Wednesday, September 16, 2015

Drugs in Clinical Pipeline: Quizartinib

Quizartinib [(N-(5-tert-butyl-isoxazol-3-yl)-N'-{4-[7-(2-morpholin-4-yl-ethoxy)imidazo[2,1-b][1,3]benzothiazol-2-yl]phenyl}urea] is a once-daily, orally administered, potent and selective inhibitor of FMS-like tyrosine kinase 3 (FLT3) kinase (Kd = 1.6 ± 0.7 nM).

Quizartinib is a novel compound expressly optimized as a FLT3 inhibitor for the treatment of Acute Myeloid Leukemia (AML). Quizartinib inhibits FLT3 with low nanomolar potency in cellular assays and is highly selective when screened against the majority of the human protein kinome [1, 2].

Quizartinib is currently in development for the treatment of relapsed or refractory FLT3-ITD positive and negative AML patients and as a maintenance therapy. Though patients of any age may be FLT3 positive, over 35% of AML patients over age 55 are estimated to harbour a genetic mutation in FLT3, known as the FLT3 internal tandem duplication (FLT3-ITD) mutation. The FLT3-ITD mutation acts like a “power switch” that causes leukemic cells, or blasts, to spread more aggressively and grow back more rapidly following chemotherapy, conferring an especially poor survival outcome. Quizartinib is designed to turn off this switch.

This novel second generation class III receptor tyrosine kinase inhibitor is currently under clinical investigation for wildtype AML and also those bearing FLT3 ITD mutation. Quizartinib with superior pharmaceutical properties and an excellent pharmacokinetic profile is a clear leader as compared to other agents in clinical trials. A study using recombinant enzyme in in vitro kinase assays identified that Quizartinib targets related class III RTKs, such as wildtype and gain-of-function mutant KIT and PDGFR isoforms. This implies that patients with CBF AML with KIT D816Y or exon 11 mutations or patients with solid tumors associated with KIT and PDGFR mutations, such as GIST too might benefit from this agent. This would not include the group of mutant-KIT CBF AML that have KIT D816V mutations [1].

The activity of Quizartinib is as follows:

Kd (FLT3 binding affinity) = 1.6 ± 0.7 nM
Kd (FLT1 binding affinity) = 41 nM
Kd (FLT3(D835H) binding affinity) = 3.7 nM
Kd (FLT3(D835Y) binding affinity) = 7.1 nM
Kd (FLT3(ITD) binding affinity) = 8.8 nM
Kd (FLT3(K663Q) binding affinity) = 2.2 nM
Kd (FLT3(N841I) binding affinity) = 4.1 nM
Kd (FLT4 binding affinity) = 41 nM
Common Name: Quizartinib
Synonyms: AC220; AC 220; AC-220
IUPAC Name: (N-(5-tert-butyl-isoxazol-3-yl)-N'-{4-[7-(2-morpholin-4-yl-ethoxy)imidazo[2,1-b][1,3]benzothiazol-2-yl]phenyl}urea
CAS Number: 950769-58-1; 1132827-21-4 (dihydrochloride)
Mechanism of Action: Kinase Inhibitor; FLT3 Inhibitor
Indication: Acute Myeloid Leukemia
Development Stage: Phase III
Company: Ambit Biosciences

To assess the potential of Quizartinib to inhibit off-target kinases, and to compare its selectivity with that of the other FLT3 inhibitors, it was screened against a KinomeScan panel of 402 kinase binding assays, representing almost 80% of “typical” human protein kinases. Quizartinib was screened against the panel at a single concentration of 10 ┬ÁM in a primary screen, and dissociation constants (Kds) were determined for all kinases identified as targets in the primary screen. For Quizartinib, researchers also measured a Kd for every kinase not found to bind in the primary screen to ensure that no targets were missed.

The highest affinity target identified for Quizartinib was FLT3. The only other kinases with binding constants within 10-fold that for FLT3 were the closely related receptor tyrosine kinases (RTKs) KIT, PDGFRA, PDGFRB, RET, and CSF1R (Kd = 4.8, 11, 7.7, 9.9 and 12 nM, respectively) and only 4 additional kinases, also related RTKs (FLT1, FLT4, DDR1, VEGFR2 Kd = 41, 41, 81, 87 nM, respectively), bound with Kds within 100-fold of that for FLT3. In follow-up cellular assays the activity of Quizartinib against KIT, RET, CSF1R, and PDGFR was at least 10-fold less potent than the cellular activity against FLT3, confirming the selectivity observed in the biochemical assays. The kinase interaction pattern for Quizartinib was therefore highly focused [1].

To determine the ability of Quizartinib to inhibit FLT3 in the cellular environment, researchers measured inhibition of FLT3 autophosphorylation in the human leukemia cell lines MV4-11 (IC50 = 0.56 ± 0.3 nM), which harbors a homozygous FLT3-ITD mutation and is FLT3 dependent and RS4;11, which expresses wild-type FLT3. Furthermore, to determine the effect of FLT3-ITD inhibition on cell growth, researchers measured MV4-11 cell proliferation in the presence of Quizartinib. The IC50s in this assay tracked very well with FLT3 autophosphorylation inhibition, with Quizartinib having values of 4.2 ± 0.3 and 1.1 ± 0.1 for wild type and ITD respectively [1].

1. Zarrinkar, P. P.; et. al. AC220 is a uniquely potent and selective inhibitor of FLT3 for the treatment of acute myeloid leukemia (AML). Blood 2009, 114(14), 2984-2992.
2. Bhagwat, S. S.; et. al. Identification of N-(5-tert-butyl-isoxazol-3-yl)-N'-{4-[7-(2-morpholin-4-yl-ethoxy)imidazo[2,1-b][1,3]benzothiazol-2-yl]phenyl}urea dihydrochloride (AC220), a uniquely potent, selective, and efficacious FMS-like tyrosine kinase-3 (FLT3) inhibitor. J Med Chem 2009, 52(23), 7808-7816.