To cite httk in publications use:

Pearce RG, Setzer RW, Strope CL, Sipes NS, Wambaugh JF (2017). “httk: R Package for High-Throughput Toxicokinetics.” Journal of Statistical Software, 79(4), 1–25. doi:10.18637/jss.v079.i04.

Corresponding BibTeX entry:

    title = {{httk}: {R} Package for High-Throughput Toxicokinetics},
    author = {Robert G. Pearce and R. Woodrow Setzer and Cory L. Strope
      and Nisha S. Sipes and John F. Wambaugh},
    journal = {Journal of Statistical Software},
    year = {2017},
    volume = {79},
    number = {4},
    pages = {1--25},
    doi = {10.18637/jss.v079.i04},