Abemaciclib

It is an inhibitor of cyclin-dependent kinases (CDK) 4 and 6. It is used to treat advanced or metastatic breast cancer, HR+ or HER2-. It is given alone or with an aromatase inhibitor or estrogen antagonist. fulvestrant. Oral administration twice a day.

At the date of the last update we did not find any published data on its excretion in breast milk.

Its pharmacokinetic data (very high percentage of protein binding, moderately high molecular weight and very wide volume of distribution) make it highly unlikely that significant quantities will pass into breast milk.

Diarrhea, loss of appetite, infections, leukopenia, neutropenia, anemia, and hepatotoxicity are common adverse reactions.

It is known via Pharmacokinetics that after 3 elimination half-lives (T½) 87.5% of the drug is eliminated from the body; after 4 T½ 94%, after 5 T½ 96.9%, after 6 T½ 98.4% and after 7 T½ 99%. Plasma drug concentrations in the body are negligible after 7 T½. In general, a period of al least five half-lives may be considered a safe waiting period before returning to breastfeeding. ^{(Anderson 2016)}

Taking the longest published T½, these 5 T½ would correspond to 5 days. Meanwhile, express and discard milk from the breast regularly to maintain production.  This does not allow breastfeeding during treatment.

If it is administered during lactation, a clinical and analytical control in the infant is necessary for the possible side effects described.