Targeting the ERK Pathway in KRAS-and BRAF-Driven Lung Cancers

BRAF突变或KRAS突变的肿瘤依赖ERK信号，对MEK抑制剂敏感，但只有BRAF突变的肿瘤对RAF抑制剂敏感。我们已经证明RAF抑制剂变构激活ras依赖性F？AF二聚体存在于大多数正常细胞和肿瘤细胞中，因此矛盾地激活信号传导。然而，在V600E BRAF突变的肿瘤中，活化的ERK引起RAS的反馈抑制。GTP水平太低，无法支持RAF二聚化，在这种情况下，V600E作为单体发出信号。RAF抑制剂与单体结合并有效抑制这些肿瘤中的ERK信号传导。这是与MEK抑制剂相比，密码子600 BRAF突变的黑色素瘤对这些药物的显著治疗反应的基础。然而，肿瘤反应是不完整的，而且往往是暂时的。肿瘤进展是由于获得性耐药，通常以ERK信号对RAF抑制剂不敏感为特征。我们已经证明，这可能是通过诱导RAS介导的。GTP或剪切以ras独立方式二聚化的RAF变体。我们还认为，最初的肿瘤反应受到肿瘤对ERK抑制的适应的限制。在RAS和BRAF突变肿瘤中，ERK激活导致其他细胞内信号通路的反馈抑制，并使细胞依赖ERK信号通路。这导致对RAF抑制剂（突变BRAF肿瘤）或MEK抑制剂（突变BRAF和一些KRAS肿瘤）过敏。然而，用这些药物抑制ERK信号可以缓解这种反馈，减弱对ERK输出的抑制，并激活其他有丝分裂信号通路，导致对ERK抑制的适应性抵抗。我们在本提案中的目标是通过将RAF或MEK抑制剂与选择性MEK和RTK抑制剂结合来开发最大限度地抑制ERK输出的疗法，以防止RAF的反馈再激活。我们假设，这些方案对ERK输出的最大抑制将减轻对受体酪氨酸激酶信号传导的反馈抑制，并以这种方式引起耐药性。我们将确定这些再激活途径，然后开发和测试基于最大ERK抑制结合关键再激活受体抑制的疗法，以防止或限制适应性抵抗。

Tumors with mutant BRAF or mutant KRAS are dependent on ERK signaling and sensitive to MEK inhibitors, but only the BRAF mutant tumors are sensitive to RAF inhibitors. We have shown that RAF inhibitors allosterically activate RAS-dependent F?AF dimers in most normal and tumor cells and thus paradoxically activate signaling. However, in tumors with V600E BRAF mutation, activated ERK causes feedback inhibition of RAS.GTP to a levels too low to support RAF dimerization and in this context V600E signals as a monomer. RAF inhibitors bind to the monomer and potently inhibit ERK signaling in these tumors. This is basis for the dramatic therapeutic response of melanomas with codon 600 BRAF mutation to these drugs compared to MEK inhibitors. However, tumor responses are incomplete and often temporary. Tumor progression is due to acquired resistance often characterized by insensitivity of ERK signaling to the RAF inhibitor. We have shown that this may be mediated by induction of RAS.GTP or to splice variants of RAF that dimerize in a Ras-independent manner. We also believe that the initial tumor response is limited by adaptation of the tumor to inhibition of ERK. In RAS and BRAF mutant tumors, ERK activation causes the feedback inhibition of other intracellular signaling pathways and renders the cell dependent on ERK signaling. This causes hypersensitivity to RAF inhibitors (mutant BRAF tumors) or MEK inhibitors (mutant BRAF and some KRAS tumors). However, inhibition of ERK signaling with these drugs relieves this feedback, attenuates inhibition of ERK output, and activates other mitogenic signaling pathway that cause adaptive resistance to ERK inhibition. Our goals in this proposal are to develop therapies that maximally inhibit ERK output by combining RAF or MEK inhibitors with selective MEK and RTK inhibitors that prevent feedback reactivation of RAF. We hypothesize that maximal inhibition of ERK output with these regimens will relieve feedback inhibition of receptor tyrosine kinase signaling and cause resistance in that manner. We will identify these reactivated pathways and then develop and test therapies based on maximal ERK inhibition combined with inhibition of key reactivated receptors to prevent or limit adaptive resistance.