A conscious dog model for assessing the absorption, enterocyte-based metabolism, and intestinal lymphatic transport of halofantrine

Intestinal Lymphatic Biology, Drug Delivery, and Therapeutics: Current Status and Future Directions

Historically, the intestinal lymphatics were considered passive conduits for fluids, immune cells, dietary lipids, lipid soluble vitamins, and lipophilic drugs. Studies of intestinal lymphatic drug delivery in the late 20th century focused primarily on the drugs' physicochemical properties, especially high lipophilicity, that resulted in intestinal lymphatic transport. More recent discoveries have changed our traditional view by demonstrating that the lymphatics are active, plastic, and tissue-specific players in a range of biological and pathological processes, including within the intestine. These findings have, in turn, inspired exploration of lymph-specific therapies for a range of diseases, as well as the development of more sophisticated strategies to actively deliver drugs or vaccines to the intestinal lymph, including a range of nanotechnologies, lipid prodrugs, and lipid-conjugated materials that "hitchhike" onto lymphatic transport pathways. With the increasing development of novel therapeutics such as biologics, there has been interest in whether these therapeutics are absorbed and transported through intestinal lymph after oral administration. Here we review the current state of understanding of the anatomy and physiology of the gastrointestinal lymphatic system in health and disease, with a focus on aspects relevant to drug delivery. We summarize the current state-of-the-art approaches to deliver drugs and quantify their uptake into the intestinal lymphatic system. Finally, and excitingly, we discuss recent examples of significant pharmacokinetic and therapeutic benefits achieved via intestinal lymphatic drug delivery. We also propose approaches to advance the development and clinical application of intestinal lymphatic delivery strategies in the future. SIGNIFICANCE STATEMENT: This comprehensive review details the understanding of the anatomy and physiology of the intestinal lymphatic system in health and disease, with a focus on aspects relevant to drug delivery. It highlights current state-of-the-art approaches to deliver drugs to the intestinal lymphatics and the shift toward the use of these strategies to achieve pharmacokinetic and therapeutic benefits for patients.

Pharmacokinetics of cycloheximide in rats and evaluation of its effect as a blocker of intestinal lymph formation

Cycloheximide (CHX) has been used to reduce the flow of intestinal lymph and as a non-surgical tool to study drug absorption via the intestinal lymphatics. Pharmacokinetic information on the agent, and its relationship to effect and toxicity, have not been examined. The goal of this study was to provide pharmacokinetic data and link it to lymph-blocking and toxicological effects. Jugular-vein cannulated (JVC) adult Sprague-Dawley male rats were administered 0.5 mg/kg CHX by oral, intraperitoneal (ip), and intravenous routes followed by blood draws, and CHX was assayed using LC-MS/MS. Another four JVC rats were given peanut oil (2 mL/kg) without and then with CHX to measure effects on lipid absorption as a surrogate indicator of lymph flow. One-week later plasma biochemistry measures were obtained. The results indicated that CHX had a high clearance and volume of distribution, and oral absolute bioavailability of 0.47 with 0.5 mg/kg. CHX was associated with dose- and route-dependent pharmacokinetics. The relative bioavailability after ip doses was over 3. CHX had low plasma protein binding and minor urinary excretion. Metabolism appeared to be occur by oxidation and glucuronidation. Reductions in plasma lipids (24-40 %) were seen after 2.5 mg/kg orally with signs of inflammation and increased liver enzymes persisting for a week after the dose. CHX was associated with a reduction in lipid absorption after oral doses of 2.5 mg/kg, which seems to justify its use as a non-surgical tool to evaluate the lymphatic pathway of absorption of drugs. However, it also possesses hepatotoxicity, which should be taken into consideration in its use.

Development of a Novel In Vitro Model to Study Lymphatic Uptake of Drugs via Artificial Chylomicrons

The lymphatic system plays a crucial role in the absorption of lipophilic drugs, making it an important route for drug delivery. In this study, an in vitro model using Intralipid® was developed to investigate the lymphatic uptake of drugs. The model was validated using cannabidiol, halofantrine, quercetin, and rifampicin. Remarkably, the uptake of these drugs closely mirrored what would transpire in vivo. Furthermore, adding peanut oil to the model system significantly increased the lymphatic uptake of rifampicin, consistent with meals containing fat stimulating lymphatic drug uptake. Conversely, the inclusion of pluronic L-81 was observed to inhibit the lymphatic uptake of rifampicin in the model. This in vitro model emerges as a valuable tool for investigating and predicting drug uptake via the lymphatic system. It marks the first phase in developing a physiologically based predictive tool that can be refined further to enhance the precision of drug interaction predictions with chylomicrons and their subsequent transport via the lymphatic system. Moreover, it can be employed to explore innovative drug formulations and excipients that either enhance or hinder lymphatic drug uptake. The insights gained from this study have significant implications for advancing drug delivery through the lymphatic system.

