Dear NONMEM users, I'm struggling for a couple of weeks against contradictory results between NONMEM and R analysis of the same data with the same model which includes a time-varying bioavailability. Here is a simplified example of my issue:
On the one hand, let's introduce a bicompartmental model with a depot compartment, where bioavailability is decreasing over time given a maximum in decrease (MAXDECR) and a first-order decay constant (LAMBDA). Instead of the variable TIME, I use a covariate TOLD (Time Of Last Dose) in order to be sure that the value of F1 computed by NONMEM will be independent of the time used for computation: --- $INPUT CID TIME EVID AMT CMT DV MDV TOLD $MODEL COMP=(DEPOT) COMP=(CENTRAL) COMP=(PERIPH) $PK MAXDECR = THETA(1) LAMBDA = THETA(2) / 24 ; TIME is in hour, Lambda in day-1 F1 = 1 - MAXDECR + MAXDECR * EXP(-LAMBDA * TOLD) $THETA (0, 0.5, 1) FIX (0, 0.15 ) FIX --- On the other hand, we have a dataset of 28 IDs with: - the same dosing regimen of 400 mg qd for 28 days (one line with EVID=1 per administration, no ADDL). - different "sampling occasions" at 0h, 6h, 12 and 18h post-dose; at day 1 for ID1, at day 1&2 for ID2, at day 1&2&3 for ID3, and so on until ID28 who has a complete PK exploration from day 1 to 28. All these lines are filled with EVID=0, DV=., and MDV=1. Then, I estimate these concentrations in maximum a posteriori Bayesian manner (MAXEVAL = 0) with ADVAN 13 (there is no inter-individual nor residual variability). My problem is that NONMEM found different concentrations in these 28 individuals, even though they received the same dose. Besides, as excepted, I found that all individuals had the same value for F1 (at a given time point). Would any of you have an idea of why NONMEM does not return the same predictions ? Thank you very much in advance Kind regards Félicien LE LOUEDEC, PharmD PhD student Centre de Recherches en Cancérologie de Toulouse (CRCT), Toulouse, FRANCE Team 14: « Dose Individualization of Anticancer Drugs » +335 31 15 55 69 lelouedec.felic...@iuct-oncopole.fr<mailto:lelouedec.felic...@iuct-oncopole.fr>
