Abstract 494: Evaluation of soluble FLT3 ligand as a marker of bone marrow stress and recovery

Depletion of bone marrow (BM)-resident myeloid progenitor cells is a consequence of various anti-cancer treatments, including bispecific T-cell engager (BiTE®) antibodies targeting myeloid cell antigens. Currently, aspirates are required to determine the extent of myeloid cell depletion in the BM. This procedure is painful and potentially invasive for patients. As a surrogate marker for BM myeloid cell depletion, we evaluated soluble FLT3 ligand (FLT3L) in non-human primates treated with myeloid cell-depleting BiTE® antibodies. While FLT3L is not detectable in serum during steady-state hematopoiesis, it is detectable in response to hematopoietic deficiency. The goal of this study was to evaluate the relationship of serum FLT3L with depletion and recovery of BM myeloid cells in non-human primates treated with BiTE® antibodies against CD33. Serum FLT3L levels were determined using an anti-human FLT3L ELISA kit. Validation studies demonstrated that both the capture and detection antibodies recognized cynomolgus FLT3L. Serum FLT3L concentrations were determined using a standard curve with rhesus FLT3L (99.6% identical to cynomolgus FLT3L). Serum samples were taken from cynomolgus monkeys before, during and after treatment with anti-CD33 BiTE® antibodies and compared with the number of BM-resident target cells. In the first study, cynomolgus monkeys were dosed once with different dose levels of anti-CD33 BiTE® antibody ranging from 1- 15 μg/kg. BM-resident myeloid cells were transiently reduced in some, but not all, cohorts. Soluble FLT3L was detected only when there was a reduction in BM myeloid cells. When FLT3L was detectable, the serum concentration correlated with the extent of target cell reduction and FLT3L levels decreased as the myeloid cells recovered. In the second study, cynomolgus monkeys were dosed three times with anti-CD33 BiTE® antibody. In these studies target cell reduction was greater and persisted longer compared to the single dose studies. Similarly, the magnitude and duration of increased FLT3L in the multidose studies was also greater. Within the multi-dose study, FLT3L levels continued to increase as more BM-resident myeloid cells were depleted, suggesting that it was possible to detect cumulative stress to the BM. In summary, serum FLT3L levels are correlated with the extent and duration of myeloid cell depletion in the BM suggesting that serum FLT3L levels reflect depletion and subsequent myeloid cell replenishment. All studies to date have involved treatment with myeloid antigen-specific BiTE® antibodies; future studies will evaluate FLT3L levels following treatment with anti-lymphoid or solid tumor antigen-specific BiTE® antibodies to determine if FLT3L increases are associated with myeloid cell depletion or indicative of general BM stress or inflammation. The potential benefit of this marker is that it could allow for repeat non-invasive BM monitoring in patients on myelosuppressive therapies. Citation Format: Christine Sastri, Mercedesz Balazs, Priya Koppikar, James R. Lipford, Angela Coxon, Tara L. Arvedson. Evaluation of soluble FLT3 ligand as a marker of bone marrow stress and recovery. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 494.