was recovered from bile as intact peptide. DPDPE disposition was best described by a two-compartment model with Michaelis-Menten elimination (Km: 37.5 +/- 11 micrograms/ml; Vmax: 1143 +/- 368 micrograms/min/kg) from the central compartment into bile, suggestive of an active hepatic transport system. DPDPE was associated with a distributional space of 486 +/- 62 ml/kg. In vitro incubation of DPDPE with whole blood showed that approximately 65% of the peptide was associated with erythrocytes. The difference between concentrations of DPDPE in erythrocytes and plasma was statistically significant (29.2 +/- 4.9 vs. 18.1 +/- 3.1 micrograms/ml, p < 0.05), but not between whole blood and plasma (21.3 +/- 2.8 vs. 18.1 +/- 3.1 micrograms/ml, p > 0.05). Concentration-independent binding of DPDPE to serum proteins was evidenced between 10 and 100 micrograms/ml, with an unbound fraction of 0.517 +/- 0.182. CONCLUSIONS: DPDPE undergoes extensive biliary excretion after i.v. administration in rats. The apparent nonlinearity in the biliary excretion of DPDPE revealed by the pharmacokinetic modeling strongly suggests the existence of an active transport system(s) in hepatocytes which may mediate the rapid disappearance of DPDPE from the systemic circulation.

Analgesics↗

PubMed1997

Hepatic distribution and clearance of antisense oligonucleotides in the isolated perfused rat liver.

PURPOSE: This study was conducted to investigate the impact of backbone modifications on the hepatobiliary disposition of oligonucleotides. METHODS: The disposition of backbone-modified antisense oligonucleotides [phosphorothioate (PS) and methylphosphonate (MP)] of the same base-length and sequence (5'-TAC-GCC-AAC-AGC-TCC-3'), complementary to the codon 12 activating mutation of Ki-ras, was investigated in the isolated perfused rat liver. Livers were perfused for 2 hr: perfusate and bile concentrations were analyzed by HPLC. Hepatocellular distribution was examined by measuring the amount of radiolabeled PS oligonucleotide associated with hepatocytes and Kupffer cells. Protein binding of the PS and MP oligonucleotides was determined in rat serum by ultrafiltration. RESULTS: MP oligonucleotide perfusate concentrations remained constant during the 2-hour perfusion. In contrast, PS oligonucleotide was eliminated slowly by the isolated perfused liver [CI = 1.05 +/- 0.21 mL/min; extraction ratio = 0.06 +/- 0.01]. Uptake of PS oligonucleotide by Kupffer cells appeared to exceed uptake by hepatocytes, based on standard cell separation techniques as well as confocal microscopy. The degree of protein binding in rat serum was greater for the PS oligonucleotide (79.9 +/- 2.2%) than for the MP oligonucleotide (53.0 +/- 4.7%). CONCLUSIONS: Backbone modifications significantly-influence the hepatic clearance of oligonucleotides. Uncharged MP oligonucleotides are not extracted by the isolated perfused rat liver, whereas the charged PS oligonucleotide is processed more readily.

Animals↗

PubMed1997

Pharmacodynamics and tolerance development during multiple intravenous bolus morphine administration in rats.

Limited information is available about how the time course of the development of tolerance to morphine-induced antinociception is related to the kinetics of drug administration and disposition. The objectives of the present experiment were to characterize the rate and extent of tolerance development during the administration of multiple increasing i.v. bolus doses of morphine to rats, and to construct a pharmacokinetic-pharmacodynamic model of morphine tolerance. Morphine was administered according to two different treatment (TXT) regimens: a 12-hr TXT, in which a total morphine exposure of 24 mg/kg was administered in seven escalating doses, and a 13-day TXT, in which escalating doses of morphine were administered daily up to a maximum of 6 mg/kg. Analgesic effect, expressed as the percent of maximum possible response, was assessed with the hot water-induced tail flick. Serum samples were collected for determination of morphine concentrations by HPLC. Concentration-normalized peak effects, measured after each morphine dose, remained constant throughout the 12-hr study period, which suggests that there was little or no tolerance development during the 12-hr TXT. In contrast, tolerance appeared more significant during administration of the 13-day TXT; a large decrease in normalized peak effect occurred between days 1 and 8. Effect remained constant thereafter, with administration of the maximum dose of morphine for the remainder of the treatment period. A pharmacokinetic-pharmacodynamic model describing the development of tolerance during the 13-day TXT was constructed. The applicability of this model of tolerance to morphine-induced antinociception with different modes of administration is discussed.

