Oral Absorption from Surfactant-Based Drug Formulations: The Impact of Molecularly Dissolved Drug on Bioavailability.

Enabling drug formulations are often required to ensure sufficient absorption after oral administration of poorly soluble drugs. While these formulations typically increase the apparent solubility of the drug, it is widely acknowledged that only molecularly dissolved, i.e., free fraction of the drug, is prone for direct absorption, while colloid-associated drug does not permeate to the same extent. In the present study, we aimed at comparing the effect of molecularly and apparently (i.e., the sum of molecularly and colloid-associated drug) dissolved drug concentrations on the oral absorption of a poorly water-soluble drug compound, Alectinib. Mixtures of Alectinib and respectively 50 %, 25 %, 12.5 %, and 3 % sodium lauryl sulfate (SLS) relative to the dose were prepared and small-scale dissolution tests were performed under simulated fed and fasted state conditions. Both the molecularly and apparently dissolved drug concentrations were assessed in parallel using microdialysis and centrifugation/filtration sampling, respectively. The data served as the basis for an in vitro-in vivo correlation (IVIVC) and as input for a GastroPlusTM physiologically-based biopharmaceutics model (PBBM). It was shown that with increasing the content of SLS the apparently dissolved drug in FeSSIF and FaSSIF increased to a linear extent and thus, the predicted in vivo performance of the 50 % SLS formulation, based on apparently dissolved drug, would outperform all other formulations. Against common expectation, however, the free (molecularly dissolved) drug concentrations were found to vary with SLS concentrations as well, yet to a minor extent. A systematic comparison of solubilized and free drug dissolution patterns at different SLS contents of the formulations and prandial states allowed for interesting insights into the complex dissolution-/supersaturation-, micellization-, and precipitation-behavior of the formulations. When comparing the in vitro datasets with human pharmacokinetic data from a bioequivalence study, it was shown that the use of molecularly dissolved drug resulted in an improved IVIVC. By incorporating the in vitro dissolution datasets into the GastroPlusTM PBBM, the apparently dissolved drug concentrations resulted in both, a remarkable overprediction of plasma concentrations as well as a misprediction of the influence of SLS on systemic exposure. In contrast, by using the molecularly dissolved drug (i.e., free fraction) as the model input, the predicted plasma concentration-time profiles were in excellent agreement with observed data for all formulations under both fed and fasted conditions. By combining an advanced in vitro assessment with PBBM, the present study confirmed that only the molecularly dissolved drug, and not the colloid-associated drug, is available for direct absorption.

Using molecularly dissolved drug concentrations in PBBMs improves the prediction of oral absorption from supersaturating formulations.

Predicting the absorption of drugs from enabling formulations is still challenging due to the limited capabilities of standard physiologically based biopharmaceutics models (PBBMs) to capture complex absorption processes. Amongst others, it is often assumed that both, molecularly and apparently dissolved drug in the gastrointestinal lumen are prone to absorption. A recently introduced method for measuring concentrations of molecularly dissolved drug in a dynamic in vitro dissolution setup using microdialysis has opened new opportunities to test this hypothesis and refine mechanistic PBBM approaches. In the present study, we compared results of PBBMs that used either molecularly or apparently dissolved concentrations in the simulated gastrointestinal lumen as input parameters. The in vitro dissolution data from three supersaturating formulations of Posaconazole (PCZ) were used as model input. The modeling outcome was verified using PCZ concentration vs. time profiles measured in human intestinal aspirates and in the blood plasma. When using apparently dissolved drug concentrations (i.e., the sum of colloid-associated and molecularly dissolved drug) the simulated systemic plasma exposures were overpredicted, most pronouncedly with the ASD-based tablet. However, if the concentrations of molecularly dissolved drug were used as input values, the PBBM resulted in accurate prediction of systemic exposures for all three PCZ formulations. The present study impressively demonstrated the value of considering molecularly dissolved drug concentrations as input value for PBBMs of supersaturating drug formulations.

