The solubility of a compound varies with factors such as type of solid form, purity, solvent, temperature and pH Information on the solubility of a molecule therefore is a valuable tool in development activities such as:
- Evaluation of viable manufacturing processes for chemical and pharmaceutical products.
- Formulation work of drugs. Solubility is a key property for the drug absorption process in the body. Since most drugs are administered orally, sufficient solubility in the aqueous intestinal fluid is a prerequisite for pharmacological activity. Solubility data of the active compound is used in the preparation of a suitable formulation, for instance a tablet.
- Interpreting biological assay results. Poor aqueous solubility can give false negatives because of precipitation in vitro and cause variable bioavailability by incomplete absorption in vivo.
- Early drug development. The solubility data of drug candidates are used in molecular design and in the building of structure-activity relationships.
Figure 1. “Eidothea”, an automated solubility equipment, available at SPPD.
Solubility in different solvents and temperatures
Much processing of organic molecules is done in organic solvents. Then the solubility of the molecule in several organic solvents, at different temperatures, is vital to know. At SP Process Development we manually determine the solubility of a substance in 10 – 30 different organic solvents, or organic solvent – water mixtures, at room temperature. We then use two, 10 position thermal blocks, to automatically determine the solubility in some of these solvents, as a function of temperature (T). Using this, not commercially available system, difficult sampling at high temperatures is avoided.
For the automatic determination 3-5 slurry samples, with increasing amounts of solid in 1.0 mL of solvent, are prepared. They are then stirred, while T is increased in steps. The turbidity is monitored continuously and from the temperature of dissolution the solubility is determined. When all samples have been measured the solubility = f(T) is plotted (see Figure 2.).
We are presently testing an automated system for the determination of solubility at room temperature, using very small amounts of substance (1 – 2 mg) and drop-wise addition of solvent.
Figure 2. Solubility as a function of concentration and temperature.
Miniaturized solubility assay
Early knowledge of parameters such as solubility, exposure and acceptable toxicity profile of candidate drug is important in drug development. To obtain extended information in early development - when drug candidates are many, but milligrams are few - the solubility method must work with small amounts of material and preferably be automated.
At SP Process Development an improved methodology will be used for solubility measurement of small substance quantities. It is based on a thermodynamic aqueous solubility screen using <1 mg material, where 1) the drug is dissolved in DMSO, 2) the solutions are added to a 96-position well-plate and evaporated to dryness, 3) phosphate buffer pH7.4 at 22°C is added, 4) the samples are stirred for 24 hours at 22°C, 5) sampling is done via centrifuge and 6) the samples are analyzed with LC-UV-MS.
Advantages of this method include the use of automation-friendly, easily dispensable DMSO solutions - without suffering from the DMSO co-solvent effect- and low compound material consumption (<1 mg). A shortcoming is however the lack of information of the solid state of the dried down material, as well as for the solid in the sampled slurry. This knowledge is crucial and to address this issue, the method will be further developed by adding microscopy and/or Raman spectroscopy examination of the dried-down material, as well as of the precipitate in the sampled slurry.
Further possible modifications to the assay include running at lower pH and using simulated intestinal fluid instead of the original buffer solutions, allowing information of compounds’ solubility properties along the gastrointestinal tract.
Figure 3. Solubility in aqueous phosphate buffer in micro-titer format for 24h is analyzed by LC-UV-MS.