BCL-2 Inhibitor Venetoclax (ABT-199) and MEK Inhibitor GDC-0973 Synergise to Target AML Progenitors and Overcome Drug Resistance with the Use of PET Scanning in a Mouse Model of HR-MDS to Monitor Response to Treatment

Introduction: Targeted drugs are needed for HR-MDS/AML, particularly in elderly patients and Venetoclax, approved for some CLL, gives promising results in elderly AML. Assays to predict response to treatment may enable us to deliver personalized treatment. We sought to determine the most informative assay to predict response; viability assays can directly measure the effects of reagents on growth. Progenitor assays can potentially determine if the reagents can target diseased primitive cells. PET scanning can be used to follow response to treatment.

Methods: Peripheral blood (PB) or bone marrow (BM) from 7 MDS/AML patients were incubated in a) no treatment, b) ABT-199 (1 µM) (Abbvie), c) GDC-0973 (1 µM) (Genentech) or d) ABT-199+GDC-0973 (1 µM of each) and assessed for viability using the MTT assay (n=2); cell death followed using the Incucyte® Zoom System (Essen Bioscience) (n=2) or methocult progenitor assays (Stem Cell Technologies) (n=4). Having shown that RAS:BCL-2 co-localization correlated with prognosis in MDS/AML patients (Leuk Res 37:312-9, 2013), immunofluorescence was undertaken. A micro PET device dedicated to mice was used to measure BM blast proliferation. After injection of 18F-FLT(a thymidine analogue) in mice untreated (n=7) or ABT-199 (75mg/kg)+GDC-0973(10mg/kg) treated (n=5) normal FVB/N, HR-MDS mice treated with vehicle (n=4), 2-month old HR-MDS before (n=5) and 3-month old before (n=4) and after ABT-199 (75mg/kg)+GDC-0973(10mg/kg) treatment (n=8), PET imaging was performed (Inveon Siemens Medical Systems), analyzed for signal and quantified.

Results: Patient details and results are summarized on Table 1. Using the MTT assay 2 PB patient samples were found to be sensitive to ABT-199 treatment (Figure 1A, AS, p=0.00042 and YA, 0.00002) and more sensitive to the combination compared to untreated (AS, p=0.00007 and YA, 0.000003). With the incucyte the BM of one patient (AE) was found to be resistant to both ABT-199 and GDC-0973, but sensitive to the combination (Figure 1B). PB and BM from patient JA were assayed for apoptosis with the incucyte and were found to be sensitive to ABT-199 with increased apoptosis, resistant to GDC-0973 with decreased apoptosis and sensitive to the combination. Four bone marrow samples were tested in the 4 conditions using the progenitor assay (Figure 1C). Three patients were sensitive to GDC-0973, inhibiting any colony formation and the fourth had reduced colony numbers. In this assay patient JA appeared to be sensitive to GDC-0973 treatment whereas the incucyte assay scored this sample to be resistant to apoptosis; thus the cytotoxic effects of GDC-0973 may not be via apoptopsis. As the progenitor assay is likely to score the primitive disease population, this assay may prove more informative than the others without prior selection. One patient (DH) was clearly resistant to ABT-199, whereas the other three (JA, CB and FL) had reduced colony growth. All patients were sensitive to the combination treatment and inhibited colony growth. The RAS:BCL-2 co-localization in the PB revealed no complex in either the Mito or PM upon treatment with ABT-199 alone and some localization in the Mito with GDC-0973. With both ABT-199 and GDC-0973, there were hardly any cells confirming the cytotoxic effects of the combination. As we have previously shown that PM co-localization of the complex is associated with drug resistance (Blood 130:2613, 2017Suppl), we used the combination on our HR-MDS mouse model, where the complex co-localizes in the PM and followed the mice by PET scanning (Figure 1D). Weak signal was visualized in the femurs of untreated and ABT-199+GDC-0973 treated FVB/N mice (FBR 1.17+/-0.34 and 1.02+/-0.08 respectively). Mild PET signal was seen in the femurs of 2 month-old HR-MDS mice, (FBR 1.79+/-0.98). Intense PET signal was seen in the femurs and proximal humerus of HR-MDS mice treated with vehicle (3 month-old, FBR=2.35+/-1.32). Low PET signals were seen in the femurs of 5/8 HR-MDS mice treated with ABT-199+GDC-0973 (FBR=1.93+/-0.84). FBRs of the 3 groups of HR-MDS mice were significantly higher than those of FBV/N groups.

Conclusion: Combined Venetoclax (ABT-199) and GDC-0973 targets MDS/AML progenitors and can potentially overcome drug resistance with the disruption of the RAS:BCL-2 complex. Bone marrow disease progression in HR-MDS mice can be monitored with 18F-FLT-PET imaging; PET data shows that the combination slows down disease progression.

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