Muscle capillary permeability for [14C]inulin and [51Cr]EDTA in human forearm

Unusual Distribution Kinetics of Gadoxetate in Healthy Human Subjects Genotyped for OATP1B1: Application of Population Analysis and a Minimal Physiological-Based Pharmacokinetic Model

Gadoxetate (Gd-EOB-DTPA) is a hepatobiliary-specific contrast agent for magnetic resonance imaging. Using a minimal physiological-based pharmacokinetic (PBPK) model, it has been shown for the first time, that the rapid initial decline of plasma concentration after intravenous injection is the result of an uptake into hepatocytes rather than of a distribution into the extravascular extracellular space. About 50% of the steady-state distribution volume is related to hepatic uptake. The hepatic extraction ratio and hepatic clearance estimated based on the liver model as a part of the PBPK model were in accordance with literature data. The same holds for the predicted time course of the amount of gadoxetate in liver parenchyma. In elucidating the impact of OATP1B1 genotype (*1a/*1a and *15/*15) on the pharmacokinetics of gadoxetate, we found that tissue uptake and back-transfer rates were significantly reduced, whereas the hepatic sinusoidal efflux rate was significantly increased in carriers of the *15/*15 haplotype compared with those of the *1a/*1a (wild type). The model is potentially useful for determining hepatic kinetic parameters and distribution properties of drugs.

Circulatory transport and capillary-tissue exchange as determinants of the distribution kinetics of inulin and antipyrine in dog

A pharmacokinetic model was developed to estimate physiologically meaningful parameters of distribution kinetics from plasma concentration-time data. The model is based on simultaneously measured disposition curves of drug and vascular marker. Employing residence time distribution theory, a recirculatory model with two subsystems, the pulmonary and systemic circulation, was constructed. In addition to intravascular mixing, the axially distributed model of the systemic circulation accounts for transcapillary transport of solutes, quantified by permeability-surface area product (PS) and diffusional equilibration time. Parameters of ICG, inulin, and antipyrine were estimated from disposition data obtained in awake dogs under control conditions and during an isoproterenol infusion or moderate hypovolemia. Results suggest that distribution kinetics is (1) governed by extravascular diffusion and (2) its dependency on cardiac output decreases with increasing diffusional resistance. Hemorrhage decreased the effective PS of inulin. In conclusion, this novel mechanistic model effectively described both the permeability-limited distribution of inulin into interstitial fluid and the flow-limited distribution of antipyrine into total body water and might be useful for other drugs.

Transit time dispersion in pulmonary and systemic circulation: effects of cardiac output and solute diffusivity

We present an in vivo method for analyzing the distribution kinetics of physiological markers into their respective distribution volumes utilizing information provided by the relative dispersion of transit times. Arterial concentration-time curves of markers of the vascular space [indocyanine green (ICG)], extracellular fluid (inulin), and total body water (antipyrine) measured in awake dogs under control conditions and during phenylephrine or isoproterenol infusion were analyzed by a recirculatory model to estimate the relative dispersions of transit times across the systemic and pulmonary circulation. The transit time dispersion in the systemic circulation was used to calculate the whole body distribution clearance, and an interpretation is given in terms of a lumped organ model of blood-tissue exchange. As predicted by theory, this relative dispersion increased linearly with cardiac output, with a slope that was inversely related to solute diffusivity. The relative dispersion of the flow-limited indicator antipyrine exceeded that of ICG (as a measure of intravascular mixing) only slightly and was consistent with a diffusional equilibration time in the extravascular space of approximately 10 min, except during phenylephrine infusion, which led to an anomalously high relative dispersion. A change in cardiac output did not alter the heterogeneity of capillary transit times of ICG. The results support the view that the relative dispersions of transit times in the systemic and pulmonary circulation estimated from solute disposition data in vivo are useful measures of whole body distribution kinetics of indicators and endogenous substances. This is the first model that explains the effect of flow and capillary permeability on whole body distribution of solutes without assuming well-mixed compartments.

