Dissolution-Microdialysis predicts oral bioavailability from phospholipid-based amorphous solid dispersions

Abstract

Here, we report dissolution of aprepitant from three phospholipid-based amorphous solid dispersions (ASDs), namely binary ASDs containing drug and either natural or hydrogenated phospholipid (HPL) and a ternary ASD containing HPL and hydroxypropyl methylcellulose acetate-succinate (HPMCAS) in fasted state simulated intestinal fluid (FaSSIF) under lipolytic and non-lipolytic conditions (with / without pancreatin). Samples were taken simultaneously by conventional sampling using centrifugation for solid/dissolved drug separation and via a microdialysis probe.

Solid-state analysis: XRPD indicated that the ternary and the binary natural phospholipid dispersion were amorphous, while the binary HPL dispersion showed small Bragg peaks indicating remaining crystallinity.

Lipolysis: did not influence aprepitant release from the natural phospholipid-based ASD.

Dissolution: Centrifugation sampling indicated a huge burst and slow decline of apparent supersaturation. Microdialysis sampling revealed the ”true” spring and parachute effect, meaning a free drug concentrations initially above molecular solubility, which declined rapidly. Supersaturation was most pronounced for the ternary ASD.

IVIVC: Comparing dissolution AUCs in terms of apparently dissolved drug with published rat plasma AUCs, a decent IVIVC was seen (R² = 0.9198), whereas IVIVC based on molecularly dissolved drug was superior (R² = 0.9677) and even on par with published in vitro dissolution/permeation data (R² = 0.9712), indicating that dissolution in conjunction with microdialysis sampling provided excellent oral bioavailability prediction. Furthermore, it allowed for unprecedented mechanistic insights.

Introduction

The fraction of new drug candidates with poor water solubility has increased during recent decades 1, necessitating the use of enabling formulations such as amorphous solid dispersions (ASDs) or lipid-based formulations.2 Phospholipid-based amorphous solid dispersions (PL-based ASDs) can be considered as both a lipid-based formulation utilizing phospholipids, forming solubilizing colloidal structures upon dissolution and ASDs stabilizing the amorphous form of an active pharmaceutical ingredient (API) within an amorphous matrix consisting of phospholipid and API (binary systems) 3 or phospholipid + polymers + API (ternary systems).4 The amorphization of the API is expected to result in both an increased dissolution rate and solubility 5-7, better described as a transient supersaturation. Such supersaturation has been described already in 2012 as an increase in the free (molecularly) dissolved drug fraction 8, which is closely related to a colloidal state, namely amorphous drug-rich nanoparticles 9, which are spontaneously forming, boost permeability and bioavailability upon oral administration.2 When comparing recent ASD dissolution studies, where microdialysis has been employed to older studies, it appears likely that in early supersaturation studies the “dissolved” fraction (passing 0.2 µm filter pores) included these drug-rich nanoparticles. 6

In classical ASDs, a polymer is employed to stabilize the amorphous state. The polymer matrix reduces the molecular mobility of the drug and thus the likelihood of nucleation, and therefore improves physical stability. 10, 11 Synthetic polymers, such as Polyvinylporrolidone-vinylacetate and Hydroxypropylmethylcellulose-acetatesuccinate are established excipients within numerous registered drug products, 12 although fall into the “generally recognized as safe” category. 13

In recent years, phospholipids have proven to be an effective alternative to synthetic polymers in ASDs inducing and preserving the amorphous state of a drug 14, although long-term stability data are missing. Their absorption-enhancing effects 3, 15 have been described under various names, including pro-liposomes 16, drug-phospholipid complexes 17, phospholipid-based ASDs 18, 19 and co-amorphous systems.20 For the purposes of this work, these systems will be referred to as phospholipid amorphous dispersions.

The behavior of phospholipid amorphous dispersions following oral ingestion is currently not fully understood. Phospholipids self-assemble into colloidal structures, vesicles in aqueous medium or mixed micelles in human intestinal fluids containing bile salts. 21-23 These structures are capable of solubilizing poorly water-soluble compounds.13, 24 Of note, intra- and interindividual variability of the gastrointestinal (GI) environment in terms of type and content of bile salts interacting with phospholipids leads to diverse colloidal associates 21, 22, 25, mostly large disk-shaped mixed micelles.21 Another key aspect of GI processing is the secretion of phospholipases, such as phospholipase A2, in the small intestine. This enzyme hydrolyzes phospholipids, generating readily absorbed monoacyl phospholipids and free fatty acids.24 A recent experimental study on hydrogenated phospholipid (HPL) amorphous dispersions suggested two competing mechanisms for colloid formation during in vitro dissolution.26 Both postulated types of particles are small enough to pass 20 nm filter pores but have a vastly different drug content. It was proposed that drug-rich amorphous nanoparticles drive true molecular supersaturation, a pathway that dominated for a pure polymeric ASD. Conversely, the formation of bile salt/phospholipid colloids (micelles) containing associated drug was predominant for the mixed and pure HPL amorphous dispersions, which notably showed similar in vitro permeation performance.26

