Pre-Clinical Study on the Dual BCL2/BCL-XL Inhibitor AZD0466 for the Treatment of Chronic Lymphocytic Leukemia

Targeting BCL2 with venetoclax has proven successful for treatment of chronic lymphocytic leukemia (CLL), but resistance develops in a subset of patients. Among other mechanisms, mutations in BCL2 and increased BCL-XL expression confer venetoclax refractoriness in patient subgroups. It is therefore important to identify alternative treatment options targeting BCL-XL and BCL2 for combating venetoclax resistance. AZD4320 was developed as a dual BCL-2/BCL-XL inhibitor. AZD0466 is a dendrimer-drug conjugate of AZD4320, designed to optimize drug release and improve drug tolerability. Hence, we used the nano drug AZD0466 for in vivo experiments and AZD4320 for in vitro and ex vivo experiments.

Firstly, we studied the efficacy of AZD0466 in vivo in the Eµ-TCL1 murine CLL adoptive transfer model. Due to prevalence of primary resistance, murine tumors with the best ex vivo response to AZD4320 were selected and 10 million cells were transferred by i.v. in syngeneic recipient mice. Upon reaching a tumor load of 10% CD19+ CD5+ cells in peripheral blood, mice were randomized and treated with 30mg/kg ibrutinib in drinking water, 34mg/kg or 70mg/kg of AZD0466 once weekly (QW) by i.v, or the vehicle. Treatments were continued until reaching a predefined humane endpoint. AZD0466 at 70mg/kg led to a significant increase in survival (median 72 days vs. 38 days, P=0.0009) similar to ibrutinib (67 days, P=0.0026) when compared to vehicle (Fig. 1). Analysis of blood parameters at euthanasia did not reveal significant difference in platelet counts between AZD0466, ibrutinib and vehicle treatments.

To evaluate if combination treatment of AZD0466 with BTK inhibitors would improve efficacy, we transplanted murine Eµ-TCL1 tumors into syngeneic recipient mice and randomized them for treatment with vehicle, ibrutinib (30mg/kg in drinking water), acalabrutinib (25mg/kg, p.o. QD), AZD0466 (70mg/kg, i.v., QW) and combination of AZD0466 with ibrutinib or acalabrutinib. AZD0466 and vehicle treatments showed similar platelet and RBC counts measured 2 or 3 days after the 2^nd and 4^th dosing but, a significant increase in platelet and RBC counts were observed with the combination treatments. A significant decrease in WBC count after 2 weeks, and in bone marrow tumor load after 4 weeks was observed with combined AZD0466 and acalabrutinib treatment compared to the single drugs. Though spleen weights did not differ after 4 weeks between vehicle and AZD0466 monotherapy, a strong decrease in spleen weight and tumor cell number in spleen was observed with the combination of AZD0466 with ibrutinib or acalabrutinib compared to vehicle. To validate the findings in human CLL, we performed dynamic BH3 profiling on primary CLL samples with mutated (N=5) and unmutated IGHV (N=7). Exposure of CLL cells to 1µM acalabrutinib for 24 hours significantly increased BCL2 dependency in all CLL samples with unmutated IGHV and in a subset of mutated IGHV samples. These tumors also showed an increased response to venetoclax and AZD4320 upon exposure to acalabrutinib.

To study the efficacy of the dual BCL2/BCL-XL inhibitor in the context of venetoclax resistance, we stably expressed the BCL2 mutations A113G, D103E, D103V, D103Y, G101V, R129L, V156D and wildtype (WT) in the venetoclax sensitive mantle cell lymphoma cell line MAVER-1. Response to AZD4320 or venetoclax was analyzed using Annexin V/7AAD staining and flow cytometry after 48 hours of treatment. The BCL2 mutants showed varying degrees of resistance to venetoclax. MAVER-1 expressing WT had an IC50 of 17.7nM while D103E, D103V, D103Y, G101V showed the strongest resistance to venetoclax with 5, 13.8, 4.5 and 7.3 fold increase in IC50 compared to WT. Of note, compared to WT (IC50 43.9nM), D103E, D103V and D103Y were sensitive to AZD4320 with a fold change in IC50 of 0.6, 1.8 and 1.3, respectively. G101V mutant cells showed a 5 fold increase in IC50 to AZD4320 compared to WT (Fig. 2). Moreover, AZD4320 was highly efficacious in MAVER-1 and MINO cell line models where resistance to venetoclax mediated by BCL-XL upregulation was modelled by an in vitro dose escalation method.

In summary, our pre-clinical study shows that the dual BCL2/BCL-XL inhibitor could represent an important treatment option for venetoclax resistance mediated by specific BCL2 mutations or BCL-XL upregulation and that its efficacy could be further improved upon combination treatment with BTKi.