Homoharringtonine Overcomes Drug Resistance and Synergises with Venetoclax in Acute Myeloid Leukemia Via Inhibiting Protein Synthesis

Abstract

Introduction: Acute myeloid leukemia (AML) is a one of the most lethal cancers worldwide. In young and fit patients, chemotherapy and allogeneic haematopoietic stem cell transplantation (HSCT) are the mainstays of treatment. In elderly and unfit patients, the treatment outcome is dismal. Venetoclax, a BCL-2 inhibitor, when used in combination with hypomethylating agent, has been shown to be effective in inducing remission and improving survival, however, de novo resistance towards venetoclax has been commonly observed clinically. Homoharringtonine (HHT), a protein synthesis inhibitor currently used in AML treatment, was found to overcome the de novo venetoclax resistance in AML. We hypothesize that protein synthesis suppressed by HHT might overcome venetoclax resistance and account for the synergism between HHT and venetoclax. Method: Nine AML cell lines, representing distinct driver events in leukemogenesis, were screened for their sensitivities towards venetoclax and the synergism with HHT. OCI-AML3, one of the venetoclax-resistant cell line was transplanted into NSG mice and tested for the synergism between venetoclax and HHT. Besides, tandem mass tag (TMT) protein labelling proteomics analysis after HHT treatment was performed in OCI-AML3 to examine the changes in protein synthesis. Candidate proteins were validated by genetic knockout in AML cell lines to demonstrate the role in regulating venetoclax resistance. Results: Different AML cell lines showed various sensitivities towards venetoclax (IC50 ranged from 5.83 nM to >1000 nM). OCI-AML3 and THP1 were resistant to venetoclax. Intriguingly, they showed significant synergism upon combined treatment of venetoclax and HHT, as calculated by the Excess Over Bliss Additivism (EOBA) and Combination Index (CI). The synergism as well as growth inhibitory effects of HHT and venetoclax combined were significantly higher than those of venetoclax combined with either cytarabine or azacitidine. Combination of venetoclax and HHT also significantly inhibited OCI-AML3 growth in vivo, compared to vehicle control or single arm treatment. We further found that the synthesis of 694 proteins was downregulated by HHT. PRICKLE1, a short half-life protein, was one of the HHT-downregulating proteins showed. Genetic knockout of PRICKLE1 and its upstream activator, VANGL1, sensitized venetoclax in OCI-AML3 and THP-1. The results suggested that the activation of VANGL1/PRICKLE1 (non-canonical WNT pathway) might be responsible for de novo venetoclax resistance in AML. Conclusion: HHT can overcome venetoclax resistance in AML by regulating protein synthesis, such as downregulating PRICKLE1. HHT, being a safe clinical medication, might serve an attractive adjuvant in better development of novel therapeutic regime in AML.