Brian Phillips

Epidural, cerebrospinal fluid, and plasma pharmacokinetics of epidural opioids (part 1): differences among opioids.

Background The pharmacokinetics of epidurally administered drugs has been the subject of many studies, yet drug concentration in the epidural space has never been measured. This study was undertaken to characterize the epidural, cerebrospinal fluid, and plasma pharmacokinetics of epidurally administered opioids on the basis of measurement of drug concentration in each of these compartments after epidural administration. Methods Morphine plus alfentanil, fentanyl, or sufentanil were administered epidurally in anesthetized pigs. Microdialysis was used to sample the epidural space and the cerebrospinal fluid for measurement of opioid concentration over time. Plasma samples were obtained from the central venous plasma and the epidural venous plasma. These data were used to calculate relevant pharmacokinetic parameters, including mean residence time, elimination half-lives, areas under the concentration versus time curves, clearance, and volume of distribution for each opioid in each compartment. Results Some of the more important findings were that the cerebrospinal fluid and plasma pharmacokinetics of the opioids did not parallel their epidural pharmacokinetics and that their hydrophobic character governed multiple aspects of their lumbar epidural pharmacokinetics. Conclusions The findings indicate that the spinal pharmacokinetics of these drugs are complex and, in some ways, counterintuitive. Also, the bioavailability of opioids in the cerebrospinal fluid and epidural space is determined primarily by their hydrophobicity, with less hydrophobic drugs having greater bioavailability.

Epidural, Cerebrospinal Fluid, and Plasma Pharmacokinetics of Epidural Opioids (Part 2)Effect of Epinephrine

Background The ability of epinephrine to improve the efficacy of epidurally administered drugs is assumed to result from local vasoconstriction and a consequent decrease in drug clearance. However, because drug concentration in the epidural space has never been measured, our understanding of the effect of epinephrine on epidural pharmacokinetics is incomplete. This study was designed to characterize the effect of epinephrine on the epidural, cerebrospinal fluid, and plasma pharmacokinetics of epidurally administered opioids. Methods Morphine plus alfentanil, fentanyl, or sufentanil was administered epidurally with and without epinephrine (1:200,000) to pigs. Opioid concentration was subsequently measured in the epidural space, central venous plasma, and epidural venous plasma, and these data were used to calculate relevant pharmacokinetic parameters. Results The pharmacokinetic effects of epinephrine varied by opioid and by sampling site. For example, in the lumbar epidural space, epinephrine increased the mean residence time of morphine but decreased that of fentanyl and sufentanil. Epinephrine had no effect on the terminal elimination half-life of morphine in the epidural space, but it decreased that of fentanyl and sufentanil. In contrast, in the lumbar intrathecal space, epinephrine had no effect on the pharmacokinetics of alfentanil, fentanyl, or sufentanil, but it increased the area under the concentration–time curve of morphine and decreased its elimination half-life. Conclusions The findings indicate that the effects of epinephrine on the spinal pharmacokinetics of these opioids are complex and often antithetical across compartments and opioids. In addition, the data clearly indicate that the pharmacokinetic effects of epinephrine in spinal “compartments” cannot be predicted from measurements of drug concentration in plasma, as has been assumed for decades.

Interactions of amoxicillin and cefaclor with human renal organic anion and peptide transporters.

Amoxicillin and cefaclor are two of the widely used β-lactam antibiotics in the treatment of urinary tract infections. Both drugs are eliminated mainly by the kidney and rely on renal excretion to exert their antibacterial activities in the urinary tract. Previous studies have suggested the involvement of organic anion and oligopeptide transporters in membrane transport of β-lactams. The objective of the current study was to examine the kinetics of amoxicillin and cefaclor interactions with human renal transporters human organic anion transporter 1 (hOAT1), human peptide transporter 1 (hPepT1), and human peptide transporter 2 (hPepT2) in detail, both as substrates and as inhibitors. Using fluorescence protein tagging and cell sorting, we established Madin-Darby canine kidney cell lines stably expressing highly functional hOAT1, hPepT1, and hPepT2. Amoxicillin and cefaclor inhibited hOAT1-mediated [3H]para-aminohippuric acid uptake (Ki = 11.0 and 1.15 mM, respectively). However, our uptake study revealed that neither drug was transported by hOAT1. Amoxicillin and cefaclor competitively inhibited hPepT2-mediated [3H]glycylsarcosine uptake (Ki = 733 and 65 μM, respectively), whereas much lower affinity for hPepT1 was observed with both antibiotics. Direct uptake studies demonstrated that amoxicillin and cefaclor were transported by hPepT1 and hPepT2. Kinetic analysis showed that hPepT2-mediated uptake of both drugs was saturable with Km of 1.04 mM for amoxicillin and 70.2 μM for cefaclor. hPepT2, and to a lesser extent hPepT1, may play an important role in apical transport of amoxicillin and cefaclor in the renal tubule. hOAT1, in contrast, is not involved in basolateral uptake of these antibiotics.

