The effect of a high-fat meal on the pharmacodynamics of a model lipophilic compound that binds extensively to triglyceride-rich lipoproteins

A high-fat meal induces transient hyperlipidemia characterized by elevated triglyceride-rich lipoproteins (TRL) which are composed mainly of chylomicrons. The purpose of this work was to investigate the effect of this transient hyperlipidemia on the pharmacodynamics of lipophilic drugs, using DDT as a model compound since it binds extensively to TRL and has a distinct neurotoxic effect. The postprandial hyperlipidemia in rats was induced by oral administration of peanut oil and was monitored by measurement of plasma triglyceride levels. The control group received water instead of oil. The rats received a continuous intravenous infusion of DDT (10 mg/h) until onset of a predefined pharmacodynamic endpoint (facial muscle tremor). Plasma and brain samples were then obtained and assayed for DDT. Rats with postprandial hyperlipidemia required higher dose of DDT to induce onset of facial muscle tremor. At the pharmacodynamic endpoint, oil treated rats had significantly higher concentrations of DDT in plasma and in the chylomicron fraction, but DDT brain concentrations were the same in both groups. In conclusion, a high-fat meal induces postprandial hyperlipidemia that may significantly alter the pharmacological profile of lipophilic compounds that bind to TRL. This is due to alteration of the distribution characteristics of the lipophilic compound through its association with postprandial lipoproteins. However, this pharmacokinetic phenomenon did not affect the concentration-effect relationship at the site of action in the brain.     

Acute hypertriglyceridemia promotes intestinal lymphatic lipid and drug transport: a positive feedback mechanism in lipid and drug absorption

Elevated systemic levels of triglyceride-rich lipoproteins (TRL) are a risk factor for the development of atherosclerosis. In patients with metabolic syndrome (MetS), intestinal TRL overproduction contributes to high systemic TRL levels, and recent studies suggest that systemic changes in MetS such as increases in plasma fatty acids and insulin resistance stimulate intestinal TRL production. The current study has examined whether increases in systemic TRL influence intestinal lipid transport and lipoprotein assembly pathways and evaluates the impact of these changes on the absorption and lymphatic transport of lipids and a model lipophilic drug (halofantrine). Mesenteric lymph-duct or bile-duct cannulated rats were administered IV saline or (14)C-labeled chylomicron (CM) (to increase systemic TRL) and intraduodenal (3)H lipids and drug. Changes to biliary lipid output and lymphatic lipid and drug transport were subsequently examined. Increasing systemic TRL concentrations stimulated a significant increase in lymphatic lipid and drug transport. The increased lipids in lymph were not derived from bile or the intestinal blood supply (fatty acid or IV infused (14)C-CM). Rather, an increase in lymphatic transport of duodenally sourced lipids was evident. Increasing plasma levels of TRL therefore stimulated lipid absorption and lymphatic transport via a positive feedback process. The data also suggest that the changes to intestinal TRL formation that result from raised systemic TRL levels may impact on the absorption of highly lipophilic drugs and therefore the reproducibility of drug treatments.     

Uptake of lipophilic drugs by plasma derived isolated chylomicrons: Linear correlation with intestinal lymphatic bioavailability

Association of a drug with chylomicrons in the enterocyte is an essential step in the lymphatic absorption pathway. In this article, the uptake of lipophilic compounds by chylomicrons ex vivo was compared to the corresponding intestinal lymphatic bioavailability reported in rats in order to elucidate the degree of correlation and to evaluate the utilization of this correlation as a predictive measurement of the lymphatic bioavailability potential of lipophilic drugs. Nine lipophilic compounds (Vitamin D₃, Vitamin E, halofantrine, probucol, diazepam, testosterone, cyclosporin A, benzo[a]pyrene and p,p′-DDT) at a concentration of 1.75 × 10⁻⁶ M were incubated for 1 h with chylomicron emulsion separated from rat blood. A strong linear correlation was found between the degree of association of compounds with chylomicrons ex vivo and the lymphatic transport reported in rats (r² = 0.94, P < 0.0001), whereas log P and solubility in long chain triglycerides showed only moderate correlation with lymphatic bioavailability. The linear correlation between the degree of uptake of compounds by isolated chylomicrons and intestinal lymphatic transport suggests that the two processes are governed by similar factors. Thus, the degree of association of lipophilic compounds with isolated chylomicrons can be used as a simple screening model for estimation of intestinal lymphatic transport potential of drug molecules. This approach is important in view of the practical difficulties in direct determination of the lymphatic bioavailability in vivo.     

