Differences in the lipoprotein distribution of halofantrine are regulated by lipoprotein apolar lipid and protein concentration and lipid transfer protein I activity: in vitro studies in normolipidemic and dyslipidemic human plasmas

Role of Passive Diffusion, Transporters, and Membrane Trafficking-Mediated Processes in Cellular Drug Transport

Intracellular drug accumulation is thought to be dictated by two major processes, passive diffusion through the lipid membrane or membrane transporters. The relative role played by these distinct processes remains actively debated. Moreover, the role of membrane-trafficking in drug transport remains underappreciated and unexplored. Here we discuss the distinct processes involved in cellular drug distribution and propose that better experimental models are required to elucidate the differential contributions of various processes in intracellular drug accumulation.

The potential of cholesteryl ester transfer protein as a therapeutic target

INTRODUCTION

Over recent decades, attempts to ascertain the pro-atherogenic nature of plasma cholesteryl ester transfer protein (CETP) and to establish the relevance of its pharmacological blockade as a promising high density lipoproteins-raising and anti-atherogenic therapy have been disappointing.

AREAS COVERED

The current review focuses on CETP as a multifaceted protein, on genetic variations at the CETP gene and on their possible consequences for cardiovascular risk in human populations. Specific attention is given to physiological modulation of endogenous CETP activity by the apoC1 inhibitor. Finally, the rationale behind the need for selection of patients to treat is discussed in the light of recent studies.

EXPERT OPINION

At this stage one can only speculate on the clinical outcome of pharmacological CETP inhibitors in high-risk populations, but recent advances give cause to adjust the expectations from now on. The CETP effect is probably largely influenced by the overall metabolic state, and whether CETP blockade may be relevant or not in promoting cholesterol disposal is still questioned. The possible need for a careful stratification of patients to treat with CETP inhibitors is outlined. Finally, manipulation of CETP activity should be considered with caution in the context of sepsis and infectious diseases.

[Bioavailability of oral drug formulations and methods for its improvement]

The recent studies in nanotechnology resulted in the development of novel formulations with improved bioavailability. This is especially important for oral administered drugs as the most convenient formulations. The current review deals with the processes occurring at the gastro-intestinal (GI) tract and their influence on the drug form. The increase of bioavailability of the drug may be achieved through designing novel formulations according to the specific drug properties. They include capsules that release pharmaceutical agents at various parts of the GI tract, floating systems that prolong the presence of the drug in the GI tract, dispersed forms with surface-active soluble polymers, micelles that carry poor-soluble drugs inside their non-polar core, agents that facilitate tight junction opening, such as caprate and chitosan, and lipid-based formulations. The own data show the stimulating influence of phospholipid nanoparticles on peroral absorption of drug indomethacin in rats and on passage of transport marker and drugs through Caco-2 cell monolayer in vitro. The review summarizes current understanding of factors that influence the bioavailability of the oral drug forms, currently used models for pharmacokinetic studies, and various approaches to developing novel pharmaceutical forms that increase the bioavailability of the drugs.

Lipids and lipid-based formulations: optimizing the oral delivery of lipophilic drugs

Highly potent, but poorly water-soluble, drug candidates are common outcomes of contemporary drug discovery programmes and present a number of challenges to drug development - most notably, the issue of reduced systemic exposure after oral administration. However, it is increasingly apparent that formulations containing natural and/or synthetic lipids present a viable means for enhancing the oral bioavailability of some poorly water-soluble, highly lipophilic drugs. This Review details the mechanisms by which lipids and lipidic excipients affect the oral absorption of lipophilic drugs and provides a perspective on the possible future applications of lipid-based delivery systems. Particular emphasis has been placed on the capacity of lipids to enhance drug solubilization in the intestinal milieu, recruit intestinal lymphatic drug transport (and thereby reduce first-pass drug metabolism) and alter enterocyte-based drug transport and disposition.

The Influence of Hyperlipoproteinemia on in Vitro Distribution of Amiodarone and Desethylamiodarone in Human and Rat Plasma

PURPOSE

To study the effect of hyperlipoproteinemia on in vitro distribution of amiodarone (AM) and its prevalent metabolite desethylamiodarone (DEA) in human and rat plasma.

