Fibroblast growth factor receptor 1 (FGFR1) is a receptor tyrosine kinase that plays a major role in developmental processes and metabolism. The dysregulation of FGFR1 through genetic aberrations leads to skeletal and metabolic diseases as well as cancer. For this reason, FGFR1 is a promising therapeutic target, yet a very challenging one due to potential on-target toxicity. More puzzling is that both agonistic and antagonistic FGFR1 antibodies are reported to exhibit similar toxicity profiles in vivo, namely weight loss. In this study, we aimed to assess and compare the mechanism of action of these molecules to better understand this apparent contradiction. By systematically comparing the binding of these antibodies and the activation or the inhibition of the major FGFR1 signaling events, we demonstrated that the molecules displayed similar properties and can behave either as an agonist or antagonist depending on the presence or the absence of the endogenous ligand. We further demonstrated that these findings translated in xenografts mice models. In addition, using time-resolved FRET and mass spectrometry analysis, we showed a functionally distinct FGFR1 active conformation in the presence of an antibody that preferentially activates the FGFR substrate 2 (FRS2)-dependent signaling pathway, demonstrating that modulating the geometry of a FGFR1 dimer can effectively change the signaling outputs and ultimately the activity of the molecule in preclinical studies. Altogether, our results highlighted how bivalent antibodies can exhibit both agonistic and antagonistic activities and have implications for targeting other receptor tyrosine kinases with antibodies.

成纤维细胞生长因子受体 1 (FGFR1) 是一种受体酪氨酸激酶，在发育过程和新陈代谢中起着重要作用。通过遗传畸变引起的 FGFR1 失调会导致骨骼和代谢疾病以及癌症。出于这个原因，FGFR1 是一个有前途的治疗靶点，但由于潜在的靶向毒性，它也是一个非常具有挑战性的靶点。更令人费解的是据报道激动性和拮抗性 FGFR1 抗体在体内表现出相似的毒性特征，即减肥。在这项研究中，我们旨在评估和比较这些分子的作用机制，以更好地理解这种明显的矛盾。通过系统地比较这些抗体的结合和主要 FGFR1 信号事件的激活或抑制，我们证明这些分子显示出相似的特性，并且可以根据内源性配体的存在或不存在而表现为激动剂或拮抗剂。我们进一步证明了这些发现转化为异种移植小鼠模型。此外，使用时间分辨 FRET 和质谱分析，我们在存在优先激活 FGFR 底物 2 (FRS2) 依赖性信号通路的抗体的情况下显示了功能上不同的 FGFR1 活性构象，证明调节 FGFR1 二聚体的几何结构可以有效地改变信号输出，并最终改变分子在临床前研究中的活性。总而言之，我们的结果强调了二价抗体如何表现出激动和拮抗活性，并且对用抗体靶向其他受体酪氨酸激酶有影响。