Focus on the Lymphatic Route to Optimize Drug Delivery in Cardiovascular Medicine

While oral agents have been the gold standard for cardiovascular disease therapy, the new generation of treatments is switching to other administration options that offer reduced dosing frequency and more efficacy. The lymphatic network is a unidirectional and low-pressure vascular system that is responsible for the absorption of interstitial fluids, molecules, and cells from the peripheral tissue, including the skin and the intestines. Targeting the lymphatic route for drug delivery employing traditional or new technologies and drug formulations is exponentially gaining attention in the quest to avoid the hepatic first-pass effect. The present review will give an overview of the current knowledge on the involvement of the lymphatic vessels in drug delivery in the context of cardiovascular disease.

Effect of Dihydropyridine Enrichment in the Microstructure of the Palisade Layer on the Stability of Fat Nano-emulsions

Relationship between the stability of fat nano-emulsions and the incorporated drug at the molecular level are rarely known. Herein, fat nano-emulsions containing dihydropyridine drugs were prepared and the microstructure of their palisade layers were investigated.The prepared 1.0 mg/mL nimodipine nano-emulsion was found to contain 65.50% drug in the palisade layer. The increasing drug concentration led to a decrease-increase-decrease trend in centrifugal stability constant, particle size and proton nuclear magnetic resonance (¹H NMR) signal intensity of the lecithin trimethyl ammonium group in the nimodipine and felodipine nano-emulsions. The ¹H NMR spectra of test solutions including nano-emulsions suggest that increasing drugs penetrated into the palisade layer, resulting in the lecithin arrangement from loose to tight, and then from monolayer to bilayer. Nimodipine and felodipine nano-emulsions showed two valley values at concentrations of 0.15 and 0.75 mg/mL, and 0.30 and 0.90 mg/mL respectively, which indicated that the nano-emulsion has two more stable states corresponding to the tightly arranged mono- and bi-palisade layer. These two concentrations are positively correlated with lipophilicity of nimodipine and felodipine. Further, nimodipine liposomes were prepared to validate the effect of drugs on the arrangement of lecithin in the palisade layer. ¹H NMR characterizations of the liposomes showed a similar profile to that of nano-emulsions. These results demonstrated that the increasing drug concentration could cause a rearrangement of lecithin in the palisade layer, thus affecting emulsion stability.

Intestinal delivery in a long-chain fatty acid formulation enables lymphatic transport and systemic exposure of orlistat

Orlistat is a pancreatic lipase (PL) inhibitor that inhibits dietary lipid absorption and is used to treat obesity. The oral bioavailability of orlistat is considered zero after administration in standard formulations. This is advantageous in the treatment of obesity. However, if orlistat absorption could be improved it has the potential to treat diseases such as acute and critical illnesses where PL transport to the systemic circulation via gut lymph promotes organ failure. Orlistat is highly lipophilic and may associate with intestinal lipid absorption pathways into lymph. Here we investigate the potential to improve orlistat lymph and systemic uptake through intestinal administration in lipid formulations (LFs). The effect of lipid type, lipid dose, orlistat dose, and infusion time on lymph and systemic availability of orlistat was investigated. After administration in all LFs, orlistat concentrations in lymph were greater than in plasma, suggesting direct transport via lymph. Lymph and plasma orlistat derivative concentrations were ~8-fold greater after administration in a long-chain fatty acid (LC-FA) compared to a lipid-free, LC triglyceride (LC-TG) or medium-chain FA (MC-FA) formulation. Overall, administration of orlistat in a LC-FA formulation promotes lymph and systemic uptake which may enable treatment of diseases associated with elevated systemic PL activity.

Physical Characterization of Halofantrine-Encapsulated Fat Nanoemulsions

We report the colloidal characterization of halofantrine (Hf)-laden soybean oil fat emulsions. Hf increased the zeta potential, at all pH values, of the fat emulsions. Concomitant with this, the isoelectric point (i.e.p.) of the emulsion increased to higher pH values. The emulsion was destabilized by a small amount of Hf; interestingly, however, this was ameliorated by increasing the amount of Hf. The particle size and polydispersity of the fat emulsion reflected this with a small Hf concentration resulting in a significant increase in both particle size and polydispersity, but less so as the Hf concentration was increased. Emulsions lost stability as the pH approached the i.e.p. and this effect was greatest for the small Hf concentration emulsions. Cryogenic transmission electron microscopy showed the presence of beading or string-like behavior leading to gross distortions of the spherical shape for highly unstable emulsions. We conclude that to maintain good stability for Hf-laden soybean oil emulsions, the pH of the emulsion should be kept away from its i.e.p, and also that the drug concentration should be maintained at a relatively high value.

