Low dose lipid formulations: effects on gastric emptying and biliary secretion

Evaluating the potential of cubosomal nanoparticles for oral delivery of amphotericin B in treating fungal infection

The oral administration of amphotericin B (AmB) has a major drawback of poor bioavailability. The aim of this study was to investigate the potential of glyceryl monoolein (GMO) cubosomes as lipid nanocarriers to improve the oral efficacy of AmB. Antifungal efficacy was determined in vivo in rats after oral administration, to investigate its therapeutic use. The human colon adenocarcinoma cell line (Caco-2) was used in vitro to evaluate transport across a model of the intestinal barrier. In vivo antifungal results showed that AmB, loaded in GMO cubosomes, could significantly enhance oral efficacy, compared against Fungizone®, and that during a 2 day course of dosage 10 mg/kg the drug reached effective therapeutic concentrations in renal tissue for treating fungal infections. In the Caco-2 transport studies, GMO cubosomes resulted in a significantly larger amount of AmB being transported into Caco-2 cells, via both clathrin- and caveolae-mediated endocytosis, but not macropinocytosis. These results suggest that GMO cubosomes, as lipid nanovectors, could facilitate the oral delivery of AmB.

Gastric pre-processing is an important determinant of the ability of medium-chain lipid solution formulations to enhance oral bioavailability in rats

The contribution of dispersion and digestion in the stomach to the bioavailability of poorly water-soluble drugs administered in lipid-based formulations was assessed by comparison of intraduodenal (ID) and peroral (p.o.) administration using cinnarizine (CZ) as a model drug. Differences in the dispersion and digestion in the gastric and intestinal compartments for medium-chain triacylglycerides (MCT) and long-chain triacylglycerides (LCT) were observed, leading to differences in the oral bioavailability of CZ. Bypassing gastric processing using ID administration of lipid solution formulations decreased drug bioavailability regardless of lipid type. Overall, bioavailability from LCT formulations was higher than MCT regardless of route of administration, consistent with past data after p.o. administration and previously reported descriptions of increases in drug precipitation after administration of medium-chain lipid formulations. The larger differences between bioavailability after both p.o. and ID administration for MCT compared with LCT formulations suggest that passage through the stomach is more critical for MCT formulations, and that gastric digestion may be more critical for MCT than LCT formulations. For MCT-based formulations, efficient dispersion and partial digestion in the stomach may be required to allow rapid transfer to intestinal-mixed micelles and absorption in the upper small intestine prior to drug precipitation.

Bioavailability of cinnarizine in dogs: effect of SNEDDS loading level and correlation with cinnarizine solubilization during in vitro lipolysis

PURPOSE

To investigate the effect of increasing the loading level of the poorly soluble drug cinnarizine in a self-nanoemulsifying drug delivery system (SNEDDS) both in vitro and in vivo.

METHODS

A fixed dose of cinnarizine was administered orally to dogs in solution in different amounts of SNEDDS vehicle. Furthermore, the SNEDDSs were characterised using the dynamic in vitro lipolysis model.

RESULTS

Statistical differences in bioavailability were not obtained between the different amounts of SNEDDS vehicle, in spite of differences in the tendency of cinnarizine to precipitate during in vitro lipolysis of the treatments. Use of the SNEDDS concept decreased the variation in cinnarizine exposure observed between dogs as compared to administering cinnarizine in an aqueous suspension.

CONCLUSIONS

Optimization of SNEDDSs towards keeping the drug compound in solution upon in vitro lipolysis of the SNEDDSs may not be as important as previously suggested.

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.

Bile salts and their importance for drug absorption

Bile salts are present in the intestines of humans as well as the animals used during the development of pharmaceutical products. This review provides a short introduction into the physical chemical properties of bile salts, a description of the bile concentration and composition of bile in different animal species and an overview of the literature investigating the influence of bile salts on the in vivo performance of different compounds and drug formulations. Generally, there is a positive effect on bioavailability when bile is present in the gastro-intestinal tract, independent of the formulation systems, e.g. suspensions, solutions, cyclodextrin complexes or lipid based formulations, but a few exceptions have also been reported.

The effect of administered dose of lipid-based formulations on the in vitro and in vivo performance of cinnarizine as a model poorly water-soluble drug

The influence of varying the amount of lipid co-administered with the drug on drug solubilisation and absorption is poorly understood. In the current study, the effect of lipid dose on the in vitro drug distribution is compared with the in vivo absorption of cinnarizine (CZ) when formulated using long-chain triacylglyceride (LCT) and medium-chain triacylglycerides (MCT). At a fixed drug-lipid ratio, in the closed in vitro model, the drug concentrations in the aqueous phase increased and decreased for MCT and LCT, respectively, with increasing lipid dose. However, in vivo, the oral bioavailability (F%) of CZ was independent of the quantity of lipid administered for both MCT and LCT, but was higher for LCT (32.1 ± 2.3%) than for MCT (16.6 ± 2.3%). Increasing the quantity of lipid relative to the dose of CZ resulted in an increase in the oral F% when the lipid mass was increased from 125 to 250 mg, but was no greater at 500 mg lipid dose. The results confirm the limitations of the in vitro model but positively indicate that the use of the rat as a pre-clinical model for studying the bioavailability of poorly water-soluble drugs is not compromised by the mass of formulation administered.

