Elucidating the Molecular Interactions Occurring during Drug Precipitation of Weak Bases from Lipid-Based Formulations: A Case Study with Cinnarizine and a Long Chain Self-Nanoemulsifying Drug Delivery System

Evaluating human milk as a drug delivery vehicle for clofazimine to premature infants

Human milk is proposed as a drug delivery vehicle suitable for use in neonatal patients. Clofazimine, traditionally used for the treatment of leprosy and tuberculosis, is emerging as a treatment for cryptosporidiosis in infants, however its poor aqueous solubility has led to its commercial formulation as a waxy lipid formulation in a capsule, a format that is not suitable for infants. In this study, the evaluation of pasteurised human milk for the delivery of clofazimine was investigated using an in vitro lipolysis model to simulate gastric and intestinal digestion. The total lipid composition of the human milk was characterised alongside the liberated fatty acid species following digestion for comparison to alternative lipid-based delivery systems. Small-angle X-ray scattering was used to measure the presence of crystalline clofazimine during digestion and hence the extent of drug solubilisation. High-performance liquid chromatography was used to quantify the mass of clofazimine solubilised per gram of human milk fat (drug-to-fat ratio) in digested and undigested human milk. The digestion process was essential for the solubilisation of clofazimine, with digested human milk solubilising a sufficient dose of clofazimine for treatment of a premature infant. Human milk solubilised the clofazimine to a greater extent than bovine milk and infant formula during digestion, most likely as a result of differing lipid composition and increased long-chain fatty acid concentrations. These findings show that human milk enhances the solubility of clofazimine as a model drug and may be a suitable drug delivery vehicle for infant populations requiring therapeutic treatment.

Study and Computational Modeling of Fatty Acid Effects on Drug Solubility in Lipid-Based Systems

Lipid-based systems have many advantages in formulation of poorly water-soluble drugs but issues of a limited solvent capacity are often encountered in development. One of the possible solubilization approaches of especially basic drugs could be the addition of fatty acids to oils but currently, a systematic study is lacking. Therefore, the present work investigated apparently neutral and basic drugs in medium chain triglycerides (MCT) alone and with added either caproic acid (C6), caprylic acid (C8), capric acid (C10) or oleic acid (C18:1) at different levels (5 - 20%, w/w). A miniaturized solubility assay was used together with X-ray diffraction to analyze the residual solid and finally, solubility data were modeled using the conductor-like screening model for real solvents (COSMO-RS). Some drug bases had an MCT solubility of only a few mg/ml or less but addition of fatty acids provided in some formulations exceptional drug loading of up to about 20% (w/w). The solubility changes were in general more pronounced the shorter the chain length was and the longest oleic acid even displayed a negative effect in mixtures of celecoxib and fenofibrate. The COSMO-RS prediction accuracy was highly specific for the given compounds with root mean square errors (RMSE) ranging from an excellent 0.07 to a highest value of 1.12. The latter was obtained with the strongest model base pimozide for which a new solid form was found in some samples. In conclusion, targeting specific molecular interactions with the solute combined with mechanistic modeling provides new tools to advance lipid-based drug delivery.

Precipitation from amorphous solid dispersions in biorelevant dissolution testing: The polymorphism of regorafenib

Amorphous Solid Dispersions (ASDs) are a major drug formulation technique to achieve higher bioavailability for poorly water-soluble active pharmaceutical ingredients. So far, dissolution tailoring and supersaturation enhancement have been studied in detail, whereas less is known about the importance of formed precipitates with amorphous or crystalline states at the site of drug absorption. Regorafenib monohydrate (RGF MH), a multikinase inhibitor drug categorized as Biopharmaceutics Classification System (BCS) class II compound, was formulated with povidone K25 and hypromellose acetate succinate (HPMCAS) as an ASD. Here, for the first time, the RGF precipitation process as well as the physicochemical properties of the arising precipitates are investigated. The formed precipitates from biorelevant dissolution showed varying drug content and were analyzed offline by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), confocal Raman microscopy (CRM), X-ray powder diffraction (XRPD), and small angle X-ray scattering (SAXS). In addition to different crystalline RGF precipitates, an amorphous co-precipitate of RGF and HPMCAS was identified, which was suppressed in the presence of PVP. Wide angle X-ray scattering (WAXS) and isothermal calorimetry (ITC) were used to track the precipitation process of RGF in-situ. From calorimetric data, the precipitation profile was calculated. RGF forms precipitates in multiple polymorphic states dependent on the environmental conditions, i.e., dissolution media composition and chosen excipients. The engineered formation of defined amorphous structures in-vivo may be a promising future drug formulation strategy.

