Anti-tumor Activity of the Type I PRMT Inhibitor, GSK3368715, Synergizes with PRMT5 Inhibition through MTAP Loss

Type I protein arginine methyltransferases (PRMTs) catalyze asymmetric dimethylation of arginines on proteins. Type I PRMTs and their substrates have been implicated in human cancers, suggesting inhibition of type I PRMTs may offer a therapeutic approach for oncology. The current report describes GSK3368715 (EPZ019997), a potent, reversible type I PRMT inhibitor with anti-tumor effects in human cancer models. Inhibition of PRMT5, the predominant type II PRMT, produces synergistic cancer cell growth inhibition when combined with GSK3368715. Interestingly, deletion of the methylthioadenosine phosphorylase gene (MTAP) results in accumulation of the metabolite 2-methylthioadenosine, an endogenous inhibitor of PRMT5, and correlates with sensitivity to GSK3368715 in cell lines. These data provide rationale to explore MTAP status as a biomarker strategy for patient selection.

Abstract 4357: Key differences revealed in NSD2 kinetics using truncated versus full-length protein

Histone lysine methyltransferases (HKMTs) are a class of enzymes that transfer a methyl group from S-adenosyl-L-methionine (SAM) to lysine residues on histone tails. The nuclear receptor SET domain-containing (NSD) family of proteins is known to methylate lysine 36 of histone H3 (H3K36). Abnormal methylation at H3K36 has been widely implicated in a variety of cancers and diseases, therefore, the enzymes responsible for this posttranslational modification are of interest from a drug-discovery standpoint. NSD2 (MMSET/WHSC1), a representative of this family, was found to be highly expressed in a multitude of human tumors and has been directly linked to multiple myeloma and Wolf-Hirschhorn syndrome. The chromosomal translocation t(4:14)(p16;q32) that occurs in 15% of myeloma patients is associated with both increased production of NSD2 and poor prognosis while loss of the NSD2 gene at the 4p16.3 region results in Wolf-Hirschhorn syndrome. In order to better understand the multi-dimensional nature of NSD2, full-length and truncated versions of the protein were generated to evaluate NSD2 kinetics. The biochemical activity of each NSD2 construct was assessed using (1) a radioactive assay measuring 3H transfer from SAM to the histone substrate or (2) LC-MS/MS analysis of the NSD2-dependent product. Full-length NSD2 prefers a nucleosomal substrate; whereas, C-terminal truncation of a highly charged region (AA 12-14-1240) resulted in a loss of nucleosomal activity and a gain of activity using a peptide derived from Histone H3. Additionally, LC-MS/MS mapping revealed a shift in the methylation site from H3K36 to H3K18 when using the truncated system. More detailed kinetic analysis revealed that the FL/nucleosome reaction catalyzes processive methylation while truncated NSD2 methylates the peptide distributively. Lastly, key changes in inhibitor specificity were observed. An alternative C terminal region, residues 1341-1365, was required to maintain potency of the product inhibitor SAH but not the close analog sinefungin. This may be indicative of the ping-pong kinetics proposed for FL NSD2 implying that SAH targets the NSD2-nucleosome bound form of the enzyme (UC vs. nucleosome) while sinefungin does not (NC vs. nucleosome). While a truncated NSD2/peptide system would be much easier to screen, profile and characterize in the search for inhibitors, the results found herein indicate that screening a full-length NSD2/nucleosome system may be more physiologically relevant.

Citation Format: Melissa B. Pappalardi, Jessica L. Schneck, Rosalie Matico, Michael Huddleston, Wangfang Hou, Patrick McDevitt, Roland Annan, Robert Kirkpatrick, Ryan Kruger. Key differences revealed in NSD2 kinetics using truncated versus full-length protein. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4357. doi:10.1158/1538-7445.AM2015-4357

