Background: Some drugs, such as cyclosporin, exhibit flat and delayed absor
ption profiles, with a correlation between the delay and the peak width. Su
ch profiles can be described by an absorption model in which the absorption
rate is derived from a gamma distribution (of which the classical first-or
der absorption model is a special case).
Objective: To develop a model for the pharmacokinetics of extravascular adm
inistration of cyclosporin and apply it to a study of the pharmacokinetics
of cyclosporin microemulsion in stable renal transplant recipients.
Patients and participants: 21 renal transplant patients receiving oral cycl
osporin microemulsion 75 to 175mg twice daily.
Methods: The equation of the plasma concentration-time curve after oral adm
inistration was expressed as a convolution product between the absorption r
ate and a multi-exponential impulse response. The convolution integral was
computed analytically and expressed in terms of the incomplete gamma functi
on. Cyclosporin was assayed by liquid chromatography/mass spectrophotometry
. The model was fitted by nonlinear regression, using a specially developed
program.
Results: The gamma model yielded a good fit in all of the 21 patients studi
ed. Attempts to fit the same data by a classical exponential with lag-time
model failed in most patients.
Conclusions: This model could simplify the Bayesian monitoring of cyclospor
in therapy.