Monitoring of FLT3 Phosphorylation and FLT3 Ligand Levels in Patients with FLT3-ITD Mutated Acute Myeloid Leukemia (AML) Treated with Midostaurin within the AMLSG 16-10 Trial of the German-Austrian Study Group
Authors: Theis Frauke , Weber Daniela , Rücker Frank G. , Paschka Peter , Gaidzik Verena I. , Thol Felicitas , Heuser Michael , Fiedler Walter , Wulf Gerald , Lübbert Michael , Salih Helmut R. , Schroeder Thomas Michael , Götze Katharina S. , Salwender Hans Jürgen , Kindler Thomas , Horst Heinz-August , Ringhoffer Mark , Wolf‎ Dominik , Krauter Jürgen , Schlenk Richard F. , Bullinger Lars , Ganser Arnold , Döhner Hartmut , Döhner Konstanze 
Published: October 2019

Abstract
Background: Target inhibition of FLT3 by therapy with the recently FDA- and EMA-approved multi-targeted tyrosine kinase inhibitor (TKI) midostaurin can be monitored by plasma inhibitor activity (PIA) analysis by visualizing the level of target-dephosphorylation as previously described. When combining intensive chemotherapy with midostaurin, we have recently shown that the TKI achieves the lowest level of FLT3 phosphorylation (p-FLT3) at the end of the 1st induction cycle, indicating a deep target inhibition. However, sufficient inhibition could not be maintained during subsequent cycles by midostaurin in combination with chemotherapy, but it was reestablished during maintenance therapy with the TKI alone. Recent data indicate that this might be due to an increase in FLT3 ligand (FL) plasma levels induced by concomitant intensive chemotherapy.
Aim: To individually measure the plasma levels of FL and to correlate the results with those from PIA analysis at defined time points during treatment in a large cohort of FLT3-ITD AML patients (pts) treated within our AMLSG 16-10 trial (NCT01477606).
Methods: FL levels were measured in plasma samples from pts (age 18-70 years) with newly diagnosed FLT3-ITD positive AML obtained at defined time points during therapy in which PIA analyses were also previously performed. All pts were enrolled in the AMLSG 16-10 trial applying intensive standard chemotherapy in combination with midostaurin. For consolidation therapy allogeneic hematopoietic cell transplantation (allo HCT) was intended whereas pts not eligible for allo HCT received 3 cycles of age-adapted high-dose cytarabine (HiDAC) in combination with midostaurin starting on day 6, followed by one year of midostaurin maintenance therapy for both groups. FL levels were measured at diagnosis, at day 15 and at the end of each treatment cycle, after allo HCT and monthly during maintenance therapy using a Quantikine® ELISA kit obtained from R&D Systems®.
Results: So far, we have analyzed 709 plasma samples from 68 pts at the time of diagnosis (n=62), during (day 15, n=73) and after (n=83) 1st and 2nd induction cycle, during (day 15, n=69) and after (n=82) consolidation therapy, after allo HCT (n=36) as well as during maintenance therapy (n=304). The median level of FL at diagnosis was 5.2pg/ml (0 - 66.2pg/ml). At day 15 of the 1st induction cycle FL levels showed a drastic increase (median 1057.3pg/ml; 23.6 - 2287.8pg/ml) which maintained high at day 15 of each following consolidation cycle, up to a maximum of 1696.6pg/ml (133.4 - 2461pg/ml) in median at day 15 of the 3rd consolidation cycle. Interestingly, at this time point p-FLT3 levels in median (80.2%; 32.6 - 100%) reached highest values indicating a loss of target inhibition. Of note, FL levels decreased at the end of each treatment cycle with a median level between 116.6pg/ml (19.7 - 1676.7pg/ml) and 184.5pg/ml (10.4 - 2398.3pg/ml) supporting the hypothesis of an induction of FL secretion during each treatment cycle due to concomitant chemotherapy. Consistent with this hypothesis, median FL levels decreased and stayed low during the 12 months of TKI maintenance therapy without concomitant chemotherapy with the lowest level after month 5 (median 186.7pg/ml; 125.2 - 468.6pg/ml) congruent with our previous results of a decrease in p-FLT3 levels and reestablished target inhibition during maintenance therapy. Interestingly, pts who received allo HCT showed significantly higher median FL levels after 6 months of maintenance therapy than pts who received consolidation chemotherapy (230.3pg/ml; (58.8 - 441pg/ml) vs 169.8pg/ml; (60.6-218.5pg/ml); P=.03). However this has no impact on the median p-FLT3 level at this time point.
