Bcl-xL–inhibitory BH3 mimetics can induce a transient thrombocytopathy that undermines the hemostatic function of platelets

Abstract

BH3 mimetics are a new class of proapoptotic anticancer agents that have shown considerable promise in preclinical animal models and early-stage human trials. These agents act by inhibiting the prosurvival function of one or more Bcl-2-related proteins. Agents that inhibit Bcl-x L induce rapid platelet death that leads to thrombocytopenia; however, their impact on the function of residual circulating platelets remains unclear. In this study, we demonstrate that the BH3 mimetics, ABT-737 or ABT-263, induce a time-and dose-dependent decrease in platelet adhesive function that correlates with ectodomain shedding of the major platelet adhesion receptors, glycoprotein Ib␣ and glycoprotein VI, and functional downregulation of integrin ␣ IIb ␤ 3 . Analysis of platelets from mice treated with higher doses of BH3 mimetics revealed the presence of a subpopulation of circulating platelets undergoing cell death that have impaired activation responses to soluble agonists. Functional analysis of platelets by intravital microscopy revealed a timedependent defect in platelet aggregation at sites of vascular injury that correlated with an increase in tail bleeding time.Overall, these studies demonstrate that Bcl-x L -inhibitory BH3 mimetics not only induce thrombocytopenia but also a transient thrombocytopathy that can undermine the hemostatic function of platelets. (Blood. 2011;118(6):1663-1674) IntroductionApoptosis is an evolutionarily conserved process important for mammalian development, tissue homeostasis, and immune tolerance. Apoptosis also acts as an important barrier against malignant transformation. Resistance to apoptosis is a hallmark feature of cancer, and several pathways regulating apoptosis are commonly altered in human malignancies. 1,2 For example, Ͼ 50% of all human cancers contain mutations in the p53 tumor suppressor gene, with nearly all of these mutations preventing p53 from triggering apoptosis. 3 Furthermore, the overexpression or enhanced function of certain prosurvival proteins, such as Bcl-2, 2 Bcl-x L , 2 PI3K/Akt/mTOR, 4 and nuclear factor-B, 5 is found in many types of human cancers. As a consequence, strategies targeting specific prosurvival pathways have gained prominence as novel anticancer therapies.Progress unraveling the complex pathways underlying apoptosis and cell survival, and elucidation of their roles in human cancers, has led to the development of several potential anticancer drugs, such as inhibitors of the Bcl-2 protein family, 6 inhibitors of apoptosis (IAPs), 7 MDM2, 8 PI3K/Akt/mTOR, 9 as well as TNFrelated apoptosis-inducing ligand. 10 A particularly promising approach is the ability to inhibit tumor cell survival through the use of agents that mimic the proapoptotic Bcl-2 homology 3 (BH3) domain proteins. BH3 proteins play an important role in inducing apoptosis by antagonizing the function of prosurvival Bcl-2 family proteins. Preclinical studies with the BH3 mimetic ABT-737, which selectively antagonizes Bcl-2, Bcl-x L , and Bcl-w, have demonstrated potent cytotoxic activity a...

Bcl-xL–inhibitory BH3 mimetic ABT-737 depletes platelet calcium stores

“…[1][2][3] These reports agree that 2 related BH3 mimetics, ABT-737 and ABT-263, induce platelet apoptosis. This is characterized by caspase-3 activation and caspase-dependent phosphatidylserine externalization, and may underlie the thrombocytopenia observed during administration of these drugs.…”

Platelet production proceeds independently of the intrinsic and extrinsic apoptosis pathways

“…Since the initial discovery that BCL-XL is the critical factor responsible for regulating murine platelet survival and lifespan 19 , it has been shown that Navitoclax, or its relative ABT-737 (ref. 23), trigger platelet apoptosis and consequent thrombocytopenia not only in mice 19,25 , but also in dogs 21 and humans 20,21,26 . Caspase inhibitors can block the short-term apoptotic effects of navitoclax and ABT-737 in human platelets 20,21 , just as they do in mouse 56 .…”

“…Moreover, pharmacological blockade of BCL-X L with the BH3 mimetic drugs ABT-737 (ref. 23) or navitoclax (ABT-263) 24 triggers BAK/BAX-mediated platelet apoptosis [19][20][21] and thrombocytopenia in mice 19,25 dogs 21 and humans 26 . Indeed, the major dose-limiting toxicity of navitoclax in clinical trials has proven to be thrombocytopenia 26 .…”

Caspase-9 mediates the apoptotic death of megakaryocytes and platelets, but is dispensable for their generation and function

“…29 Although it is now well established that Bcl-x L and Bak regulate platelet survival and lifespan at steady state, [30][31][32][33][34] the requirement for caspases in mediating platelet turnover is unclear. Certainly, induction of platelet apoptosis results in caspase activation, and caspase inhibition can block PS exposure, 33 but the physiologic significance of this phenomena remains to be clarified.…”