Nanoparticle oral absorption and its clinical translational potential

Oral administration of pharmaceuticals is the most preferred route of administration for patients, but it is challenging to effectively deliver active ingredients (APIs) that i) have extremely high or low solubility in intestinal fluids, ii) are large in size, iii) are subject to digestive and/or metabolic enzymes present in the gastrointestinal tract (GIT), brush border, and liver, and iv) are P-glycoprotein substrates. Over the past decades, efforts to increase the oral bioavailability of APIs have led to the development of nanoparticles (NPs) with non-specific uptake pathways (M cells, mucosal, and tight junctions) and target-specific uptake pathways (FcRn, vitamin B12, and bile acids). However, voluminous findings from preclinical models of different species rarely meet practical standards when translated to humans, and API concentrations in NPs are not within the adequate therapeutic window. Various NP oral delivery approaches studied so far show varying bioavailability impacted by a range of factors, such as species, GIT physiology, age, and disease state. This may cause difficulty in obtaining similar oral delivery efficacy when research results in animal models are translated into humans. This review describes the selection of parameters to be considered for translational potential when designing and developing oral NPs.

Smart design approaches for orally administered lipophilic prodrugs to promote lymphatic transport

Challenges to effective delivery of drugs following oral administration has attracted growing interest over recent decades. Small molecule drugs (<1000 Da) are generally absorbed across the gastrointestinal tract into the portal blood and further transported to the systemic circulation via the liver. This can result in a significant reduction to the oral bioavailability of drugs that are metabolically labile and ultimately lead to ineffective exposure and treatment. Targeting drug delivery to the intestinal lymphatics is attracting increased attention as an alternative route of drug transportation providing multiple benefits. These include bypassing hepatic first-pass metabolism and selectively targeting disease reservoirs residing within the lymphatic system. The particular physicochemical requirements for drugs to be able to access the lymphatics after oral delivery include high lipophilicity (logP>5) and high long-chain triglyceride solubility (> 50 mg/g), properties required to enable drug association with the lipoprotein transport pathway. The majority of small molecule drugs, however, are not this lipophilic and therefore not substantially transported via the intestinal lymph. This has contributed to a growing body of investigation into prodrug approaches to deliver drugs to the lymphatic system by chemical manipulation. Optimised lipophilic prodrugs have the potential to increase lymphatic transport thereby improving oral pharmacokinetics via a reduction in first pass metabolism and may also target of disease-specific reservoirs within the lymphatics. This may provide advantages for current pharmacotherapy approaches for a wide array of pathological conditions, e.g. immune disease, cancer and metabolic disease, and also presents a promising approach for advanced vaccination strategies. In this review, specific emphasis is placed on medicinal chemistry strategies that have been successfully employed to design lipophilic prodrugs to deliberately enable lymphatic transport. Recent progress and opportunities in medicinal chemistry and drug delivery that enable new platforms for efficacious and safe delivery of drugs are critically evaluated.

Validity of cycloheximide chylomicron flow blocking method for the evaluation of lymphatic transport of drugs

BACKGROUND AND PURPOSE

Lymphatic transport of drugs after oral administration is an important mechanism for absorption of highly lipophilic compounds. Direct measurement in lymph duct cannulated animals is the gold standard method, but non-invasive cycloheximide chylomicron flow blocking method has gained popularity recently. However, concerns about its reliability have been raised. The aim of this work was to investigate the validity of cycloheximide chylomicron flow blocking method for the evaluation of lymphatic transport using model compounds with high to very high lipophilicity, that is, abiraterone and cinacalcet.

EXPERIMENTAL APPROACH

Series of pharmacokinetic studies were conducted with abiraterone acetate and cinacalcet hydrochloride after enteral/intravenous administration to intact, lymph duct cannulated and/or cycloheximide pre-treated rats.

KEY RESULTS

Mean total absolute oral bioavailability of abiraterone and cinacalcet was 7.0% and 28.7%, respectively. There was a large and significant overestimation of the lymphatic transport extent by the cycloheximide method. Mean relative lymphatic bioavailability of abiraterone and cinacalcet in cycloheximide method was 28-fold and 3-fold higher than in cannulation method, respectively.