Real-time Dose Adjustment of Cyclophosphamide in a Preparative Regimen for Hematopoietic Cell Transplant: A Bayesian Pharmacokinetic Approach

Purpose: Dose-related toxicity of cyclophosphamide may be reduced and therapeutic efficacy may be improved by pharmacokinetic sampling and dose adjustment to achieve a target area under the curve (AUC) for two of its metabolites, hydroxycyclophosphamide (HCY) and carboxyethylphosphoramide mustard (CEPM). To facilitate real-time dose adjustment, we developed open-source code within the statistical software R that incorporates individual data into a population pharmacokinetic model. Experimental Design: Dosage prediction performance was compared to that obtained with nonlinear mixed-effects modeling using NONMEM in 20 cancer patients receiving cyclophosphamide. Bayesian estimation of individual pharmacokinetic parameters was accomplished from limited (i.e., five samples over 0-16 hours) sampling of plasma HCY and CEPM after the initial cyclophosphamide dose. Conditional on individual pharmacokinetics, simulations of the AUC of both HCY and CEPM were provided for a range of second doses (i.e., 0-100 mg/kg cyclophosphamide). Results: The results compared favorably with NONMEM and returned accurate predictions for AUCs of HCY and CEPM with comparable mean absolute prediction error and root mean square prediction error. With our method, the mean absolute prediction error and root mean square prediction error of AUC CEPM were 11.0% and 12.8% and AUC HCY were 31.7% and 44.8%, respectively. Conclusions: We developed dose adjustment software that potentially can be used to adjust cyclophosphamide dosing in a clinical setting, thus expanding the opportunity for pharmacokinetic individualization of cyclophosphamide. The software is simple to use (requiring no programming experience), reads individual patient data directly from an Excel spreadsheet, and runs in less than 5 minutes on a desktop PC.

Metabolism-based cyclophosphamide dosing for hematopoietic cell transplant

When cyclophosphamide (120 mg/kg) is used for hematopoietic cell transplant, the increased area under the curve of carboxyethylphosphoramide mustard (AUCCEPM) is related to liver toxicity and death. We determined the feasibility of dose‐adjusting cyclophosphamide to a preset metabolic endpoint (AUCCEPM, 325 ± 25 μmol/L · h). In 20 patients blood sampling was done over a 16‐hour period after administration of 45 mg/kg cyclophosphamide; AUCCEPM from 0 to 16 hours was calculated by noncompartmental analysis. The expected AUCCEPM for 0 to 48 hours was estimated, and the second cyclophosphamide dose was determined. The mean second cyclophosphamide dose was 42 mg/kg, and the mean total cyclophosphamide dose was 86 mg/kg (range, 54–120 mg/kg). The mean AUCCEPM for the time from 0 to 48 hours was 296 μmol/L · h (95% confidence interval, 275–317 μmol/L · h). A retrospective analysis indicated that AUCCEPM could be more accurately predicted by use of a population pharmacokinetic model. We conclude that metabolism‐based dosing of cyclophosphamide is feasible and that a lower cyclophosphamide dose does not affect engraftment.

A novel phenotypic method to determine fludarabine triphosphate accumulation in T-lymphocytes from hematopoietic cell transplantation patients

PurposeFludarabine is an integral anticancer agent for patients with chronic lymphocytic leukemia (CLL) and those receiving conditioning regimens prior to allogeneic hematopoietic cell transplantation (HCT). An individual’s response to fludarabine may be influenced by the amount of CD4+ and CD8+ T-lymphocyte suppression. Fludarabine undergoes cellular uptake and activation to form the cytotoxic metabolite, fludarabine triphosphate (F-ara-ATP).MethodsWe have previously developed a highly sensitive LC–MS method to quantitate intracellular F-ara-ATP concentrations in a leukemic cell line. However, quantitation of F-ara-ATP concentrations within CD4+ and CD8+ T-lymphocytes from pharmacokinetic blood samples obtained from patients receiving fludarabine therapy is not feasible because of the limited number of T-lymphocytes that can be isolated from each blood sample. Thus, we sought to determine F-ara-ATP accumulation after ex vivo exposure of freshly isolated human CD4+ or CD8+ T-lymphocytes to fludarabine. The method was optimized in T-lymphocytes obtained from healthy volunteers, and proved to be a feasible method to determine F-ara-ATP accumulation in patients undergoing HCT.ResultsConsiderable variability was observed in F-ara-ATP accumulation in HCT patients (10.5- and 12.5-fold in CD4+ and CD8+ cells, respectively), compared to healthy volunteers (1.6- and 1.9-fold in CD4+ and CD8+ cells, respectively). Larger variability was also observed in gene expression of transporters and enzymes involved in F-ara-ATP accumulation in HCT patients; however, F-ara-ATP accumulation was not correlated with gene expression, which is in agreement with previous studies.ConclusionsThe quantitation of F-ara-ATP accumulation in T-lymphocytes provides a novel tool to evaluate patient sensitivity to fludarabine. This tool can be used in future studies to evaluate whether intracellular F-ara-ATP accumulation is associated with efficacy and/or toxicity in patients receiving fludarabine.

Effects of Cranberry Juice on Pharmacokinetics of β-Lactam Antibiotics following Oral Administration

ABSTRACT Cranberry juice consumption is often recommended along with low-dose oral antibiotics for prophylaxis for recurrent urinary tract infection (UTI). Because multiple membrane transporters are involved in the intestinal absorption and renal excretion of β-lactam antibiotics, we evaluated the potential risk of pharmacokinetic interactions between cranberry juice and the β-lactams amoxicillin (amoxicilline) and cefaclor. The amoxicillin-cranberry juice interaction was investigated in 18 healthy women who received on four separate occasions a single oral test dose of amoxicillin at 500 mg and 2 g with or without cranberry juice cocktail (8 oz) according to a crossover design. A parallel cefaclor-cranberry juice interaction study was also conducted in which 500 mg cefaclor was administered with or without cranberry juice cocktail (12 oz). Data were analyzed by noncompartmental methods and nonlinear mixed-effects compartmental modeling. We conclude that the concurrent use of cranberry juice has no significant effect on the extent of oral absorption or the renal clearance of amoxicillin and cefaclor. However, delays in the absorption of amoxicillin and cefaclor were observed. These results suggest that the use of cranberry juice at usual quantities as prophylaxis for UTI is not likely to alter the pharmacokinetics of these two oral antibiotics.

Pharmacokinetics of metoprolol during pregnancy and lactation

The objective of this study was to evaluate the steady‐state pharmacokinetics of metoprolol during pregnancy and lactation. Serial plasma, urine, and breast milk concentrations of metoprolol and its metabolite, α‐hydroxymetoprolol, were measured over 1 dosing interval in women treated with metoprolol (25–750 mg/day) during early pregnancy (n = 4), mid‐pregnancy (n = 14), and late pregnancy (n = 15), as well as postpartum (n = 9) with (n = 4) and without (n = 5) lactation. Subjects were genotyped for CYP2D6 loss‐of‐function allelic variants. Using paired analysis, mean metoprolol apparent oral clearance was significantly higher in mid‐pregnancy (361 ± 223 L/h, n = 5, P < .05) and late pregnancy (568 ± 273 L/h, n = 8, P < .05) compared with ≥3 months postpartum (200 ± 131 and 192 ± 98 L/h, respectively). When the comparison was limited to extensive metabolizers (EMs), metoprolol apparent oral clearance was significantly higher during both mid‐ and late pregnancy (P < .05). Relative infant exposure to metoprolol through breast milk was <1.0% of maternal weight‐adjusted dose (n = 3). Because of the large, pregnancy‐induced changes in metoprolol pharmacokinetics, if inadequate clinical responses are encountered, clinicians who prescribe metoprolol during pregnancy should be prepared to make aggressive changes in dosage (dose and frequency) or consider using an alternate beta‐blocker.

