Background: The results obtained so far in studies designed to neutralize P glycoprotein (PGP)-related multidrug resistance (MDR) by using MDR reversal agents, have not yet fulfilled the promise of the experiments which were performed in vitro. In order to improve PGP-related MDR neutralization, we tested in vitro the activity of the cyclosporine derivative SDZ PSC 833 (PSC) together with doxorubicin (DOX) and with two new DOX derivatives named 4' iodo 4' deoxy-doxorubicin (IODODOX) and methoxy-morpholynil-doxorubicin (MMDOX, FCE 23762) using four different human cell lines and their multi-drug resistant variants.
Methods: Anthracycline toxicity was evaluated by using the MTT method after a 7-day culture with continuous exposure to the antitumor drugs with or without the addition of PSC.
Results: PSC significantly downmodulated the toxicity of all three anthracyclines in all the four cell systems. However, despite the great increase caused by PSC in the toxicity of DOX and a more modest effect on the toxicity of the two DOX derivatives, this MDR reversal agent could only completely block the PGP mediated MMDOX resistance whereas DOX refractoriness was only decreased.
Conclusions: The combination of MMDOX or IODODOX with PSC 1.6 microM is more efficient than the combination of DOX plus PSC for the full reversion of PGP-mediated drug resistance. Careful clinical studies are required to evaluate if these associations can also effectively and safely neutralize MDR in vivo.