High throughput solubility measurement in drug discovery and development

Self-emulsifying therapeutic system: a potential approach for delivery of lipophilic drugs

Self-emulsifying therapeutic system (SETs) provide an effective and intelligent solution to the various issues related to the formulation of hydrophobic drugs with limited solubility in gastrointestinal fluid. Although the potential utility of SETs is well known, only in recent years has a mechanistic understanding of the impact of these systems on drug disposition emerged. These in situ emulsion-forming systems have a high stability when incorporated in various dosage forms. SETs are being looked upon as systems which can overcome the problems associated with delivery of poorly water soluble drugs. An in-depth knowledge about lipids and surfactants that can contribute to these systems, criterion for their selection and the proportion in which they can be used, represent some crucial factors determining the in vivo performance of these systems. This article presents a comprehensive account of various types of self-emulsifying formulations with emphasis on their composition and examples of currently marketed preparations.

Exploratory analysis of kinetic solubility measurements of a small molecule library

Kinetic solubility measurements using prototypical assay buffer conditions are presented for a ∼58,000 member library of small molecules. Analyses of the data based upon physical and calculated properties of each individual molecule were performed and resulting trends were considered in the context of commonly held opinions of how physicochemical properties influence aqueous solubility. We further analyze the data using a decision tree model for solubility prediction and via a multi-dimensional assessment of physicochemical relationships to solubility in the context of specific 'rule-breakers' relative to common dogma. The role of solubility as a determinant of assay outcome is also considered based upon each compound's cross-assay activity score for a collection of publicly available screening results. Further, the role of solubility as a governing factor for colloidal aggregation formation within a specified assay setting is examined and considered as a possible cause of a high cross-assay activity score. The results of this solubility profile should aid chemists during library design and optimization efforts and represent a useful training set for computational solubility prediction.

Nanosizing for oral and parenteral drug delivery: a perspective on formulating poorly-water soluble compounds using wet media milling technology

A significant percentage of active pharmaceutical ingredients identified through discovery screening programs is poorly soluble in water. These molecules are often difficult to formulate using conventional approaches and are associated with innumerable formulation-related performance issues, e.g. poor bioavailability, lack of dose proportionality, slow onset of action and other attributes leading to poor patient compliance. In addition, for parenteral products, these molecules are generally administered with co-solvents and thus have many undesirable side effects. Wet media milling is one of the leading particle size reduction approaches that have been successfully used to formulate these problematic compounds. The approach is a water-based media milling process where micron-sized drug particles are shear-fractured into nanometer-sized particles. Nanoparticle dispersions are stable and typically have a mean diameter of less than 200 nm with 90% of the particles being less than 400 nm. The formulation consists only of water, drug and one or more GRAS excipients. Drug concentrations approaching 300-400mg/g can be targeted with the use of minimal amounts stabilizer. Typically, on average, the drug to stabilizer ratio on a weight basis ranges from 2:1 to 20:1. These liquid nanodispersions exhibit acceptable shelf-life and can be post-processed into various types of solid dosage forms. Nanoparticulate-based drug products have been shown to improve bioavailability and enhance drug exposure for oral and parenteral dosage forms. Suitable formulations for the most commonly used routes of administration can be identified with milligram quantities of drug substance providing the discovery scientist an alternate avenue for screening and identifying superior leads. In the last few years, formulating poorly water soluble compounds as nanosuspensions has evolved from a conception to a realization. The versatility and applicability of this drug delivery platform are just beginning to be realized.

Non-stoichiometric inhibition in biochemical high-throughput screening

INTRODUCTION

Over the last 2 decades, high-throughput screening (HTS) has become one of the key strategies for the generation of new leads. Non-stoichiometric inhibition is one of the most extensively studied mechanisms responsible for the large percentage of hit compounds from biochemical screens that cannot be developed into leads. Therefore, HTS hit lists need to be sorted rapidly and efficiently into stoichiometrically binding inhibitors and compounds that affect enzyme activity non-stoichiometrically.

AREAS COVERED

This article explores the non-stoichiometric inhibition of enzymatic activity in biochemical screens, particularly by compound aggregation, and the authors explain the terminology they use to describe such compound behavior. The paper then provides a short historical overview of both academic and industrial research on compound aggregation specifically. Finally, the article discusses the implications for industrial drug discovery and the measures that can be taken to identify non-stoichiometric and aggregating inhibitors early in this process.

