Lymphatically Transported Research Articles

Oral absorption and lymphatic transport of baicalein following drug-phospholipid complex incorporation in self-microemulsifying drug delivery systems.

PurposeThe aims of this study were to prepare a baicalein self-microemulsion with baicalein-phospholipid complex as the intermediate (BAPC-SMEDDS) and to compare its effects with those of conventional baicalein self-microemulsion (CBA-SMEDDS) on baicalein oral absorption and lymphatic transport.MethodsTwo SMEDDS were characterized by emulsifying efficiency, droplet size, zeta potential, cloud point, dilution stability, physical stability, and in vitro release and lipolysis. Different formulations of 40 mg/kg baicalein were orally administered to Sprague-Dawley rats to investigate their respective bioavailabilities. The chylomicron flow blocking rat model was used to evaluate their lymphatic transport.ResultsThe droplet sizes of BAPC-SMEDDS and CBA-SMEDDS after 100x dilution were 9.6±0.2 nm and 11.3±0.4 nm, respectively. In vivo experiments indicated that the relative bioavailability of CBA-SMEDDS and BAPC-SMEDDS was 342.5% and 448.7% compared to that of free baicalein (BA). The AUC0–t and Cmax of BAPC-SMEDDS were 1.31 and 1.87 times higher than those of CBA-SMEDDS, respectively. The lymphatic transport study revealed that 81.2% of orally absorbed BA entered the circulation directly through the portal vein, whereas approximately 18.8% was transported into the blood via lymphatic transport. CBA-SMEDDS and BAPC-SMEDDS increased the lymphatic transport ratio of BA from 18.8% to 56.2% and 70.2%, respectively. Therefore, self-microemulsion not only significantly improves oral bioavailability of baicalein, but also increases the proportion lymphatically transported. This is beneficial to the direct interaction of baicalein with relevant immune cells in the lymphatic system and for proper display of its effects.ConclusionThis study demonstrates the oral absorption and lymphatic transport characteristics of free baicalein and baicalein SMEDDS with different compositions. This is of great significance to studies on lymphatic targeted delivery of natural immunomodulatory compounds.

The Impact of Lymphatic Transport on the Systemic Disposition of Lipophilic Drugs

This work investigates the influence of drug absorption route (intestinal lymphatics vs. blood supply) on drug pharmacokinetics and tissue distribution. To achieve this aim, the pharmacokinetics and tissue distribution of model compounds [1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane, DDT; halofantrine] and lipids were assessed following intravenous delivery in lymph lipoproteins or plasma, and were found to differ significantly. For DDT, the clearance (CL) and volume of distribution (Vd ) were higher, whereas for halofantrine, CL and V(d) were lower, after entry in lymph versus plasma due, in particular, to differences in adipose tissue and liver uptake. In a recent study, halofantrine CL and V(d) were similar following entry in lymph or entry in plasma into the systemic circulation of animals predosed with lymph, whereas in the current study, predosing lymph did not influence DDT CL and V(d). For compounds such as DDT, changes to the route of absorption may thus directly impact on pharmacokinetics and tissue distribution, whereas for halofantrine factors that influence lymphatic transport may, by altering systemic lipoprotein concentrations, indirectly impact pharmacokinetics and tissue distribution. Ultimately, careful control of dosing conditions (formulation, prandial state), and thus the extent of lymphatic transport, may be important in assuring reproducible efficacy and toxicity for lymphatically transported drugs.

Intravenous Dosing Conditions May Affect Systemic Clearance for Highly Lipophilic Drugs: Implications for Lymphatic Transport and Absolute Bioavailability Studies

The clearance (Cl) and volume of distribution (V(ss)) of a lipophilic, lymphatically transported drug, halofantrine (Hf) have been evaluated after intravenous delivery to the systemic circulation in ex vivo lymph and plasma, and compared with the data obtained after administration of a lipid-based emulsion and a lipid-free cosolvent formulation. Systemic Cl and V(ss) were significantly lower (approximately twofold) after delivery of Hf in lymph or the emulsion when compared with the administration in plasma or the cosolvent formulation. Preadministration of drug-free lymph, immediately before administration of drug in plasma, however, resulted in plasma profiles consistent with that obtained after administration of drug in lymph/emulsion. Where drug and lipid entered the systemic circulation coincidentally, systemic Cl of Hf, therefore, appeared to be relatively unaffected by the route of entry to the systemic circulation (i.e. via the lymph or the blood), but more significantly altered by total plasma lipid levels. Because temporal changes to plasma lipid levels occur as a result of the absorption of formulation or food-derived lipids and the infusion of intravenous lipid emulsions, the current data suggest that a mismatch in plasma lipid levels after intravenous and oral administrations may lead to differences in drug Cl and errors in bioavailability assessment.

