Ewing sarcoma (ES) is an aggressive bone and soft tissue malignancy that predominantly affects children and adolescents. CD99 is a cell surface protein that is highly expressed on ES cells and is required to maintain their malignancy. We screened small molecule libraries for binding to extracellular domain of recombinant CD99 and subsequent inhibition of ES cell growth. We identified two structurally similar FDA-approved compounds, clofarabine and cladribine that selectively inhibited the growth of ES cells in a panel of 14 ES vs. 28 non-ES cell lines. Both drugs inhibited CD99 dimerization and its interaction with downstream signaling components. A membrane-impermeable analog of clofarabine showed similar cytotoxicity in culture, suggesting that it can function through inhibiting CD99 independent of DNA metabolism. Both drugs drastically inhibited anchorage-independent growth of ES cells, but clofarabine was more effective in inhibiting growth of three different ES xenografts. Our findings provide a novel molecular mechanism for clofarabine that involves direct binding to a cell surface receptor CD99 and inhibiting its biological activities.

中文翻译：

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Clofarabine 通过涉及直接结合 CD99 的新型分子机制抑制尤文肉瘤的生长。

尤文肉瘤 (ES) 是一种侵袭性骨和软组织恶性肿瘤，主要影响儿童和青少年。CD99 是一种细胞表面蛋白，在 ES 细胞上高度表达，是维持其恶性肿瘤所必需的。我们筛选了与重组 CD99 的细胞外结构域结合并随后抑制 ES 细胞生长的小分子文库。我们确定了两种结构相似的 FDA 批准的化合物，氯法拉滨和克拉屈滨，它们在一组 14 种 ES 与 28 种非 ES 细胞系中选择性地抑制 ES 细胞的生长。这两种药物都抑制 CD99 二聚化及其与下游信号成分的相互作用。氯法拉滨的膜不可渗透类似物在培养物中表现出相似的细胞毒性，表明它可以通过抑制 CD99 发挥作用，而与 DNA 代谢无关。这两种药物都极大地抑制了 ES 细胞的贴壁非依赖性生长，但氯法拉滨在抑制三种不同 ES 异种移植物的生长方面更有效。我们的研究结果为氯法拉滨提供了一种新的分子机制，涉及直接结合细胞表面受体 CD99 并抑制其生物活性。