Cell-free permeation systems are gaining interest in drug discovery and development as tools to obtain a reliable prediction of passive intestinal absorption without the disadvantages associated with cell- or tissue-based permeability profiling. Depending on the composition of the barrier, cell-free permeation systems are classified into two classes including (i) biomimetic barriers which are constructed from (phospho)lipids and (ii) non-biomimetic barriers containing dialysis membranes. This review provides an overview of the currently available cell-free permeation systems including Parallel Artificial Membrane Permeability Assay (PAMPA), Phospholipid Vesicle-based Permeation Assay (PVPA), Permeapad®, and artificial membrane based systems (e.g. the artificial membrane insert system (AMI-system)) in terms of their barrier composition as well as their predictive capacity in relation to well-characterized intestinal permeation systems. Given the potential loss of integrity of cell-based permeation barriers in the presence of food components or pharmaceutical excipients, the superior robustness of cell-free barriers makes them suitable for the combined dissolution/permeation evaluation of formulations. While cell-free permeation systems are mostly applied for exploring intestinal absorption, they can also be used to evaluate non-oral drug delivery by adjusting the composition of the membrane.

Archaea contain membrane lipids that differ from those found in the other domains of life (Eukarya and Bacteria). These lipids consist of isoprenoid chains attached via ether bonds to the glycerol carbons at the sn-2,3 positions. Two types of ether lipids are known, polar diether lipids and bipolar tetraether lipids. The inherent chemical stability and unique membrane-spanning characteristics of tetraether lipids render them interesting for oral drug delivery purposes. Archaeal lipids form liposomes spontaneously (archaeosomes) and may be incorporated in conventional liposomes (mixed vesicles). Both types of liposomes are promising to protect their drug cargo, such as therapeutic peptides, against the acidic environment of the stomach and proteolytic degradation in the intestine. They appear to withstand lipolytic enzymes and bile salts and may thus deliver orally administered therapeutic peptides to distant sections of the intestine or to the colon, where they may be absorbed, eventually by the help of absorption enhancers. Archaeal lipids and their semisynthetic derivatives may thus serve as biological source for the next generation oral drug delivery systems. The aim of this review is to present a systematic overview over existing literature on archaea carrying diether and tetraether lipids, lipid diversity, means of lipid extraction and purification, preparation and in vitro stability studies of archaeal lipid-based liposomal drug carriers and in vivo proof-of concepts studies.

Replacing in vivo with in vitro studies can increase sustainability in the development of medicines. This principle has already been applied in the biowaiver approach based on the biopharmaceutical classification system, BCS. A biowaiver is a regulatory process in which a drug is approved based on evidence of in vitro equivalence, i.e., a dissolution test, rather than on in vivo bioequivalence. Currently biowaivers can only be granted for highly water-soluble drugs, i.e., BCS class I/III drugs. When evaluating poorly soluble drugs, i.e., BCS class II/IV drugs, in vitro dissolution testing has proved to be inadequate for predicting in vivo drug performance due to the lack of permeability interpretation. The aim of this review was to provide solid proofs that at least two commercially available cell-free in vitro assays, namely, the parallel artificial membrane permeability assay, PAMPA, and the PermeaPad^® assay, PermeaPad, in different formats and set-ups, have the potential to reduce and replace in vivo testing to some extent, thus increasing sustainability in drug development. Based on the literature review presented here, we suggest that these assays should be implemented as alternatives to (1) more energy-intense in vitro methods, e.g., refining/replacing cell-based permeability assays, and (2) in vivo studies, e.g., reducing the number of pharmacokinetic studies conducted on animals and humans. For this to happen, a new and modern legislative framework for drug approval is required.

The current study documents enhanced apparent solubility of the BCS class II drug celecoxib (CXB) when formulated as solid phospholipid dispersion (SPD) with either mono- or diacyl-phospholipids by freeze drying from hydro-alcoholic solvent. The enhanced solubility upon dispersion in buffer or fasted state simulated intestinal fluid (FaSSIF) is interpreted to be due to two effects: (1) amorphization of CXB, inducing supersaturation, which is also observed when CXB is freeze dried in the absence of phospholipids and (2) association of CXB with spontaneously forming colloidal structures, such as vesicles and/or micelles, promoting solubilization. The latter effect depended on the CXB-to-phospholipid ratio, where monoacyl-phospholipid was a more efficient solubilizer than diacyl-phospholipid. In the case of diacyl-phospholipid, solubilization also depended strongly on the dispersion medium, where FaSSIF induced a more pronounced solubilization effect than buffer. In contrast, a significantly enhanced in-vitro permeability of CXB across a biomimetic barrier (Permeapad®) was found only with low lipid contents up to a CXB to phospholipid mass-ratio of 1:10 or in the absence of phospholipid; above this critical ratio, permeability was not enhanced, i.e. comparable to that observed with a suspension of non-processed (crystalline) drug. This non-linear dissolution-/permeation-behavior was observed independently of (1) the type of phospholipid (monoacyl- or diacyl-) employed and (2) the dispersion medium (buffer or FaSSIF), despite the fact that different patterns of co-existing colloidal states were observed from mono-/diacyl-phospholipid formulations in buffer/FaSSIF (small bile salt micelles, intermediate size mixed micelles and large vesicular structures), assessed by asymmetric flow field-flow fractionation/multi angle laser light scattering. A uniform mechanistic hypothesis is presented to describe the impact of phospholipids on CXB permeation behavior: Obviously, the critical drug-to-phospholipid ratio represents a compromise between optimal stabilization of the amorphous state-induced supersaturation and reduced thermodynamic activity of CXB due to association with colloidal states, where the type of colloidal state (vesicle or micelle) appears to be of minor importance.

Supersaturated lipid-based drug delivery systems have recently been investigated for oral administration for a variety of lipophilic drugs and have shown either equivalent or superior oral bioavailability compared to conventional non-supersaturated lipid-based drug delivery systems. The aim of the present work was to explore supersaturated versus non-supersaturated lipid-based systems at equivalent lipid doses, on in vivo bioavailability in rats and on in vitro permeation across a biomimetic Permeapad^Ⓡ membrane to establish a potential in vivo - in vitro correlation. A secondary objective was to investigate the influence of lipid composition on in vitro and in vivo performance of lipid systems. Results obtained indicated that increasing the celecoxib load in the lipid-based formulations by thermally-induced supersaturation resulted in increased bioavailability for medium and long chain mono-/di-glycerides systems relative to their non-supersaturated (i.e. 85%) reference formulations, albeit only significant for the medium chain systems. Long chain systems displayed higher celecoxib bioavailability than equivalent medium chain systems, both at supersaturated and non-supersaturated drug loads. In vitro passive permeation of celecoxib was studied using both steady-state and dynamic conditions and correlated well with in vivo pharmacokinetic results with respect to compositional effects. In contrast, permeation studies indicated that flux and percentage permeated of supersaturated systems, either at steady-state or under dynamic conditions, decreased or were unchanged relative to non-supersaturated systems. This study has shown that by using two cell-free Permeapad^Ⓡ permeation models coupled with rat-adapted gastro-intestinal conditions, bio-predictive in vitro tools can be developed to be reflective of in vivo scenarios. With further optimization, such models could be successfully used in pharmaceutical industry settings to rapidly screen various prototype formulations prior to animal studies.