Enhanced intracellular accumulation of a non-nucleoside anti-cancer agent via increased uptake of its valine ester prodrug through amino acid transporters.

The phenomenon known as multiple-drug resistance, whereby anti-cancer agents are expelled from cancer cells, makes it necessary to develop methods that will reliably increase the accumulation of anti-cancer agents within cancer cells. To accomplish this goal, a new model compound, Val-SN-38, was synthesized by introducing valine to SN-38, an active ingredient of irinotecan. Val-SN-38 improved intracellular accumulation approximately 5-fold in MCF7 cells, compared with SN-38, and rather than changes in membrane permeability, the amino acid transporter ATB(0,+) played a role, whereas the dipeptide transporter PEPT1 did not. Other sodium-dependent amino acid transporters, namely ATA1, ATA2, and ASCT2, were unexpectedly involved in the uptake of Val-SN-38 as well. The efflux of Val-SN-38 by major efflux transporters was variably changed, but not significantly. In summary, the enhanced accumulation of Val-SN-38 in cancer cells was due to augmented uptake via various amino acid transporters. The results of the present study make a compelling argument in favour of a prodrug concept that can improve intracellular accumulation and take advantage of amino acid transporters without significantly inducing multiple-drug resistance.