Drugs overlapping peaks

If reference materials of the suspected impurities are available, the drug substance or finished drug product should be spiked at an appropriate level to demonstrate that the result is unaffected by the addition of the impurity. Figure 2 shows examples of individual chromatograms of the API and three known process impurities. As shown here, none of the three process impurities interfere with the API peak, although peaks for impurities A and C appear to overlap and could co-elute if both were present in the sample. This specificity may be acceptable if the method was designated as an assay method for the quantitation of the API. For a method intended to quantitate process impurities, the overlap of these two components would in most cases be unacceptable due to the inability of the method to accurately measure the two individual components. 

When matrix effect exists, it is usually preferable to coeluate the analyte and its internal standard to better reduce the impact of matrix effect on quantitation. The more their chromatographic peaks overlap, the better the correction is. Since the concentration of the analyte varies while the amount of IS added is constant, a choice must be made as to match which part of a calibration curve. Usually, the segment between 1/3 and 1/2 of the ULOQ is most important because this segment is expected to cover the average Cma for most drugs and metabolites. This is probably why other researchers have proposed to use IS concentrations around 1/3 or 1/2 of the ULOQ of an analyte. 

The cases of peak delivery at 10 a.m. (Fig. 10.5b) or 10 p.m. (Fig. 10.5d) are intermediate between the two preceding cases. Overlap between the peak of 5-FU and the peak of cells in S phase is only partial, but it is still greater in the case of the peak at 10 a.m., so that this pattern is the second most toxic, followed by the circadian delivery centered around 10 p.m. The comparison of the four panels Fig. 10.5a-d explains the results of Fig. 10.4a on the marked differences in cytotoxic effects of the four 5-FU circadian delivery schedules. The use of the cell cycle automaton helps clarify the dynamic bases that underlie the distinctive effects of the peak time in the circadian pattern of anticancer drug delivery. 

The cell cycle automaton model permits us to clarify the reason why circadian delivery of 5-FU is least or most toxic when it peaks at 4 a.m. or 4 p.m., respectively. Indeed, the model allows us to determine the position of the peak in S-phase cells relative to that of the peak in 5-FU. As shown in Fig. 10.5, 5-FU is least cytotoxic when the fraction of S-phase cells oscillates in antiphase with 5-FU (when 5-FU peaks at 4 a.m.) and most toxic when both oscillate in phase (when 5-FU peaks at 4 p.m). Intermediate cytotoxicity is observed for other circadian patterns of 5-FU (when the drug peaks at 10 a.m. or 10 p.m.), for which the peak of 5-FU partially overlaps with the peak of S-phase cells. For the continuous infusion of 5-FU, the peak in S-phase cells necessarily occurs in the presence of a constant amount of 5-FU. Hence, the constant delivery pattern is nearly as toxic as the circadian pattern peaking at 4 p.m. 

Typically, excipients found in drug formulations resonate in limited spectral regions. Most excipient peaks can be found between 60 and 100 ppm, and as such do not completely overlap the active ingredient resonances. Spectral regions containing only drug signals can often be found even in complex formulation. 

This section illustrates the resolution of components in strongly overlapping CE peak is described. Data correspond to a capillary zone electrophoresis (CZE) method for the determination of an antihistaminic drug and its... 

The optimization of mobile phase composition should take into account not only the retention of the compound to be analyzed, but also the retention of the matrix. The determination of anticancer 6-thiopurine drugs and Iheir metabolites in untreated serum is a usefiil example (Fig. 11.6) [ 19]. With a mobile phase of 0.04 M SDS in 0.01 M phosphate buffer at pH 2.2, the blank serum produced a background response that had completely eluted after 6 min, with the exception of a peak at 8 min. The peaks of 6-thioguanidine riboside and 6-thioguanine were well resolved, but unfortunately, the three earlier-eluted compounds (6-mercaptopurine riboside, 6-thioxanthine and 6-mercaptopurine) were overlapped by the matrix peaks. A lower pH (2.0) permitted complete separation of 6-mercaptopurine from the serum background signal. Under these conditions, 6-thioguanine eluted too late (at around 40 min). 

Richer information from variable temperature and hetero-nuclear re-laxometry (e.g., F Ti/rinearest neighbor distance) Molecular mechanism of miscibihty from 2D-correlation contours (strength, i.e., distance of drug-polymer H-bonding) Convoluted spectral profile of partially crystalline dispersions due to overlapping sharp signals of crystalline fraction and the broad peaks of the amorphous fraction Expensive, time and resource intensive technique... 

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