Phosphate uptake in rat skeletal muscle is reduced during isometric contractions

During contractions, there is a net efflux of phosphate from skeletal muscle, likely because of an elevated intracellular inorganic phosphate (P(i)) concentration. Over time, contracting muscle could incur a substantial phosphate deficit unless P(i) uptake rates were increased during contractions. We used the perfused rat hindquarter preparation to assess [(32)P]P(i) uptake rates in muscles at rest or over a range of energy expenditures during contractions at 0.5, 3, or 5 Hz for 30 min. P(i) uptake rates were reduced during contractions in a pattern that was dependent on contraction frequency and fiber type. In soleus and red gastrocnemius, [(32)P]P(i) uptake rates declined by approximately 25% at 0.5 Hz and 50-60% at 3 and 5 Hz. Uptake rates in white gastrocnemius decreased by 65-75% at all three stimulation frequencies. These reductions in P(i) uptake are not likely confounded by changes in precursor [(32)P]P(i) specific activity in the interstitium. In soleus and red gastrocnemius, declines in P(i) uptake rates were related to energy expenditure over the contraction duration. These data imply that P(i) uptake in skeletal muscle is acutely modulated during contractions and that decreases in P(i) uptake rates, in combination with expected increases in P(i) efflux, exacerbate the net loss of phosphate from the cell. Enhanced uptake of P(i) must subsequently occur because skeletal muscle typically maintains a relatively constant total phosphate pool.

PKQuest: capillary permeability limitation and plasma protein binding - application to human inulin, dicloxacillin and ceftriaxone pharmacokinetics

BACKGROUND

It is generally assumed that the tissue exchange of antibiotics is flow limited (complete equilibration between the capillary and the tissue water). This assumption may not be valid if there is a large amount of plasma protein binding because the effective capillary permeability depends on the product of the intrinsic capillary permeability (PS) and the fraction of solute that is free in the blood (fwB). PKQuest, a new generic physiologically based pharmacokinetic software routine (PBPK), provides a novel approach to modeling capillary permeability in which the only adjustable parameter is the PS of muscle.

METHODS

All the results were obtained by applying PKQuest to previously published human pharmacokinetic data.

RESULTS

The PKQuest analysis suggests that the highly protein bound antibiotics dicloxacillin and ceftriaxone have a significant capillary permeability limitation. The human muscle capillary PS of inulin, dicloxacillin and ceftriaxone was 0.6, 13 and 6 ml/min/100 gm, respectively. The ceftriaxone protein binding is non-linear, saturating at high plasma concentrations. The experimental ceftriaxone data over a wide range of intravenous inputs (0.15 to 3 gms) was well described by PKQuest. PKQuest is the first PBPK that includes both permeability limitation and non-linear binding.

CONCLUSIONS

Because of their high degree of plasma protein binding, dicloxacillin and ceftriaxone appear to have a diffusion limited exchange rate between the blood and tissue and are not flow limited as had been previously assumed. PKQuest and all the examples are freely available at http:\\www.pkquest.com.

Pharmacokinetics and pharmacodynamics of melphalan in isolated limb infusion for recurrent localized limb malignancy

Isolated limb infusion (ILI) is an attractive, less complex alternative to isolated limb perfusion (ILP). It has a lower morbidity in treating localized recurrences and in transit metastases of the limb for tumours such as melanoma, Merkel cell tumour and Kaposi's sarcoma, allowing administration of high concentrations of cytotoxic agent to the affected limb under hypoxic conditions. Melphalan is the preferred cytotoxic agent for the treatment of melanoma by ILP or ILI. We report pharmacokinetic data from 12 patients treated by ILI for tumours of the limb in Brisbane. The kinetics of drug distribution in the limb was calculated using a two-compartment vascular model, where both tissue and infusate act as well-stirred compartments. Analysis of melphalan concentrations in the perfusate during ILI showed good agreement between the values measured and the concentrations predicted by the model. Recirculation and wash-out flow rates, tissue concentrations and the permeability surface area product (PS) were calculated. Correlations between the PS value and the drug concentrations in the perfusate and tissue were supported by the results. These data contribute to a better understanding of the distribution of melphalan during ILI in the limb, and offer the opportunity to optimize the drug regimen for patients undergoing ILI.

