Exploration for development of more effective drugs by modern technologies has led to many new drug candidates that are poorly water-soluble [1,2]. As a result, 40% of marketed drugs and 70 to 90% of drugs in development are classified as biopharmaceutics classification system (BCS) class II/IV [[3], [4], [5]], requiring some formulation development for them to be absorbed after oral administration [1,6]. Furthermore, some of the drugs with low water-solubility are also poorly soluble in lipoidal components; these compounds are called “brick dust” [1,7,8]. “Brick dust” compounds have tight crystal lattices with high crystal packing energy [7] and a melting point (Tm) of over 200°C [8], and based on this categorization, approximately 27% of the approved drugs would be categorized into “brick dust” [8,9]. “Brick dust” compounds need to overcome more formulation-related challenges to ensure adequate pharmacological effect [7]. Several approaches have recently been proposed for formulating “brick dust” drugs, including lipid-based formulations (LBF) [2,4,6,7,[10], [11], [12], [13], [14], [15], [16], [17], [18]]. Although LBF has been a promising approach for poorly water-soluble drugs for many years [2], additional elaborate efforts [4,19,20] are needed for “brick dust” drugs due to their limited solubility in lipoidal components used for preparing LBF including SNEDDS [6]. SNEDDS/SMEDDS is one of LBF and isotropic mixtures of an oil, surfactant, and co-surfactant [21]. After oral dosing of the mixtures, an oil-in-water nano- or micro-emulsion can be formed with gentle agitation by gastrointestinal motility and with dilution by drinking water and gastrointestinal fluid. A very large surface area due to the small particle size of the nano- or micro-emulsions can lead to rapid lipid digestion and drug release, resulting in rapid drug absorption from the small intestine [22,23]. The term SNEDDS is often used as an umbrella name for preconcentrate mixtures that result in either microemulsions or nanoemulsions from a biopharmaceutical perspective, although some academic distinctions between nanoemulsions and microemulsions [2,24].

Previously, we have successfully improved oral absorption of three “brick dust” drugs, including rebamipide [25], mebendazole (MBZ) [26], and nintedanib ethansulfonate [27] by combining co-amorphization with SNEDDS. In brief, the high crystallinity of a “brick dust” drug is destroyed by the molecular interaction with its counter ion, resulting in the formation of co-amorphous complex. The co-amorphization with the counter ion improves the solubility of the “brick dust” drugs in lipoidal components, thereby makes it possible to prepare SNEDDS for “brick dust” drugs by using their co-amorphous complexes [[25], [26], [27]]. This strategy resulted in a 70- to 670-fold improvement in the dissolution of the three “brick dust” drugs over their crystal powders in the in-vitro non-sink dissolution method, referred to as an in-vitro conventional method; however, it led to only 3.6- to 10.5-fold improvement in the in-vivo oral absorption [[25], [26], [27]]. This large discrepancy between in-vitro and in-vivo studies should be reduced to estimate the in-vivo performance more precisely and make more efficient formulations for “brick dust” compounds. It is also well known that there is some discrepancy between the in-vitro dissolution behavior and the in-vivo performance of formulations prepared to improve drug dissolution [[28], [29], [30]], including the issue of thermodynamic activity [[31], [32], [33], [34]].

As discussed previously [27], this discrepancy can be attributed to several factors, as listed below: a) Oral absorption from crystal or co-amorphous powders is higher than expected due to “spontaneous supersaturation” in vivo [29]. b) Oral absorption from SNEDDS is lower than expected due to higher precipitation rate dependent on higher degree of supersaturation [29], which might be superior to the absorption rate. c) The maintenance of supersaturation, as observed in the in-vitro conventional method, does not occur in vivo after oral dosing due to i) possible changes in physicochemical properties of nanoemulsion oil droplets, including digestion and/or absorption [35] of lipoidal components, ii) subsequent changes in the release properties of drugs from nanoemulsion droplets, and iii) the precipitation of drugs dissolved directly in the aqueous phase under acidic conditions after being transferred to the small intestine facilitates the nucleation of drugs newly released from nanoemulsion droplets and/or micelles afterward. d) “nucleation in the gastrointestinal lumen is facilitated by various surfaces and interfaces, only existing in vivo, that may act a catalyst for nucleation” [29].

In the present study, to investigate the reason for such a large discrepancy, SNEDDS with or without 2% HPMCP-50 for MBZ co-amorphized with (+)-10-camphorsulufonic acid (CSA) (MBZ-CSA) was selected as a model “brick dust” drug formulation. MBZ is a marketed anti-helminthic drug [36] that has recently been evaluated as an anti-cancer drug against several types of cancer cells [36]. MBZ is a basic compound with a Tm of 288.5°C (Supplemental Table 1) [26] and possesses very low solubility in water [37]. Furthermore, its solubility in olive oil is also very low [38], even though its clogP is 2.64 (Supplemental Table 1) [26]. In fact, MBZ is very poorly soluble in several aqueous vehicles, ethanol, and SNEDDS vehicle (Supplemental Table 2) [26]. These characteristics are typical for “brick dust” compounds [1,7,8]. To improve the solubility of MBZ in lipoidal vehicles, we formed a complex of MBZ and CSA, an anionic compound (Supplemental Table 1) [26], resulting in the co-amorphous salt, MBZ-CSA [26]. MBZ-CSA possesses much higher solubility in distilled water, ethanol, and SNEDDS vehicle compared to MBZ crystals (Supplemental Table 2) [26]. Additionally, 2% HPMCP-50 SNEDDS for MBZ-CSA provided significant improvement in both in-vitro dissolution and in-vivo oral absorption of MBZ, compared with SNEDDS without HPMCP-50 [26], and provided the highest improvement in oral absorption among the three “brick dust” drugs examined before [[25], [26], [27]].

We first focused on drug transit from the stomach to the small intestine (GI-transit) and the subsequent absorption process. Gastric emptying to the small intestine sequentially changes the environmental conditions to which the ingested drugs are exposed. We attempted to investigate the effect of the sequential environmental change on the dissolution behavior of MBZ by using a novel in-vitro “Sequential Gastro-Intestinal Exposure” (SGIE) method. The absorption process involves the absorption of drugs as well as lipoidal components forming nanoemulsion droplets, which changes the distribution equilibrium of drugs in nanoemulsions [39]. We attempted to evaluate the effect of the absorption process on the dissolution of MBZ using the “Egg phosphatidylcholine-Monolayer-CHCl3 Partition” (EMCP) method [39]. Thereafter, the effects of the GI-transit and the subsequent absorption processes were evaluated using the SGIE-EMCP method, where SGIE was combined with EMCP.