Mechanisms of drug solubilization by polar lipids in biorelevant media

Despite the widespread use of lipid excipients in both academic research and oral formulation development, rational selection guidelines are still missing. In the current study, we aimed to establish a link between the molecular structure of commonly used polar lipids and drug solubilization in biorelevant media. The solubilization of fenofibrate by 13 phospholipids, 11 fatty acids and 2 monoglycerides was studied by an in vitro model of the upper GI tract. The main trends were verified with progesterone and danazol. It was revealed that to alter drug solubilization in biorelevant media, the polar lipids must form mixed colloidal aggregates with the bile. Such aggregates are formed when: (1) the polar lipid is used at a sufficiently high concentration (relative to its mixed critical micellar concentration) and (2) its hydrophobic chain has a melting temperature (T_m) < 37 °C. When these two conditions are met, the increased polar lipid chain length increases the drug solubilization capacity. Hence, long chain (C18) unsaturated polar lipids show best drug solubilization, due to the combination of long chain length and low T_m. Polar lipids with T_m significantly higher than 37 °C (e.g. C16 and C18 saturated compounds) do not impact drug solubilization in biorelevant media, due to limited association in mixed colloidal aggregates. The hydrophilic head group also has a dramatic impact on the drug solubilization enhancement, with polar lipids performance decreasing in the order [choline phospholipids] > [monoglycerides] > [fatty acids]. As both the acyl chain and head group types are structural features of the polar lipids, and not of the solubilized drugs, the described trends in drug solubilization should hold true for a variety of hydrophobic molecules.

Current Status of Supersaturable Self-Emulsifying Drug Delivery Systems

Self-emulsifying drug delivery systems (SEDDSs) are a vital strategy to enhance the bioavailability (BA) of formulations of poorly water-soluble compounds. However, these formulations have certain limitations, including in vivo drug precipitation, poor in vitro in vivo correlation due to a lack of predictive in vitro tests, issues in handling of liquid formulation, and physico-chemical instability of drug and/or vehicle components. To overcome these limitations, which restrict the potential usage of such systems, the supersaturable SEDDSs (su-SEDDSs) have gained attention based on the fact that the inclusion of precipitation inhibitors (PIs) within SEDDSs helps maintain drug supersaturation after dispersion and digestion in the gastrointestinal tract. This improves the BA of drugs and reduces the variability of exposure. In addition, the formulation of solid su-SEDDSs has helped to overcome disadvantages of liquid or capsule dosage form. This review article discusses, in detail, the current status of su-SEDDSs that overcome the limitations of conventional SEDDSs. It discusses the definition and range of su-SEDDSs, the principle mechanisms underlying precipitation inhibition and enhanced in vivo absorption, drug application cases, biorelevance in vitro digestion models, and the development of liquid su-SEDDSs to solid dosage forms. This review also describes the effects of various physiological factors and the potential interactions between PIs and lipid, lipase or lipid digested products on the in vivo performance of su-SEDDSs. In particular, several considerations relating to the properties of PIs are discussed from various perspectives.

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.

An update on polymer-lipid hybrid systems for improving oral drug delivery

INTRODUCTION

A promising approach that has recently emerged to overcome the complex biobarriers and interrelated challenges associated with oral drug absorption is to combine the benefits of polymeric and lipid-based nanocarriers within one hybrid system. This multifaceted formulation strategy has given rise to a plethora of polymer-lipid hybrid (PLH) systems with varying nanostructures and biological activities, all of which have demonstrated the ability to improve the biopharmaceutical performance of a wide range of challenging therapeutics.

AREAS COVERED

The multitude of polymers that can be combined with lipids to exert a synergistic effect for oral drug delivery have been identified, reviewed and critically evaluated. Specific focus is attributed to preclinical studies performed within the past 5 years that have elucidated the role and mechanism of the polymer phase in altering the oral absorption of encapsulated therapeutics.

