Phosphoinositide-3-kinase-γ (PI3Kγ) is implicated as a target to repolarize tumour-associated macrophages and promote antitumour immune responses in solid cancers^1,2,3,4. However, cancer cell-intrinsic roles of PI3Kγ are unclear. Here, by integrating unbiased genome-wide CRISPR interference screening with functional analyses across acute leukaemias, we define a selective dependency on the PI3Kγ complex in a high-risk subset that includes myeloid, lymphoid and dendritic lineages. This dependency is characterized by innate inflammatory signalling and activation of phosphoinositide 3-kinase regulatory subunit 5 (PIK3R5), which encodes a regulatory subunit of PI3Kγ⁵ and stabilizes the active enzymatic complex. We identify p21 (RAC1)-activated kinase 1 (PAK1) as a noncanonical substrate of PI3Kγ that mediates this cell-intrinsic dependency and find that dephosphorylation of PAK1 by PI3Kγ inhibition impairs mitochondrial oxidative phosphorylation. Treatment with the selective PI3Kγ inhibitor eganelisib is effective in leukaemias with activated PIK3R5. In addition, the combination of eganelisib and cytarabine prolongs survival over either agent alone, even in patient-derived leukaemia xenografts with low baseline PIK3R5 expression, as residual leukaemia cells after cytarabine treatment have elevated G protein-coupled purinergic receptor activity and PAK1 phosphorylation. Together, our study reveals a targetable dependency on PI3Kγ–PAK1 signalling that is amenable to near-term evaluation in patients with acute leukaemia.

磷酸肌醇-3-激酶-γ (PI3Kγ) 是实体瘤中肿瘤相关巨噬细胞复极化和促进抗肿瘤免疫反应的靶标^1,2,3,4。然而，PI3Kγ 的癌细胞内在作用尚不清楚。在这里，通过将无偏见的全基因组 CRISPR 干扰筛选与急性白血病的功能分析相结合，我们定义了包括骨髓、淋巴和树突谱系在内的高风险亚群中对 PI3Kγ 复合物的选择性依赖性。这种依赖性的特征是先天炎症信号传导和磷酸肌醇 3-激酶调节亚基 5 ( PIK3R5 ) 的激活，PIK3R5 编码 PI3Kγ ⁵的调节亚基并稳定活性酶复合物。我们将 p21 (RAC1) 激活激酶 1 (PAK1) 确定为 PI3Kγ 的非典型底物，介导这种细胞内在依赖性，并发现通过 PI3Kγ 抑制使 PAK1 去磷酸化会损害线粒体氧化磷酸化。使用选择性 PI3Kγ 抑制剂 eganelisib 治疗PIK3R5激活的白血病有效。此外，与单独使用任一药物相比，eganelisib 和阿糖胞苷的组合可以延长生存期，即使在基线 PIK3R5 表达较低的患者来源的白血病异种移植物中也是如此，因为阿糖胞苷治疗后残留的白血病细胞具有升高的 G 蛋白偶联嘌呤能受体活性和 PAK1 磷酸化。总之，我们的研究揭示了对 PI3Kγ-PAK1 信号传导的可靶向依赖性，适合急性白血病患者的近期评估。