Precipitation as Key Parameter for Early Oral Drug Absorption Predictions, Part I: Acquisition of critical data

Abstract

Despite recent advancements in the development of in vitro precipitation assays for weakly basic drugs, their relevance for predicting in vivo oral absorption remains ambiguous, highlighting the need for further research. The objective of this study was to develop a predictive tool for drug absorption of basic drugs, grounded in data derived from in vitro solubility, permeability, and precipitation assays which is particularly suited for application in drug discovery. The study aimed to compile a comprehensive dataset comprising in vitro solubility, permeability, and precipitation data, and in vivo absorption data for a set of 17 model compounds. The dataset was designed to serve as foundation for the development of an absorption prediction tool specifically tailored for drug discovery. For this purpose, solubility and permeability data were measured using a consistent experimental set-up for each, ensuring data comparability. Furthermore, a micro-scale precipitation assay (µPA) was developed that enables streamlined screening of drug candidates, thereby enhancing the lead optimization process during drug discovery. In vivo absorption data for the 17 model compounds were gathered through a comprehensive literature review, addressing the inherent challenges associated with acquiring high-quality in vivo absorption data. The results of this study indicate a decent relationship between the extent of precipitation in vitro, and the fraction of a drug absorbed in vivo. In-depth statistical analysis of the dataset and the development of the absorption prediction tool will be described in a subsequent publication.

Highlights

Comprehensive dataset with key data for development of absorption prediction tool.

In vitro permeability, solubility, precipitation, and in vivo absorption data.

Data highlighted the importance of precipitation testing for absorption predictions.

Micro-scale precipitation assay enables efficient screening in drug discovery.

Existing literature revealed a shortage of high-quality absorption data.

Introduction

One critical factor influencing the bioavailability of orally administered drugs is the fraction absorbed, which determines the amount of drug that passes through the intestinal epithelial membrane (Sugano, 2012). Two of the key determinants for oral drug absorption are I) permeability through biological membranes and II) aqueous solubility (Dahan et al., 2009). Intestinal permeability is a biopharmaceutical parameter reflecting the drug’s potential to cross the intestinal epithelial barrier (Dubbelboer et al., 2019). Likewise, it is crucial that a drug is dissolved in the gastrointestinal fluids prior to its absorption into the systemic circulation. Given their importance for the overall absorption process, drug solubility and permeability build the foundation for the Biopharmaceutical Classification System (BCS) (Amidon et al., 1995).

A factor potentially impacting the intestinal absorption of especially weakly basic drugs is precipitation in the small intestine (O’Dwyer et al., 2019). Weakly basic drugs generally exhibit high solubility in acidic environments such as the gastric fluids in the stomach. However, upon entering the upper small intestine, they encounter a pH shift towards more neutral conditions, leading to a reduction in overall drug solubility. This pH shift can result in supersaturation and an increased likelihood of precipitation. Approximately 75% of commercially available drugs are estimated to be either weak bases or salts thereof (Fiolka et al., 2020). Furthermore, 60-70% of compounds in drug development pipelines belong to BCS class II (Ting et al., 2018). These data underscore the relevance of investigating the precipitation behavior of weakly basic drugs.

Over the past years, extensive research has been conducted to examine the precipitation behavior of compounds, both in vivo (Brouwers et al., 2017; Hens et al., 2020; Rubbens et al., 2016) and in vitro (Berlin et al., 2014; Fiolka and Dressman, 2018; Kostewicz et al., 2004; O’Dwyer et al., 2019). However, the likelihood of precipitation occurring in vivo, along with its potential impact on intestinal absorption, remains uncertain. Additionally, the translation of in vitro findings to into vivo precipitation poses considerable challenges due to the complexity of the precipitation process. Consequently, the applicability of in vitro assays for predicting in vivo absorption in humans during drug discovery and development remains challenging.