EXPERT OPINION

The potential of PLH systems has been clearly identified; however, improved understanding of the structure-activity relationship between PLH systems and oral absorption is fundamental for translating this promising delivery approach into a clinically relevant formulation. Advancing research within this field to identify optimal polymer, lipid combinations and engineering conditions for specific therapeutics are therefore encouraged.

In vitro digestion models to evaluate lipid based drug delivery systems; present status and current trends

During the past two decades, a range of in vitro models simulating the digestion processes occurring in the stomach and small intestine have been developed to characterize lipid based drug delivery systems (LbDDSs). This review describes the presently existing range of in vitro digestion models and their use in the field of oral drug delivery. The models are evaluated in terms of their suitability to assess LbDDSs, and their ability to produce in vitro - in vivo correlations (IVIVCs). While the pH-stat lipolysis model is by far the most commonly utilized in vitro digestion model in relation to characterizing LbDDSs, a series of recent studies have shown a lack of IVIVCs limiting its future use. Presently, no single in vitro digestion model exists which is able to predict the in vivo performance of various LbDDSs. However, recent research has shown the potential of combined digestion-permeation models as well as species specific digestion models.

Drug supersaturation during formulation digestion, including real-time analytical approaches

Self-emulsifying and other lipid-based drug delivery systems have drawn considerable interest from pharmaceutical scientists for managing oral delivery of poorly water-soluble compounds. Following administration, self-emulsifying systems exhibit complex aqueous dispersion and digestion in the gastro-intestinal tract. These processes generally result in drug supersaturation, which leads to enhanced absorption or the high drug concentrations may cause precipitation with erratic and variable oral bioavailability. This review briefly outlines drug supersaturation obtained from self-emulsifying and other lipid-based formulations; recent advancements of in vitro lipolysis testing are also discussed. Further, a main focus is mechanisms by which supersaturation is triggered from gastro-intestinal processes, as well as analytical techniques that are promising from a research and development perspective. Comparatively simple approaches are presented together with more sophisticated process analytics to enable direct examination of kinetic changes. The analytical methods together with their sensor probes are discussed in detail to clarify opportunities as well as technical limitations. Some of the more sophisticated methods, including those based on synchrotron radiation, are primarily research oriented despite interesting experimental findings from an industrial viewpoint. The availability of kinetic data further opens the door to mathematical modeling of supersaturation and precipitation versus permeation, which lays the groundwork for better in vitro to in vivo correlations as well as for physiologically-based modeling of lipid-based systems.

The impact of digestion is essential to the understanding of milk as a drug delivery system for poorly water soluble drugs

Milk has previously been considered as a potential lipid-based drug delivery system for poorly water soluble drugs but it has never gained significant attention. This is in part because relying on solubility in lipid-based formulations (in this case milk) does not provide a complete picture of the behavior of such systems upon digestion. Herein, we demonstrate using time resolved X-ray scattering that the digestion of milk is actually crucial to the solubilisation of a poorly water-soluble drug, halofantrine. Halofantrine was chosen because its behaviour in lipid-based formulations has been widely investigated and because of its close structural relationship to lumefantrine, an antimalarial drug of current interest for the treatment of paediatric malaria. The transformation of the drug from a crystalline solid form in suspension in milk, to a solubilised form as a direct consequence of lipolysis highlights that consideration of digestion of the milk lipids as a critical process that influences drug solubilisation and availability for absorption is vital.

Impact of Drug Physicochemical Properties on Lipolysis-Triggered Drug Supersaturation and Precipitation from Lipid-Based Formulations

