The Ottawa Hospital
Drs. William Stanford (left) and Mitchell Sabloff Photo courtesy of
Preclinical research has revealed a potential treatment for chemotherapy-resistant acute myeloid leukemia (AML).
Researchers characterized a mechanism of chemotherapy resistance in AML and found that MDM2 is a key player in this dysregulated signaling pathway.
They tested MDM2 inhibitors and found these drugs could sensitize resistant AML to chemotherapy in vitro and in vivo.
In fact, mice with refractory AML responded to standard induction therapy when combined with an MDM2 inhibitor, showing no signs of disease and prolonged survival.
These results were published in Cancer Discovery.
“We were blown away when we saw the results,” said study author William Stanford, PhD, of Ottawa Hospital Research Institute in Ontario, Canada.
“If these findings hold up in clinical trials, we could have a new treatment for people who would almost certainly die of their disease today.”
Mechanism of resistance
Dr. Stanford’s research began with the protein MTF2. He and his colleagues previously found that MTF2 plays a role in erythropoiesis, and the team wanted to determine if MTF2 also plays a role in AML.
Using AML samples from patients treated at The Ottawa Hospital, the researchers found the mean survival was three times longer in patients with normal MTF2 activity than in patients with low MTF2 activity.
“Initially, we thought that MTF2 could be an important biomarker to identify patients who might benefit from experimental therapies,” Dr. Stanford said. “But then we started thinking that if we could understand what MTF2 was doing, maybe we could use this information to develop a new treatment.”
Dr. Stanford and his colleagues discovered that MTF2 represses MDM2, a protein that helps cells resist chemotherapy.
The team found that MTF2-deficient cells overexpress MDM2, which inhibits p53, and this leads to defects in cell-cycle regulation and apoptosis that enable resistance to chemotherapy.
Testing MDM2 inhibitors
Since MDM2 inhibitors are already being tested in clinical trials for other cancers, Dr. Stanford and his colleagues tested these inhibitors in vitro and in mouse models of chemotherapy-resistant AML.
The in vitro experiments included two MDM2 inhibitors—Nutlin3A and MI-773—combined with daunorubicin or cytarabine.
The researchers found that refractory, MTF2-deficient AML cells underwent apoptosis when treated with either daunorubicin or cytarabine in combination with Nutlin3A or MI-773. The effect was comparable to that observed in AML cells with normal MTF2.
The team found that Nutlin3A was more efficient at sensitizing refractory, MTF2-deficient AML cells to daunorubicin, so they used Nutlin3A in the in vivo experiments.
For these experiments, the researchers tested Nutlin3A in mice injected with either chemotherapy-responsive AML cells (with normal MTF2) or refractory, MTF2-deficient AML cells.
Once the mice had “a substantial leukemic burden” (≥ 20% CD45+CD33+ leukemic blasts in their peripheral blood), they were randomized to receive vehicle control, Nutlin3A, standard induction therapy, or induction plus Nutlin3A.
The mice engrafted with chemotherapy-responsive AML cells did not respond to vehicle control or Nutlin3A alone. However, they did respond to standard induction and induction plus Nutlin3A, surviving until the end of the experiment at 16 weeks.
Among the mice engrafted with refractory, MTF2-deficient AML cells, only those animals treated with induction plus Nutlin3A survived until the end of the experiment.
The researchers also noted a “dramatic loss” in the blast-containing CD45+CD33+ and CD34+CD38− populations in mice treated with induction plus Nutlin3A.
To assess residual disease, the researchers performed secondary transplants with cells from mice that had engrafted with refractory, MTF2-deficient AML cells but responded to induction plus Nutlin3A.
The recipient mice had no evidence of AML at 16 weeks after transplant when the experiment ended.
Dr. Stanford and his colleagues are now trying to obtain pharmaceutical-grade MDM2 inhibitors to conduct trials in AML patients at The Ottawa Hospital.
The researchers are also screening libraries of approved drugs to see if any of these can block MDM2, and they are working with a biotech company to develop a test to identify chemotherapy-resistant AML patients who would respond to MDM2 inhibitors.
The current research was supported by grants from the Canadian Cancer Society Research Institute, Canadian Institutes of Health Research, Cancer Research Society, National Institutes of Health, and a Tier 1 Canada Research Chair in Integrative Stem Cell Biology. One study author reported a relationship with Epicypher, Inc. No other conflicts of interest were reported.