SNEDDS Containing Poorly Water Soluble Cinnarizine; Development and in Vitro Characterization of Dispersion, Digestion and Solubilization

Self-Nanoemulsifying Drug Delivery Systems (SNEDDSs) were developed using well-defined excipients with the objective of mimicking digested SNEDDSs without the use of enzymes and in vitro lipolysis models and thereby enabling studies of the morphology and size of nanoemulsions as well as digested nanoemulsions by Cryo-TEM imaging and Dynamic Light Scattering. Four SNEDDSs (I-IV) were developed. Going from SNEDDS I to IV lipid content and solubility of the model drug cinnarizine decreased, which was also the case for dispersion time and droplet size. Droplet size of all SNEDDS was evaluated at 1% (w/w) dispersion under different conditions. Cinnarizine incorporation increased the droplet size of SNEDDSs I and II whereas for SNEDDSs III and IV no difference was observed. At low pH cinnarizine had no effect on droplet size, probably due to increased aqueous solubility and partitioning into the aqueous phase. Dispersion of the SNEDDSs in Simulated Intestinal Media (SIM) containing bile salts and phospholipids resulted in a decrease in droplet size for all SNEDDS, as compared to dispersion in buffer. Increasing the bile salt/phospholipid content in the SIM decreased the droplet sizes further. Mimicked digested SNEDDS with highest lipid content (I and II) formed smaller nanoemulsion droplet sizes upon dispersion in SIM, whereas droplet size from III and IV were virtually unchanged by digestion. Increasing the bile acid/phosphatidylcholine content in the SIM generally decreased droplet size, due to the solubilizing power of the endogenous surfactants. Digestion of SNEDDSs II resulted in formation of vesicles or micelles in fasted and fed state SIM, respectively. The developed and characterized SNEDDS provide for a better knowledge of the colloid phases generated during digestion of SNEDDS and therefore will enable studies that may yield a more detailed understanding of SNEDDS performance.

Characterising lipid lipolysis and its implication in lipid-based formulation development

Facing the increasing number of poorly water-soluble drugs, pharmaceutical scientists are required to break new grounds for the delivery of these pharmaceutically problematic drugs. Lipid-based drug delivery systems (LBDDS) have received increased interest as a novel drug delivery platform during the last decades and several successfully marketed products have shown the potential for LBDDS. However, there exists a discrepancy between the clear need for innovative delivery forms and their rational design. In the case of LBDDS, this can be attributed to the complexity of LBDDS after administration. Unlike conventional formulations, LBDDS are susceptible to digestion in the gastrointestinal tract, the interplay of delivery system, drug and physiology ultimately effecting drug disposition. In vitro lipolysis has become an important technique to mimic the enzymatic degradation. For the better understanding of how LBDDS promote drug delivery, in vitro lipolysis requires advanced characterisation methods. In this review, the physiological background of lipid digestion is followed by a thorough summary of the techniques that are currently used to characterise in vitro lipolysis. It would be desirable that the increasing knowledge about LBDDS will foster their rationale development thereby increasing their broader application.

In vitro and in vivo performance of novel supersaturated self-nanoemulsifying drug delivery systems (super-SNEDDS)

Novel supersaturated self-nanoemulsifying drug delivery systems (super-SNEDDS) containing the poorly water-soluble drug halofantrine above equilibrium solubility (150% S(eq)) were compared in vitro and in vivo with conventional SNEDDS containing the drug below equilibrium solubility (75% S(eq)). Pre-concentrates comprising of either medium chain lipids (Captex 300/Capmul MCM) or long chain lipids (soybean oil/Maisine), Cremophor RH40 and ethanol were formulated maintaining the lipid-to-surfactant-to-cosolvent ratio constant (55:35:10, w/w %). The ability of super-SNEDDS to increase the absorption of halofantrine in dogs, as well as the predictivity of the dynamic in vitro lipolysis model was studied. In vitro lipolysis of SNEDDS and super-SNEDDS showed rapid drug precipitation from all formulations while the same drug concentrations in the digestion medium were found during digestion of equal amounts of SNEDDS and super-SNEDDS. Elevated halofantrine solubilisation during in vitro lipolysis was observed only when multiple capsules of conventional SNEDDS were subjected to in vitro digestion. After lipolysis the isolated super-SNEDDS pellets were characterised by XRPD revealing no crystalline halofantrine from any of the investigated formulations. Subsequent dissolution studies of the super-SNEDDS pellet in the lipolysis medium demonstrated enhanced dissolution of halofantrine suggesting that halofantrine in the pellet was amorphous. The enhanced dissolution of the amorphous halofantrine was also reflected in vivo since two capsules of conventional SNEDDS were needed to achieve similar AUC and C(max) as obtained after dosing of a single capsule of super-SNEDDS. The study demonstrated that the absorption of halofantrine was not hampered by drug precipitation. Super-SNEDDS lead to precipitation of halofantrine in an amorphous form, which can be the driving force for enhanced absorption. Since super-SNEDDS were also physically stable for at least 6 months they represent a potential novel oral lipid-based drug delivery system for low aqueous soluble compounds.

