Intracellular expression of FLT3 in Purkinje cells: implications for adoptive T-cell therapies

Nonclinical Safety Assessment of AMG 553, an Investigational Chimeric Antigen Receptor T-Cell Therapy for the Treatment of Acute Myeloid Leukemia

“…Therefore, to understand potential off-tumor, on-target liabilities, FLT3 mRNA and FLT3 protein expression were thoroughly assessed in normal human tissues using numerous complementary methods. Full results are presented elsewhere ( Brauchle et al , 2020 ) but were consistent with published results ( Çakmak-Görür et al , 2019 ; Kikushige et al , 2008 ). Briefly, cell surface FLT3 protein in human tissues was only detected in a portion of bone marrow hematopoietic stem and progenitor cells in addition to rare, scattered cells in the tonsil; FLT3 protein in other cell types was cytoplasmic.…”

“…In vitro AMG 553-mediated cytotoxicity was observed for a subset of CD34 + bone marrow cells (Goldstein, Frank, Dunn, Kim, Wang, Karbowski, Balazs, Coxon, Koppikar, Lebrec, and Arvedson, in preparation) and was not observed against other cells, including cortical neurons. Despite some tissues that were previously reported to have detectable FLT3 transcript or FLT3 intracellular protein ( Brauchle et al , 2020 ; Çakmak-Görür et al , 2019 ), many of the cells obtained for this study did not have detectable FLT3 transcript or FLT3 protein. Donor variability, heterogeneity of cell types in a tissue, and/or resident immune cells may explain the lack of FLT3 transcript and FLT3 protein detection in the cultured cells in this study.…”

“…To determine if FLT3 expression in cynomolgus monkey was representative of human tissue expression, FLT3 mRNA and FLT3 protein expression were assessed in a variety of normal cynomolgus monkey tissues. Overall, FLT3 expression distribution across tissues and cellular localization was similar between human ( Brauchle et al , 2020 ; Çakmak-Görür et al , 2019 ; Kikushige et al , 2008 ) and cynomolgus monkey. Cell surface FLT3 protein expression in cynomolgus monkey was restricted to bone marrow hematopoietic stem and progenitor cells ( Brauchle et al , 2020 ); in other tissues, FLT3 protein was cytoplasmic and, therefore not expected to be accessible to AMG 553.…”

“…Due to the specificity of AMG 553 for FLT3 (Goldstein, Frank, Dunn, Kim, Wang, Karbowski, Balazs, Coxon, Koppikar, Lebrec, and Arvedson, in preparation), AMG 553 is not anticipated to exhibit off-target cytotoxicity against cells that do not express FLT3 protein. To further assess the activity of AMG 553, in vitro cytotoxicity experiments were performed to assess potential “off-tumor, on-target” effects of AMG 553 on human cells from tissue types with previous evidence of FLT3 transcript and/or FLT3 intracellular protein but no detectable cell surface FLT3 protein expression ( Brauchle et al , 2020 ; Çakmak-Görür et al , 2019 ). Cells from additional tissues representing major organs with no previous evidence of FLT3 transcript or FLT3 protein were also evaluated to assess potential “off-target” activity.…”

Peer Review #1 of "Risk assessment of FLT3 and PAX5 variants in B-acute lymphoblastic leukemia: a case–control study in a Pakistani cohort (v0.2)"

“…At carboxyl terminus, there are two kinase domains, separated by a kinase insert region (Gilliland and Griffin, 2002). FLT3 is expressed in normal human bone marrow especially in CD34+ hematopoietic stem, brain (Çakmak-Görür et al, 2019) and gonads (Matthews et al, 1991;Small et al, 1994) and encodes 1000 amino acid protein in humans. In the hematopoietic tissues, Binding of FL with its receptor causes auto-phosphorylation of tyrosine residues present in the kinase domain and stimulates growth of progenitor cells in the marrow and blood (Marhäll et al, 2018).…”