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R.S. Mali et al.
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Figure 3. Venetoclax combined with quizartinib to reduce disease burden in the tumor microenvironment of patient-derived FLT3-ITD+ xenograft models.
NOD/SCID/IL-2Rγnull (NSG) mice were engrafted with primary samples from Fms-like tyrosine kinase 3 wild-type (FLT3-WT) or FLT3 internal tandem duplication (FLT3- ITD+) patients and were treated orally with 5 mg/kg quizartinib, 100 mg/kg venetoclax or the combination once daily for 28 days. (A) Spleen images and spleen weight at the end of 28 days of dosing. Data is represented as average + standard deviation (n=3). *P=0.0004 for venetoclax vs. quizartinib for FLT3-WT model and **P=0.0068 for quizartinib vs. venetoclax for FLT3-ITD+ model by one-way ANOVA with Tukey post hoc test. (B) Percentage of human CD45+ cells in peripheral blood, bone marrow and spleen at the end of dosing. Data is represented as average + standard deviation (n=3). *P<0.0001 for venetoclax vs. quizartinib in the FLT3-WT model in the peripheral blood, bone marrow and spleen; **P=0.0005, P=0.01 and P<0.0001 for quizartinib vs. venetoclax for peripheral blood, bone marrow and spleen, respectively, for FLT3-ITD+ model; and *** P=0.0002 and P=0.0159 for quizartinib + venetoclax vs. quizartinib for bone marrow and spleen, respectively, for FLT3-ITD+ model by one-way ANOVA with Tukey post hoc test.
of mice remained disease-free showing long-term survival following combination treatment with venetoclax that was not observed with quizartinib monotherapy. All drug treatments were well tolerated based on minimal changes in body weight (Online Supplementary Figure S1). Importantly, combination activity was demonstrated at clinically relevant doses, as 5 mg/kg dose of quizartinib achieved Cmax of ~1 mM in NSG mice (Online Supplementary Figure S2), similar to the clinically effica- cious dose of 60 mg/day in patients.33
FLT3-ITD inhibition combines with venetoclax in primary patient samples
Studies were expanded to primary AML patient samples to confirm combination activity ex vivo. FLT3-WT and
FLT3-ITD+ primary samples were treated with quizar- tinib, venetoclax or the combination of both drugs and colony forming units were determined after 14 days. In 3 of 4 FLT3-ITD+ samples, venetoclax plus quizartinib impaired colony formation greater than each agent alone (Figure 2A), suggesting that the co-treatment targeted more FLT3-ITD+ progenitor cells, which is an important factor related to greater clinical response.
In vivo efficacy of quizartinib and venetoclax was also investigated in FLT3-WT and FLT3-ITD+ patient-derived xenograft (PDX) models (see Online Supplementary Figure S3A for cytogenetic characteristics). Following engraftment, mice were treated orally with vehicle, venetoclax (100 mg/kg), quizartinib (5 mg/kg), or the combination for 28 continuous days. Quizartinib prolonged survival of the
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haematologica | 2021; 106(4)