Troubleshooting stability

The level 3 method is used until synthetic routes and formulations have been finalized and forced degradation and preliminary stability studies have been conducted i.e., until the components that need to be separated in the final DS and in the final DP have been clearly determined. At this juncture, the focus shifts to the development of fast, robust and transferable final methods to be used for primary stability studies and post-approval analyses. Freqnently, separate methods are developed for DS and DP since the goals of each method are different (see Section I). Orthogonal methods continue to be of importance to troubleshoot any questions that may arise during the subsequent life cycle of the drug. 

In general, cocoa butter is considered a stabilizing fat and all others nonstabilizing fats. For more detail on this issue, see Critical Parameters and Troubleshooting. 

When inadequate in vivo exposure or PK properties are observed, pharmaceutical profiles can be used to troubleshoot the cause of poor in vivo exposure or PK [26]. Assays for solubility, permeability, and stability (metabolic, plasma, acid) can help to track down the inadequate properties responsible for poor in vivo performance. Property optimization synthesis can then be initiated. Subsequent series analogs can be assayed to rank order compounds by properties for subsequent in vivo tests, in order to give the highest likelihood of success. Often animal studies are expensive and time-consuming, especially if they are performed using the animal activity model. Simple in vitro profiling assays can provide information for improved decisions and efficiency. 

It should be pointed out that, for some electrodes, chemical etching is not sufficient to obtain the optimal surface roughness for SERS, as shown in Fig. 16. One may consider further SERS activation to chemically etched electrodes by ORC, either in the chemical etching solution (ex situ) or in the measured solution (in situ). However, the electrode treated by the in situ ORC, in general, has the higher activity, but poorer stability, as the intensity deceases with time irreversibly. A detailed discussion of how to deal with this problem will be given in the Sect. 3.6.3.7.4 on Troubleshooting. 

Various in vitro assays are widely available for profiling distribution, metabolism, and pharmacokinetics (DMPK, also referred to as ADME absorption, distribution, metabolism, and excretion). Such properties of molecules are measured to ultimately predict their in vivo behavior. The metabolic stability of molecules is assessed routinely in drug discovery units by way of medium- to high-through-put assays using hepatic microsomes or hepatocytes obtained from different species (usually rat and/or human). Permeability assays (e.g., utilizing Caco-2 or MDCK cells) together with an assessment of efflux potential are also useful to troubleshoot unexpectedly low cell activity or can help select candidates for subsequent in vivo studies. 

Other polar phases, i.e., silicas with covalently bonded nitrile, nitro, amine, or diol groups, are valuable alternatives to silica. They are less polar (hence the mobile phase can be weaker) and have different selectivities. However, in most cases, they are used in the reversed-phase mode with aqueous mobile phases. Normal-phase applications of some importance are the separation of aromatics on nitro-derivatized phases, and of non-ionic surfactants on diol-derivatized phases. The theoretical plate numbers are usually lower than with non-derivatized sUica. Activity control (see Section 2.2.4 on troubleshooting of NP separations) is not necessary. The stability of the chemical bonding can be limited. 

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