Studies supporting the development of a physiologically based pharmacokinetic (PBPK) model for methyl iodide: pharmacokinetics of sodium iodide (NaI) in pregnant rabbits

Methyl iodide (MeI) is a water soluble monohalomethane that is metabolized in vivo to release iodide (I^?). A physiologically based pharmacokinetic (PBPK) model exists for iodide in adult rats, pregnant rats and fetuses, and lactating rats and neonates, but not for pregnant rabbits and fetuses, which have been used extensively for toxicity testing with MeI. Thus, this study was conducted to determine the blood and tissue distribution kinetics of radioiodide in pregnant rabbits and fetuses. Timed-pregnant New Zealand White rabbits received a single intravenous injection of the sodium salt of iodine-131 (Na¹³¹I) at either a high (10?mg/kg body weight) or low (0.75?mg/kg body weight) dose on gestation day 25. At various intervals ranging from 0.5 to 24?h post- injection, blood and tissues (thyroid, stomach contents, and skin) were collected from each doe, and blood, stomach contents, thyroid, trachea, and amniotic fluid were collected from a random sampling of three fetuses per doe per time point. Radioiodide accumulated as expected in the thyroid of maternal animals, where concentrations were the highest of any maternal tissues measured in both dose groups. Radioiodide also accumulated in fetal blood and tissues; levels were consistently higher than maternal levels and, unlike maternal tissues, showed no evidence of clearance over the 24-h sampling period. In contrast to observations in the maternal animals, fetal stomach contents showed the highest accumulation of radioiodide for both dose groups by 1–2?h after dosing, followed by the trachea and thyroid tissues, with the lowest concentrations of radioiodide in the amniotic fluid and blood. There was no evidence for preferential accumulation of radioiodide in fetal thyroid tissues.     

Microdialysis in peripheral tissues

The objective of this review is to survey the recent literature regarding the applications of microdialysis in pharmacokinetic studies and facilitating many other studies in peripheral tissues such as muscle, subcutaneous adipose tissue, heart, lung, etc. It has been reported extensively that microdialysis is a useful technique for monitoring free concentrations of compounds in extracellular fluid (ECF), and it is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses the use of microdialysis technique for ECF sampling in peripheral tissues in animal studies. The second part of the review describes the use of microdialysis for ECF sampling in peripheral tissues in human studies. Microdialysis has been applied extensively to measure both endogenous and exogenous compounds in ECF. Of particular benefit is the fact that microdialysis measures the unbound concentrations in the peripheral tissue fluid which have been shown to be responsible for the pharmacological effects. With the increasing number of applications of microdialysis, it is obvious that this method will have an important place in studying drug pharmacokinetics and pharmacodynamics.     

Physiologically based pharmacokinetics of radioiodinated human beta-endorphin in rats. An application of the capillary membrane-limited model

In order to simulate the distribution and elimination of radioiodinated human beta-endorphin (125I-beta-EP) after iv bolus injection in rats, we proposed a physiologically based pharmacokinetic model incorporating diffusional transport of 125I-beta-EP across the capillary membrane. This model assumes that the distribution of 125I-beta-EP is restricted only within the blood and the tissue interstitial fluid, and that a diffusional barrier across the capillary membrane exists in each tissue except the liver. The tissue-to-blood partition coefficients were estimated from the ratios of the concentration in tissues to that in arterial plasma at the terminal (pseudoequilibrium) phase. The total body plasma clearance (9.0 ml/min/kg) was appropriately assigned to the liver and kidney. The transcapillary diffusion clearances of 125I-beta-EP were also estimated and shown to correlate linearly with that of inulin in several tissues. Numerically solving the mass-balance differential equations as to plasma and each tissue simultaneously, simulated concentration curves of 125I-beta-EP corresponded well with the observed data. It was suggested by the simulation that the initial rapid disappearance of 125I-beta-EP from plasma after iv injection could be attributed in part to the transcapillary diffusion of the peptide.     

