Novel KIT inhibitor also targets a new suspect; clinical trials to begin this year

By Jerry Call

Researchers, led by Daruka Mahadevan M.D., Ph.D., at the University of Arizona in Tucson, have characterized a novel mechanism of resistance to Gleevec that may be present in some GIST patients. Clinical trials that test this discovery are expected to start in the first half of 2007. Call

Dr. Mahadevan and his colleagues have found that some GISTs may switch from being dependent on the KIT signaling pathway to being dependent on another tyrosine kinase receptor called AXL. Mahadevan and colleagues took an existing GIST cell line, GIST882, developed in the labs of Dr. Jonathan Fletcher and subjected it to subtherapeutic levels of Gleevec and then slowly increased the levels of Gleevec. This resulted in Gleevec-resistant cells called GIST-R.

The GIST-R cells had lost expression of KIT and were no longer responsive to Gleevec. They had also changed in appearance, changing from spindle-shaped cells (longer, more tubular looking) to epithelioid shaped (more rounded). With the loss of KIT expression, they no longer stained positive for KIT and had become KIT-negative GISTs. Mahadevan and his team also found this same mechanism of resistance in two of their Gleevec-resistant patients.

Mutations in KIT are the primary cause of GIST and being able to inhibit the signal of the mutated KIT protein is what makes Gleevec so effective in GIST. In both the GIST-R cells and the two Gleevec-resistant patients, KIT expression was more than 100-fold downregulated, whereas AXL was overexpressed by 49-fold in GIST-R, 11-fold for the two GIST patients; c-Met, another receptor tyrosine kinase, was overexpressed by 19-fold in GIST-R. In the GIST-R cells, AXL was phosphorylated, implying an activated receptor, thus maintaining a proliferative signal despite Gleevec. Furthermore, the growth factor that normally stimulates AXL (GAS-6) was overexpressed by 6-fold. In addition, the growth factor for c-Met (HGF) was 14-fold overexpressed suggesting a dual mechanism for cell proliferation. The cells had undergone what Mahadevan called a “novel tyrosine kinase switch.” They had lost dependence on KIT and appeared to be dependent on AXL and perhaps to c-Met as well.

The Mahadevan team, which also included David Bearss, Ph.D., tested the GIST-R cells against a new compound that had been developed at the University of Arizona and found that the Gleevec-resistant cells were sensitive to this new compound, which they called MP-470.

In 2003, Bearss and Dallin Anderson founded Montigen Pharmaceuticals to further develop and commercialize the research conduced by Bearss in collaboration with Daniel Von Hoff M.D., including the development of MP-470. In 2006, Montigen was acquired by Super- Gen Pharmaceuticals.

MP-470 is a new tyrosine kinase inhibitor that inhibits several kinases, including KIT, PDGFR, AXL, c-Met and c-Ret. Interestingly, MP-470 appears to also potently inhibit some of the secondary mutations that cause resistance to Gleevec. In fact, it inhibits all of the ten or so secondary mutations that it has been tested against.

There are several things that potentially make MP-470 different from other kinase inhibitors. First, the drug inhibits AXL and c-Met in addition to KIT and PDGFRA. This has the potential to make it effective in the Gleevec-resistant GISTs that have lost KIT expression, if the AXL hypothesis is correct (estimates based on small numbers suggest that 10 to 20 percent of GISTs become resistant by losing KIT expression). The activity profile also suggests it could be effective against secondary KIT mutations as well.

The second thing that potentially separates MP-470 from other kinase inhibitors is that it indirectly inhibits another protein, known as RAD51. This protein is involved in the repair of DNA breaks. In-vitro studies with MP-470 have shown that it often has synergistic effects with other, more traditional therapies including radiation, cisplatin, topo I inhibitors and doxorubicin. In the GISTR cells, it was synergistic with docetaxel, a common microtubuletargeting chemotherapy.

MP-470 has been well-tolerated in animal studies and appears to have a wide therapeutic dosage range. Phase I, firstin- human trials are expected to start soon at two locations: Johns Hopkins University under Manual Hidalgo and Wells Messersmith and the University of Arizona in Scottsdale under Dan Von Hoff. While the preliminary work by the Arizona team is interesting, it is based on small sample sizes (one cell line and two patients).

The phase I trial is open to patients with advanced solid tumors but like other phase I solid tumors trials with KIT inhibitors, it is hoped that some GIST patients will enroll. Clinical trials should help to further define whether or not AXL activation is important in some patients, whether or not MP-470 is effective at inhibiting AXL and mutant KIT signaling and most importantly, whether or not MP-470 is effective in Gleevec/ Sutent-resistant GIST patients.