Dear forum, Please advise how to address the following in phoenix using indirect response models:

an endogenous target "E".

On a weekly basis and only on a fixed day of each week (e.g., monday) for 8 weeks, 20% of the target "E" is chemically modified to "E_mod". The chemical modification is assumed to be instantaneous. "E_mod" has a different(slower) turnover rate (kout) than "E".

"E" is an endogenous enzyme. "E_mod" is not endogenous.

Observation is total amount of "E" + "E_mod"

how do I write an indirect response model to describe this type discrete (noncontinuous) target conversion?

many thanks