The effect of high-dose, short-term caffeine intake on the renal clearance of calcium, sodium and creatinine in healthy adults.

The consumption of caffeine has been linked to osteoporosis, believed to be due to enhanced bone resorption as a result of increased calcium excretion in the urine. However, the amount of calcium in the urine may not necessarily reflect the true effect of caffeine on calcium clearance. This study therefore examined the impact of high-dose, short-term caffeine intake on renal clearance of calcium, sodium and creatinine in healthy adults. In a double-blind clinical study, participants chewed caffeine (n = 12) or placebo (n = 12) gum for 5 minutes at 2-hour intervals over a 6-hour treatment period (800 mg total caffeine). Caffeine increased renal calcium clearance by 77%. Furthermore, the effect was positively correlated with sodium clearance and urine volume, suggesting that caffeine may act through inhibition of sodium reabsorption in the proximal convoluted tubule. This study confirmed that caffeine does increase renal calcium clearance and fosters further investigation into safe consumption of caffeine.

Population pharmacokinetics of ribavirin in lung transplant recipients and examination of current and alternative dosing regimens.

BACKGROUND: Ribavirin is used in the treatment of respiratory paramyxovirus infection in lung transplant recipients; however, its pharmacokinetic profile in the transplant population is unknown despite the potential for alterations due to underlying pathology. Furthermore, the ability of current regimens to meet exposure targets has not been established. OBJECTIVES: This study examined the pharmacokinetics of ribavirin in a lung transplant population for which current and alternative dosing regimens were assessed. METHODS: Population pharmacokinetic modelling was conducted in NONMEM using concentration-time data from 24 lung transplant recipients and 6 healthy volunteers. Monte Carlo simulation was used to assess the ability of dosing regimens to achieve pre-specified target concentrations. RESULTS AND CONCLUSIONS: A three-compartment model with first-order elimination most adequately described ribavirin concentration-time data, with CLCR and patient type (i.e. lung transplant) identified as significant covariates in the model. Simulations indicate that current regimens achieve efficacious concentrations within 24 h of treatment initiation that increase to supra-therapeutic levels over the treatment period. A regimen of 8 mg/kg q6h orally for 48 h followed by 8 mg/kg q24h orally for the remainder of the treatment period was predicted to result in >90% of patients exhibiting concentrations within the defined target range throughout the entire treatment course. Additional work to formally establish target therapeutic concentrations is required; however, this study provides a valuable first step in determining optimal ribavirin treatment regimens for paramyxovirus infections in the lung transplant population.

Reducing Palivizumab Dose Requirements Through Rational Dose Regimen Design.

Palivizumab for respiratory syncytial virus (RSV) immunoprophylaxis in premature infants poses a significant economic challenge. Although standard dosing of palivizumab results in unnecessary drug accumulation without additional clinical benefit, some clinicians have moved outside of evidence-based practice by implementing untested dose modifications, potentially jeopardizing efficacy. Using an industry-developed population pharmacokinetic model, this study evaluated the previously published alternate dosing regimens and developed a revised regimen that minimizes palivizumab dose requirements while maintaining established therapeutic concentrations. All published dose modifications resulted in unacceptably high proportions of infants not attaining minimum protective concentrations, compromising efficacy. Through intelligent dose regimen design, a clinically practical palivizumab regimen was devised that reduces drug use by 25%, while enabling a greater proportion of infants attaining early season target concentrations, particularly those at greatest risk of the consequences of RSV infection. This novel regimen has the potential to substantially change clinical practice and increase drug availability.

Effect of food on the pharmacokinetics of piperaquine and dihydroartemisinin.

