Analysis of nonlinear tissue distribution of quinidine in rats by physiologically based pharmacokinetics

The nonlinear tissue distribution of quinidine in rats was investigated by a physiologically based pharmacokinetic model. Serum protein binding of quinidine showed a nonlinearity over the in vivo plasma concentration range. The blood-to-plasma concentration ratio (Cb/Cp) of quinidine also showed a concentration dependence. The steady-state volume of distribution (Vss) determined over the plasma concentration range from 0.5 to 10 micrograms/ml was 6.0 +/- 0.45 L/kg. The tissue-to-plasma partition coefficient (Kp) of muscle, skin, liver, lung, and gastrointestinal tract (GI) showed a nonlinearity over the in vivo plasma concentration range of quinidine, suggesting saturable tissue binding. The concentration of quinidine in several tissues and plasma was predicted by a physiologically based pharmacokinetic model using in vitro plasma protein binding and the Cb/Cp of quinidine. The tissue binding parameters were estimated from in vivo Kp values. The predicted concentration curves of quinidine in each tissue and in plasma showed good agreement with the observed values.     

Physiologically Based Pharmacokinetic Models of 2′,3′-Dideoxyinosine

Purpose. The goal of this study was to develop physiologically based pharmacokinetic (PBPK) models for 2,3-dideoxyinosine (ddI) in rats when the drug was administered alone (ddI model) and with pentamidine (ddI + pentamidine model), and to use these models to evaluate the effect of our previously reported pentamidine-ddI interaction on tissue ddI exposure in humans. Methods. The PBPK models consisted of pharmacologically relevant tissues (blood, brain, gut, spleen, pancreas, liver, kidney, lymph nodes, muscle) and used the assumptions of perfusion-rate limited tissue distribution and linear tissue binding of ddI. The required physiologic model parameters were obtained from the literature, whereas the pharmacokinetic parameters and the tissue-to-plasma partition coefficients were calculated using plasma and tissue data. Results. The ddI model in rats yielded model-predicted concentration-time profiles that were in close agreement with the experimentally determined profiles after an intravenous ddI dose (5% deviation in plasma and 20% deviation in tissues). The ddI + pentamidine model incorporated the pentamidine-induced increases of ddI partition in pancreas and muscle. The two PBPK models were scaled-up to humans using human physiologic and pharmacokinetic parameters. A comparison of the model-predicted plasma concentration-time profiles with the observed profiles in AIDS patients who often received ddI with pentamidine showed that the ddI model underestimated the terminal half-life (t_1/2,) by 39% whereas the ddI + pentamidine model yielded identical t_1/2, and area-under-the-curve as the observed values (<1% deviation). Simulations of ddI concentration-time profiles in human tissues using the two models showed that pancreas and lymph nodes received about 2- to 30-fold higher ddI concentration than spleen and brain, and that coadministration of pentamidine increased the AUC of ddl in the pancreas by 20%. Conclusions. Data of the present study indicate that the plasma ddI concentration-time profile in patients were better described by the ddI + pentamidine model than by the ddI model, suggesting that the pentamidine-induced changes in tissue distribution of ddI observed in rats may also occur in humans.     

Pharmacokinetics of quinidine related to plasma protein binding in man

Summary The disposition and plasma protein binding of quinidine after intravenous administration were studied in 13 healthy subjects. Plasma protein binding, expressed as the fraction of quinidine unbound ranged from 0.134–0.303 (mean 0.221). Elimination rate constant () varied from 0.071 to 0.146 h^–1 (mean 0.113), and apparent volume of distribution (V) varied from 1.39–3.20 l · kg^–1 (mean 2.27). Total body clearance was 2.32–6.49 ml min^–1 · kg^–1. There was a positive linear correlation between the plasma fraction of unbound quinidine and both V (r=0.885, p<0.01) and total body clearance (r=0.668, p<0.05). No significant correlation existed between the fraction of unbound quinidine in plasma and the elimination rate constant. The results show that both the apparent volume of distribution and total body clearance of quinidine are proportional to the unbound fraction in plasma. This implies that the total plasma concentration of quinidine at steady state will change with alterations in plasma binding, whilst the concentration of unbound compund and its elimination rate will remain unaffected.     

