Multidimensional analysis of human intestinal fluid composition

Characterisation of colloidal structures and their solubilising potential for BCS class II drugs in fasted state simulated intestinal fluid

A suite of fasted state simulated intestinal fluid (SIF), based on variability observed in a range of fasted state human intestinal fluid (HIF) samples was used to study the solubility of eight poorly soluble drugs (three acidic drugs (naproxen, indomethacin and phenytoin), two basic drugs (carvedilol and tadalafil) and three neutral drugs (felodipine, fenofibrate, griseofulvin)). Particle size of the colloidal structures formed in these SIF in the presence and absence of drugs was measured using dynamic light scattering and nanoparticle tracking analysis. Results indicate that drug solubility tends to increase with increasing total amphiphile concentration (TAC) in SIF with acidic drugs proving to be more soluble than basic or neutral drug in the media evaluated. Dynamic light scattering showed that as the amphiphile concentration increased, the hydrodynamic diameters of the structures decreased. The scattering distribution confirmed the polydispersity of the simulated intestinal fluids compared to the monodisperse distribution observed for FaSSIF v1). There was a large difference in the size of the structures found based on the composition of the SIF, for example, the diameter of the structures measured in felodipine in the minimum TAC media was measured to be 170 ± 5 nm which decreased to 5.1 ± 0.2 nm in the maximum TAC media point. The size measured of the colloidal structures of felodipine in the FaSSIF v1 was 86 ± 1 nm. However, there was no simple correlation between solubility and colloidal size. Nanoparticle tracking analysis was used for the first time to characterise colloidal structures within SIF and the results were compared to those obtained by dynamic light scattering. The particle size measured by dynamic light scattering was generally greater in media with a lower concentration of amphiphiles and smaller in media of a higher concentration of amphiphiles, compared to that of the data yielded by nanoparticle tracking analysis. This work shows that the colloidal structures formed vary depending on the composition of SIF which affects the solubility. Work is ongoing to determine the relationship between colloidal structure and solubility.

PBBM Considerations for Base Models, Model Validation, and Application Steps: Workshop Summary Report

The proceedings from the 30th August 2023 (Day 2) of the workshop "Physiologically Based Biopharmaceutics Models (PBBM) Best Practices for Drug Product Quality: Regulatory and Industry Perspectives" are provided herein. Day 2 covered PBBM case studies from six regulatory authorities which provided considerations for model verification, validation, and application based on the context of use (COU) of the model. PBBM case studies to define critical material attribute (CMA) specification settings, such as active pharmaceutical ingredient (API) particle size distributions (PSDs) were shared. PBBM case studies to define critical quality attributes (CQAs) such as the dissolution specification setting or to define the bioequivalence safe space were also discussed. Examples of PBBM using the credibility assessment framework, COU and model risk assessment, as well as scientific learnings from PBBM case studies are provided. Breakout session discussions highlighted current trends and barriers to application of PBBMs including: (a) PBBM credibility assessment framework and level of validation, (b) use of disposition parameters in PBBM and points to consider when iv data are not available, (c) conducting virtual bioequivalence trials and dealing with variability, (d) model acceptance criteria, and (e) application of PBBMs for establishing safe space and failure edges.

Using the refined Developability Classification System (rDCS) to guide the design of oral formulations

The refined Developability Classification System (rDCS) provides a comprehensive animal-free approach for assessing biopharmaceutical risks associated with developing oral formulations. This work demonstrates practical application of a recently advanced rDCS framework guiding formulation design for six diverse active pharmaceutical ingredients (APIs) and compares rDCS classifications with those of the Biopharmaceutics Classification System (BCS). While the BCS assigns five of the APIs to class II/IV, indicating potentially unfavorable biopharmaceutical attributes, the rDCS provides a more nuanced risk assessment. Both BCS and rDCS assign acetaminophen to class I at therapeutic doses. Voriconazole and lemborexant (both BCS II) are classified in rDCS class I at therapeutic doses, indicating suitability for development as conventional oral formulations. Fedratinib is classified as BCS IV but the rDCS indicates a stratified risk (class I, IIa or IIb), depending on the relevance of supersaturation/precipitation in vivo. Voxelotor and istradefylline (both BCS II) belong to rDCS class IIb, requiring solubility enhancement to achieve adequate oral bioavailability. Comparing the rDCS analysis with literature on development and pharmacokinetics demonstrates that the rDCS reliably supports oral formulation design over a wide range of API characteristics, thus providing a strong foundation for guiding development.