CONCLUSION AND IMPLICATIONS

Cycloheximide chylomicron flow blocking method did not provide reliable results on lymphatic absorption and substantially overestimated lymphatic transport for both molecules, that is, abiraterone and cinacalcet. This non-invasive method should not be used for the assessment of lymphatic transport and previously obtained data should be critically revised.

An update on oral drug delivery via intestinal lymphatic transport

Orally administered drug entities have to survive the harsh gastrointestinal environment, penetrate the enteric epithelia and circumvent hepatic metabolism before reaching the systemic circulation. Whereas the gastrointestinal stability can be well maintained by taking proper measures, hepatic metabolism presents as a formidable barrier to drugs suffering from first-pass metabolism. The pharmaceutical academia and industries are seeking alternative pathways for drug transport to circumvent problems associated with the portal pathway. Intestinal lymphatic transport is emerging as a promising pathway to this end. In this review, we intend to provide an updated overview on the rationale, strategies, factors and applications involved in intestinal lymphatic transport. There are mainly two pathways for peroral lymphatic transport-the chylomicron and the microfold cell pathways. The underlying mechanisms are being unraveled gradually and nowadays witness increasing research input and applications.

Comparison of Canine and Human Physiological Factors: Understanding Interspecies Differences that Impact Drug Pharmacokinetics

This review is a summary of factors affecting the drug pharmacokinetics (PK) of dogs versus humans. Identifying these interspecies differences can facilitate canine-human PK extrapolations while providing mechanistic insights into species-specific drug in vivo behavior. Such a cross-cutting perspective can be particularly useful when developing therapeutics targeting diseases shared between the two species such as cancer, diabetes, cognitive dysfunction, and inflammatory bowel disease. Furthermore, recognizing these differences also supports a reverse PK extrapolations from humans to dogs. To appreciate the canine-human differences that can affect drug absorption, distribution, metabolism, and elimination, this review provides a comparison of the physiology, drug transporter/enzyme location, abundance, activity, and specificity between dogs and humans. Supplemental material provides an in-depth discussion of certain topics, offering additional critical points to consider. Based upon an assessment of available state-of-the-art information, data gaps were identified. The hope is that this manuscript will encourage the research needed to support an understanding of similarities and differences in human versus canine drug PK.

Impact of gastrointestinal tract variability on oral drug absorption and pharmacokinetics: An UNGAP review

The absorption of oral drugs is frequently plagued by significant variability with potentially serious therapeutic consequences. The source of variability can be traced back to interindividual variability in physiology, differences in special populations (age- and disease-dependent), drug and formulation properties, or food-drug interactions. Clinical evidence for the impact of some of these factors on drug pharmacokinetic variability is mounting: e.g. gastric pH and emptying time, small intestinal fluid properties, differences in pediatrics and the elderly, and surgical changes in gastrointestinal anatomy. However, the link of colonic factors variability (transit time, fluid composition, microbiome), sex differences (male vs. female) and gut-related diseases (chronic constipation, anorexia and cachexia) to drug absorption variability has not been firmly established yet. At the same time, a way to decrease oral drug pharmacokinetic variability is provided by the pharmaceutical industry: clinical evidence suggests that formulation approaches employed during drug development can decrease the variability in oral exposure. This review outlines the main drivers of oral drug exposure variability and potential approaches to overcome them, while highlighting existing knowledge gaps and guiding future studies in this area.

Mechanistic Oral Absorption Modeling of Halofantrine: Exploring the Role of Intestinal Lymphatic Transport

Absorption via the intestinal lymphatic system is known to be important for some highly lipophilic compounds, and can be associated with unique pharmacokinetic properties due to evasion of hepatic first-pass metabolism. This work aimed to develop a physiologically-based pharmacokinetic model incorporating the role of lymphatic transport in a physiologically-based, mechanistic oral absorption model, using halofantrine as a model compound. Simcyp V19 was used for model development; oral absorption was characterized using the multi-layer gut wall (M-ADAM) model, and the model was constructed and verified using parameters derived from in vitro experiments and clinical PK data. The final model appeared to adequately capture halofantrine pharmacokinetics in the fasted state and the magnitude of the effect of food on halofantrine total exposure; the effect of food on peak exposure was slightly underpredicted, which may be due to transient post-prandial changes in protein binding. The model simulated halofantrine fraction absorbed (f_a) via the lymph in the fed state was 0.26, representing 62% of the increase in f_a in the fed state over fasting. This work demonstrates that a PBPK modeling approach can be used to mechanistically describe oral absorption incorporating intestinal lymphatic transport.