A687V EZH2 is a driver of histone H3 lysine 27 (H3K27) hypertrimethylation

The EZH2 methyltransferase silences gene expression through methylation of histone H3 on lysine 27 (H3K27). Recently, EZH2 mutations have been reported at Y641, A677, and A687 in non-Hodgkin lymphoma. Although the Y641F/N/S/H/C and A677G mutations exhibit clearly increased activity with substrates dimethylated at lysine 27 (H3K27me2), the A687V mutant has been shown to prefer a monomethylated lysine 27 (H3K27me1) with little gain of activity toward H3K27me2. Herein, we demonstrate that despite this unique substrate preference, A687V EZH2 still drives increased H3K27me3 when transiently expressed in cells. However, unlike the previously described mutants that dramatically deplete global H3K27me2 levels, A687V EZH2 retains normal levels of H3K27me2. Sequencing of B-cell-derived cancer cell lines identified an acute lymphoblastic leukemia cell line harboring this mutation. Similar to exogenous expression of A687V EZH2, this cell line exhibited elevated H3K27me3 while possessing H3K27me2 levels higher than Y641- or A677-mutant lines. Treatment of A687V EZH2-mutant cells with GSK126, a selective EZH2 inhibitor, was associated with a global decrease in H3K27me3, robust gene activation, caspase activation, and decreased proliferation. Structural modeling of the A687V EZH2 active site suggests that the increased catalytic activity with H3K27me1 may be due to a weakened interaction with an active site water molecule that must be displaced for dimethylation to occur. These findings suggest that A687V EZH2 likely increases global H3K27me3 indirectly through increased catalytic activity with H3K27me1 and cells harboring this mutation are highly dependent on EZH2 activity for their survival.

Smyd3 regulates cancer cell phenotypes and catalyzes histone H4 lysine 5 methylation

Smyd3 is a lysine methyltransferase implicated in chromatin and cancer regulation. Here we show that Smyd3 catalyzes histone H4 methylation at lysine 5 (H4K5me). This novel histone methylation mark is detected in diverse cell types and its formation is attenuated by depletion of Smyd3 protein. Further, Smyd3-driven cancer cell phenotypes require its enzymatic activity. Thus, Smyd3, via H4K5 methylation, provides a potential new link between chromatin dynamics and neoplastic disease.

GSK1120212 (JTP-74057) is an inhibitor of MEK activity and activation with favorable pharmacokinetic properties for sustained in vivo pathway inhibition

PURPOSE

Despite their preclinical promise, previous MEK inhibitors have shown little benefit for patients. This likely reflects the narrow therapeutic window for MEK inhibitors due to the essential role of the P42/44 MAPK pathway in many nontumor tissues. GSK1120212 is a potent and selective allosteric inhibitor of the MEK1 and MEK2 (MEK1/2) enzymes with promising antitumor activity in a phase I clinical trial (ASCO 2010). Our studies characterize GSK1120212' enzymatic, cellular, and in vivo activities, describing its unusually long circulating half-life.

EXPERIMENTAL DESIGN

Enzymatic studies were conducted to determine GSK1120212 inhibition of recombinant MEK, following or preceding RAF kinase activation. Cellular studies examined GSK1120212 inhibition of ERK1 and 2 phosphorylation (p-ERK1/2) as well as MEK1/2 phosphorylation and activation. Further studies explored the sensitivity of cancer cell lines, and drug pharmacokinetics and efficacy in multiple tumor xenograft models.

RESULTS

In enzymatic and cellular studies, GSK1120212 inhibits MEK1/2 kinase activity and prevents Raf-dependent MEK phosphorylation (S217 for MEK1), producing prolonged p-ERK1/2 inhibition. Potent cell growth inhibition was evident in most tumor lines with mutant BRAF or Ras. In xenografted tumor models, GSK1120212 orally dosed once daily had a long circulating half-life and sustained suppression of p-ERK1/2 for more than 24 hours; GSK1120212 also reduced tumor Ki67, increased p27(Kip1/CDKN1B), and caused tumor growth inhibition in multiple tumor models. The largest antitumor effect was among tumors harboring mutant BRAF or Ras.

CONCLUSIONS

GSK1120212 combines high potency, selectivity, and long circulating half-life, offering promise for successfully targeting the narrow therapeutic window anticipated for clinical MEK inhibitors.

Novel mechanism of lapatinib resistance in HER2-positive breast tumor cells: activation of AXL

HER2-directed therapies, such as trastuzumab and lapatinib, are important treatments for breast cancer. However, some tumors do not respond or develop resistance to these agents. We isolated and characterized multiple lapatinib-resistant, HER2-positive, estrogen receptor (ER)-positive breast cancer clones derived from lapatinib-sensitive BT474 cells by chronic exposure to lapatinib. We show overexpression of AXL as a novel mechanism of acquired resistance to HER2-targeted agents in these models. GSK1363089 (foretinib), a multikinase inhibitor of AXL, MET, and vascular endothelial growth factor receptor currently in phase II clinical trials, restores lapatinib and trastuzumab sensitivity in these resistant cells that exhibit increased AXL expression. Furthermore, small interfering RNA to AXL, estrogen deprivation, or fulvestrant, an ER antagonist, decreases AXL expression and restores sensitivity to lapatinib in these cells. Taken together, these data provide scientific evidence to assess the expression of AXL in HER2-positive, ER-positive patients who have progressed on either lapatinib or trastuzumab and to test the combination of HER2-targeted agents and GSK1363089 in the clinic.