Understanding the mechanisms of food effect on omaveloxolone pharmacokinetics through physiologically based biopharmaceutics modeling

Omaveloxolone is a nuclear factor (erythroid-derived 2)-like 2 activator approved in the United States and the European Union for the treatment of patients with Friedreich ataxia aged ≥16 years, with a recommended dosage of 150 mg orally once daily on an empty stomach. The effect of the US Food and Drug Administration (FDA) high-fat breakfast on the pharmacokinetic profile of omaveloxolone observed in study 408-C-1703 (NCT03664453) deviated from the usual linear correlation between fed/fasted maximum plasma concentration (Cmax) and area under the concentration-time curve (AUC) ratios reported for various oral drugs across 323 food effect studies. Here, physiologically based biopharmaceutics modeling (PBBM) was implemented to predict and explain the effect of the FDA high-fat breakfast on a 150-mg dose of omaveloxolone. The model was developed and validated based on dissolution and pharmacokinetic data available across dose-ranging, food effect, and drug-drug interaction clinical studies. PBBM predictions support clinical observations of the unique effect of a high-fat meal on omaveloxolone pharmacokinetic profile, in which the Cmax increased by 350% with only a 15% increase in the AUC. Key parameters influencing omaveloxolone pharmacokinetics in the fasted state based on a parameter sensitivity analysis included bile salt solubilization, CYP3A4 activity, drug substance particle size distribution, and permeability. Mechanistically, in vivo omaveloxolone absorption was solubility and dissolution rate limited. However, in the fed state, higher bile salt solubilization led to more rapid dissolution with predominant absorption in the upper gastrointestinal tract, resulting in increased susceptibility to first-pass gut extraction; this accounts for the lack of correlation between Cmax and AUC for omaveloxolone.

A novel simulated media system for in vitro evaluation of bioequivalent intestinal drug solubility

Orally administered solid drug must dissolve in the gastrointestinal tract before absorption to provide a systemic response. Intestinal solubility is therefore crucial but difficult to measure since human intestinal fluid (HIF) is challenging to obtain, varies between fasted (Fa) and fed (Fe) states and exhibits inter and intra subject variability. A single simulated intestinal fluid (SIF) cannot reflect HIF variability, therefore current approaches are not optimal. In this study we have compared literature Fa/FeHIF drug solubilities to values measured in a novel in vitro simulated nine media system for either the fasted (Fa9SIF) or fed (Fe9SIF) state. The manuscript contains 129 literature sampled human intestinal fluid equilibrium solubility values and 387 simulated intestinal fluid equilibrium solubility values. Statistical comparison does not detect a difference (Fa/Fe9SIF vs Fa/FeHIF), a novel solubility correlation window enclosed 95% of an additional literature Fa/FeHIF data set and solubility behaviour is consistent with previous physicochemical studies. The Fa/Fe9SIF system therefore represents a novel in vitro methodology for bioequivalent intestinal solubility determination. Combined with intestinal permeability this provides an improved, population based, biopharmaceutical assessment that guides formulation development and indicates the presence of food based solubility effects. This transforms predictive ability during drug discovery and development and may represent a methodology applicable to other multicomponent fluids where no single component is responsible for performance.

Meta-analysis guided development of a standard artificial urine

In this study, we present the first meta-analysis of human urine reported in the literature, drawing data from a total of 35 articles with a combined participant count of 14,021. Through this analysis, we have developed an artificial urine (AU) composition that can be adjusted within typical physiological parameters for in vitro applications. Our findings demonstrate the utility of this AU in determining the solubility of nitrofurantoin, particularly in the context of crystalluria. Notably, we observe that in saline, nitrofurantoin solubility, within the framework of its urinary pharmacokinetics, suggests a risk of crystalluria. However, in AU, this risk is mitigated due to complexation with urea. More broadly, we anticipate that our developed formulation will serve as a foundation for translational studies across biomedical and pharmaceutical sciences.