The influence of multiple oral administration on the pharmacokinetics and distribution profile of dalcetrapib in rats

We investigated the influence of multiple oral administration on the accumulation of dalcetrapib (JTT-705/RO4607381), a novel cholesteryl ester transfer protein inhibitor, in rats. It is well known that orally administered dalcetrapib is rapidly hydrolysed to its active form, which has a sulfhydryl group, in the body. The active form then binds covalently to endogenous thiols via mixed disulfide bonds. Following multiple once daily oral administration of ¹⁴C-dalcetrapib for seven days to rats, the concentration of radioactivity in the plasma and almost all tissues reached the steady state by day 4. At 24 h after the last dose, there was a relatively high concentration of radioactivity in the mesenteric lymph nodes, liver, adrenal glands and fat. After the last dose to rats, the radioactivity was almost completely recovered in the urine and faeces, indicating that dalcetrapib is not retained in the body, probably due to the reversibility of the disulfide bonds despite being covalent bonds.

Lymphatic Transport of Drugs after Intestinal Absorption: Impact of Drug Formulation and Physicochemical Properties

PURPOSE

To provide a comprehensive and up-to-date overview focusing on the extent of lymphatic transport of drugs following intestinal absorption and to summarize available data on the impact of molecular weight, lipophilicity, formulation and prandial state.

METHODS

Literature was searched for in vivo studies quantifying extent of lymphatic transport of drugs after enteral dosing. Pharmacokinetic data were extracted and summarized. Influence of molecular weight, log P, formulation and prandial state was analyzed using relative bioavailability via lymph (F_RL) as the parameter for comparison. The methods and animal models used in the studies were also summarized.

RESULTS

Pharmacokinetic data on lymphatic transport were available for 103 drugs. Significantly higher F_RL [median (IQR)] was observed in advanced lipid based formulations [54.4% (52.0)] and oil solutions [38.9% (60.8)] compared to simple formulations [2.0% (27.1)], p < 0.0001 and p = 0.004, respectively. Advanced lipid based formulations also provided substantial F_RL in drugs with log P < 5, which was not observed in simple formulations and oil solutions. No relation was found between F_RL and molecular weight. There were 10 distinct methods used for in vivo testing of lymphatic transport after intestinal absorption so far.

CONCLUSION

Advanced lipid based formulations provide superior ability to increase lymphatic absorption in drugs of various molecular weights and in drugs with moderate to low lipophilicity.

Intestinal Lymph Flow, and Lipid and Drug Transport Scale Allometrically From Pre-clinical Species to Humans

The intestinal lymphatic system transports fluid, immune cells, dietary lipids, and highly lipophilic drugs from the intestine to the systemic circulation. These transport functions are important to health and when dysregulated contribute to pathology. This has generated significant interest in approaches to deliver drugs to the lymphatics. Most of the current understanding of intestinal lymph flow, and lymphatic lipid and drug transport rates, comes from in vitro studies and in vivo animal studies. In contrast, intestinal lymphatic transport studies in human subjects have been limited. Recently, three surgical patients had cannulation of the thoracic lymph duct for collection of lymph before and during a stepwise increase in enteral feed rate. We compared these data to studies where we previously enterally administered controlled quantities of lipid and the lipophilic drug halofantrine to mice, rats and dogs and collected lymph and blood (plasma). The collected lymph was analyzed to compare lymph flow rate, triglyceride (TG) and drug transport rates, and plasma was analyzed for drug concentrations, as a function of enteral lipid dose across species. Lymph flow rate, TG and drug transport increased with lipid administration in all species tested, and scaled allometrically according to the equation A = aM ^E where A is the lymph transport parameter, M is animal body mass, a is constant and E is the allometric exponent. For lymph flow rate and TG transport, the allometric exponents were 0.84-0.94 and 0.80-0.96, respectively. Accordingly, weight normalized lymph flow and TG mass transport were generally lower in larger compared to smaller species. In comparison, mass transport of drug via lymph increased in a greater than proportional manner with species body mass with an exponent of ∼1.3. The supra-proportional increase in lymphatic drug transport with species body mass appeared to be due to increased partitioning of drug into lymph rather than blood following absorption. Overall, this study proposes that intestinal lymphatic flow, and lymphatic lipid and drug transport in humans is most similar to species with higher body mass such as dogs and underestimated by studies in rodents. Notably, lymph flow and lipid transport in humans can be predicted from animal data via allometric scaling suggesting the potential for similar relationships with drug transport.