Dexmedetomidine reduces cranial temperature in hypothermic neonatal rats

Background:The α2-adrenergic agonist dexmedetomidine (DEX) is increasingly used for prolonged sedation of critically ill neonates, but there are currently no data evaluating possible consequences of prolonged neonatal DEX exposure. We evaluated the pharmacokinetics and histological consequences of neonatal DEX exposure.Methods:DEX was administered (s.c.) to naive (uninjured) neonatal Lewis rats to provide acute (25 µg/kg, ×1) or prolonged (25 µg/kg three times daily, ×2 or ×4 d) exposure. Therapeutic hypothermia was simulated using a water-cooled blanket. Cranial temperatures were measured using an infrared thermometer. DEX concentrations were measured by LC-MS in plasma and homogenized brainstem tissue for pharmacokinetic analysis. Cortex, cerebellum, and brainstem were evaluated for evidence of inflammation or injury.Results:Prolonged neonatal DEX exposure was not associated with renal or brain pathology or indices of gliosis, macrophage activation, or apoptosis in either hypothermic or control rats. Plasma and brain DEX concentrations were tightly correlated. DEX peaked within 15 min in brain and reduced cranial temperature from 32 to 30 °C within 30 min after injection in cooled rats.Conclusion:Prolonged DEX treatment in neonatal rats was not associated with abnormal brain histology. These data provide reassuring preliminary results for using DEX with therapeutic hypothermia to treat near-term brain injury.

P-gp/ABCB1 Exerts Differential Impacts On Brain and Fetal Exposure to Norbuprenorphine.

Graphical abstract Figure. No Caption available. ABSTRACT Norbuprenorphine is the major active metabolite of buprenorphine which is commonly used to treat opiate addiction during pregnancy. Norbuprenorphine produces marked respiratory depression and was 10 times more potent than buprenorphine. Therefore, it is important to understand the mechanism that controls fetal exposure to norbuprenorphine, as exposure to this compound may pose a significant risk to the developing fetus. P‐gp/ABCB1 and BCRP/ABCG2 are two major efflux transporters regulating tissue distribution of drugs. Previous studies have shown that norbuprenorphine, but not buprenorphine, is a P‐gp substrate. In this study, we systematically examined and compared the roles of P‐gp and BCRP in determining maternal brain and fetal distribution of norbuprenorphine using transporter knockout mouse models. We administered 1 mg/kg norbuprenorphine by retro‐orbital injection to pregnant FVB wild‐type, Abcb1a−/−/1b−/−, and Abcb1a−/−/1b−/−/Abcg2−/− mice on gestation day 15. The fetal AUC of norbuprenorphine was ˜64% of the maternal plasma AUC in wild‐type mice, suggesting substantial fetal exposure to norbuprenorphine. The maternal plasma AUCs of norbuprenorphine in Abcb1a−/−/1b−/− and Abcb1a−/−/1b−/−/Abcg2−/− mice were ˜2 times greater than that in wild‐type mice. Fetal AUCs in Abcb1a−/−/1b−/− and Abcb1a−/−/1b−/−/Abcg2−/− mice were also increased compared to wild‐type mice; however, the fetal‐to‐maternal plasma AUC ratio remained relatively unchanged by the knockout of Abcb1a/1b or Abcb1a/1b/Abcg2. In contrast, the maternal brain‐to‐maternal plasma AUC ratio in Abcb1a−/−/1b−/− or Abcb1a−/−/1b−/−/Abcg2−/− mice was increased ˜30‐fold compared to wild‐type mice. Protein quantification by LC–MS/MS proteomics revealed significantly higher amounts of P‐gp protein in the wild‐type mice brain than that in the placenta. These results indicate that fetal exposure to norbuprenorphine is substantial and that P‐gp has a minor impact on fetal exposure to norbuprenorphine, but plays a significant role in restricting its brain distribution. The differential impacts of P‐gp on norbuprenorphine distribution into the brain and fetus are likely, at least in part, due to the differences in amounts of P‐gp protein expressed in the blood‐brain and blood‐placental barriers. BCRP is not as important as P‐gp in determining both the systemic and tissue exposure to norbuprenorphine. Finally, fetal AUCs of the metabolite norbuprenorphine‐&bgr;‐d‐glucuronide were 3–7 times greater than maternal plasma AUCs, while the maternal brain AUCs were <50% of maternal plasma AUCs, suggesting that a reversible pool of conjugated metabolite in the fetus may contribute to the high fetal exposure to norbuprenorphine.