Different impacts of intestinal lymphatic transport on the oral bioavailability of structurally similar synthetic lipophilic cannabinoids: Dexanabinol and PRS-211,220

The aim of this article was to investigate the role of intestinal lymphatic transport in the oral bioavailability of two structurally similar synthetic lipophilic cannabinoids: dexanabinol and PRS-211,220. For this purpose, the long chain triglyceride (LCT) solubility and affinity to chylomicrons ex vivo of both cannabinoids were evaluated. Their oral bioavailability was assessed in rats following administration in a lipid-free and a LCT-based formulation. The intestinal lymphatic transport of these two molecules was also directly measured in a freely moving rat model. LCT solubility of dexanabinol and PRS-211,220 was 7.9 ± 0.2 and 95.8 ± 5.3 mg/g, respectively. The uptake by chylomicrons was moderate (31.6 ± 5.2%) and high (66.1 ± 2.4%), respectively. The bioavailability of dexanabinol (37%) was not affected by LCT solution, whereas administration of PRS-211,220 in LCT improved the absolute oral bioavailability three-fold (from 13 to 35%) in comparison to the lipid-free formulation. The intestinal lymphatic transport of dexanabinol and PRS-211,220 was 7.5 ± 0.8 and 60.7 ± 6.8% of the absorbed dose, respectively. In conclusion, despite structural similarity and similar lipophilicity, dexanabinol and PRS-211,220 exhibited a very diverse pattern of oral absorption, and the lymphatic system played quite a different role in the oral bioavailability of these molecules. The low lymphatic transport of dexanabinol is likely driven by relatively lower affinity to chylomicrons and lower LCT solubility.     

Factors affecting the lymphatic transport of penclomedine (NSC-338720), a lipophilic cytotoxic drug: Comparison to DDT and hexachlorobenzene

Penclomedine (NSC-338720) is a low melting point, poorly water soluble cytotoxic agent with good solubility in triglycerides and a high octanol/water partition coefficient. Because of these physical properties, an attempt was made to assess the role that intestinal lymphatic transport plays in the systematic absorption of penclomedine and to compare its behavior to those of DDT and hexachlorobenzene, two other xenobiotics with somewhat similar properties. After intraduodenal administration to anesthetized rats as either a 10% o/w emulsion prepared from tributyrin, trioctanoin, triolein, soybean oil or mineral oil, or as a suspension, the rank order of lymphatic transport for penclomedine was soybean oil ≈ triolein > trioctanoin ≈ mineral oil ≈ tributyrin ≈ suspension. Correcting for bioavailability, approx. 3% of the absorbed dose was lymphatically transported for both soybean oil and triolein emulsions. Less than 0.75% of the absorbed dose was transported after administration in the other vehicles. The low percent of the absorbed dose transported in the intestinal lymphatics may be related to the high extent of plasma protein and red blood cell binding of penclomedine and lower partition coefficient compared to DDT, whereas the low degree of lymphatic transport of hexachlorobenzene may be related to its more limited water and lipid solubility combined with considerable plasma protein and red blood cell binding. A high degree of plasma protein and red blood cell binding is proposed to compete with chylomicron affinity in the dynamics of intestinal lymph versus portal blood transport.     

Comparison of the lymphatic transport of a lipophilic drug from vehicles containing alpha-tocopherol and/or triglycerides in rats.

