Discovery of a Potent, Selective, and Orally Bioavailable Pyridinyl-Pyrimidine Phthalazine Aurora Kinase Inhibitor

Abstract

The discovery of aurora kinases as essential regulators of cell division has led to intense interest in identifying small molecule aurora kinase inhibitors for the potential treatment of cancer. A high-throughput screening effort identified pyridinyl-pyrimidine 6a as a moderately potent dual inhibitor of aurora kinases -A and -B. Optimization of this hit resulted in an anthranilamide lead (6j) that possessed improved enzyme and cellular activity and exhibited a high level of kinase selectivity. However, this anthranilamide and subsequent analogues suffered from a lack of oral bioavailability. Converting the internally hydrogen-bonded six-membered pseudo-ring of the anthranilamide to a phthalazine (8a-b) led to a dramatic improvement in oral bioavailability (38-61%F) while maintaining the potency and selectivity characteristics of the anthranilamide series. In a COLO 205 tumor pharmacodynamic assay measuring phosphorylation of the aurora-B substrate histone H3 at serine 10 (p-histone H3), oral administration of 8b at 50 mg/kg demonstrated significant reduction in tumor p-histone H3 for at least 6 h.

Aurora B: A New Prognostic Marker and Therapeutic Target in Cancer

“…The efficacy of the inhibitor was further tested in vivo against COLO205 tumor xenografts , in tumor tissues the reduction of H3 phosphorylation was also observed. All these data indicate that this novel inhibitor acts through the block of Aurora B kinase rather than blocking Aurora A [135].…”

“…In addition, intraperitoneal administration of GSK1070916 at a dose of 100 mg/kg (maximum tolerated dose) for 5 consecutive days with two cycles induced tumor regression [140]. Furthermore, human-leukemia-cellimplanted SCID-NOD mice were sensitive to GSK1070916 treatment at a dose of 25 mg/kg with the same schedule of administration [135].This compound is being progressed to Phase I clinical trials in patients with advanced solid tumors. This inhibitor may represent an "ultimate physiological inhibitor" that exerts a constant inhibition of Aurora B until the enzyme is degraded and replaced by new protein synthesis [137].…”

Quantitative conformational profiling of kinase inhibitors reveals origins of selectivity for Aurora kinase activation states

“…The type II binding mode has been observed in a relatively small subset of protein kinases, and AurA is not generally considered one of them. However, inspection of an X-ray structure of an analog of AMG-900 from early stages of lead optimization (43) reveals the classical type II binding mode and suggests that the hydrophobic tail group of AMG-900 indeed binds in the hydrophobic back pocket of AurA (Fig. 5 B ).…”

“…However, these studies utilized AurB trapped in the DFG-in state by its activator protein INCENP (47). Aligning the AurB:BI-847325 structure (DFG-in) (44) with the structure of AurA bound to the AMG-900 analog (DFG-out) (43) shows that the tail groups of the two compounds superimpose, suggesting that BI-847325 and AMG-900 could bind to similar DFG-out states in AurA (Fig. 5 C , Inset ).…”

The fully‐extended conformation in peptides and proteins

“…It is evident that the fully‐extended, flattened strand is almost invariably flanked by conventional β‐strand segments providing opportunities for interstrand H‐bonding. The flattened fully‐extended strand appears to be stabilized in many examples [including the four‐residue segments ‐Gly 198 ‐Tyr 199 ‐Phe 200 ‐His 201 ‐ in the aurora‐A kinase domain (PDB code: 3O50) and ‐Tyr 306 ‐Phe 307 ‐Gly 308 ‐Ala 309 ‐ in the extracellular domain of the murine protein RANKL (PDB code: 1S55)] by aromatic side chains which may contribute both intrastrand and interstrand aromatic···amide interactions. However, such interactions do not represent a necessary condition for the onset of long fully‐extended strands, as exemplified by the four‐residue segment ‐Gly 64 ‐Ser 65 ‐Gly 66 ‐Ala 67 ‐ in the enzyme catalase from Micrococcus lysodeikticus (PDB code: 1HBZ).…”