Speeding up code with explicit, piecewise ODE solutions

Hi, I'm trying to solve an indirect response model in Nonmem where the drug effect depends upon the dose, which can change every day. Currently, I'm using the ADVAN6 routine, but because I have thousands of time points for about a thousand patients, Nonmem is quite slow. I thought that in principle, it should be possible to speed this up. Rather than use the ADVAN6 routine, it would be nice if I could just somehow enter the piecewise, analytical solution into the model. Instead of the DADT(1) equation shown below, it would be nice to be able to use something like the pseudo-code SSA = KIN*(1-EFF)/KOUT ; the steady state for A given the effect at a certain dose). note that EFF can change with time due to dose changes A(this time step) = (A(last time step) - SSA)*EXP(-KOUT*TSTEP) + SSA ; this is the explicit solution to the indirect response equation from the model below, TSTEP=size of time step Does anyone know if this is possible? If so, if you have sample code for a similar problem that would be extremely helpful. Thanks, Andy $PROBLEM this is the essential part of the current indirect response code I'm using $SUBS ADVAN6 TOL=5 $PK ; PD model KIN = THETA(1)*EXP(ETA(1)); KOUT = THETA(2)*EXP(ETA(2)); E10 = THETA(3)*EXP(ETA(3)); %effect of 10mg ; Initialize ODE A_0(1) = Y0; baseline value (as covariate) $DES IF (DOSE.EQ.10) EFF=E10 IF (DOSE.EQ.0) EFF=0 DADT(1) = KIN*(1-EFF)-KOUT*A(1)