The Mechanism of Lymphatic Access of Two Cholesteryl Ester Transfer Protein Inhibitors (CP524,515 and CP532,623) and Evaluation of Their Impact on Lymph Lipoprotein Profiles

To explore the mechanism of lymphatic access of the CETP inhibitors (CETPi) CP524,515 and CP532,623 and probe their potential effect on lymph lipoprotein development. Lymphatic access mechanisms were examined via correlation of lymphatic drug transport profiles with drug affinity for lymph lipoproteins and drug solubility in representative combinations of lipoprotein lipids. The effects of the CETPi on lymph lipoprotein profiles were evaluated by ultracentrifugation and flow cytometry. Both CETPi were highly lymphatically transported (22-28% of dose), and lymphatic transport was closely correlated with drug affinity for ex-vivo lymph lipoproteins or triglyceride emulsions and poorly related to solubility in mixtures of lipoprotein core and/or surface lipids. Both CETPi altered the kinetics of lymph lipid transport and decreased lymph lipid transport in chylomicrons. Lymphatic transport of the CETPi appears to reflect high affinity for the interface of lymph lipoproteins rather than solubilisation in the lipoprotein core and confirms that triglyceride solubilities >50 mg/g are not necessarily a pre-requisite for lymphatic transport. The CETPi also led to changes to lipoprotein processing in the enterocyte including a reduction in lipid transport in chylomicrons. Changes to intestinal lipoprotein profiles may contribute to the changes in systemic lipoprotein levels seen during CETPi therapy.

The Role of the Intestinal Lymphatics in the Absorption of Two Highly Lipophilic Cholesterol Ester Transfer Protein Inhibitors (CP524,515 and CP532,623)

To evaluate the potential role of intestinal lymphatic transport in the absorption and oral bioavailability of members of an emerging class of anti-atherosclerosis drugs (CETP inhibitors). CP524,515 and CP532,623 are structurally related with eLogD(7.4) >5; however, only CP524,515 (and not CP532,623) had sufficient solubility (>50 mg/g) in long-chain triglyceride (LCT) to be considered likely to be lymphatically transported. CP524,515 and CP532,623 were administered intravenously and orally to fasted or fed lymph-cannulated or non-cannulated dogs. Oral bioavailability and lymphatic transport of drug (and triglyceride) was subsequently quantified. Both CETP inhibitors were substantially transported into the lymphatic system (>25% dose) in fed and fasted dogs. Food enhanced oral bioavailability (from 45 to 83% and 44 to 58% for CP524,515 and CP532,623, respectively) and the proportion of the absorbed dose transported via the lymph (from 61 to 86% and from 68 to 83%, respectively). Lymphatic triglyceride transport was significantly lower in fed dogs administered CP532,623. Intestinal lymphatic transport is the major absorption pathway for CP524,515 and CP532,623, suggesting that a LCT solubility >50 mg/g is not an absolute requirement for lymphatic transport. The effect of CP532,623 on intestinal lipid transport may suggest a role in the activity/toxicity profiles of CETP inhibitors.

AN EXAMINATION OF THE INTERPLAY BETWEEN ENTEROCYTE-BASED METABOLISM AND LYMPHATIC DRUG TRANSPORT IN THE RAT

The current study has examined whether drugs that are transported to the systemic circulation via the intestinal lymph (and therefore associate with lipoproteins within the enterocyte) are accessible to enterocyte-based metabolic processes. The impact of changes to the mass of lipid present within the enterocyte-based lymph lipid precursor pool (LLPP) on the extent of enterocyte-based drug metabolism has also been addressed. Low (5 mg oleic acid/h) or high [20 mg oleic acid/5.2 mg lyso-phosphatidylcholine/h] lipid dose formulations containing halofantrine (which is lymphatically transported and metabolized) or dichlorodiphenyltrichloroethane (DDT) (which is lymphatically transported and relatively metabolically inert) and radiolabeled oleic acid were infused into the duodenum of lymph duct-cannulated rats. After 5 h, drug and radiolabeled oleic acid were removed from the infusions, allowing calculation of the first order turnover rate constants describing drug and oleic acid transport from the LLPP into lymph from the washout profiles. In one group of animals, bolus doses of ketoconazole were also administered to inhibit cytochrome P450-based metabolism. The rate constant describing halofantrine transport from the LLPP into the lymph was lower than that of oleic acid, whereas these differences were abolished in the presence of ketoconazole. DDT and oleic acid exhibited similar turnover rate constants. The data therefore suggest that enterocyte-based metabolism removes halofantrine from the LLPP before transport into the lymph. Furthermore, enhancing the lymphatic transport of halofantrine by coadministration of larger quantities of lipid reduced the difference between the turnover rate constant for halofantrine and oleic acid and seemed to reduce the extent of enterocyte-based metabolism.