MATERIALS AND METHODS

Human and rat normolipidemic (NL) and hyperlipidemic (HL) plasma were spiked with AM and DEA. The fractions (high and low density lipoproteins, triglyceride rich lipoproteins and lipoprotein deficient plasma) were separated using ultracentrifugation.

RESULTS

Human and rat displayed similar patterns in terms of association of AM and DEA in NL plasma, in which the highest and lowest associations were observed in lipoprotein deficient (LPDP) and triglyceride (TRL) rich plasma fractions, respectively. In HL a substantial shift was observed in partitioning of AM and DEA mostly to TRL. The shift of AM and DEA into TRL fraction of HL plasma was more drastic for rat than human. In HL, association of AM with rat LPDP and HDL fractions were 10 and 26-fold lower than in the corresponding human fractions, respectively. The DEA:AM ratio in rat, but not human, was significantly affected by HL.

CONCLUSION

HL caused a major shift of AM and DEA to TRL fraction in both species. The findings were consistent with the higher AM concentrations previously noted in HL rats given the drug.

Plasma protein distribution and its impact on pharmacokinetics of liposomal amphotericin B in paediatric patients with malignant diseases

OBJECTIVE

This study investigates the association of liposomal amphotericin B (L-AmB) with plasma proteins and its impact on the pharmacokinetics of L-AmB in paediatric patients with malignant diseases.

METHODS

Paediatric oncology patients (n = 39) who received multiple-doses of L-AmB were recruited into this study. The association of the drug with plasma lipoprotein was investigated using single vertical spin density gradient ultracentrifugation and quantitated with a validated HPLC assay. The unbound amphotericin B (AmB) in the plasma was separated by ultrafiltration and determined with a validated LC/MS/MS assay.

RESULTS

The ex vivo lipoprotein distribution of L-AmB found that 68.3 +/- 11.8% of the drug was associated with the high density lipoprotein (HDL) fraction, which demonstrated a significant inverse correlation with posterior Bayesian estimates of L-AmB clearance (r = -0.690, p < 0.01). The average of unbound fraction of AmB in plasma of patients administered with L-AmB was 0.005, but its relationship with L-AmB clearance did not reach a statistical significance.

CONCLUSION

L-AmB displays different lipoprotein distribution profile from that of the conventional AmB formulation, with L-AmB preferentially associated with HDL in plasma. The inverse correlation of L-AmB clearance to its HDL distribution contributes to the difference in the pharmacokinetic profile of L-AmB.

Influence of Dyslipidemia on Moxidectin Distribution in Plasma Lipoproteins and on its Pharmacokinetics

PURPOSE

We studied the influence of dyslipemia on the distribution of moxidectin, a potent antiparasitic drug of the macrocyclic lactone (ML) family, in plasma lipoproteins and on its pharmacokinetic behaviour.

MATERIALS AND METHODS

Plasma samples from normolipidemic or dyslipidemic subjects were spiked with moxidectin (20 ng/ml). Rabbits fed with standard (n = 5) or cholesterol-enriched diet (n = 5) were injected subcutaneously with moxidectin (300 microg/kg) and blood samples were collected over 32 days. Lipoproteins were separated from plasma samples by ultracentrifugation on density gradients. Moxidectin and lipids were measured in plasma and in lipoproteins and the pharmacokinetic parameters calculated.

RESULTS

In normolipidemic subjects or rabbits, the drug bound preferentially to HDL. In hyperlipidemic samples, moxidectin shifted to the VLDL-LDL fraction. In addition, hyperlipidemic rabbits had a 2.8-fold higher area under the plasma concentration versus time curve (AUC) and a lower clearance and volume of distribution when compared with controls.

CONCLUSION

Dyslipidemia led to major changes in moxidectin plasma distribution and in drug disposition. Therefore, a high variability in moxidectin disposition might be expected in humans or animals liable to develop dyslipidemia, with a possible impact on the efficacy and safety of this class of drugs.

