Cytosine Arabinoside

Cytarabine is an analogue of pyrimidine nucleoside with antineoplastic, antiviral and immunosuppressive properties.
It is used in the treatment of leukemia and lymphomas intravenously, intrathecally or subcutaneously in very varied patterns depending on the type of treatment and the patient's response.

Since the last update we have not found any published data on its excretion in breast milk.

Its pharmacokinetic data (low molecular weight, low protein binding and high pKa) make transfer into milk probable, although its rapid elimination would mean it would be present in very low concentrations and for a short period of time.

It is known from pharmacokinetics that after 3 elimination half-lives (T½), 87.5% of the drug is eliminated from the organism; after 4 T½ it is 94%, after 5 T½, 96.9%, after 6 T½, 98.4% and after 7 T½, 99%. From 7 T½ the plasma concentrations of the drug in the body are negligible. In general, a period of at least five half-lives can be considered a safe waiting period before breastfeeding again (Anderson 2016).

Taking the longest published T½ of all the active metabolites as a reference, these 5 T½ would correspond to 15 hours. Due to major side effects, it would be advisable to wait 7 T½, which would correspond to 21 hours. Meanwhile, express and discard milk from the breast regularly to maintain production.

The LIPOSOMAL FORM of cytarabine or cytarabine DEPOT is used in intrathecal injection in lymphomatous meningitis. Although this liposomal form has a long elimination period (4 to 11 days), it slowly passes from the CSF to plasma, rapidly degrading into an inactive metabolite, so the systemic exposure is negligible (EMA 2011) and the passage to breast milk in significant amounts very unlikely.

This medication is not effective when administered orally (AEMPS 2017). Its low oral bioavailability complicates the passage to infant plasma from the ingested breast milk, except in premature infants and the immediate neonatal period in which there may be increased intestinal permeability.

When it is possible to do so, milk detections of each patient to determine the total elimination of the drug would be the best indicator to resume breastfeeding between two cycles of chemotherapy.

During the treatment of cancer, breastfeeding should be interrupted temporarily due to potentially serious side effects for the infant. Chemotherapy does not affect milk production during or after treatment.
Abrupt weaning can be psychologically traumatic for both the mother and the infant (Pistilli 2013). If the mother wishes, production of milk can be maintained by regularly expressing breastmilk, being able to resume breastfeeding in the periods in which no significant traces of the drug remain in the milk (Anderson 2016) or at the end of the treatment (Pistilli 2013).

Some chemotherapeutic agents with an antibiotic effect can alter the composition of the microbiota (bacterial set or bacterial flora) of the milk and the concentration of some of its components (Urbaniak 2014). This possibly occurs temporarily with subsequent recovery, although no harmful effects are assumed or have been reported in breastfed infants.

Women undergoing chemotherapy during pregnancy have lower rates of breastfeeding because they experience difficulties in breastfeeding (Stopenski 2017), needing more support to achieve it.

Given the strong evidence that exists regarding the benefits of breastfeeding for the development of babies and the health of mothers, it is advisable to evaluate the risk-benefit of any maternal treatment, including chemotherapy, individually advising each mother who wishes to continue with breastfeeding (Koren 2013).