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Maokoto Yamagishi, PhD
Photo by Larry Young

SAN FRANCISCO—Preclinical research suggests a compound that inhibits both EZH1 and EZH2 could be effective against adult T-cell leukemia/lymphoma (ATLL).

The compound, known as OR-S1, has demonstrated activity against ATLL in vitro and in vivo.

Researchers said OR-S1 reversed epigenetic disruption in ATLL cells, selectively eliminated both ATLL cells and cells infected with human T-cell leukemia virus type I (HTLV-1), and inhibited tumor growth in mouse models of ATLL.

Based on these results, the researchers are planning a phase 1 study of the compound.

Makoto Yamagishi, PhD, of The University of Tokyo in Japan, described the preclinical research with OR-S1 and discussed the rationale for developing the compound at the 8th Annual T-cell Lymphoma Forum. The work was carried out in collaboration with Daiichi Sankyo Co., Ltd.

“We do not precisely understand the molecular mechanism of ATLL development, including genetic and epigenetic abnormalities,” Dr Yamagishi noted.

To gain some insight, he and his colleagues performed microRNA profiling, gene expression profiling, and histone methylation/epigenetic factor profiling on cells from ATLL patients and CD4+ T cells from healthy donors.

The team found that PRC2 factors were significantly upregulated in ATLL. EZH2 was the most upregulated histone methyltransferase, but ATLL cells did not have active mutations in the EZH2 gene. Dr Yamagishi said this suggests EZH2 upregulation is critical for the ATLL-specific epigenome.

“At long last, we determined the epigenetic pattern of ATLL,” he said. “ATLL cells showed specific and significant reprogramming of the epigenome, especially H3K27me3 gain. We found abnormal H3K27me3 change in half of genes, and gain was dominant.”

“But, interestingly, the methylated genes are specific in ATLL and do not overlap with other EZH2-dependent cell types, such as embryonic stem cells and diffuse large B-cell lymphoma cells. So ATLL has a very unique epigenome.”

Further investigation revealed that both EZH1 and EZH2 contribute to ATLL-specific epigenetic deregulation. More than 80% of H3K27me3 accumulated genes are occupied by EZH1 and/or EZH2.

So the researchers decided to examine the effects of knocking down EZH1 and EZH2 in ATLL cells.

Compared with knockdown of either gene alone, double knockdown synergistically influenced target gene expression. It led to complete dysfunction of the Polycomb family and had a significant impact on ATLL cell survival.

The researchers also found that EZH1 depletion enhanced ATLL cells’ sensitivity to the EZH2 inhibitor GSK126.

So the team decided to develop a dual EZH1/EZH2 inhibitor. They created OR-S1, which showed “strong activity” against EZH1 and EZH2 but none of the other histone methyltransferases tested.

In in vitro experiments, OR-S1 completely removed H3K27me3 and significantly reduced cell growth in the ATLL-derived cell line TL-Om1.

The drug also reduced cell viability in primary ATLL cells. All 15 samples tested proved sensitive to OR-S1. In addition, OR-S1 treatment selectively removed HTLV-1-infected cells from samples taken from 16 asymptomatic carriers.

Finally, OR-S1 proved active in mice. The drug prevented engraftment of ATLL cells in immunocompromised mice. All 6 OR-S1-treated mice were alive and tumor-free at 49 days, whereas 5 of 6 control mice had died (P=0.0041).

In mice treated after ATLL cell engraftment, OR-S1 reduced tumor growth without causing notable weight loss.

“Synthetic lethality by targeting EZH1 and EZH2 is promising [for ATLL],” Dr Yamagishi said. “Toxicity tests suggest the EZH1/2 dual inhibitor may be sufficient for clinical use, so we are now planning a phase 1 study.” end hematology article

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