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E.C. Matheson et al.
In addition, a phase I trial of selumetinib in children with neurofibromatosis type 1 and inoperable plexiform neu- rofibromas showed partial responses in 17 of 24 children (71%) and no excess toxicity was reported.32
While we have previously shown preclinical activity of selumetinib in ALL, this drug, like other MEK inhibitors, is likely to show maximal therapeutic benefit in combina- tion. Indeed, in phase III clinical trials for advanced non- small-cell lung cancer and uveal melanoma, selumetinib has been evaluated in combination with docetaxel and dacarbazine, respectively.33 In this current study, we show significant synergy of selumetinib with the synthetic glu- cocorticoid dexamethasone in vitro and in an orthotopic mouse model engrafted with RAS pathway-activated pri- mary-derived ALL cells. Importantly, we demonstrate this across a range of cytogenetic subgroups, including high hyperdiploidy, B-other, t(17;19) and t(1;19) ALL. Pharmacokinetic data showed clinically relevant drug lev- els and optimal scheduling and in vivo pharmacodynamic analyses confirmed an impact on drug targets and apopto- sis. Mechanistically, the synergism was associated with enhanced induction of the pro-apoptotic protein, BIM, and decreases in the anti-apoptotic BH3-only protein, MCL1. BIM is a BH3-only protein that binds to anti-apop- totic BCL2 family members, including MCL1 and BCL2, to liberate and directly activate bax and bak, which then elicit caspase-dependent apoptosis. BIM is an effector pro- tein in both glucocorticoid and MEK inhibitor responses and reducing pERK activity enhances BIM levels as well as decreasing MCL1 protein levels by increasing its turnover.34 Therefore, we propose that the drug combina- tion enables BIM to inhibit anti-apoptotic BH3-only pro- teins more completely and directly activate BAX and BAK. Others have reported a direct effect of MEK inhibition on glucocorticoid receptor transcriptional activity which may also contribute to the synergism, but this did not appear significant in our experiments at the time point chosen.22 Epigenetic regulation of the BIM locus due to acetylation has been described in a subgroup of glucocorticoid-resis- tant ALL and is associated with BIM under-expression. Such individuals may be expected to have a suboptimal response to the selumetinib/dexamethasone drug combi- nation.35 However, the incidence of acetylated BIM in the relapsed setting and in the context of RAS pathway muta- tions has not been described to date. Our synergism data are supported by a study by Jones et al., who used an inte- grated approach to understand glucocorticoid resistance and relapse and identified MAPK pathways as a contribu- tory factor.22 In this study, knockdown of MEK2 or MEK inhibition enhanced responses not only to glucocorticoids but also to other chemotherapeutics and was not depend- ent on the presence of RAS pathway mutations, a finding suggestive of activation of the pathway through alterna- tive routes. We have previously shown an excellent corre- lation between pERK activation and the presence of RAS pathway mutations, although we too noted some rare exceptions which in our study were in part explained by the presence of chromosomal translocations, including the Philadelphia chromosome and 11q23.6
Drug synergies have also been shown for MEK inhibitors with both traditional chemotherapeutics such as gemc- itabine and targeted agents including PI3K/AKT inhibitors36,37 and the BCL-XL inhibitor, navitoclax (ABT263).38 Inhibiting the other effector pathways of RAS is clearly a rational strategy; however, while we have
observed synergism of MEK and AKT inhibition in RAS pathway-mutated ALL in vitro, the synergism was consid- erably weaker than that observed with dexamethasone (unpublished observations). In solid cancers, increased lev- els of BIM protein are also observed with MEK inhibition, but the protein is inactive due to sequestration by high lev- els of BCL-XL. In the presence of navitoclax, BIM is released, triggering an apoptotic response.38
We have previously reported a reduction of CNS leukemia in selumetinib-treated mice and now confirm this in additional primagraft samples and show complete absence of leukemic infiltration in the leptomeninges of mice treated with the selumetinib/dexamethasone drug combination.6 The identification of CNS disease in mice with similar levels of ALL engraftment prior to drug dosing suggests that the drug combination completely eradicated the leukemia in situ. This is a highly significant finding given the association of RAS pathway mutations and CNS disease at relapse that we previously reported in the IBFM- REZ2002 clinical trial and the fact that in contemporary regimens, the proportion of CNS relapses is increasing.39
A key question, relevant to MEK inhibitor therapy, is whether Ras pathway mutations are initiating events in ALL or secondary, cooperating genetic events and there is evidence for both.13 However, for targeted therapies to be successful, the target is ideally present on all tumor cells and we and others have reported that mutations can be subclonal, particularly at diagnosis, and can be gained or lost at relapse.6,40-42 Importantly, we have also shown that mutations at relapse are in the major ALL clone, are often selected from a minor subclone at diagnosis and that apparent ‘loss’ of a Ras pathway mutation can be ‘replace- ment’ of one for another.6,8,43 This suggests a dependence on the pathway that can be exploited by MEK inhibition and, as we show here, is enhanced with co-exposure to dexamethasone.
Based on these promising data, an international phase I/II clinical trial of oral dexamethasone and selumetinib (Seludex) is underway in RAS pathway-mutated, multiply relapsed/refractory ALL. A parallel, national study in adult disease at first relapse is also ongoing, since the prevalence of RAS pathway mutations and association with poor prognosis has also been noted.44 One relevant observation from selumetinib and other Mek inhibitor trials is that the most common toxicity is inflammatory rash. In severe cases, the recommended treatment is oral glucocorticoids and no adverse effects of drug co-administration have been reported.45 Thus, if efficacy is seen in the proposed clinical trials, selumetinib and other Mek inhibitors may be a much needed novel therapy for a substantial number of children with high-risk, relapsed disease. There may also be a role for the drug combination in the upfront treatment of RAS-driven, high-risk ALL, to avert relapse.
Acknowledgments
The authors gratefully acknowledge Cancer Research UK (project grant to JAEI, HN and JV, number 18780), Bloodwise (previously known as the Leukaemia and Lymphoma Research Fund, project grant to JAEI, number 11007), the North of England Children’s Cancer Research Fund and the Newcastle Haematology Biobank for ALL samples. We are grateful to AstraZeneca for their kind donation of selumetinib. CH is funded by the Chief Scientist Office (ETM/374). We thank Clare Orange and Lynn Stevenson, University of Glasgow and Think Pink, Scotland for help with histology and slide scanning.
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haematologica | 2019; 104(9)