Dear Community,

I have an issue in re-estimating parameter values in Phoenix of a 2-CMT model that was initially developed using NONMEM.

The NONMEM model:

* 2-CMT (ADVAN3 TRANS4)

* IIV on Vc, Q, Vp and an additional baseline parameter

* No IOV

* Combined error model

* OMEGA block

* FOCE (with "INTER")

I brought everthing to Phoenix NLME using PML. I re-estimated the parameter values and I constantly obtain parameter values (robust!) which are somewhat different from the NONMEM results. The Phoenix model diagnostics in general look very good. The IPREDs of obtained with Phoenix and NONMEM are comparably good! The differences are big when comparing PRED vs. IVAR of Phoenix vs. NONMEM. I would really like to know where the differences in the estimations come from!

To give you an impression on the differences in parameter values:

NONMEM Phoenix

baseline parameter: 0.8 0.83

CL: 0.04 0.03

Vc: 4.6 27.2

Vp: 7.9 49

Q: 0.18 0.33

error add: 0.011 (variance?) 0.094 (standard deviation?)

error prop: 0.0029 (variance?) 0.095 (standard deviation?)

The OMEGA then shows more differences. Although the values of the IIVs (diagonal) are somewhat similar, the covariances are very different!

NONMEM:

positive ; IIV_Baseline

negative positive ; IIV_Vc

positive negative positive ; IIV_Q

negative positive negative positive ; IIV_Vp

Phoenix:

positive ; IIV_Baseline

positive positive ; IIV_Vc

negative positive positive ; IIV_Q

negative negative positive positive ; IIV_Vp

May be the differences come from the missing "INTER" option in Phoenix?

What could be other reasons?

Thank you very much in advance - Every thought/idea is very much appreciated!

Best

Daniel