Prediction of Phase I single-dose pharmacokinetics using recombinant cytochromes P450 and physiologically based modelling

1. Ten compounds from the Merck Research Laboratories pipeline were selected to evaluate the utility of using intrinsic clearance derived from recombinantly expressed cytochromes P450 (CYP) and physiologically based pharmacokinetic modelling to predict Phase I pharmacokinetics using simCYP. The compounds selected were anticipated to be eliminated predominantly by P450 metabolism.

2. There was a reasonable agreement between the predicted and actual clinical exposure with 80% of the predicted exposures being within three-fold of the observed values. Furthermore, prediction of C(t) (plasma concentration at a specified time point) and T_max were acceptable with greater than or equal to 70% of the predicted data being within three-fold of the observed values. However, prediction of C_max was unreliable and may have been due to error in predicting the time-dependent change in volume of distribution and/or error in estimating absorption rate.

3. Although it is acknowledged that research is needed to improve predictive performance, the data presented are supportive of using recombinant P450 intrinsic clearance and physiologically based pharmacokinetic modelling to predict Phase I pharmacokinetics for compounds eliminated by P450 metabolism.

     

Tissue distribution kinetics of tetraethylammonium ion in the rat

Tissue distribution kinetics of tetraethylammonium (TEA) ion in rats were studied following both constant-rate intravenous infusion and rapid intravenous injection of the drug. At a steady-state plasma concentration of 0.2 g/ml, the tissue-to-plasma (T/P) concentration ratio of the kidneys, liver, heart, gut, and lungs exceeded 1,indicating that TEA is localized in these tissues. In vitrotissue homogenate binding and slice uptake experiments provided no evidence of TEA binding to tissue constituents, suggesting that the high T/P concentration gradient is due to an active transport process. The maximum concentration of TEA in all tissues occurred within 5–15 min after rapid injection of a 2-mg dose. Except for the liver, the subsequent decline of TEA concentration in various tissues over a 5-hr period was slow compared to that in plasma. Consequently, the T/P ratio of liver and kidney remained relatively constant, while those of the other tissues increased continually with time. These features of TEA tissue distribution kinetics can be predicted by a physiologically based pharmacokinetic model which incorporates both active and passive transport processes for the passage of TEA between blood and the tissue mass.This work was supported in part by United States Public Health Service Grants No. GM-20852 (M. G. and D. D. S.), GM-15956 (D. D. S.), and RR-07037 (K. J. H.).     

Analysis of Tissue Distribution of Valproate in Guinea-Pigs: Evidence for Its Capacity-Limited Tissue Distribution

The tissue distribution of valproic acid (VPA) was investigated over a wide range of steady-state plasma levels (C_ss) in guinea-pigs. The VPA concentrations in various tissues, except the kidney, were all lower than in plasma. Tissue-to-unbound plasma concentration ratios (K_pu) of VPA for adipose, heart, kidney, liver, lung, muscle, pancreas and skin all decreased significantly with increasing unbound plasma concentration (C_uss). The K_pu for brain (0.5–0.9), intestine, spleen and stomach failed to show significant change with C_uss. The disposition of VPA in tissues is adequately described by a model in which VPA was distributed in interstitial and intracellular fluid and bound to interstitial albumin, with limited tissue binding. Tissue binding was extensive only in the kidney. Most of the measured apparent K_pu values agreed well with simulated K_pu values. Steady-state tissue concentration of VPA can be predicted from C_ss and C_uss when reference data for interstitial albumin and tissue total water are available.     

DOSE-DEPENDENT DISTRIBUTION VOLUMES OF TOTAL AND UNBOUND VALPROATE IN GUINEA-PIGS: CONSEQUENCE OF NON-LINEAR PLASMA PROTEIN BINDING

The response of steady-state distribution volume (V_dss for total and V_dssu for unbound drug) of valproate (VPA) to dose-dependent plasma protein binding was studied in guinea-pigs. Various steady-state plasma concentrations of VPA were achieved by intravenous constant infusion. The concentrations of VPA in plasma (C_ss for total and C_uss for unbound drug) and various tissues (C_T) were determined. The V_dss and the V_dssu were estimated based upon the apparent tissue-to-plasma concentration ratio of VPA. The results showed that the plasma unbound fraction (f_u) of VPA increased significantly with dose. The V_dss was significantly increased with, while the V_dssu was significantly decreased against the increasing dose. The increase in V_dss with dose indicated an increase in tissue-to-plasma concentration ratio, which may be attributed to the increase in distribution of unbound drug from plasma to tissues subsequent to non-linear plasma protein binding. The decrease in V_dssu against the increasing dose indicated a decrease in tissue-to-unbound plasma concentration ratio, which suggests that the extravascular distribution of unbound VPA might be capacity limited and the tissue binding of VPA negligible.     

