Prediction of the precipitation profiles of weak base drugs in the small intestine using a simplified transfer (“dumping”) model coupled with in silico modeling and simulation approach

Physiologically based in vitro - In vivo correlation of modified release oral formulations with non-linear intestinal absorption: A case study using mirabegron

Establishing an in vitro - in vivo correlation (IVIVC) for oral modified release (MR) formulations would make it possible to substitute an in vitro dissolution test for human bioequivalence (BE) studies when changing the formulation or manufacturing methods. However, the number of IVIVC applications and approvals are reportedly low. One of the main reasons for failure to obtain IVIVCs using conventional methodologies may be the lack of consideration of the dissolution and absorption mechanisms of drugs in the physiological environment. In particular, it is difficult to obtain IVIVC using conventional methodologies for drugs with non-linear absorption processes. Therefore, the aim of the present study was to develop a physiologically based biopharmaceutics model (PBBM) that enables Level A IVIVCs for mirabegron MR formulations with non-linear absorption characteristics. Using human pharmacokinetic (PK) data for immediate-release formulations of mirabegron, the luminal drug concentration-dependent membrane permeation coefficient was calculated through curve fitting. The membrane permeation coefficient data were then applied to the human PK data of the MR formulations to estimate the in vivo dissolution rate by curve fitting. It was assumed that in vivo dissolution could be described using a zero-order rate equation. Furthermore, a Levy plot was generated using the estimated in vivo dissolution rate and the in vitro dissolution rate obtained from the literature. Finally, the dissolution rate of the MR formulations from the Levy plot was applied to the PBBM to predict the oral PK of the mirabegron MR formulations. This PB-IVIVC approach successfully generated linear Levy plots with slopes of almost 1.0 for MR formulations with different dose strengths and dissolution rates. The Cmax values of the MR formulations were accurately predicted using this approach, whereas the prediction errors for AUC exceeded the Level A IVIVC criteria. This can be attributed to the incomplete description of colonic absorption in the current PBBM.

Evaluation of the impact of mucin on supersaturation and permeation of BCS class 2 basic drugs

This study evaluated the impact of mucin on supersaturation and permeation of BCS Class 2 basic drugs in a pH-shift, 2-stage model using three model compounds, dipyridamole, ricobendazole, and Compound A. The three compounds showed various degrees of supersaturation (DoS) in Stage 2 and modest to no increases in flux with the presence of mucin in the dissolution media. Mucin's impact on DoS and flux, if any, appeared to be compound specific and possibly related to its pKa and ionization state. Overall, the increases in supersaturation and permeation due to mucin ranged from modest to minimal for the three model compounds under the conditions tested. The pH-shift model using MacroFLUX was able to monitor gastric and intestinal dissolution and simultaneously assess the effect of intestinal mucin on supersaturation and flux.

Parameterization of Physiologically Based Biopharmaceutics Models: Workshop Summary Report

This Article shares the proceedings from the August 29th, 2023 (day 1) workshop "Physiologically Based Biopharmaceutics Modeling (PBBM) Best Practices for Drug Product Quality: Regulatory and Industry Perspectives". The focus of the day was on model parametrization; regulatory authorities from Canada, the USA, Sweden, Belgium, and Norway presented their views on PBBM case studies submitted by industry members of the IQ consortium. The presentations shared key questions raised by regulators during the mock exercise, regarding the PBBM input parameters and their justification. These presentations also shed light on the regulatory assessment processes, content, and format requirements for future PBBM regulatory submissions. In addition, the day 1 breakout presentations and discussions gave the opportunity to share best practices around key questions faced by scientists when parametrizing PBBMs. Key questions included measurement and integration of drug substance solubility for crystalline vs amorphous drugs; impact of excipients on apparent drug solubility/supersaturation; modeling of acid-base reactions at the surface of the dissolving drug; choice of dissolution methods according to the formulation and drug properties with a view to predict the in vivo performance; mechanistic modeling of in vitro product dissolution data to predict in vivo dissolution for various patient populations/species; best practices for characterization of drug precipitation from simple or complex formulations and integration of the data in PBBM; incorporation of drug permeability into PBBM for various routes of uptake and prediction of permeability along the GI tract.

The Use of Global Sensitivity Analysis to Assess the Oral Absorption of Weakly Basic Compounds: A Case Example of Dipyridamole

OBJECTIVE

To utilize the global system analysis (GSA) in oral absorption modeling to gain a deeper understanding of system behavior, improve model accuracy, and make informed decisions during drug development.