Potential role of the low-density lipoprotein receptor family as mediators of cellular drug uptake

We highlight the importance of the low-density lipoprotein (LDL) receptor family and its pharmaceutical implications in the field of drug delivery. The members of the LDL receptor family are a group of cell surface receptors that transport a number of macromolecules into cells through a process called receptor-mediated endocytosis. This process involves the receptor recognizing a ligand from the extracellular membrane (ECM), internalizing it through clathrin-coated pits and degrading it upon fusion with lysosomes. There are nine members of the receptor family, which include the LDL receptor, low-density lipoprotein-related protein (LRP), megalin, very low-density lipoprotein (VLDL) receptor, apoER2 and sorLA/LRP11, LRP1b, MEGF7, LRP5/6; the former six having been identified in humans. Each member is expressed in a number of different tissues and has a wide range of different ligands, not specific to the recognition of the LDL particle. Thus, rather than the original hypothesis that the receptor is only a mediator of cholesterol uptake, it may also be involved in a number of other physiological functions, including the progression of certain disease states and, potentially, cellular drug uptake. A number of studies have suggested that the LDL receptors are involved in endocytosis of drugs and drug formulations including aminoglycosides, anionic liposomes and cyclosporine A (CsA). This article reviews the importance of lipoproteins as a drug delivery system and how LDL receptors are relevant to the design and targeting of specific drugs.

A stepwise surgical procedure to investigate the lymphatic transport of lipid-based oral drug formulations: Cannulation of the mesenteric and thoracic lymph ducts within the rat

INTRODUCTION

A number of animal models have been described for the assessment of intestinal lymphatic drug transport. Lymphatic transport studies are commonly first conducted in the laboratory rat, with larger more complicated models (i.e., dog or pig) subsequently investigated. However, the utility of lymph fistulation in large animals is limited by considerable logistical and economic constraints.

METHODS

This paper describes a stepwise surgical procedure for cannulating the thoracic and mesenteric lymph ducts in male Sprague-Dawley rats.

RESULTS

Following surgery, thoracic and mesenteric lymph flow rates during the 24-h period immediately following surgery averaged 12.5+/-2.5 and 2.4+/-1.1 ml/h, respectively. This flow rate is greater than that obtained with previously described methods, which require restraint of the animals and/or a 24-h recovery period and are reported to produce average intestinal lymph flow rates of 2 ml/h.

DISCUSSION

This animal model can be utilized for the assessment of drug transport by the lymphatics and for determining what percentage of lymphatic transport is a result of only intestinal lymphatics.

Cholesteryl ester transfer protein facilitates the movement of water-insoluble drugs between lipoproteins: a novel biological function for a well-characterized lipid transfer protein

This review article addresses the recently discovered finding that cholesteryl ester transfer protein (CETP) can facilitate the transfer of water-insoluble drugs between different lipoprotein subclasses. This protein, which is often referred to as lipid transfer protein I (LTP I), is involved in the lipid regulation of lipoproteins. It is responsible for the facilitated transfer of core lipoprotein lipids, cholesteryl ester and triglycerides, and approximately one-third of the coat lipoprotein lipid, phosphatidylcholine, between different plasma lipoproteins. The human body appears to recognize exogenous water-insoluble drugs as lipid-like particles, which suggests that these compounds may interact with lipoproteins just like endogenous plasma lipids, and thus their transfer between lipoproteins may be facilitated by plasma CETP. Patients with a variety of diseases (i.e. diabetes, cancer, AIDS) often exhibit hypo- and/or hypercholesterolemia and triglyceridemia, commonly referred to as dyslipidemias, which result in changes in their plasma lipoprotein-lipid composition and concentration. The interaction of water-insoluble drugs with these dyslipidemic lipoproteins may be responsible for the differences seen in the pharmacokinetics and pharmacodynamics of the drug within different diseased patient populations. It is possible that these differences may be linked to the ability of CETP to transfer these compounds from one lipoprotein to another. This review examines the current understanding of the relationship between CETP activity and the lipoprotein distribution of a number of compounds (e.g. amphotericin B and cyclosporine A). It further suggests that additional research will expand our understanding of the role of CETP to explain other functions in lipophilic drug distribution and metabolism.

The role of lymphatic transport in enhancing oral protein and peptide drug delivery

The gastrointestinal lymphatic system is a specific transport pathway through which dietary lipids, fat-soluble vitamins, and water-insoluble peptide-type molecules (e.g., cyclosporine A) can gain access to the systemic circulation. Drugs transported by way of the gastrointestinal lymphatic system bypass the liver and avoid potential hepatic first-pass metabolism. Lymphatic delivery of immunomodulatory and low therapeutic index protein and peptide drugs used in the treatment of cancer cell metastases and HIV presents an opportunity to maximize therapeutic benefit while minimizing general systemic drug exposure. Furthermore, lymphatic drug transport may promote drug incorporation into the body's lipid-handling system, thus offering the potential to manipulate drug distribution and residence time within the body. This review article will discuss the potential utilization of lymphatic transport in enhancing the oral absorption of protein- and peptide-like drugs.

