PharmPK Discussion - N-desmethyl sildenafil ISRA

• On 7 May 2011 at 22:21:06, "chandramohan.-a-.gmail.com" (mohancra.at.gmail.com) sent the message
  

  
  The following message was posted to: PharmPK
  
  Dear all,
  We have developed a Bioanalytical method for the determination of
  sildenafil and  N-desmethyl sildenafil. During the method validation
  all parameters were found to be well within the acceptable limits as
  mentioned in the USFDA guidelines.After study sample analysis ISRA
  (incurred sample reanalysis) was performed for both sildenafil and its
  metabolite N-desmethyl sildenafil. The ISRA results of sildenafil were
  found to be matching with initial concentration, but  ISRA results of
  N-desmethyl sildenafil has shown 20 to 50% increase in the
  concentration  when compared to initial concentration. Same bulk
  spiked Calibration standards and QCs were were used for both study
  sample analysis and ISRA. Calibration  standards and Quality control
  samples were comfortably passing for both sildenafil and N-desmethyl
  sildenafil. Using same samples second ISRA was performed.During second
  ISRA the values of sildenafil were matching with initial values, but
  N-desmethyl sildenafil results has shown 50 to 100% increase in the
  concentration when compared to the concentration of initial analysis.
  Using same bulk spiked calibration standards and QC samples ISRA was
  performed for 6 times on different days.  The concentration of
  sildenafil was found to be matching with initial analysis during all
  six times, but N-desmethyl sildenafil concentration was found to be
  increasing 20 to 50% during each analysis. The N-desmethyl sildenafil
  ISRA results of 5th and 6th time analysis were matched and  found to
  be with in 20%.
  Please share yours experience to resolve this problem.
  Freeze thaw stability has been proved for 6 cycles by spiking the
  sildenafil and N-desmethyl sildenafil  to harvested plasma.
  
  Thank you,
  
  Regards
  Mohan
  
  

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