Effect of Food Composition on the PK of Isoniazid Quantitatively Explained Using Physiologically Based Biopharmaceutics Modeling

This work shows the utilization of a physiologically based biopharmaceutics model (PBBM) to mechanistically explain the impact of diverse food types on the pharmacokinetics (PK) of isoniazid (INH) and acetyl-isoniazid (Ac-INH). The model was established and validated using published PK profiles for INH along with a combination of measured and predicted values for the physico-chemical and biopharmaceutical propertied of INH and Ac-INH. A dedicated ontogeny model was developed for N-acetyltransferase 2 (NAT2) in human integrating Michaelis Menten parameters for this enzyme in the physiologically based pharmacokinetic (PBPK) model tissues and in the gut, to explain the pre-systemic and systemic metabolism of INH across different acetylator types. Additionally, a novel equation was proposed to calculate the luminal drug degradation related to the presence of reducing sugars, using individual sugar molar concentrations in the meal. By incorporating luminal degradation into the model, adjusting bile salt concentrations and gastric emptying according to food type and quantity, the PBBM was able to accurately predict the negative effect of carbohydrate-rich diets on the PK of INH.

In vitro optimization of crushed drug-sensitive antituberculosis medication when administered via a nasogastric tube

IMPORTANCE

The incidence of tuberculosis (TB) in intensive care units (ICUs) can be as high as 3% in high-burden settings, translating to more than 7,500 patients admitted to the ICU annually. In resource-limited settings, the lack or absence of intravenous formulations of drug-sensitive antituberculosis medications necessitates healthcare practitioners to crush, dissolve, and administer the drugs to critically ill patients via a nasogastric tube (NGT). This off-label practice has been linked to plasma concentrations below the recommended target concentrations, particularly of rifampicin and isoniazid, leading to clinical failure and the development of drug resistance. Optimizing the delivery of crushed drug-sensitive antituberculosis medication via the NGT to critically ill patients is of utmost importance.

Structured solubility behaviour in fed simulated intestinal fluids

Intestinal drug solubility is a key parameter controlling absorption after the administration of a solid oral dosage form. The ability to measure fed state solubility in vitro is limited and multiple simulated intestinal fluid recipes have been developed but with no consensus which is optimal. This study has utilised nine bioequivalent simulated fed intestinal media recipes that cover over 90% of the compositional variability of sampled fed human intestinal fluid. The solubility of 24 drugs (Acidic; furosemide, ibuprofen, indomethacin, mefenamic acid, naproxen, phenytoin, piroxicam, valsartan, zafirlukast: Basic; aprepitant, atazanavir, bromocriptine, carvedilol, dipyridamole, posaconazole, tadalafil: Neutral; acyclovir, carbamazepine, felodipine, fenofibrate, griseofulvin, itraconazole, paracetamol, probucol) has been assessed to determine if structured solubility behaviour is present. The measured solubility behaviour can be split into four categories and is consistent with drug physicochemical properties and previous solubility studies. For acidic drugs (category 1) solubility is controlled by media pH and the lowest and highest pH media identify the lowest and highest solubility in 90% of cases. For weakly acidic, basic and neutral drugs (category 2) solubility is controlled by media pH and total amphiphile concentration (TAC), a consistent solubility pattern is evident with variation related to individual drug media component interactions. The lowest and highest pH × TAC media identify the lowest and highest solubility in 70% and 90% of cases respectively. Four drugs, which are non-ionised in the media systems (category 3), have been identified with a very narrow solubility range, indicating minimal impact of the simulated media on solubility. Three drugs exhibit solubility behaviour that is not consistent with the remainder (category 4). The results indicate that the use of two bioequivalent fed intestinal media from the original nine will identify in vitro the maximum and minimum solubility values for the majority of drugs and due to the media derivation this is probably applicable in vivo. When combined with a previous fasted study, this introduces interesting possibilities to measure a solubility range in vitro that can provide Quality by Design based decisions to rationalise drug and formulation development. Overall this indicates that the multi-dimensional media system is worthy of further investigation as in vitro tool to assess fed intestinal solubility.