Combining in vitro dissolution/permeation with microdialysis sampling: Capabilities and limitations for biopharmaceutical assessments of supersaturating drug formulations.

Many novel small drug molecules are poorly water-soluble and thus, enabling drug formulations may be required to ensure sufficient absorption upon oral administration. Biopharmaceutical assessment and absorption prediction of enabling formulations, however, remains challenging. Combined in vitro dissolution/permeation (D/P) assays have gained increasing interest since they may provide a more realistic formulation ranking based on the drug permeation profiles from different formulations as compared to conventional dissolution, which captures both readily permeable and not readily permeable fractions of "dissolved" drug. Moreover, the combined in vitro D/P assays allow to better predict intestinal supersaturation and precipitation processes as compared to simple dissolution setups due to the effect of an absorptive sink. Microdialysis on the other hand has proven useful to determine molecularly dissolved drug in colloidal dispersions, thus allowing for a deeper mechanistic insight into the mechanism of drug release from supersaturating formulations. Here, microdialysis sampling from the donor compartment was used in combination with the dissolution/permeation (D/P) tool PermeaLoop™ to study commercial supersaturating drug formulations of the poorly soluble and weakly basic drug Posaconazole (PCZ). An amorphous solid dispersion (ASD)-based tablet, as well as a crystalline suspension in acidified and neutral dilution medium, respectively, were tested. Microdialysis sampling allowed for differentiation between molecularly dissolved and micellar drug concentration, as expected, but, surprisingly, it was found that the presence of the microdialysis probe affected the precipitation behavior of a crystalline suspension within the two-stage D/P setup, simulating the oral administration of the acidified PCZ (Noxafil®) suspension: the extent and duration of supersaturation in the donor decreased significantly, which also affected permeation. Similarly, for the ASD-based tablet, a less pronounced supersaturation was observed during the first 120 min of the experiment. Hence, in this case, the formulation ranking and the prediction of intestinal supersaturation in the in vitro D/P assay became less predictive as compared to a conventional PermeaLoop™ study without microdialysis sampling. It was concluded that valuable mechanistic insights into the molecularly dissolved drug profiles over time can be obtained by microdialysis. However, since the presence of the probe may affect the degree of supersaturation and precipitation, a conventional D/P assay (without microdialysis sampling) is preferred for formulation ranking of supersaturating drug formulations.

Leveraging the use of in vitro and computational methods to support the development of enabling oral drug products: An InPharma commentary.

Due to the strong tendency towards poorly soluble drugs in modern development pipelines, enabling drug formulations such as amorphous solid dispersions, cyclodextrins, co-crystals and lipid-based formulations are frequently applied to solubilize or generate supersaturation in gastrointestinal fluids, thus enhancing oral drug absorption. Although many innovative in vitro and in silico tools have been introduced in recent years to aid development of enabling formulations, significant knowledge gaps still exist with respect to how best to implement them. As a result, the development strategy for enabling formulations varies considerably within the industry and many elements of empiricism remain. The InPharma network aims to advance a mechanistic, animal-free approach to the assessment of drug developability. This commentary focuses current status and next steps that will be taken in InPharma to identify and fully utilize 'best practice' in vitro and in silico tools for use in physiologically based biopharmaceutic models.

Biopredictive capability assessment of two dissolution/permeation assays, µFLUX™ and PermeaLoop™, using supersaturating formulations of Posaconazole.