EXPERT OPINION

The most pragmatic approach in a lead finding campaign is to focus on the early identification of selective and stoichiometric inhibitors. The combination of multiple approaches (assessing both activity and binding) allows the enrichment of stoichiometric inhibitors at each stage of the flowchart.

Current oscillations generated by precipitate formation in the mixing zone between two solutions inside a nanopore

Unlike biological protein pores in lipid membranes, nanopores fabricated in synthetic materials can withstand a wide range of environmental conditions including the presence of organic solvents. This capability expands the potential of synthetic nanopores to monitor chemical reactions occurring at the interface between solutions of organic and aqueous character. In this work, nanopores fabricated in borosilicate glass or silicon nitride connected a predominantly organic solvent to an aqueous solvent, thereby generating a mixing zone between these solutions inside the pore. This configuration was exploited to precipitate small organic molecules with low aqueous solubility inside the nanopores, and concomitantly, to monitor this precipitation by the decrease of the ionic conductance through the nanopores over time. Hence, this method provides a means to induce and investigate the formation of nanoprecipitates or nanoparticles. Interestingly, precipitates with a slight electric charge were cleared from the pore, causing the conductance of the pore to return to its original value. This process repeated, resulting in stable oscillations of the ionic current. Although such oscillations might be useful in fluidic logic circuits, few conditions capable of generating oscillations in ionic currents have been reported. The frequency and amplitude of oscillations could be tuned by changing the concentration of the precipitating molecule, the pH of the electrolyte, and the applied potential bias. In addition to generating oscillations, nanopores that separate two different solutions may be useful for monitoring and mediating chemical reactions in the mixing zone between two solutions.

Electron density guided fragment-based lead discovery of ketohexokinase inhibitors

A fragment-based drug design paradigm has been successfully applied in the discovery of lead series of ketohexokinase inhibitors. The paradigm consists of three iterations of design, synthesis, and X-ray crystallographic screening to progress low molecular weight fragments to leadlike compounds. Applying electron density of fragments within the protein binding site as defined by X-ray crystallography, one can generate target specific leads without the use of affinity data. Our approach contrasts with most fragment-based drug design methodology where solution activity is a main design guide. Herein we describe the discovery of submicromolar ketohexokinase inhibitors with promising druglike properties.

A modified physiological BCS for prediction of intestinal absorption in drug discovery

In this study, the influence of physiologically relevant media on the compound position in a biopharmaceutical classification system (BCS) which resembled the intestinal absorption was investigated. Both solubility and permeability limited compounds (n = 22) were included to analyze the importance of each of these on the final absorption. Solubility was determined in three different dissolution media, phosphate buffer pH 6.5 (PhB 6.5), fasted state simulated intestinal fluid (FaSSIF), and fed state simulated intestinal fluid (FeSSIF) at 37 °C, and permeability values were determined using the 2/4/A1 cell line. The solubility data and membrane permeability values were used for sorting the compounds into a BCS modified to reflect the fasted and fed state. Three of the seven compounds sorted as BCS II in PhB 6.5 (high permeability, low solubility) changed their position to BCS I when dissolved in FaSSIF and/or FeSSIF (high permeability, high solubility). These were low dosed (20 mg or less) lipophilic molecules displaying solvation limited solubility. In contrast, compounds having solid-state limited solubility had a minor increase in solubility when dissolved in FaSSIF and/or FeSSIF. Although further studies are needed to enable general cutoff values, our study indicates that low dosed BCS Class II compounds which have solubility normally restricted by poor solvation may behave as BCS Class I compounds in vivo. The large series of compounds investigated herein reveals the importance of investigating solubility and dissolution under physiologically relevant conditions in all stages of the drug discovery process to push suitable compounds forward, to select proper formulations, and to reduce the risk of food effects.

What is modulating solubility in simulated intestinal fluids?