Tissue distribution of YM758, a novel If channel inhibitor,in pregnant and lactating rats

In this study the tissue distribution of radioactivity in pregnant and lactating rats was investigated by quantitatively determining radioactivity concentrations and by whole-body autoradioluminograms after a single oral administration of ¹⁴C-YM758. In addition, the transfer of radioactivity into the reproductive tissues, foetus, and milk is discussed in terms of the localization of transporters in syncytiotrophoblast and mammary gland. The radioactivity concentrations in the liver were the highest of all the tissues and organs tested at all the sampling times. The radioactivity in main tissues (liver and kidney), including reproductive tissues (amniotic fluid, placenta, ovary, and uterus), was not retained for a long time, as in the plasma. The tissue/plasma (T/P) ratio of radioactivity in the foetus was below 1.0, which might be due to Mdr1-mediated export of YM758 into blood via the blood–placenta barrier since YM758 is a substrate for hMDR1, not for hBCRP/rBcrp. The T/P ratio of radioactivity in the maternal milk 1 and 4 h after oral administration of ¹⁴C-YM758 was 7.2 and 11.0, respectively. To understand better the distribution of new drugs into the reproductive tissues/milk, and to interpret further the results of reproductive safety studies for drug development, the contribution of transporters expressed in the blood–placenta barrier and mammary gland to the drug-transfer into placenta and milk should be considered.     

Application of microdialysis to characterize drug disposition in tumors

Microdialysis is an in vivo sampling technique that was initially developed to measure endogenous substances in the field of neurotransmitter research. In the past decade, microdialysis has been increasingly applied to study the pharmacokinetics and drug metabolism in the blood and various tissues of both animals and humans. This paper describes the general aspects of this in vivo sampling technique followed by the survey of the recent papers regarding the application of microdialysis to characterize anticancer drug disposition in solid tumors. It can be concluded that microdialysis is a very suitable method to obtain drug concentration-time profiles in the interstitial fluid of solid tumors as well as of other variety of tissues.     

Tissue distribution of YM758, a novel If channel inhibitor, in pregnant and lactating rats

In this study the tissue distribution of radioactivity in pregnant and lactating rats was investigated by quantitatively determining radioactivity concentrations and by whole-body autoradioluminograms after a single oral administration of 14C-YM758. In addition, the transfer of radioactivity into the reproductive tissues, foetus, and milk is discussed in terms of the localization of transporters in syncytiotrophoblast and mammary gland. The radioactivity concentrations in the liver were the highest of all the tissues and organs tested at all the sampling times. The radioactivity in main tissues (liver and kidney), including reproductive tissues (amniotic fluid, placenta, ovary, and uterus), was not retained for a long time, as in the plasma. The tissue/plasma (T/P) ratio of radioactivity in the foetus was below 1.0, which might be due to Mdr1-mediated export of YM758 into blood via the blood-placenta barrier since YM758 is a substrate for hMDR1, not for hBCRP/rBcrp. The T/P ratio of radioactivity in the maternal milk 1 and 4 h after oral administration of 14C-YM758 was 7.2 and 11.0, respectively. To understand better the distribution of new drugs into the reproductive tissues/milk, and to interpret further the results of reproductive safety studies for drug development, the contribution of transporters expressed in the blood-placenta barrier and mammary gland to the drug-transfer into placenta and milk should be considered.     

Control of alcohol addiction by SKV therapy--its action on water, food intake, brain function and cell membrane composition

Alcohol being easily permeable through cell membrane causes toxic damage to many tissues. Rats drinking aqueous ethanol (25% v/v) for 120 days and 240 days showed an initial rise in body weight. The reduced rate in weight gain in chronic alcoholism is associated with a fall in food intake. Ethanol ingesting animals showed slow response to stimuli and increase in blood ethanol and serum GGTP levels. Liver plasma membrane, kidney brush-border membrane and pancreatic plasma membrane from alcoholic rats showed significant alterations in cholesterol/phospholipid molar ratio and membrane ATPases. Water retention with the enlargement of liver and kidney associated with increased fluid consumption are also seen during alcoholism. SKV by breaking alcohol dependence reduces drinking, lowers blood ethanol level and fluid intake without developing withdrawal symptoms. Restriction of ethanol intake by SKV therapy resulted in the reversal of organ enlargement and membrane composition in alcoholics.     