The applicability of alpha-tocopherol as a lymphotropic carrier for a highly lipophilic drug has been evaluated. Transport to the intestinal lymph of the highly lipophilic model drug, Lu28-179, in rats after administration to the stomach in an alpha-tocopherol emulsion was compared with lymphatic transport after administration of a sesame oil emulsion and an alpha-tocopherol/sesame oil emulsion. Lymphatic transport of the triglycerides and of alpha-tocopherol was determined. A conscious rat model was used, and the mesenteric lymph was collected. There was no significant difference between the cumulative masses of triglyceride from the two emulsions containing triglyceride 24 h after administration. Administration of an alpha-tocopherol emulsion seemed to induce mobilization of endogenous triglyceride. The lymphatic transport of alpha-tocopherol was less than 1 mg 24 h after administration of both emulsions containing alpha-tocopherol. The absorption of Lu28-179 from the alpha-tocopherol emulsion was very low, with a lymphatic recovery of 0.05%. When administered in an alpha-tocopherol/sesame oil emulsion, the recovery of Lu28-179 increased sevenfold to 0.35%. However, after administration of Lu28-179 in a sesame oil emulsion, the lymphatic recovery increased a further 13-fold to 4.5%. In conclusion, the study showed that alpha-tocopherol did not promote lymphatic absorption of Lu28-179 and thus was not a good lymphotropic carrier, as compared with sesame oil. Alpha-tocopherol in combination with sesame oil was not a good lymphotropic carrier either. The non-absorbed alpha-tocopherol fraction in the intestine might be able to prevent the absorption of Lu28-179.     

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 effect of oils on the lymphatic absorption of DDT

Plasma concentrations of DDT were measured in conscious rats following oral administration in arachis oil, Miglyol 812 (fractionated coconut oil), liquid paraffin or as a fine suspension in water. The total absorption of DDT, calculated as the area under the plasma concentration time curve, was significantly greater for the arachis oil formulation compared to the other vehicles. In anaesthetized rats it was shown that DDT absorption was almost totally via the lymphatic system, and that lymph levels of DDT were highest following administration in arachis oil. Measurement of lymph flow showed that this enhanced absorption of DDT in the presence of arachis oil was not due to an increased flow as both arachis oil and liquid paraffin stimulated lymph production to the same extent.     

Effect of a high-fat meal on the bioavailability of a polymer-coated erythromycin particle tablet formulation

The effect of food on the relative bioavailability of an erythromycin particles-in-tablet formulation was studied in 27 healthy volunteers, using a four-way, crossover study design with the following treatments: one or two erythromycin capsules USP (Eryc, Parke-Davis), or one polymer-coated erythromycin particles-in-tablet (PCE, Abbott) administered fasting or with a high-fat meal. Under fasting conditions the erythromycin particles-in-tablet and erythromycin capsule formulations are bioequivalent based on similar tmax and dose-normalized Cmax and AUC values. The rate and extent of absorption from the particles-in-tablet formulation, however, are dramatically reduced following administration with a meal. Mean Cmax and AUC values decreased by 73% and 72%, respectively, and seven subjects had no detectable erythromycin plasma concentrations for 16 hours following administration of the particles-in-tablet formulation with the high-fat meal. Greater than 40% of the subjects had nonfasting Cmax and AUC values that were less than 10% of those values following administration of the dose fasting. Cmax and AUC values in nonfasting subjects were within 75% to 125% of fasting values in only two and one of 27 subjects, respectively. The erythromycin particles-in-tablet formulation therefore should not be administered with meals.     

