Translate ALAG1 and D1 in ADVAN2 to differential equations

Dear NMusers, I'm using ADVAN2 to model the PK of a drug. To best fit the profile, the model includes ALAG1 and D1. For the next step, the ADVAN2 needs to be translated into differential equation to include a PD compartment. How should I write the differential equation for ALAG1 and D1? As you can see, there is Ka in the ADVAN2 model. I'm not very clear what's the meaning of D1 and KA when both are modeled. Any hint or previous link would be appreciated. Thanks in advance! $SUBROUTINES ADVAN2 $PK KASC=THETA(4) VSC=THETA(5) CLSC=THETA(6) LGSC=THETA(8) MU_4=KASC MU_5=VSC MU_6=CLSC MU_8=THETA(11) SD=THETA(9) SIG=SD KA=EXP(MU_4+ETA(4)) V=EXP(MU_5+ETA(5)) CL=EXP(MU_6+ETA(6)) K=CL/V S2=V/1000; DOSE IN 1000 U, CONC in U, VOLUME IN mL ALAG1=LGSC*EXP(ETA(10)) D1=EXP(MU_8+ETA(8)) Final parameter estimates: $THETA 12 ;THETA5 8.89 ;THETA6 0.0015;THETA8 0.263 ; THETA9 2.87; THETA11 Best regards, Shelley ======================================== Xiao Hu (Shelley), Ph.D. Scientist, Development Pharmacokinetics & Disposition Biogen Idec, Inc. 14 Cambridge Center Cambridge, MA 02142