Comparative physiologically based pharmacokinetics of hexobarbital,phenobarbital, and thiopental in the rat

A physiologically based pharmacokinetic model, which is an extension of the Bischoff-Dedrick multiorgan model, was developed to describe the kinetics of barbiturates (hexobarbital, phenobarbital, and thiopental) in the rat. The model is composed of 11 organ or tissue compartments. The brain compartment was featured as a nonflow-limited organ for some low lipid soluble barbiturates. Michaelis-Menten constants for drug metabolism (K_m, V_max were determined from in vitroexperiments using liver microsomes. Binding of drugs to plasma and tissue proteins was measured in vitrousing an equilibrium dialysis method. Distribution of drugs to red blood cells was measured in vitrowith thiopental exhibiting a concentration dependent distribution. Penetration rates of the barbiturates into the brain were predicted on the basis of their lipid solubilities. A set of mass balance equations included terms for the inflow and outflow of drug carried by the perfusing blood, drug metabolism, protein binding, and penetration rate into the brain as well as blood flow rate and tissue mass. Solution of the system of equations yielded the time courses of drugs in each organ. However, predicted time courses of drugs in plasma and brain were not in good agreement with those observed. Therefore, the tissue to plasma distribution ratios evaluated from in vivoexperiments were substituted for the in vitrovalues, resulting in fairly good agreement between predicted and observed values.     

Neuropharmacokinetics of two investigational compounds in rats: Divergent temporal profiles in the brain and cerebrospinal fluid

Two investigational compounds (FRM-1, (R)-7-fluoro-N-(quinuclidin-3-yl)benzo[b]thiophene-2-carboxamide and FRM-2, (R)-7-cyano-N-(quinuclidin-3-yl)benzo[b]thiophene-2-carboxamide) resided in rat brain longer than in systemic circulation. In Caco-2 directional transport studies, they both showed good intrinsic passive permeability but differed significantly in efflux susceptibility (efflux ratio of <2 and ∼7, respectively), largely attributed to P-glycoprotein (P-gp). Capitalizing on these interesting properties, we investigated how cerebrospinal fluid (CSF) concentration (C_CSF) would be shaped by unbound plasma concentration (C_u,p) and unbound brain concentration (C_u,b) in disequilibrium conditions and at steady state. Following subcutaneous administration, FRM-1C_CSF largely followed C_u,p initially and leveled between C_u,p and C_u,b. However, it gradually approached C_u,b and became lower than, but parallel to C_u,b at the terminal phase. In contrast, FRM-2C_CSF temporal profile mostly paralleled the C_u,p but was at a much lower level. Upon intravenous infusion to steady state, FRM-1C_CSF and C_u,b were similar, accounting for 61% and 69% of the C_u,p, indicating a case of largely passive diffusion-governed brain penetration where C_CSF served as a good surrogate for C_u,b. On the contrary, FRM-2C_CSF and C_u,b were remarkably lower than C_u,p (17% and 8% of C_u,p, respectively), suggesting that FRM-2 brain penetration was severely impaired by P-gp-mediated efflux and C_CSF underestimated this impact. A semi-physiologically based pharmacokinetic (PBPK) model was constructed that adequately described the temporal profiles of the compounds in the plasma, brain and CSF. Our work provided some insight into the relative importance of blood–brain barrier (BBB) and blood–CSF barrier (BCSFB) in modulating C_CSF.     

The pharmacokinetics and organ distribution of ajmaline and quinidine in the mouse

Summary After i.v. infusion into mice (lasting 10 s) the time courses of ajmaline and quinidine concentrations in blood, heart, lung, liver, and brain were studied. The drugs were assayed by a spectrofluorophotometric procedure. Blood concentration data obtained were fitted graphically and calculations were performed in accordance with an open two compartment model.Blood kinetic data were very similar for both alkaloids. A rapid distribution phase with a t _0.5 of 3.0 min for ajmaline and 2.5 min for quinidine was followed by a disposition phase with a t0.5 of 16 min for ajmaline and 20 min for quinidine. High tissue accumulation of both alkaloids was found in lung, liver, and heart and this is also reflected by the volume of distribution V _d, which was 136 ml for ajmaline and 116 ml for quinidine (body weight of the mice=31 g). With equilibrium dialysis a 62% binding of ajmaline and a 77% binding of quinidine to mouse blood constituents was found. Both drugs were highly metabolized since only 5% of a given dose was excreted unchanged in the urine.     