BACKGROUND AND OBJECTIVE: Piperaquine-dihydroartemisinin combination therapy has established efficacy for the treatment of malaria; however, a more comprehensive understanding of the pharmacokinetic properties and factors contributing to inter- and intra-individual variability is critical to optimize clinical use. This study assessed the effects of food on the pharmacokinetics of combination piperaquine-dihydroartemisinin administration in healthy volunteers. METHODS: This was an open-label, single-dose, parallel-group study. Participants were randomly allocated to receive oral piperaquine-dihydroartemisinin either after an overnight fast or immediately after a standardized, high-fat, high-calorie meal. Blood samples were collected for analysis of plasma piperaquine and dihydroartemisinin concentrations, which were utilized for calculation of pharmacokinetic parameters, using a standard model-independent approach. RESULTS: Consumption of a high-fat, high-calorie meal resulted in substantial increases in the extent of exposure to piperaquine (ratio between area under the plasma concentration-time curve [AUC] values from 0 to 168 h in the fed and fasted states [AUC0-168 h FED/AUC0-168 h FASTED] = 299 %, 90 % confidence interval [CI] 239-374 %). This likely reflects an increase in the oral bioavailability of the drug, directly related to the fat content of the meal. Co-administration of food was also found to result in both delayed and enhanced absorption of dihydroartemisinin (ratio between AUC values from time zero to infinity in the fed and states [AUC∞ FED/AUC∞ FASTED] = 142 %, 90 % CI 113-178 %; ratio between mean transit time [MTT] values in the fed and fasted states [MTTFED/MTTFASTED] = 135 %, 90 % CI 114-160 %). CONCLUSION: Although food was found to significantly impact on the pharmacokinetics of piperaquine and dihydroartemisinin, given the low fat content of standard meals within endemic regions and the anorexic effects of malaria infection, these results are unlikely to impact on the clinical utility of these drugs. However, co-administration of food with these anti-malarials by populations consuming a typical Western diet should be avoided to reduce the risk of toxic side effects. It is therefore a general recommendation that piperaquine-dihydroartemisinin not be administered within ±3 h of food consumption.

Population pharmacokinetics of orally administered mefloquine in healthy volunteers and patients with uncomplicated Plasmodium falciparum malaria.

BACKGROUND: The determination of dosing regimens for the treatment of malaria is largely empirical and thus a better understanding of the pharmacokinetic/pharmacodynamic properties of antimalarial agents is required to assess the adequacy of current treatment regimens and identify sources of suboptimal dosing that could select for drug-resistant parasites. Mefloquine is a widely used antimalarial, commonly given in combination with artesunate. PATIENTS AND METHODS: Mefloquine pharmacokinetics was assessed in 24 healthy adults and 43 patients with Plasmodium falciparum malaria administered mefloquine in combination with artesunate. Population pharmacokinetic modelling was conducted using NONMEM. RESULTS: A two-compartment model with a single transit compartment and first-order elimination from the central compartment most adequately described mefloquine concentration-time data. The model incorporated population parameter variability for clearance (CL/F), central volume of distribution (VC/F) and absorption rate constant (KA) and identified, in addition to body weight, malaria infection as a covariate for VC/F (but not CL/F). Monte Carlo simulations predict that falciparum malaria infection is associated with a shorter elimination half-life (407 versus 566 h) and T>MIC (766 versus 893 h). CONCLUSIONS: This is the first known population pharmacokinetic study to show falciparum malaria to influence mefloquine disposition. Protein binding, anaemia and other factors may contribute to differences between healthy individuals and patients. As VC/F is related to the earlier portion of the concentration-time profiles, which occurs during acute malaria, and CL/F is more related to the terminal phase during convalescence after treatment, this may explain why malaria was found to be a covariate for VC/F but not CL/F.

Application site affects the pharmacokinetics of topical testosterone applied to the axilla compared with the inner arm.

PURPOSE: This study compared the pharmacokinetics of a single dose of 1% testosterone solution after application to the inner arm or the axilla as application sites for transdermal testosterone therapy. METHODS: Healthy, not pregnant, premenopausal women, 18 to 45 years of age with a body mass index of 20 to 28 kg/m(2) were enrolled into a single-center, open-label, randomized, 2-way crossover study. Serum total testosterone (TT), free testosterone (fT), and sex hormone binding globulin concentrations were measured. Pharmacokinetic parameters determined from serum TT and fT included area under the serum concentration versus time curve from time zero (pre-dose) until 72 hours post-dose (AUC0-72), Cmax, and Tmax. Descriptive statistics were performed on serum concentrations of TT and fT for each site. ANOVA was performed on AUC0-72 and Cmax. FINDINGS: A single-dose application of 1% testosterone solution to the inner arm and the axilla produced clear increases in TT and fT. Slower and lower increases in TT and fT were observed after treatment to the inner arm. Based on baseline-corrected AUC versus time curves, the bioavailability of 1% testosterone solution was increased 2-fold for the axilla compared with the inner arm. IMPLICATIONS: The absorption of a 1% testosterone solution was significantly greater after application to the axilla than to the inner arm. Study number DDS16; Australian Therapeutic Goods Administration, CTN 2005/158.