Intestinal lymphatic drug transport: an update

The trend towards identification of poorly water-soluble and highly lipophilic candidate drug molecules has led to an increase in interest in intestinal lymphatic drug transport. In this article we provide a brief background to the mechanism of access of drugs to the intestinal lymph and the role of lipid digestion and absorption in the stimulation of lymphatic transport. The ability of different lipid types to stimulate lymphatic drug transport, is addressed, concentrating specifically on the impact of the class, chain length and degree of unsaturation of co-administered lipids. Comment is also made as to the relevance of dosing different lipid volumes to the rat and the possible complications this may provide when trying to assess the likely extent of intestinal lymphatic transport. Recent studies are described in which the extent of lymphatic transport of a highly lipophilic antimalarial, halofantrine, was investigated after post-prandial administration to greyhound dogs. Finally the possible future directions for studies of intestinal lymphatic transport are discussed, including the use of cell culture models and genetically modified animals.     

Effect of different surfactants in biorelevant medium on the secretion of a lipophilic compound in lipoproteins using Caco-2 cell culture

The impact of a pharmaceutical relevant metabolizable, ionic surfactant or two synthetic, nonionic surfactants on the absorption and lipoprotein incorporation of a lipophilic drug, retinol, was studied in the Caco-2 cell culture. Filter-grown monolayers of Caco-2 cells were incubated for 20 h with (3)H-retinol and (14)C-oleic acid and with increasing concentrations of lyso-phosphatidylcholine (lyso-PC), Cremophor RH40, or Tween 80. The concentration of (3)H-retinol and (14)C-lipid was measured in the apical, intracellular, and basolateral compartments. The basolateral medium was ultracentrifugated into different lipoprotein classes and their (3)H-retinol and (14)C-lipid concentrations were determined. The cells incubated with lyso-PC and Tween 80 increased the incorporation of (3)H-retinol and (14)C-lipid into chylomicrons and very low density lipoproteins (VLDL). The explored surfactants impacted the incorporation of (3)H-retinol and (14)C-lipid in chylomicrons and VLDL in a concentration-dependent manner. As these surfactants interfere with the intestinal lipoprotein secretion, inclusion of high concentrations of the surfactants in lipid-based formulations of poorly aqueous soluble drugs might impact the degree of intestinal lymphatic transport of the drug after oral administration.     

Food intake increases the relative oral bioavailability of vanoxerine.

Each of 12 healthy male subjects received single oral doses of 100 mg vanoxerine (GBR 12909), a dopamine reuptake inhibitor with potential antidepressant activity, on three different occasions (fasting, after a low-fat meal and after a high-fat meal) according to a randomized, cross-over design. The mean tmax value increased from 0.82 h after fasting to 1.44 h after a low-fat meal and to 2.46 h after a high-fat meal. Only modest food effects were seen on mean Cmax values (55 nM, 52 nM and 84 nM, after fasting, after the low-fat meal and after the high-fat meal, respectively) but values of AUC up to the last measurable concentration (AUC(0,t)) increased by 76% (from 110 to 194 nM h) after the low-fat meal and by 255% (from 110 to 391 nM h) after the high-fat meal compared with fasting. All of these effects were statistically significant except for the differences in tmax and Cmax between fasting and the low-fat meal. The mechanism of these changes is unclear, but it seems likely that food may lower the first-pass metabolism of vanoxerine, as has been shown for other lipophilic basic drugs.     

Intestinal lymphatic transport for drug delivery

Intestinal lymphatic transport has been shown to be an absorptive pathway following oral administration of lipids and an increasing number of lipophilic drugs, which once absorbed, diffuse across the intestinal enterocyte and while in transit associate with secretable enterocyte lipoproteins. The chylomicron-associated drug is then secreted from the enterocyte into the lymphatic circulation, rather than the portal circulation, thus avoiding the metabolically-active liver, but still ultimately returning to the systemic circulation. Because of this parallel and potentially alternative absorptive pathway, first-pass metabolism can be reduced while increasing lymphatic drug exposure, which opens the potential for novel therapeutic modalities and allows the implementation of lipid-based drug delivery systems. This review discusses the physiological features of the lymphatics, enterocyte uptake and metabolism, links between drug transport and lipid digestion/re-acylation, experimental model (in vivo, in vitro, and in silico) of lymphatic transport, and the design of lipid- or prodrug-based drug delivery systems for enhancing lymphatic drug transport.     