The mesenteric lymph duct cannulated rat model: application to the assessment of intestinal lymphatic drug transport

The intestinal lymphatic system plays key roles in fluid transport, lipid absorption and immune function. Lymph flows directly from the small intestine via a series of lymphatic vessels and nodes that converge at the superior mesenteric lymph duct. Cannulation of the mesenteric lymph duct thus enables the collection of mesenteric lymph flowing from the intestine. Mesenteric lymph consists of a cellular fraction of immune cells (99% lymphocytes), aqueous fraction (fluid, peptides and proteins such as cytokines and gut hormones) and lipoprotein fraction (lipids, lipophilic molecules and apo-proteins). The mesenteric lymph duct cannulation model can therefore be used to measure the concentration and rate of transport of a range of factors from the intestine via the lymphatic system. Changes to these factors in response to different challenges (e.g., diets, antigens, drugs) and in disease (e.g., inflammatory bowel disease, HIV, diabetes) can also be determined. An area of expanding interest is the role of lymphatic transport in the absorption of orally administered lipophilic drugs and prodrugs that associate with intestinal lipid absorption pathways. Here we describe, in detail, a mesenteric lymph duct cannulated rat model which enables evaluation of the rate and extent of lipid and drug transport via the lymphatic system for several hours following intestinal delivery. The method is easily adaptable to the measurement of other parameters in lymph. We provide detailed descriptions of the difficulties that may be encountered when establishing this complex surgical method, as well as representative data from failed and successful experiments to provide instruction on how to confirm experimental success and interpret the data obtained.

The influence of intestinal lymphatic transport on the systemic exposure and brain deposition of a novel highly lipophilic compound with structural similarity to cholesterol

Objectives

To assess the role of intestinal lymphatic transport in the oral bioavailability and brain deposition of a highly lipophilic, centrally acting drug candidate (Org 49209) in comparison to cholesterol, a close structural analogue.

Methods

The intestinal lymphatic transport of Org 49209 and cholesterol was assessed in lymph-cannulated anaesthetised rats and total bioavailability evaluated in non-lymph-cannulated animals. Parallel groups were employed to examine the brain deposition of Org 49209 after intraduodenal and intraperitoneal administrations.

Key findings

The contribution of intestinal lymphatic transport to total bioavailability was similar for Org 49209 and cholesterol (approximately 40% of the absorbed dose). However, the oral bioavailability of Org 49209 was significantly (fourfold) lower than cholesterol. Brain deposition of Org 49209 was similar after intraduodenal and intraperitoneal administration. Systemic exposure, however, was higher after intraduodenal administration and brain-to-plasma ratios were therefore reduced.

Conclusion

The oral bioavailability of Org 49209 was significantly lower than that of its structural analogue cholesterol; however, intestinal lymphatic transport played a similar role in oral bioavailability for both compounds. Brain to plasma ratios were lower after intraduodenal versus intraperitoneal administration, suggesting that drug association with intestinal lymph lipoproteins may limit central nervous system access for highly lipophilic drugs.

Strategies to address low drug solubility in discovery and development

Drugs with low water solubility are predisposed to low and variable oral bioavailability and, therefore, to variability in clinical response. Despite significant efforts to "design in" acceptable developability properties (including aqueous solubility) during lead optimization, approximately 40% of currently marketed compounds and most current drug development candidates remain poorly water-soluble. The fact that so many drug candidates of this type are advanced into development and clinical assessment is testament to an increasingly sophisticated understanding of the approaches that can be taken to promote apparent solubility in the gastrointestinal tract and to support drug exposure after oral administration. Here we provide a detailed commentary on the major challenges to the progression of a poorly water-soluble lead or development candidate and review the approaches and strategies that can be taken to facilitate compound progression. In particular, we address the fundamental principles that underpin the use of strategies, including pH adjustment and salt-form selection, polymorphs, cocrystals, cosolvents, surfactants, cyclodextrins, particle size reduction, amorphous solid dispersions, and lipid-based formulations. In each case, the theoretical basis for utility is described along with a detailed review of recent advances in the field. The article provides an integrated and contemporary discussion of current approaches to solubility and dissolution enhancement but has been deliberately structured as a series of stand-alone sections to allow also directed access to a specific technology (e.g., solid dispersions, lipid-based formulations, or salt forms) where required.

A mouse model to evaluate the impact of species, sex, and lipid load on lymphatic drug transport

PURPOSE

To establish a lymph-cannulated mouse model, and use the model to investigate the impact of lipid dose on exogenous and endogenous lipid recruitment, and drug transport, into the lymph of males versus females. Finally, lymphatic transport and drug absorption in the mouse were compared to other pre-clinical models (rats/dogs).

METHODS

Animals were orally or intraduodenally administered 1.6 mg/kg halofantrine in low or high (14)C-lipid doses. For bioavailability calculation, animals were intravenuosly administered halofantrine. Lymph or blood samples were taken and halofantrine, triglyceride, phospholipid and (14)C-lipid concentrations measured.

RESULTS

Lymphatic lipid transport increased linearly with lipid dose, was similar across species and in male/female animals. In contrast, lymphatic transport of halofantrine differed markedly across species (dogs>rats>mice) and plateaued at higher lipid doses. Lower bioavailability appeared responsible for some species differences in halofantrine lymphatic transport; however other systematic differences were involved.

CONCLUSIONS

A contemporary lymph-cannulated mouse model was established which will enable investigation of lymphatic transport in transgenic and disease models. The current study found halofantrine absorption and lymphatic transport are reduced in small animals. Future analyses will investigate mechanisms involved, and if similar trends occur for other drugs, to establish the most relevant model(s) to predict lymphatic transport in humans.