DESCRIPTION:
The administration is extravascular with a first order absorption (rate constant ka).
The PK model for Parent has a central compartment (volume V), a peripheral compartment 
(rate of transfer to and from k12 and k21), and a linear elimination (elimination rate k).
The PK model for Metabolite has one compartment (volume V) and a linear elimination (elimination rate km).
The transfer between parent and metabolite is uni-directional  (transfer rate Kpm).


[LONGITUDINAL]
input = {ka, V, k, k12, k21, km, Kpm}

PK:

; PK model definition for Parent
compartment(cmt = 1, volume = V, concentration = Cp)
oral(cmt = 1, ka)
peripheral(k12, k21)
elimination(cmt = 1, k)

; PK model definition for Metabolite
compartment(cmt = 3, volume = V, concentration = Cm)
elimination(cmt = 3, k = km)
transfer(from = 1, to = 3, kt = Kpm)

OUTPUT:
output = {Cp, Cm}
