Efflux is the process of removing toxic materials from cells.
The number of hydrogen bond donors and the topological polar surface area (TPSA) are factors in efflux. In both cases, a smaller value is better. See Small Molecule Properties for background on these values.
Foldit Lab Report #27: Foldit gets a makeover looks at a small molecule design by a Foldit player, and discusses efflux.
See the wikipedia article Efflux (microbiology) for more on efflux, including the "efflux pumps" that cells use to remove toxins.