PGP and neoplasm: Interestingly, the novel DHPIQ Mannich base 8c, prepared as a HCl salt, stable at pHs 2 and 7.4 at room temperature, proved to be cytotoxic to all of the tested tumor cell lines in the low micromolar range (IC50 < 20 μM) and to inhibit in vitro the efflux pumps P-gp and MRP1 responsible for MDR, with IC50 of 0.45 and 12.1 μM, respectively.