The majority of new drug entities exhibits poor water solubility and therefore enabling formulations are often needed to ensure sufficient in vivo bioavailability upon oral administration. Several in vitro tools have been proposed for biopredictive screening of such drug formulations to facilitate formulation development. Among these, combined dissolution/permeation (D/P) assays have gained increasing interest in recent years, since they are presumed to better predict the absorption behavior as compared to single-compartment dissolution assays. Moreover, especially for supersaturating formulations, it has been demonstrated that the presence of an absorption sink better mimics the intraluminal supersaturation performance. The present study aimed to investigate the biopredictive abilities of two in vitro D/P setups to predict intestinal supersaturation and systemic absorption of supersaturable systems. Experiments were performed with a µFLUX™ and PermeaLoop™ apparatus, respectively, which differ primarily in their volume-to-area ratios between donor compartment and membrane as well as in the type of biomimetic barrier. A two-stage dissolution protocol was adopted to mimic the transit from acidic stomach to more neutral intestinal fluids using biomimetic media. Three formulations of the weakly basic compound Posaconazole (PCZ), namely an acidified and a neutral suspension and an amorphous solid dispersion (ASD) tablet, were tested. Under the present conditions, and for the specific set of formulations studied here, PermeaLoop™ showed a better biopredictive ability for intestinal supersaturation and systemic absorption for the three formulations than the µFLUX™ D/P setup. Interestingly, minor modifications of the two-stage D/P protocol in terms of medium transfer rates from simulated gastric fluid (SGF) to fasted state simulated intestinal fluid (FaSSIF) had a substantial impact particularly on the permeation of the crystalline PCZ suspension ("acidified suspension"). The ASD tablet was less sensitive to gradual medium changes than the crystalline PCZ suspensions. The current study confirms the usefulness of D/P assays for formulation ranking of weakly basic compounds and supersaturating formulations.

Microdialysis and nanofiltration allow to distinguish molecularly dissolved from colloid-associated drug concentrations during biomimetic dissolution testing of supersaturating formulations.

Many new drug entities are poorly water-soluble and thus require solubility-enhancing formulations to ensure sufficient bioavailability. On the other hand, it is more and more accepted that not all "dissolved" states of a drug contribute equally to enhanced absorption, i.e. an increase in apparent solubility does not necessarily go in parallel with an increase in molecularly dissolved drug, the latter being regarded as the key driving force for absorption. Our study aimed to provide time-resolved information on the dissolution, supersaturation, and precipitation behavior of molecularly dissolved drug as released from an amorphous solid dispersion and a surfactant-containing crystalline suspension of Posaconazole (PCZ), a weakly basic and poorly water-soluble drug. Thereby, we aimed to gain a deeper mechanistic understanding of enabling formulation principles and possibly establish a dynamic biopharmaceutical assessment tool for molecularly dissolved drug released from enabling formulations. A two-staged dissolution test, with media transition from simulated gastric fluid (SGF) to fasted state simulated intestinal fluid (FaSSIF), was performed with three alternative sampling approaches in parallel: the classical bench centrifugation approach was used to assess total dissolved concentrations, while a nanofiltration method and a microdialysis setup were tested for their ability to discriminate molecularly and colloid-associated drug concentrations over time. For comparison, a single-stage dissolution setup was performed, where a marketed PCZ suspension was dispersed in biomimetic media with increasing amounts of solubilizing agents to understand their effect on the concentration of molecularly dissolved drug. It was demonstrated that the microdialysis setup allowed to follow the molecularly dissolved drug concentration in a time-resolved manner during the single-and two-stage dissolution tests with marginal delays. Interestingly, the PCZ concentrations measured by the nanofiltration approach differed from both, the molecularly dissolved (assessed by microdialysis) and apparently dissolved (assessed by centrifuge) PCZ concentrations, indicating that nanofiltration may allow to differentiate between different colloid-associated (apparently) dissolved drug species. Moreover, it was shown that the release of the molecularly dissolved drug from an amorphous solid dispersion did not correlate at all with the results obtained by the centrifugation method: While this conventional sampling revealed a classical spring and parachute concentration/time-profile with a high degree of (apparent) supersaturation, the concentration of molecularly dissolved drug (assessed by the microdialysis setup) indicated an initial short decline of PCZ concentration, followed by a prolonged (moderate) molecular supersaturation. This observation may give rise to a re-thinking of the current mechanistic understanding of how amorphous solid dispersions enhance oral bioavailability. In essence, the current study provides data which indicate a benefit of both the microdialysis sampling and nanofiltration approaches for the in vitro biopharmaceutical assessment of enabling drug formulations.