In vitro activity: Navoximod (formerly known as IDO-IN-7 or NLG919) is a potent inhibitor of the IDO (indoleamine-(2,3)-dioxygenase) pathway with Ki/EC50 of 7 nM/75 nM. It has potential immunomodulating and antineoplastic activities. Navoximod was obtained by rational structural design based on the X-ray crystal structure for IDO complexed with 4-phenyl-imidazole (PIM). It is a strong competitive IDO inhibitor and potently inhibits IDO pathway in vitro and in cellular assays. In Phase Ia clinical trial for the treatment of advanced-stage solid tumors, Navoximod demonstrated the high dose tolerance (well tolerated up to 800 mg BID on a 21/28 day cycle), but revealed that single-agent therapy with an IDO inhibitor failed to cause tumor eradication and to prevent disease progression (best response was limited to stable disease in 7 out of 17 patients). Navoximod is now being evaluated in phase Ib in combination with PD-L1 inhibitor atezolizumab.

Kinase Assay: Navoximod inhibits the IDO activity in a concentration-dependent manner with an EC50 of 0.95 μM. PEG2k-Fmoc-NLG(L) is less active (EC50 of 3.4 μM) in inhibiting IDO compared with free Navoximod while PEG2k-Fmoc-NLG(S) is least active (EC50>10 μM). Coculture of IDO+tumor cells with splenocytes isolated from BALB/c mice leads to significant inhibition of T-cell proliferation. This inhibition is significantly attenuated when the mixed cells are treated with Navoximod. PEG2k-Fmoc-NLG(L) is also active in reversing the inhibitory effect of tumor cells although slightly less potent than Navoximod.

Cell Assay: Using IDO-expressing human monocyte-derived dendritic cells (DCs) in allogeneic mixed lymphocyte reaction (MLR) reactions, Navoximod (NLG919) potently blocks IDO-induced T cell suppression and restores robust T cell responses with an ED50=80 nM. Similarly, using IDO-expressing mouse DCs from tumor-draining lymph nodes, Navoximod abrogates IDO-induced suppression of antigen-specific T cells (OT-I) in vitro, with ED50=120 nM. The IDO inhibitory effect of PEG2k-Fmoc-NLG is tested by an in vitro IDO assay. Briefly, HeLa cells are seeded in a 96-well plate at a cell density of 5000 cells per well and allowed to grow overnight. Recombinant human IFN-γ is then added to each well with a final concentration of 50 ng/mL. At the same time, various concentrations of PEG2k-Fmoc-NLG(L), PEG2k-Fmoc-NLG(S) or Navoximod (NLG919) (50 nM-20 μM) are added to the cells. After 48 h of incubation, 150 μL of the supernatants per well is transferred to a new 96-well plate. Seventy-five μL of 30% trichloroacetic acid is added into each well and the mixture is incubated at 50°C for 30 min to hydrolyse N-formylkynurenine to kynurenine. For colorimetric assay, supernatants are transferred to a new 96-well plate, mixed with equal volume of Ehrlich reagent (2% p-dimethylamino-benzaldehyde w/v in glacial acetic acid), and incubated for 10 min at RT. Reaction product is measured at 490 nm by a plate reader