Conclusions: In our study of FLT3-ITD positive AML pts treated with midostaurin in combination with intensive chemotherapy or allo HCT we could observe a drastic increase of FL plasma levels promptly after start of chemotherapy followed by loss of stable target inhibition. In contrast, during maintenance therapy with the TKI alone FL plasma levels decreased and remained low. This correlated with a decrease of p-FLT3 levels as well indicating target inhibition. Further studies are needed to evaluate if different scheduling of the TKI in combination with chemotherapy might overcome the loss of target inhibition and if this might improve clinical outcome. These pharmacodynamic data may provide support for single-agent TKI maintenance therapy.

Disclosures
Paschka: Astellas: Membership on an entity's Board of Directors or advisory committees, Travel support; Agios: Membership on an entity's Board of Directors or advisory committees; Sunesis: Membership on an entity's Board of Directors or advisory committees; Jazz: Speakers Bureau; Bristol-Meyers Squibb: Other: Travel support, Speakers Bureau; Pfizer: Membership on an entity's Board of Directors or advisory committees; Otsuka: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Other: Travel support, Speakers Bureau; Celgene: Membership on an entity's Board of Directors or advisory committees, Other: Travel support, Speakers Bureau; Astex: Membership on an entity's Board of Directors or advisory committees; Amgen: Other: Travel support; Janssen: Other: Travel support; Takeda: Other: Travel support. Fiedler:Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees; ARIAD/Incyte: Membership on an entity's Board of Directors or advisory committees, support for meeting attendance; Novartis: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Patents & Royalties; Amgen: Research Funding; Pfizer: Research Funding; Amgen: Other: support for meetíng attendance; Gilead: Other: support for meeting attendance; GSO: Other: support for meeting attendance; Teva: Other: support for meeting attendance; JAZZ Pharmaceuticals: Other: support for meeting attendance; Daiichi Sankyo: Other: support for meeting attendance. Lübbert:Janssen: Honoraria, Research Funding; Celgene: Other: Travel Grant; Teva: Other: Study drug. Salih:Several patent applications: Patents & Royalties: e.g. EP3064507A1. Schroeder:Celgene: Consultancy, Honoraria, Research Funding. Götze:JAZZ Pharmaceuticals: Honoraria; Celgene: Honoraria, Research Funding; Takeda: Honoraria, Other: Travel aid ASH 2017; Novartis: Honoraria. Salwender:Amgen: Honoraria, Other: travel suppport, Research Funding; Novartis: Honoraria, Other: travel suppport, Research Funding; Celgene: Honoraria, Other: travel suppport, Research Funding; Takeda: Honoraria; Bristol-Myers Squibb: Honoraria, Other: travel suppport, Research Funding; Janssen: Honoraria, Other: travel support, Research Funding. Schlenk:Pfizer: Research Funding, Speakers Bureau. Bullinger:Amgen: Honoraria, Speakers Bureau; Bristol-Myers Squibb: Speakers Bureau; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sanofi: Research Funding, Speakers Bureau; Bayer Oncology: Research Funding; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Pfizer: Speakers Bureau; Janssen: Speakers Bureau. Ganser:Novartis: Membership on an entity's Board of Directors or advisory committees. Döhner:Pfizer: Research Funding; Agios: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Astex Pharmaceuticals: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; AROG Pharmaceuticals: Research Funding; Pfizer: Research Funding; Bristol Myers Squibb: Research Funding; AbbVie: Consultancy, Honoraria; Astex Pharmaceuticals: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding; AROG Pharmaceuticals: Research Funding; Astellas: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria; Astellas: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding; Bristol Myers Squibb: Research Funding; Jazz: Consultancy, Honoraria; Celator: Consultancy, Honoraria; Seattle Genetics: Consultancy, Honoraria; Celator: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Sunesis: Consultancy, Honoraria, Research Funding; Sunesis: Consultancy, Honoraria, Research Funding; Jazz: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; Seattle Genetics: Consultancy, Honoraria; Agios: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding.