Nature-Inspired 1-Phenylpyrrolo[2,1- a]isoquinoline Scaffold for Novel Antiproliferative Agents Circumventing P-Glycoprotein-Dependent Multidrug Resistance

Abstract

Previous studies have shown 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 some tumor cells and may reverse multidrug resistance (MDR) mediated by P-glycoprotein (P-gp). Herein, several novel substituted 1-Ph-DHPIQ derivatives were synthesized which carry carboxylate groups (COOH, COOEt), nitrile (CN) and Mannich bases (namely, morpholinomethyl derivatives) in the C2 position, as replacements of the already reported aldehyde group. They were evaluated for antiproliferative activity in four tumor cell lines (RD, HCT116, HeLa, A549) and for the ability of selectively inhibiting P-gp-mediated MDR. Lipophilicity descriptors and molecular docking calculations helped us in rationalizing the structure-activity relationships in the P-gp inhibition potency of the investigated 1-Ph-DHPIQs. As a main outcome, a morpholinomethyl Mannich base (8c) was disclosed which proved to be cytotoxic to all 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 IC₅₀s of 0.45 and 12.1 μM, respectively.

Keywords: Mannich bases; P-glycoprotein; cytotoxicity; inhibition; multidrug resistance reversal; pyrrolo[2,1-a]isoquinolines.

Figure 1

Structure of the 1-phenyl-5,6-dihydropyrrolo[2,1-a]isoquinoline (1-Ph-DHPIQ) scaffold and natural (1, lamellarins) and synthetic derivatives (2–4).

Scheme 1

Synthesis of diversely R^1–4-substituted 1-Ph-DHPIQ derivatives. Colors blue, red and pink highlight the new fragments/functional groups in C2 position.

Figure 2

Surface plasmon resonance (SPR) sensorgrams (left panels), obtained at different concentrations of 1-Ph-DHPIQ derivatives 6c (top) and 8c (bottom) to immobilized human serum albumin (HSA); sigmoidal binding isotherms (right panels), with compound concentrations in logarithm scale. Response units (RU), expressed as the averages with SD from three repeated experiments, were fitted using nonlinear regression in GraphPad Prism software (vers. 5.01), and the log of the equilibrium dissociation constants (KD) was calculated by the curve interpolation.

Figure 2

Surface plasmon resonance (SPR) sensorgrams (left panels), obtained at different concentrations of 1-Ph-DHPIQ derivatives 6c (top) and 8c (bottom) to immobilized human serum albumin (HSA); sigmoidal binding isotherms (right panels), with compound concentrations in logarithm scale. Response units (RU), expressed as the averages with SD from three repeated experiments, were fitted using nonlinear regression in GraphPad Prism software (vers. 5.01), and the log of the equilibrium dissociation constants (KD) was calculated by the curve interpolation.

Figure 3

Front (left) and extracellular view (right) of the binding mode of 7c to CryoEM P-gp structure. The transmembrane-spanning helices are depicted as orange, ligand in meshes.

Figure 4

Detailed view of 4a (a), 6a (b), 7c (c) and 8c (d) highest scored docking modes. In the interaction pattern scheme, hydrogen bonds are depicted in cyan, Van der Waals contacts and π–π stackings in magenta.