Constitutively activated AKT kinase is a common feature of T-cell acute lymphoblastic leukemia (T-ALL). Here, we report that the novel AKT inhibitor, A443654, leads to rapid cell death of T-ALL lines and patient samples. Treatment of CEM, Jurkat, and MOLT-4 cells with nanomolar doses of the inhibitor led to AKT phosphorylation accompanied by dephosphorylation and activation of the downstream target, glycogen synthase kinase-3beta. Effects were time and dose-dependent resulting in apoptotic cell death. Treatment of Jurkat cells with A443654 resulted in activation of caspase-2, -3, -6, -8, and -9. Apoptotic cell death was mostly dependent on caspase-2 activation, as demonstrated by pre-incubation with a selective pharmacological inhibitor. Remarkably, A443654 was highly effective against the drug-resistant cell line CEM-VBL100, which expresses 170 kDa P-glycoprotein. Moreover, A443654 synergized with the DNA damaging agent etoposide in both drug-sensitive and drug-resistant cell lines when co-administered (CI=0.39), or when pre-treated with etoposide followed by A443654 (CI=0.689). The efficacy of A443654 was confirmed using blasts from 6 patients with T-ALL, all of whom displayed low levels of PTEN and constitutive phosphorylation of Akt on S473. At 1 microM concentration, the inhibitor was able to induce apoptotic cell death of T-ALL blast cells, as indicated by flow cytometric analysis of samples immunostained for active (cleaved) caspase-3. As activated AKT is seen in a large percentage of T-ALL patients, A443654, either alone or in combination with existing drugs, may be a useful therapy for primary and drug-resistant T-ALL.