The influence of lipids on stereoselective pharmacokinetics of halofantrine: Important implications in food-effect studies involving drugs that bind to lipoproteins

The objective of this study was to determine the effect of lipids on the pharmacokinetics of halofantrine enantiomers. Rats were given (+/-)-halofantrine HCl 2 mg/kg i.v., or 7 mg/kg orally. Some rats were rendered hyperlipidemic by intraperitoneal administration of poloxamer 407 1 g/kg, followed by (+/-)-halofantrine HCl intravenously. In other normolipidemic rats, (+/-)-halofantrine was administered under fasted conditions, or after peanut oil given orally. Halofantrine enantiomer plasma concentrations were considerably (>10-fold) increased in hyperlipidemia. Decreases were noted in the clearance, volume of distribution and the unbound fraction in plasma of the hyperlipidemic rats. Peanut oil caused a significant 28% reduction in clearance of the (-), but not the (+) enantiomer (mean clearance reduced 11%) of halofantrine. After oral halofantrine, peanut oil resulted in a two- to threefold increase in the plasma area under the curves of halofantrine enantiomers. Halofantrine enantiomer pharmacokinetics are highly dependent upon plasma lipid concentrations. Oral lipids may result in a stereoselective interaction at the level of clearance. Because lipids may affect clearance of drugs that bind to lipoproteins, in determining bioavailability of such drugs in food-effect studies, reference intravenous groups should be included to separate true increase in bioavailability from the effects of decreased clearance.

Cyclosporine A transfer between high- and low-density lipoproteins: independent from lipid transfer protein I-facilitated transfer of lipoprotein-coated phospholipids because of high affinity of cyclosporine a for the protein component of lipoproteins

The objectives of this study were to determine if lipid transfer protein I (LTP I)-facilitated phospholipid (PC) transfer activity regulates the plasma lipoprotein distribution of cyclosporine (CSA) and if the association of CSA with high-density lipoproteins (HDL) is due to the high protein and/or alterations in coat lipid content of HDL. To assess if LTP I-facilitated PC transfer activity regulates the plasma lipoprotein distribution of CSA, (14)C-PC- or (3)H-CSA-enriched HDL or low-density lipoproteins (LDL) were incubated in T150 buffer [pH 7.4, containing a (14)C-PC- or (3)H-CSA-free lipoprotein counterpart +/- exogenous LTP I (1.0 microg protein/mL)] or in delipidated human plasma that contained 1.0 microg protein/mL of endogenous LTP I in the presence or absence of a monoclonal antibody TP1 (30 microg protein/mL) directed against LTP I for 90 min at 37 degrees C. To assess the influence of HDL subfraction lipid composition and structure on the plasma distribution of CSA, CSA at 1000 ng of drug/mL of plasma was incubated in human plasma pretreated for 24 h with a lecithin:cholesterol acyltransferase (LCAT) inhibitor, dithionitrobenzoate (DTNB; 3 mM). To assess the binding of CSA to apolipoproteins AI, AII, and B, increasing concentrations of CSA were added to a constant concentration of either apolipoprotein AI, AII, or B. Equilibrium dialysis was used to determine free and bound fractions and Scatchard plot analysis was used to determine binding coefficients. To assess the influence of hydrophobic core lipid volume on the plasma distribution of CSA, CSA was incubated in plasma from patients with well-characterized dyslipidemias. The hydrophobic core lipid volume (CE + TG) within each lipoprotein subfraction was correlated to the amount of CSA recovered in each plasma sample from the different human subjects. The percent transfer of PC from LDL to HDL was different than the percent transfer of CSA in T150 buffer or human plasma source. In the presence of TP1, only PC transfer from LDL to HDL decreased. For plasma incubated with CSA and separated into HDL(2) and HDL(3), 35-50% of drug originally incubated was recovered in the HDL(3) fraction, with the remaining drug being found within the other fractions. When CSA was incubated in plasma pretreated with DTNB, the percentage of CSA recovered in the HDL(3) and HDL(2) fractions was not significantly different compared with that in the HDL(3) and HDL(2) fractions from untreated control plasma. CSA distribution into HDL inversely correlated with the hydrophobic core lipid volume of HDL, whereas distribution into LDL and triglyceride-rich lipoproteins directly correlated with their respective hydrophobic core lipid volumes. We further observed that CSA has high binding affinity and multiple binding sites with apolipoproteins AI (k(d) = 188.9 nM; n = 2), AII (k(d) = 184.7 nM; n = 2), and B (k(d) = 191 nM; n = 3). These findings suggest that the transfer of CSA between different lipoprotein particles is not influenced by LTP I-facilitated PC transfer activity probably because of the high affinity of CSA for the protein components of HDL and LDL.

