Effect of binding protein surface charge on palmitate uptake by hepatocyte suspensions

Plasma Protein-Mediated Uptake and Contradictions to the Free Drug Hypothesis: A Critical Review

According to the free drug hypothesis (FDH), only free, unbound drug is available to interact with biological targets. This hypothesis is the fundamental principle that continues to explain the vast majority of all pharmacokinetic and pharmacodynamic processes. Under the FDH, the free drug concentration at the target site is considered the driver of pharmacodynamic activity and pharmacokinetic processes. However, deviations from the FDH are observed in hepatic uptake and clearance predictions, where observed unbound intrinsic hepatic clearance (CL_int,u) is larger than expected. Such deviations are commonly observed when plasma proteins are present and form the basis of the so-called plasma protein-mediated uptake effect (PMUE). This review will discuss the basis of plasma protein binding as it pertains to hepatic clearance based on the FDH, as well as several hypotheses that may explain the underlying mechanisms of PMUE. Notably, some, but not all, potential mechanisms remained aligned with the FDH. Finally, we will outline possible experimental strategies to elucidate PMUE mechanisms. Understanding the mechanisms of PMUE and its potential contribution to clearance underprediction is vital to improving the drug development process.

Consideration of albumin-mediated hepatic uptake for highly protein-bound anionic drugs: Bridging the gap of hepatic uptake clearance between in vitro and in vivo

The accuracy in predicting in vivo hepatic clearance of drugs from in vitro data (often termed as in vitro-to-in vivo extrapolation, IVIVE) has improved in part by applying the extended-clearance concept that considers the interplay between hepatic metabolism and uptake/efflux processes. However, the IVIVE-based prediction performs poorly in predicting the hepatic uptake clearance of highly albumin-bound anionic drugs. Their hepatic uptake clearances tend to be much higher than expected based on the free-drug theory. Such observation can be attributable to a phenomenon called albumin-mediated hepatic uptake, for which various models have been thus far proposed. Our group has been applying a facilitated-dissociation model, which assumes the enhanced dissociation of the drug-albumin complex upon interaction with the cell surface. By considering the albumin-mediated hepatic uptake (using the facilitated-dissociation model or alternative kinetic models), a number of investigations demonstrated the improvement in the prediction accuracy for the hepatic clearance of highly protein-bound anionic drugs that are substrates for hepatic uptake transporters. This review summarizes the reported kinetic analyses of the albumin-mediated hepatic uptake of highly albumin-bound drugs concerning the IVIVE and the clinical and physiological relevance.

The Presence of a Transporter-Induced Protein Binding Shift: A New Explanation for Protein-Facilitated Uptake and Improvement for In Vitro-In Vivo Extrapolation

Accurately predicting hepatic clearance is an integral part of the drug-development process, and yet current in vitro to in vivo (IVIVE) extrapolation methods yield poor predictions, particularly for highly protein-bound transporter substrates. Explanations for error include inaccuracies in protein-binding measurements and the lack of recognition of protein-facilitated uptake, where both unbound and bound drug may be cleared, violating the principles of the widely accepted free drug theory. A new explanation for protein-facilitated uptake is proposed here, called a transporter-induced protein binding shift High-affinity binding to cell-membrane proteins may change the equilibrium of the nonspecific binding between drugs and plasma proteins, leading to greater cellular uptake and clearance than currently predicted. The uptake of two lower protein-binding organic anion transporting polypeptide substrates (pravastatin and rosuvastatin) and two higher binding substrates (atorvastatin and pitavastatin) were measured in rat hepatocytes in incubations with protein-free buffer versus 100% plasma. Decreased unbound K _m values and increased intrinsic clearance values were seen in the plasma incubations for the highly bound compounds, supporting the new hypothesis and mitigating the IVIVE underprediction previously seen for highly bound transporter substrates.

An examination of protein binding and protein-facilitated uptake relating to in vitro-in vivo extrapolation

As explained by the free drug theory, the unbound fraction of drug has long been thought to drive the efficacy of a molecule. Thus, the fraction unbound term, or f_u, appears in equations for fundamental pharmacokinetic parameters such as clearance, and is used when attempting in vitro to in vivo extrapolation (IVIVE). In recent years though, it has been noted that IVIVE does not always yield accurate predictions, and that some highly protein bound ligands have more efficient uptake than can be explained by their unbound fractions. This review explores the evolution of f_u terms included when implementing IVIVE, the concept of protein-facilitated uptake, and the mechanisms that have been proposed to account for facilitated uptake.

