Science Editor

Two papers published this week described a novel target for fighting mixed-lineage leukemia. By inhibiting the histone methyltransferase DOT1L, scientists were able to kill mixed lineage leukemia cells both in cell culture and in animal models.

The work, Robert Copeland told BioWorld Today, has implications beyond mixed-lineage leukemia. It is "the first demonstration of in vivo efficacy for a histone methyltransferase inhibitor," and "portends the success of the entire class."

Copeland is chief scientific officer of Cambridge, Mass.-based biotech Epizyme Inc., and a co-author on one of the papers, "Selective Killing of Mixed Lineage Leukemia Cells by a Potent Small-Molecule DOT1L Inhibitor."

The other paper, which was published by a partially overlapping team of scientists and also includes authors from both Epizyme and academic co-authors, is titled "MLL-Rearranged Leukemia Is Dependent on Aberrant H3K79 Methylation by DOT1L."

Both appeared in the July 11, 2011, online edition of Cancer Cell. The papers are "complimentary," in their approach, Copeland said.

While his team developed a pharmacological inhibitor, the other paper described experiments investigating DOT1L's role in leukemia by using genetic methods to knock out DOT1L expression.

Mixed-lineage leukemia – which gets its name, Copeland said, because it seems to "bridge" the acute myelogenous and acute lymphoblastic leukemia populations in terms of its underlying genetic error – results from a chromosome shuffling, or translocation in the MLL gene.

The cancer, which can appear in both children and adults, is difficult to treat. While the remission rates for some other forms of pediatric leukemia now approach 80 percent to 90 percent, less than half of all children with mixed-lineage leukemia are helped by standard chemotherapies.

The underlying chromosomal trouble is somewhat similar to the BCR-ABL or Philadelphia chromosome that is Gleevec's target, though with a subtle but important difference: The enzyme that is the net cause of trouble is not part of the translocation.

The MLL gene normally encodes for a histone methyltransferase – but not DOT1L, which is ultimately the troublemaker in mixed-lineage leukemia. It consists both of the active site of the enzyme, and binding sites that direct the enzyme to the right histones.

During the translocation, Copeland explained, "the active site of that enzyme is lost, and the remainder fuses to other protein partners."

DOT1L gets recruited to the sites by binding to the fusion protein, and "acts on genes that it normally wouldn't act on" by adding methyl groups to specific histones. Addition of such methyl groups tends to activate genes, and in the case of the MLL fusion gene, the genes that are activated include two leukemia genes: HOXA9 and MEIS1.

In their experiments, Copeland and his colleagues designed a small molecule, EPZ004777, which inhibits DOT1L.

That inhibitor was able to take out cells with the MLL translocation in cell culture, as well as in animal models of mixed-lineage leukemia, and is the first time, Copeland said, that evidence of the effectiveness of inhibiting DOT1L in in animals has been published.

On the other hand, there's "really not much" of an effect of the DOT1L inhibitor on cells without the MLL translocation. Although DOT1L activity is blocked by the inhibitor in the cells as well, "the cells continue to grow and proliferate just fine."

Copeland said that MLL cells' dependency on the enzyme is an example of oncogene addiction, where cancer cells become dependent on one particular cell signal – often a kinase – for survival.

Epizyme's program – like all others targeting histone methyltransferases – is still in the preclinical stage. The company has partnerships with GlaxoSmithKline plc and Eisai Co. Ltd. to develop other histone methyltransferase inhibitors. (See BioWorld Today, March 11, 2011.)

Copeland said the company is "progressing very aggressively" toward the clinic. "We are optimistic that we will be the first to enter clinical trials with a histone methyltransferase inhibitor."

It won't be EPZ004777, though. The compound has poor pharmacokinetics and, in the experiments described in Cancer Cell, had to be administered via implanted pumps.

Epizyme scientists consider EPZ004777 to be a "very interesting tool compound," Copeland said. "But we have much better compounds that we are pursuing for the clinic."