Characterizing interspecies differences in gastric fluid properties to improve understanding of in vivo oral drug formulation performance

An in-depth understanding of the properties of gastric fluid(s) prior to an in vivo pharmacokinetic investigation can vastly improve predictions of in vivo performance. Previously, properties of animal and human gastric fluids have been characterized with varying methods. Unfortunately, characterization has often not been thorough, and some properties, such as density and viscosity, have not been reported. Here, human, porcine and canine gastric fluids were harvested and characterized for pH, viscosity, surface tension, density, and osmolarity. We found that the variability of pH and surface tension between dogs was significantly higher than the variability between pigs, and, furthermore, gastric fluids collected from the same canine species (beagles) housed in two different countries (Denmark and China) had surprisingly different pH values. Next, an in vitro dissolution study in diluted gastric fluids from each species was performed using minitablets containing ibuprofen. Human gastric fluids and porcine gastric fluids showed similar dissolution profiles and corroborated well with biorelevant human Fasted State Simulated Gastric Fluid (FaSSGF). In contrast, differences in canine gastric fluids caused highly variable dissolution results. We systematically compared our findings to those in the literature and based on this evaluation, propose obtaining aspirates from the animals used for in vivo studies to ensure knowledge on the fluid properties affecting the performance of the formulated drug in question.

Fed Intestinal Solubility Limits and Distributions Applied to the Developability Classification System

For solid oral dosage forms drug solubility in intestinal fluid is an important parameter influencing product performance and bioavailability. Solubility along with permeability are the two parameters applied in the Biopharmaceutics and Developability Classification Systems (DCS) to assess a drug's potential for oral administration. Intestinal solubility varies with the intestinal contents and the differences between the fasted and fed states are recognised to influence solubility and bioavailability. In this study a novel fed state simulated media system comprising of nine media has been utilised to measure the solubility of seven drugs (ibuprofen, mefenamic acid, furosemide, dipyridamole, griseofulvin, paracetamol and acyclovir) previously studied in the fasted state DCS. The results demonstrate that the fed nine media system provides a range of solubility values for each drug and solubility behaviour is consistent with published design of experiment studies conducted in either the fed or fasted state. Three drugs (griseofulvin, paracetamol and acyclovir) exhibit very narrow solubility distributions, a result that matches published behaviour in the fasted state, indicating that this property is not influenced by the concentration of simulated media components. The nine solubility values for each drug can be utilised to calculate a dose/solubility volume ratio to visualise the drug's position on the DCS grid. Due to the derivation of the nine media compositions the range and catergorisation could be considered as bioequivalent and can be combined with the data from the original fed intestinal fluid analysis to provide a population based solubility distribution. This provides further information on the drugs solubility behaviour and could be applied to quality by design formulation approaches. Comparison of the fed results in this study with similar published fasted results highlight that some differences detected match in vivo behaviour in food effect studies. This indicates that a combination of the fed and fasted systems may be a useful in vitro biopharmaceutical performance tool. However, it should be noted that the fed media recipes in this study are based on a liquid meal (Ensure Plus) and this may not be representative of alternative fed states achieved through ingestion of a solid meal. Nevertheless, this novel approach provides greater in vitro detail with respect to possible in vivo biopharmaceutical performance, an improved ability to apply risk-based approaches and the potential to investigate solubility based food effects. The system is therefore worthy of further investigation but studies will be required to expand the number of drugs measured and link the in vitro measurements to in vivo results.

Colonic drug delivery: Formulating the next generation of colon-targeted therapeutics

Colonic drug delivery can facilitate access to unique therapeutic targets and has the potential to enhance drug bioavailability whilst reducing off-target effects. Delivering drugs to the colon requires considered formulation development, as both oral and rectal dosage forms can encounter challenges if the colon's distinct physiological environment is not appreciated. As the therapeutic opportunities surrounding colonic drug delivery multiply, the success of novel pharmaceuticals lies in their design. This review provides a modern insight into the key parameters determining the effective design and development of colon-targeted medicines. Influential physiological features governing the release, dissolution, stability, and absorption of drugs in the colon are first discussed, followed by an overview of the most reliable colon-targeted formulation strategies. Finally, the most appropriate in vitro, in vivo, and in silico preclinical investigations are presented, with the goal of inspiring strategic development of new colon-targeted therapeutics.

