Tissue-plasma Ratios Research Articles

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

In this study, total body clearance (CLt), volume of distribution at steady state (Vss) and plasma concentration–time profiles in humans of model compounds were predicted using chimeric mice with humanized livers.On the basis of assumption that unbound intrinsic clearance (CLUint) per liver weight in chimeric mice was equal to those in humans, CLt were predicted by substituting human liver blood flow and liver weights in well-stirred model. Vss were predicted by Rodgers equation using scaling factors of tissue-plasma concentration ratios (SFKp) in chimeric mice estimated from a difference between the observed and predicted Vss. These physiological approaches showed high prediction accuracy for CLt and Vss values in humans.We compared the predictability of CLt and Vss 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.Simulation of human plasma concentration–time profiles were generally successful with physiologically based pharmacokinetic (PBPK) model incorporating CLUint and SFKp obtained from chimeric mice.Combined application of chimeric mice and PBPK modeling is effective for prediction of human PK in various compounds.

Assessment of lung function in asthma and COPD using hyperpolarized 129Xe chemical shift saturation recovery spectroscopy and dissolved-phase MRI.

Magnetic-resonance spectroscopy and imaging using hyperpolarized xenon-129 show great potential for evaluation of the most important function of the human lung -- gas exchange. In particular, chemical shift saturation recovery (CSSR) xenon-129 spectroscopy provides important physiological information for the lung as a whole by characterizing the dynamic process of gas exchange, while dissolved-phase (DP) xenon-129 imaging captures the time-averaged regional distribution of gas uptake by lung tissue and blood. Herein, we present recent advances in assessing lung function using CSSR spectroscopy and DP imaging in a total of 45 subjects (23 healthy, 13 chronic obstructive pulmonary disease (COPD) and 9 asthma). From CSSR acquisitions, the COPD subjects showed red blood cell to tissue-plasma (RBC-to-TP) ratios below the average for the healthy subjects (p < 0.001), but significantly higher septal wall thicknesses as compared with the healthy subjects (p < 0.005); the RBC-to-TP ratios for the asthmatic subjects fell outside two standard deviations (either higher or lower) from the mean of the healthy subjects, although there was no statistically significant difference for the average ratio of the study group as a whole. Similarly, from the 3D DP imaging acquisitions, we found that all the ratios (TP to gas phase (GP), RBC to GP, RBC to TP) measured in the COPD subjects were lower than those from the healthy subjects (p < 0.05 for all ratios), while these ratios in the asthmatic subjects differed considerably between subjects. Despite having been performed at different lung inflation levels, the RBC-to-TP ratios measured by CSSR and 3D DP imaging were fairly consistent with each other, with a mean difference of 0.037 (ratios from 3D DP imaging larger). In ten subjects the RBC-to-GP ratios obtained from the 3D DP imaging acquisitions were also highly correlated with their diffusing capacity of the lung for carbon monoxide per unit alveolar volume ratios measured by pulmonary function testing (R = 0.91).

Correlation of Tissue-Plasma Partition Coefficients Between Normal Tissues and Subcutaneous Xenografts of Human Tumor Cell Lines in Mouse as a Prediction Tool of Drug Penetration in Tumors

Understanding drug distribution and accumulation in tumors would be informative in the assessment of efficacy in targeted therapy; however, existing methods for predicting tissue drug distribution focus on normal tissues and do not incorporate tumors. The main objective of this study was to describe the relationships between tissue-plasma concentration ratios (Kp ) of normal tissues and those of subcutaneous xenograft tumors under nonsteady-state conditions, and establish regression equations that could potentially be used for the prediction of drug levels in several human tumor xenografts in mouse, based solely on a Kp value determined in a normal tissue (e.g., muscle). A dataset of 17 compounds was collected from the literature and from Genentech. Tissue and plasma concentration data in mouse were obtained following oral gavage or intraperitoneal administration. Linear regression analyses were performed between Kp values in several normal tissues (muscle, lung, liver, or brain) and those in human tumor xenografts (CL6, EBC-1, HT-29, PC3, U-87, MCF-7-neo-Her2, or BT474M1.1). The tissue-plasma ratios in normal tissues reasonably correlated with the tumor-plasma ratios in CL6, EBC-1, HT-29, U-87, BT474M1.1, and MCF-7-neo-Her2 xenografts (r(2) in the range 0.62-1) but not with the PC3 xenograft. In general, muscle and lung exhibited the strongest correlation with tumor xenografts, followed by liver. Regression coefficients from brain were low, except between brain and the glioblastoma U-87 xenograft (r(2) in the range 0.62-0.94). Furthermore, reasonably strong correlations were observed between muscle and lung and between muscle and liver (r(2) in the range 0.67-0.96). The slopes of the regressions differed depending on the class of drug (strong vs. weak base) and type of tissue (brain vs. other tissues and tumors). Overall, this study will contribute to our understanding of tissue-plasma partition coefficients for tumors and facilitate the use of physiologically based pharmacokinetics (PBPK) modeling for chemotherapy in oncology studies. © 2013 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 102:1355-1369, 2013.

