Eliza Majewska, PhD

DUBROVNIK, CROATIA—The CDK8 inhibitor SEL120 has demonstrated preclinical activity against acute myeloid leukemia (AML), but the agent’s mechanism of action is still unclear.

Researchers found that several AML cell lines were “highly sensitive” to SEL120, and the inhibitor was active in primary patient samples.

SEL120 also reduced tumor growth in mouse models of AML and demonstrated synergy with venetoclax.

The researchers believe SEL120 works by affecting the maintenance of AML cells and leukemic stem cells (LSCs), inducing differentiation and, sometimes, apoptosis. However, the mechanism is not well defined.

Eliza Majewska, PhD, of Selvita S.A. in Krakow, Poland, discussed research with SEL120 at Leukemia and Lymphoma: Europe and the USA, Linking Knowledge and Practice.

Dr. Majewska explained that CDK8 is a transcriptional kinase working in the context of the Mediator complex, and previous research¹ indicated that CDK8 drives oncogenic transcription in AML.

In a prior study², researchers found that SEL120 inhibits CDK8 activity in AML cells with high levels of STAT phosphorylation.

Dr. Majewska said the MV4-11 cell line responds particularly well to SEL120, and other sensitive cell lines include SKNO-1, Oci-AML5, GDM-1, KG-1, MOLM-16, and Oci-AML3.

“The fact that STAT signaling was upregulated in those cell lines that were very sensitive to SEL120 gave us the hint that perhaps we are looking at a mechanism of action of the compound that has something to do with leukemic stem cells,” Dr. Majewska said.

In fact, she and her colleagues found that cell lines sensitive to SEL120 had upregulation of genes linked to LSCs and high levels of CD34 surface expression.

Experiments in CD34+ TEX cells showed that SEL120 specifically depletes CD34+ cells, leads to downregulation of stemness-related genes, and induces myeloid differentiation.

After 6 days of treatment with SEL120, TEX cells showed decreased expression of the LSC-linked genes MEIS1 and LILRB2, enrichment of gene sets downregulated in LSCs and linked to differentiation, and increased expression of differentiation markers and immune response genes.

SEL120 also demonstrated antileukemic activity in vivo. The researchers tested SEL120 in a CD34+ model of AML (KG-1) and a FLT3-ITD model of AML (MV4-11).

In both models, SEL120 induced “significant tumor regression” of about 80%. In some cases, the researchers observed apoptosis.

Toxicities observed in the mice included weight loss and upregulation of inflammation.

The researchers also found that SEL120 was synergistic with venetoclax. In fact, the combination of these drugs resulted in “almost complete remission cures” in the MV4-11 model, according to Dr. Majewska.

Finally, she and her colleagues discovered that SEL120 was active against primary patient cells. Samples from 3 of 4 AML patients had a significant reduction in cell numbers after 7 days of treatment with SEL120. For one patient, there were no viable cells on day 7.

Dr. Majewska said a phase 1 trial of SEL120 is planned for 2019 or 2020, and SEL120’s mechanism of action is still under investigation.

“The mechanism of action . . . is, in our mind, at least in some cases, linked to the fact that CDK8 functions within the context of the Mediator complex, which contributes to gene expression related to leukemic stem cells,” Dr. Majewska said.

“And when we inhibit this specific transcription, of course, the Mediator complex still works because this is just one of the components of the complex. However, the function that it has is suddenly very different, and it’s actually linked to lack of maintenance of leukemic stem cells, resulting in differentiation [and], in some cases, the induction of apoptosis, but we do not fully understand the mechanism of this induction.”

Dr. Majewska works for Selvita, the company developing SEL120. This research was funded by Selvita, the Leukemia & Lymphoma Society, and the National Centre for Research and Development.

1. Pelish HE et al. Nature. 2015 Oct 8;526(7572):273-276. doi: 10.1038/nature14904

2. Rzymski T et al. Oncotarget. 2017 May 16;8(20):33779-33795. doi: 10.18632/oncotarget.16810.

CDK8 inhibitor can fight AML, though it’s unclear how