Bioequivalence and Bioavailability Forum

Dear Ohlbe!

❝ No guideline jumps to my mind on this topic.

Not quite; FDA (2001) states for chromatographic methods (http://www.fda.gov/cder/guidance/4252fnl.pdf Section IV.A. Selectivity, 2^nd paragraph):

'Potential interfering substances in a biological matrix include endogenous matrix components, metabolites, decomposition products, and in the actual study, concomitant medication and other exogenous xenobiotics. If the method is intended to quantify more than one analyte, each analyte should be tested to ensure that there is no interference.'

... and for ligand binding assays (Section V.A.1):

'Cross-reactivity of metabolites, concomitant medications, or endogenous compounds should be evaluated individually and in combination with the analyte of interest.'

ANVISA (2003) states (Sections 3.1.3/3.1.4):

The interferents may be components of the biological matrix, metabolites, decomposition products and drug products used concomitantly with the study.
If the method is intended for quantification of more than one drug, each one must be injected separately to determine the individual retention times and to ensure that the impurities of a drug do not interfere with the analysis of the other.

❝ What I would suggest...

Your suggestions are quite reasonable, but metabolites may make the life of the analyst quite miserable.
Since metabolites generally are more polar (less lipophilic) than the parent compound(s), they will show up in the commonly used RP-LC at earlier retention times. If analyzing combinations of drugs, interferences (let's say by the metabolite of a more lipophilic drug 1 with the less lipophilic parent drug 2) may occur. Looking at the parent compounds only may lead to surprises with 'real world' samples.
The ideal solution would be checking the retention times of metabolites, but since they may not be available...

• I think that some chromatographic prediction software is 'out there'.
  
• At least a thorough understanding of the polarity/ionisation state (pH <-> pK) of both the metabolite(s)
  
• and the extraction/separation technique should help.

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Edit: Link corrected for FDA’s new site. [Helmut]