In a patient with ER+/HER2+ breast cancer with significant residual disease post neoadjuvant TCHP, is there a role of using CDK4/6i in the adjuvant setting with T-DM1?

This is an open question. As has been true in oncology for some time, most of the adjuvant therapies we offer have shown benefit first in the metastatic setting, and then are tested in the adjuvant setting. Abemaciclib has been shown to have benefit when combined with herceptin and endocrine therapy in the metastatic setting (Tolaney et al., PMID 32353342).

However, what you are asking is yet unanswered. There is a possibility there will be benefit. However, given that TDM1 already has some benefit, it's not clear if the addition of abemaciclib will be worth the cost/toxicity. We will have to await answers from the eMonarcHER trial. (Tolaney et al., Journal of Clinical Oncology 2021). Until results from this trial are published, I do not th...