To date, numerous in vitro precipitation assays aiming to simulate in vivo drug precipitation have been proposed in the literature (Fiolka and Dressman, 2018; O’Dwyer et al., 2019). These assays differ based on their intended purpose and the phase of drug development in which they are applied. For instance, the volumes utilized in the gastric and intestinal compartments can range from microliter-scale to physiological levels, and the media used in these compartments may also vary (Fiolka and Dressman, 2018; O’Dwyer et al., 2019). Some assays employ dumping or solvent-shifting techniques, while others implement transfer pumps to mimic gastric emptying kinetics (Fiolka and Dressman, 2018; O’Dwyer et al., 2019). In contrast to pharmacopeial dissolution techniques, there is no standardization or harmonization for in vitro precipitation assays (Thakral et al., 2021).

The lack of standardization complicates their application during the various stages of drug discovery and development, as well as the interpretation of the results obtained. Particularly, their relevance for quantitatively assessing absorption in humans have not been studied systematically. In 2010, Carlert et al. (Carlert et al., 2010) evaluated the predictive accuracy of in vitro methods for assessing in vivo intestinal precipitation of a basic BCS class II drug in humans. They argued that existing studies had not adequately verified in vivo precipitation or distinguished it from other absorption-related factors, such as slow dissolution. Although several in vitro models predicted rapid drug precipitation for the drug under investigation, they found no evidence of precipitation occurring in vivo in humans. They observed a dose-proportional increase in drug plasma exposure, indicating that precipitation had no significant impact on the rate and extent of drug absorption in the tested range. By contrast, some studies indicate a strong correlation between in vitro precipitation and both in vivo precipitation and absorption. For example, Psachoulias et al. and Kourentas et al. reported good agreement between concentrations obtained from an in vitro set-up and luminal concentrations for various compounds and formulations (Kourentas et al., 2016a; Psachoulias et al., 2012). Additionally, Higashino et al. demonstrated that a supersaturated state achieved in vitro enhanced in vivo intestinal absorption for several drugs in rats (Higashino et al., 2014).

Assessing the impact of precipitation on oral drug absorption can be advantageous particularly in drug discovery as it facilitates the prioritization of drug candidates based on their individual absorption risks, and allows for the early elimination of less promising candidates. Given the time and cost implications associated with measuring precipitation in vivo in humans, animal testing offers a valid alternative. However, the translatability of such data is challenging due to differences in gastrointestinal anatomy and physiological conditions between humans and animals, alongside ethical considerations. Consequently, in vitro techniques are often preferred (Thakral et al., 2021). Nevertheless, further investigations on a more systematic approach to exploring the applicability of in vitro precipitation test results for predicting human absorption remains crucial.

To address these challenges, the overarching aim of this study was to develop a useful tool to improve absorption predictions for weakly basic drugs especially in drug discovery. To achieve this objective, the research was divided into two parts. Part I focused on the acquisition of a comprehensive set of in vitro precipitation, solubility, and permeability and in vivo absorption data, providing the foundation for developing the prediction tool. This involved identifying suitable basic drugs and collecting in vivo absorption data through a thorough literature search. Subsequently, in vitro tests were conducted to acquire the solubility, permeability, and precipitation data for the selected model drugs. In the context of this work, a novel precipitation assay designed specifically for the drug discovery phase was developed. Subsequently, Part II of this work will address the statistical analysis and evaluation of the data generated in Part I, the development of the absorption prediction tool, and elucidation of the predictive performance of the model to predict the impact of precipitation on oral drug absorption.

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Following excipients are mentioned in the study besides other: Sodium dihydrogen phosphate monohydrate,

Sara Mayer, Christian Wagner, Andreas Lehmann, Zhizhou Fang, Werner Weitschies, Katharina Krollik, Precipitation as Key Parameter for Early Oral Drug Absorption Predictions, Part I: Acquisition of critical data, European Journal of Pharmaceutical Sciences, 2025, 107210, ISSN 0928-0987, https://doi.org/10.1016/j.ejps.2025.107210.