Small scale in vitro method to determine a potential bioequivalent equilibrium solubility range for fed human intestinal fluid

Intestinal drug solubility is a key parameter controlling oral absorption but varies both intra and inter individuals and between the fasted and fed states, with food intake known to alter the bioavailability of many compounds. Intestinal solubility can be measured in vitro either using sampled fed human intestinal fluid (FeHIF) or simulated fed intestinal fluid (SIF) but neither approach is optimal. FeHIF is difficult to obtain and variable, whilst for fed SIF multiple recipes are available with no consensus on the ideal version. A recent study characterised FeHIF aspirates using a multidimensional approach and calculated nine simulated media recipes that covered over ninety percent of FeHIF compositional variability. In this study the equilibrium solubility of thirteen drugs have been measured using the nine simulated media recipes and compared to multiple previous design of experiment (DoE) studies, which have examined the impact of fed SIF media components on solubility. The measured nine media solubility data set is only statistically equivalent to the large scale 92 media DoE in 4 out of 13 drug comparisons, but has improved equivalence against small scale DoEs (9 or 10 media) with 6 out of 9 or 10 out of 12 (9 and 10 media respectively) equivalent. Selective removal of non-biorelevant compositions from the 92 media DoE improves statistical equivalence to 9 out of 13 comparisons. The results indicate that solubility equivalence is linked to media component concentrations and compositions, the nine media system is measuring a similar solubility space to previous systems, with a narrower solubility range than the 92 point DoE but equivalent to smaller DoE systems. Phenytoin and tadalafil display a narrow solubility range, a behaviour consistent with previous studies in fed and fasted states and only revealed through the multiple media approach. Custom DoE analysis of the nine media results to determine the most statistically significant component influencing solubility does not detect significant components. Indicating that the approach has a low statistical resolution and is not appropriate if determination of media component significance is required. This study demonstrates that it is possible to assess the fed intestinal equilibrium solubility envelope using the nine media recipes obtained from a multi-dimensional analysis of fed HIF. The derivation of the nine media compositions coupled with the results in this study indicate that the solubility results are more likely to reflect the fed intestinal solubility envelope than previous DoE studies and highlight that the system is worthy of further investigation.

Structured solubility behaviour in bioequivalent fasted simulated intestinal fluids

Drug solubility in intestinal fluid is a key parameter controlling absorption after the administration of a solid oral dosage form. To measure solubility in vitro simulated intestinal fluids have been developed, but there are multiple recipes and the optimum is unknown. This situation creates difficulties during drug discovery and development research. A recent study characterised sampled fasted intestinal fluids using a multidimensional approach to derive nine bioequivalent fasted intestinal media that covered over 90% of the compositional variability. These media have been applied in this study to examine the equilibrium solubility of twenty one exemplar drugs (naproxen, indomethacin, phenytoin, zafirlukast, piroxicam, ibuprofen, mefenamic acid, furosemide, aprepitant, carvedilol, tadalafil, dipyridamole, posaconazole, atazanavir, fenofibrate, felodipine, griseofulvin, probucol, paracetamol, acyclovir and carbamazepine) to determine if consistent solubility behaviour was present. The bioequivalent media provide in the majority of cases structured solubility behaviour that is consistent with physicochemical properties and previous solubility studies. For the acidic drugs (pKa < 6.3) solubility is controlled by media pH, the profile is identical and consistent and the lowest and highest pH media identify the lowest and highest solubility in over 70% of cases. For weakly acidic (pKa > 8), basic and neutral drugs solubility is controlled by a combination of media pH and total amphiphile concentration (TAC), a consistent solubility behaviour is evident but with variation related to individual drug interactions within the media. The lowest and highest pH x TAC media identify the lowest and highest solubility in over 78% of cases. A subset of the latter category consisting of neutral and drugs non-ionised in the media pH range have been identified with a very narrow solubility range, indicating that the impact of the simulated intestinal media on their solubility is minimal. Two drugs probucol and atazanavir exhibit unusual behaviour. The study indicates that the use of two appropriate bioequivalent fasted intestinal media from the nine will identify in vitro the maximum and minimum solubility boundaries for drugs and due to the media derivation this is probably applicable in vivo. These media could be applied during discovery and development activities to provide a solubility range, which would assist placement of the drug within the BCS/DCS and rationalise drug and formulation decisions.

