Towards mesoporous silica as a pharmaceutical treatment for obesity - impact on lipid digestion and absorption.

Mesoporous silica particles (MSPs) are emerging as an interesting option to reduce calorific uptake as a treatment for obesity and other metabolic conditions. However, their further development under the pharmaceutical regulatory framework is hindered by poor understanding of the mechanisms by which they exert their effects. In the current study the interaction of MSPs with the lipid digestion process is investigated, specifically interactions with lipase enzymes and lipid digestion products as a key contributing factor to lipid absorption and calorific intake. The impact of exposing lipase to MSPs on the enzyme activity was assessed directly using the tributyrin digestion test. The extent of interaction of digestion products with MSPs was studied using selectively radiolabeled bile components and lipids, while the impact on in vivo absorption of lipids was studied by incorporation of radiolabelled lipid (triolein) into milk and administration with and without particles. The studies showed that particles that inhibited lipase activity also tended to interact more extensively with lipid digestion products. In vitro X-ray scattering studies revealed the interaction of some MSPs with lipid digestion products through changes in lipid self-assembly during digestion. The MSPs led to reduced lipid absorption in vivo compared to the control particles and MSP-free milk. While the specific properties of the MSPs that drive the differences between the behavior of MSPs during lipid digestion remain elusive, the studies highlight that interactions with the lipid digestion and absorption pathways are a likely mechanism for reducing calorific uptake.

Formation of Self-Assembled Mesophases During Lipid Digestion.

Lipids play an important role in regulating bodily functions and providing a source of energy. Lipids enter the body primarily in the form of triglycerides in our diet. The gastrointestinal digestion of certain types of lipids has been shown to promote the self-assembly of lipid digestion products into highly ordered colloidal structures. The formation of these ordered colloidal structures, which often possess well-recognized liquid crystalline morphologies (or "mesophases"), is currently understood to impact the way nutrients are transported in the gut and absorbed. The formation of these liquid crystalline structures has also been of interest within the field of drug delivery, as it enables the encapsulation or solubilization of poorly water-soluble drugs in the aqueous environment of the gut enabling a means of absorption. This review summarizes the evidence for structure formation during the digestion of different lipid systems associated with foods, the techniques used to characterize them and provides areas of focus for advancing our understanding of this emerging field.

Prolonged Plasma Exposure of the Kv1.3-Inhibitory Peptide HsTX1[R14A] by Subcutaneous Administration of a Poly(Lactic-co-Glycolic Acid) (PLGA) Microsphere Formulation.

This study evaluated the impact of poly(lactic-co-glycolic acid) (PLGA) microsphere formulations on in vitro release and in vivo plasma exposure of HsTX1[R14A], a potent inhibitor of the voltage-gated potassium channel Kv1.3, with potential to treat autoimmune conditions. Microspheres containing HsTX1[R14A] were prepared using different PLGA materials, including Resomer® RG502H, RG503H and PURASORB® PDLG 5004 (Purac). After assessing encapsulation efficiency and in vitro release, plasma concentrations of HsTX1[R14A] were quantified by LCMS/MS following subcutaneous administration of HsTX1[R14A]-loaded RG503H microspheres (15 mg/kg) or HsTX1[R14A] solution (4 mg/kg) to Sprague-Dawley rats. Microspheres prepared with Purac exhibited the greatest encapsulation efficiency (45.5 ± 2.4% (mean ± SD)) and RG502H the lowest (22.0 ± 6.4%). Release of HsTX1[R14A] was fastest in vitro for RG502H microspheres (maximum release at 31 days) and slowest for Purac (82 days). With a relatively rapid burst release of 20.0 ± 0.4% and a controlled release profile of up to 41 days, HsTX1[R14A]-loaded RG503H microspheres were selected for subcutaneous administration, resulting in detectable plasma concentrations for 11 days relative to 8 h following subcutaneous administration of HsTX1[R14A] solution. Therefore, subcutaneous administration of RG503H PLGA microspheres is a promising approach to be exploited for delivery of this immune modulator.

Sustained absorption of delamanid from lipid-based formulations as a path to reduced frequency of administration.

Delamanid is a poorly water-soluble drug currently being used for the treatment of tuberculosis. The high frequency of dosing leads to poor adherence for patients who live in lower economic and nomadic populations. Non-digestible self-assembling lipids as a formulation approach for poorly water-soluble drugs have previously been shown to extend the window of absorption through gastric retention. We hypothesise that this approach could lead to the reduction of dosing frequency for delamanid and thereby has potential to improve adherence. Formulations of delamanid were prepared in selachyl alcohol and phytantriol as non-digestible self-assembling lipid vehicles, and their behaviour was compared with reconstituted milk powder, as a digestible lipid-based formulation, and an aqueous suspension. The self-assembly of selachyl alcohol and phytantriol in aqueous media in the presence of delamanid was studied using small angle X-ray scattering and produced the inverse hexagonal (H2) and inverse bicontinuous cubic (V2) liquid crystal structures, respectively. The times at which maximum delamanid levels in plasma were observed (Tmax) after oral administration of the phytantriol, selachyl alcohol and reconstituted milk powder formulations of delamanid to rats were 27 ± 3, 20 ± 4 and 6.5 ± 1.0 h, respectively, compared with the aqueous suspension formulation with a Tmax of 3.4 ± 1 h, which confirms the hypothesis of an extended duration of absorption after administration in non-digestible self-assembling lipids. The digestion products of the triglycerides in the milk formulation increased the solubilisation of delamanid in the gastrointestinal tract, leading to an increase in exposure compared with the aqueous suspension formulation but did not significantly extend Tmax. Overall, the non-digestible nanostructured lipid formulations extended the duration of absorption of delamanid well beyond that from milk or suspension formulations. Graphical abstract.

