# Transit Model estimating compartment number

**URL:** <https://forum.certara.com/t/transit-model-estimating-compartment-number/2656>\
**Category:** Modelling and Simulation\
**Created:** [November 29, 2013, 1:27pm UTC](https://forum.certara.com/t/transit-model-estimating-compartment-number/2656 "2013-11-29T13:27:17Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![roland123](https://avatars.discourse-cdn.com/v4/letter/r/a9a28c/32.png) [@roland123](https://forum.certara.com/u/roland123)\
**Post date:** [November 29, 2013, 1:27pm UTC](https://forum.certara.com/t/transit-model-estimating-compartment-number/2656/1 "2013-11-29T13:27:17Z")

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Dear All, I have a question regarding the explicit usage of a „dose“ in a PML script. A second question regards controlling of the time parameter. I am trying to set up transit compartments in Phoenix/NLME to model delayed drug absorption for a Pop-PK model of a once daily dosed small molecule drug. I would like to code it according to Savic et al. (2007), where the number of transit compartments is a estimated parameter. The key equation reads: dA1/dt = Dose x F x ktr x (Ktr x t)\*\*n x exp(-ktr x t)/n! - Ka x A1 with ktr = transit rate constant, n is number of transit compartments and n! is approximated by Sterling’s formula. How can I implement an explicit dose - as it appears in the above equation - in the PML script? In order to consider multiple dosing in this model one needs to implement a relative time, something like (ktr x (t – tdose))\*\*n., so that time is reset to zero a the next dose. How can this be done in Phoenix/NLME? I think it could be achieved using the reset flag but I am not sure how to do this correctly. I was able to run the model in WinNonLin for single PK profiles, a NONMEM code for this model can be found as a free supplement to Savic et al. paper. For sake of clarity I include the relevant parts of the NONMEM code. Thanks for your comments Roland ;—Setup for multidose transit compartment IF (NEWIND.NE.2) THEN ; beginning of dataset, or new individual TNXD = TIME ; TIME will be the time of the first record of that subject even if it’s not a dose, but PNXD = AMT ; the amount will be 0 if the first record is not a dose ENDIF TDOS = TNXD PD = PNXD IF(AMT.GT.0) THEN TNXD=TIME PNXD=AMT ENDIF ;—Transit compartment KTR =(NN+1)/MTT ;ktr as function of mean transition time MTT F1 = 0 L = 0.9189385+(NN+.5)_LOG(NN)-NN+LOG(1+1/(12_NN)) ; logarithm of the approximation to the gamma function LBPD = LOG(BIO_PNXD) LKTR = LOG(KTR) PFUNC = LBPD+LKTR-L $DES TEMPO = T-TDOS ; time after dose IF(TEMPO.GT.0)THEN KTT = KTR_TEMPO DADT(1) = EXP(PFUNC+NN_LOG(KTT)-KTT)-KTR_A(1) ; dapagliflozin absorption ELSE DADT(1)=0 ENDIF DADT(2) = KTR_A(1) - K26_A(2) - K23_A(2) + K32_A(3) -K24_A(2) - K20_A(2) ;