Concentration and composition dependent aggregation of Pluronic- and Poly-(2-oxazolin)-Efavirenz formulations in biorelevant media

Many drugs and drug candidates are poorly water-soluble. Intestinal fluids play an important role in their solubilization. However, the interactions of intestinal fluids with polymer excipients, drugs and their formulations are not fully understood. Here, diffusion ordered spectroscopy (DOSY) and nuclear Overhauser effect spectroscopy (NOESY), complemented by cryo-TEM were employed to address this. Efavirenz (EFV) as model drug, the triblock copolymers Pluronic® F-127 (PF127) and poly(2-oxazoline) based pMeOx-b-pPrOzi-b-pMeOx (pOx/pOzi) and their respective formulations were studied in simulated fed-state intestinal fluid (FeSSIF). For the individual polymers, the bile interfering nature of PF127 was confirmed and pure pOx/pOzi was newly classified as non-interfering. A different and more complex behaviour was however observed if EFV was involved. PF127/EFV formulations in FeSSIF showed concentration dependent aggregation with separate colloids at low formulation concentrations, a merging of individual particles at the solubility limit of EFV in FeSSIF and joint aggregates above this concentration. In the case of pOx/pOzi/EFV formulations, coincident diffusion coefficients for pOx/pOzi, lipids and EFV indicate joint aggregates across the studied concentration range. This demonstrates that separate evaluation of polymers and drugs in biorelevant media is not sufficient and their mixtures need to be studied to learn about concentration and composition dependent behaviour.

Fasted Intestinal Solubility Limits and Distributions Applied to the Biopharmaceutics and Developability Classification Systems

After oral administration, a drug’s solubility in intestinal fluid is an important parameter influencing bioavailability and if the value is known it can be applied to estimate multiple biopharmaceutical parameters including the solubility limited absorbable dose. Current in vitro measurements may utilise fasted human intestinal fluid (HIF) or simulated intestinal fluid (SIF) to provide an intestinal solubility value. This single point value is limited since its position in relation to the fasted intestinal solubility envelope is unknown. In this study we have applied a nine point fasted equilibrium solubility determination in SIF, based on a multi-dimensional analysis of fasted human intestinal fluid composition, to seven drugs that were previously utilised to investigate the developability classification system (ibuprofen, mefenamic acid, furosemide, dipyridamole, griseofulvin, paracetamol and acyclovir). The resulting fasted equilibrium solubility envelope encompasses literature solubility values in both HIF and SIF indicating that it measures the same solubility space as current approaches with solubility behaviour consistent with previous SIF design of experiment studies. In addition, it identifies that three drugs (griseofulvin, paracetamol and acyclovir) have a very narrow solubility range, a feature that single point solubility approaches would miss. The measured mid-point solubility value is statistically equivalent to the value determined with the original fasted simulated intestinal fluid recipe, further indicating similarity and that existing literature results could be utilised as a direct comparison. Since the multi-dimensional approach covered greater than ninety percent of the variability in fasted intestinal fluid composition, the measured maximum and minimum equilibrium solubility values should represent the extremes of fasted intestinal solubility and provide a range. The seven drugs all display different solubility ranges and behaviours, a result also consistent with previous studies. The dose/solubility ratio for each measurement point can be plotted using the developability classification system to highlight individual drug behaviours. The lowest solubility represents a worst-case scenario which may be useful in risk-based quality by design biopharmaceutical calculations than the mid-point value. The method also permits a dose/solubility ratio frequency distribution determination for the solubility envelope which permits further risk-based refinement, especially where the drug crosses a classification boundary. This novel approach therefore provides greater in vitro detail with respect to possible biopharmaceutical performance in vivo and an improved ability to apply risk-based analysis to biopharmaceutical performance. Further studies will be required to expand the number of drugs measured and link the in vitro measurements to in vivo results.

Small scale in vitro method to determine a bioequivalent equilibrium solubility range for fasted human intestinal fluid

