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compound stability under different storage conditions
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However, when we examined the 52 most unstable compounds, we found that for samples stored at 40 C versus 22 C, those exhibiting observable degradation peaks in the UV chromatogram following storage to have much higher decreases than a factor of 2 (Figure 13.5). A similar analysis of 45 unstable compounds without new UV peaks after storage showed a smaller difference in the rate of compound loss between 22 C and 40 C (Figure 13.6). It is possible that the change in sample concentration for the latter set of unstable compounds resulted from some other mechanism than thermal degradation, such as precipitation or absorption, and that these processes did not abide by the same Arrhenius kinetics as thermal degradation. Nevertheless, the our observations indicate a limitation to projecting the data obtained at elevated temperatures in predicting compound stability at lower temperatures, especially for large and diverse compound collections. As compound loss processes may have weak temperature dependence or different mechanisms of compound loss at different temperatures, recognition of such confounding factors will be important in the design and utility of large repository storage systems. Pure DMSO has a melting point of 18.4 C at normal atmospheric pressure. DMSO-solubilized compounds are often stored at or below 4 C in a frozen form to prolong their shelf life beyond a few months (e.g., 6 9
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1.2 Dry, 40 C 1 Concentration 0.8 0.6 0.4 0.2 0 0 5 10 15 Storage time (wk) Figure 13.5. Average concentration for 52 unstable compounds for which new UV peaks were observed after storage. 20 25 30 Wet, 40 C RT
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organic compound stability in large, diverse pharmaceutical
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Dry, 40 C 1 0.8 0.6 0.4 0.2 0 0 5 10 15 Storage time (wk) 20 25 30 Wet, 40 C RT
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Figure 13.6. Average concentration for 45 unstable compounds for which no new UV peaks were observed after storage.
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months), which is appropriate for room temperature storage. However, it is uncertain as to how different the stability would be for compounds stored in a frozen form at much lower temperatures than just below the DMSO freezing point (e.g., 4 C vs. -20 C vs. -80 C). Previous studies have indicated that compound storage at -20 C is preferable to 4 C, resulting in a more stable storage environment for the sample as the DMSO solution is presumably frozen. Heaton et al. studied 297 compounds with lidded plate seals and observed approximately 12% degradation at -20 C compared with 23% degradation at 4 C after six months of storage.24 However, there is uncertainty about the sample state at 4 C as the melting temperature of the DMSO solution will depend greatly on the concentration and physiochemical properties of the analytes, as well as level of impurities and water in the solution. In fact 4 C may be relatively close to the melting point of the DMSO solutions for many compounds depending on the analyte concentrations. Nevertheless, the preponderance of the available data indicates that storage as a truly frozen DMSO solution is generally much more stable than in liquid format and offers satisfactory retention of compounds for up to ve years when humidity and temperature are well controlled to minimize water uptake. In a subsequent study Heaton et al. observed only 3.5%
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compound degradation by LC/MS analysis for 342 compounds stored as DMSO solutions at -20 C for two years with proper sealing.24 Cheng et al. observed that compounds stored in DMSO at -80 C for approximately four years exhibited only 13% notable degradation in an HPLC analysis. Aside from the absolute temperature, an equally important, yet unaddressed factor in uencing the rate of compound loss is the rate at which the solubilized samples are frozen. The freezing rate has not been addressed in any of the aforementioned studies and the authors are not aware of work that has been presented or published on this issue or on subsequent compound integrity and precipitation rates. However, it should be noted that research on compound storage in combinatorial chemistry and high-throughput biological screening has only recently become important to the pharmaceutical industry, and therefore most published work has focused on predicting compound behavior within a framework of the existing storage systems and parameters. No doubt, as future studies focus more broadly on the theoretical aspects of compound integrity, the issue of freezing rates will be appropriately addressed. Humidity The presence of water can affect compound stability in several different ways. Water can participate or catalyze degradation reactions in solid or DMSO solution format. Additionally, in solid-state storage, water absorption can alter the morphology of the sample and the mobility of the molecules in the sample and thereby affect the rate of degradation or compound loss. In liquid storage, the presence of water can change the solubility of compounds in DMSO and, depending on the nature and concentration of the analyte in solution, cause compound precipitation. Savchuk et al. recently showed the effect of relative humidity (RH) on the stability of 2212 compounds selected from a collection of combinatorial library compounds stored as frozen DMSO solutions at 4 C.Their results indicated that after only one year of storage there was signi cant degradation in 2% of the compounds stored at 20% RH, while there was a marked increase in the level and number of compounds exhibiting signi cant degradation in compounds maintained at a similar storage temperature but at higher relative humidity, namely an increase from 2% to 20% degradation at 40% RH and an increase to 24% degradation at 60% RH.26 Additionally, Cheng et al. have demonstrated in an accelerated study conducted at 40 C that the presence of 5% water in the DMSO samples has a clear detrimental effect on compound stability compared with waterfree DMSO samples (Figure 13.4).9,19 The authors suggest that compounds
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