METHODS

GSA was utilized to give insight into which drug substance (DS), drug product (DP), and/or physiological parameter would have an impact on peak plasma concentration (Cmax) and area under the curve (AUC) of dipyridamole as a model weakly basic compound. GSA guided the design of in vitro experiments and oral absorption risk assessment using FormulatedProducts v2202.1.0. The solubility and precipitation profiles of dipyridamole in different bile salt concentrations were measured. The results were then used to build a mechanistic oral absorption model.

RESULTS

GSA warranted further investigation into the precipitation kinetics and its link to the levels of bile salt concentrations. Mechanistic modeling studies demonstrated that a precipitation-integrated modeling approach appropriately predicted the mean plasma profiles, Cmax, and AUC from the clinical studies.

CONCLUSIONS

This work shows the value of GSA utilization in early development to guide in vitro experimentation and build more confidence in identifying the critical parameters for the mathematical models.

Model-Informed Drug Development: In Silico Assessment of Drug Bioperformance following Oral and Percutaneous Administration

The pharmaceutical industry has faced significant changes in recent years, primarily influenced by regulatory standards, market competition, and the need to accelerate drug development. Model-informed drug development (MIDD) leverages quantitative computational models to facilitate decision-making processes. This approach sheds light on the complex interplay between the influence of a drug's performance and the resulting clinical outcomes. This comprehensive review aims to explain the mechanisms that control the dissolution and/or release of drugs and their subsequent permeation through biological membranes. Furthermore, the importance of simulating these processes through a variety of in silico models is emphasized. Advanced compartmental absorption models provide an analytical framework to understand the kinetics of transit, dissolution, and absorption associated with orally administered drugs. In contrast, for topical and transdermal drug delivery systems, the prediction of drug permeation is predominantly based on quantitative structure-permeation relationships and molecular dynamics simulations. This review describes a variety of modeling strategies, ranging from mechanistic to empirical equations, and highlights the growing importance of state-of-the-art tools such as artificial intelligence, as well as advanced imaging and spectroscopic techniques.

The biomolecular gastrointestinal corona in oral drug delivery

The formation of a biomolecular corona on exogenous particles in plasma is well studied and is known to dictate the biodistribution and cellular interactions of nanomedicine formulations. In contrast, while the oral route is the most favorable administration method for pharmaceuticals, little is known about the formation and composition of the corona formed by biomolecules on particles within the gastrointestinal tract. This work reviews the current literature understanding of (1) the formation of drug particles after oral administration, (2) the formation of a biomolecular corona within the gastrointestinal tract ("the gastrointestinal corona"), and (3) the possible implications of the formation of a gastrointestinal corona on the interactions of drug particles with their biological environment. In doing so, this work aims to establish the significance of the formation of a gastrointestinal corona in oral drug delivery to ultimately arrive at new avenues to control the behavior of orally administered pharmaceuticals.

Development of an In Vitro Methodology to Assess the Bioequivalence of Orally Disintegrating Tablets Taken without Water

To assess the probability of bioequivalence (BE) between orally disintegrating tablets (ODTs) taken without water and conventional tablets (CTs) taken with water, an in vitro biorelevant methodology was developed using the BE Checker, which reproduces fluid shifts in the gastrointestinal tract and drug permeation. In addition to the fluid shift from the stomach to the small intestine, the process of ODT disintegration in a small amount of fluid in the oral cavity and the difference in gastric emptying caused by differences in water intake were incorporated into the evaluation protocol. Assuming a longer time to maximum plasma concentration after oral administration of ODTs taken without water than for CTs taken with water due to a delay in gastric emptying, the fluid shift in the donor chamber of the BE Checker without water was set longer than that taken with water. In the case of naftopidil ODTs and CTs, the values of the f2 function, representing the similarity of the permeation profiles, were 50 or higher when the fluid shift in ODTs taken without water was set at 1.5 or 2 times longer than that of the CTs taken with water. The values of the f2 function in permeation profiles of pitavastatin and memantine ODTs were both 62 when the optimized experimental settings for naftopidil formulations were applied. This methodology can be useful in formulation studies for estimating the BE probability between ODTs and CTs.

