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Abstract Chemotherapy-induced drug resistance remains a major cause of cancer recurrence and patient mortality. ATP binding cassette subfamily B member 1 (ABCB1) transporter overexpression in tumors contributes to resistance, yet current ABCB1 inhibitors have been unsuccessful in clinical trials. To address this challenge, we propose a new strategy using tryptophan as a lead molecule for developing ABCB1 inhibitors. Our idea stems from our studies on bat cells, as bats have low cancer incidences and high ABCB1 expression. We hypothesized that potential ABCB1 substrates in bats could act as competitive inhibitors in humans. By molecular simulations of ABCB1-substrate interactions, we generated a benzylatedCyclo-tryptophan (C3N-Dbn-Trp2) that inhibits ABCB1 activity with efficacy comparable to or better than the classical inhibitor, verapamil. C3N-Dbn-Trp2 restored chemotherapy sensitivity in drug-resistant human cancer cells with no adverse effect on cell proliferation. Our unique approach presents a promising lead toward developing effective ABCB1 inhibitors to treat drug-resistant cancers. 

Abstract The emergence of drug resistance in cancer cells eventually causing relapse is a serious threat that demands new advances. Upregulation of the ATP‐dependent binding cassette (ABC) transporters, such as ABCB1, significantly contributes to the emergence of drug resistance in cancer. Despite more than 30 years of therapeutic discovery, and several generations of inhibitors against P‐gp, the search for effective agents that minimize toxicity to human cells, while maintaining efflux pump inhibition is still underway. Leads derived from natural product scaffolds are well‐known to be effective in various therapeutic approaches. Inspired by the biosynthetic pathway to Nocardioazine A, a marine alkaloid known to inhibit the P‐gp efflux pump in cancer cells, we devised a regioselective pathway to create structurally unique indole‐C3‐benzylcyclo‐L‐Trp‐L‐Trp diketopiperazines (DKPs). Using bat cells as a model to derive effective ABCB1 inhibitors for targeting human P‐gp efflux pumps, we have recently identifiedexo‐C3‐N‐Dbn‐Trp2 (13)as a lead ABCB1 inhibitor. This C3‐benzylated lead inhibited ABCB1 better than Verapamil.^[21]Additionally,C3‐N‐Dbn‐Trp2restored chemotherapy sensitivity in drug‐resistant human cancer cells and had no adverse effect on cell proliferation in cell cultures. For a clearer structure‐activity relationship, we developed a broader screen to test C3‐functionalized pyrroloindolines as ABCB1 inhibitors and observed that C3‐benzylation is outperforming respective isoprenylated derivatives. Results arising from the molecular docking studies indicate that the interactions at the access tunnel between ABCB1 and the inhibitor result in a powerful predictor for the efficacy of the inhibitor. Based on fluorescence‐based assays, we conclude that the most efficacious inhibitor is thep‐cyano‐derivedexo‐C3‐N‐Dbn‐Trp2 (33 a), closely followed by thep‐nitro substituted analogue. By combining assay results with molecular docking studies, we further correlate that the predictions based on the inhibitor interactions at the access tunnel provide clues about the design of improved ABCB1 inhibitors. As it has been well documented that ABCB1 itself is powerfully engaged in multi‐drug resistance, this work lays the foundation for the design of a new class of inhibitors based on the endogenous amino acid‐derivedcyclo‐L‐Trp‐L‐Trp DKP scaffold.