Computational prediction of drug solubility in fasted simulated and aspirated human intestinal fluid.

PURPOSE: To develop predictive models of apparent solubility (Sapp) of lipophilic drugs in fasted state simulated intestinal fluid (FaSSIF) and aspirated human intestinal fluid (HIF). METHODS: Measured Sapp values in FaSSIF, HIF and phosphate buffer pH 6.5 (PhBpH6.5) for 86 lipophilic drugs were compiled and divided into training (Tr) and test (Te) sets. Projection to latent structure (PLS) models were developed through variable selection of calculated molecular descriptors. Experimentally determined properties were included to investigate their contribution to the predictions. RESULTS: Modest relationships between Sapp in PhBpH6.5 and FaSSIF (R(2) = 0.61) or HIF (R(2) = 0.62) were found. As expected, there was a stronger correlation obtained between FaSSIF and HIF (R(2) = 0.78). Computational models were developed using calculated descriptors alone (FaSSIF, R(2) = 0.69 and RMSEte of 0.77; HIF, R(2) = 0.84 and RMSEte of 0.81). Accuracy improved when solubility in PhBpH6.5 was added as a descriptor (FaSSIF, R(2) = 0.76 and RMSETe of 0.65; HIF, R(2) = 0.86 and RMSETe of 0.69), whereas no improvement was seen when melting point (Tm) or logDpH 6.5 were included in the models. CONCLUSION: Computational models were developed, that reliably predicted Sapp of lipophilic compounds in intestinal fluid, from molecular structures alone. If experimentally determined pH-dependent solubility values were available, this further improved the accuracy of the predictions.

Physiologically based biopharmaceutics modeling of regional and colon absorption in humans.

Colon absorption is a key determinant for successful development of extended release and colon targeted drug products. This is the first systematic evaluation of the ability to predict in vivo regional differences in absorption and the extent of colon absorption in humans using mechanistic physiologically based biopharmaceutics modeling (PBBM). A new dataset, consisting of 19 drugs with a wide range of biopharmaceutics properties and extent of colon absorption in humans, was established. Mechanistic predictions of the extent of absorption and plasma exposure after oral, or jejunal and direct colon administration were performed in GastroPlus and GI-Sim using an a priori approach. Two new colon models developed in GI-Sim, were also evaluated to assess if the prediction performance could be improved. Both GastroPlus and GI-Sim met the pre-defined criteria for accurate predictions of regional and colon absorption for high permeability drugs irrespective of formulation type, while the prediction performance was poor for low permeability drugs. For solutions, the two new GI-Sim colon models improved the colon absorption prediction performance for the low permeability drugs while maintaining the accurate prediction performance for the high permeability drugs. In contrast, the prediction performance decreased for non-solutions using the two new colon models. In conclusion, PBBM can be used with sufficient accuracy to predict regional and colon absorption in humans for high permeability drugs in candidate selection as well as early design and development of extended release or colon targeted drug products. The prediction performance of the current models needs to be improved to allow high accuracy predictions for commercial drug product applications including highly accurate predictions of the entire plasma concentration-time profiles as well as for low permeability drugs.

Ethanol effects on apparent solubility of poorly soluble drugs in simulated intestinal fluid.

Ethanol intake can lead to an unexpected and possibly problematic increase in the bioavailability of druglike compounds. In this work we investigated the effect of ethanol on the apparent solubility and dissolution rate of poorly soluble compounds in simulated intestinal fluid representing a preprandial state. A series of 22 structurally diverse, poorly soluble compounds were measured for apparent solubility and intrinsic dissolution rate (37 °C) in phosphate buffer pH 6.5 (PhB6.5) and fasted state simulated intestinal fluid (FaSSIF, pH 6.5) with and without ethanol at 5% v/v or 20% v/v. The obtained data were used to understand for which molecules ethanol results in an increased apparent solubility and, therefore, may increase the amount of drug absorbed. In FaSSIF20%ethanol 59% of the compounds displayed >3-fold higher apparent solubility than in pure FaSSIF, whereas the effects of 5% ethanol on solubility, in most cases, were negligible. Acidic and neutral compounds were more solubilized by the addition of ethanol than by lecithin/taurocholate aggregates, whereas bases showed a more substance-specific response to the additives in the buffer. The stronger solubilizing capacity of ethanol as compared to the mixed lipid aggregates in FaSSIF was further identified through Spearman rank analyses, which showed a stronger relationship between FaSSIF20%ethanol and PhB6.5,20%ethanol (rS of 0.97) than FaSSIF20%ethanol and FaSSIF (rS of 0.86). No relationships were found between solubility changes in media containing ethanol and single physicochemical properties, but multivariate data analysis showed that inclusion of ethanol significantly reduced the negative effect of compound lipophilicity on solubility. For this data set the higher concentration of ethanol gave a dose number (Do) <1 for 30% of the compounds that showed incomplete dissolution in FaSSIF. Significant differences were shown in the melting point, lipophilicity, and dose profiles between the compounds having a Do < 1 and Do > 1, with the latter having higher absolute values in all three parameters. In conclusion, this study showed that significant effects of ethanol on apparent solubility in the preprandial state can be expected for lipophilic compounds. The results herein indicate that acidic and neutral compounds are more sensitive to the addition of ethanol than to the mixed lipid aggregates present in the fasted intestine.

