Initial Characterization and Toxicology of an Nmt Inhibitor in Development for Hematologic Malignancies

N-myristoylation, the addition of the 14-carbon fatty acid to proteins, plays a fundamental role in cell signaling. Over 200 proteins are myristoylated, including the Src Family Kinases (SFK) Src, Lyn, Lck, Hck, and Fgr, as well as c-Abl, G_α subunits, caspase truncated (ct-) Bid and ct-PAK2, regulating cell growth and apoptosis. Human myristoylation is performed by two ubiquitously expressed N-myristoyl-transferases NMT1 and NMT2.

PCLX-001 is a new, orally bioavailable, small-molecule, dual NMT inhibitor that is under investigation as a novel and selective treatment for B-cell malignancies. In vitro, PCLX-001 inhibits NMT1 and NMT2 at IC₅₀ of 5nM and 8nM, respectively, inhibits the growth of hematological cancer cells at concentrations 10-fold lower than dasatinib and ibrutinib, and inhibits SFK recruitment to B cell receptor signaling, reducing survival signals and triggering apoptosis. PCLX-001 causes complete tumor regression in NOD/SCID mouse xenograft models of Acute Myeloid Leukemia (AML), Burkitts Lymphoma (BL), and Diffuse Large B-cell Lymphoma (DLBCL), including drug-resistant human tumor in a PDX model.

When screened in vitro for its ability to inhibit the activity of 468 kinases and kinase mutants in a KINOMEscan®, PCLX-001 did not inhibit any kinase at up to 10 µM (5380 ng/mL) and produced modest inhibition of only three kinases (MRCKA, PIP5K2B, and SRPK1) at 100 µM (53800 ng/mL). Male rats tolerated single oral doses of PCLX-001 at 100 mg/kg without clear effects, but one of three rats died after a single dose at 1000 mg/kg. Dogs tolerated single oral doses of PCLX-001 at 10 mg/kg without effects, but both dogs showed emesis and diarrhea and lost weight after a single dose at 50 mg/kg. In 21-day xenograft efficacy studies in mice, the maximum tolerated dose level (MTD) was 50mg/kg, which also produced complete tumor remission of most xenografts. Administered daily, the 14-day MTD is >75mg/kg (highest dose tested) in rats and between 5 and 25mg/kg in dogs. The dose-limiting toxicities in dogs involved the gastrointestinal tract and hematopoietic bone marrow.

When administered orally, PCLX-001 is 26% bioavailable in rats and >90% bioavailable in mice and dogs. The plasma half-life was 5.7h in mice, 1-2h in rats, and 3.9h in dogs. With repeated daily administration, systemic exposure did not change in dogs but decreased in rats.

Based on KINOMEscan® results, PCLX-001 is very unlikely to produce adverse effects in patients due to off-target kinase inhibition. The oral bioavailability and half-life in mice and dogs are consistent with a once-a-day dosing regimen. Repeat dosing studies of up to 21 days in mice and 14 days in rats and dogs determined preliminary MTDs. When extrapolated to human doses based on a body surface area scaling, these MTDs are equivalent to 150mg/m² in mice, >450mg/m² in rats, and between 100 and 500 mg/m² in dogs. These results are consistent with a therapeutic window with potential for tumor response in humans, and support proceeding with 28-day GLP toxicology studies and an IND filing for first in human testing of oral PCLX-001.