Carnitine and acylcarnitines: pharmacokinetic, pharmacological and clinical aspects.

L-Carnitine (levocarnitine) is a naturally occurring compound found in all mammalian species. The most important biological function of L-carnitine is in the transport of fatty acids into the mitochondria for subsequent ß-oxidation, a process which results in the esterification of L-carnitine to form acylcarnitine derivatives. As such, the endogenous carnitine pool is comprised of L-carnitine and various short-, medium- and long-chain acylcarnitines. The physiological importance of L-carnitine and its obligatory role in the mitochondrial metabolism of fatty acids has been clearly established; however, more recently, additional functions of the carnitine system have been described, including the removal of excess acyl groups from the body and the modulation of intracellular coenzyme A (CoA) homeostasis. In light of this, acylcarnitines cannot simply be considered by-products of the enzymatic carnitine transfer system, but provide indirect evidence of altered mitochondrial metabolism. Consequently, examination of the contribution of L-carnitine and acylcarnitines to the endogenous carnitine pool (i.e. carnitine pool composition) is critical in order to adequately characterize metabolic status. The concentrations of L-carnitine and its esters are maintained within relatively narrow limits for normal biological functioning in their pivotal roles in fatty acid oxidation and maintenance of free CoA availability. The homeostasis of carnitine is multifaceted with concentrations achieved and maintained by a combination of oral absorption, de novo biosynthesis, carrier-mediated distribution into tissues and extensive, but saturable, renal tubular reabsorption. Various disorders of carnitine insufficiency have been described but ultimately all result in impaired entry of fatty acids into the mitochondria and consequently disturbed lipid oxidation. Given the sensitivity of acylcarnitine concentrations and the relative carnitine pool composition in reflecting the intramitochondrial acyl-CoA to free CoA ratio (and, hence, any disturbances in mitochondrial metabolism), the relative contribution of L-carnitine and acylcarnitines within the total carnitine pool is therefore considered critical in the identification of mitochondria dysfunction. Although there is considerable research in the literature focused on disorders of carnitine insufficiency, relatively few have examined relative carnitine pool composition in these conditions; consequently, the complexity of these disorders may not be fully understood. Similarly, although important studies have been conducted establishing the pharmacokinetics of exogenous carnitine and short-chain carnitine esters in healthy volunteers, few studies have examined carnitine pharmacokinetics in patient groups. Furthermore, the impact of L-carnitine administration on the kinetics of acylcarnitines has not been established. Given the importance of L-carnitine as well as acylcarnitines in maintaining normal mitochondrial function, this review seeks to examine previous research associated with the homeostasis and pharmacokinetics of L-carnitine and its esters, and highlight potential areas of future research.

Particle size reduction to the nanometer range: a promising approach to improve buccal absorption of poorly water-soluble drugs.

Poorly water-soluble drugs, such as phenylephrine, offer challenging problems for buccal drug delivery. In order to overcome these problems, particle size reduction (to the nanometer range) and cyclodextrin complexation were investigated for permeability enhancement. The apparent solubility in water and the buccal permeation of the original phenylephrine coarse powder, a phenylephrine-cyclodextrin complex and phenylephrine nanosuspensions were characterized. The particle size and particle surface properties of phenylephrine nanosuspensions were used to optimize the size reduction process. The optimized phenylephrine nanosuspension was then freeze dried and incorporated into a multi-layered buccal patch, consisting of a small tablet adhered to a mucoadhesive film, yielding a phenylephrine buccal product with good dosage accuracy and improved mucosal permeability. The design of the buccal patch allows for drug incorporation without the need to change the mucoadhesive component, and is potentially suited to a range of poorly water-soluble compounds.

Effects of ketamine on human UDP-glucuronosyltransferases in vitro predict potential drug-drug interactions arising from ketamine inhibition of codeine and morphine glucuronidation.

