PharmPK Discussion - V/F and CL/F

• On 13 Jun 2013 at 15:23:18, "Prof. Dr. Zafar Iqbal" (zafar_iqbal.-at-.upesh.edu.pk) sent the message
  

  
  Dear Friends,
  what is the importance of Vd/F and Cl/F over VD and Cl, if F is not known then how to calculate these parameters.
  
  Dr Zafar
  
  

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• On 13 Jun 2013 at 20:27:59, Stefan Soback (stefan.soback.-at-.gmail.com) sent the message
  

  
  CL/F can be a very valuable parameter. By itself it does not mean much,
  but if you compare two groups and you can assume equal bioavailability,
  you can get some very important information about the clearance. You can
  also use the same subjects and do a crossover study, which makes it even
  more robust. Clearance is a model independent parameter.
  
  Vd/F however is totally useless (or more accurately incorrect)
  parameter. It will be correct only in the case when your drug can best
  be described by one-compartment distribution. If we take Vbeta (or
  Varea), it will be correct only if the half-life after non-IV
  administration is identical to that after IV bolus administration (Vbeta
  = (D*t1/2)/(AUC*ln2)). In real life you can't make that assumption.
  
  Vss/F (Vss is the "real" volume term) is even more erroneous. After
  non-IV administration the equation becomes Vss/F = CL/F*(MRTnon-IV -
  MAT). You don't have the MAT because you need the MRT after IV
  administration to calculate it. Remember Vss = CL * MRT. If you divide
  the Vss and CL by F, it is totally legitimate, but the MRT is still the
  MRT after IV administration. MRTnon-IV - MAT equals MRT after IV
  administration.
  
  We wrote a short communication of this in the J.Vet.Pharm.Therap., but I
  guess nobody reads that Journal.
  
  Best regards,
  
  Stefan Soback, DVM, PhD
  
  

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• On 13 Jun 2013 at 18:41:56, William Wolowich (wwolowic.-a-.nova.edu) sent the message
  

  
  The following message was posted to: PharmPK
  
  Dr. Zafar:
  CL/F and V/F are hybrid parameters. They are formally known as "apparent
  oral clearance" and "apparent (oral) volume of distribution". They are
  useful in the same way that CL and V are useful, ie you can use them to
  develop a dosage regimen, even when F is not known. I'm sure Dr. Bourne
  discusses them in his online PK text. I know Bauers text has examples of
  use.
  
  Wolowich
  NSUCOP
  
  

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