HSP70 Inhibitor, YK5, Synergizes with Chemotherapeutic Agents and Prevents Chemoresistance in Acute Myelogenous Leukemia (AML).

Abstract 2476

Stress-inducible heat shock protein 70 (HSP70) is a major cytoprotective factor and a molecular chaperone that interacts with HSP90 to form a multi-chaperone complex. Cancer cells are highly dependent on this complex due to their increased demand for protein synthesis. HSP70 overexpression inhibits apoptosis and has been associated with drug resistance and poor prognosis. YK5, a novel inhibitor of tumor-HSP70, has been shown to induce potent cell death in AML blast, progenitor, and stem cell populations with minimal effects in normal hematopoietic cells. Due to the role of HSP70 in drug resistance, we examined the effect of combining YK5 with other chemotherapeutic agents, including arsenic trioxide, cytarabine, suberoylanilide hydroxamic acid (SAHA) and PU-H71, a novel tumor-specific HSP90 inhibitor.

We tested the ability of YK5 to synergize with either AsO₃, AraC, SAHA, or PU-H71 in primary AML samples. Using multiparameter flow cytometry to measure viability after 48 hours of treatment, we found that combining 1μM YK5 with either 500nM AsO₃ or PU-H71 resulted in a significant increase in cell death when compared to either agent alone (n=9, mean viability: 51.8, 67.2, and 13.4% for AsO₃, YK5, and AsO₃/YK5, respectively, P = 0.0018; mean viability: 57.1 and 20.8% for PU-H71 and PU-H71/YK5, respectively, P = 0.0029). A synergistic relationship between YK5 and both AsO₃ and PU-H71 was found in all nine primary samples (combination indexes 0.29 – 0.76 with YK5/AsO₃, 0.33 – 0.83 with YK5/PU-H71). In contrast, the combination of YK5 with either AsO₃ or PU-H71 in CD34+ cord blood mononuclear cells did not result in a significant increase in cell death when compared to either agent alone (mean viability: 42.4, 72.4, and 37.2% for AsO₃, YK5, and AsO₃/YK5, respectively; mean viability: 61.1 and 51.1% for PU-H71 and PU-H71/YK5, respectively). YK5 in combination with either AraC or SAHA, however, did not result in a significant increase in cell death when compared to either drug alone, with an additive effect being demonstrated with a 1:1 YK5 to AraC/SAHA drug ratio (Mean CI = 0.9918).

To determine the mechanism of the observed synergistic activity, intracellular HSP70 and active caspase-3, a client of HSP70, were measured using flow cytometry. Both AsO₃ and PU-H71 significantly increased intracellular HSP70 and caspase-3 (Mean fold change = 18.3, 21.0 of HSP70 and 9.9, 8.3 of Caspase-3 for AsO₃ and PU-H71 treatment, respectively), while treatment with AraC or SAHA resulted in no change in HSP70 levels. Furthermore, quantitative PCR revealed that treatment with either AsO₃ or PU-H71 strongly upregulated HSPA1A and HSPA6, the main stress-inducible isoforms of HSP70 (Mean fold change = 15.9, 14.1 of HSPA1A, and 20.8, 23.4 of HSPA6 for AsO₃ and PU-H71 treatment, respectively). AraC and SAHA had no significant upregulation of these genes. We have previously shown that increased levels of HSPA1A correlate with sensitivity to HSP70 inhibition via YK5.

To further explore the mechanism of this observed synergy, flow cytometry was used to measure the levels of reactive oxygen species (ROS). Treatment with AsO₃, PU-H71, AraC, or SAHA resulted in a significant increase in ROS (Mean fold change = 2.75, 1.92, 2.89, 1.67, respectively). Quantitative PCR also confirmed the activation of the oxidative stress response by the upregulation of heme oxygenase 1 (HMOX1) by treatment with these drugs (Mean fold change = 10.9, 8.7, 11.2, 7.7, respectively). YK5, however, did not induce ROS or upregulate HMOX1. Interestingly, pretreatment with NAC in primary AML samples (n=4) resulted in no protection from YK5 synergistic effect when combined with either AsO₃ or PU-H71. These results suggest that YK5 synergizes with AsO₃ and PU-H71 due to the increase in intracellular HSP70 caused by these drugs. This synergy is most likely due to the activation of the heat shock response and independent of the production of ROS due to drug treatment.

In summary, we have found that the novel tumor-HSP70 inhibitor YK5 can synergize with AsO₃ and PU-H71 in primary human AML, and that the basis of this synergism is due to the increase in intracellular HSP70 caused by these chemotherapeutic agents. HSP70 inhibition represents a novel approach in AML treatment and can be particularly significant to drug-resistant patients when combined with other chemotherapy.