A physicochemical basis for the extensive intestinal lymphatic transport of a poorly lipid soluble antimalarial, halofantrine hydrochloride, after postprandial administration to dogs

The highly lipid soluble, free-base form of halofantrine (Hf base; approximately 50 mg/mL in triglyceride lipids), a highly lipophilic (calculated log P approximately 8.5) antimalarial, has recently been shown to undergo significant intestinal lymphatic transport (54% of administered dose) after postprandial administration to dogs. In contrast, the clinically available hydrochloride salt of Hf (Hf small middle dot HCl), was not considered to be a likely substrate for lymphatic transport because its solubility in long-chain triglyceride lipids is low (< 1 mg/mL). This paper reports the lymphatic transport of Hf after postprandial administration of Hf.HCl, which was surprisingly high at 47% of the administered dose, and not significantly different from that of Hf base. It was postulated that partial conversion of solubilized Hf.HCl to the highly lipid soluble Hf base within the intestinal lumen might account for the extensive lymphatic transport. However, as Hf is a tertiary amine with an expected pK(a) of > 10, at gastrointestinal pH, the fraction of Hf present as the free base form is likely to be extremely low. Physicochemical studies exploring the solubility and pK(a) of Hf suggest that Hf.HCl was extensively solubilized following fed administration. When solubilized in representative fed state mixed micellar solutions, its apparent pK(a) was 6.92 and considerably lower than anticipated for a tertiary amine. It appears that the extensive lymphatic transport of Hf observed after postprandial administration of Hf.HCl was likely to be due to the conversion of solubilized Hf.HCl to the free base. Therefore, in addition to indicators such as log P and triglyceride solubility, factors such as drug solubilization in representative fed state intestinal conditions and the possible conversion to the un-ionized form should be considered when predicting the potential lymphatic transport of salts of poorly water soluble acids and bases.     

Lipid-based formulations for intestinal lymphatic delivery

The current state of the art of intestinal lymphatic transport is given by reviewing the more recent publications, which have utilized lipid-based vehicles. The results published often show variable trends depending on, the design of the vehicle, the components used, the physicochemical properties of the drug, the animal model and experimental techniques, these variables often make direct comparisons difficult. Traditionally intestinal lymphatic delivery has been expressed as a percentage of the dose transported in the lymph. Using this parameter results obtained to date, with lipid-based vehicles, are somewhat disappointing maximising at approximately 20–30%, for highly lipophilic compounds including DDT and halofantrine (Hf). Recent data, monitoring Hf, in a fed versus fasted dog study, have shown that a higher degree of lymphatic transport is possible (>50% dose) in the postprandial state, this study should result in stimulating renewed interest in the potential of achieving significant levels of lymphatic targeting. Although some relevant features controlling lymphatic transport have been identified over the years a deeper appreciation of all the mechanisms, which is vital for therapeutic exploitation of lymphatic transport, is still unrealized. This review analyses the success and limitations of a formulation approach using lipid-based vehicles and highlights potential areas for further research.     

Pharmacokinetics of Amiodarone in hyperlipidemic and simulated high fat-meal rat models

The objective of this study was to examine the effect of a high fat meal and hyperlipidemia on the pharmacokinetic behavior of amiodarone. To evaluate these effects, single doses of amiodarone were administered to rats i.v. (25 mg/kg) or orally (50 mg/kg). Some rats were rendered hyperlipidemic by intraperitoneal doses of poloxamer 407 followed by amiodarone i.v. In other normolipidemic rats, amiodarone was administered i.v. in a fasted state or after the administration of 1% cholesterol in peanut oil. Amiodarone plasma concentrations were considerably (>11-fold) increased in hyperlipidemia. Substantial decreases were noted in the clearance, volume of distribution and unbound fraction (11.6, 23 and 24.7-fold, respectively) in plasma of hyperlipidemic rats. Oral lipid caused a significant increase in plasma AUC(0-infinity) (1.38-fold) and a significant decrease in clearance (1.5-fold) of amiodarone after intravenous doses. Oral consumption of 1% cholesterol in peanut oil significantly increased the plasma AUC (1.83-fold) and bioavailability of amiodarone (1.31-fold) after oral doses. In determining oral bioavailability of lipophilic drugs such as amiodarone in food effect studies, in addition to the increase in absorption of drugs, other factors such as a decrease in clearance due to increases in lipoprotein levels should be taken into account.     