Contribution of lymphatically transported testosterone undecanoate to the systemic exposure of testosterone after oral administration of two andriol formulations in conscious lymph duct-cannulated dogs.

Orally administered testosterone (T) is ineffective in the treatment of male androgen deficiency syndromes due to extensive presystemic first-pass metabolism. In contrast, the lipophilic long-chain ester testosterone undecanoate (TU) exhibits androgenic activity that has been attributed to formation of T via systemic hydrolysis of lymphatically transported TU. However, there are no definitive data regarding the oral bioavailability of TU or the extent to which lymphatically transported TU contributes to the systemic availability of T after oral TU administration. This report describes the application of stable isotope methodology in a thoracic lymph duct-cannulated dog model to study the oral bioavailability and lymphatic transport of TU after postprandial administration. When administered as either Andriol or Andriol Testocaps, the mean (+/-S.E., n = 4) absolute bioavailability of TU was 3.25 +/- 0.48 and 2.88 +/- 0.88%, respectively, and lymphatically transported TU accounted for between 91.5 and 99.7% of the systemically available ester. Model-independent pharmacokinetic analysis indicated that 83.6 +/- 1.6 and 84.1 +/- 8.2% of the systemically available T, resulting from Andriol or Andriol Testocaps, respectively, was due to systemic hydrolysis of lymphatically transported TU. These data demonstrate that intestinal lymphatic transport of TU produces increased systemic exposure of T by avoiding the extensive first-pass effect responsible for the inactivation of T after oral administration.

Intestinal lymphatic transport of halofantrine occurs after oral administration of a unit-dose lipid-based formulation to fasted dogs.

To examine whether the small quantities of lipid present in unit-dose microemulsion formulations comprising medium- (C8-10) or long-chain (C18) glyceride lipids can stimulate the intestinal lymphatic transport of halofantrine (Hf), a model lymphatically transported drug. Hf (50 mg) was administered to thoracic lymph duct- and cephalic vein-cannulated fasted greyhound dogs. Drug was formulated as a single soft gelatin capsule containing approximately 1 g of a microemulsion preconcentrate based on either medium- or long-chain glycerides. Thoracic lymph was collected, and systemic plasma samples taken over 10 h postdose. The extent of lymphatic transport of Hf after administration of the long-chain lipid formulation was high (28.3% of dose), and significantly higher than that seen after administration of the medium-chain formulation (5.0% of dose). Plasma levels of Hf were not significantly different across the two formulations when assessed by AUC0-10h. This is the first study to demonstrate that the small amounts of lipid present within a single lipid-based dose form can support substantial intestinal lymphatic transport in the fasted state. Furthermore, microemulsions based on long-chain glycerides appear to be more effective with respect to lymphatic transport than the equivalent medium-chain formulation.

Effect of Short‐, Medium‐, and Long‐Chain Fatty Acid‐Based Vehicles on the Absolute Oral Bioavailability and Intestinal Lymphatic Transport of Halofantrine and Assessment of Mass Balance in Lymph‐Cannulated and Non‐cannulated Rats

The contribution of lymphatic transport and absorption directly into the portal blood to the overall oral bioavailability of a model lipophilic drug, halofantrine (Hf), was determined in lymph‐cannulated, conscious, unrestrained rats after administration in lipidic vehicles with different fatty acid chain lengths. Both lymphatic transport (C18‐based vehicle, 15.8% of dose > C8–10, 5.5% > C4, 2.22% > C0, 0.34%) and total systemic exposure (C18, 22.7% of dose > C8–10, 19.2% > C4, 15.2% > C0, 6.4%) of Hf were enhanced by the presence of lipids in the formulation and specifically by an increase in the fatty acid chain length of the coadministered lipid. Increases in lymphatic drug transport appeared to correlate with increases in lymphatic lipid transport. Surprisingly, where lymphatic transport was the primary mechanism of drug transport to the systemic circulation (i.e., after administration in a C18‐based lipid vehicle), Hf bioavailability assessed in nonlymph‐cannulated animals was lower than the extent of total availability measured in lymph‐cannulated animals (estimated as percent appearing in the intestinal lymph plus percent transported directly into the blood), suggesting either presystemic drug clearance within the lymphatics or an altered systemic clearance pattern for lymphatically transported drug. © 2000 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 89: 1073–1084, 2000

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.