Therapeutic delivery to the peritoneal lymphatics: Current understanding, potential treatment benefits and future prospects

The interest in approaches to deliver therapeutics to the lymphatic system has increased in recent years as the lymphatics have been discovered to play an important role in a range of disease states such as cancer metastases, inflammatory and metabolic disease, and acute and critical illness. Therapeutic delivery to lymph has the potential to enhance treatment of these conditions. Currently much of the existing data explores therapeutic delivery to the lymphatic vessels and nodes that drain peripheral tissues and the intestine. Relatively little focus has been given to understanding the anatomy, function and therapeutic delivery to the peritoneal lymphatics. Gaining a better understanding of peritoneal lymphatic structure and function would contribute to the understanding of disease processes involving these lymphatics and facilitate the development of delivery systems to target therapeutics to the peritoneal lymphatics. This review explores the basic anatomy and ultrastructure of the peritoneal lymphatics system, the lymphatic drainage pathways from the peritoneum, and therapeutic and delivery system characteristics (size, lipophilicity and surface properties) that favour lymph uptake and retention after intraperitoneal delivery. Finally, techniques that can be used to quantify uptake into peritoneal lymph are outlined, providing a platform for future studies.

Liposomal delivery systems for intestinal lymphatic drug transport

Intestinal lymphatic drug delivery has been widely studied because drugs can bypass the first-pass metabolism in the liver via the lymphatic route, which increases oral bioavailability. Various lipid-based nanoparticles have been used to deliver hydrophobic drugs to the lymphatic pathway. This review focuses on the liposomal delivery systems used for intestinal lymphatic drug transport. Liposomal formulations have attracted particular attention because they can stimulate the production of chylomicrons and the incorporated drugs readily associate with enterocyte-derived chylomicrons, enhancing lymphatic drug transport. We believe that a full understanding of their contribution to intestinal drug translocation will lead to effective oral delivery with liposomal formulations.

Lipid-associated oral delivery: Mechanisms and analysis of oral absorption enhancement

The majority of newly discovered oral drugs are poorly water soluble, and co-administration with lipids has proven effective in significantly enhancing bioavailability of some compounds with low aqueous solubility. Yet, lipid-based delivery technologies have not been widely employed in commercial oral products. Lipids can impact drug transport and fate in the gastrointestinal (GI) tract through multiple mechanisms including enhancement of solubility and dissolution kinetics, enhancement of permeation through the intestinal mucosa, and triggering drug precipitation upon lipid emulsion depletion (e.g., by digestion). The effect of lipids on drug absorption is currently not quantitatively predictable, in part due to the multiple complex dynamic processes that can be impacted by lipids. Quantitative mechanistic analysis of the processes significant to lipid system function and overall impact on drug absorption can aid in the understanding of drug-lipid interactions in the GI tract and exploitation of such interactions to achieve optimal lipid-based drug delivery. In this review, we discuss the impact of co-delivered lipids and lipid digestion on drug dissolution, partitioning, and absorption in the context of the experimental tools and associated kinetic expressions used to study and model these processes. The potential benefit of a systems-based consideration of the concurrent multiple dynamic processes occurring upon co-dosing lipids and drugs to predict the impact of lipids on drug absorption and enable rational design of lipid-based delivery systems is presented.

Biological voyage of solid lipid nanoparticles: a proficient carrier in nanomedicine

This review projects the prospects and issues faced by solid lipid nanoparticles (SLNs) in current scenarios, specially related to its clinical implementation and effectiveness. We re-examine the basic concept of biobehavior and movement of SLNs as a nanomedicine carrier. The extensive survey of the uptake and absorption mechanism from different routes, distribution pattern, targeting efficiency, effect of surface functionalization on biodistribution, elimination pathways and toxic effects have been documented. In general, the objective of this review is to boost our knowledge about the interaction of SLNs with the bioenvironment, their movement in, and effect on, a living system and future prospects.

Formulation, Pharmacokinetic, and Efficacy Studies of Mannosylated Self-Emulsifying Solid Dispersions of Noscapine

Purpose

To formulate hydroxypropyl methylcellulose-stabilized self-emulsifying solid dispersible carriers of noscapine to enhance oral bioavailability.