A Combined In-Vitro and GastroPlus® Modeling to Study the Effect of Intestinal Precipitation on Cinnarizine Plasma Profile in a Fasted State

Poorly water-soluble weak base molecules such as cinnarizine often exhibit pH-dependent solubility within the gastrointestinal tract. This means that their solubility can be influenced by the pH of the surrounding environment, and this can affect their oral absorption. The differential pH solubility between the fasted-state stomach and intestine is an important consideration when studying the oral absorption of cinnarizine. Cinnarizine has moderate permeability and is known to exhibit supersaturation and precipitation in fasted-state simulated intestinal fluid (FaSSIF), which can significantly impact its oral absorption. The present work is aimed at studying the precipitation behavior of cinnarizine in FaSSIF using biorelevant in vitro tools and GastroPlus^® modeling, to identify the factors contributing to the observed variability in clinical plasma profiles. The study found that cinnarizine demonstrated variable precipitation rates under different bile salt concentrations, which could impact the concentration of the drug available for absorption. The results also showed that a precipitation-integrated modeling approach accurately predicted the mean plasma profiles from the clinical studies. The study concluded that intestinal precipitation may be one of the factors contributing to the observed variability in C_max but not the AUC of cinnarizine. The study further suggests that the integration of experimental precipitation results representing a wider range of FaSSIF conditions would increase the probability of predicting some of the observed variability in clinical results. This is important for biopharmaceutics scientists, as it can help them evaluate the risk of in vivo precipitation impacting drug and/or drug product performance.

Application of Population Balance Model to Simulate Precipitation of Weak Base and Zwitterionic Drugs in Gastrointestinal pH Environment

The purpose of the present study was to evaluate whether the population balance model (PBM) could be a suitable model for the precipitation of weak base and zwitterionic drugs in the gastrointestinal pH environment. Five poorly soluble drugs were used as model drugs (dipyridamole, haloperidol, papaverine, phenazopyridine, and tosufloxacin). PBM consists of the equations for primary nucleation, secondary nucleation, and particle growth. Each equation has two empirical parameters. The pH shift (pH-dumping) precipitation test (pH 3.0 to 6.5) was used to determine the model parameters for each drug. It was difficult to determine all six parameters by simultaneously fitting them to the precipitation profiles. Therefore, the number of model parameters was reduced from six to three by neglecting the secondary nucleation process and applying a common exponent number for the particle growth equation. Despite reducing the parameter number, PBM appropriately described the precipitation profiles in the pH shift tests. The constructed PBM model was then used to predict the precipitation profiles in an artificial stomach-intestine transfer (ASIT) test. PBM appropriately predicted the precipitation profiles in the ASIT test. These results suggested that PBM can be a suitable model to represent the precipitation of weak base and zwitterionic drugs in the gastrointestinal pH environment for biopharmaceutics modeling and simulation.

Current State and Challenges of Physiologically Based Biopharmaceutics Modeling (PBBM) in Oral Drug Product Development

Physiologically based biopharmaceutics modeling (PBBM) emphasizes the integration of physicochemical properties of drug substance and formulation characteristics with system physiological parameters to predict the absorption and pharmacokinetics (PK) of a drug product. PBBM has been successfully utilized in drug development from discovery to postapproval stages and covers a variety of applications. The use of PBBM facilitates drug development and can reduce the number of preclinical and clinical studies. In this review, we summarized the major applications of PBBM, which are classified into six categories: formulation selection and development, biopredictive dissolution method development, biopharmaceutics risk assessment, clinically relevant specification settings, food effect evaluation and pH-dependent drug-drug-interaction risk assessment. The current state of PBBM applications is illustrated with examples from published studies for each category of application. Despite the variety of PBBM applications, there are still many hurdles limiting the use of PBBM in drug development, that are associated with the complexity of gastrointestinal and human physiology, the knowledge gap between the in vitro and the in vivo behavior of drug products, the limitations of model interfaces, and the lack of agreed model validation criteria, among other issues. The challenges and essential considerations related to the use of PBBM are discussed in a question-based format along with the scientific thinking on future research directions. We hope this review can foster open discussions between the pharmaceutical industry and regulatory agencies and encourage collaborative research to fill the gaps, with the ultimate goal to maximize the applications of PBBM in oral drug product development.

Integrating Forward and Reverse Translation in PBPK Modeling to Predict Food Effect on Oral Absorption of Weakly Basic Drugs

INTRODUCTION

Ketoconazole and posaconazole are two weakly basic broad-spectrum antifungals classified as Biopharmaceutics Classification System class II drugs, indicating that they are highly permeable, but exhibit poor solubility. As a result, oral bioavailability and clinical efficacy can be impacted by the formulation performance in the gastrointestinal system. In this work, we have leveraged in vitro biopharmaceutics and clinical data available in the literature to build physiologically based pharmacokinetic (PBPK) models for ketoconazole and posaconazole, to determine the suitability of forward in vitro-in vivo translation for characterization of in vivo drug precipitation, and to predict food effect.

METHODS

A stepwise modeling approach was utilized to derive key parameters related to absorption, such as drug solubility, dissolution, and precipitation kinetics from in vitro data. These parameters were then integrated into PBPK models for the simulation of ketoconazole and posaconazole plasma concentrations in the fasted and fed states.