Human subcutaneous tissue distribution of fluconazole: comparison of microdialysis and suction blister techniques

AIMS: To investigate uptake of fluconazole into the interstitial fluid of human subcutaneous tissue using the microdialysis and suction blister techniques. METHODS: A sterile microdialysis probe (CMA/60) was inserted subcutaneously into the upper arm of five healthy volunteers following an overnight fast. Blisters were induced on the lower arm using gentle suction prior to ingestion of a single oral dose of fluconazole (200 mg). Microdialysate, blister fluid and blood were sampled over 8 h. Fluconazole concentrations were determined in each sample using a validated HPLC assay. In vivo recovery of fluconazole from the microdialysis probe was determined in each subject by perfusing the probe with fluconazole solution at the end of the 8 h sampling period. Individual in vivo recovery was used to calculate fluconazole concentrations in subcutaneous interstitial fluid. A physiologically based pharmacokinetic (PBPK) model was used to predict fluconazole concentrations in human subcutaneous interstitial fluid. RESULTS: There was a lag-time (approximately 0.5 h) between detection of fluconazole in microdialysate compared with plasma in each subject. The in vivo recovery of fluconazole from the microdialysis probe ranged from 57.0 to 67.2%. The subcutaneous interstitial fluid concentrations obtained by microdialysis were very similar to the unbound concentrations of fluconazole in plasma with maximum concentration of 4.29 +/- 1.19 microg ml(-1) in subcutaneous interstitial fluid and 3.58 +/- 0.14 microg ml(-1) in plasma. Subcutaneous interstitial fluid-to-plasma partition coefficient (Kp) of fluconazole was 1.16 +/- 0.22 (95% CI 0.96, 1.35). By contrast, fluconazole concentrations in blister fluid were significantly lower (P < 0.05, paired t-test) than unbound plasma concentrations over the first 3 h and maximum concentrations in blister fluid had not been achieved at the end of the sampling period. There was good agreement between fluconazole concentrations derived from microdialysis sampling and those estimated using a blood flow-limited PBPK model. CONCLUSIONS: Microdialysis and suction blister techniques did not yield comparable results. It appears that microdialysis is a more appropriate technique for studying the rate of uptake of fluconazole into subcutaneous tissue. PBPK model simulation suggested that the distribution of fluconazole into subcutaneous interstitial fluid is dependent on tissue blood flow.     

Distribution study of 3,4-methylenedioxymethamphetamine and 3,4-methylenedioxyamphetamine in a fatal overdose

In this study, regional tissue distributions of the amphetamine analogue 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") and its metabolite 3,4-methylenedioxyamphetamine (MDA) in a fatal overdose are presented. Quantitation of MDMA and MDA levels occurred in blood samples taken centrally (right and left heart and main adjacent great vessels) and peripherally (subclavian and femoral blood). In addition, MDMA and MDA concentrations were determined in cardiac and iliopsoas muscle, both lungs, liver, both kidneys, spleen, the four brain lobes, cerebellum and brainstem, and adipose tissue. Finally, MDMA and MDA levels were determined in serum, vitreous humor, urine, and bile. For all samples, a fully validated high-pressure liquid chromatography procedure with fluorescence detection was used. The found substances were also identified with liquid chromatography-tandem mass spectrometry. Our data confirm that blood sampling from an isolated peripheral vein is recommended for MDMA and MDA. In addition, the vitreous humor MDMA level indicates that this fluid can be an interesting alternative when a suitable blood sample is missing. Considering the substantial differences in concentrations in blood samples taken from various sites in the body and the high levels in some tissues (e.g., in liver), we concluded that the influence of postmortem redistribution should be taken into account in the interpretation of toxicological data when an appropriate peripheral sample cannot be obtained or when blood samples are not available because of putrefaction.     