The aim of this study was to understand which parameters are responsible for the selective modulation of compounds solubility in simulated intestinal fluids. The solubility of 25 chemically diverse reference compounds was measured in simulated intestinal fluid (FaSSIF-V2) and in aqueous phosphate and maleate buffers. Electrostatic interactions between compounds and the bio-relevant medium components seem to explain the different solubility behavior observed for acids and bases. The solubility of ionized acids is not increased in FaSSIF-V2 probably due to electrostatic repulsions with the media components. Lipophilicity plays an important role but mainly for charged bases with a logP>4 (or logD(6.5)>1.9). When the aqueous solubility is mainly driven by lipophilicity, the FaSSIF-V2 components seem to improve the solubility of basic compounds to a greater extent than for compounds whose solubility is limited by crystal packing. These results suggest that ionization, lipophilicity and crystal packing play important but peculiar roles in controlling solubility in FaSSIF-V2 compared to that in aqueous buffer and this information could be useful to guide medicinal chemists and formulation scientists.

Chitosan-based nanostructures: a delivery platform for ocular therapeutics

Nanoscience and nanotechnology has caused important breakthroughs in different therapeutic areas. In particular, the application of nanotechnology in ophthalmology has led to the development of novel strategies for the treatment of ocular disorders. Indeed, the association of an active molecule to a nanocarrier allows the molecule to intimately interact with specific ocular structures, to overcome ocular barriers and to prolong its residence in the target tissue. Over the last decade, our group has designed and developed a delivery platform based on the polysaccharide chitosan, which suits the requirements of the topical ocular route. These nanosystems have been specifically adapted for the delivery of hydrophilic and lipophilic drugs and also polynucleotides onto the eye surface. The results collected up until now suggest the potential of this delivery platform and the subsequent need of a full preclinical evaluation in order to satisfy the specific regulatory demands of this mode of administration.

Improvement of pharmacological properties of irreversible thyroid receptor coactivator binding inhibitors

We have previously reported the discovery and preliminary structure activity relationships of a series of beta-aminoketones that disrupt the binding of coactivators to TR. However, the most active compounds had moderate inhibitory potency and relatively high cytotoxicity, resulting in narrow therapeutic index. Additionally, preliminary evaluation of in vivo toxicology revealed a significant dose related cardiotoxicity. Here we describe the improvement of pharmacological properties of thyroid hormone receptor coactivator binding inhibitors. A comprehensive survey of the effects of substitutents in key areas of the molecule was carried out based on mechanistic insight from the earlier report. This study revealed that both electron withdrawing and hydrophobic substituents on the aromatic ring led to higher potency. On the other hand, moving from an alkyl to a sulfonyl alkyl side chain led to reduced cytotoxicity. Finally, utilization of amine moieties having low pK(a)'s resulted in lowered ion channel activity without any loss of pharmacological activity.

Initial salt screening procedures for manufacturing ibuprofen

The aim of this paper is to design initial salt screening procedures for manufacturing ibuprofen. Salt forms of a pharmaceutical acid racemic (R,S)-(+/-)-ibuprofen and their "developable" synthetic routes were ferreted out simultaneously through the screening of seven bases of sodium hydroxide, potassium hydroxide, L-arginine, L-histidine, L-lysine, diethanolamine, and tris(hydroxymethyl)aminomethane (THAM), and the match with the use of nine organic solvents of methanol, dimethyl sulfoxide, ethanol, N, N-dimethylformamide, acetonitrile, isopropyl alcohol, 1,4-dioxane, acetone, and tetrahydrofuran mainly in the presence of water in 20 mL scintillation vials. Racemic (R,S)-(+/-)-sodium ibuprofen dihydrate, a well-known ibuprofen salt and the newly discovered racemic (R,S)-(+/-)-THAM ibuprofen, appeared as white-squared powders with a molecular weight of 327.42 g/mol, a melting point of 160.17 degrees C, and the apparent solubility product, K'(sp), of 6.0 x 10(-4) M(2) at 25 degrees C were successfully synthesized by the initial salt screening methods. The new amine salt of ibuprofen was monoclinic and had a space group of P2(1)/c and lattice parameters of a = 17.578(8) degrees, b = 10.428(4) degrees, c = 9.991(4) A, alpha = 90.00 degrees , beta = 97.17(1) degrees, gamma = 90.00 degrees, and V = 1,817.05(244) A(3). The aspect ratio of the amine salt crystals of ibuprofen of approximately 1.0 implied that the crystals had a better flowability than the sodium salt counterparts. This amine salt of ibuprofen was more stable in moist or dried atmospheres and was more hydrophobic than the sodium salt of ibuprofen. Moreover, the slow dissolution of this amine salt of ibuprofen might have made it less bitter and more suitable as a sustained release drug than the sodium salt of ibuprofen. The future work is to search for the different polymorphs of this amine salt of ibuprofen and to extend the initial salt screening working logics to the formation of co-crystals.

