Scientists have known that the cancer drug ONC201 blocks cells from proliferating and kills tumors in cell and animal models—but they haven’t known exactly how it works, or what its molecular target is. Nevertheless, multiple clinical trials of the drug, in various cancer types, are underway. In 2018, ONC201, made by the Philadelphia-based company Oncoceutics, received a fast track designation from the Food and Drug Administration, meaning it gets expedited review, for the treatment of certain gliomas in adults. Now, two studies published independently this month reveal the drug’s mechanism of action: ONC201 works by activating ClpP, an enzyme that chews up misfolded proteins in mitochondria.
“It’s important to understand how a drug works to understand how best to use it in patients, and in these two studies, they identified ClpP, a mitochondrial protease, as an important target of ONC201, which seems to account for some if not most of its clinical activity,” says Stanley Lipkowitz, chief of the Women’s Malignancy Branch of the National Cancer Institute’s Center for Cancer Research, who was not involved in either study but is currently collaborating with Lee Graves, who led one of the studies.
ONC201 was originally identified as a potential cancer drug in a screen for molecules that induce the transcription of TRAIL, a gene that leads to apoptosis in tumors. Yet ONC201 doesn’t activate TRAIL in all of the cancer cells against which it is effective: In a 2018 paper, Yoshimi Greer and colleagues in the Lipkowitz lab reported that ONC201 worked against several lines of breast cancer cells—without upregulating TRAIL. Instead, they found, ONC201 was hindering the cancers’ mitochondrial function. But just how ONC201 was doing its mitochondrial damage was unclear.
To find out, Graves, a University of North Carolina School of Medicine pharmacologist, and colleagues studied ONC201 as well as very similar molecules, called ONC201 analogs, generated by the Chapel Hill–based company Madera Therapeutics, of which one of Graves’s coauthors is president. They attached the ONC201 analog TR-80 to agarose beads to construct a column and ran the innards of HeLa cells, the immortal cervical cancer–derived cell line, through it to see what would stick.
“We came at it from an old-fashioned affinity-chromatography approach—you know, ‘let’s make some bait and go fishing and see what we catch,’” Graves tells The Scientist. Mass spectrometry identified the protein they caught as ClpP. They repeated the experiments using cell lysates from other cancers, including breast, pancreatic, and lung, and in every case, they found that the ONC201 analogs bound ClpP, they report in ACS Chemical Biology….