RESULTS

Forward in vitro-in vivo translation of intestinal precipitation kinetics for both model drugs resulted in poor predictions of PK profiles. Therefore, a reverse translation approach was applied, based on limited fitting of precipitation-related parameters to clinical data. Subsequent simulations for ketoconazole and posaconazole demonstrated that fasted and fed state PK profiles for both drugs were adequately recapitulated.

CONCLUSION

The two examples presented in this paper show how middle-out modeling approaches can be used to predict the magnitude and direction of food effects provided the model is verified on fasted state PK data.

Dynamic Changes in Gastrointestinal Fluid Characteristics after Food Ingestion Are Important for Quantitatively Predicting the In Vivo Performance of Oral Solid Dosage Forms in Humans in the Fed State

The aim of this study was to develop a simulation model to predict the in vivo performance of solid oral dosage forms in humans in the fed state. We focused on investigating the effect of dynamic changes in gastrointestinal (GI) fluid characteristics in the fed state on the in vivo performance of solid dosage forms. We used six solid dosage forms containing weak base drugs as model formulations, two with positive food effects in humans, two with negative food effects, and two which are not affected by food ingestion. These model drug formulations were used to perform biorelevant dissolution tests in the stomach and small intestine under both prandial states. The in vitro properties of the drug products obtained from these tests were then coupled with in silico models (fasted or fed) to predict food effects in humans. We successfully incorporated the dynamic changes in GI fluid characteristics and their effects on the in vivo dissolution of drugs into the prediction model for the fed state. This newly designed physiologically based biopharmaceutics modeling approach provided the precise and quantitative prediction of food effects (i.e., changes in C_max and AUC after food ingestion) in humans while considering the dynamic changes in fluid characteristics in the fed state.

An Artificial Gut/Absorption Simulator: Simultaneous Evaluation of Desupersaturation and Absorption from Ketoconazole Supersaturated Solutions

For supersaturating formulations of BCS-II compounds, which by definition have high intestinal permeability, a closed USP apparatus does not provide the necessary absorptive conditions during dissolution. To address this, an artificial gut simulator (AGS) has been constructed consisting of a 2.5 mL donor compartment in which a hollow fiber-based absorption module is suspended. Drug from donor diffuses across the hollow fiber membrane to be absorbed by the continuously flowing intraluminal receiver fluid. The membrane surface area and intraluminal fluid flow rate are tuned to obtain the physiologically observed absorption rate constant for a weakly basic, poorly water-soluble model compound, ketoconazole (KTZ). Supersaturated solutions of KTZ were generated in the donor in pH 6.5 phosphate buffer by the pH-shift method in the absence (closed system, control) and presence (open system, biorelevant) of an optimally or suboptimally tuned absorption module. Drug concentrations in the donor and intraluminal fluids were determined by in-line UV spectroscopy. The presence of an absorptive sink reduced the supersaturated solution's crystallization propensity, more so in the case of the optimally tuned AGS. This study demonstrates the significance of simulating absorption of drug at a physiological rate during dissolution studies, especially to predict the performance of formulations of BCS-II drugs.

Quantitative assessment of disintegration rate is important for predicting the oral absorption of solid dosage forms containing poorly soluble weak base drugs

This study aimed to develop a novel in silico modeling and simulation that considers the disintegration rate in the stomach to predict the in vivo performance of oral solid dosage forms with slow disintegration rates containing poorly soluble weak base drugs. Oxatomide and manidipine hydrochloride were used as model drugs. First, the in vitro disintegration rate and dissolution rate were determined in biorelevant media that simulate the gastrointestinal fluids in fasted humans using a USP apparatus II paddle dissolution tester. Next, the oral absorption of the dosage forms was predicted using the novel simulation model coupled with not only the dissolution rate but also the estimated disintegration rate. As the in vitro disintegration time was 45 min or longer for both drugs in Fasted State Simulated Gastric Fluid, the disintegration rate of these dosage forms was considered slow as immediate release (IR) tablets. While the predicted and observed pharmacokinetic profiles of both drugs were comparable using the new model, the conventional model, which did not consider the disintegration step, underestimated the oral absorption of both drugs. Thus, our novel simulation model coupled with the disintegration rate estimated from in vitro tests is promising for predicting the in vivo performance of oral solid dosage forms with slow disintegration rates containing poorly soluble weak base drugs.