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.

Characterising the behaviour of poorly water soluble drugs in the intestine: application of biorelevant media for solubility, dissolution and transport studies

OBJECTIVES

Based on the knowledge of human intestinal fluids, compositions of biorelevant media and their impact on solubility, dissolution and permeability studies of poorly soluble drug compounds are discussed.

KEY FINDINGS

Human intestinal fluids show large variations with regard to composition and pH, which complicate the selection of biorelevant media. The influence of concentration and ratio of bile salts, phospholipids and hydrolysis products, such as monoglycerides and free fatty acids, in well characterised media, on the solubility, dissolution and permeability of a given drug provides valuable information on the behaviour of the drug in the intestine, thus enabling the prediction of the in-vivo absorption.

SUMMARY

This review discusses the implications of biorelevant media composition on the solubility, dissolution and permeability of poorly soluble drug compounds. Biorelevant media contain bile salts and phospholipids and when simulating the fed state also monoglycerides and free fatty acids. Solubility of some poorly soluble drugs increase independently of the type of surfactants included in the biorelevant media, while others have a higher solubility in monoglyceride- and fatty acid-containing media. This is independent of the log P (the octanol-water partition coefficient) of the drug. The use of biorelevant dissolution media improves the correlation to in-vivo data, compared with compendial media, and although the field of permeability studies is complex the use of biorelevant media in this setting shows promise with respect to a better prediction of absorption.

New perspectives on lipid and surfactant based drug delivery systems for oral delivery of poorly soluble drugs

Objectives

The aim of this review is to highlight relevant considerations when implementing a rational strategy for the development of lipid and surfactant based drug delivery system and to discuss shortcomings and challenges to the current classification of these delivery systems. We also aim to offer suggestions for an improved classification system that will accommodate lipid based formulations that are not currently accommodated in the lipid formulation classification system.

Key findings

When categorising lipid and surfactant based drug delivery systems, the current Lipid Formulations Classifications System is a useful tool. However, it does not apply to all marketed lipid and surfactant systems or those reported in research papers. A more profound understanding of the functionalities of lipids and surfactants and their role in emulsion formation will enable a rational development strategy and will create the basis for a revised classification system encompassing all employed lipid and surfactant drug delivery systems.

Summary

The ever-increasing number of poorly soluble compounds in drug discovery and development calls for the serious need for effective and affordable drug delivery strategies that will enhance bioavailability and decrease variability. Lipid and surfactant based drug delivery systems offer these advantages; however, the development of these systems requires proper understanding of the physicochemical nature of the compound as well as the lipid excipients and gastrointestinal digestion. One major challenge of lipid excipients and delivery systems is the varying range of compounds they contain. This has contributed to the challenge of proper characterisation and evaluation of these delivery systems, their stability, classification and regulatory issues, which consequently have affected the number of these formulations that eventually reach the market. Suggestions as to proper classification of these delivery systems based on their main lipid component and recommended use are put forward. The prospect of these delivery systems looks promising.

Enhancing intestinal drug solubilisation using lipid-based delivery systems

Lipid-based delivery systems are finding increasing application in the oral delivery of poorly water-soluble, lipophilic drugs. Whilst lipidic dose forms may improve oral bioavailability via several mechanisms, enhancement of gastrointestinal solubilisation remains argueably the most important method of absorption enhancement. This review firstly describes the mechanistic rationale which underpins the use of lipid-based delivery systems to enhance drug solubilisation and briefly reviews the available literature describing increases in oral bioavailability after the administration of lipid solution, suspension and self-emulsifying formulations. The use of in vitro methods including dispersion tests and more complex models of in vitro lipolysis as indicators of potential in vivo performance are subsequently described, with particular focus on recent data which suggests that the digestion of surfactants present in lipid-based formulations may impact on formulation performance. Finally, a series of seven guiding principles for formulation design of lipid-based delivery systems are suggested based on an analysis of recent data generated in our laboratories and elsewhere.