Affinity of Lipophilic Drugs to Mixed Lipid Aggregates in Simulated Gastrointestinal Fluids.

Mixed lipid aggregates, comprising of bile salts and phospholipids, present in the small intestine assist in drug solubilization and subsequent drug dissolution and absorption through the intestinal epithelium. The increased variability in their levels, observed physiologically, may create challenges not only for in vivo bioavailability and bioequivalence studies, but also for in vitro bio-predictive studies as correlations between in vitro and in vivo data are not always successful. The current study investigated the impact of biorelevant dissolution media, with physiologically relevant sodium taurocholate and lecithin levels, on the apparent solubility and affinity of lipophilic compounds with a wide range of physicochemical properties (drug ionization, drug lipophilicity, molecular weight) to mixed lipid aggregates. Apparent solubility data in biorelevant dissolution media for the studied neutral drugs, weak bases and weak acids were compared against a phosphate buffer pH 6.5 in the absence of these lipidic components. Presence of mixed lipid aggregates enhanced the apparent solubility of the majority of compounds and the use of multivariate data analysis identified the significant parameters affecting drug affinity to mixed lipid aggregates based on the chemical class of the drug. For neutral drugs, increasing bile salt concentrations and/or drug lipophilicity resulted in greater enhancement in apparent solubility at 24-hr. For weak bases and weak acids, the effect of increasing bile salt levels on apparent solubility depended mostly on an interplay between drug lipophilicity and drug ionization.

Dissolution rate and apparent solubility of poorly soluble drugs in biorelevant dissolution media.

A series of poorly soluble BCS class II compounds with "grease ball" characteristics were assessed for solubility and dissolution rate in biorelevant dissolution media (BDM) with the purpose of investigating which molecular structures gain most in solubility when dissolved under physiologically relevant conditions. The compounds were studied in four media (simulated intestinal fluid in fasted (FaSSIF pH 6.5) and fed state (FeSSIF pH 5.0), and their corresponding blank buffers (FaSSIF(blk) and FeSSIF(blk))) at a temperature of 37 °C. The experimental results were used to analyze which molecular characteristics are of importance for the solubility in BDM and for in silico modeling using multivariate data analysis. It was revealed that a majority of the compounds exhibited a higher dissolution rate and higher solubility in the FaSSIF and FeSSIF than in their corresponding blank buffers. Compounds which were neutral or carried a positive charge were more soluble in FeSSIF than FaSSIF. The acidic compounds displayed clear pH dependency, although the higher concentration of solubilizing agents in FeSSIF than FaSSIF also improved the solubility. Five of the ten compounds were upgraded to BCS class I when dissolved in FaSSIF or FeSSIF, i.e., the maximum dose of these compounds given orally was soluble in 250 mL of these BDMs. Lipophilicity as described by the log D(oct) value was identified as a good predictor of the solubilization ratio (R(2) = 0.74), and computed molecular descriptors were also shown to successfully predict the solubilities in BDM for this data set. To conclude, the physiological solubility of "grease ball" molecules may be largely underestimated in in vitro solubility assays unless BDM is used. Moreover, the results herein indicate that the improvement obtained in BDM may be possible to predict from chemical features alone.

Intestinal solubility and absorption of poorly water soluble compounds: predictions, challenges and solutions.

We have explored for which type of compounds biorelevant dissolution profiling in simulated intestinal fluids would accurately predict solubility in human intestinal fluid. In total, 474 solubility values in simulated and aspirated human intestinal fluid for 78 drugs were compiled and analyzed. Significant solubilization in the colloidal structures was obtained in fasted and fed state fluids for drug compounds with a logD(oct)>3. Highly lipophilic compounds with high melting points (Tm > 200 °C) could also be significantly solubilized, but typically such compounds had solubility values in the lower µg/ml range also in the presence of the colloidal structures. On the basis of our analysis, compounds with a logD(oct)>3 should be explored in biorelevant dissolution media to better predict in vivo performance after oral dosing.

Concomitant intake of alcohol may increase the absorption of poorly soluble drugs.