Jacobsen and co-workers compared monoacyl phosphatidylcholine and diacyl phosphatidylcholine phospholipid amorphous dispersions of celecoxib, finding both types performed equally in terms of in vitro permeation.19 Interestingly, a meta-analysis of literature data indicated that high phospholipid content can substantially increase the solubility of poorly soluble APIs without proportionally increasing in vitro permeation or oral bioavailability.15 High phospholipid content has even been reported to decrease in vitro permeation, suggesting an optimal phospholipid-to-drug-ratio exists for maximizing permeation. However, in vivo data did not always reflect these in vitro observations.19 This in vitro-in vivo discrepancy may stem from the colloidal structures formed when phospholipids are dispersed in GI-fluids, which reduce the overall free drug fraction in vitro. In contrast, in vivo digestion (lipolysis), catalyzed by phospholipase A2, may liberate the API from this colloidal entrapment.14,19 A recent study demonstrated that combining in vitro permeation experiments with in vitro lipolysis to simulate digestion did, in fact, result in enhanced permeation of indomethacin from phospholipid amorphous dispersions, when compared to non-lipolytic conditions. 27

As hypothesized in 2013, only the molecularly dissolved drug fraction, and not the drug solubilized within colloidal species (e.g., micelle-bound drug), is readily available for absorption 2, which highlights the need for an approach to quantify the free drug concentration. Microdialysis has recently been demonstrated ideal for time-resolved measurements of the molecularly dissolved drug fraction in biomimetic media and under digestive conditions.28-36 Microdialysis is a non-equilibrium sampling technique that relies on a semi-permeable membrane. This membrane is characterized by a specific molecular-weight-cut-off. Continuous perfusion of the probe with a perfusion medium establishes a concentration gradient, the driving force for free drug to diffuse. Time-resolved collection of the emerging dialysate, in small aliquots allows near real-time observation of the free drug concentration/time-profile.

A recent study demonstrated that dissolution data obtained by microdialysis sampling correlated better with human in vivo plasma curves (AUCs) than any other sampling technique (i.e., bench top centrifugation, micro-and nanofiltration and 2nd derivative UV-spectroscopy). 34

Tønning and co-workers recently developed a microdialysis method capable of quantifying the molecularly dissolved (free) concentration of indomethacin during in vitro lipolysis. The method proved compatible with standard lipolysis buffer and lipolysis media supplemented with pancreatin extract.36 This method was subsequently applied to binary phospholipid amorphous dispersions of indomethacin and phosphatidylcholine.27 The study demonstrated that microdialysis, when compared to classical centrifugation sampling, revealed a critical role of lipolysis for in vitro drug permeation regarding drug release from diacyl phosphatidylcholine. These insights provided an increased understanding in the performance of, and the mechanism of action of phospholipid amorphous dispersions. 27

Aprepitant is a poorly water-soluble weak base (biopharmaceutics classification system class IV), with a molecular mass of 543.435 g/mol, a pKa value of 8.1 and a clogP value of 4.6. 37 Polymer-based ASD have been described in literature, e.g. in ref.38 Amorphous dispersions of aprepitant using HPLs have also been described in literature including animal data on oral bioavailability. 39 So far, microdialysis sampling has not been applied to HPL amorphous dispersions under digestive conditions. HPLs are known to be less affected by digestion (lipolysis) than their natural counterparts. 40 In essence, this study aimed to 1) develop a biopharmaceutical method to assess the mechanism of release of model drug aprepitant from natural and HPL amorphous dispersions during GI processing, 2) correlate the in vitro data obtained by microdialysis sampling of HPL amorphous dispersions with existing in vivo data for validation.

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Chemicals

The model drug used in this study, aprepitant, was purchased from abcr (Karlsruhe, Germany). Trifluoroacetic acid, CaCl2·2H2O, NaH2PO4·2H2O, tris-maleate, pancreatin from porcine pancreas (8×USP specifications), polysorbate 80 (PS80) were ordered from Sigma-Aldrich® Denmark ApS (Brøndby, Denmark). NaCl and methanol (HPLC-grade) were from VWR™ International A/S (Søborg, Denmark). NaOH pellets were from Merck A/S (Hellerup, Denmark). Lipoid S75 (70% phosphatidylcholine from soybean) and Lipoid Phospholipon® 90H (≥90% hydrogenated phosphatidylcholine from soybean) were kindly gifted by Lipoid GmbH (Ludwigshafen, Germany). HPMCAS-MF was donated from Shin-Etsu Chemical Co. Ltd. (Japan). FaSSIF/FeSSIF/FaSSGF powder comprising lecithin and sodium taurocholate was purchased from biorelevant.com (London, UK). All buffers were prepared using highly purified water from a Milli-Q® reference A+ water purification system (Merck KGaA, Darmstadt, Germany) and salts of analytical grade.

Martin Brandl , Ann-Christin Jacobsen , Dissolution-Microdialysis predicts oral bioavailability from
phospholipid-based amorphous solid dispersions, Journal of Pharmaceutical Sciences (2026), doi:
https://doi.org/10.1016/j.xphs.2026.104236