BCL-2-Selective BH3 Mimetic ABT-199 Is a Potent Agent For Acute Myeloid Leukemia

Evasion of apoptosis is a key hallmark of cancer. BCL-2 family proteins, the central regulators of apoptosis, are often aberrantly expressed in tumors. Pro-apoptotic BCL-2 members bind and sequester anti-apoptotic BCL-2 proteins via their BH3 domains. Thus, BH3 mimetics represent a promising direction in cancer drug development. ABT-263, designed as a BH3 mimetic to inhibit BCL-2, BCL-X_L, and BCL-W, has demonstrated efficacy in preclinical and clinical studies. However, thrombocytopenia is common in patients treated with ABT-263 due to the inhibition of BCL-X_L, which is indispensable for platelet survival. ABT-199 (GDC-0199), a second-generation BH3 mimetic, has higher affinities for BCL-2 protein (Ki < 0.01 nM), which enhances the specificity of this agent to kill cancer cells without provoking unwanted thrombocytopenia (Souers, et al, Nature Med, 2013). Since BCL-2 is often overexpressed in hematological malignancies including acute myeloid leukemia (AML), we evaluated the anti-cancer effects of ABT-199 on AML cells. As a measure of BCL-2 specificity, BCL-X_L overexpression in sensitive HL-60 cells resulted in complete resistance to ABT-199, while BCL-2 overexpression in these cells conferred moderate resistance to apoptosis induction. Moreover, OCI-AML3 cells with high MCL-1 levels were highly resistant to ABT-199, while knockdown of this protein greatly sensitized cells to this BH3 mimetic. Among 12 genetically diverse AML cell lines tested, seven were sensitive to ABT-199-induced apoptosis with 48-h EC₅₀ ranging from 1.5 nM to 145 nM. In these seven sensitive, BCL-2 dependent cell lines, ABT-199 was uniformly more potent than ABT-737 (another BCL-2 inhibitor with a spectrum similar to ABT-263, p = 0.016). Next, we tested ABT-199 in 15 primary samples from relapsed/refractory AML patients. Twelve patient samples showed high sensitivity to apoptosis induction following 48-h exposure to ABT-199 (EC₅₀ < 10 nM). In a larger set of 23 cryopreserved AML patient samples, including AML cells with diploid cytogenetics and mutations in FLT3, NRAS, and NPM1 genes, 18 (78%) were sensitive to ABT-199 (100 nM). However, samples from patients with complex cytogenetics, t(8;21) and JAK2 mutation (n = 12) were largely insensitive to ABT-199 (17% response rate). Interestingly, in five of six primary AML samples with high blast counts, ABT-199 induced marked apoptosis in CD34⁺/CD38^- AML stem/progenitor cells compared to bulk AML blasts (p = 0.01). Quantitative Western blot was used to determine BCL-2 protein levels in AML cell lines. Spearman analysis showed that EC₅₀ of ABT-199 correlated negatively with BCL-2 protein expression (r = -0.605, p = 0.0143) and correlated positively with BCL-X_L protein expression (r = 0.633, p = 0.0101). Similar correlations were also observed in primary AML samples, suggesting that pre-treatment cellular BCL-2 and BCL-X_L levels might have utility as predictive markers of ABT-199 sensitivity. We next examined the in vivo anti-leukemic efficacy of ABT-199 in NOD SCID gamma (NSG) mice injected with luciferase-labeled MOLM-13 cells. The mice were treated with ABT-199 by daily oral gavage (a 2-wk treatment at dose of 100 mg/kg). Bioluminescence imaging showed that ABT-199 treatment significantly inhibited leukemia burden, which was also manifested by smaller spleen size and prolonged overall survival (p = 0.0004) when compared to the vehicle-treated mice. Furthermore, a 2-wk ABT-199 treatment significantly reduced leukemia burden (> 50%) in bone marrows of NSG mice engrafted with primary FLT3-mutated AML cells (i.e., a mean of 70 ± 16% human CD45⁺ cells in bone marrow of control mice (n = 9) versus 32.7 ± 12% in ABT-treated mice (n = 11), p = 0.00002). Conclusions: the in vitro and in vivo efficacy data indicates that ABT-199 is a selective BCL-2 inhibitor, a potent apoptogenic agent, and hence a promising candidate for AML BCL-2-targeted therapy.