Correlating Digestion-Driven Self-Assembly in Milk and Infant Formulas with Changes in Lipid Composition.

Lipids in mammalian milks such as bovine milk and human breast milk have been shown to self-assemble into various liquid crystalline materials during digestion. In this study, the direct correlation between the composition of the lipids from three types of mammalian milk, three brands of infant formulas (IFs), and soy milk and the liquid crystalline structures that form during their digestion was investigated to link the material properties to the composition. The self-assembly behavior was assessed using in vitro digestion coupled with in situ small-angle X-ray scattering (SAXS). Lipid composition was determined during in vitro digestion using ex situ liquid chromatography-mass spectrometry. All tested milks self-assembled into ordered structures during digestion, with the majority of milks displaying nonlamellar phases. Milks that released mostly long-chain fatty acids (>95 mol % of the top 10 fatty acids released) with more than 47 mol % unsaturation predominantly formed a micellar cubic phase during digestion. Other milks released relatively more medium-chain fatty acids and medium-chain monoglycerides and produced a range of ordered liquid crystalline structures including the micellar cubic phase, the hexagonal phase, and the bicontinuous cubic phase. One infant formula did not form liquid crystalline structures at all as a consequence of differences in fatty acid distributions. The self-assembly phenomenon provides a powerful discriminator between different classes of nutrition and a roadmap for the design of human milklike systems and is anticipated to have important implications for nutrient transport and the delivery of bioactives.

A Nonionic Polyethylene Oxide (PEO) Surfactant Model: Experimental and Molecular Dynamics Studies of Kolliphor EL.

Polyethoxylated, nonionic surfactants are important constituents of many drug formulations, including lipid-based formulations. In an effort to better understand the behavior of formulation excipients at the molecular level, we have developed molecular dynamics (MD) models for the widely used surfactant Kolliphor EL (KOL), a triricinoleate ester of ethoxylated glycerol. In this work, we have developed models based on a single, representative molecular component modeled with 2 force field variations based on the GROMOS 53A6DBW and 2016H66 force field parameters for polyethoxylate chains. To compare the computational models to experimental measurements, we investigated the phase behavior of KOL using nephelometry, dynamic light scattering, cross-polarized microscopy, small-angle X-ray scattering, and cryogenic transmission electron microscopy. The potential for digestion of KOL was also evaluated using an in vitro digestion experiment. We found that the size and spherical morphology of the KOL colloids at low concentrations was reproduced by the MD models as well as the growing interactions between the aggregates to from rod-like structures at high concentrations. We believe that this model reproduces the phase behavior of KOL relevant to drug absorption and that it can be used in whole formulation simulations to accelerate the formulation development.

The impact of digestion is essential to the understanding of milk as a drug delivery system for poorly water soluble drugs.

Milk has previously been considered as a potential lipid-based drug delivery system for poorly water soluble drugs but it has never gained significant attention. This is in part because relying on solubility in lipid-based formulations (in this case milk) does not provide a complete picture of the behavior of such systems upon digestion. Herein, we demonstrate using time resolved X-ray scattering that the digestion of milk is actually crucial to the solubilisation of a poorly water-soluble drug, halofantrine. Halofantrine was chosen because its behaviour in lipid-based formulations has been widely investigated and because of its close structural relationship to lumefantrine, an antimalarial drug of current interest for the treatment of paediatric malaria. The transformation of the drug from a crystalline solid form in suspension in milk, to a solubilised form as a direct consequence of lipolysis highlights that consideration of digestion of the milk lipids as a critical process that influences drug solubilisation and availability for absorption is vital.

Characterization of Solubilizing Nanoaggregates Present in Different Versions of Simulated Intestinal Fluid.

