Effect of plasma protein binding on kinetics of capillary uptake and efflux

A Complete Extension of Classical Hepatic Clearance Models Using Fractional Distribution Parameter fd in Physiologically Based Pharmacokinetics

The classical organ clearance models have been proposed to relate the plasma clearance CLp to probable mechanism(s) of hepatic clearance. However, the classical models assume the intrinsic capability of drug elimination (CLu,int) that is physically segregated from the vascular blood but directly acts upon the unbound drug concentration in the blood (fubCavg), and do not handle the transit-time delay between the inlet/outlet concentrations in their closed-form clearance equations. Therefore, we propose unified model structures that can address the internal blood concentration patterns of clearance organs in a more mechanistic/physiological manner, based on the fractional distribution parameter fd operative in PBPK. The basic partial/ordinary differential equations for four classical models are revisited/modified to yield a more complete set of extended clearance models, i.e., the Rattle, Sieve, Tube, and Jar models, which are the counterparts of the dispersion, series-compartment, parallel-tube, and well-stirred models. We demonstrate the feasibility of applying the resulting extended models to isolated perfused rat liver data for 11 compounds and an example dataset for in vitro-in vivo extrapolation of the intrinsic to the systemic clearances. Based on their feasibilities to handle such real data, these models may serve as an improved basis for applying clearance models in the future.

Determination of the Number of Tissue Groups of Kinetically Distinct Transit Time in Whole-Body Physiologically Based Pharmacokinetic (PBPK) Models I: Theoretical Consideration of Bottom-Up Approach of Lumping Tissues in Whole-Body PBPK

Minimal physiologically based pharmacokinetic (mPBPK) models, consisting of system-specific (e.g., tissue volume and blood flow) and drug-related (e.g., tissue-to-plasma partition coefficient) parameters, are practically useful for pharmacokinetic analyses. However, biopharmaceutical principles were not clear on how peripheral tissues, adopted in whole-body physiologically based pharmacokinetic (WB-PBPK) models, could be kinetically consolidated into one or two tissue groups in the mPBPK models. In this theoretical examination, we studied the relationship between the progressive tissue lumping in the direction from the longest mean transit time (MTT_max) to the shorter one(s) and the slopes of the terminal (λ_ter)/distributional phases, assuming tissues with comparable MTTs are kinetically combined. The appropriateness of lumping was ascertained by evaluating the impact of difference in tissue MTTs during the lumping on the analytical solution of WB-PBPK models. We found that the ratio of MTT_max to the mean residence time in the body, viz., K_det, is related to the change in λ_ter by the progressive lumping and can serve as an index for the robustness of λ_ter. Calculations with two extreme cases revealed that, for caffeine at K_det < 0.03, the change in λ_ter was minimal even when all peripheral tissues were collectively lumped, whereas for artesunic acid at K_det > 50, the tissue of MTT_max could not be kinetically combined even with the tissue having the second-longest MTT without significantly affecting λ_ter. Therefore, we proposed K_det as an index for the robustness of λ_ter during tissue lumping and for the number of tissue groups with distinct transit times in WB-PBPK.

Consideration of Fractional Distribution Parameter fd in the Chen and Gross Method for Tissue-to-Plasma Partition Coefficients: Comparison of Several Methods

PURPOSE

The tissue-to-plasma partition coefficient (K_p) describes the extent of tissue distribution in physiologically-based pharmacokinetic (PBPK) models. Constant-rate infusion studies are common for experimental determination of the steady-state K_p,ss, while the tissue-plasma concentration ratio (C_T/C_p) in the terminal phase after intravenous doses is often utilized. The Chen and Gross (C&G) method converts a terminal slope C_T/C_p to K_p,ss based on assumptions of perfusion-limited distribution in tissue-plasma equilibration. However, considering blood flow (Q_T) and apparent tissue permeability (f_upPS_in) in the rate of tissue distribution, this report extends the C&G method by utilizing a fractional distribution parameter (f_d).

METHODS

Relevant PBPK equations for non-eliminating and eliminating organs along with lung and liver were derived for the conversion of C_T/C_p values to K_p,ss. The relationships were demonstrated in rats with measured C_T/C_p and K_p,ss values and the model-dependent f_d for 8 compounds with a range of permeability coefficients. Several methods of assessing K_p were compared.

RESULTS

Utilizing f_d in an extended C&G method, our estimations of K_p,ss from C_T/C_p were improved, particularly for lower permeability compounds. However, four in silico methods for estimating K_p performed poorly across tissues in comparison with measured K_p values. Mathematical relationships between K_p and K_p,ss that are generally applicable for eliminating organs with tissue permeability limitations necessitates inclusion of an extraction ratio (ER) and f_d.

