Tandutinib (MLN518), previously known as CT53518, is a small-molecule inhibitor of the type III receptor tyrosine kinases, including the Fms-like tyrosine kinase 3 receptor (FLT3), platelet-derived growth factor receptor (PDGFR), and c-Kit receptor tyrosine kinase. Tandutinib is relatively selective, having no appreciable activity against EGFR, FGFR, KDR, or several non-receptor kinases. FLT3, c-Kit, and ßPDGFR are equally inhibited by tandutinib, while the agent is much less active against CSF-1R (in vitro IC50 of 3.4 µM).
MLN518 is a potent, ATP-competitive and reversible inhibitor of Type III receptor tyrosine kinases (RTKs). In vitro, MLN518 inhibits FLT3, cKIT and PDGFRß with a median IC50 of approximately 30 nM. In cellular assays, MLN518 inhibited the autophosphorylation of these receptors with an IC50 of approximately 200 nM. MLN518 is quite selective; of the other receptor tyrosine kinases tested, only CSF-1R was inhibited to any appreciable degree, with a cellular IC50 of 3.4 µM. Non-RTKs, serine-threonine kinases and MAP kinases were not inhibited at tested concentrations up to 30 µM.
Based on MLN518’s activity against the FLT3 receptor, MLN518 was tested against a panel of clinically isolated FLT3 ITD gain of function mutants expressed in BaF3 cells. Both the W51 mutant and the WT FLT3 receptor showed a significant inhibition of receptor autophosphorylation with MLN518 treatment. In studies of 5 different FLT3/ITD expressing cells, MLN518 inhibited cell growth with an IC50 of 10-30 nM in all cases.
Four AML cell lines were characterized for their level of FLT3 expression, the presence of FLT3/ITD mutations, and their sensitivity to inhibition of cell proliferation by MLN518. All cell lines expressed FLT3, albeit at different levels (1000 to 20,000 receptors per cell), whereas only Molm-13 and Molm-14 had an FLT3/ITD mutation that resulted in consitituitive receptor phosphorylation. In Molm cells only, MLN518 blocked FLT3/ITD phosphorylation and cell proliferation with an IC50 of 10-40 nM and an IC90 of 300 nM, which is consistent with the values previously obtained in the BaF3 cell system. These data provide additional evidence that FLT3/ITD signaling can play a critical role in the proliferation and survival of AML cells and that MLN518 is a potent and selective inhibitor of this pathway.
MLN518 is also a potent inhibitor of both ßPDGFR and c-Kit in vitro. Dysregulated autocrine PDGF stimulation is thought to contribute both to the early transformation events and to the maintenance of glioma tumorigenesis. Virtually all glioma cell lines and surgical specimens express the PDGF A and B ligands. In addition, PDGFR genes are typically expressed in both glioma tumor cells and in the endothelial cells of the tumor vasculature. C-Kit is also expressed by a percentage of primary glioblastoma tumors. Disruption of the PDGF and c-Kit autocrine and paracrine loops present in malignant glioma by MLN518 could therefore have a therapeutic benefit.
An in vivo model of FLT3/ITD mediated myeloproliferative disease was established using BaF3 cells stably transfected with a FLT3/ITD cDNA cloned from an AML patient. This myeloproliferative disease evolves through the proliferation of BaF3 cells and their infiltration into various organs. MLN518 treatment of 60 mg/kg BID PO led to a significant increase in survival in this model. Plasma concentrations were determined from MLN518 treated mice prior to and 1 hour after the administration of the last dose. At 60 mg/kg BID, where 20% of the mice were cured, the mean nadir MLN 518 concentration ranged from 25 to 43 ng/mL, well below the IC90 of 150 ng/mL.
MLN518 was also evaluated in a mouse bone marrow transplantation model developed by Dr. Gary Gilliland. Mice were dosed with MLN518 at 60 mg/kg BID PO from day 32 through 64 post transplantation. MLN518 gave a statistically significant reduction in mortality in this model.
In preclinical studies, MLN518 shows dose dependent single agent activity in the C6 rat glioma model in nude mice. There was an 80% reduction of glioma tumor volume at the highest dose of MLN518 tested, 60 mg/kg BID PO.
Acute administration of high oral doses of tandutinib in dogs produced symptoms of central nervous system or neuromuscular toxicity such as lack of coordination and tremors. However, under conditions of chronic oral dosing, tandutinib was generally well tolerated in both rats and dogs. Principal toxicologic findings at high chronic doses in both species included (1) reversible hypocellularity of the bone marrow with associated anemia and leucopenia and (2) reversible inflammatory infiltrates in hepatic portal triads, associated with reversible increases in liver serum chemistry parameters.
Non-clinical Pharmacokinetics and Drug Metabolism
In animals, tandutinib (MLN518) is characterized by high plasma clearance, a large volume of distribution, and a long apparent terminal half-life. After oral administration, tandutinib is absorbed relatively slowly and has moderate (24-49%) bioavailability in rats and monkeys and high bioavailability (98%) in dogs. The major driver of the pharmacokinetics of tandutinib is its disposition by transporters (P-gp and BCRP), with metabolism appearing to play an insignificant role in its clearance. The potential for drug-drug interactions (DDIs) due to the inhibition of cytochrome P450 (CYP) -mediated metabolism of concomitant medications is low. However, there is the potential for tandutinib to have DDIs with drugs that are substrates for, or inhibitors of, P-glycoprotein (P-gp) and/or breast cancer resistance protein (BCRP).