Importance of Gastric Secretion and the Rapid Gastric Emptying of Ingested Water along the Lesser Curvature ("Magenstraße") in Predicting the In Vivo Performance of Liquid Oral Dosage Forms in the Fed State Using a Modeling and Simulation

The objective of the present study was to develop an in silico model of the stomach for predicting oral drug absorption in fed humans. We focused on a model capable of simulating dynamic fluid volume changes and included a simulated Magenstraße "stomach road," a route along the lesser curvature that often carries fluids rapidly to assess the gastric emptying of drugs. Two types of model liquid drug formulations, liquid-filled soft gelatin capsules (enzalutamide, cyclosporine, and nifedipine) and oral solutions (levofloxacin and fenfluramine), were used. An in silico model was assembled, and simulations were performed using Stella Professional software. The secretion rate of the gastric juice induced by food ingestion was assessed along with the gastric emptying of the ingested water via the Magenstraße in the fed state. The model for the fed state successfully described the in vivo performance of the model drug formulations. These results clearly indicate the importance of including gastric secretion and the kinetics of Magenstraße when predicting the in vivo performance of dosage forms using an in silico modeling and simulation of fed humans. This simulation model should be further optimized to allow for the different physiological mechanisms following the ingestion of different types of meals, as well as modifications for interindividual and intraindividual variabilities in gastrointestinal physiology in the fed state in the future.

In vitro models to evaluate ingestible devices: Present status and current trends

Orally ingestible medical devices offer significant opportunity in the diagnosis and treatment of gastrointestinal conditions. Their development necessitates the use of models that simulate the gastrointestinal environment on both a macro and micro scale. An evolution in scientific technology has enabled a wide range of in vitro, ex vivo and in vivo models to be developed that replicate the gastrointestinal tract. This review describes the landscape of the existing range of in vitro tools that are available to characterize ingestible devices. Models are presented with details on their benefits and limitations with regards to the evaluation of ingestible devices and examples of their use in the evaluation of such devices is presented where available. The multitude of models available provides a suite of tools that can be used in the evaluation of ingestible devices that should be selected on the functionality of the device and the mechanism of its function.

Prediction of in vivo supersaturation and precipitation of poorly water-soluble drugs: Achievements and aspirations

This review focuses on options available to a pharmaceutical scientist to predict in vivo supersaturation and precipitation of poorly water-soluble drugs. As no single device or system can simulate the complex gastrointestinal environment, a combination of appropriate in vitro tools may be utilized to get optimal predictive information. To address the empirical issues encountered during small-scale and full-scale in vitro predictive testing, theoretical background and relevant case studies are discussed. The practical considerations for selection of appropriate tools at various stages of drug development are recommended. Upcoming technologies that have potential to further reduce in vivo studies and expedite the drug development process are also discussed.

Mechanisms for the Slowing of Desupersaturation of a Weak Acid at Elevated pH

Supersaturating drug delivery systems are used to achieve higher oral bioavailability for poorly soluble drugs. However, supersaturated solutions often decline to lower concentrations by precipitation and crystallization. The purpose of the current research is to provide a mechanistic understanding of drug crystallization as a function of pH, using indomethacin (IMC, pK_a 4.18) as a model compound. Desupersaturation kinetics to the γ-form of IMC was measured at pH 2.0, 3.0, 4.0, and 4.5 from an initial degree of supersaturation of 2.5-6. At equivalent levels of supersaturation, crystal growth rates decreased with an increase in solution pH. Two mechanisms for this phenomenon, reactive diffusion (resulting in a higher surface pH as compared to bulk pH) and inhibition of crystallization by structurally similar ionized IMC at higher pH, were explored. Non-steady-state models for reactive diffusion showed that the surface pH was only 0.01 units above that of the bulk solution pH. Mass transport models for reactive diffusion during crystallization could not explain the decrease in desupersaturation kinetics at higher pH. However, zeta potentials as high as -70 mV suggested that IMC^- is adsorbed on the surface of the particles. A mathematical model for inhibition of crystal growth by IMC^- accounted for the pH effect suggesting that ionized IMC acts as an effective crystallization inhibitor of IMC.

In Vitro Biopredictive Methods: A Workshop Summary Report

This workshop report summarizes the proceedings of Day 1 of a three-day workshop on "Current State and Future Expectations of Translational Modeling Strategies to Support Drug Product Development, Manufacturing Changes and Controls". Physiologically based biopharmaceutics models (PBBM) are tools which enable the drug product quality attributes to be linked to the in vivo performance. These tools rely on key quality inputs in order to provide reliable predictions. After introducing the objectives of the workshop and the expectations from the breakout sessions, Day 1 of the workshop focused on the best practices and challenges in measuring in vitro inputs needed for modeling, such as the drug solubility, the dissolution rate of the drug product, potential precipitation of the drug and drug permeability. This paper reports the podium presentations and summarizes breakout session discussions related to A) the best strategies for determining solubility, supersaturation and critical supersaturation; B) the best strategies for the development of biopredictive (clinically relevant) dissolution methods; C) the challenges associated with describing gastro-intestinal systems parameters such as mucus, liquid volume and motility; and D) the challenges with translating biopharmaceutical measures of drug permeability along the gastrointestinal tract to a meaningful model parameter.