In this study, the selectivity of UDP-glucuronosyltransferase (UGT) enzyme inhibition by ketamine (KTM) and the kinetics of KTM inhibition of human liver microsomal morphine (MOR) and codeine (COD) glucuronidation were characterized to explore a pharmacokinetic basis for the KTM-opioid interaction. With the exception of UGT1A4, KTM inhibited the activities of recombinant human UGT enzymes in a concentration-dependent manner. However, IC(50) values were <100 µM only for UGT2B4, UGT2B7, and UGT2B15. UGT2B7 catalyzes MOR 3- and 6-glucuronidation and the 6-glucuronidation of COD, with an additional substantial contribution of UGT2B4 to the latter reaction. Consistent with the effects of KTM on the activities of recombinant UGT2B enzyme activities, KTM competitively inhibited human liver microsomal MOR and COD glucuronidation. K(i) values for KTM inhibition of MOR 3- and 6-glucuronidation and COD 6-glucuronidation by human liver microsomes supplemented with 2% bovine serum albumin were 5.8 ± 0.1, 4.6 ± 0.2, and 3.5 ± 0.1 µM, respectively. Based on the derived inhibitor constants, in vitro-in vivo extrapolation was used to predict the effects of anesthetic and analgesic doses of KTM on MOR and COD clearances. Potentially clinically significant interactions (>50% increases in the in vivo area under the curve ratios) with MOR and COD were predicted for anesthetic doses of KTM and for a subanesthetic dose of KTM on COD glucuronidation.

A novel tri-layered buccal mucoadhesive patch for drug delivery: assessment of nicotine delivery.

OBJECTIVES: The aim of this study was to assess the potential of a novel delivery device for administering drugs that suffer from a high degree of first-pass metabolism. METHODS: A tri-layered buccal mucoadhesive patch, comprising a medicated dry tablet adhered to a mucoadhesive film, was prepared and characterized by its physicochemical properties and mucoadhesive strength. Nicotine was used as a model drug for the characterization of drug release and drug permeation. The influence of different adsorbents on the release of nicotine base from the patches was evaluated in vitro. Different molecular forms of nicotine (base and complex salt) were evaluated for their effect on release performance and permeation in vitro. KEY FINDINGS: Results demonstrated acceptable physicochemical and mucoadhesive properties for the tri-layered patch. Rapid release of nicotine was observed when nicotine base was incorporated with calcium sulfate dihydrate as the adsorbent. Patches incorporating nicotine base showed distinct advantages over those containing nicotine polacrilex, in terms of drug release (complete drug release achieved at 30 vs 60 min) and transmucosal permeation (37.28 ± 4.25 vs 2.87 ± 0.26% of the dose permeating through mucosa within 120 min). CONCLUSIONS: The novel tri-layered patch can effectively adhere to, and deliver an active ingredient through the buccal mucosa, confirming its potential for buccal mucoadhesive drug delivery.

Differential disposition of intra-renal generated and preformed glucuronides: studies with 4-methylumbelliferone and 4-methylumbelliferyl glucuronide in the filtering and nonfiltering isolated perfused rat kidney.

OBJECTIVES: This study was designed to investigate the renal disposition of 4-methylumbelliferone (4MU) and 4-methylumbelliferyl glucuronide (4MUG) to characterise the contribution of excretion and metabolic clearance to total clearance in the kidney. METHODS: The isolated perfused kidney (IPK) from the male Sprague-Dawley rat was used in filtering and non-filtering mode to study the renal disposition of 4MU, renally generated 4MUG and preformed 4MUG. Perfusate and urine (filtering IPK only) was collected for up to 120 min and 4MU and 4MUG in perfusate and urine were determined by HPLC. Analytes were also measured in kidney tissue collected at 120 min. Non-compartmental analysis was used to derive pharmacokinetic parameters. KEY FINDINGS: The concentration of 4MU in perfusate declined with a terminal half-life of approximately 120 min following administration to the filtering IPK and nonfiltering IPK. There was a corresponding increase in the concentration of 4MUG. Metabolic clearance of 4MU accounted for 92% of total renal clearance. After bolus dosing of preformed 4MUG in the perfusion reservoir of the filtering IPK, the perfusate concentration declined with the terminal half-life of approximately 260 min. The renal excretory clearance of preformed 4MUG accounted for 96% of total renal clearance. 4MU was extensively metabolized by glucuronidation in the filtering and nonfiltering IPK, and the total renal clearance of 4MU was far greater than its renal excretory clearance. This indicated that glucuronidation was the major elimination pathway for 4MU in the kidney. CONCLUSIONS: The data confirmed an important role for the kidney in the metabolic clearance of xenobiotics via glucuronidation and signalled the lack of impact of impaired glomerular filtration on renal drug metabolism.

