Finally, with respect to drug stability and associated analytical
methods, mass balance is defined by the International Conference
on Harmonization (ICH) as ‘‘the process of adding together the assay
value and levels of degradation products to see how closely
these add up to 100% of the initial value, with due consideration of
the margin of analytical error’’ [3]. It is this latter definition of mass
balance, in the context of drug-product degradation, which is the
focus of this paper.
The ICH definition of mass balance is deceptively simple.
Although it appears straightforward, its practical application requires
careful consideration of what is being measured. For example,
assay values and levels of degradation products are commonly
measured by HPLC with UV absorbance detection at a particular
wavelength, in which case the analytical measurement is typically
peak area. If a given decrease in peak area of the API is matched by
a corresponding increase in peak areas of degradation products,
then mass balance is often presumed. If, on the other hand, the
changes in peak areas are dissimilar, then a mass imbalance is concluded.
Both conclusions are presumptuous and potentially erroneous
if inherent assumptions are not examined. Mass-balance
determination by simple comparison of peak areas is based on
the assumption that all reactants and degradation products are detected
as peaks and that the peak areas are uniformly proportional
to the actual amount of each compound present. This assumption
is more often than not incorrect and can lead to an inaccurate
assessment of mass balance
Finally, with respect to drug stability and associated analytical
methods, mass balance is defined by the International Conference
on Harmonization (ICH) as ‘‘the process of adding together the assay
value and levels of degradation products to see how closely
these add up to 100% of the initial value, with due consideration of
the margin of analytical error’’ [3]. It is this latter definition of mass
balance, in the context of drug-product degradation, which is the
focus of this paper.
The ICH definition of mass balance is deceptively simple.
Although it appears straightforward, its practical application requires
careful consideration of what is being measured. For example,
assay values and levels of degradation products are commonly
measured by HPLC with UV absorbance detection at a particular
wavelength, in which case the analytical measurement is typically
peak area. If a given decrease in peak area of the API is matched by
a corresponding increase in peak areas of degradation products,
then mass balance is often presumed. If, on the other hand, the
changes in peak areas are dissimilar, then a mass imbalance is concluded.
Both conclusions are presumptuous and potentially erroneous
if inherent assumptions are not examined. Mass-balance
determination by simple comparison of peak areas is based on
the assumption that all reactants and degradation products are detected
as peaks and that the peak areas are uniformly proportional
to the actual amount of each compound present. This assumption
is more often than not incorrect and can lead to an inaccurate
assessment of mass balance
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