Rat and rabbit plasma distribution of free and chylomicron-associated BIRT 377, a novel small molecule antagonist of LFA-1-mediated cell adhesion

PURPOSE

The objectives of this study are to determine the plasma distribution of free and chylomicron-associated BIRT 377 within rats and rabbits.

METHODS

For the rat studies free and chylomicron-associated BIRT 377 was incubated in plasma from CD 1 non-fasted rats for 60 minutes at 37 degrees C. Following incubation the plasma was separated into its lipoprotein and lipoprotein-deficient plasma (LPDP) fractions by three different methods and analyzed for BIRT 377 content by HPLC. For the rabbit studies New Zealand fasted white rabbits (3 kg; n=4) were administered an intravenous dose of free BIRT 377 (1 mg/kg). Following administration, serial blood samples were obtained and the plasma was analyzed for BIRT 377. The plasma conected at the 0.083-h time point was separated into each of its lipoprotein fractions and analyzed for BIRT 377.

RESULTS

37.8 +/- 1.2% of the original drug amount incubated in rat plasma was recovered within the lipoprotein-rich fraction. 41.5 +/- 0.4% of the original chylomicron-associated drug concentration incubated was recovered within the lipoprotein-rich fraction. The percentage of drug recovered within the TRL fraction was significantly greater following the incubation of chylomicron-associated BIRT 377 compared to free BIRT 377. In addition, BIRT 377 apparently follows a two-compartment pharmacokinetic model following single intravenous dose administration to rabbits.

CONCLUSIONS

These findings suggest that plasma lipoprotein binding of BIRT 377 is evident and may be a factor in evaluating the pharmacological fate of this drug when administered to patients that exhibit changes in their plasma lipoprotein lipid.

The stereoselective distribution of halofantrine enantiomers within human, dog, and rat plasma lipoproteins

PURPOSE

To study the in vitro distribution of the enantiomers of the antimalarial drug halofantrine in human, dog and rat plasma lipoprotein-fractions.

METHODS

Plasma was spiked with racemic halofantrine (1,000 ng/ml) and incubated for 1 h at 37 degree C. The fractions (high and low density lipoproteins, triglyceride-rich lipoproteins and lipoprotein deficient plasma) were separated using density gradient ultracentrifugation. Fractions were assayed for halofantrine enantiomer using stereospecific high performance liquid chromatography.

RESULTS

The (-) enantiomer of halofantrine displayed higher affinity for the lipoprotein-deficient fraction than the (+) enantiomer in all three species. The (+) enantiomer was predominately located in the lipoprotein rich fractions of dog and human plasma (the (+):(-) ratio ranging from 1.2-9.6). In contrast, the (+):(-) ratio was consistently < 1 in lipoprotein-deficient fractions. Dog displayed a large magnitude of stereoselectivity in halofantrine distribution to the plasma fractions tested. There were substantial interspecies differences in the pattern of distribution of halofantrine enantiomers within the different fractions. A significant positive relationship was observed between halofantrine uptake into lipoprotein-rich fractions and the percent of apolar core lipid in those fractions. There was also a strong negative correlation between total protein concentration and the enantiomeric ratio in the lipoprotein-deficient plasma fraction.

CONCLUSION

Distribution of halofantrine enantiomer to plasma lipoprotein-fractions is stereoselective and species specific. This differential binding of halofantrine enantiomers to lipoproteins may need to be considered in viewing pharmacokinetic and pharmacodynamic data involving the drug.