Reliability and validity of indices of hand-grip strength and endurance.

BACKGROUND/AIM: Grip strength is useful in clinical practice for the assessment of disease and/or rehabilitation progression. Brief maximal gripping is seldom required in everyday occupations, with repeated or sustained gripping at sub-maximal power more commonly involved. It has been proposed that assessment of both maximal hand-grip force and endurance is utilised. While the suitability of maximal contraction measures has been clearly established, the reliability and validity of other hand-grip indices have not been investigated. This study examined the reliability of various hand-grip indices and their validity in relation to distance walked during the six-minute walk test, a standardised exercise capacity test. METHODS: Subjects undertook static sub-maximal (50%) and maximal force contraction hand-grip testing from which various indices were derived, and six-minute walk testing from which distance walked was determined. Testing was repeated on three separate occasions for determination of test-retest reliability. RESULTS: Pre- and post-fatigue maximal contraction measurements demonstrated excellent test-retest reliability and validity. Conversely, other hand-grip indices were shown to be unreliable and exhibited no relationship with distance walked and hence concurrent validity could not be established. CONCLUSIONS: Based on the results of this study, it is recommended that pre- and post-fatigue maximal contraction may be utilised for the assessment of client ability and progression due to their established validity and test-retest reliability. However, previously proposed measures of fatigue such as endurance (duration of sustained contraction), Strength Decrement Index and work performed (function of endurance and force of contraction) are unreliable and invalid and may have limited use in clinical practice.

Inhibition of morphine metabolism by ketamine.

Clinical observation of a synergistic effect of ketamine on morphine analgesia remains controversial. Although a pharmacodynamic basis for an interaction has been explored in animal and clinical studies, the possibility of a pharmacokinetic mechanism has not been investigated. Whereas both morphine and morphine-6-glucuronide are effective analgesics, morphine-3-glucuronide (M3G) lacks activity. Thus, changes in the metabolism and disposition of morphine may result in an altered response. First, we investigated the interaction between morphine and ketamine in the isolated perfused rat liver preparation. The clearance of morphine was decreased from 16.8 +/- 4.6 ml/min in the control period to 7.7 +/- 2.8 ml/min in the ketamine-treatment period, with the formation clearance of M3G decreasing from 8.0 +/- 4.1 ml/min to 2.1 +/- 1.1 ml/min. Fractional conversion of morphine to M3G was significantly decreased from 0.46 +/- 0.17 in the control period to 0.28 +/- 0.14 upon the addition of ketamine. The possible mechanism of the interaction was further investigated in vitro with rat liver microsomes as the enzyme source. The formation of M3G followed single-enzyme Michaelis-Menten kinetics, with a mean apparent K(m) of 2.18 +/- 0.45 mM and V(max) of 8.67 +/- 0.59 nmol/min/mg. Ketamine inhibited morphine 3-glucuronidation noncompetitively, with a mean K(i) value of 33.3 +/- 7.9 microM. The results demonstrate that ketamine inhibits the glucuronidation of morphine in a rat model.

Endogenous plasma carnitine pool composition and response to erythropoietin treatment in chronic haemodialysis patients.

BACKGROUND: Anaemia is a common complication associated with haemodialysis and is usually managed by treatment with recombinant human erythropoietin (rHuEPO). However, many patients remain hyporesponsive to rHuEPO treatment despite adequate iron therapy. The effect of L-carnitine administration on rHuEPO dose and/or haematocrit in haemodialysis patients has been previously reported with equivocal results. This study examined the relationship between endogenous carnitine pool composition and rHuEPO requirements in long-term haemodialysis patients. METHODS: Pre-dialysis blood samples were collected from 87 patients and analysed for plasma L-carnitine and individual acylcarnitine levels by LCMS/MS. As an indication of rHuEPO responsiveness, erythropoietin resistance index (ERI) was calculated as rHuEPO dose/kg/week normalized for haemoglobin levels. RESULTS: A significant negative correlation between L-carnitine levels and ERI was found (P = 0.0421). All patients categorized as high ERI (>0.02 microg/kg/week/gHb) exhibited subnormal L-carnitine levels (<30 microM); conversely, patients with normal L-carnitine levels (>30 microM) displayed low ERI values (<0.02 microg/kg/week/gHb). More importantly, the ratio of non-acetyl acylcarnitines/total carnitine was significantly positively correlated with ERI (P = 0.0062). CONCLUSIONS: These data illustrate the relationship between carnitine levels and response to rHuEPO treatment in haemodialysis patients, in particular, the importance of the proportion of long-chain acylcarnitines within the plasma carnitine pool. This proportion may be more indicative of the response to L-carnitine supplementation than absolute L-carnitine levels alone.

