Journal of Analytical & Bioanalytical Techniques
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Lately growing attention has been dedicated to the occurrence
of back-conversion, intended as the transformation of a drug metabolite to the
parent compound during samples handling. Back-conversion brings to inaccurate
quantitative determinations and EMA is imposing the validation of back-conversion
in analytical methods for bioequivalence (Guideline on Bioequivalence 2010).
While it is clear now that validation procedures shall check back-conversion, other
methods than the use of biological samples spiked with metabolite standards, are
unavailable. Unfortunately the metabolism of several well-known drugs is unclear
and new molecules are coming: what to do? A validation approach based on
incurred samples of subjects treated with studied drugs was developed; the results
collected on 20 drugs will be presented.
Materials and methods:
In all validations, HPLC-MS/MS and reversed phase
or ion exchange columns separations were employed. These data compare the
evaluation of back-conversion by classical procedures (spiked samples) with the
findings gathered using incurred samples analyzed soon after sampling, then after
different period/conditions of storage.
Results and conclusions:
With one drug, only the incurred samples permitted
to validate back-conversion and with another-one they allowed to perform a
validation when metabolite standards were unavailable. In all other cases both
approaches gave similar results; it never occurred that incurred samples results
were contradictory with those based on spiked samples. In conclusion the incurred
samples approach is very useful, if not the only one in some cases, to adequately
validate analytical methods for back-conversion.
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