Coding for Bioavailability

From: "Liesbeth Rook" Subject: [NMusers] Coding for Bioavailability Date:Tue, 22 Apr 2003 09:54:09 +0200 Dear NM users At the moment I am modelling data of a compound administered to patients intravenously and by inhalation. The bioavailability after inhalation is known to be in the order of 0.5. Because of the expected large inter-individual variation in inhalation technique, we did not want to fix the F but estimate it. Bioavailability however should be restricted to a value varying between 0-1. Therefore, we have defined F in the control stream as follows F1=EXP((LOG(TVF1*(1-TVF1))+ETA(5))/(1+EXP(LOG(TVF1*(1-TVF1))+ETA(5)))) However, according to this equation, when F is estimated as 0.5 and ETA=0, the calculated outcome for F1 is 0.33. The PRED are indeed much lower than the DV when I apply this equation. If I use F without any restriction in the control stream, then NM has problems in finding the solution. Has anybody tackled this problem before or have I made a mistake in the equation (which was cited from Karlson et al., Assumption testing in PPK models, J. of Pharmacokinetics and Biopharmaceutics, 1998, page 211) ? Kind regards Liesbeth Rook