I would continue TKI at full dose. I have now done this with chemotherapy in a couple of dozen cases and only in one case of bladder cancer, did the drug have to stop. Remember, it should be active only against CML clones, not the normal clones in a patient who has been on it for a while and thus is not likely going to have an impact on the normal stem cells which will be affected by the chemo. Unless there is a potential interaction between the chemo and the TKI you should be fine.
Check for all the reasons for loss of TKI response. If a switch is necessary, you could make a theoretical argument about using dasatinib because of the src activity which may have some impact on the breast cancer as well.

From CML point of view check compliance and a mutation screen.

For breast cancer continue imatinib 400mg but be on the look out for greater degree of neutropenia with each cycle of chemo - use G-CSF if required to support neutrophil counts. If severe neutropenia may need to stop imatinib temporarily (7 days) at expected neutropenia.

Tessa Holyoake

I would suggest to continue the therapy with imatinib 400 mg. I would switch to a second generation TKI only if CCyR is lost and after testing for the presence of BCR-ABL mutations as it is possible that the present loss of MMR is simply due to a lack of proper adherence, due to the concomitant discovery of breast cancer and maybe the thinking that breast cancer has been caused by imatinib. This patient needs a strong support, by doctors, family and psycologists. The intake of the imatinib therapy can be continued during the days of the chemotherapy cycles unless excessive gastrointestinal toxicity will develop. The imatinib can be discontinued only during the days of the post-chemo aplasia to avoid hematotoxicity and to facilitate recovery of the WBC and platelets. Multiple possible interactions between tamoxifen and imatinib have been described, as both are metabolised by CYP450. Other possible mechanisms of interaction have been reported. In this case the imatinib dosage should be modulated based on the response and on the side effects observed and (if possible) by imatinib blood level testing.

53 y-old lady with CML diagnosed February 2013. She achieved MMR on imatinib 400mg daily but lost it in September 2014, BCR-ABL/ABL being now 0.24% (IS). Last week was diagnosed with breast cancer still being evaluated. Any particular suggestions regarding TKI during chemotherapy she will likely need?
Manuel Abecasis