Ethanol can increase the solubility of poorly soluble and hence present a higher drug concentration in the gastrointestinal tract. This may produce a faster and more effective absorption resulting in variable and/or high drug plasma concentrations, both of which can lead to adverse drug reactions. In this work we therefore studied the solubility and absorption effects of nine diverse compounds when ethanol was present. The apparent solubility was measured using the µDiss Profiler Plus (pION, MA) in four media representing gastric conditions with and without ethanol. The solubility results were combined with in-house data on solubility in intestinal fluids (with and without ethanol) and pharmacokinetic parameters extracted from the literature and used as input in compartmental absorption simulations using the software GI-Sim. Apparent solubility increased more than 7-fold for non-ionized compounds in simulated gastric fluid containing 20% ethanol. Compounds with weak base functions (cinnarizine, dipyridamole and terfenadine) were completely ionized at the studied gastric pH and their solubility was therefore unaffected by ethanol. Compounds with low solubility in intestinal media and a pronounced solubility increase due to ethanol in the upper gastric compartments showed an increased absorption in the simulations. The rate of absorption of the acidic compounds indomethacin and indoprofen was slightly increased but the extent of absorption was unaffected as the complete doses were readily absorbed even without ethanol. This was likely due to a high apparent solubility in the intestinal compartment where the weak acids are ionized. The absorption of the studied non-ionizable compounds increased when ethanol was present in the gastric and intestinal media. These results indicate that concomitant intake of alcohol may significantly increase the solubility and hence, the plasma concentration for non-ionizable, lipophilic compounds with the potential of adverse drug reactions to occur.

Is the full potential of the biopharmaceutics classification system reached?

In this paper we analyse how the biopharmaceutics classification system (BCS) has been used to date. A survey of the literature resulted in a compilation of 242 compounds for which BCS classes were reported. Of these, 183 compounds had been reported to belong to one specific BCS class whereas 59 compounds had been assigned to multiple BCS classes in different papers. Interestingly, a majority of the BCS class 2 compounds had fraction absorbed (FA) values >85%, indicating that they were completely absorbed after oral administration. Solubility was computationally predicted at pH 6.8 for BCS class 2 compounds to explore the impact of the pH of the small intestine, where most of the absorption occurs, on the solubility. In addition, the solubilization capacity of lipid aggregates naturally present in the intestine was studied computationally and experimentally for a subset of 12 compounds. It was found that all acidic compounds with FA>85% were completely dissolved in the pH of the small intestine. Further, lipids at the concentration used in fasted state simulated intestinal fluid (FaSSIF) dissolved the complete dose given of the most lipophilic (logD6.5>3) compounds studied. Overall, biorelevant dissolution media (pure buffer of intestinal pH or FaSSIF) identified that for 20 of the 29 BCS class 2 compounds with FA>85% the complete dose given orally would be dissolved. These results indicate that a more relevant pH restriction for acids and/or dissolution medium with lipids present better forecast solubility-limited absorption in vivo than the presently used BCS solubility criterion. The analysis presented herein further strengthens the discussion on the requirement of more physiologically relevant dissolution media for the in vitro solubility classification performed to reach the full potential of the BCS.

Formulation of the microbicide INP0341 for in vivo protection against a vaginal challenge by Chlamydia trachomatis.

The salicylidene acylhydrazide (SA) compounds have exhibited promising microbicidal properties. Previous reports have shown the SA compounds, using cell cultures, to exhibit activity against Chlamydia trachomatis, herpes simplex virus and HIV-1. In addition, using an animal model of a vaginal infection the SA compound INP0341, when dissolved in a liquid, was able to significantly protect mice from a vaginal infection with C. trachomatis. To expand upon this finding, in this report INP0341 was formulated as a vaginal gel, suitable for use in humans. Gelling agents (polymers) with inherent antimicrobial properties were chosen to maximize the total antimicrobial effect of the gel. In vitro formulation work generated a gel with suitable rheology and sustained drug release. A formulation containing 1 mM INP0341, 1.6 wt% Cremophor ELP (solubility enhancer) and 1.5 wt% poly(acrylic acid) (gelling and antimicrobial agent), was chosen for studies of efficacy and toxicity using a mouse model of a vaginal infection. The gel formulation was able to attenuate a vaginal challenge with C. trachomatis, serovar D. Formulations with and without INP0341 afforded protection, but the inclusion of INP0341 increased the protection. Mouse vaginal tissue treated with the formulation showed no indication of gel toxicity. The lack of toxicity was confirmed by in vitro assays using EpiVaginal tissues, which showed that a 24 h exposure to the gel formulation did not decrease the cell viability or the barrier function of the tissue. Therefore, the gel formulation described here appears to be a promising vaginal microbicide to prevent a C. trachomatis infection with the potential to be expanded to other sexually transmitted diseases.

Optimizing solubility and permeability of a biopharmaceutics classification system (BCS) class 4 antibiotic drug using lipophilic fragments disturbing the crystal lattice.

Esterification was used to simultaneously increase solubility and permeability of ciprofloxacin, a biopharmaceutics classification system (BCS) class 4 drug (low solubility/low permeability) with solid-state limited solubility. Molecular flexibility was increased to disturb the crystal lattice, lower the melting point, and thereby improve the solubility, whereas lipophilicity was increased to enhance the intestinal permeability. These structural changes resulted in BCS class 1 analogues (high solubility/high permeability) emphasizing that simple medicinal chemistry may improve both these properties.