Comparison of the permeability surface product (PS) of the blood capillary wall in skeletal muscle tissue of various species and in vitro porous membranes using hydrophilic drugs

The aim of this study was twofold: Firstly, to compare the PS, the permeability surface product or organ clearance, of various hydrophilic molecules in the muscle blood capillary wall of different animal species, including humans and, secondly, to design an in vitro diffusion cell to identify similar PS values using the respective membranes. The flux rates of solutes with a wide range of molecular weights measured across porous membranes (Millipore(R) VM (MVM), and Nadir(R) UFC (NUFC)) was associated with high reproducibility (<+/-10% SD). The findings were as follows: (1) For the first time ever it was demonstrated that the log molecular weight dependency (between 180 and 10,000 Dalton) of the log of the PS product of the animal muscle capillary wall (slope = -0.51 +/- 0. 16, and a regression coefficient r(2) = 0.90) reported in the literature was similar to that observed by various authors in humans (slope = -0.50 +/- 0.25 r(2) = 0.77). (2) PS of the MVM membrane with a surface area of 3.8 cm(2) recorded for the newly developed diffusion cell ranged at approximately 50% below (slope -0.58 +/- 0. 12, r(2) = 0.85) the value observed in the in vivo human experiments after intramuscular injection. No linear relationship was established for the NUFC membrane due to solute exclusion effects of the membrane. (3) The diameter of the inner donor chamber (resembling the interstice between the muscle fiber membrane and the capillary wall membrane) was calculated to be sufficiently small to eliminate the possibility of creating a statistically significant diffusion barrier to the test membranes, which was corroborated by theoretical modeling. The structure of the in vitro diffusion cell prevents any significant contribution from the unstirred water layer (UWL) to overall resistance, thus reflecting in vivo diffusion properties of hydrophilic solutes deposited intramuscularly to enter the systemic circulation. It may be concluded, that MVM membrane in the vitro diffusion cell mimics the in vivo blood capillary PS for hydrophilic solutes of up to 10,000 Da.

Capillary diffusion capacity for Cr-EDTA and cyanocobalamine in spontaneously beating rat hearts

In order to obtain a functional estimate of the diffusional capacity of the myocardial capillary bed, the permeability surface area product (PS) for Cr-EDTA (mol. wt = 341) and cyanocobalamine (vitamin B12, mol. wt = 135) was determined in spontaneously beating Langendorff-perfused rat hearts over a wide range of coronary flow rates (700-3000 ml min-1 100 g-1). PS was determined by a single injection, colorimetric indicator dilution technique, allowing multiple, rapid and accurate determinations to be made in the same heart. During maximal vasodilation with nitroprusside Na PS averaged 535 +/- 33 and 220 +/- 22 ml min-1 100 g-1 for Cr-EDTA and vitamin B12 respectively at the highest flow (2917 +/- 74 ml min-1 100 g-1). The vasculature of the heart was found to be highly heterogeneous, since PS increased with flow and there were marked variations of extraction over transit times. A functional estimate of 'equivalent pore radius' was obtained from the ratio PSCr-EDTA/PSB12, which was 2.61 +/- 0.15 demonstrating a marked restriction to diffusion corresponding to a pore radius of 51 (41-75) A. This value is similar to that from skeletal muscle determined by the same method while PS-values are 40-45 times higher in the heart (Haraldsson & Rippe 1986). Taken together with morphological estimations of capillary surface area and endothelial path depth, these data indicate a 3-fold increase in the density of pores available for diffusion in the myocardium, compared to skeletal muscle.