PGP and lymphoma: In terms of insight into the role of the three modified moieties, at least three main modes of the biological action should be considered, i.e., (1) the potency to inhibit 123 rhodamine efflux in the accumulation assay, (2) the substrate “force” of compounds, observed as a difference in the cytotoxic action between PAR and MDR T-lymphoma cells, and (3) the substrate properties seen as the ability to increase the intrinsic ATPase activity of Pgp.