Visualization of plasma and tissue binding using dose fractions parameter

A novel concept of dose fractions, based on the distribution of total bioavailable dose between the six combinations of location and binding state in Øie-Tozer's model is suggested as a way to visualize the distribution pharmacokinetics of a drug. The concept of dose fractions provides a sharper terminology in discussions of drug distribution allowing for a more precise description of the state and location of a drug within a system. In medicinal chemistry literature, the free fraction of a drug in plasma is a commonly discussed factor affecting the exposure to free drug while tissue binding is less well addressed. The free dose fraction, defined as the fraction of the bioavailable dose existing in free form, is suggested as a potentially valuable term for such discussions. Presently, drugs with high (>95%) plasma protein binding are viewed with skepticism, the rational behind which is questioned. The plasma protein bound dose fraction defined as the fraction of the total available dose, which is bound to plasma proteins, is suggested as a measure of the risk for problems related to fluctuations in free drug exposure due to variations in the concentration of drug binding plasma protein.

Membrane binding proteins are the major determinants for the hepatocellular transmembrane flux of long-chain fatty acids bound to albumin

PURPOSE

The hepatic transmembrane flux of long-chain fatty acids (LCFA) occurs through passive and fatty acid transport protein facilitated processes from blood. The extent that these transport processes can be related to the unbound and protein-bound fractions of LCFA in blood is not clear.

METHODS

We used hepatocyte suspensions, hepatoma monolayers, and perfused rat livers to quantitate the transport of purified [(3)H]palmitate ([(3)H]PA) and 12-(N-methyl)-N-[(7-nitrobenz-2oxa-1,3-diazol-4yl-)amino]octadecanoicacid (12-NBDS) from solutions with a constant unbound LCFA concentration with varying bovine serum albumin (BSA) concentrations and in the presence and absence of antisera raised against cytosolic liver fatty acid binding protein (L-FABP).

RESULTS

In the absence of L-FABP antisera, using an unbound ligand concentration that was adjusted to remain constant at each BSA concentration, hepatocyte [(3)H]PA and 12-NBDS uptake rates increased linearly with an increase in BSA concentration (p < 0.0001). In the presence of L-FABP antisera, [(3)H]PA uptake showed a greater reduction in the presence of 100 muM BSA than 5 muM BSA. The calculated permeability surface area product (PS) confirmed that both unbound and bound fractions of LCFA contributed to the overall flux, but only the PS for the protein-bound fraction was reduced in the presence of L-FABP antisera. In situ rat liver perfusion studies showed that the only rate process for the disposition of [(3)H]PA in the liver inhibited by L-FABP antisera was that for influx, as defined by PS, and that it reduced PS in the perfused liver by 42%.

CONCLUSION

These results suggest that, at physiological albumin concentrations, most of the LCFA uptake is mediated from that bound to albumin by a hepatocyte basolateral membrane transport protein, and uptake of unbound LCFA occurring by passive diffusion contributes a minor component.

Assessing the cellular transmembrane electrical potential difference on the hepatic uptake of palmitate

Understanding the driving forces for the hepatic uptake of endogenous and exogenous substrates in isolated cells and organs is fundamental to describing the underlying hepatic physiology/pharmacology. In this study we investigated whether uptake of plasma protein-bound [3H]-palmitate across the hepatocyte wall is governed by the transmembrane electrical potential difference (PD). Uptake was studied in isolated hepatocytes and isolated perfused rat livers (IPL). Protein-binding and vasoactive properties of the different perfusates were determined using in vitro heptane/buffer partitioning studies and the multiple indicator dilution (MID) technique in the IPL, respectively. Altering hepatocyte PD by perfusate ion substitution resulted in either a substantial depolarization (-14 +/- 1 mV, n = 12, mean +/- S.E., substituting choline for Na+) or hyperpolarization (-46 +/- 3 mV, n = 12, mean +/- S.E., substituting nitrate for Cl-). Perfusate ion substitution also affected the equilibrium binding constant for the palmitate-albumin complex. IPL studies suggested that, other than with gluconate buffer, hepatic [3H]-palmitate extraction was not affected by the buffer used, implying PD was not a determinant of extraction. [3H]-Palmitate extraction was much lower (p < 0.05) when gluconate was substituted for Cl- ion. This work contrasts with that for the extraction of [3H]-alanine where hepatic extraction fraction was significantly reduced during depolarization. Changing the albumin concentration did not affect hepatocyte PD, and [3H]-palmitate clearance into isolated hepatocytes was not affected by the buffers used. MID studies with vascular and extravascular references revealed that, with the gluconate substituted buffer, the extravascular volume possibly increased the diffusional path length thus explaining reduced [3H]-palmitate extraction fraction in the IPL.

