Dear Arindam
I made a simple project to help you with your issue.
the idea is in the data set to do a copy of your dosing schedule and one column will be called dosefirstorder and the other dosezeroorder. You then create a graphical model and put a dose in the extravascular and also in the central compartment. In the central compartment you select the option of zero order absorption and here I used rate as the input (I could have used also duration). In both compartment , you have the option to select bioavailability that also emulate fraction of the drug. I defined F for the fraction going out of the extravascular and 1-F from the central compartment. In the central compartment I added a tlag.
This will emulate what you are trying to do. The project has both the simulation and the fitting procedure of the simulate data set.
I hope it helps.
best Regards;
Serge [file name=combined_first_order_zero_order_absorption.phxproj size=1717629]http://www.pharsight.com/extranet/media/kunena/attachments/legacy/files/combined_first_order_zero_order_absorption.phxproj[/file]