In this study we investigated lipolysis-triggered supersaturation and precipitation of a set of model compounds formulated in lipid-based formulations (LBFs). The purpose was to explore the relationship between precipitated solid form and inherent physicochemical properties of the drug. Eight drugs were studied after formulation in three LBFs, representing lipid-rich (extensively digestible) to surfactant-rich (less digestible) formulations. In vitro lipolysis of drug-loaded LBFs were conducted, and the amount of dissolved and precipitated drug was quantified. Solid form of the precipitated drug was characterized with polarized light microscopy (PLM) and Raman spectroscopy. A significant solubility increase for the weak bases in the presence of digestion products was observed, in contrast to the neutral and acidic compounds for which the solubility decreased. The fold-increase in solubility was linked to the degree of ionization of the weak bases and thus their attraction to free fatty acids. A high level of supersaturation was needed to cause precipitation. For the weak bases, the dose number indicated that precipitation would not occur during lipolysis; hence, these compounds were not included in further studies. The solid state analysis proved that danazol and griseofulvin precipitated in a crystalline form, while niclosamide precipitated as a hydrate. Felodipine and indomethacin crystals were visible in the PLM, whereas the Raman spectra showed presence of amorphous drug, indicating amorphous precipitation that quickly crystallized. The solid state analysis was combined with literature data to allow analysis of the relationship between solid form and the physicochemical properties of the drug. It was found that low molecular weight and high melting temperature increases the probability of crystalline precipitation, whereas precipitation in an amorphous form was favored by high molecular weight, low melting temperature, and positive charge.

Understanding the generation and maintenance of supersaturation during the dissolution of amorphous solid dispersions using modulated DSC and 1H NMR

In this study, the dissolution behaviour of dipyridamole (DPM) and cinnarizine (CNZ) spray-dried amorphous solid dispersions (ASDs) using polyvinyl pyrrolidone (PVP) and polyacrylic acid (PAA) as a carrier matrix were evaluated and compared. The drug concentrations achieved from the dissolution of PVP and PAA solid dispersions were significantly greater than the equilibrium solubility of crystalline DPM and CNZ in phosphate buffer pH 6.8 (PBS 6.8). The maximum drug concentration achieved by dissolution of PVP and PAA solid dispersions did not exceed the theoretically calculated apparent solubility of amorphous DPM and CNZ. However, the degree of supersaturation of DPM and CNZ increased considerably as the polymer weight fraction within the solid dispersion increased. In addition, the supersaturation profile of DPM and CNZ were studied in the presence and absence of the polymers. PAA was found to maintain a higher level of supersaturation compared to PVP. The enhanced drug solution concentration following dissolution of ASDs can be attributed to the reduced crystal growth rates of DPM and CNZ at an equivalent supersaturation. We have also shown that, for drugs having high crystallization tendency and weak drug-polymer interaction, the feasible way to increase dissolution might be increase the polymer weight fraction in the ASD. Solution ¹H NMR spectra were used to understand dissolution mechanism and to identify drug-polymer interaction. The change in electron densities of proton attached to different groups in DPM and CNZ suggested drug-polymer interaction in solution. The relative intensities of peak shift and nature of interaction between drug and polymer in different systems are different. These different effects suggest that DPM and CNZ interacts in a different way with PVP and PAA in solution which goes some way towards explaining the different polymeric effect, particularly in terms of inhibition of drug recrystallization and dissolution of DPM and CNZ ASDs. These results established that the different drug/polymer interactions in the solid state and in solution give rise to the variation in dissolution profile observed for different systems.

Using Potentiometric Free Drug Sensors to Determine the Free Concentration of Ionizable Drugs in Colloidal Systems

The present study investigates the use of free drug sensors (FDS) to measure free ionized drug concentrations in colloidal systems, including micellar solutions, emulsions, and lipid formulations during in vitro lipolysis. Diphenhydramine hydrochloride (DPH) and loperamide hydrochloride (LOP) were selected as model drugs. Self-diffusion nuclear magnetic resonance studies were performed and confirmed the entrapment of drugs in micelles in Brij 35 and sodium taurodeoxycholate (TDC)/phosphatidylcholine (PC) micellar solutions. The FDS measurements indicated that with a constant level of drug, the percentage of free DPH and LOP decreased from 84% to 57% and from 51% to 18%, respectively, as the concentration of Brij 35 was increased from 4.7 to 22 mM; and from 99% to 46% and from 100% to 21%, respectively, as the concentration of TDC/PC was increased from 0.49/0.04 to 8.85/0.78 mM. During the in vitro lipolysis of a lipid formulation, free drug concentration decreased with lipolysis time. The percentage of free DPH was higher than for LOP in the same colloidal system because DPH is less lipophilic than LOP. The study showed that FDS can be used to monitor the free drug concentration in colloidal systems with fast response, no sample treatment and simple data analysis.