Prediction of human pharmacokinetics of typical compounds by a physiologically based method using chimeric mice with humanized liver

1. In this study, total body clearance (CL_t), volume of distribution at steady state (V_ss) and plasma concentration–time profiles in humans of model compounds were predicted using chimeric mice with humanized livers.

2. On the basis of assumption that unbound intrinsic clearance (CL_Uint) per liver weight in chimeric mice was equal to those in humans, CL_t were predicted by substituting human liver blood flow and liver weights in well-stirred model. V_ss were predicted by Rodgers equation using scaling factors of tissue-plasma concentration ratios (SF_Kp) in chimeric mice estimated from a difference between the observed and predicted V_ss. These physiological approaches showed high prediction accuracy for CL_t and V_ss values in humans.

3. We compared the predictability of CL_t and V_ss determined by the physiologically based predictive approach using chimeric mice with those from predictive methods reported by Pharmaceutical Research Manufacturers of America. The physiological approach using chimeric mice indicated the best prediction accuracy in each predictive method.

4. Simulation of human plasma concentration–time profiles were generally successful with physiologically based pharmacokinetic (PBPK) model incorporating CL_Uint and SF_Kp obtained from chimeric mice.

5. Combined application of chimeric mice and PBPK modeling is effective for prediction of human PK in various compounds.

     

Repeated dose pharmacokinetics of pancopride in human volunteers

The aim of this study was to assess the pharmacokinetic profile of pancopride after repeated oral dose administration of 20 mg pancopride in tablet form once a day for 5 d in 12 healthy male volunteers. Plasma levels were measured by HPLC using a solid phase extraction method and automated injection. The minimum quantification limit of pancopride in plasma was 2 ng mL^?1. The maximum plasma concentration (mean ± SD) after the first dose was 92.5 ± 41.5 ng mL^?1 and t_max was 1.7 ± 0.9 h. The elimination half-life (t_1/2) was 14.3 ± 6.9 h. The area under the concentration-time curve from zero to infinity (AUC) was 997 ± 396 ng h mL^?1. The maximum plasma concentration (mean ± SD) at steady state (day 5) was 101.8 ± 36.9 ng mL^?1 and t_max was 2.2 ± 1.2 h. The elimination half-life (t_1/2) was 16.3 ± 2.7 h and the minimum plasma concentration (C) was 16.6 ± 6.9 ng mL^?1. The area under the concentration-time curve during the dosing interval (AUC) was 995 ± 389 ng h mL^?1. The average plasma concentration at steady state (C) was 43.3 ± 16.1 ng mL^?1 and the experimental accumulation ratio (R_AUC) was 1.34 ± 0.19, whereas the mean theoretical value (R) was 1.40 ± 0.29. The results obtained showed a good correlation between the experimental plasma levels and the expected values calculated using a repeated dose two-compartment model assessed by means of the Akaike value. It is concluded that the pharmacokinetics of pancopride are not modified after repeated dose administration. The safety parameters showed no clinically relevant alterations.     

Effect of serum protein binding on pharmacokinetics and anticoagulant activity of phenprocoumon in rats

