Hi! Any ideas to set up the following model: 1comp, but the input rate is not a constant (zero order), but decreases with time following a simple exponential (like Dose/time=A ·exp(-B ·t). Background: I try to model formulations (like patches, implants), where the release rate decreases with time. Some of these formulations deliver at a constant rate and therefore can be modeled like an infusiuon, but others not…
I would suggest to consider this as a first-order model with extravascular input. in your formula, the B that’s being estimated is Ka; the A is bioavailability. If there is also a “bolus” component, that can go directly into the central compartment. I hope that makes sense, Simon.
Hi Simon! [quote]I would suggest to consider this as a first-order model with extravascular input. in your formula, the B that’s being estimated is Ka; the A is bioavailability.[/quote] Nice to call what I posted a ‘formula’ 
I was thinking (in conventional WNL code) of something like:f = kin*exp(-kdeg*x)/(V*k10)*(1-exp(-k10*x))which would reduce to f = kin/(V*k10)*(1-exp(-k10*x))if kdeg approaches zero.
The suggestion is not to use a closed form solution but to write derivs and choose non-stiff solver to start. e.g. Derivs for compartments, same as whatever regular model you’d choose. Stparm(ka = ka0exp(-kaet)) Please let us know how that works out for your problem. DO you perhaps have a snippet of data you could share? Simon