CONCLUSION

Since many different types/sources of K_p are present in the literature and used in PBPK models, these perspectives and equations should provide better insights in measuring and interpreting K_p values in PBPK.

Selectivity requirements for diagnostic imaging of neurofibrillary lesions in Alzheimer's disease: a simulation study

Whole-brain imaging is a promising strategy for premortem detection of tau-bearing neurofibrillary lesions that accumulate in Alzheimer's disease. However, the approach is complicated by the high concentrations of potentially confounding binding sites presented by beta-amyloid plaques. To predict the contributions of relative binding affinity and binding site density to the imaging-dynamics and selectivity of a hypothetical tau-directed radiotracer, a nonlinear, four-tissue compartment pharmacokinetic model of diffusion-mediated radiotracer uptake and distribution was developed. Initial estimates of nonspecific binding and brain uptake parameters were made by fitting data from a previously published kinetic study of Pittsburgh Compound B, an established amyloid-directed radiotracer. The resulting estimates were then used to guide simulations of tau binding selectivity while assuming early-stage accumulation of disease pathology. The simulations suggest that for tau aggregates to represent at least 80% of specific binding signal, binding affinity or density selectivities for tau over beta-amyloid should be at least 20- or 50-fold, respectively. The simulations also suggest, however, that overcoming nonspecific binding will be an additional challenge for tau-directed radiotracers owing to low concentrations of available binding sites. Overall, nonlinear modeling can provide insight into the performance characteristics needed for tau-directed radiotracers in vivo.

Analysis of chiral non-steroidal anti-inflammatory drugs flurbiprofen, ketoprofen and etodolac binding with HSA

The protein binding of non-steroidal anti-inflammatory drugs flurbiprofen, ketoprofen and etodolac with human serum albumin (HSA) was investigated using indirect chiral high performance liquid chromatography (HPLC) and ultrafiltration techniques. S-(–)-1-(1-naphthyl)-ethylamine (S-NEA) was utilized as chiral derivatization reagent and pre-column derivatization RP-HPLC method was established for the separation and assay of the three pairs of enantiomer. The method had good linear relationship over the investigated concentration range without interference. The average extraction efficiency was higher than 85% in different systems, and the intra-day and inter-day precisions were less than 15%. In serum albumin, the protein binding of etodolac enantiomers showed significant stereoselectivity that the affinity of S-enantiomer was stronger than R-enantiomer, and the stereoselectivity ratio reached 6.06; Flurbiprofen had only weak stereoselectivity in HSA, and ketoprofen had no stereoselectivity at all. Scatchard curves showed that all the three chiral drugs had two types of binding sites in HSA.

Preclinical prediction of human brain target site concentrations: considerations in extrapolating to the clinical setting

The development of drugs for central nervous system (CNS) disorders has encountered high failure rates. In part, this has been due to the sole focus on blood-brain barrier permeability of drugs, without taking into account all other processes that determine drug concentrations at the brain target site. This review deals with an overview of the processes that determine the drug distribution into and within the CNS, followed by a description of in vivo techniques that can be used to provide information on CNS drug distribution. A plea follows for the need for more mechanistic understanding of the mechanisms involved in brain target site distribution, and the condition-dependent contributions of these mechanisms to ultimate drug effect. As future direction, such can be achieved by performing integrative cross-compare designed studies, in which mechanisms are systematically influenced (e.g., inhibition of an efflux transporter or induction of pathological state). With the use of advanced mathematical modeling procedures, we may dissect contributions of individual mechanisms in animals as links to the human situation.

Brain uptake of nonsteroidal anti-inflammatory drugs: ibuprofen, flurbiprofen, and indomethacin

PURPOSE

To determine the roles of blood-brain barrier (BBB) transport and plasma protein binding in brain uptake of nonsteroidal anti-inflammatory drugs (NSAIDs)-ibuprofen, flurbiprofen, and indomethacin.

METHODS

Brain uptake was measured using in situ rat brain perfusion technique.

RESULTS

[14C]Ibuprofen, [3H]flurbiprofen, and [14C]indomethacin were rapidly taken up into the brain in the absence of plasma protein with BBB permeability-surface area products (PS(u)) to free drug of (2.63 +/- 0.11) x 10(-2), (1.60 +/- 0.08) x 10(-2), and (0.64 +/- 0.05) x 10(-2) mL s(-1) g(-1) (n = 9-11), respectively. BBB [14C]ibuprofen uptake was inhibited by unlabeled ibuprofen (Km = 0.85 +/- 0.02 mM, Vmax = 13.5 +/- 0.4 nmol s(-1) g(-1)) and indomethacin, but not by pyruvate, probenecid, digoxin, or valproate. No evidence was found for saturable BBB uptake of [3H]flurbiprofen or [14C]indomethacin. Initial brain uptake for all three NSAIDs was reduced by the addition of albumin to the perfusion buffer. The magnitude of the brain uptake reduction correlated with the NSAID free fraction in the perfusate.