Advances in improving the quality and flexibility of compound management

The process of drug discovery has evolved considerably since the advent of high-throughput screening (HTS) in the 1980s. Experts and opinion leaders today are agreeing that the current trend in the field is a focus on increasing overall quality (target, screening, and compounds), use of multiple screening approaches for lead discovery, and more flexibility in the process. The associated need for increased flexibility and quality control to support existing HTS paradigms as well as lower throughput approaches such as fragment screening, computational chemistry, focused library building, and centralized lead optimization support has required an evolution in compound management (CM, aka sample management or library management). Although there is much less published peer-reviewed data in CM, due to its historical links to HTS, it has followed very similar trends. In recent years, the focus in CM has been increasingly in compound quality and increased flexibility of the process, as opposed to number of compounds dispensed and speed of dispensing, which were standard metrics and indicators used not so long ago. Ideally, to screen the highest quality sample for every assay, one would start with a correct identity and pure solid, make a correct concentration solution in water or water-soluble/assay-compatible solvent that would allow 100% solubilization, and screen it immediately in a biological assay. Neither CM nor screening has advanced sufficiently to deliver this ideal scenario, but many significant advancements have been made in recent years both in terms of quality of compounds in stores and flexibility of the process, which will be reviewed herein.

Applications of physiologically based absorption models in drug discovery and development

This article describes the use of physiologically based models of intestinal drug absorption to guide the research and development of new drugs. Applications range from lead optimization in the drug discovery phase through clinical candidate selection and extrapolation to human to phase 2 formulation development. Early simulations in preclinical species integrate multiple screening data and add value by transforming these individual properties into a prediction of in vivo absorption. Comparison of simulations to plasma levels measured after oral dosing in animals highlights unexpected behavior, and parameter sensitivity analysis can explore the impact of uncertainties in key properties, point toward factors which are limiting absorption and contribute to assessment of compound developability. Physiological models provide reliable prediction of human absorption and with refinement based on phase 1 data are useful guides to further market formulation development. Improvements in the accuracy of simulations are expected as better in vitro methods generate more in vivo relevant solubility and permeability data, and modeling will play a central role in the development of more predictive methods for transporter-related effects on drug absorption.

Principles of encapsulating hydrophobic drugs in PLA/PLGA microparticles

Injectable biodegradable and biocompatible copolymers of lactic and glycolic acid (PLGA) are an important advanced delivery system for week-to-month controlled release of hydrophobic drugs (e.g., from biopharmaceutical classification system class IV), which often display poor oral bioavailability. The basic principles and considerations to develop such microparticle formulations is reviewed here based on a comprehensive study of papers and patents from the beginnings of hydrophobic drug encapsulation in polylactic acid and PLGA up through the very recent literature. Challenges with the diversity of drug properties, microencapsulation methods, and organic solvents are evaluated in light of the precedence of commercialized formulations and with a focus on decreasing the time to lab-scale encapsulation of water-insoluble drug candidates in the early stage of drug development. The influence of key formulation variables on final microparticle characteristics, and how best to avoid undesired microparticle properties, is analyzed mechanistically. Finally, concepts are developed to manage the common issues of maintaining sink conditions for in vitro drug release assays of hydrophobic compounds. Overall, against the backdrop of an increasing number of new, poorly orally available drug entities entering development, microparticle delivery systems may be a viable strategy to rescue an otherwise undeliverable substance.

Estimating drug solubility in the gastrointestinal tract

Solubilities measured in water are not always indicative of solubilities in the gastrointestinal tract. The use of aqueous solubility to predict oral drug absorption can therefore lead to very pronounced underestimates of the oral bioavailability, particularly for drugs which are poorly soluble and lipophilic. Mechanisms responsible for enhancing the luminal solubility of such drugs are discussed. Various methods for estimating intra-lumenal solubilities are presented, with emphasis on the two most widely implemented methods: determining solubility in fluids aspirated from the human gastrointestinal tract, and determining solubility in so-called biorelevant media, composed to simulate these fluids. The ability of the biorelevant media to predict solubility in human aspirates and to predict plasma profiles is illustrated with case examples.