Methods

Formulation of noscapine (Nos) self-emulsifying solid dispersible microparticles (SESDs) was afforded by emulsification using an optimized formula of Labrafil M1944, Tween-80, and Labrasol followed by spray-drying with hydroxypropyl methylcellulose (HPMC), with and without mannosamine (Mann-Nos_SESDs and Nos_SESDs respectively); self-microemulsifying liquid dispersions (SMEDDs) with and without mannosamine (Mann-Nos_SMEDDs and Nos_SMEDDs respectively) were also prepared. SMEDDs and SESDs were characterized for size, polydispersity, surface charge, entrapment efficiency, in vitro permeability, in vitro release kinetics, and oral pharmacokinetics in Sprague-Dawley rats (10 mg/kg p.o). The antitumor efficacy of Mann-Nos_SESDs on the basis of chemosensitization to cisplatin (2.0 mg/kg, IV) was investigated in a chemorefractory lung tumor Nu/Nu mouse model up to a maximal oral dose of 300 mg/kg.

Results

The oil/surfactant/co-surfactant mixture of Labrafil M1944, Tween-80, and Labrasol optimized at weight ratios of 62.8:9.30:27.90% produced stable self-microemulsifying dispersions (SMEDDs) at a SMEDD to water ratio of 1–3:7–9 parts by weight. SMEDDs had hydrodynamic diameters between 231 and 246 nm; surface charges ranged from -16.50 to -18.7 mV; and entrapment efficiencies were between 32 and 35%. SESDs ranged in size between 5.84 and 6.60 μm with surface charges from -10.62 to -12.40 mV and entrapment efficiencies of 30.96±4.66 and 32.05±3.72% (Nos_SESDs and Mann-Nos_SESDs respectively). Mann-Nos_SESDs exhibited saturating uptake across Caco-2 monolayers (P_app = 4.94±0.18 × 10⁻⁶ cm/s), with controlled release of 50% of Nos in 6 hr at pH 6.8 following Higuchi kinetics. Mann-Nos_ SESDs was 40% more bioavailable compared to Nos_SESDs; and was effective in sensitizing H1650 SP cells to Cisplatin in vitro and in an orthotopic lung tumor model of H1650 SP origin.

Conclusions

Mannosylated noscapine self-emulsifying solid dispersions (Mann-Nos_SESDs) are bioavailable and potentiate the antineoplastic effect of cisplatin-based chemotherapy in cisplatin-resistant NSCLC.

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.

The role of lymphatic transport on the systemic bioavailability of the Bcl-2 protein family inhibitors navitoclax (ABT-263) and ABT-199

Navitoclax (ABT-263), a Bcl-2 family inhibitor and ABT-199, a Bcl-2 selective inhibitor, are high molecular weight, high logP molecules that show low solubility in aqueous media. While these properties are associated with low oral bioavailability (F), both navitoclax and ABT-199 showed moderate F in preclinical species. The objective of the described study was to determine if lymphatic transport contributes to the systemic availability of navitoclax and ABT-199 in dogs. The intravenous pharmacokinetics of navitoclax and ABT-199 were determined in intact (noncannulated) dogs. In oral studies, tablets (100 mg) of navitoclax and ABT-199 were administered to both intact and thoracic lymph duct-cannulated (TDC) dogs. The clearance of navitoclax and ABT-199 was low; 0.673 and 0.779 ml/min per kilogram, respectively. The volume of distribution of both compounds was low (0.5-0.7 l/kg). The half-lives of navitoclax and ABT-199 were 22.2 and 12.9 hours, respectively. The F of navitoclax and ABT-199 were 56.5 and 38.8%, respectively, in fed intact dogs. In fed TDC dogs, 13.5 and 4.67% of the total navitoclax and ABT-199 doses were observed in lymph with the % F of navitoclax and ABT-199 of 21.7 and 20.2%, respectively. The lower lymphatic transport of ABT-199 corresponds to the lower overall % F of ABT-199 versus navitoclax despite similar systemic availability via the portal vein (similar % F in TDC animals). This is consistent with the higher long chain triglyceride solubility of navitoclax (9.2 mg/ml) versus ABT-199 (2.2 mg/ml). In fasted TDC animals, lymph transport of navitoclax and ABT-199 decreased by 1.8-fold and 10-fold, respectively.

