Thermodynamics solubility measurements

This chapter is not simply a treatise on the highest capacity methods for measuring aqueous solubility in a discovery setting that most closely approximate a lower capacity thermodynamic solubility measurement. Rather it is the author s viewpoint that dealing with the problem of poor drug solubility in an early discovery setting requires an appreciation of recent changes in three related but distinct areas. The areas to be considered are ... 

We have studied the performance of several prediction methods to see how well in-house thermodynamic solubility measurements could be predicted. Among the prediction methods we studied were Huuskonen s method [26], ACD/Solubility DB [38], Meylan s method [21] as implemented in QMPRPlus, and the SimulationsPlus solubility prediction as implemented in QMPRPlus [39]. In general, we foimd the predictions to... 

In the lead identification and lead optimization phases of discovery, there is greater focus on thermodynamic solubility measurements. Thermodynamic solubility assays are designed to determine the solubility of the stable crystalline form of the compound, since this is the physical form that will be sought in the development phase for orally administered drugs. As such, thermodynamic solubilities provide discovery projects with a better risk assessment of likely formulation issues in development. Thermodynamic solubilities, unlike kinetic solubilities, are less dependent on the initial physical form of the compound and being less time critical also tend to be more reproducible. This is particularly important from a molecular design perspective where chemists are seeking to modify molecular structure to improve solubility. 

Another approach to increase the throughput of thermodynamic solubility measurement in discovery involves evaporating a DMSO stock solution to dryness at the start of the assay [12, 22, 31]. Aqueous buffer is added to the dried-down solid, which is then agitated for 24h followed by HPLC-UV or UV analysis of the supernatant. Removal of all the DMSO solvent ensures that the solubility value is not enhanced by... 

The thermodynamic solubility product for Pbl2 is determined in this experiment by measuring its solubility at several ionic strengths. 

Measuring compound remaining in solution has the advantage that the endpoint is quantitative and similar to that in a thermodynamic solubility assay. Within reasonable experimental parameters so that Beer s law is followed a UV absorbance is linearly related to concentration in solution. Measuring compound concentration in solution is very well established technology with a wealth of available instrumentation. 

In the following section, the calculation of the VolSurf parameters from GRID interaction energies will be explained and the physico-chemical relevance of these novel descriptors demonstrated by correlation with measured absorption/ distribution/metabolism/elimination (ADME) properties. The applications will be shown by correlating 3D molecular structures with Caco-2 cell permeabilities, thermodynamic solubilities and metabolic stabilities. Special emphasis will be placed on interpretation of the models by multivariate statistics, because a rational design to improve molecular properties is critically dependent on an understanding of how molecular features influence physico-chemical and ADME properties. 

The thermodynamic solubility of a drug is the concentration of the compound that is dissolved in aqueous solution in equilibrium with the undissolved amount, when measured at 25°C after an appropriate time period. Aqueous solubility has long been recognized as a key molecular property in pharmaceutical science. Drug delivery, transport and distribution phenomena depend on solubility thus, it is of considerable value to possess information of the solubility value of a drug candidate, to be able to predict the solubility for unknown compounds and, finally, to be able to modify the structure of a compound in order to modulate its solubility value in an appropriate manner. 

TABLE I Comparison of Solubility Measurements by UF-LC/MS,Thermodynamic Approach (TD), and Nephelometry ... 

With liquid feed solutions, however, it is possible to work in a manner analogous to traditional solvent extraction. Pressurized columns can be of the packed-bed type or agitated by magnetic stirrers. Because of the efforts of pilot plant tests, much of the scale-up work has to be carried out in laboratory extractors. From solubility measurements, it is possible to determine parameters in thermodynamic models (e.g., equations of state), which can be used for the simulation of large-scale applications. 

As an alternative to laboratory solubility measurements, solubility product constants (KSp), which are derived from thermodynamic data, can be used to calculate the solubility of solids in water (Table 2.9). Each solubility product constant describes a disassociation of a solid in water and calculates the activities or concentrations of the dissolution products in the saturated solution. The solubility product constant or another equilibrium constant of a reaction may be derived from the Gibbs free energy of the reaction (AG"K) as shown in the following equation ... 

Other important applications include the generation of a model to predict thermodynamic water solubility (Cruciani et al. 2003). This model is based on consistent solubility data from literature plus additional measurements for 970 compounds. Its quality allows to differentiate between very poorly/poorly/medium/ highly and very highly soluble molecules while exact rankings within individual classes are not possible. However, given the different factors influencing experimental thermodynamic solubility data, it is not likely that significantly improved models for this key property in pharmaceutical sciences can be derived. 

The actual in vitro measurements of thermodynamic solubility correspond to the idealized titration regime conditions only to some extent. The closest method seems to be a labor-intensive shake-flask experiment requiring relatively large amounts of a dry crystalline drag and long equilibration times. Moreover, each pH point requires a separate measurement in different buffer. Different buffers usually represent different ionic conditions and may lead to internal inconsistency of the solubility profile thus obtained. The buffer issue deserves an entire subsection of this review and will be discussed later. The real question is whether this investment of resources is worthwhile. The answer depends on who is asking the question. A drag-development... 

Solubility and speciation. Minimum requirements for reliable thermodynamic solubility studies include (i) solution equilibrium conditions (ii) effective and complete phase separation (iii) well-defined solid phases and (iv) knowledge of the speciation/oxidation state of the soluble species at equilibrium. Ideally, radionuclide solubilities should be measured in both oversaturation experiments, in which radionuclides are added to a solution untU a solid precipitates, and undersaturation experiments, in which a radionuchde solid is dissolved in aqueous media. Due to the difference in solubilities of crystalline versus amorphous solids and different kinetics of dissolution, precipitation, and recrystalhzation, the results of these two types of experiments rarely agree. In some experiments, the maximum concentrahon of the radionuchde source term in specific water is of interest, so the sohd that is used may be SF or nuclear waste glass rather than a pure radionuclide solid phase. 

DSC analysis represents a superior method of thermal analysis, in that the area under a DSC peak is directly proportional to the heat absorbed or evolved by the thermal event, and integration of these peak areas yields the enthalpy of reaction (in units of calor-ies/gram or Joules/gram). Even though conclusions reached on the basis of enthalpies of fusion are possibly compromised by their omission of the entropy contribution, an indication of the thermodynamic trends inherent in the system is often possible. For instance, the same polymorphic form of moricizine hydrochloride was deduced on the basis of thermal analysis and equilibrium solubility measurements. On the other hand, auranofin represents a compound for which one anhydrous polymorphic form is predicted to be the most stable by virtue of its melting point and heat of fusion but for which solubility measurements demonstrate that the other polymorph was in fact the thermodynamically stable form. ... 

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