Surfactants enhance recovery of poorly soluble drugs during microdialysis sampling: Implications for in vitro dissolution-/permeation-studies

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.

In vitro dissolution/permeation tools for amorphous solid dispersions bioavailability forecasting II: Comparison and mechanistic insights

Along with the increasing demand for complex formulations comes the need for appropriate in vitro methodologies capable of predicting their corresponding in vivo performance and the mechanisms controlling the drug release which can impact on in vivo drug absorption. In vitro dissolution-permeation (D/P) methodologies that can account for the effects of enabling formulations on the permeability of drugs are increasingly being used in performance ranking during early development stages. This work comprised the application of two different cell-free in vitro D/P setups: BioFLUX™ and PermeaLoop™ to evaluate the dissolution-permeation interplay upon drug release from itraconazole (ITZ)- HPMCAS amorphous solid dispersions (ASDs) of different drug loads. A solvent-shift approach was employed, from a simulated gastric environment to a simulated intestinal environment in the donor compartment. PermeaLoop™ was then combined with microdialysis sampling to separate the dissolved (free) drug from other species present in solution, like micelle-bound drug and drug-rich colloids, in real time. This setup was applied to clarify the mechanisms for drug release and permeation from these ASDs. In parallel, a pharmacokinetic study (dog model) was conducted to assess the drug absorption from these ASDs and to compare the in vivo results with the data obtained from each in vitro D/P setup, allowing to infer which would be the most adequate setup for ASD ranking. Even though both D/P systems resulted in the same qualitative ranking, BioFLUX™ overpredicted the difference between the in vivo AUC of two ASDs, whereas PermeaLoop™ permeation flux resulted in a good correlation with the AUC observed in pharmacokinetic studies (dog model) (R2 ≈ 0.98). Also, PermeaLoop™ combined with a microdialysis sampling probe clarified the mechanisms for drug release and permeation from these ASDs. It demonstrated that the free drug was the only driving force for permeation, while the drug-rich colloids kept permeation active for longer periods by acting as drug reservoirs and maintaining constant high levels of free drug in solution, which are then immediately able to permeate. Hence, the data obtained points BioFLUX™ and PermeaLoop™ applications to different momentums in the drug product development pipeline: while BioFLUX™, an automated standardized method, poses as a valuable tool for initial ASD ranking during the early development stages, PermeaLoop™ combined with microdialysis sampling allows to gain mechanistic understanding of the dissolution-permeation interplay, being crucial to fine tune and identify leading ASD candidates prior to in vivo testing.

In-vitro dynamic dissolution/bioconversion/permeation of fosamprenavir using a novel tool with an artificial biomimetic permeation barrier and microdialysis-sampling

Fosamprenavir is a phosphate ester prodrug that, upon dissolution, is cleaved to the poorly soluble yet readily absorbable parent drug amprenavir. In this study, a novel cell-free in vitro setup with quasi-continuous monitoring of the dynamic dissolution/bio-conversion/permeation of fosamprenavir was designed and tested. It consists of side-by-side diffusion cells, where the donor and acceptor compartments are separated by the biomimetic barrier PermeaPad®, and sampling from the donor compartment is accomplished via a microdialysis probe. Externally added bovine alkaline phosphatase induced bioconversion in the donor compartment. Microdialysis sampling allowed to follow the enzymatic conversion of fosamprenavir to amprenavir by the bovine alkaline phosphatase in an (almost) real-time manner eliminating the need to remove or inactivate the enzyme. Biomimetic conversion rates in the setup were established by adding appropriate amounts of the alkaline phosphatase. A substantial (6.5-fold) and persistent supersaturation of amprenavir was observed due to bioconversion at lower (500 µM) concentrations, resulting in a substantially increased flux across the biomimetic barrier, nicely reflecting the situation in vivo. At conditions with an almost 10-fold higher dose than the usual human dose, some replicates showed premature precipitation and collapse of supersaturation, while others did not. In conclusion, the proposed novel tool appears very promising in gaining an in-depth mechanistic understanding of the bioconversion/permeation interplay, including transient supersaturation of phosphate-ester prodrugs like fosamprenavir.

A rapid screening method to select microdialysis carriers for hydrophobic compounds

Microdialysis is a minimally invasive sampling technique which is widely applied in many fields including clinical studies. This technique usually has limitation on sampling hydrophobic compounds as aqueous solutions are commonly used as the perfusates. The relative recovery of hydrophobic compounds is often low and irreproducible because of the non-specific binding to microdialysis membranes or catheter tubing. Carriers such as cyclodextrins have been used to improve the recovery and consistency, however the identification of an optimal carrier can only be achieved after time-consuming and costly microdialysis experiments. We therefore developed a rapid, convenient, and low-cost method to identify the optimal carriers for sampling hydrophobic compounds with the use of centrifugal ultrafiltration. Doxorubicin was used as the model compound and its relative recoveries obtained from centrifugal ultrafiltration and from microdialysis were compared. The results show that the relative recoveries are highly correlated (correlation coefficient ≥ 0.9) between centrifugal ultrafiltration and microdialysis when different types or different concentrations of cyclodextrins were used as the carriers. In addition to doxorubicin, this method was further confirmed on three other drugs with different hydrophobicity. This method may facilitate and broaden the use of microdialysis perfusion on sampling or delivering hydrophobic substances in various applications.

Successful oral delivery of poorly water-soluble drugs both depends on the intraluminal behavior of drugs and of appropriate advanced drug delivery systems

Poorly water-soluble drugs continue to be a problematic, yet important class of pharmaceutical compounds for treatment of a wide range of diseases. Their prevalence in discovery is still high, and their development is usually limited by our lack of a complete understanding of how the complex chemical, physiological and biochemical processes that occur between administration and absorption individually and together impact on bioavailability. This review defines the challenge presented by these drugs, outlines contemporary strategies to solve this challenge, and consequent in silico and in vitro evaluation of the delivery technologies for poorly water-soluble drugs. The next steps and unmet needs are proposed to present a roadmap for future studies for the field to consider enabling progress in delivery of poorly water-soluble compounds.

Metal-organic frameworks as affinity agents to enhance the microdialysis sampling efficiency of fatty acids

Microdialysis (MD) has been extensively used for in vivo sampling of hydrophilic analytes such as neurotransmitters and drug metabolites. In contrast, there have been few reports on sampling of lipophilic analytes by MD. Lipophilic analytes are easily adsorbed on the surfaces of the dialysis membrane and the inner wall of tubing, which leads to a very low relative recovery (RR). In this work, a strategy to develop an enhanced MD sampling of fatty acids (FAs) by using metal-organic frameworks (MOFs) as affinity agents in the perfusion fluid was investigated. Two MOFs, MIL-101 and ZIF-8, were synthesized and tested for the first time. A 2 times higher RR, about 70% RR, was obtained. The FT-IR experiment showed that the unsaturated metal sites in MOFs could coordinate with FAs, therefore the FAs were encapsulated into MOFs, avoiding FAs to be absorbed on the surfaces of the dialysis membrane and the inner wall of tubing. Moreover, incorporation of FAs into MOFs led to a decrease of free concentration of FAs inside the MD membrane and an increase of concentration gradient, allowing more FAs to diffuse across the membrane. Consequentially, an enhanced RR was obtained. The approach was successfully used to monitor the time profile of targeted FAs in cell culture media after lipopolysaccharide (LPS)-induced inflammation.

Drug permeability profiling using cell-free permeation tools: Overview and applications

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.