Hitting the snooze button: Inducing quiescence with the FLT3 inhibitor quizartinib protects hematopoietic progenitors from chemotherapy

ABSTRACT

Myelosuppression is one of the most severe and limiting side effects of chemotherapy. Our recent work outlines a strategy to prevent chemotherapy-induced myelosuppression by administering a priming dose of the FMS-Like Tyrosine kinase 3 (FLT3) inhibitor quizartinib. Furthermore, by administering sequential quizartinib primed injections of fluorouracil (5-FU), we demonstrated a novel and effective strategy to eliminate disease in two mouse models of quizartinib resistant acute myeloid leukemia (AML).

Chemotherapy-induced myelosuppression poses a serious problem for cancer patients. The numbers of circulating blood cells often become critically low during chemotherapy, leading to cessation of treatment and compromised patient survival.¹ In our recent study we described a combination therapy that prevents myelosuppression by protecting multipotent progenitors (MPPs) in the bone marrow.²

Quizartinib (AC220) is a potent small molecule inhibitor of FMS-Like Tyrosine kinase 3 (FLT3), originally designed to treat acute myeloid leukemia (AML) patients with FLT3-ITD (-Internal Tandem Duplication) mutations.³ Quizartinib is also effective in combatting wild type FLT3-driven leukemia that occurs in mice with a mutation in the RING finger domain of c-Cbl.⁴ In fact, disease development in these mice was prevented by the ability of quizartinib to suppress the proliferative activity of the overactive population of FLT3-expressing MPPs. From these studies of c-Cbl mutant mice we also observed that a single dose of quizartinib induced a rapid but transient quiescence in MPPs.

We continued our investigation with quizartinib in C57BL/6 mice and found that the induced quiescence of MPPs was also evident, and therefore was not an effect associated with the c-Cbl mutation.² Furthermore, the quizartinib-induced quiescence was found to be robust enough to protect these cells from the toxic effects of the chemotherapeutics, fluorouracil (5-FU) and gemcitabine (See Fig. 1). Whereas MPPs in mice that were not dosed with quizartinib were destroyed by chemotherapy, the quizartinib-protected MPPs were able to rapidly repopulate the bone marrow and facilitate a dramatic recovery of blood cells. We also found that quizartinib provided significant protection to committed progenitors within the lineage negative, c-Kit⁺, Sca-1⁻ population (i.e. LK cells), as well as hematopoietic stem cells (CD48⁻/CD150⁻ and CD48⁻/CD150⁺ cells) within the lineage negative, c-Kit⁺, Sca-1⁺ population (i.e. LSK cells). Perhaps the most obvious improvement was the marked increase in survival of quizartinib-primed mice receiving sequential injections of 5-FU. Overall it was found that a single priming dose of quizartinib could drastically reduce myelosuppression caused by chemotherapy.

Figure 1.

Quizartinib-mediated protection of hematopoietic progenitor cells from chemotherapy. Quizartinib induces quiescence in multipotent progenitors (MPPs) which confers protection from the cytotoxic killing of fluorouracil (5-FU) and gemcitabine.

Quizartinib has been shown to be safe for small duration exposure in healthy volunteers.⁵ Despite this, translation of our quizartinib-prime approach to the clinic will still require careful consideration of quizartinib pharmacokinetics in order for successful hematopoietic protection to be realized. Given that prolonged exposure to quizartinib can cause myelosuppression,⁶ it is essential that initial trials establish the dose or doses of quizartinib that can induce sufficient protection to hematopoietic progenitors over the duration of the cytotoxic therapy without eliciting myelosuppression itself.

The concept of using small molecule inhibitors to induce quiescence in bone marrow cells, and provide protection from chemotherapy, is gaining momentum as a viable treatment option. Earlier this year, He and colleagues described a way to prevent myelosuppression using a cyclin dependent kinase 4/6 (CDK4/6 inhibitor).⁷ However, additional studies are required to determine the selectivity of this approach, with it being essential that the agent only shields the hematopoietic progenitors, and not the cancer cells, from chemotherapy. Fortunately, FLT3 is almost exclusively expressed on hematopoietic cells, therefore the likelihood that quizartinib would induce quiescence, and therefore protect cancer cells, is minimal. Furthermore, it would be interesting to investigate whether dual inhibition of FLT3 and CDK4/6 would provide additive hematopoietic protection.

Although untested, it seems likely that the quizartinib-primed chemotherapy approach has the potential to be successfully combined with other experimental cancer treatments, or current treatments for myelosuppression. For example, the most commonly used therapy to combat neutropenia, granulocyte colony stimulating factor (G-CSF) therapy, would likely synergize with quizartinib-mediated protection. The larger pool of surviving progenitor cells would provide a greater amplification of neutrophil production following G-CSF treatment. Furthermore, it seems feasible that quizartinib-primed chemotherapy would more successfully combine with immunotherapy than chemotherapy alone, due to the increase in numbers of immune cells post treatment.

Finally, we also identified that quizartinib-priming in combination with 5-FU is a novel approach to treat preclinical models of AML. The key feature for the successful treatment of these models was that the AML cells were resistant to quizartinib-induced quiescence, and therefore remained sensitive to 5-FU cytotoxicity. The treatment regimen was well tolerated and was significantly more effective than standard induction chemotherapy in mice. Given that induction chemotherapy is associated with meek 30% overall survival rates, new therapeutic regimens are desperately needed. Although slightly unconventional, the robust results from our study provide a strong impetus for an alternative approach: to use quizartinib-primed 5-FU, or gemcitabine, as a clinical therapy for AML.