Unfortunately, none of the provided sources provide information on how long lurbinectedin stays in breast milk. The first source [1] only provides information on the patent status of the drug, while the second source [2] and third source [3] provide general information about the drug's uses, side effects, and precautions. However, both sources do mention that it is not known whether lurbinectedin passes into breast milk and that women who are breastfeeding should consult with their healthcare provider before taking the drug. It is important to note that lurbinectedin is a chemotherapy drug and may be harmful to a nursing infant. Therefore, healthcare providers may recommend that women stop breastfeeding while taking the drug.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LURBINECTEDIN
[2] https://medlineplus.gov/druginfo/meds/a620049.html
[3] https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=632bb50c-3bcb-4c85-9056-fc33410550ae&audience=consumer