In vivo and in vitro microdialysis sampling of free fatty acids

Microdialysis is a technique that allows continuous sampling of compounds from the interstitial fluid of different tissues with minimal influence on surrounding tissues and/or whole body function. In the present study, the feasibility of using microdialysis as a technique to sample free fatty acids (FFA) was investigated both in vitro and in vivo, by use of a high molecular weight (MW) cut-off membrane (3 MDa) and a push–pull system to avoid loss of perfusion fluid through the dialysis membrane. The relative recovery was examined in vitro for three different concentrations of radiolabelled oleic acid-BSA solutions (oleic acid:BSA molar ratio 1:1) and for various temperatures and flow rates. The recovery of oleic acid was found to be dependent on the concentration of analyte in the medium surrounding the membrane (17.3%, 29.0% and 30.6% for 50, 100 and 200 μM oleic acid-BSA solutions, respectively). Addition of 0.25% BSA to the perfusion fluid resulted, however, in a concentration-independent recovery of 31.4%, 28.1% and 28.1% for the 50, 100 and 200 μM solutions, respectively.

The capability of the method to measure FFA together with glycerol was investigated in vivo in visceral adipose tissue of rats, before and after lipolytic treatment with the β₃-adrenergic agent, BRL37344. BRL37344 caused an increase in both dialysate FFA and glycerol, although the increase was markedly higher for glycerol, amounting to 24.5% and 329.2% increase from baseline, respectively. Subsequent in vitro test of probe performance revealed a decrease in the dialysing properties with regard to FFA, but not glycerol. This suggests that clogging of the membrane pores after 110 min prevented the measurement of the full FFA response in vivo.     

A novel pharmacokinetic method for analysis of placental transfer of latamoxef in humans

A novel pharmacokinetic method was developed for analysing the behaviour of a drug in tissues. The absolute transfer ratio of a drug to a tissue was defined using the pharmacokinetic parameters obtained by this method. Composite data of latamoxef (moxalactam) concentration in maternal blood, umbilical cord blood and amniotic fluid following a 2g intravenous injection to pregnant women at delivery were analysed by this method to study the drug behaviour in pregnant women, fetuses and amniotic fluid. Latamoxef kinetics in pregnant women at full term were generally similar to that in previously reported healthy subjects. The concentration of latamoxef in umbilical cord blood peaked about 2 hours after dosing then decreased in parallel with the maternal blood concentration. The amniotic fluid concentration peaked about 7 hours after administration, then decreased slowly. The absolute transfer ratios to fetus and amniotic fluid were calculated to be about 2.5 and 0.37% respectively.     

Effect of plasma cholesterol on red blood cell oxygen transport

1. Oxygen (O2) transfer from the blood to tissues is a function of the red blood cell (RBC) O2 saturation (SO2), the plasma O2 content being negligible. Under conditions of increased tissue O2 demand, the SO2 of arterial blood does not change appreciably (97%); however, the SO2 of mixed venous blood, equal to that of the perfused tissues, can go as low as 20%. 2. Tissue O2 availability is limited by the exposure time to a RBC, which decreases under conditions of maximum stress (< 1 s). If the O2 unloading time was to increase significantly, because of a decrease in the RBC diffusion constant or an increase in the RBC membrane thickness, the RBC O2 unloading time would exceed tissue (e.g. cardiac) transit time and O2 transfer would be impaired. 3. Cholesterol constitutes the non-polar, hydrophobic lipid of the enveloping layer of the RBC membrane. As the cholesterol content of the RBC increases, the fluidity of the membrane decreases and the lipid shell stiffens. 4. Early studies demonstrated that high blood cholesterol concentrations were associated with reduced blood O2 transport; in essence, the haemoglobin dissociation curve was shifted to the left. 5. Current investigations have shown that the cholesterol RBC membrane barrier to O2 diffusion delayed O2 entry into the RBC during saturation and delayed O2 release from the RBC during desaturation. In an analysis of 93 patients divided by their cholesterol concentration into five groups, the percentage change in blood O2 diffusion was inversely proportional to the cholesterol concentration. 6. The RBC membrane cholesterol is in equilibrium with the plasma cholesterol concentration. It stands to reason that as the plasma cholesterol increases, the RBC membrane becomes impaired and O2 transport is reduced. 7. The implications of this new perspective on O2 transport include the ability to increase tissue oxygenation by lowering plasma cholesterol.     

