MTT assay. pLuc705 cells were treated at concentrations ranging from 2 to 60 mM for 24 h. As shown in Figure 6 , all conjugates, except (RX) 8 and (RXR) 4 , had no toxicity at up to 60 mM. Up to 10 mM, (RX) 8 and (RXR) 4 conjugates exhibited no toxicity, at higher concentrations they reduced cell viability in a concentration-dependent manner, with (RX) 8 being more toxic than (RXR) 4 ( Figure 6C and D) . Replacement of L-arginine with D-arginine in R 8 À, (RB) 8 À and (RXR) 4 ÀPMO did not change the viability profiles of these conjugates ( Figure 6A-C) . Surprisingly, the L!D replacement in (RX) 8 ÀPMO decreased the toxicity. Cell viability with 60 mM treatment was 40% for (RX) 8 ÀPMO, but 80% for (rX) 8 ÀPMO ( Figure 6D ). The eight conjugates containing fewer than 5 X residues did not inhibit cell proliferation up to 60 mM ( Figure 6E ). Monomers of arginine or X, individually or in combination, at 500 mM each, produced no inhibition of cell proliferation ( Figure 6F ). The toxicities of the CPPÀPMO conjugates, (RXR) 4 ÀPMO, 3dÀPMO and 4cÀPMO, were also evaluated in human liver HepG2 cells. We found that only (RXR) 4 ÀPMO caused dose-dependent inhibition of cell proliferation while other two conjugates had no toxicity up to 60 mM, the highest concentration tested in this study (data not shown).