Background While deregulation of the cyclin D1-CDK4/6-retinoblastoma pathway is common in hormone receptor positive (HR+) breast cancer, Rb is usually intact in HR+ breast cancer, and targeted CDK 4/6 inhibitors that act upstream of Rb, are routinely being utilized in clinical practice. However, factors that can lead to clinical resistance to CDK 4/6 inhibitors are not known. Patients and methods We identified patients who had pre- and post-genotyping in tissue and peripheral blood samples after receiving CDK 4/6 inhibitors. Genotyping was carried out in tumor tissue or blood collected before start of CDK 4/6 inhibitor and after disease progression on CDK 4/6 inhibitor, covering more than 90% of the coding region in RB1. Results We identified detectable acquired RB1 mutations in circulating tumor DNA (ctDNA) after exposure to CDK4/6 inhibitor (palbociclib, palbociclib, ribociclib) for 5, 8, and 13 months, respectively, in three patients. The RB1 mutations included substitution in donor splicing site of exon 8 of the RB1 gene in patient #1; substitution in donor splicing site of exon 22 of RB1 gene, exon 19 deletion, exon 3 insertion in patient #2; and RB1 exon 16 H483Y mutation in patient #3. None of these RB1 mutations were present in the pre-CDK 4/6 specimen highlighting these molecular alterations, which lead to functional loss of Rb1, likely emerged under selective pressure from the CDK4/6 inhibitor potentially confering therapeutic resistance. Conclusion This is the first clinical report to describe the emergence of somatic RB1 mutations after exposure to palbociclib or ribociclib, in patients with metastatic breast cancer. Further research is needed to validate these findings, identify how these mutations temporally emerge under selective pressure of CDK 4/6 inhibitor, and develop rational therapeutic strategies.

中文翻译：

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转移性乳腺癌患者的多克隆RB1突变和对CDK 4/6抑制剂的获得性耐药。

背景虽然在激素受体阳性（HR +）乳腺癌中常见细胞周期蛋白D1-CDK4 / 6-视网膜母细胞瘤途径的失调，但Rb在HR +乳腺癌中通常是完整的，并且在Rb上游起作用的靶向CDK 4/6抑制剂是常规的在临床实践中被利用。但是，尚不知道可能导致对CDK 4/6抑制剂产生临床耐药性的因素。患者和方法我们确定了接受CDK 4/6抑制剂后在组织和外周血样本中进行过基因分型的患者。在开始CDK 4/6抑制剂之前和在疾病进展之后在CDK 4/6抑制剂上进行的肿瘤组织或血液中进行基因分型，覆盖RB1中90％以上的编码区。结果我们在三例患者中分别暴露于CDK4 / 6抑制剂（palbociclib，palbociclib，ribociclib）分别5、8和13个月后，在循环肿瘤DNA（ctDNA）中发现了可检测到的获得性RB1突变。RB1突变包括＃1患者中RB1基因外显子8供体剪接位点的替换；RB1基因外显子22供体剪接位点的替换，外显子19缺失，2号患者外显子3插入；＃3患者的RB1外显子和RB1外显子16 H483Y突变。这些RB1突变均未出现在前CDK 4/6标本中，突显了这些分子改变，这导致Rb1的功能丧失，很可能是在选择性压力下从CDK4 / 6抑制剂中出现的，可能赋予治疗抵抗力。结论这是描述转移性乳腺癌患者暴露于palbociclib或ribociclib后体细胞RB1突变出现的第一份临床报告。需要进一步的研究来验证这些发现，确定这些突变如何在CDK 4/6抑制剂的选择性压力下暂时出现，并制定合理的治疗策略。