Research Articles
Development of a partially automated solubility screening (PASS) assay for early drug development

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ABSTRACT

A medium‐throughput, compound‐saving, thermodynamic solubility assay for early drug development was developed. Solid compound suspended in heptane was used for simple, time‐saving, and flexible compound distribution into 96‐well plates, with minor risk to generate new physical forms during dispensing. Low volume, well‐stirred incubation vessels were generated by using a combination of V‐shaped wells, well caps, and vertically inserted stir bars. This allowed solubility determination up to 100 mg/mL in 40–80 µL volumes in aqueous and nonaqueous, low‐ and high‐viscosity solvents. After removal of residual solid through syringe filters mounted on microtiter plates, the filtrate was quantified by ultra performance liquid chromatography (UPLC) using a 1.2 min gradient. Combined with a robotic liquid handling system, throughput was 45 samples per hour and >600 solubility measurements per week. Results from the partially automated solubility screening (PASS) assay correlated well with reported solubility values (r2 = 0.882). The PASS assay is useful for compound‐saving, thermodynamic solubility measurement at the discovery–development interface where maximal solubility in many commonly used solvents needs to be determined. PASS results provide a basis for the identification of formulation strategies, the selection of appropriate excipients, and for the prediction of the potential in vivo behavior of compounds. © 2007 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 1748–1762, 2007

Section snippets

INTRODUCTION

Drug solubility is a crucial factor for absorption in the gastrointestinal tract, and compounds with too low aqueous solubility often have inappropriate pharmacokinetic properties and carry a higher risk to fail during early or late development. Therefore, phase appropriate solubility measurements are performed along the drug discovery and development process, the assays and their focus varying with the phase.

In drug discovery, in silico prediction and high‐throughput screening (HTS) of aqueous

Materials

Sodium dihydrogen phosphate, ethanol, sodium dodecyl sulfate (SDS), polyethylene glycol 400 (PEG 400), propylene glycol, Cremophor EL®, Tween 80®, olive oil, Danazol, ketoconazol, and sodium taurocholate were obtained from Sigma‐Aldrich Chemie GmbH (Deissenhofen, Germany). Digoxin was purchased from Fluka (Buchs, Switzerland). Mefenamic acid was kindly provided by Amphray Laboratories (Mumbai, India). L‐alpha‐egg lecithin as Lipoid EPC 98% pure was obtained from Lipoid (Ludwigshafen, Germany).

Compound Distribution into 96‐Well Microtiter Plates

A low‐tech, manual method was established for rapid and flexible distribution of compounds into 96‐well plates (Fig. 1). In a typical experiment, crystalline compound was dispersed in heptane at a concentration of 25 mg/mL and, while constantly stirring, 20–160 µL volumes were transferred into 96‐well plates using Gilson positive air‐displacement pipettes. Alternatively, amorphous compound was dissolved in organic solvent or suspended in heptane. Solvent or heptane was removed by evaporation in

DISCUSSION

The described PASS assay is adapted to the needs of a modern drug discovery–development interface setting with respect to compound consumption, volume, speed, and throughput. Compound is distributed into individual wells as suspension in heptane and heptane is rapidly removed by evaporation. This approach is cheap, fast, flexible, and avoids the time‐ and labor‐intensive process of weighing of hundreds of samples at submilligram quantities into 96‐well plates, if an automated weighing station

Acknowledgements

We gratefully acknowledge R. Raso for performing the solubility studies.

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    Eva Meister's present address is Department of Pharmaceutics, University of Erlangen‐Nuremberg, 91058 Erlangen, Germany.

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