Analgesics, Opioid↗

PubMed1997

Blood-brain disposition and antinociceptive effects of -D-penicillamine2,5-enkephalin in the mouse.

Although intravenous administration of [D-penicillamine2, 5]-enkephalin (DPDPE) produces significant antinociception in rodents, the duration of antinociception is short ( approximately 15 min). The present study was conducted to test the hypothesis that duration of antinociception for DPDPE is determined by both systemic and regional disposition (i.e., blood-brain translocation), and that the magnitude of antinociception is related more closely to concentrations in brain tissue than in blood. Systemic disposition was examined after i.v. administration of DPDPE (10-100 mg/kg) to male CD-1 mice. The relationship between antinociception and concentration in blood and brain tissue was assessed by determining antinociception 10 min after administration of DPDPE (10-100 mg/kg); effect versus brain tissue concentration data were fit with pharmacodynamic models to recover EC50 estimates. In addition, the time course of antinociception, as well as blood and brain tissue concentrations, were examined after an i.v. bolus dose (40 mg/kg) of DPDPE. The systemic disposition of DPDPE was nonlinear; both clearance and volume of distribution were dose-dependent. Antinociception increased proportionately with increasing concentrations of DPDPE in blood or brain tissue, with an EC50 of 1.42 +/- 0.06 microg/g expressed as brain tissue concentration. However, the brain-to-blood concentration ratio also increased with increasing dose, suggestive of saturable translocation of DPDPE across the blood-brain barrier. Antinociception appeared rapidly (within 5 min) and dissipated within approximately 15 min after a 40 mg/kg i.v. dose. These results suggest that rapid elimination from blood and active efflux from brain limit the duration of action of DPDPE.

Analgesics, Opioid↗

PubMed1996

Evidence for reversible sequestration of morphine in rat liver.

The residence of morphine in the systemic circulation is prolonged despite a high systemic clearance, suggestive of significant extravascular sequestration. The present study was conducted to test the hypothesis that morphine binds significantly in tissues, and that the liver plays an important role in morphine binding. [14C]Morphine was administered to male Sprague-Dawley rats 55 min before unlabeled morphine or saline. Blood 14C increased immediately after injection of unlabeled morphine; the area under the blood concentration-time curve (AUC) for 14C increased approximately 2-fold after morphine compared with saline injection. Residual radioactivity in the liver was lower in morphine-treated rats than in controls, suggesting that unlabeled drug displaced [14C]morphine (or a metabolite) from binding sites. To examine this phenomenon more directly, a recirculating isolated perfused liver system was employed. [14C]Morphine was added to the perfusate reservoir 15 min before unlabeled morphine or saline; perfusate and bile samples were collected for 120 min. Upon termination of perfusion, the liver was fractionated to identify the hepatic subcellular fraction(s) in which morphine was sequestered. The perfusate AUC for [14C]morphine was increased approximately 2-fold in response to unlabeled drug, consistent with the in vivo experiment. Morphine was associated preferentially with the cytosolic fraction, and [14C]morphine in all relevant fractions was reduced after administration of unlabeled morphine. In contrast, unlabeled drug had no influence on derived [14C]morphine-3-beta,D-glucuronide. These data are consistent with significant, reversible binding of morphine in hepatic tissue.

Animals↗

PubMed1996

Development of a capillary zone electrophoresis assay to examine the disposition of [D-pen2,5]enkephalin in rats.

A novel capillary zone electrophoresis (CZE) assay method was developed to evaluate the systemic disposition of [D-pen2,5]enkephalin (DPDPE) in rats. DPDPE was recovered from serum samples (200 microliters) by solid-phase extraction. Complete resolution of DPDPE and the internal standard ([D-ser2]leucine-enkephalin; DSLET) from other serum components was achieved within 15 min on a 50-microns I.D. capillary column with borate buffer (25 mM, pH 8.3). The peak-height ratio (DPDPE to DSLET) was linear through 100 micrograms/ml, with a detection limit of 250 ng/ml in serum, when absorbance of the column eluent was monitored at 210 nm. Serum samples obtained from rats after a 10 mg/kg intravenous bolus dose of DPDPE were analyzed with the present CZE method. The results suggest that CZE is a useful technique for quantitating therapeutic peptides in biological matrices.

Acetonitriles↗