CONCLUSIONS

Free ibuprofen, flurbiprofen, and indomethacin rapidly cross the BBB, with ibuprofen exhibiting a saturable component of transport. Plasma protein binding limits brain NSAID uptake by reducing the free fraction of NSAID in the circulation.

Role of Site-Specific Binding to Plasma Albumin in Drug Availability to Brain

Many studies have reported greater drug uptake into brain than that predicted based upon existing models using the free fraction (f(u)) of drug in arterial serum. To explain this difference, circulating plasma proteins have been suggested to interact with capillary membrane in vivo to produce a conformational change that favors net drug dissociation and elevation of f(u). Albumin, the principal binding protein in plasma, has two main drug binding sites, Sudlow I and II. We tested this hypothesis using drugs that bind selectively to either site I (warfarin) or site II (ibuprofen), as well as mixed ligands that have affinity for both sites (tolbutamide and valproate). Brain uptake was determined in the presence and absence of albumin using the in situ rat brain perfusion technique. Unidirectional brain uptake transfer constants (K(in)) were measured and compared with those predicted using the modified Kety-Crone-Renkin model: K(in) = F(1-e(-f(u) x PS(u)/F)), where F is perfusion flow and PS(u) is the permeability-surface area product to free drug of brain capillaries. The results demonstrated good agreement between measured and predicted K(in) over a 100-fold range in perfusion fluid albumin concentration using albumin from three different species (i.e., human, bovine, and rat), as well as whole-rat serum. K(in) decreased in the presence of albumin in direct proportion to perfusion fluid f(u) with constant PS(u). The results show that brain uptake of selected Sudlow site I and II ligands matches that predicted by the modified Kety-Crone-Renkin model with no evidence for enhanced dissociation.

Concentration-EEG effect relationship of propofol in rats

Propofol is a unique highly lipid-soluble anesthetic that is formulated in a fat emulsion (Diprivan) for intravenous (i.v.) use. It has the desirable properties of rapid onset and offset of effect following rapid i.v. administration and minimal accumulation on long-term administration. Based on physicochemical properties and preliminary brain solubility data, propofol should have an extended effect-site turnover and a resulting prolonged effect. However, a preliminary study in humans has reported a rapid blood-brain equilibration half-time (T1/2 kE0) of only 2.9 min. We used a chronically instrumented rat model to examine the unique disposition and electroencephalographic (EEG) pharmacodynamics of propofol. Although the pharmacokinetics were variable, there was low interindividual variability in the concentration-EEG effect relationship. The duration of EEG sleep was 26 (+/- 44% CV) min following 11-15 mg/kg doses of propofol. The T1/2 kE0 was 1.7 (+/- 32%) min. Apparent effect-site concentrations of 0.5-1 microg/mL were required to maintain sleep in rats. Like other general anesthetics, the concentration-EEG effect relationship of propofol is biphasic. At a propofol concentration of 0.6 (+/- 35%) microg/mL, the number of EEG waves/s was maximal at 175% of baseline awake state. Further increases in the concentration of propofol depressed EEG activity until complete suppression occurred at 7 (+/- 22%) microg/mL.

Myocardial uptake of thiopental enantiomers by the isolated perfused rat heart

Myocardial uptake of thiopental enantiomers by an isolated perfused rat heart preparation was examined after perfusion with protein-free perfusate. Outflow perfusate samples were collected at frequent intervals for 20 min during single-pass perfusion with 10 micrograms/ml racemic thiopental (washin phase) and for another 45 min during perfusion with drug-free perfusate (washout phase). (+)- and (-)-thiopental concentrations were assayed by chiral high-performance liquid chromatography. Heart rate, perfusion pressure, and electro-cardiogram were also monitored. During the washin phase, there was no significant difference between the mean values of the equilibration rate constants of (+)- and (-)-thiopental enantiomers (0.44 +/- 0.07 min-1 and 0.43 +/- 0.09 min-1, respectively, P > 0.05). Mean volumes of distribution of (+)- and (-)-thiopental enantiomers were similar (6.34 +/- 1.20 and 6.45 +/- 1.29 ml/g for the washin phase and 7.22 +/- 0.71 and 7.47 +/- 0.81 ml/g for the washout phase, respectively, P > 0.05). This indicates that tissue accumulation of thiopental enantiomers in the isolated perfused rat heart was not stereoselective. Uptake of thiopental by the heart was perfusion flow rate-limited and independent of capillary permeability. These findings suggest that myocardial tissue concentration of racemic thiopental should be an accurate predictor of myocardial drug effect.

