- Pregnant women are excluded from this study because abemaciclib is an agent with the potential for teratogenic or abortifacient effects. Because there is an unknown but potential risk for adverse events in nursing infants secondary to treatment of the mother with abemaciclib, breastfeeding should be discontinued if the mother is treated with abemaciclib. A negative serum pregnancy test is required for women of childbearing potential prior to study entry.