Drug solubility is a key parameter controlling oral absorption, but intestinal solubility is difficult to assess in vitro. Human intestinal fluid (HIF) aspirates can be applied but they are variable, difficult to obtain and expensive. Simulated intestinal fluids (SIF) are a useful surrogate but multiple recipes are available and the optimum is unknown. A recent study characterised fasted HIF aspirates using a multi-dimensional approach and determined nine bioequivalent SIF media recipes that represented over ninety percent of HIF compositional variability. In this study these recipes have been applied to determine the equilibrium solubility of twelve drugs (naproxen, indomethacin, phenytoin, piroxicam, aprepitant, carvedilol, zafirlukast, tadalafil, fenofibrate, griseofulvin, felodipine, probucol) previously investigated using a statistical design of experiment (DoE) approach. The bioequivalent solubility measurements are statistically equivalent to the previous DoE, enclose literature solubility values in both fasted HIF and SIF, and the solubility range is less than the previous DoE. These results indicate that the system is measuring the same solubility space as literature systems with the lower overall range suggesting improved equivalence to in vivo solubility, when compared to DoEs. Three drugs (phenytoin, tadalafil and griseofulvin) display a comparatively narrow solubility range, a behaviour that is consistent with previous studies and related to the drugs' molecular structure and properties. This solubility behaviour would not be evident with single point solubility measurements. The solubility results can be analysed using a custom DoE to determine the most statistically significant factor within the media influencing solubility. This approach has a lower statistical resolution than a formal DoE and is not appropriate if determination of media factor significance for solubilisation is required. This study demonstrates that it is possible to assess the fasted intestinal equilibrium solubility envelope using a small number of bioequivalent media recipes obtained from a multi-dimensional analysis of fasted HIF. The derivation of the nine bioequivalent SIF media coupled with the lower measured solubility range indicate that the solubility results are more likely to reflect the fasted intestinal solubility envelope than previous DoE studies and highlight that intestinal solubility is a range and not a single value.

Current challenges and future perspectives in oral absorption research: An opinion of the UNGAP network

Although oral drug delivery is the preferred administration route and has been used for centuries, modern drug discovery and development pipelines challenge conventional formulation approaches and highlight the insufficient mechanistic understanding of processes critical to oral drug absorption. This review presents the opinion of UNGAP scientists on four key themes across the oral absorption landscape: (1) specific patient populations, (2) regional differences in the gastrointestinal tract, (3) advanced formulations and (4) food-drug interactions. The differences of oral absorption in pediatric and geriatric populations, the specific issues in colonic absorption, the formulation approaches for poorly water-soluble (small molecules) and poorly permeable (peptides, RNA etc.) drugs, as well as the vast realm of food effects, are some of the topics discussed in detail. The identified controversies and gaps in the current understanding of gastrointestinal absorption-related processes are used to create a roadmap for the future of oral drug absorption research.

Impact of gastrointestinal tract variability on oral drug absorption and pharmacokinetics: An UNGAP review

The absorption of oral drugs is frequently plagued by significant variability with potentially serious therapeutic consequences. The source of variability can be traced back to interindividual variability in physiology, differences in special populations (age- and disease-dependent), drug and formulation properties, or food-drug interactions. Clinical evidence for the impact of some of these factors on drug pharmacokinetic variability is mounting: e.g. gastric pH and emptying time, small intestinal fluid properties, differences in pediatrics and the elderly, and surgical changes in gastrointestinal anatomy. However, the link of colonic factors variability (transit time, fluid composition, microbiome), sex differences (male vs. female) and gut-related diseases (chronic constipation, anorexia and cachexia) to drug absorption variability has not been firmly established yet. At the same time, a way to decrease oral drug pharmacokinetic variability is provided by the pharmaceutical industry: clinical evidence suggests that formulation approaches employed during drug development can decrease the variability in oral exposure. This review outlines the main drivers of oral drug exposure variability and potential approaches to overcome them, while highlighting existing knowledge gaps and guiding future studies in this area.

Exploring the Impact of Intestinal Fluid Components on the Solubility and Supersaturation of Danazol

Eleven simulated intestinal fluids (SIF) were designed using a Design of Experiment (DoE) approach. The DoE SIF covered a range of compositions of fasted state human intestinal fluid (FaHIF) with regard to pH, bile salt (BS), and phospholipid (PL). Using the model compound danazol, the apparent crystalline solubility (aCS) and apparent amorphous solubility (aAS), as well as the supersaturation propensity was determined in the DoE SIF media. The aCS of danazol was dependent on the composition of the SIF, with PL as the main factor, and a small effect from BS and an interaction between BS and PL. From the DoE solubility data a model was derived, which could predict aCS in commercially available SIF (FaSSIF-V1 and -V2) and in a range of FaHIF. The aAS of danazol was differently affected by the SIF composition than the aCS; PL was again the main factor influencing the aAS, but interactions between BS and pH, as well as pH and PL were also important. The supersaturation propensities of danazol in the DoE SIF media were affected by the same factors as the aCS. Hence, the supersaturation behaviour and aCS of danazol, were found to be closely related.