Aqueous Solubility from DMSO Solutions

Kinetic solubilities are by definition very time dependent and as such results can be less reproducible than thermodynamic solubility values. The short timescale also means that they are more dependent on the physical form of the initial precipitate. Consequentially, the correlation between kinetic and thermodynamic solubility is generally poor, with the kinetic measurement usually giving higher values [12, 16]. However, an advantage this can bring is that there will be few compounds excluded as false negatives in this phase. 

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In general the rate of false negatives are by definition difficult to ascertain. There are two general approaches to get a handle on false negatives. The first approach is based on what is known about the aqueous solubility of screening compounds since truly active compounds out of solution are the most common cause of false negatives. One can infer that perhaps 15% of true positives will be missed in an HTS. This inference comes from an analysis of the concordance or lack of concordance between nominal concentrations in DMSO stocks and nominal... 

AMCA-hydrazide is soluble in DMSO or DMF and may be dissolved as a concentrated stock solution in either of these solvents prior to the addition of a small aliquot to an aqueous reaction medium. The solid and all solutions made from the fluorophore must be protected from light to avoid photo-decomposition. Prepare the stock solution fresh immediately before use. A suggested protocol on the use of this fluorescent probe may be obtained from the following method on the labeling of periodate-oxidized IgG. Optimization may be necessary to achieve the best level of fluorescent modification (F/P ratio) for a particular application. 

In the early phase the solid state of discovery compounds is usually not characterized and powders are often not crystalline. When starting with stock solutions the solid material obtained after evaporation of DMSO is mostly amorphous. However, there is evidence of crystallization upon incubation in the aqueous medium if the incubation time is long enough [17]. It has been reported that solubility data obtained from DMSO stock solutions are getting dose to the values obtained from crystalline material after 20 h equilibration [17]. Quantitative aspects of solubility/dissolution are discussed in details in Chapter 4. 

High throughput in-well sonication has been applied to dissolve compounds that are insoluble in DMSO in 96, 384, and 1536 well formats (Oldenburg et ah, 2005). Compounds that precipitated from DMSO stocks, due to either water uptake that reduced solubility or low instrinsic solubility that promoted crystallization, can be re-dissolved by low energ) sonication. Sonication can accelerate compound dissolution in seconds and, in some cases, drive the solution to super saturation, due to energy input and elevated temperature. This process can bring many precipitates back into solution and has no effect on compound stability. Sonication of DMSO stocks or concentrated aqueous solutions can improve HTS hit rates and enhance biological assay results. 

The quantitation of solubility in the current discovery setting differs markedly from that of a traditional aqueous solubility assay. In the traditional solubility assay the crystalline well-characterized solid is equilibrated in aqueous solution for sufficient time to reach equilibrium (generally 24-48 hours). The solution composition is important in the comparison with a discovery solubility assay. The traditional thermodynamic solubility experiment is performed in the absence of any organic co-solvent. By comparison the discovery solubility assay is frequently performed in the presence of a co-solvent, usually DMSO. The physical state of the starting solid in the development thermodynamic assay is relevant to the comparison with discovery solubility assays. As was previously mentioned, the solids in discovery today are frequently noncrystalline and therefore invariably more aqueous soluble. 

Absolute molecular weight of 36 kD was determined by membrane osmometry in toluene. Surprisingly, the final polymer was not soluble in a basic aqueous solution, but it was soluble in DMSO. In a subsequent report from the same research group [104,105] this synthetic approach was extended to produce other isomeric structures. 

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