Among the expanding population of elderly AML patients, aggressive therapy options are often limited due to other medical conditions and the decreased tolerance of these patients to conventional chemotherapy. Internal tandem duplications (ITD) of the FLT3 juxtamembrane domain occur in 20–30% of AML patients and predict poor outcome. Monotherapy with FLT3-tyrosine kinase inhibitors (TKIs) has produced biological and some clinical responses. Side effects of TKI and chemoagents are only moderately overlapping. We hypothesized that the use of FLT3 inhibitors may allow dose reduction of chemotherapy with resultant decreased side effects, but without loss of antileukemic activity. Using primary AML blasts, we found that combination of the TKI tandutinib (Tn) (or sunitinib (Sn)) with cytarabine (Cy) and/or daunorubicin (Dn) has a favorable growth inhibitory activity compared to Cy/Dn alone. Of note, the effects were only seen in cells expressing a FLT3 ITD mutation. In order to perform more detailed mathematical analyses, we utilized three cell lines containing FLT3 ITD mutations: a BaF3 cell line transduced with an ITD mutant FLT3 as well as MV4-11 and MOLM14 cell lines which have naturally occurring FLT3 ITD mutations. Dilution series were set up with the single agents as well as different drug combinations to create isobolograms using the method of Chou and Talalay. Next, dose reduction indices (DRI) were computed using calcusyn® software. DRI for the combination of Tn with Cy demonstrated a 6 (MV4:11) - 19 (Ba/F3 FLT3 ITD) fold potential dose reduction of Cy at ED90 for induction of apoptosis. In addition, the DRI of cytarabine in combination with Sn would allow a 2–145 fold cytarabine dose reduction for the various cell lines. The combination of Dn with Tn or Sn demonstrated a DRI up to 6 fold. Similar DRI were seen when inhibition of cellular proliferation was the final endpoint. Of note, Sn plus Cn or Dn displayed a - moderate but beneficial - DRI of 2 in Ba/F3 cells expressing the FLT3 wildtype isoform. Also, triple-combination allowed up to 10 fold dose reduction of the chemoagents - in cell lines as well as native blasts. We conclude that dose reduction strategies by adding TKI may be clinically useful, particularly for elderly AML patients or patients with co-existent illnesses that would predict for a poor tolerance of standard chemotherapy regimens.

Author notes

Disclosure:Consultancy: Novartis, Pfizer. Ownership Interests:; MolecularMD. Research Funding: Novartis, Pfizer, Merit Review Grant from the Department of Veterans Affairs, Doris Duke Charitable Foundation, Leukemia and Lymphoma Society, Deutsche Krebshilfe Foundation, fortüne-Program of the Tübingen Medical Faculty. Paid Export Testimony Information: Novartis. Membership Information: Novartis.