Synthesis and evaluation of 5′ alkyl ester prodrugs of zidovudine for directed lymphatic delivery

The butanoic, lauric and oleic acid ester prodrugs of the anti-AIDS drug zidovudine (AZT) have been synthesised and assessed for their ability to promote the transport of AZT through the intestinal lymph (a major reservoir for the human immunodeficiency virus (HIV)). The octanol/water partition co-efficient and triglyceride solubility of the AZT prodrugs increased with increasing chain length of the alkyl pro-moiety, and the observed values were consistent with that required for potential intestinal lymphatic transport after oral administration. The intestinal lymphatic transport of AZT and the ester prodrugs was assessed after intraduodenal administration as a micellar lipid solution in an anesthetised rat model. Systemic blood was also sampled in order to estimate the overall extent of absorption.The lymphatic transport of AZT was similar when administered as either AZT alone or the lipophilic ester prodrugs, where the amount of AZT collected in fistulated mesenteric lymph was approximately 0.1-0.2% of the administered dose (15 mg/kg AZT). The extent of absorption of AZT, estimated from the area under the plasma concentration time profiles of AZT, when dosed as either parent compound or the lipophilic esters, was essentially complete. These data suggest that rapid bioconversion of the ester prodrugs to AZT in either the intestinal lumen or the enterocyte limits exploitation of this approach as a means of enhancing the selective lymphatic delivery of AZT.     

Effect of lipid vehicle on the intestinal lymphatic transport of isotretinoin in the rat

Many lipophilic compounds are absorbed to some degree via the lymphatics, however, the mechanisms and factors controlling this absorption process are unclear. In order to provide some information on this area we have studied the effect of lipid vehicle on the lymphatic transport of isotretinoin following oral dosing to the rat. Oils containing higher percentages of the linoleate triglyceride ester appeared to promote both enhanced lymph flow and chylomicron concentration. Long chain triglyceride oils proved to be the most effective vehicles for increasing lymphatic transport - especially cottonseed oil and peanut oil. The solubility of the drug in the oil was also shown to be a key factor in lymphatic transport     

Linezolid absolute bioavailability and the effect of food on oral bioavailability.

Linezolid is a novel oxazolidinone antibiotic that has a spectrum of activity encompassing a variety of Gram-positive bacteria. The objectives of this study were twofold: (1) to compare the absorption of linezolid tablets given immediately following a high-fat meal with the absorption of tablets administered while fasting, and (2) to assess the bioavailability of a 375-mg oral dose given while fasting relative to a 375-mg dose of linezolid sterile solution given intravenously. Venous blood samples were taken over the 48 h following the single dose administration of both the oral and intravenous (IV) treatment. Samples were subsequently frozen for the determination of linezolid concentrations by HPLC. The only statistically significant difference between the fasted and the fed treatment was in peak plasma concentration, with the mean C(max) for fasted subjects being 23% greater than that for subjects after consumption of a high-fat meal. Comparable AUC(0-infinity) values were measured under both conditions, indicating that the overall extent of absorption is the same. Therefore, the difference in C(max), while statistically significant, should not affect the therapeutic efficacy of linezolid when it is administered with food. There were no statistically significant differences in AUC(0-infinity), CL or half-life between the fasted oral treatment and the intravenous treatment. As expected, C(max) was statistically different between the two treatments. However, the mean absolute bioavailability (F) of the tablet, using the IV sterile solution as the reference treatment, was 103% (+/-20%).