Previous studies showed that some lamellarin-resembling annelated azaheterocyclic carbaldehydes and related imino adducts, sharing the 1-phenyl-5,6-dihydropyrrolo[2,1-a]isoquinoline (1-Ph-DHPIQ) scaffold, are cytotoxic in tumor cells and may reverse multidrug resistance (MDR) mediated by P-glycoprotein (P-gp). Herein, we synthesized and evaluated for their antiproliferative activity in four tumor cell lines (RD, HCT116, HeLa, A549) and ability of inhibiting P-gp-mediated MDR some ten diversely substituted 1-Ph-DHPIQ derivatives bearing at position 2 carboxylate groups (COOH, COOEt), nitriles (CN) and Mannich bases (e.g., morpholinomethyl derivatives). Lipophilicity parametrization and molecular docking calculations improved the understanding of the structure-activity relationships in cytotoxicity and P-gp inhibition of the investigated 1-Ph-DHPIQs, which led us to disclose a novel Mannich base HCl salt (8b’), which proved to be cytotoxic to all the tested tumor cell lines in the low micromolar range (IC50 < 20 M) and to inhibit in-vitro the efflux pumps P-gp and MRP1 responsible for MDR, with IC50s of 0.45 and 12.1 M, respectively.