HONG KONG – The in vivo antileukemic efficacy and safety of a novel combination of apoptosis-regulating drugs seen in a recent study by Australian researchers suggests that inhibition of apoptosis and activation of another type of cell death, necroptosis, warrants clinical investigation as a treatment for acute myeloid leukemia (AML).

AML is the most common type of acute leukemia in adults and its incidence increases with age. Therefore, while AML is a relatively rare disease, accounting for approximately 1.2 percent of all cancer deaths in the U.S., this incidence is expected to increase with the aging of the population, with AML being particularly difficult to treat in older patients.

"The current treatments for AML haven't changed for the last 40 years and revolve around the cytotoxic agent cytarabine," said study corresponding author John Silke, professor and joint head of the Division of Cell Signaling and Cell Death at the Walter and Eliza Hall Institute of Medical Research (WEHI) in Melbourne, Australia.

"We need better treatments, particularly seeing as the incidence of AML increases with age and, among those age 65 or older, only around 5 percent will survive for five years or more after diagnosis," said Silke, who led the research together with Paul Ekert, associate professor and group leader, Cancer Research, at Murdoch Children's Research Institute, the Royal Children's Hospital, Melbourne.

"There are also different types of AML, such as the [mixed lineage leukemia] MLL translocations that that occur in children and have a dismal prognosis," Ekert told BioWorld Today. "We looked at these and they seemed to respond to the new combination."

Resistance to chemotherapy is a major problem in cancer treatment, and is frequently associated with failure of tumor cells to undergo apoptosis. That has led to the development of new therapies that resensitize cancers by targeting the inhibitory proteins of apoptosis.

Apoptosis or programmed cell death in both normal and abnormal cells is important for health, with multiple molecular mechanisms ensuring healthy cells do not undergo apoptosis and abnormal cells, such as cancer cells, die and are cleared from the body.

Such key regulatory protein molecules include inhibitors of apoptosis (IAPs), which protect cells from apoptosis, whereas second mitochondrial activator of caspases (SMAC)/DIABLO is a naturally occurring IAP inhibitor that potentiates some forms of apoptosis.

Birinapant (Tetralogic Pharmaceuticals Inc.), a clinical SMAC mimetic, had been designed to mimic the interaction between IAP and SMAC/DIABLO, thereby relieving IAP-mediated caspase inhibition and promoting the apoptosis of cancer cells.

"We believe that birinapant should be targeted at particular types of AML," said Gabriella Brumatti, the study's lead author and a senior postdoctoral researcher at WEHI. "We have shown, for example, that MLL translocation AMLs are sensitive to birinapant, whereas the [core binding factor] CBF type of AML is resistant. So in order to make birinapant more generic in its action, we feel that it will have to be combined with other drugs."

The Australian researchers used a flow cytometry-based assay to show that AML cells were sensitive to birinapant-induced cell death. Remarkably, however, they also found that blocking apoptosis with the clinical caspase inhibitor, emricasan (IDN-6556, Conatus Pharmaceuticals Inc.) augmented, rather than prevented, birinapant-induced cell death.

Formerly, while it was known that inducing necroptosis could kill tumor cells in vitro, it was merely hypothesized that this might be an effective strategy in vivo.

"We have not only shown that the birinapant and emricasan combination is tolerated in vivo, both in mice and by primary human cells, but we have also shown that these clinically used drugs are very effective," Ekert said.

The researchers also demonstrated that birinapant activated caspases 3 and 8, two members of a family of protease enzymes that play an essential role in programmed cell death, including apoptosis and necroptosis, they reported in the May 18, 2016, issue of Science Translational Medicine.

Deletion of caspase-8, a target of emricasan, also sensitized AML to birinapant, whereas combined loss of caspase-8 and the apoptosis effector called mixed lineage kinase domain-like (MLKL) protein prevented birinapant/emricasan–induced cell death, showing that inhibition of caspase-8 sensitizes AML cells to birinapant-induced apoptosis.

However, loss of MLKL alone did not prevent a caspase-dependent birinapant/emricasan-induced death, suggesting AML may be less likely to acquire resistance to this drug combination.

"To become cancers, cells must become resistant to apoptosis, so clinicians are always fighting an uphill battle when they try to kill cancers via apoptosis," explained Silke. "Cancers do not, as far as we know, have to be selected to resist necroptosis and we showed that cells that cannot die via chemotherapy-induced apoptosis were killed very well by the birinapant plus emricasan combination."

They also showed that even if cells were unable to die by necroptosis, "when we added birinapant and emricasan, because we had deleted the genes required for necroptosis, [we showed] that they now died by apoptosis, just more slowly," he said. Moreover, "we generated birinapant-resistant cells, such as might occur during treatment, and even these were sensitive to the birinapant/emricasan treatment."

Although the researchers concede that the clinical use of emricasan for treating AML is limited by the short half-life of the caspase inhibitor, their research results suggest that caspase inhibition could be used successfully in the clinic together with birinapant.

In the future either emricasan will have to be used with an alternative dosing schedule or a caspase inhibitor with the same inhibitory profile as emricasan but with better pharmacokinetics will need to be developed.

"Given that the half-life of emricasan in the circulation is only 50 minutes, I think the protection that we observed from a very aggressive model of AML is stunning and promising. If I had a million dollars, I would start making a better emricasan today," Silke told BioWorld Today.

"We are fundamental mechanism researchers and will continue to try and work out how birinapant kills cancer cells. We believe that by understanding how birinapant kills we can make sure that it is correctly applied and to the correct patients," he added.

"Birinapant is the most clear pathway into the clinic that we can see, because it has already been shown to be well tolerated by patients, but like any anticancer drug, in the end, if we want to eradicate cancer not just postpone it, then we have to understand more."