The Introduction of a New Flexible In Vivo Predictive Dissolution Apparatus, GIS-Alpha (GIS-α), to Study Dissolution Profiles of BCS Class IIb Drugs, Dipyridamole and Ketoconazole

The physiological pH changes and peristalsis activities in gastrointestinal (GI) tract have big impact on the dissolution of oral drug products, when those oral drug products include APIs with pH-dependent solubility. It is well documented that predicting the bioperformance of those oral drug products can be challenging using compendial methods. To overcome this limitation, in vivo predictive dissolution apparatuses, such as the transfer model, have been developed to predict bioperformance of oral formulation candidates and drug products. In this manuscript we utilize a new transfer-model dissolution apparatus, the gastrointestinal simulator-α (GIS-α), to characterize its behavior in terms of transfer kinetics and pH, assess its reproducibility and adaptability to mimic different transfer conditions, as well as study dissolution of ketoconazole and dipyridamole as model BCS class IIb compounds. Availability of commercially available dissolution transfer systems with similar configuration to compendial dissolution apparatus, may be helpful to simplify and standardize in vivo predictive dissolution methodologies for BCS class IIb compounds in the future.

Bottom-Up Physiologically Based Oral Absorption Modeling of Free Weak Base Drugs

In this study, we systematically evaluated "bottom-up" physiologically based oral absorption modeling, focusing on free weak base drugs. The gastrointestinal unified theoretical framework (the GUT framework) was employed as a simple and transparent model. The oral absorption of poorly soluble free weak base drugs is affected by gastric pH. Alternation of bulk and solid surface pH by dissolving drug substances was considered in the model. Simple physicochemical properties such as pKa, the intrinsic solubility, and the bile micelle partition coefficient were used as input parameters. The fraction of a dose absorbed (Fa) in vivo was obtained by reanalyzing the pharmacokinetic data in the literature (15 drugs, a total of 85 Fa data). The AUC ratio with/without a gastric acid-reducing agent (AUCr) was collected from the literature (22 data). When gastric dissolution was neglected, Fa was underestimated (absolute average fold error (AAFE) = 1.85, average fold error (AFE) = 0.64). By considering gastric dissolution, predictability was improved (AAFE = 1.40, AFE = 1.04). AUCr was also appropriately predicted (AAFE = 1.54, AFE = 1.04). The Fa values of several drugs were slightly overestimated (less than 1.7-fold), probably due to neglecting particle growth in the small intestine. This modeling strategy will be of great importance for drug discovery and development.

Understanding the Oral Absorption of Irbesartan Using Biorelevant Dissolution Testing and PBPK Modeling

Poorly soluble weak bases form a significant proportion of the drugs available in the market thereby making it imperative to understand their absorption behavior. This work aims to mechanistically understand the oral absorption behavior for a weakly basic drug, Irbesartan (IRB), by investigating its pH dependent solubility, supersaturation, and precipitation behavior. Simulations performed using the equilibrium solubility could not accurately predict oral absorption. A multi-compartmental biorelevant dissolution testing model was used to evaluate dissolution in the stomach and duodenal compartment and mimic oral drug administration. This model exhibited sustained intestinal supersaturation (2-4-fold) even upon varying flow rates (4 mL/min, 7 mL/min, and mono-exponential transfer) from gastric to intestinal compartment. Simulation of oral absorption using GastroPlus™ and dissolution data collectively predicted plasma exposure with higher accuracy (% prediction error values within ± 15%), thereby indicating that multi-compartment dissolution testing enabled an improved prediction for oral pharmacokinetics of Irbesartan. Additionally, precipitates obtained in the intestinal compartment were characterized to determine the factors underlying intestinal supersaturation of Irbesartan. The solid form of these precipitates was amorphous with considerable particle size reduction. This indicated that following gastric transit, precipitate formation in the amorphous form coupled with an approximately 10 times particle size reduction could be potential factors leading to the generation and sustenance of intestinal drug supersaturation.