Estimating Blood and Brain Concentrations and Blood-to-Brain Influx by Magnetic Resonance Imaging with Step-Down Infusion of Gd-DTPA in Focal Transient Cerebral Ischemia and Confirmation by Quantitative Autoradiography with Gd-[14C]DTPA

An intravenous step-down infusion procedure that maintained a constant gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) blood concentration and magnetic resonance imaging (MRI) were used to localize and quantify the blood-brain barrier (BBB) opening in a rat model of transient cerebral ischemia (n=7). Blood-to-brain influx rate constant (K(i)) values of Gd-DTPA from such regions were estimated using MRI-Patlak plots and compared with the K(i) values of Gd-[(14)C]DTPA, determined minutes later in the same rats with an identical step-down infusion, quantitative autoradiography (QAR), and single-time equation. The normalized plasma concentration-time integrals were identical for Gd-DTPA and Gd-[(14)C]DTPA, indicating that the MRI protocol yielded reliable estimates of plasma Gd-DTPA levels. In six rats with a BBB opening, 14 spatially similar regions of extravascular Gd-DTPA enhancement and Gd-[(14)C]DTPA leakage, including one very small area, were observed. The terminal tissue-plasma ratios of Gd-[(14)C]DTPA tended to be slightly higher than those of Gd-DTPA in these regions, but the differences were not significant. The MRI-derived K(i) values for Gd-DTPA closely agreed and correlated well with those obtained for Gd-[(14)C]DTPA. In summary, MRI estimates of Gd-DTPA concentration in the plasma and brain and the influx rate are quantitatively and spatially accurate with step-down infusions.

Influence of ammonium chloride on the tissue distribution of anticholinergic drugs in rats.

Ammonium chloride (NH4Cl) increases lysosomal pH and thereby abolishes intralysosomal accumulation of drugs. Its effect on the tissue distribution of biperiden and trihexyphenidyl in rats has been investigated. The tissue-plasma concentration ratios (Kp) of these drugs in various tissues were determined by infusion studies at steady-state in the presence or absence of NH4Cl. Treatment with NH4Cl reduced the Kp values for both drugs, causing the largest reduction in Kp in the lung (52.1 to 11.8 for biperiden and 59.5 to 18.9 for trihexyphenidyl; ratios of decrease 0.77 and 0.68, respectively), followed by the heart and kidneys, with relatively small reductions in the brain, gut, muscle and fat. Subcellular fractionation studies in the lung indicated that the subcellular fraction-plasma concentration ratio of each drug at the steady state (K(p,sf)) was reduced by treatment with NH4Cl, with the largest decrease in the post-nuclear fraction (ratio of decrease 0.82 for biperiden and 0.74 for trihexyphenidyl), followed by the nucleus, microsomes and supernatant, in that order. A strong correlation was found between the ratio of decrease in K(p,sf) after NH4Cl treatment and the specific activity of acid phosphatases, a marker of lysosomes, in each fraction (biperiden, r = 0.948; trihexyphenidyl, r = 0.945). These results suggest that acidic organelles contribute significantly to the distribution kinetics of anticholinergic drugs.

Altered tissue distribution and elimination of amikacin encapsulated in unilamellar, low-clearance liposomes (MiKasome).

Amikacin in small unilamellar liposomes (MiKasome) has prolonged plasma residence (half-life > 24hr) and sustained efficacy in Gram-negative infection models. Since low-clearance liposomes may be subject to a lower rate of phagocytic uptake, we hypothesized this formulation may enhance amikacin distribution to tissues outside the mononuclear phagocyte system. Rats received one intravenous dose (50 mg/kg) of conventional or liposomal amikacin. Amikacin was measured for ten days in plasma, twelve tissues, urine and bile. Liposomal amikacin increased and prolonged drug exposure in all tissues. Tissue half-lives (63-465 hr) exceeded the plasma half-life (24.5 hr). Peak levels occurred within 4 hours in some tissues, but were delayed 1-3 days in spleen, liver, lungs and duodenum, demonstrating the importance of characterizing the entire tissue concentration vs. time profile for liposomal drugs. Predicted steady-state tissue concentrations for twice weekly dosing were >100 microg/g. Less than half the liposomal amikacin was recovered in tissues and excreta, suggesting metabolism occurred. Amikacin was not detected in plasma ultrafiltrates. Tissue-plasma partition coefficients (0.2-0.8 in most tissues) estimated from tissue-plasma ratios at Tmax were similar to those estimated from tissue AUCs. Low-clearance liposomal amikacin increased and prolonged drug residence in all tissues compared to conventional amikacin. The long tissue half-lives suggest liposomal amikacin is sequestered within tissues, and that an extended dosing interval is appropriate for chronic or prophylactic therapy with this formulation.