Rational design and engineering of delivery systems for therapeutics: biomedical exercises in colloid and surface science

Engineering delivery systems of therapeutic agents has grown into an independent field, transcending the scope of traditional disciplines and capturing the interest of both academic and industrial research. At the same time, the acceleration in the discovery of new therapeutic moieties (chemical, biological, genetic and radiological) has led to an increasing demand for delivery systems capable of protecting, transporting, and selectively depositing those therapeutic agents to desired sites. The vast majority of delivery systems physically reside in the colloidal domain, while their surface properties and interfacial interactions with the biological milieu critically determine the pharmacological profiles of the delivered therapeutic agents. Interestingly though, the colloidal and surface properties of delivery systems are commonly overlooked in view of the predominant attention placed on the therapeutic effectiveness achieved. Moreover, the development and evaluation of novel delivery systems towards clinical use is often progressed by serendipity rather than a systematic design process, often leading to failure. The present article will attempt to illustrate the colloid and interfacial perspective of a delivery event, as well as exemplify the vast opportunities offered by treating, analysing and manipulating delivery systems as colloidal systems. Exploring and defining the colloid and surface nature of the interactions taking place between the biological moieties in the body and an administered delivery vehicle will allow for the rational engineering of effective delivery systems. A design scheme is also proposed on the way in which the engineering of advanced delivery systems should be practiced towards their transformation from laboratory inventions to clinically viable therapeutics. Lastly, three case studies are presented, demonstrating how rational manipulation of the colloidal and surface properties of delivery systems can lead to newly engineered systems relevant to chemotherapy, gene therapy and radiotherapy.

Mobilization of intracellular iron by analogs of pyridoxal isonicotinoyl hydrazone (PIH) is determined by the membrane permeability of the iron-chelator complexes

In the ongoing search for an effective, orally active iron-chelator, the capacity of a series of halogenated analogs of pyridoxal isonicotinoyl hydrazone (PIH) to bind intracellular 59Fe and cause its release from cells was investigated. Reticulocytes labeled with 59Fe(2)-transferrin in which heme synthesis was inhibited by succinylacetone were used as a model of 59Fe mobilization. The kinetics of iron binding were similar for all the chelators tested (half-time of approximately 1 hr), and all bound more than twice as much 59Fe as PIH. The rate of release of the 59Fe-chelator complexes from cells depended upon the structure of the chelators. Ortho-substituted analogs were more effective at mobilizing cellular iron than meta and para isomers, due to a more efficient release of the iron complexes from the cell. The iron-chelator complexes which were released slowly from cells had a high affinity for erythrocyte ghost membranes, indicating the role of membrane permeability in the release mechanism of the complexes. The addition of BSA to the extracellular medium increased the extent of iron release by lipophilic analogs in a concentration-dependent manner, presumably by acting as a sink for the lipophilic complexes. The affinity of BSA for the chelators and their Fe(3+) complexes, determined spectrophotometrically, demonstrated that all chelators and their iron complexes bound BSA with dissociation constants ranging from 7,000 to >500,000 M(-1). Understanding the importance of the rate of release of the iron-chelator complex will direct the search for iron-chelators with improved efficacy.

Hepatocyte [3H]-palmitate uptake: effect of albumin surface charge modification

The role of plasma proteins on the cellular uptake of lipophilic substrates has perplexed investigators for many years. We tested the hypothesis that an ionic interaction between the protein–ligand complex and hepatocyte surface may be responsible for supplying more ligand to the cell for uptake. The surface-charged groups on albumin were modified to yield proteins having a range of isoelectric points (ALB, ALBs, ALBm, ALBe had values of 4.8–5.0, 4.5–4.7, 3.0–3.5, 8.4–8.6, respectively). [3H]-Palmitate uptake studies were performed with adult rat hepatocyte suspensions using similar unbound ligand fractions in the presence of the different binding proteins. Mass spectrometry, isoelectric focusing (pI), and heptane : water partitioning were used to determine protein molecular weight, pI, and protein-palmitate equilibrium binding constant, respectively. Hepatocyte [3H]-palmitate clearance in the presence of ALBs and ALBm were significantly lower (p < 0.05) than ALB, whereas [3H]-palmitate clearance in the presence of ALBe was significantly higher (p < 0.05) than ALB. The data were consistent with the notion that ionic interactions between extracellular protein–ligand complexes and the hepatocyte surface facilitate the uptake of long-chain fatty acids.Key words: uptake, long-chain fatty acid, hepatic, protein, albumin, surface charge, facilitation, ionic interaction.