Determination of the reference range of endogenous plasma carnitines in healthy adults.

BACKGROUND: l-carnitine is an endogenous substance, vital in the transport of fatty acids across the inner mitochondrial membrane for oxidation. Disturbances in carnitine homeostasis can have a significant impact on human health; therefore, it is critical to define normal endogenous concentrations for l-carnitine and its esters to facilitate the diagnosis of carnitine deficiency disorders. This study was conducted to determine the normal concentrations of a number of carnitines in healthy adults using three analytical methods. The impact of age and gender on carnitine concentrations was also examined. METHODS: Blood samples were collected from 60 healthy subjects of both genders and various ages. Plasma samples were analysed for endogenous carnitine concentrations by radioenzymatic assay, high-performance liquid chromatography and electrospray tandem mass spectrometry. RESULTS: Precision and accuracy of results obtained for each assay were within acceptable limits. Average endogenous concentrations obtained from the three analytical methods in this study were in the range of 38-44, 6-7 and 49-50 mumol/L for l-carnitine, acetyl-l-carnitine and total carnitine, respectively. Comparison of results between the genders indicated that males had significantly higher endogenous plasma l-carnitine and total carnitine concentrations than females. Age was found to have no impact on plasma carnitine concentrations. CONCLUSION: These results are useful in the evaluation of biochemical or metabolic disturbances and in the diagnosis and treatment of patients with carnitine deficiency.

L-carnitine supplementation in the dialysis population: are Australian patients missing out?

It has been widely established that patients with end-stage renal disease undergoing chronic haemodialysis therapy exhibit low endogenous levels of L-carnitine and elevated acylcarnitine levels; however, the clinical implication of this altered carnitine profile is not as clear. It has been suggested that these disturbances in carnitine homeostasis may be associated with a number of clinical problems common in this patient population, including erythropoietin-resistant anaemia, cardiac dysfunction, and dialytic complications such as hypotension, cramps and fatigue. In January 2003, the Centers for Medicare and Medicaid Services (USA) implemented coverage of intravenous L-carnitine for the treatment of erythropoietin-resistant anaemia and/or intradialytic hypotension in patients with low endogenous L-carnitine concentrations. It has been estimated that in the period of 1998-2003, 3.8-7.2% of all haemodialysis patients in the USA received at least one dose of L-carnitine, with 2.7-5.2% of patients receiving at least 3 months of supplementation for one or both of these conditions. The use of L-carnitine within Australia is virtually non-existent, which leads us to the question: Are Australian haemodialysis patients missing out? This review examines the previous research associated with L-carnitine administration to chronic dialysis patients for the treatment of anaemia, cardiac dysfunction, dyslipidaemia and/or dialytic symptoms, and discusses whether supplementation is warranted within the Australian setting.

Glucuronidation of mycophenolic acid by Wistar and Mrp2-deficient TR- rat liver microsomes.

