If you’ve done method development
for a pharmaceutical impurities analysis, you know it can be a tricky endeavor.
I find there are two main obstacles to deal with. The first issue is the
sensitivity. Impurities are typically present in low levels in the formulation,
yet must be adequately quantitated to ensure safety to the consumer. In order
to obtain the required sensitivity, it may likely be necessary to overload the
main peak. If a gradient is used, interference from the baseline can also be a
problem as well in detecting impurity peaks. The second issue pertains to the
chromatographic resolution. Impurities tend to share many structural
similarities with the API, as they are often side-products from the synthesis,
degradants, etc., and therefore exhibit similar chromatographic behavior. As
such, a traditional C8 or C18 column may not be enough to obtain separation.
I used a Cogent UDC-Cholesterol™ column in
this case, which I found was necessary to separate promethazine from the
impurity isopromethazine. Shape selectivity from the UDC moiety can help
separate isomers in many instances. As for the problem of sensitivity, I found
it hard to observe the phenothiazine peak, which was the last to elute, due to
the sloping baseline from the gradient. Here, I chose to switch the wavelength
to a local UV max of 320 nm, in which interference from the baseline is
negligible.
I used this method, as well as a
separate isocratic assay, for quantitation of the API and the specified
degradant promethazine sulfoxide. Further assessment of the methods was
conducted by comparing the data to established system suitability criteria. I
found the methods to be reliable for routine analyses of promethazine
formulations in a QC environment.