The relationship among serum protein binding, kinetics of elimination, distribution, and anticoagulant activity of phenprocoumon was investigated in 25 selected outbred Sprague-Dawley rats which differed in the extent of serum protein binding of this drug. In addition, the serum protein binding of phenprocoumon was altered in inbred Lewis rats by continuous treatment with tolbutamide. This drug was found to displace phenprocoumon from serum proteins without affecting its intrinsic clearance. The serum free fraction values (f_s)of the selected Sprague-Dawley rats ranged from 0.0053 to 0.0145. There were positive and linear correlations between f_s and the first-order elimination rate constant (k), f_s and total clearance (CL _total ),and f_s and the liver/plasma concentration ratio (L/P ratio) of phenprocoumon. The free fraction values in the liver tissue (f _I )showed twofold variations and were not related to f_s.The half-effective plasma concentrations (C _p50% )of total phenprocoumon (i.e., the concentrations necessary to inhibit the prothrombin complex synthesis rate by 50%) decreased with increasing f_s.The C_p50% values of total drug varied eightfold between the animals but those of free drug only 3.5- fold. The total anticoagulant effect per dose (AE/dose), as reflected by the magnitude of the area above the prothrombin complex activity vs. time curve in the plasma, varied only 1.5- fold between the rats and was not related to f_s.Continuous treatment of inbred Lewis rats with tolbutamide led to an increase of f_s (twofold), k (1.3-fold), V_d (1.5-fold), and CL_total (twofold). The intrinsic clearance (CL _intr )remained unaffected. There was no significant increase of f_L but a twofold increase of the L/P ratio. AE/dose and the C_p50% values of free drug in tolbutamide-treated rats were not significantly different from those of control rats. Thus an increase of the free fraction of phenprocoumon in the serum of rats is followed by a proportional increase of the total clearance. This prevents a concomitant rise of the free drug concentration. Consequently, the total anticoagulant effect per dose remains almost unaffected by about threefold variations in the serum free fraction values of this drug.This work was supported by the Deutsche Forschungsgemeinschaft: it is part of the Ph.D. thesis for D. T.     

Phosphatidylserine as a Determinant for the Tissue Distribution of Weakly Basic Drugs in Rats

Interorgan variation in tissue distribution of weakly basic drugs such as quinidine, propranolol, and imipramine was investigated as a function of binding to phosphatidylserine (PhS) in tissues. Tissue distributions of these drugs were determined using 10 different tissues at a steady-state plasma concentration and were expressed as tissue-to-plasma partition coefficients (K _p values). The concentration of PhS in the tissue was determined by two-dimensional thin-layer chromatography. Plotting of K _p values, except for brain, against the tissue PhS concentrations showed a linear relationship, indicating that PhS is a determinant in the interorgan variation of these tissue distributions. Further, differences in tissue distribution among the drugs was considered to be due to the difference in binding potency to PhS. Drug binding parameters to individual standard phospholipid were determined using a hexane-pH 4.0 buffer partition system. Binding was highest to PhS, and a linear relationship was found between the log nK [product of the number of binding sites (n) and the association constant (K) for PhS binding] obtained in vitro and K _p values of drugs in tissues in vivo. The empirically derived equation, K _p = 14.3 × (log nK) × (PhS cone.) – 8.09, was found to predict K _p values in vivo of weakly basic drugs. Thus, a determinant of interorgan variation in the tissue distribution of the weakly basic drugs studied was the tissue concentration of PhS and the drug binding affinity to PhS.     

Effect of mdr1a P-glycoprotein gene disruption, gender, and substrate concentration on brain uptake of selected compounds

Purpose. This study assessed the influence of mdr1a P-glycoprotein (P-gp) gene disruption, gender and concentration on initial brain uptake clearance (Cl _up) of morphine, quinidine and verapamil. Methods. Cl _up of radiolabeled substrates was determined in P-gp-competent and deficient [mdr1a(–/–)] mice by in situ brain perfusion. Brain:plasma distribution of substrates after i.v. administration was determined in both strains. Results. Genetic disruption of mdr1a P-gp resulted in 1.3-, 6.6- and 14-fold increases in Cl _up for morphine, verapamil and quinidine, respectively. With the exception of small differences for verapamil, gender did not affect Cl _up. Saturable transport of verapamil and quinidine was observed only in P-gp-competent mice, with apparent IC ₅₀ values for efflux of 8.6 ± 2.3 M and 36 ± 2 M, respectively. VerapamilCl _up was 50% higher in mdr1a(+/–) vs. mdr1a(+/+) mice; no such difference was observed for quinidine. In P-gp-competent mice, uptake of verapamil and quinidine was unaffected by organic vehicles. Plasma decreased VER Cl _up to a greater extent in the presence of P-gp. The influence of P-gp in situ was lower than, but correlated with, the effect in vivo. Conclusions. P-gp decreases Cl _up of morphine, verapamil and quinidine in situ with little or no influence of gender, but this effect cannot fully account for the effects of P-gp in vivo. P-gp is the only saturable transport mechanism for verapamil and quinidine at the murine blood-brain barrier. The influence of protein binding on Cl _up may be enhanced by P-gp-mediated efflux.     