A STELLA simulation model for in vitro dissolution testing of respirable size particles

In vitro dissolution testing is a useful quality control tool to discriminate the formulations and to approximate the in vivo drug release profiles. A dissolution apparatus has been custom-made for dissolution testing of dry powder formulations in a small volume of stationary medium (25 μL spread over 4.91 cm² area i.e. ~50 μm thick). To understand the system and predict the key parameters which influence the dissolution of respirable size particles, a simulation model was constructed using STELLA modeling software. Using this model, the permeation (dissolution followed by diffusion through the membrane) of two anti-tubercular drugs of differing solubilities, moxifloxacin (17.68 ± 0.85 mg mL^-1) and ethionamide (0.46 ± 0.02 mg mL^-1), from the respirable size particles and their diffusion from a solution were simulated. The simulated permeation profiles of moxifloxacin from solution and respirable size particles were similar, indicating fast dissolution of the particles. However, the simulated permeation profile of ethionamide from respirable size particles showed slower permeation compared to the solution indicating the slow dissolution of the respirable size particles of ethionamide. The sensitivity analysis suggested that increased mucus volume and membrane thickness decreased the permeation of drug. While this model was useful in predicting and distinguishing the dissolution behaviours of respirable size moxifloxacin and ethionamide, further improvement could be made using appropriate initial parameter values obtained by experiments.

Automated small-scale in vitro transfer model as screening tool for the prediction of in vivo-dissolution and precipitation of poorly solubles

Precipitation testing, especially for weakly basic APIs, represents a key parameter in drug substance characterization during early development stages, where the amount of API available is limited. Therefore, it was the aim of this study to develop an automated small-scale in vitro transfer model to characterize the supersaturation and precipitation behavior of two poorly water-soluble drugs. Following automation and scale-down of the standard transfer model, the developed small-scale model was used to assess the impact of gastrointestinal variability, i.e. gastric pH, gastric emptying, and gastrointestinal fluid volumes, on supersaturation and precipitation of two weakly basic model compounds, ketoconazole and a new chemical entity from the research laboratories of Merck KGaA, MSC-A. The experiments revealed that variations in gastrointestinal parameters affected the in vitro behavior of ketoconazole, but not of MSC-A. Elevated gastric pH, as it can result from co-medication with acid-reducing drugs, resulted in lower degrees of supersaturation for both substances. This result is in agreement with the observation that the oral bioavailability of ketoconazole is lowered when proton pump inhibitors are co-administered. The small-scale transfer model presented herein represents a valuable in vitro tool to assess the risk of drug precipitation, additionally covering a broad range of gastrointestinal parameters.

In vitro methods to assess drug precipitation in the fasted small intestine – a PEARRL review

Objectives

Drug precipitation in vivo poses a significant challenge for the pharmaceutical industry. During the drug development process, the impact of drug supersaturation or precipitation on the in vivo behaviour of drug products is evaluated with in vitro techniques. This review focuses on the small and full scale in vitro methods to assess drug precipitation in the fasted small intestine.

Key findings

Many methods have been developed in an attempt to evaluate drug precipitation in the fasted state, with varying degrees of complexity and scale. In early stages of drug development, when drug quantities are typically limited, small-scale tests facilitate an early evaluation of the potential precipitation risk in vivo and allow rapid screening of prototype formulations. At later stages of formulation development, full-scale methods are necessary to predict the behaviour of formulations at clinically relevant doses. Multicompartment models allow the evaluation of drug precipitation after transfer from stomach to the upper small intestine. Optimisation of available biopharmaceutics tools for evaluating precipitation in the fasted small intestine is crucial for accelerating the development of novel breakthrough medicines and reducing the development costs.

Summary

Despite the progress from compendial quality control dissolution methods, further work is required to validate the usefulness of proposed setups and to increase their biorelevance, particularly in simulating the absorption of drug along the intestinal lumen. Coupling results from in vitro testing with physiologically based pharmacokinetic modelling holds significant promise and requires further evaluation.

In Vitro, in Silico, and in Vivo Assessments of Intestinal Precipitation and Its Impact on Bioavailability of a BCS Class 2 Basic Compound

In this study, a multipronged approach of in vitro experiments, in silico simulations, and in vivo studies was developed to evaluate the dissolution, supersaturation, precipitation, and absorption of three formulations of Compound-A, a BCS class 2 weak base with pH-dependent solubility. In in vitro 2-stage dissolution experiments, the solutions were highly supersaturated with no precipitation at the low dose but increasing precipitation at higher doses. No difference in precipitation was observed between the capsules and tablets. The in vitro precipitate was found to be noncrystalline with higher solubility than the crystalline API, and was readily soluble when the drug concentration was lowered by dilution. A gastric transit and biphasic dissolution (GTBD) model was developed to better mimic gastric transfer and intestinal absorption. Precipitation was also observed in GTBD, but the precipitate redissolved and partitioned into the organic phase. In vivo data from the phase 1 clinical trial showed linear and dose proportional PK for the formulations with no evidence of in vivo precipitation. While the in vitro precipitation observed in the 2-stage dissolution appeared to overestimate in vivo precipitation, the GTBD model provided absorption profiles consistent with in vivo data. In silico simulation of plasma concentrations by GastroPlus using biorelevant in vitro dissolution data from the tablets and capsules and assuming negligible precipitation was in line with the observed in vivo profiles of the two formulations. The totality of data generated with Compound-A indicated that the bioavailability differences among the three formulations were better explained by the differences in gastric dissolution than intestinal precipitation. The lack of intestinal precipitation was consistent with several other BCS class 2 basic compounds in the literature for which highly supersaturated concentrations and rapid absorption were also observed.

