The main characteristic of gastric fluids is their acidic pH which has a profound effect on the solubility of ionizable compounds. The FaSSGF used to mimic human gastric fluid contains 80 μM taurocholate and 20 μM lecithin, derived from soybean oil. Lecithin has a critical micelle concentration (CMC) well below 1 nM (King and Marsh, 1987) whereas taurocholate has a reported CMC of 6.3 mM (Yang et al., 2010). The low concentration of taurocholate in FaSSGF in relation to its CMC implies that the bile salt may primarily have wetting effects during dissolution in the medium. A large

fraction of the bile salt is likely to be dissolved selleck in the bulk of the medium whereas the lecithin is likely found in liposomes together with the

remainder of the taurocholate. The addition of ethanol to aqueous systems leads to a lower dielectric constant of the resulting mixture, which in turn leads to an increase in Sapp of nonpolar compounds. Indeed this was confirmed by our study since drugs that were non-ionized at the studied pH (2.5) generally had higher solubility in media containing 20% ethanol. The two most lipophilic compounds, tolfenamic acid and felodipine, were the compounds with the strongest positive effect on solubility by the presence of lipids and/or ethanol. Tolfenamic acid showed a slight increase in Sapp in media with ethanol. This was the only compound in the study that appeared to be effectively solubilized by the low concentrations of taurocholate and bile salt present in FaSSGF, with a close to 20 times

higher Sapp in FaSSGF compared to that selleck screening library observed in the corresponding blank medium (NaClpH2.5). This could potentially be a result of the high lipophilicity in combination with its relatively small size; tolfenamic acid had the lowest molecular weight (261.7) of the compounds. The larger substance, felodipine, was also solubilized by phospholipid aggregates in FaSSGF but its Sapp was only doubled compared to that in NaClpH2.5. On the other hand, the effect of ethanol on felodipine Sapp was more pronounced. The addition of 20% ethanol to NaClpH2.5 or FaSSGF led to a 25-fold and 15-fold increase, respectively. In comparison, the less lipophilic neutral compounds, griseofulvin and progesterone, were both unaffected by the lipids in FaSSGF. however However, they exhibited an 8–10-fold increase in solubility after the inclusion of 20% ethanol to either NaClpH2.5 or FaSSGF. The compounds with basic functions were highly charged and had considerably lower lipophilicity at pH 2.5 (log DpH2.5) compared to the other drugs. They all exhibited a relatively high Sapp due to being completely ionized and they were therefore unaffected by either lipid content or ethanol in the media. The observation that Sapp of uncharged and lipophilic compounds significantly increases in response to ethanol is in agreement with our previous results regarding ethanol effects in intestinal media ( Fagerberg et al., 2012).