[R-sig-ME] Two compartment model PK model using nlme

[martin highfield]

Dear list members,

Can anyone point me in the right direction for implementing a two
–compartment exponential model with instantaneous input and
first-order elimination kinetics using the nlme package. I have been
asked (informally)  to replicate some pharmacokinetic data analysis
which was previously analysed using the NONMEM program but have little
experience with these types of models (how do I get myself into these
situations?) I know there is a function that implements a one
compartment open model (phenoModel) but am unable to find a function
specific to two compartment exponential with instantaneous input and
first order elimination. Would the SSbiexp function be equivalent or
would I have to construct a function myself?

The data comes from 33 subjects although the number of observations
per subjectis quite sparse and with some non consecutive observations
within a subject.

table(table(subset(dataf.stack, !is.na(dataf.stack$Concentration))$Subject))

 2  3  4  5  6  7
 4  5  5  8 10  1

Many thanks (and I hope I havent asked too many daft questions)
Best
Martin




sessionInfo()
R version 2.11.0 (2010-04-22)
i386-pc-mingw32

locale:
[1] LC_COLLATE=English_United Kingdom.1252  LC_CTYPE=English_United
Kingdom.1252    LC_MONETARY=English_United Kingdom.1252
[4] LC_NUMERIC=C                            LC_TIME=English_United
Kingdom.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] reshape_0.8.3  plyr_0.1.9     lattice_0.18-8 nlme_3.1-96

loaded via a namespace (and not attached):
[1] grid_2.11.0  tools_2.11.0