Development, current applications and future roles of biorelevant two-stage in vitro testing in drug development

Objectives

Various types of two stage in vitro testing have been used in a number of experimental settings. In addition to its application in quality control and for regulatory purposes, two-stage in vitro testing has also been shown to be a valuable technique to evaluate the supersaturation and precipitation behavior of poorly soluble drugs during drug development.

Key findings

The so-called ‘transfer model’, which is an example of two-stage testing, has provided valuable information about the in vivo performance of poorly soluble, weakly basic drugs by simulating the gastrointestinal drug transit from the stomach into the small intestine with a peristaltic pump. The evolution of the transfer model has resulted in various modifications of the experimental model set-up. Concomitantly, various research groups have developed simplified approaches to two-stage testing to investigate the supersaturation and precipitation behavior of weakly basic drugs without the necessity of using a transfer pump.

Summary

Given the diversity among the various two-stage test methods available today, a more harmonized approach needs to be taken to optimize the use of two stage testing at different stages of drug development.

The impact of supersaturation level for oral absorption of BCS class IIb drugs, dipyridamole and ketoconazole, using in vivo predictive dissolution system: Gastrointestinal Simulator (GIS)

The development of formulations and the assessment of oral drug absorption for Biopharmaceutical Classification System (BCS) class IIb drugs is often a difficult issue due to the potential for supersaturation and precipitation in the gastrointestinal (GI) tract. The physiological environment in the GI tract largely influences in vivo drug dissolution rates of those drugs. Thus, those physiological factors should be incorporated into the in vitro system to better assess in vivo performance of BCS class IIb drugs. In order to predict oral bioperformance, an in vitro dissolution system with multiple compartments incorporating physiologically relevant factors would be expected to more accurately predict in vivo phenomena than a one-compartment dissolution system like USP Apparatus 2 because, for example, the pH change occurring in the human GI tract can be better replicated in a multi-compartmental platform. The Gastrointestinal Simulator (GIS) consists of three compartments, the gastric, duodenal and jejunal chambers, and is a practical in vitro dissolution apparatus to predict in vivo dissolution for oral dosage forms. This system can demonstrate supersaturation and precipitation and, therefore, has the potential to predict in vivo bioperformance of oral dosage forms where this phenomenon may occur. In this report, in vitro studies were performed with dipyridamole and ketoconazole to evaluate the precipitation rates and the relationship between the supersaturation levels and oral absorption of BCS class II weak base drugs. To evaluate the impact of observed supersaturation levels on oral absorption, a study utilizing the GIS in combination with mouse intestinal infusion was conducted. Supersaturation levels observed in the GIS enhanced dipyridamole and ketoconazole absorption in mouse, and a good correlation between their supersaturation levels and their concentration in plasma was observed. The GIS, therefore, appears to represent in vivo dissolution phenomena and demonstrate supersaturation and precipitation of dipyridamole and ketoconazole. We therefore conclude that the GIS has been shown to be a good biopredictive tool to predict in vivo bioperformance of BCS class IIb drugs that can be used to optimize oral formulations.

Biorelevant test for supersaturable formulation

Supersaturable formulation can generate supersaturation after dissolution, providing kinetic advantage in vivo. However, the supersaturation may precipitate before being absorbed, which makes it difficult to ensure and predict its in vivo performance. The traditional USP method is typically for Quality Control (QC) purpose and cannot be used to predict the formulation in vivo performance. Therefore, there is generally a lack of a predictive biorelevant testing method. In this review, different types of supersaturable formulations are described, including amorphous dispersions, polymorphs, salts/co-crystals, weak base and supersaturable solubilized formulations. Different kinds of in vitro dissolution methods for supersaturable formulations are also reviewed and discussed. Most of the methods take the physiology of gastrointestinal (GI) track into consideration, allowing reasonable prediction of the in vivo performance of supersaturable formulation. However, absorbing drug from GI track into blood stream is a complicate process, which can be affected by different in vivo processes such as transporter and metabolism. These factors cannot be captured by the in vitro testing. Thus, combining in vitro biorelevant dissolution methods with physiology-based pharmacokinetic modeling is a better way for the product development of supersaturable formulation.