Lymphatic absorption, metabolism, and excretion of a therapeutic peptide in dogs and rats

The objective of the current study was to evaluate the mechanism of absorption and metabolism of a PEGylated peptide, MRL-1 (46 kDa), after s.c. dosing in dogs and rats. Thoracic lymph duct-cannulated (LDC) dog and rat models were developed that allowed continuous collection of lymph for up to 8 days. When [(3)H]MRL-1 was administered s.c. to LDC dogs, ∼73% of the administered radioactivity was recovered in pooled lymph over a period of 120 hours, suggesting that lymphatic uptake is the major pathway of s.c. absorption for this peptide. In agreement with these data, the systemic exposure of radioactivity related to [(3)H]MRL-1 in LDC dogs was decreased proportionately when compared with that in noncannulated control dogs. After i.v. dosing with [(3)H]MRL-1 in LDC dogs, 20% of the administered radioactivity was recovered in pooled lymph over 168 hours, suggesting some level of recirculation of radioactivity related to [(3)H]MRL-1 from the plasma compartment into the lymphatic system. Experiments conducted in the LDC rat model also resulted in similar conclusions. Analysis of injection site s.c. tissue showed significant metabolism of [(3)H]MRL-1, which provides an explanation for the <100% bioavailability of therapeutic proteins and peptides after s.c. dosing. After s.c. dosing, the major circulating components in plasma were the parent peptide and the PEG-linker [(3)H]MRL-2. The metabolism profiles in lymph were similar to those in plasma, suggesting that the loss of peptide was minimal during lymphatic transport. After i.v. dosing in rats, [(3)H]MRL-1 was metabolized and excreted primarily in the urine as metabolites.

Advanced drug delivery to the lymphatic system: lipid-based nanoformulations

The delivery of drugs and bioactive compounds via the lymphatic system is complex and dependent on the physiological uniqueness of the system. The lymphatic route plays an important role in transporting extracellular fluid to maintain homeostasis and in transferring immune cells to injury sites, and is able to avoid first-pass metabolism, thus acting as a bypass route for compounds with lower bioavailability, ie, those undergoing more hepatic metabolism. The lymphatic route also provides an option for the delivery of therapeutic molecules, such as drugs to treat cancer and human immunodeficiency virus, which can travel through the lymphatic system. Lymphatic imaging is useful in evaluating disease states and treatment plans for progressive diseases of the lymph system. Novel lipid-based nanoformulations, such as solid lipid nanoparticles and nanostructured lipid carriers, have unique characteristics that make them promising candidates for lymphatic delivery. These formulations are superior to colloidal carrier systems because they have controlled release properties and provide better chemical stability for drug molecules. However, multiple factors regulate the lymphatic delivery of drugs. Prior to lymphatic uptake, lipid-based nanoformulations are required to undergo interstitial hindrance that modulates drug delivery. Therefore, uptake and distribution of lipid-based nanoformulations by the lymphatic system depends on factors such as particle size, surface charge, molecular weight, and hydrophobicity. Types of lipid and concentration of the emulsifier are also important factors affecting drug delivery via the lymphatic system. All of these factors can cause changes in intermolecular interactions between the lipid nanoparticle matrix and the incorporated drug, which in turn affects uptake of drug into the lymphatic system. Two lipid-based nanoformulations, ie, solid lipid nanoparticles and nanostructured lipid carriers, have been administered via multiple routes (subcutaneous, pulmonary, and intestinal) for targeting of the lymphatic system. This paper provides a detailed review of novel lipid-based nanoformulations and their lymphatic delivery via different routes, as well as the in vivo and in vitro models used to study drug transport in the lymphatic system. Physicochemical properties that influence lymphatic delivery as well as the advantages of lipid-based nanoformulations for lymphatic delivery are also discussed.

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.

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.

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.

Targeted drug delivery to lymphocytes: a route to site-specific immunomodulation?