From piecewise to full physiologic pharmacokinetic modeling: Applied to thiopental disposition in the rat

Physiologically based pharmacokinetic modeling procedures employ anatomical tissue weight, blood flow, and steady tissue/blood partition data, often obtained from different sources, to construct a system of differential equations that predict blood and tissue concentrations. Because the system of equations and the number of variables optimized is considerable, physiologic modeling frequently remains a simulation activity where fits to the data are adjusted by eye rather than with a computer-driven optimization algorithm. We propose a new approach to physiological modeling in which we characterize drug diposition in each tissue separately using constrained numerical deconvolution. This technique takes advantage of the fact that the drug concentration time course, C_T(t), in a given tissue can be described as the convolution of an input function with the unit disposition function (UDF _T) of the drug in the tissue, (i.e., C _T (t)=(C _a (t)Q _r )*UDF _r (t) whereC _a(t) is the arterial concentration,Q _T is the tissue blood flow and * is the convolution operator). The obtained tissue unit disposition function (UDF) for each tissue describes the theoretical disposition of a unit amount of drug injected into the tissue in the absence of recirculation. From theUDF, a parametric model for the intratissue disposition of each tissue can be postulated. Using as input the product of arterial concentration and blood flow, this submodel is fit separately utilizing standard nonlinear regression programs. In a separate step, the entire body is characterized by reassembly of the individuals submodels. Unlike classical physiologic modeling the fit for a given tissue is not dependent on the estimates obtained for other tissues in the model. Additionally, because this method permits examination of individualUDF _s, appropriate submodel selection is driven by relevant information. This paper reports our experience with a piecewise modeling approach for thiopental disposition in the rat. Supported in part by Grant RO1-AG04594 from the National Institute of Aging and the Anesthesia/Pharmacology Research Foundation.     

Significance of Binding to Na,K-ATPase in the Tissue Distribution of Ouabain in Guinea Pigs

Ouabain binds specifically to Na,K-ATPase on the plasma membrane and therefore serves to measure the tissue concentration of Na,K-ATPase. We examined the role of ouabain binding to Na,K-ATPase in its overall tissue distribution. The tissue-to-plasma concentration ratio (K _p,vivo) was defined in each tissue after intravenous administration of ³H-ouabain in guinea pigs, and specific binding of ouabain to Na,K-ATPase was measured in tissue homogenate to obtain the dissociation constant and binding capacity in each tissue. A predicted tissue-to-plasma concentration ratio (K _p,vitro) was calculated using the obtained binding parameters and the volume of extracellular space in each tissue. The absolute values of K _p,vitro were comparable to those of K _{p vivo}, except in brain. Regression analysis showed that the specific binding capacity of Na,K-ATPase in each tissue is the main factor in the tissue variation of K _p,vivo. Therefore, the binding of ouabain to Na,K-ATPase plays a significant role in the tissue distribution of ouabain.     

Perioperative penetration of metronidazole into muscle tissue: a microdialysis study

Objective To evaluate the concentration of metronidazole in muscle tissue using microdialysis and to compare it with plasma concentration and in vitro-defined MIC₉₀ (minimal inhibiting concentration) for the most frequent anaerobic bacteria isolated in our hospital.Materials and methods Six female patients scheduled for elective gynaecological surgery were included. Exclusion criteria were active inflammatory process and being overweight (BMI more than 30). Microdialysis catheters (CMA 60 catheters with 20 kDa cut-off membrane) were placed into the m. vastus lateralis. The microdialysis perfusion rate was 2 l/min. To assess in vivo recovery of the drug, retrodialysis with a 5-mg/l solution of metronidazole was performed. Microdialysis and blood samples were collected simultaneously 10 h after metronidazole administration. MIC₉₀ data were obtained from the database of the microbiology laboratory of the local hospital.Results Data from five patients were included in analysis. The metronidazole concentration in blood achieved a value of 16.5±4.6 mg/l at 30 min (first available data), while in muscle a maximum level of 7.8±1.5 mg/l was achieved at 114 min. The mean MIC₉₀ for the Bacteroides fragilis group was 0.25±0.26 mg/l. Data from mean plasma concentrations were fitted into the two-compartmental model and time over MIC₉₀ and time over four times MIC₉₀ were calculated, which were 52.1±13.5 h and 33.2±8.7 h, respectively. The C_max/MIC₉₀ ratio was 65.8±18.5 for plasma and 31.1±6.2 for muscle.Conclusion The present data demonstrate that metronidazole penetrates well into muscle tissue. Muscle tissue concentrations reach values far greater than MIC₉₀ for the Bacteroides fragilis group and persist at such high levels for at least 10 h.     