In humans, mycophenolic acid (MPA) is metabolized primarily by glucuronidation in the liver to mycophenolate ether glucuronide (MPAGe) and mycophenolate acyl glucuronide (MPAGa). We have previously reported that in perfused livers of TR(-) rats (lacking the Mrp2 transporter), the clearance and hepatic extraction ratio of MPA were significantly lower compared with control Wistar rats, suggesting a difference in the capacity of the TR(-) rats to metabolize MPA in situ. There is very little information regarding the phase II metabolic capabilities of TR(-) rats; therefore, the aim of this study was to investigate the in vitro glucuronidation of MPA in Wistar and TR(-) rat liver microsomal protein. A second aim was to determine whether MPAGa, cyclosporine (CsA), and/or its metabolites AM1, AM1c, and AM9 inhibit the metabolism of MPA to MPAGe in rat liver microsomes. MPAGe formation rates by Wistar and TR(-) microsomes were 0.48 and 0.65 nmol/min/mg, respectively (p = 0.33). K(m) values for control and TR(-) microsomes were 0.47 and 0.50 mM, respectively (p = 0.81). The mean (S.E.M.) ratios of MPAGe formation by Wistar rat liver microsomes incubated with 50 microM MPA plus inhibitor versus 50 microM MPA alone were MPAGa 1.2 (0.1), CsA 0.7 (0.1) (p < 0.05), AM1 2.2 (0.3) (p < 0.05), AM1c 1.2 (0.2), and AM9 1.0 (0.2). Our results suggest that lower in situ glucuronidation of MPA in TR(-) rats may be because of inhibition of glucuronidation by endogenous and exogenous compounds that accumulate in the transporter-deficient rat. Whereas CsA inhibits glucuronidation of MPA, its metabolite AM1 enhances MPAGe formation by rat liver microsomes.

A pharmacokinetic model for L-carnitine in patients receiving haemodialysis.

AIMS: Patients requiring chronic haemodialysis may develop a secondary carnitine deficiency through dialytic loss of L-carnitine. A previous report has described the plasma concentrations of L-carnitine in 12 such patients under baseline conditions and after L-carnitine administration (20 mg kg(-1)). A three-compartment pharmacokinetic model was developed to describe these data to make inferences about carnitine supplementation in these patients. METHODS: L-carnitine removal was mediated solely by intermittent haemodialysis, which was incorporated into the model as an experimentally derived dialysis clearance value that was linked to an on-off pulse function. Data were described by a model with a central compartment linked to 'fast'- and 'slow'-equilibrating peripheral compartments. RESULTS: The model adequately described the changing plasma concentrations of endogenous L-carnitine in individual haemodialysis patients. Based on pooled data (mean +/- SD; n = 12), the volume of the central compartment was 10.09 +/- 0.72 l and the transfer rate constants into and out of the slowly equilibrating pool were 0.100 +/- 0.018 h(-1) and 0.00014 +/- 0.00016 h(-1), respectively. The turnover time of L-carnitine in the slow pool (which was assumed to represent muscle) was approximately 300 days. The model was in general agreement with separate data on the measured loss of carnitine from muscle in dialysis patients. CONCLUSIONS: Haemodialysis causes rapid reductions in plasma L-carnitine concentrations with each dialysis session. Plasma concentrations are restored between sessions by redistribution from peripheral compartments. However, during chronic haemodialysis, the ongoing dialytic loss of L-carnitine may lead to a slow depletion of the compound, contributing to a possible secondary deficiency.

The cardioprotective effect of isosteviol on rats with heart ischemia-reperfusion injury.

This study was designed to assess the cardioprotective effect of isosteviol on rats with heart ischemia-reperfusion (IR) injury and to explore the mechanism of action of the compound. Sprague Dawley rats were divided into 8 groups (n=10-12): a sham-operated control and 7 ischemia-reperfusion groups (IR control, 3 isosteviol pre-treated (0.5, 1.0 and 2.0 mg kg(-1)), ligustrazine pre-treated, 5-hydroxydecanoate (5-HD) pre-treated and 5-HD+ isosteviol pre-treated groups). IR was produced by occluding the left coronary artery for 30 min followed by re-opening the artery for 90 min. The compounds under investigation were administered intravenously 10 min prior to occluding the artery. Hemodynamic parameters (+/-dp/dt(max), LVSP, LVDevP, MAP), heart rate, ventricular tachycardia (VT) and ventricular fibrillation (VF) were determined during the IR period. The myocardial infarct size, activities of serum lactate dehydrogenase and creatine kinase were determined at the end of the experiment. In the isosteviol pre-treated groups, the hemodynamic parameters were improved and the myocardial infarct size, the activities of serum enzymes, and the incidences of VT and VF were all decreased when compared to the control group. These effects of isosteviol were similar to that of a traditional cardioprotective agent, ligustrazine. The 5-HD+ isosteviol group displayed parameters that were between those in the equivalent isosteviol pre-treated group and the IR control group. In conclusion, damage due to a standard rat heart IR injury was reduced by pretreatment with intravenous isosteviol, and this effect was partly attenuated by a mitochondrial ATP-sensitive potassium channel blocker, 5-HD.