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Physicochemical properties
Solubility
Solubility is one of the most important physicochemical properties when evaluating compounds as potential drug candidates. The generation of valid in vitro test data relies upon the compound being in solution under the appropriate experimental conditions. For in vivo studies, poor solubility will make dose formulation difficult, particularly for intravenous dosing, presenting problems for reliable determination of pharmacokinetic or pharmacodynamic parameters and potentially resulting in adverse events. Solubility is an important factor in drug absorption from the intestine and it may be difficult to obtain good oral bioavailability for compounds with poor solubility.
Consequently, an assessment of compound solubility, and whether this is sufficient for the subsequent planned testing regime and the intended Product Profile, should be obtained at the earliest stages of the drug discovery process.
Solubility assays available at BioFocus
Kinetic solubility is typically a measure of compound solubility in aqueous buffer when added from an existing stock solution in DMSO. Kinetic solubility measurements are designed to determine if compounds possess sufficient solubility to produce reliable data in future biological and ADME testing. As solid test material is not used, direct correlation between thermodynamic aqueous solubility and kinetic solubility measured for a compound series is not often seen. However, it is considered that good solubility in a kinetic assay (in the upper range of the assay described here) should facilitate the generation of reliable in vitro and in vivo data in drug discovery.
Thermodynamic solubility is performed using the solid crystalline form of a compound and defines the maximum amount of compound in solution once equilibrium has been reached. Such studies are typically performed during the latter stages of drug discovery or early development and may help to design formulation strategies for future in vivo studies in animals and man.
The standard protocols are shown here and BioFocus is happy to modify these conditions, such as modification of the aqueous buffer used in the kinetic assay format, to suit individual customer requirements.
Additional physicochemical assays, such as LogD measurements , available at BioFocus can provide information on a whether a compound is likely to be ionized at physiological pH values.
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Standard kinetic solubility protocol
| Compound concentration |
200 µM or 50 µM |
| Dynamic range |
200 µM to 10 µM or 50 µM to 1 µM |
| Aqueous buffer |
0.1 M phosphate buffered saline pH 7.4 (2% DMSO) |
| Mixing period |
2 hours at room temperature |
| Sample preparation |
Filtration |
| Analysis |
LC-UV with mass confirmation by mass spectrometry |
| Control compounds |
Hydrocortisone and reserpine |
| Compound requirements |
40 µL of 10 mM DMSO solution |
Standard thermodynamic solubility protocol
| Compound concentration |
1 mg/mL |
| Aqueous buffer |
0.1 M phosphate buffered saline pH 7.4 |
| Mixing period |
24 hours at room temperature |
| Analysis |
LC-UV with mass confirmation by mass spectrometry |
| Compound requirements |
2 times 1 mg for each assay condition plus 1 mg for preparation of calibration lines |
Kinetic solubility validation data
| Figure 1. Assay reproducibility, kinetic solubility. The validation data shown in figure 1 demonstrates the sustainability of this method to provide reproducible, high throughput, kinetic solubility measurements to determine if compounds posses sufficient solubility to produce reliable data in future biological and ADME testing. |
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