In the never-ending
endeavor to produce stable and efficacious
protein therapeutics, biopharmaceutical companies often employ numerous
analytical techniques to characterize and quantify a drug candidate’s
stability. Mass spectrometry, due to the information-rich data it
produces, is commonly used in its numerous configurations to ascertain
chemical and structural stability. At issue is the comparison of the
various configurations utilized, that is, comparing bottom-up methods
such as proteolytic digest followed by reversed phase LC-MS with intact
LC-MS methods. Similar issues also arise when using capillary isoelectric
focusing to see how charge variants change over time, that is, monitoring
the progression of charge altering modifications like deamidation.
To this end, site-specific degradations as quantified from bottom-up
methods like peptide mapping can be used to build reconstructions
of both theoretical intact mass spectra as well as theoretical electropherograms.
The result can then be superimposed over the experimental data to
qualitatively, and perhaps quantitatively, evaluate differences. In
theory, if both experimental bottom-up data and intact data are accurate,
the theoretical reconstruction produced from the bottom-up data should
perfectly overlay with that of the experimental data. Valuable secondary
information can also be ascertained from reconstructions, such as
whether modifications are stochastic, as well as a detailed view of
all possible combinations of modifications and their quantities used
in the reconstruction. This comparison is also useful in determining
unknown mass differences in deconvoluted intact protein spectra that
may be a result of multiple modifications in combination. The comparison
of data from alternate sources provides a holistic and more comprehensive
view of the molecule under study.