New hepatocellular diffusion model for analysis of hepatobiliary transport processes of drugs

Spinnokinetic Analyses of Blood Disposition and Biliary Excretion of Nitric Oxide (NO)-Fe(II)-N-(Dithiocarboxy)sarcosine Complex in Rats: BCM-ESR and BEM-ESR Studies

Nitric oxide (NO) is well known to have a wide variety of biological and physiological functions in animals. On the basis of the fact that Fe(II)-dithiocarbamates react with NO, a Fe(II)-N-(dithiocarboxy)sarcosine complex (Fe(II)-DTCS) was proposed as a trapping agent for endogenous NO. However, quantitative pharmacokinetic investigation for NO-Fe(II)-dithiocarbamate complexes in experimental animals has been quite limited. This paper describes the results on the quantitative pharmacokinetic features of a NO-Fe(II)-N-DTCS in both the blood and bile of rats following intravenous (i.v.) administration of the complex. For this purpose, we applied two in vivo methods, i.e. (1) in vivo blood circulation monitoring-electron spin resonance (BCM-ESR) which previously developed, and (2) in vivo biliary excretion monitoring-electron spin resonance (BEM-ESR). We monitored real-time ESR signals due to nitrosyl-iron species in the circulating blood and bile flow. The ESR signal due to NO-Fe(II)-DTCS was stable in biological systems such as the fresh blood and bile. In in vivo BCM- and BEM-ESR, the pharmacokinetic parameters were calculated on the basis of the two-compartment and hepatobiliary transport models. The studies also revealed that the compound is widely distributed in the peripheral organs and partially excreted into the bile. We named a kinetic method to follow spin concentrations as spinnokinetics and this method will be useful for detecting and quantifying the endogenously generated NO in Fe(II)-DTCS administered animals.

Quantitative evaluation of capacity-limited hepatobiliary transport based on hepatocellular diffusion model by MULTI(FEM)

The dose-dependency of hepatic uptake and hepatobiliary transport of a drug was evaluated by means of a nonlinear least square program incorporating the finite element method, MULTI(FEM). A perfusion experiment using isolated rat livers following a pulse input (i.e., under nonsteady-state conditions) was performed at three dose levels of cefpiramide as a model drug. The hepatic extraction ratio (E(H)) of cefpiramide decreased with an increase in dose, which demonstrates that the hepatic uptake is capacity-limited. The outflow time-profiles from the liver were represented by a two-compartment dispersion model with central Michaelis-Menten elimination, and the maximal elimination rate per central compartment volume (Vmax) and the Michaelis constant (Km) were estimated to be 1420 microg/ml/min and 235 microg/ml, respectively. The biliary mean transit time (t(bile)) increased slightly with an increase in dose. The hepatocellular diffusion model under non-steady-state conditions considering nonlinear transport across the bile canalicular membrane was adopted to evaluate dose-dependency in the biliary excretion of cefpiramide. The maximal penetration velocity across the bile canalicular membrane per liver (V=(bcm)max) and the affinity constant of penetration across the bile canalicular membrane (k(bcm)m = K(bcm)m A(H)L) were estimated to be 40.1 microg/min and 123 microg, respectively. Considering that the volume of a rat liver (A(H)L) is approximately 10 ml, the Michaelis constant of penetration (K(bcm)m), which is an apparent parameter, was estimated to be approximately 12.3 microg/ml. In conclusion, MULTI(FEM) is useful for evaluation of capacity-limited local disposition.

Dose-dependency in local disposition of 5-fluorouracil under non-steady-state condition in rat liver

Dose-dependency in local disposition of 5-fluorouracil (5-FU) was evaluated using the rat liver in the once-through perfusion system under the non-steady-state condition. A curve-fitting program based on Finite Element Method, MULTI(FEM), was adopted to evaluate the capacity-limited elimination in the hepatic local disposition of 5-FU, which is described by a nonlinear dispersion model with a pulse input. The dose of 5-FU was increased from 10.4 to 208 microg for rat livers at five dose levels. Two nonlinear models, two-compartment dispersion models with Michaelis-Menten elimination from the central compartment or from the peripheral compartment, were employed for comparison. The hepatic recovery ratio, F(H), increased from 13.1% to 68.3% and the mean transit time, t(H), increased from 5.1 to 13.1 s with an increase in the dose. It was found that the hepatic local disposition of 5-FU was well represented by the peripheral elimination model. V(max) and K(m) in the model with peripheral Michaelis-Menten elimination were estimated to be 9.04 mg/h and 2.88 mg/L, respectively. V(max) estimated by the present investigation in situ was about seven times greater than that in vitro, whereas K(m) in situ was close to that in vitro. This result suggests that the elimination activity by the hepatocytes in vitro may be lowered, compared with that in situ.

Pharmacokinetic curve fitting using numerical inverse Laplace transformation

The combination of the nonlinear regression program ADAPT II with Talbot's method of numerical Laplace transformation, that allows parameter estimation when the model function is given only in the Laplace domain, is described and successfully applied to pharmacokinetic problems. The accuracy and precision of the method has been found satisfactory; its performance is comparable to that achieved in parameter estimation based on functions defined in the time domain.

Pharmacokinetics in organs and the intact body: model validation and reduction

Physiological pharmacokinetic models are based on the structure of the circulatory system reflecting the convective transport of drug by blood flow to the various organs and tissues. Distribution kinetics at the organ level is mostly simplified as transfer between well-stirred compartments neglecting a priori the effects of intravascular dispersion and diffusion within tissue parenchyma. Recirculatory models based on residence time theory overcome these structural limitations since they allow in a most general way the decomposition of the body into its natural subsystems. Because of the unidentifiability of the global multi-organ model on the basis of plasma concentration-time curves the following methods/experimental designs will be discussed which provide quantitative information regarding the subsystems under in vivo conditions: (i) determination of tissue concentration-time profiles (destructive sampling), (ii) estimation of the organ transit time density from input/output profiles and (iii) application of a recirculatory model with reduced complexity to clinical pharmacokinetic data.

Tissue distribution kinetics as determinant of transit time dispersion of drugs in organs: application of a stochastic model to the rat hindlimb

A stochastic theory of drug transport in a random capillary network with permeation across the endothelial barrier is coupled with a model of tissue residence time of drugs assuming radial intratissue diffusion. Axial diffusion is neglected both in tissue as well as in the radially well-mixed vascular phase. The convective transport through the microcirculatory network is characterized by an experimentally determined transit time distribution of a nonpermeating vascular indicator. This information is used to identify three adjustable model parameters characterizing permeation, diffusion, and steady-state distribution into tissue. Predictions are made for the influence of distribution volume, capillary permeability, and tissue diffusion on transit time distributions. The role of convection (through the random capillary network), permeation, and diffusion as determinants of the relative dispersion of organ transit times has been examined. The relationship to previously proposed models of capillary exchange is discussed. Results obtained for lidocaine in the isolated perfused hindleg in rats indicate that although the contribution of intratissue diffusion to the dispersion process is relatively small in quantitative terms, it has a pronounced influence on the shape of the impulse response curve. The theory suggests that the rate of diffusion in muscle tissue is about two orders of magnitude slower than in water.