biexp {PK}R Documentation

Two-Phase Half-Life Estimation by Biexponential Model

Description

Estimation of inital and terminal half-life by fitting a biexponential model.

Usage

biexp(conc, time, prev=0, tol=1E-9)          

Arguments

time time points of concentration assessments.
conc levels of concentrations.
prev pre-dosing value.
tol relative error tolerance.

Details

Estimation of inital and terminal half-life using the biexponential y=a1*exp(-b1*x)+a2*exp(-b2*x) model with a parametrization to ensure b1 > b2 > 0 fitted by the least squares criteria with function optim of package base with method "Nelder-Mead". Curve peeling (Foss, 1969) is used get start values for nonlinear model fitting. When no adequate start values are determined by curve peeling, a single exponential model is fitted with start values obtained from an OLS regression on log transformed values with a parametrization to ensure b > 0.

If the pre-dosing value indicating the intrinsinc level is greater than 0, the pre-dosing value is subtracted from all concentration levels before calculation of inital and terminal half-life.

Value

parms half-life and model estimates.
time time points of concentration assessments.
conc levels of concentrations.
method "biexp".

Note

Records including missing values and values below or equal to zero are omitted.

Author(s)

Martin Wolfsegger

References

Foss S. D. (1969). A Method for Obtaining Initial Estimates of the Parameters in Exponential Curve Fitting. Biometrics. 25:580-584

Pinheiro J. C. and Bates D. M. (200). Mixed-Effects Models in S and S-PLUS. Springer, New York.

Examples

## examples from Pinheiro J.C. and Bates D.M. (2000, page 279) 
time <- c(0.25, 0.5, 0.75, 1, 1.25, 2, 3, 4, 5, 6, 8, 0.25, 0.5, 0.75, 1, 1.25, 2, 3, 4, 5, 6, 8, 0.25, 
0.5, 0.75, 1, 1.25, 2, 3, 4, 5, 6, 8, 0.25, 0.5, 0.75, 1, 1.25, 2, 3, 4, 5, 6, 8, 0.25, 0.5, 0.75, 1, 1.25, 
2, 3, 4, 5, 6, 8, 0.25, 0.5, 0.75, 1, 1.25, 2, 3, 4, 5, 6, 8)

conc <- c(1.5, 0.94, 0.78, 0.48, 0.37, 0.19, 0.12, 0.11, 0.08, 0.07, 0.05, 2.03, 1.63, 0.71, 0.7, 0.64, 0.36, 
0.32, 0.2, 0.25, 0.12, 0.08, 2.72, 1.49, 1.16, 0.8, 0.8, 0.39, 0.22, 0.12, 0.11, 0.08, 0.08, 1.85, 1.39, 1.02, 
0.89, 0.59, 0.4, 0.16, 0.11, 0.1, 0.07, 0.07, 2.05, 1.04, 0.81, 0.39, 0.3, 0.23, 0.13, 0.11, 0.08, 0.1, 0.06, 
2.31, 1.44, 1.03, 0.84, 0.64, 0.42, 0.24, 0.17, 0.13, 0.1, 0.09)

result <- biexp(conc=conc, time=time)
print(result)
plot(result)


[Package PK version 0.02 Index]