Effect of perfusate pH on reduction of quinidine capillary permeability by albumin in isolated perfused rat heart

It has been suggested that albumin reduces quinidine capillary permeability (PS) in the single-pass perfused heart preparation by reducing paracellular transport of quinidine ions. Using this preparation, we examined the effect of albumin (0.1 per cent) on quinidine PS at perfusate pH's of 7.1 and 7.9 during uptake of quinidine (19 microM) and also during washout of the drug using a randomized design. Quinidine PS was approximately 16 ml/min/g heart at pH 7.9 and was not altered by the presence of albumin in perfusate. At pH 7.1, in the absence of albumin, quinidine PS was also 16 ml/min/g, but in the presence of albumin (0.1 per cent) PS was reduced significantly to approximately 5 ml/min/g (P < 0.001). In the absence of albumin PS was the same at pH 7.1 and 7.9 in spite of a greater degree of ionisation of quinidine at pH 7.1. This suggests that there is significant uptake of ionised quinidine at pH 7.1. The greater effect of albumin on PS at pH 7.1 supports the hypothesis that albumin reduces paracellular transport of quinidine ions.

Effect of alpha 1-acidglycoprotein on myocardial uptake and pharmacodynamics of quinidine in perfused rat heart

The myocardial uptake and pharmacodynamics of quinidine were examined in the isolated perfused rat heart preparation under conditions of varying concentrations of bovine alpha 1-acidglycoprotein (AAG) in the perfusate. Three hearts were perfused for five consecutive 35 min phases with buffer containing quinidine and AAG in concentrations of 0, 0.1, 0.5, 1.5 and 0 g/L, in that order, with a 55 min washout period between each phase. The equilibration rate constant for the quinidine output concentration increased with increasing AAG concentration, but not as much as predicted by the conventional pharmacokinetic uptake model, which assumes constant capillary permeability among the phases. Estimates of the permeability surface product for the two zero AAG phases (17.7 +/- 1.91 and 19.1 +/- 0.82 mL/min/g) were significantly greater than those for the three AAG phases (8.94 +/- 0.99, 8.70 +/- 0.26, 9.01 +/- 0.26 mL/min/g; P < 0.05). This effect of AAG is the same as that observed previously by us with bovine serum albumin in this same experimental preparation. This suggests that the mechanism of reduced capillary permeability is the same for both proteins, i.e., the formation of a steric barrier to paracellular transport rather than an electrostatic barrier. There was a direct, linear relationship between lengthening of the QT interval of the electrocardiogram and total and unbound quinidine concentrations, but the relationship for unbound concentration was independent of quinidine unbound fraction. Therefore, the electrocardiogram effect of quinidine was directly related to the circulating unbound rather than total drug concentration.

Influence of pH on the uptake and pharmacodynamics of quinidine in the isolated perfused rat heart

Using the single-pass isolated perfused rat heart preparation we examined the effect of perfusate pH (pH 7.05, 7.46, 7.71, 7.92) on quinidine output concentration (C(out)) and delta QT. Eight hearts were perfused at 2.5 ml/min. with quinidine (20 microM) for 35 min. followed by a 35-40 min. washout period with drug-free perfusate. This procedure was repeated four times in each preparation with the pH sequence varied and the same pH used in the first and last phases. Increasing pH slowed the rate of equilibration of C(out), the equilibration rate constant (k) decreasing from 0.273 min.-1 at pH 7.05 to 0.095 min.-1 at pH 7.92. A modified Kety-Renkin-Crone equation was fitted to the C(out) versus time data for each pH. The estimated volume of distribution (V) increased significantly with pH from 11.5 +/- 1.1 to 32.5 +/- 2.9 ml/g, but the permeability surface product did not change with pH (mean 17.7 ml/min./g). There was a linear relationship between V and calculated un-ionised quinidine C(out), with an intercept of 5.70 ml/g corresponding to the V of ionised drug. This indicates that ionised and un-ionised drug readily enter the heart and that the slower equilibration with pH is due to the increased V which results from increased partitioning of un-ionised quinidine into myocardial tissue. Perfusion pH did not directly affect baseline QT interval, but the rate of attainment of maximum delta QT decreased with increasing perfusate pH. Plots of delta QT versus calculated coronary output quinidine concentration did not change with pH, showing that this drug effect was due to both ionised and un-ionised moieties. This study shows that myocardial permeability and pharmacodynamic effect (delta QT) of quinidine are not influenced by perfusion pH over the range 7.0 to 7.9, although rate of equilibration of both C(out) and effect vary with pH.