Selectivity of boron carriers for boron neutron capture therapy: pharmacological studies with borocaptate sodium, l-boronophenylalanine and boric acid in murine tumors

Borocaptate sodium (BSH) and l-boronophenylalanine (L-BPA) are two boron carriers used for boron neutron capture therapy (BNCT) in the treatment of glioblastoma and melanoma, respectively. The suitability of these two compounds was evaluated on the basis of pharmacokinetic studies aiming at characterizing their biodistribution, tumor uptake and tumor selectivity. Boric acid was also used as a reference compound since it is nonselective and relatively freely diffusible. The compounds were investigated in two tumor models, a B16 pigmented melanoma and the RIF1 sarcoma. Mice were sacrificed after different boron doses at various post-injection times and tissue and plasma levels measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The proposed minimum effective tumor boron concentration of 15 ppm. was achieved in both tumor models for the three compounds tested, although only for l-BPA in the melanoma was this achieved when tumor-plasma ratios were above 1. In the RIF1 model, maximum tumor concentrations of 44 and 31 ppm B were reached after administration of 50 μg B/g body weight for boric acid and BSH, respectively. After administration of 12.5 μg B/g of l-BPA, maximum concentrations of 15 and 21 ppm were found in the RIF1 and B16 models, respectively. Tumor-plasma ratios (TPR) for BSH remained close to or below unity at all times studied in both tumors. Brain levels of BSH were very low, however, leading to tumor-brain ratios markedly greater than 1 at all times. l-BPA and boric acid showed TPR values above unity in both tumor models, reaching 3.2 in B16. Tissue-plasma ratios after l-BPA reached maxima of 1.3 and 3.1 for brain and muscle, in contrast to the low values for BSH. In conclusion, the study showed only modest selectivity for tumor type for the boron compounds tested, although l-BPA showed significant selectivity for the melanoma and would appear to better than BSH for this tumor type. Although tumor-plasma ratios were lower for BSH than l-BPA, the very low normal brain concentrations with BSH would favor this compound for tumors such as glioblastoma. Data on intracellular distributions, both for tumor and relevant normal tissue subpopulations, will be needed to supplement gross tissue boron data for accurate assessment of potential for BNCT.

Prediction of drug distribution in vivo on the basis of in vitro binding data.

For 11 drugs it was investigated whether tissue distribution in vivo can be predicted by use of binding data obtained in vitro. The selection of drugs represented a broad spectrum of physicochemical and pharmacokinetic properties thought to be important for distribution of drugs in vivo. The extent of binding to plasma and to tissue-homogenates of rabbits was determined in vitro. The drug concentrations in plasma, liver, lungs, kidneys, and skeletal muscle of rabbits were determined in vivo after i.v. administration of the drug. The tissue-plasma partition ratios measured in vivo were compared with the theoretical tissue-plasma partition ratios calculated from the in vitro binding data. For all drugs investigated the muscle-plasma partition ratio could be reasonably well predicted by the in vitro binding data. In liver, lungs, and kidneys good agreement was found between measured and predicted tissue-plasma ratio for anionic drugs; marked differences, however, were observed between measured and predicted tissue-plasma ratios of lipophilic cationic drugs. A significant correlation was found between binding of drugs to muscle tissue in vitro and the volume of distribution of the unbound drug (Vf), opening the possibility to approximate Vf from in vitro binding studies with rabbit muscle tissue.

Pharmacokinetics of phenobarbital and propylhexedrine after administration of barbexaclone in the mouse.

