To cite package PK in publications use:

  Jaki T and Wolfsegger MJ (2011) Estimation of pharmacokinetic
  parameters with the R package PK. Pharmaceutical Statistics. Vol
  10(No 3), 294-288. DOI: 10.1002/pst.449

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Estimation of pharmacokinetic parameters with the R package PK},
    author = {Thomas Jaki and Martin Wolfsegger},
    year = {2011},
    journal = {Pharmaceutical statistics},
    volume = {10},
    number = {3},
    pages = {284-288},
    note = {DOI: 10.1002/pst.449},
    url = {https://CRAN.R-project.org/package=PK},
  }