Reversal of Chemoresistance in Leukemia Cells Using Synthetic Bisbenzylisoquinoline Derivatives
Authors: Müller Martin , Gerndt Susanne , Arner Anja , Chao Yu-Kai , Schömig Linus , Atzberger Carina , Biel Martin , Bracher Franz , Vollmar Angelika , Binder Vera , Keller Marco , Grimm Christian , Bartel Karin 
Published: October 2019

Abstract
Introduction
Chemoresistance represents a major problem for the treatment of leukemia patients, which frequently manifests in the multidrug resistance (MDR) phenotype1. The drug refractoriness to structurally related and unrelated drugs can be caused by the P-glycoprotein (P-gp), an efflux pump belonging to the superfamily of ATP-binding cassette (ABC) proteins1, 2. Multiple large clinical studies have evaluated the effectiveness of potential P-gp inhibitors, but all of them failed clinically, except for the alkaloid tetrandrine (TET)3. Given the high structural complexity and the pulmonary toxicity of the plant-derived TET in animal studies4, we are seeking to develop new bisbenzylisoquinoline derivatives (BBIQDs) with similar potency and reduced toxicity to overcome P-gp-mediated chemoresistance in leukemia.
Methods
Calcein-AM, Cell-TiterBlue® cell viability and propidium iodide cell death were performed as described by the manufacturers. TUNEL assays were conducted as described previously5. For the assessment of in vivo toxicity, female C57BL/6-Tyr mice (Envigo) were treated with 57 or 90 nmol/kg of the corresponding P-gp inhibitor on three consecutive days.
Results
BBIQDs were synthesized and screened for their potential to inhibit P-gp-mediated drug efflux using calcein-AM as model substrate in vincristine-resistant leukemia cells (VCR-R CEM). The in vitro characterization revealed small molecule hits with a similar potency compared with the reference molecule TET. Consequently, the lead compounds were able to overcome chemoresistance to vincristine (VCR) when applied as a combination therapy, as assessed by cell viability and cell death assays. More specifically, the usage of P-gp inhibitors as add-on to VCR enabled a decrease of the IC50 value from 0.73 µM to up to 0.03 µM, corresponding to a maximum dose modifying factor of 24. Additionally, the ability of BBIQDs to reverse chemoresistance to imatinib in MDA cells overexpressing the breast cancer resistance protein (BCRP), another major efflux transporter responsible for MDR in leukemia6, 7, was tested. As no relevant chemosensitization was observed here, we concluded that our BBIQD hits are specific inhibitors of P-gp. Furthermore, in vivo toxicity was evaluated in a mouse model, showing that the BBIQDs were well-tolerated at the highest tested dose of 90 nmol/kg. Additionally, TUNEL assays on organ slides indicated lack of toxicity in lungs, heart and brain. Finally, the P-gp inhibitors will be tested for their efficacy to overcome chemoresistance in vivo using a zebra fish leukemia model.
Conclusion
Taken together, we have identified derivatives of the alkaloid TET with a similar potency to inhibit P-gp mediated drug efflux. Remarkably, the application of the BBIQDs as add-on to VCR was able to reverse chemoresistance in P-gp-mediated, VCR-resistant leukemia cells, while no toxicity issues were observed in a mouse model.
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Disclosures
No relevant conflicts of interest to declare.