The antiepileptic barbexaclone is the salt of the base propylhexedrine (indirect sympathomimetic) and phenylethylbituric acid. After i.v. and oral administration of 66 mg/kg barbexaclone to mice the time course of propylhexedrine and phenobarbital concentrations was studied in plasma, brain, lung, liver, kidney, spleen, heart, and skeletal muscle. Furthermore, the kinetics of phenobarbital were studied after treatment with an equimolar dose of phenobarbital-Na (40 mg/kg). In contrast to the i.v. bolus of phenobarbital-Na, barbexaclone was non-lethal only when infused over a period of 3 min. After the i.v. administration of either salt, phenobarbital plasma levels declined monoexponentially with a half-life of 7.5 h; the volume of distribution was 0.78 l/kg. After oral application absorption of phenobarbital was complete with both salts, though it was delayed after barbexaclone. The latter was the result of a delayed gastro-intestinal passage. Brain uptake of phenobarbital was a slow process, equilibrium with plasma concentrations being reached only 30 min after injection. Propylhexedrine reduced phenobarbital concentrations in brain as evident from steady state tissue-plasma ratios. This was observed after i.v. as well as after oral application. After i.v. application of barbexaclone the following pharmacokinetic parameters for propylhexedrine were determined: t0.5 alpha 0.31 h, t0.5 beta 2.5 h, Vd beta 19.3 l/kg; bioavailability (AUC oral/AUC i.v.) 0.37. Propylhexedrine penetrated the blood-brain barrier rapidly. High but unequal tissue accumulation was observed: lung = kidney greater than liver = brain greater than spleen greater than heart greater than skeletal muscle.

Toxicity and distribution of lidocaine in nonasphyxiated and asphyxiated baboon fetuses.

The dosage and blood concentration of lidocaine required to produce central nervous and cardiovascular system toxicity in both nonasphyxiated and asphyxiated fetuses were determined in ten pregnant baboons with fetuses of average gestation of 158 days (term, 185 days). Lidocaine was infused into the fetal jugular vein until cardiac arrest occurred, following which fetal brain, heart, lungs liver and kidneys were obtained. Mean dosage and blood concentration of lidocaine associated with seizures were 9.4 mg/kg and 15.2 micrograms/ml, respectively, in the nonasphyxiated fetuses, and 3.9 mg/kg and 5.6 micrograms/ml, respectively, in the asphyxiated ones. The dosage and blood concentration of the drug required to produce cardiac arrest were significantly higher in the nonasphyxiated group (35 mg/kg and 269 micrograms/ml, respectively) compared to the sphyxiated group (9 mg/kg and 40 micrograms/ml, respectively). Tissue-plasma ratios of lidocaine were significantly higher (P less than 0.05) in the brain, heart, and liver of the asphyxiated fetuses as compared with the nonasphyxiated ones. The relative proportion of the injected dose found in the organs was also higher (P less than 0.05) in the asphyxiated group. These results indicate that the increased sensitivity of the asphyxiated fetus to lidocaine may be related in part to a greater uptake of local anesthetics by the fetal organs.

Differential effects of digoxin at comparable concentrations in tissues of fetal and adult sheep.

Electrocardiogram (ECG) electrodes and carotid arterial and superior vena caval (SVC) catheters were placed in eight nonpregnant ewes and 11 fetuses (109-129 days gestation) to measure heart rate, arterial presssure, P-R interval, left ventricular pre-ejection period (PEP), and left ventricular ejection time (LVET), before and after digoxin infusion into the SVC. After the ewes were killed, the steady state concentration of digoxin in plasma was related to the concentration in midbrain and left ventricular free wall. Although concentrations of digoxin in tissue differed between fetuses and ewes, tissue-plasma ratios were similar; the myocardial-plasma ratio was 87 for fetuses and 90 for ewes and the midbrain-plasma ratios were 6.4 and 5.3, respectively. In spite of these similarities, physiological and toxic effects differed at comparable plasma concentrations. Reduction in PEP/LVET ratio was greater in ewes than fetuses, and P-R interval prolongation was linearly related to digoxin concentration in fetuses but uncommon at plasma concentrations below 2 ng/ml in ewes. Arrhythmias occurred in six ewes, but in only one fetus, even though the mean steady state concentration of digoxin in plasma was 4.5 ng/ml in the fetuses and 2.3 ng/ml in the ewes. Atropine had little effect on digoxin-induced P-R interrval prolongation, and isoproterenol produced no tachyarrhythmias in the fetuses. Age-related differences in inotropic and arrhythmogenic effects of digoxin exist exist and are related to differences in drug response rather than drug kinetics; this provides experimental support for the different dosage responses.

Pharmacokinetics of Mercaptopurine

The anatomical distribution of mercaptopurine was investigated in rats at dose levels of 2.5 and 25 mg/kg iv. The plasma and tissues were analyzed by radioisotopic dilution and spectrofluorometric techniques. The tissue-plasma ratios were: liver–plasma, ≃4.0; kidney–plasma, ≃2.4; spleen–plasma, ≃1.7; muscle–plasma, ≃1.4; gut lumen- plasma, ≃3; and bone marrow-plasma, ≃0.35. Physiologically based pharmacokinetic models were developed to simulate concentrations of mercaptopurine in plasma, kidneys, liver, muscle, spleen, bone marrow, and gut lumen. The agreement between experimental and predicted plasma and tissue profiles was good. Human plasma levels of mercaptopurine were predicted and, when compared with clinical data, demonstrated reasonable agreement.