In Vitro Model Simulating Gastro-Intestinal Digestion in the Pediatric Population (Neonates and Young Infants)

The focus on drug delivery for the pediatric population has been steadily increasing in the last decades. In terms of developing in vitro models simulating characteristics of the targeted pediatric population, with the purpose of predicting drug product performance after oral administration, it is important to simulate the gastro-intestinal conditions and processes the drug will encounter upon oral administration. When a drug is administered in the fed state, which is commonly the case for neonates, as they are typically fed every 3 h, the digestion of the milk will affect the composition of the fluid available for drug dissolution/solubilization. Therefore, in order to predict the solubilized amount of drug available for absorption, an in vitro model simulating digestion in the gastro-intestinal tract should be utilized. In order to simulate the digestion process and the drug solubilization taking place in vivo, the following aspects should be considered; physiologically relevant media, media volume, use of physiological enzymes in proper amounts, as well as correct pH and addition of relevant co-factors, e.g., bile salts and co-enzymes. Furthermore, physiological transit times and appropriate mixing should be considered and mimicked as close as possible. This paper presents a literature review on physiological factors relevant for digestion and drug solubilization in neonates. Based on the available literature data, a novel in vitro digestion model simulating digestion and drug solubilization in the neonate and young infant pediatric population (2 months old and younger) was designed.

In vivo evaluation of supersaturation/precipitation/re-dissolution behavior of cinnarizine, a lipophilic weak base, in the gastrointestinal tract: the key process of oral absorption

The aim of this study is to evaluate how supersaturation, precipitation, and re-dissolution processes influence the intestinal absorption of cinnarizine (CNZ), a lipophilic weak base, by monitoring its plasma and luminal concentration-time profile, after oral administration as a HCl solution containing fluorescein isothiocyanate dextran (FD-4), a non-absorbable marker. In the in vitro pH shift experiment, the supersaturation stability was significantly lower when the higher-concentration solution of CNZ (pH1.5) was added to the simulated intestinal fluid. However, although the in vivo bioavailability after oral administration of high and low dose as HCl solutions was greatly improved compared to those as neutral suspensions, the difference in the supersaturation stability was not reflected in the improvement of the in vivo bioavailability. Analysis of CNZ and FD-4 concentrations in each segment of the gastrointestinal tract revealed that most of the CNZ precipitated in the duodenum after gastric emptying, and supersaturation was observed only in the duodenum. Thereafter, the precipitate was rapidly re-dissolved and absorbed in the upper and middle small intestine. The rapid re-dissolution may be caused by smaller particles of the precipitate. In this case, it is considered that the key process for the absorption of CNZ was re-dissolution, not supersaturation. Therefore, different supersaturation stabilities in different doses observed in in vitro precipitation experiment was not reflected to in vivo absorption. These findings may be useful to design efficient supersaturable formulations and to validate and improve current prediction methods.

Lipid-Based Formulations Can Enable the Model Poorly Water-Soluble Weakly Basic Drug Cinnarizine To Precipitate in an Amorphous-Salt Form During In Vitro Digestion

The tendency for poorly water-soluble weakly basic drugs to precipitate in a noncrystalline form during the in vitro digestion of lipid-based formulations (LBFs) was linked to an ionic interaction between drug and fatty acid molecules produced upon lipid digestion. Cinnarizine was chosen as a model weakly basic drug and was dissolved in a medium-chain (MC) LBF, which was subject to in vitro lipolysis experiments at various pH levels above and below the reported pK_a value of cinnarizine (7.47). The solid-state form of the precipitated drug was analyzed using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and crossed polarized light microscopy (CPLM). In addition, the phase distribution of cinnarizine upon lipolysis was analyzed using high-performance liquid chromatography (HPLC). Cinnarizine precipitated in a noncrystalline form during lipolysis experiments at pH 6.5, pH 5.5, and pH 4.0 but precipitated in a crystalline form at pH 8.0 according to XRD measurements on the pellets. Differences were also observed in the FTIR spectra of the pellet phases at pH 8.0 and pH 6.5, with the absorption bands in the C-N stretch region of the IR spectra supporting a shift from the starting free base crystalline material to the hydrochloride salt, thus supporting the case that ionic interactions between weak bases and fatty acid molecules during digestion are responsible for producing amorphous-salts upon precipitation. The conclusion has wide implications for understanding past in vitro and in vivo data for lipid-based formulations of basic drugs, as well as future formulation design and optimization.