The absorption of hydrophobic drugs and nutrients from the intestine is principally determined by the amount that can be dissolved by the endogenous fluids present in the gut. Human intestinal fluids (HIFs) comprise a complex mixture of bile salts, phospholipids, steroids and glycerides that vary in composition in the fed and fasted state and between subjects. A number of simulated intestinal fluid (SIF) compositions have been developed to mimic fasted and fed state intestinal conditions and allow the in vitro determination of drug solubility as a proxy for the maximum dissolved concentration it is possible to reach. In particular these solvents are used during the development of lipophilic and poorly water-soluble drugs but questions remain around the differences that may arise from the source and methods of preparation of these fluids. In this work, a range of SIFs were studied using small-angle X-ray scattering (SAXS), cryogenic-transmission electron microscopy (cryo-TEM) and molecular dynamics (MD) simulations in order to analyze their structures. In-house prepared SIFs based on sodium taurodeoxycholate (NaTDC) and 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) formed oblate ellipsoidal micelles irrespective of lipid concentration and preparation conditions. In contrast, commercially available SIFs based on sodium taurocholate and lecithin formed prolate ellipsoidal micelles in the fed state and vesicles in the fasted state. These structural variations are the likely reason for the dramatic differences sometimes observed in the solubility enhancements for hydrophobic drugs, nutrients and digestion products when using different SIFs. However, the structural homogeneity of the NaTDC/DOPC micelles makes them ideal candidates for standardizing SIF formulations as the structures of the solubilizing nanoaggregates therein are not sensitive to the preparation method.

A new lipid excipient, phosphorylated tocopherol mixture, TPM enhances the solubilisation and oral bioavailability of poorly water soluble CoQ10 in a lipid formulation.

Phosphorylated tocopherols are a new class of lipid excipients that have demonstrated potential in pharmaceutical applications. Their ability to solubilise poorly water soluble drugs indicates their potential utility in improving bioavailability of drugs where solubility limits their bioavailability. In this study a commercial mixture of phosphorylated tocopherols, TPM was combined with medium chain triglyceride (MCT) as a formulation for CoQ10, and in vitro and in vivo performance compared to the effect of addition of alternative tocopherol-based excipients. In in vitro digestion experiments, CoQ10 was poorly solubilised in the digesting MCT as anticipated. Addition of TPM facilitated the enhanced solubilisation of CoQ10 as did vitamin E TPGS (TPGS). Other tocopherol derivatives (tocopherol acetate, tocopherol) were less effective at solubilising the active during the digestion process. The trends in in vitro solubilisation were conserved in the in vivo bioavailability of CoQ10 after oral administration to rats, with TPM and TPGS formulations providing approximately double the exposure of MCT alone, while the addition of the other tocopherol derivatives reduced the overall exposure. Collectively, the results indicate potential of TPM as a new solubilising excipient for use in oral drug delivery for poorly water soluble drugs.

In Vivo Formation of Cubic Phase in Situ after Oral Administration of Cubic Phase Precursor Formulation Provides Long Duration Gastric Retention and Absorption for Poorly Water-Soluble Drugs.

Lipid-based liquid crystalline systems based on the combination of digestible and nondigestible lipids have been proposed as potential sustained release delivery systems for oral delivery of poorly water-soluble drugs. The potential for cubic phase liquid crystal formation to induce dramatically extended gastric retention in vivo has been shown previously to strongly influence the resulting pharmacokinetics of incorporated drug. In vitro studies showing the in situ formation of cubic phase from a disordered precursor comprising a mixture of digestible and nondigestible lipids under enzymatic digestion have also recently been reported. Combining both concepts, here we show the potential for such systems to form in vivo, increasing gastric retention, and providing a sustained release effect for a model poorly water-soluble drug cinnarizine. A mixture of phytantriol and tributyrin at an 85:15 mass ratio, shown previously to form cubic phase under the influence of digestion, induced a similar pharmacokinetic profile to that in the absence of tributyrin, but completely different from tributyrin alone. The gastric retention of the formulation, assessed using micro-X-ray CT imaging, was also consistent with the pharmacokinetic behavior, where phytantriol alone and with 15% tributyrin was greater than that of tributyrin in the absence of phytantriol. Thus, the concept of precursor lipid systems that form cubic phase in situ during digestion in vivo has been demonstrated and opens new opportunities for sustained release of poorly water-soluble drugs.

Examining the gastrointestinal transit of lipid-based liquid crystalline systems using whole-animal imaging.

Lipid-based liquid crystalline (LC) systems have the potential to sustain the oral absorption of poorly water-soluble drugs in vivo, facilitating slow drug release from their complex internal structure. To further evaluate the dynamic relationship between gastric retention and sustained drug absorption for these systems, this study aimed to explore non-invasive X-ray micro-CT imaging as an approach to assess gastric retention. Pharmacokinetic studies were also conducted with cinnarizine-loaded LC formulations to correlate gastric retention of the formulation to drug absorption. The in vivo studies demonstrated the interplay between gastric retention and drug absorption based on the digestibility of the LC structures. An increase in non-digestible phytantriol (PHY) composition in the formulation relative to digestible glyceryl monooleate (GMO) increased the gastric retention, with 68 ± 4 % of formulation intensity remaining at 8 h for 85 % w/w PHY, and 26 ± 9 % for 60 % w/w PHY. Interestingly, it was found that PHY 30 % w/w in GMO provided the highest bioavailability for cinnarizine (CZ) amongst the other combinations, including GMO alone. The studies demonstrated that combining digestible and non-digestible lipids into LC systems allowed for an optimal balance between sustaining drug absorption whilst increasing plasma concentration (C max) over time, leading to enhanced oral bioavailability. The results demonstrate the potential for utilising non-invasive X-ray micro-CT imaging to dynamically assess the GI transit of orally administered liquid crystal-forming formulations.