Microdialysis in neurointensive care

Microdialysis is a technique for sampling the chemistry of the interstitial fluid of tissues and organs in animal and man. It is minimally invasive and simple to perform in a clinical setting. Although microdialysis samples essentially all small molecular substances present in the interstitial fluid the use of microdialysis in neurointensive care has focused on markers of ischemia and cell damage. The lactate/pyruvate ratio is a well-known marker of changes in the redox state of cells caused by ischemia Glycerol is an integral component of cell membranes. Loss of energy due to ischemia eventually leads to an influx of calcium and a decomposition of cell membranes, which liberates glycerol into the interstitial fluid. Thus the lactate/pyruvate ratio and glycerol have become the most important markers of ischemia and cell membrane damage. While the primary insult at the site of the accident is beyond our control, secondary insults during intensive care should be avoided by all means. Therefore, the single most important finding from microdialysis studies is the dramatic difference in the vulnerability of the penumbra surrounding a lesion as compared to normal brain tissue allowing early detection of secondary insults after traumatic brain injury as well as the onset of vasospasm after subarachnoid hemorrhage.     

Measurement of the endogenous adenosine concentration in humans in vivo: methodological considerations

The endogenous nucleoside adenosine has profound tissue protective effects in situations of ischaemia or inflammation. Animal studies have shown that various drugs can activate this protective mechanism by interfering with the metabolism of adenosine. Translation of this concept to the clinical arena is hampered by the difficulties encountered in measuring the adenosine concentration, due to the rapid cellular uptake and degradation of adenosine, which continues unabated after blood sampling, and due to the metabolically active endothelial barrier for adenosine. In the current paper, we critically discuss the various methods to measure the adenosine concentration in humans in vivo. For the measurement of circulating adenosine, we conclude that the use of a pharmacological blocker solution (containing inhibitors of the enzymes ecto-5'-nucleotidase, adenosine deaminase, and adenosine kinase, and of the equilibrative nucleoside transporter) and a purpose-built syringe which mixes the blood with this solution immediately at the tip of the needle, seems to be the most sensitive technique. However, for the measurement of adenosine concentrations in interstitial tissue, microdialysis is a suitable method, when used with an appropriate method to determine the recovery of adenosine across the semipermeable membrane to calculate the absolute adenosine concentration. Consistent use of these methods could help in the comparison of the various studies focussed on endogenous adenosine and could help to facilitate the use of drugs that modulate the adenosine concentration to protect tissues in the clinical arena.     

FAILURE OF THE NITROUS OXIDE TISSUE EQUILIBRATION METHOD FOR THE DETERMINATION OF BRAIN AND MYOCARDIAL BLOOD FLOW UNDER CONTROLLED CONDITIONS

1. Two adult merino ewes were prepared with intravascular cannule for sampling aortic root blood, sagittal sinus blood and coronary sinus blood. 2. One week after preparation the animals were anaesthetized then ventilated with a gas mixture containing 10% nitrous oxide (N2O) for 60 min. Serial measurements of brain and myocardial blood flow were made using the N2O tissue equilibration method of Kety and Schmidt. 3. N2O failed to achieve matching arteriovenous blood concentration equality and saturation of the relevant tissues. Valid use of the Kety-Schmidt method, therefore, could not be confirmed. 4. Because of the failure of the arteriovenous equilibration, serially determined brain and myocardial blood flows were found to decrease with time. 5. The use of this method in circumstances where tissue saturation with the indicator gas cannot be ascertained is arbitrary.     

Application of Microdialysis in Pharmacokinetic Studies

The objective of this review is to survey the recent literature regarding the various applications of microdialysis in pharmacokinetics. Microdialysis is a relatively new technique for sampling tissue extracellular fluid that is gaining popularity in pharmacokinetic and pharmacodynamic studies, both in experimental animals and humans. The first part of this review discusses various aspects of the technique with regard to its use in pharmacokinetic studies, such as: quantitation of the microdialysis probe relative recovery, interfacing the sampling technique with analytical instrumentation, and consideration of repeated procedures using the microdialysis probe. The remainder of the review is devoted to a survey of the recent literature concerning pharmacokinetic studies that apply the microdialysis sampling technique. While the majority of the pharmacokinetic studies that have utilized microdialysis have been done in the central nervous system, a growing number of applications are being found in a variety of peripheral tissue types, e.g. skin, muscle, adipose, eye, lung, liver, and blood, and these are considered as well. Given the rising interest in this technique, and the ongoing attempts to adapt it to pharmacokinetic studies, it is clear that microdialysis sampling will have an important place in studying drug disposition and metabolism.     