Pharmacokinetic analysis of sparse in vivo NMR spectroscopy data using relative parameters and the population approach

NMR spectroscopy in vivo when applied to studying drugs and their metabolites usually measures relative concentration in a tissue over time. Only ratios of clearance and volume parameters can be estimated from these data. Low drug dosages (relative to the sensitivity of in vivo NMR) or rapid drug elimination create the additional problem of data sparsity where a pharmacokinetic model cannot be fitted individually. We have investigated whether relative and absolute pharmacokinetic parameters can be estimated from such data by applying a population model.The data analysed were relative concentractions of 5-fluorouracil (FU) and of the sum of its catabolits -fluoro--ureido-propanoic acid (FUPA) and -fluoro--alanine (FBAL) in te liver, as monitored in 16 cancer patients by [¹⁹F]-NMP spectroscopy during and after a 10-min intravenous infusion of 650 mg FU·m^–2. The structural part of the population model was a non-linear, two-compartment model featuring one FU compartment with volume V _FU , a saturable clearance of FU by conversion into the catabolites where CL=v _max /(k _M +C _FU ), a catabolite compartment with volume V _cat , and a concentration-independent clearance of the catabolites, CL _cat . The parameters actually fitted were: , v _max , k _M ·V _FU , V _cat /V _FU , and CL _cat /V _cat where is a proportionality factor relating the NMR signal intensity of FU to the amount of FU in the body and, therefore, has no purely pharmacokinetic interpretation. All parameters were checked for random interindividual variation; and v _max were also tested for inter-occasion variation. The program system NONMEM was used for model fitting.The estimated mean population parameters were: v _max =121 mol·min^–1, k _M ·V _FU =2590 mol, V _cat /V _FU =0.0648, CL _cat /V _cat =0.0555·min^–1. The proportionality factor was found to depend on body weight and, in addition, to have an inter-occasion random variation (within patients, between examinations). No other random variation of a kinetic parameter could be identified. The estimated v _max is similar to a reported estimate of 2.02 mol·min^–1·kg^–1 derived from FU plasma kinetics.This study shows that sparse relative concentration data can be analysed by using relative parameters in a population model. Only one parameter has no unequivocal pharmacokinetic meaning due to the lack of absolute concentration information. Any contribution of the measuring procedure to the inter-occasion variation of in vivo NMP spectroscopy measurements should be minimized in order to allow the detection of possible inter-individual variances of the pharmacokinetic parameters.Dedicated to Prof. Dr. Rudolf Preussmann on the occasion of his 65th birthday     

Rapid determination of isocorydine in rat plasma and tissues using liquid chromatography – tandem mass spectrometry and its applications to pharmacokinetics and tissue distribution

1. A rapid and sensitive method for the determination of isocorydine in rat plasma and tissues was developed using liquid chromatography–tandem mass spectrometry (LC–MS/MS).

2. The biological samples were processed by extracting with diethyl ether–dichloromethane (3:2, v/v) and tetrahydropulmatine was used as the internal standard (IS). Detection of the analytes was achieved using positive ion mode electrospray ionization in the multiple reaction monitoring mode. The MS/MS ion transitions monitored were m/z 342.0→279.0 and 356.0→191.9 for isocorydine and IS, respectively.

3. The maximum plasma concentration (C_max 2496.8?±?374.4 µg/L) was achieved at 0.278?±?0.113?h (T_max) and the half-life (t_1/2) of isocorydine was 0.906?±?0.222?h after a 20?mg/kg oral administration. As for a 2?mg/kg intravenous (i.v.) administration, the C_max and clearance (CL) were 1843.3?±?338.3 µg/L and 2.381?±?0.356?L/h/kg, respectively. Based on the AUC_0–∞ obtained from oral and i.v. administration, the absolute bioavailability (F) was estimated as 33.4%. Tissue distribution results indicated that isocorydine underwent a rapid and wide distribution into tissues and it could effectively cross the blood-brain barrier.