Lymphocytes are central to the progression of autoimmune disease, transplant rejection, leukemia, lymphoma and lymphocyte-resident viral diseases such as HIV/AIDs. Strategies to target drug treatments to lymphocytes, therefore, represent an opportunity to enhance therapeutic outcomes in disease states where many current treatment regimes are incompletely effective and promote significant toxicities. Here we demonstrate that highly lipophilic drug candidates that preferentially access the intestinal lymphatics after oral administration show significantly enhanced access to lymphocytes leading to improved immunomodulatory activity. When coadministered with such drugs, lipids enhance lymphocyte targeting via a three tiered action: promotion of drug absorption from the gastrointestinal tract, enhancement of lymphatic drug transport and stimulation of lymphocyte recruitment into the lymphatics. This strategy has been exemplified using a highly lipophilic immunosuppressant (JWH015) where coadministration with selected lipids led to significant increases in lymphatic transport, lymphocyte targeting and IL-4 and IL-10 expression in CD4+ and CD8+ lymphocytes after ex vivo mitogen stimulation. In contrast, administration of a 2.5-fold higher dose of JWH015 in a formulation that did not stimulate lymph transport had no effect on antiinflammatory cytokine levels, in spite of equivalent drug exposure in the blood. The current data suggest that complementary drug design and delivery strategies that combine highly lipophilic, lymphotropic drug candidates with lymph-directing formulations provide enhanced selectivity, potency and therapeutic potential for drug candidates with lymphocyte associated targets.

Oral bioavailability of DN101, a concentrated formulation of calcitriol, in tumor-bearing dogs

PURPOSE

High-dose calcitriol (1,25-dihydroxyvitamin D(3)) has antineoplastic activity against a range of tumors and potentiates chemotherapeutic agents. In an earlier canine study, the MTD of intravenous (i.v.) calcitriol was 3.75 μg/kg, but polysorbate-associated hypersensitivity reactions were common. Use of commercially available oral calcitriol is limited by the absence of a formulation of suitable strength to allow administration of a reasonable number of caplets. This study evaluated the bioavailability of DN101, a concentrated oral calcitriol formulation specifically developed for anticancer applications.

METHODS

An open-label, single-dose, 2-way crossover study was conducted. Dogs randomly received a single 3.75 μg/kg dose of calcitriol either i.v. or oral (as DN101), followed by cisplatin (60 mg/m(2)). Three weeks later, the alternate form of calcitriol was given prior to another dose of cisplatin. Dogs received antihistamines and corticosteroids prior to both treatments. Food was withheld for 12 h before and after therapy. Serum calcitriol concentrations were measured by radioimmunoassay.

RESULTS

Ten tumor-bearing dogs received both i.v. and oral calcitriol. Six dogs experienced hypersensitivity reactions during i.v. calcitriol. Sequence of calcitriol administration (day-1 vs. day-21) by either i.v. or oral routes had no effect on the major calcitriol pharmacokinetic parameters. Oral calcitriol resulted in significantly lower values for AUC (P = 0.05) and prolonged T (1/2) (P = 0.003) when compared to i.v. Calcitriol oral bioavailability was highly variable among dogs (mean ± SEM, 71 ± 12.6%).

CONCLUSIONS

This study demonstrates that a high-dose formulation of calcitriol has a moderate bioavailability in dogs, but inter-individual variability in PK parameters is similar to that observed in people. With this bioavailability, serum concentrations of calcitriol that exhibit antitumor activity in a preclinical murine model were achieved in some dogs. Exploration of methods to minimize variation in calcitriol systemic exposure is warranted.

Plasma profiles of lycopene after single oral and intravenous administrations in dogs

The objectives of this study were to identify the factors limiting the absorption of purified lycopene after oral administration, and to comparatively assess plasma data sets after single oral and intravenous administrations in dogs to define the conditions for performing an absolute bioavailability study. Solubility of purified lycopene (all-trans, 93.5%) was determined in media simulating the conditions in the fasted and in the fed upper gastrointestinal lumen. After evaluating the plasma levels achieved following single administrations of purified lycopene powder to fasted and fed dogs at escalating doses (75–750 mg), a crossover study was performed in four fed female mongrel dogs at two phases. In phase I, one soft gelatine capsule (10 mg lycopene) with 500 mL milk was administered orally. In phase II, 500 mL milk was administered orally and 250 mL 5% dextrose containing 5mg lycopene in the form of a binary system with hydroxypropyl-β-cyclodextrin (HP-β-CyD) was administered intravenously over 3.5 h. In-vitro and preliminary canine studies confirmed that, after oral administration of lycopene in solid form, arrival of lycopene into the systemic circulation was limited by lymphatic transport and, in addition, if the administered dose was higher than approximately 2 mg, by intralumenal solubility. During the first 50 h after single administrations to fed dogs, lycopene plasma levels were lower after intravenous than after oral administration. This could have been related to capacity limited elimination of lycopene and/or route-dependent disposition kinetics. Estimation of the amount of lycopene reaching the systemic circulation after oral and after intravenous administration requires separate estimations of total body clearance of lycopene.