Rapid determination of drug solubilization versus supersaturation in natural and digested lipids

Lipid-based formulations (LBFs) represent one of the successful formulation approaches that enable oral delivery of poorly water-soluble drugs. This work presents a simple equilibrium approach based on solubility in lipids and their corresponding digestion media to estimate a maximum drug supersaturation ratio (SR_max). This value of drug concentration normalized by the solubility in the aqueous digestion phase indicates the propensity for drug precipitation. A set of 16 structurally diverse drugs was first measured for their solubility in tricaprin and tricaprylin and results were compared to an empirical model based on molecular predictors. In the next step, digestion media were either prepared by in vitro lipolysis or by assembling a composition to mimic the endpoint of digestion. It was found that drug solubility in the pure lipids mainly was related to the melting point in that increased values resulted in reduced solubility. The solubility values measured in the lipolysis media correlated well with those obtained from assembled digestion media. Interestingly, the solubilization upon digestion was typically higher when using tricaprin than tricaprylin in spite of that the latter oil (as pure excipient) generally was a more potent solvent. This work suggests that a simplified digestion screen can be used to facilitate evaluation of formulations during early development. Estimation of SR_max provides an early risk assessment of drug precipitation for LBFs. The method is easily scaled down to the microtiter plate format and can be used for selecting candidate formulations that merit further evaluation in more complex and dynamic in vitro tests.

50years of oral lipid-based formulations: Provenance, progress and future perspectives

Lipid based formulations (LBF) provide well proven opportunities to enhance the oral absorption of drugs and drug candidates that sit close to, or beyond, the boundaries of Lipinski's 'rule-of-five' chemical space. Advantages in permeability, efflux and presystemic metabolism are evident; however, the primary benefit is in increases in dissolution and apparent intestinal solubility for lipophilic, poorly water soluble drugs. This review firstly details the inherent advantages of LBF, their general properties and classification, and provides a brief retrospective assessment of the development of LBF over the past fifty years. More detailed analysis of the ability of LBF to promote intestinal solubilisation, supersaturation and absorption is then provided alongside review of the methods employed to assess formulation performance. Critical review of the ability of simple dispersion and more complex in vitro digestion methods to predict formulation performance subsequently reveals marked differences in the correlative ability of in vitro tests, depending on the properties of the drug involved. Notably, for highly permeable low melting drugs e.g. fenofibrate, LBF appear to provide significant benefit in all cases, and sustained ongoing solubilisation may not be required. In other cases, and particularly for higher melting point drugs such as danazol, where re-dissolution of crystalline precipitate drug is likely to be slow, correlations with ongoing solubilisation and supersaturation are more evident. In spite of their potential benefits, one limitation to broader use of LBF is low drug solubility in the excipients employed to generate formulations. Techniques to increase drug lipophilicity and lipid solubility are therefore explored, and in particular those methods that provide for temporary enhancement including lipophilic ionic liquid and prodrug technologies. The transient nature of these lipophilicity increases enhances lipid solubility and LBF viability, but precludes enduring effects on receptor promiscuity and off target toxicity. Finally, recent efforts to generate solid LBF are briefly described as a means to circumvent the need to encapsulate in soft or hard gelatin capsules, although the latter remain popular with consumers and a proven means of LBF delivery.

Trends in the Assessment of Drug Supersaturation and Precipitation In Vitro Using Lipid-Based Delivery Systems

The generation of drug supersaturation close to the absorptive site is an important mechanism of how several formulation technologies enhance oral absorption and bioavailability. Lipid-based formulations belong to the supersaturating drug delivery systems although this is not the only mechanism of how drug absorption is promoted in vivo. Different methods to determine drug supersaturation and precipitation from lipid-based formulations are described in the literature. Experimental in vitro setups vary according to their complexity and proximity to the in vivo conditions and, therefore, some tests are used for early formulation screening, while others better qualify for a later stage of development. The present commentary discusses this rapidly evolving field of in vitro testing with a special focus on the advancements in analytical techniques and new approaches of mechanistic modeling. The importance of considering a drug absorption sink is particularly emphasized. This commentary should help formulators in the pharmaceutical industry as well as in academia to make informed decisions on how to conduct in vitro tests for lipid-based delivery systems and to decide on the implications of experimental results.