Potential error in the measurement of tissue to blood distribution coefficients in physiological pharmacokinetic modeling. Residual tissue blood. I. Theoretical considerations.

Physiological pharmacokinetic models require the determination of tissue to blood distribution coefficients. A theoretical model has been developed and the resulting equations indicate that under certain conditions it is necessary to correct for the presence of drug in the residual blood remaining in the tissue. The potential error in ignoring this residual blood is expressed mathematically in terms of several important factors that include the anatomical features of the tissue (volume fractions of the blood, interstitial fluid, and cellular space) as well as the physicochemical properties of the drug (extent of binding in the blood and tissues). These theoretical considerations and resulting simulations have been applied to experimental literature data for several compounds (methotrexate, digoxin, and biperiden). We conclude that correction for the residual blood is necessary when the values of tissue to blood distribution coefficients are very small or large (relative to one) and when the volume fraction of the blood in tissue is substantial.     

Pharmacokinetic model for salicyclate in cerebrospinal fluid, blood, organs, and tissues

The developed pharmacokinetic model, an extension of the Bischoff--Dedrick model, simultaneously predicts the kinetic behavior of salicylate in cerebrospinal fluid, blood, organs, and tissues. The model, which is entirely different from conventional compartment models, is derived from basic considerations of drug distribution with biochemical and physiological meaning. The dog was studied at three different dosages of salicylate: therapeutic, moderate intoxication, and severe intoxication. The predicted kinetics of salicylate in cerebrospinal fluid, blood, plasma, liver, muscle, and adipose tissue by the model agreed well with the experimental data. The effectiveness of hemoperfusion treatment for the severely intoxicated dog by albumin-coated activated carbon and its effect on the kinetic behavior of salicylate in cerebrospinal fluid, blood, organs and tissues were studied. The model was also applied to predict the kinetic changes of salicylate in the body during and after the extracorporeal treatment. The predicted results also agreed with the experimental data.     

Studies on the mechanism of drug distribution in tissues]

The importance to elucidate the mechanisms of drug distribution in tissues has been discussed in this review. By considering several factors such as tissue binding, plasma binding, membrane permeability and tissue interstitial fluid space, the tissue distribution states of drug have been characterized based on the biochemical and pharmacokinetic studies. These physiological pharmacokinetic analyses have been applied to evaluate the determinant factors of inter-organ, drug to drug, interspecies or age related changes of tissue distribution of drug. The mechanisms of membrane permeability of drug which could play the most important role to regulate the drug distribution have been discussed. The drug distribution to the brain is strictly restricted by the function of the blood-brain barrier (BBB). The drug transport mechanisms at the BBB have been discussed. Especially, the role of carrier-mediated and absorptive-mediated transport systems at the BBB has been focused on.     

Tissue targeted metabonomics: metabolic profiling by microdialysis sampling and microcoil NMR

The concentration of low molecular weight compounds in tissues can yield valuable information about the metabolic state of an organism. Studies of changes in the metabolic state or metabonomics can reflect disease pathways, drug action, or toxicity. This research aims to develop a new approach, tissue targeted metabonomics. Microdialysis sampling and microcoil NMR analysis are employed to compare basal and ischemic metabolic states of various tissues (blood, brain, and heart) of Sprague–Dawley rats. Microdialysis sampling is localized, making the metabolic profile tissue specific. Coupling to NMR analysis is highly advantageous, because a complete metabolic profile is obtained in a single spectrum. However, small sample volumes and low analyte concentrations make analysis of microdialysis samples challenging. Microcoil NMR uses low sample volumes and has improved mass sensitivity, relative to standard 5 mm probes. The coupling of these techniques is a potentially powerful tool for metabonomics analysis.