In vitro activity: AX20017 is a highly selective low-molecular-weight inhibitor of protein kinase G (PknG) with an IC50 of 0.39 μM. The pathogenicity of mycobacteria such as Mycobacterium tuberculosis is closely associated with their capacity to survive within host macrophages. A crucial virulence factor for intracellular mycobacterial survival is protein kinase G (PknG), a eukaryotic-like serine/threonine protein kinase expressed by pathogenic mycobacteria that blocks the intracellular degradation of mycobacteria in lysosomes. In the 2.4 A x-ray crystal structure of PknG in complex with AX20017, the compound AX20017 inhibitor is bound deep within a narrow pocket formed by the inter lobe cleft of the PknG domain. The structure of PknG-AX20017 further reveals that the inhibitor is buried deep within the adenosine-binding site, targeting an active conformation of the kinase domain. Remarkably, although the topology of the kinase domain is reminiscent of eukaryotic kinases, the AX20017-binding pocket is shaped by a unique set of amino acid side chains that are not found in any human kinase. Directed mutagenesis of the unique set of residues resulted in a drastic loss of the compounds inhibitory potency.The results explain the specific mode of action of AX20017 and demonstrate that virulence factors highly homologous to host molecules can be successfully targeted to block the proliferation of M. tuberculosis.

Kinase Assay: In vitro phosphorylation by PknG (0.5 μg) is in 25 mM Tris (pH 7.5), 2 mM MnCl2, and 0.5 μCi [γ-32P]ATP in the absence or presence of the reagents. To monitor kinase activity of PknGΔN, the protein is combined with equal amounts of the kinase-dead mutant of full-length PknG, PknG-K181M. To analyze kinase activity of PknG-I87S/A92S and PknG-C/S, the PknG-N-terminal fragment of PknG (2 μg) is included. Phosphorylated proteins are separated on 12.5% SDS/PAGE and analyzed by autoradiography or quantitated by PhosphorImage analysis. IC50 values are determined by using a radiometric ATP consumptive assay. Twelve concentrations of AX20017 in the range from 5 × 10-5M to 1.5 × 10-10 M are tested in each kinase assay.

Cell Assay: Phagocytosis is analyzed after incubation of J774 cells for 30 min in the presence of the indicated concentration of AX20017 (0, 10, 20 μM), followed by incubating the cells for 2 h with latex beads at a ratio of 10:1 beads/cells in the continued presence of the inhibitor, followed by fixation in 3% paraformaldehyde as described. Cells are observed with a Axiophot using a ×63 objective. Proliferation of J774 cells is analyzed by incorporation of tritiated thymidine (0.1 μCi) for 12 h as described of cells that had been incubated for 48 h in the absence or presence of the AX20017(0, 10, 20 μM)