Palmitate uptake by neonatal rat myocytes and hepatocytes. Role of extracellular protein

The role of extracellular binding proteins in the rate of [3H]palmitate uptake by neonatal cardiac myocytes and hepatocytes was investigated using a model-independent approach. Binding proteins used in this study included alpha1-acid glycoprotein [isoelectric point (pI) approximately 2.7], conalbumin (pI approximately 6.4), lysozyme (pI approximately 11.0), albumin (pI approximately 4.9), and albumin which had been modified to yield proteins with pI values of 3.5, 4.7, 7.5 and 8.6. All uptake studies were conducted at similar unbound ligand fractions. There was a linear relationship between the rate of neonatal hepatocyte [3H]palmitate clearance and protein pI (r2 = 0.98). In contrast, there was an overall poor relationship between neonatal cardiac myocyte [3H]palmitate-clearance rate and protein pI (r2 = 0.48). However, the relationship improved when the data on [3H]palmitate-clearance were analyzed using only the modified albumins. The study indicates that an ionic interaction between extracellular proteins and the hepatocyte surface enhances the overall uptake of [3H]palmitate. This interaction may be limited to albumin for neonatal cardiac myocytes.

Cytosolic fatty acid binding protein enhances rat hepatocyte [3H]palmitate uptake

Liver cytosolic fatty acid binding protein (FABP) represents the intracellular equivalent to extracellular serum albumin, participating in the intracellular transport of long-chain fatty acids. In this study we observed the effect of increasing and decreasing FABP levels on hepatocyte [3H]palmitate uptake in male Sprague-Dawley rats. We also were interested to determine whether uptake, from either the unbound or unbound and protein-bound fractions, was fundamentally different at the different FABP levels. FABP levels were modified by hypophysectomy and clofibrate treatment (50 mg/100 g body weight for 10 days). Results showed that the [3H]palmitate clearance rates paralleled the 54% decrease and 73% increase in FABP levels in hypophysectomy and clofibrate-treated animals, respectively. In the presence of 2 and 20 µM albumin, hepatocyte clearance rates of unbound [3H]palmitate from hypophysectomized animals (0.16 ± 0.01 and 0.64 ± 0.01 mL·s-1·10-6 cells, respectively) were significantly lower (p < 0.01) than those of the sham group (0.30 ± 0.02 and 1.00 ± 0.06 mL·s-1·10-6 cells, respectively). However, the unbound [3H]palmitate clearance rates from the clofibrate-treated group (0.39 ± 0.04 and 1.18 ± 0.12 mL·s-1·10-6 cells) were significantly higher (p < 0.01) than the control group (0.29 ± 0.02 and 0.81 ± 0.05 mL·s-1·10-6 cells) for 2 and 20 µM albumin, respectively. To investigate whether uptake was fundamentally different between the hypophysectomized and clofibrate-treated groups, we expressed the clearance rates as enhancement factors, i.e., EF = CL20µM/CL2µM. No statistical difference was observed between EF of the hypophsectomized (3.8 ± 0.4) and EF of the clofibrate-treated (3.1 ± 0.3) groups, suggesting that the extracted ligand originated from similar fractions.Key words: palmitic acid, albumin, growth hormone, liver, fatty acid binding protein, uptake, hepatic, long-chain fatty acids, clofibrate, hypophysectomy.

Altered transmembrane ionic flux in hepatocytes isolated from cirrhotic rats

BACKGROUND/AIMS

Although cirrhosis is known to be associated with many hepatocyte abnormalities, there is no well-established model to study the cellular drug uptake process independent of hemodynamic effects. The purpose of the present study was to test the following hypothesis: hepatocytes isolated from cirrhotic animals may be used as a model to study the cellular abnormalities associated with cirrhosis. Our hypothesis was tested by comparing the membrane potential (PD) of hepatocytes in anesthetized healthy and cirrhotic animals, and the PD and [3H]palmitic acid clearance rate of hepatocytes isolated from healthy and cirrhotic animals.

METHODS

Mild to moderate cirrhosis was induced in female Sprague-Dawley rats by CCl4 administration. PD was recorded in anesthetized animals using intracellular microelectrodes. Hepatocytes from those livers were subsequently isolated by collagenase perfusion for further determinations of PD and [3H]palmitic acid uptake.

RESULTS

The mean (+/-SEM) hepatocyte PD from intact rat livers was 38+/-1 mV (control) and -32+/-1 mV (cirrhosis; n=6/group, p<0.01). The PD (mean+/-SEM) in isolated hepatocytes was -21+/-1 mV (control) and -15+/-1 mV (cirrhosis, n=13/group, p<0.01). The clearance rate of [3H]palmitic acid was lower in hepatocytes isolated from cirrhotic animals (26%) than in those isolated from healthy control animals (p<0.01).

CONCLUSION

The results of this study indicate that hepatocytes isolated from cirrhotic animals may be used to study the cellular abnormalities associated with cirrhosis.