NEW YORK – By measuring the functional MAPK activity of different RAS mutations, researchers have developed a method they believe could help identify patients with RAS-mutated cancers likely to benefit from MEK inhibitors like trametinib (Novartis' Mekinist).
The method is detailed in a study published recently in the European Journal of Cancer, in which researchers from the University of California, San Diego and Fore Biotherapeutics (previously Novellus Dx) found that patients whose RAS mutations had higher levels of functional MAPK activity experienced improved progression-free survival, overall survival, and clinical benefit rates after treatment with MEK inhibitors.
"A lot of people say that trametinib doesn't work for RAS-mutant patients, and that's actually not true," Razelle Kurzrock of UCSD, one of the lead authors on the European Journal of Cancer paper, explained. "We already knew that there were patients who responded, [but] the problem was that it was a relatively small subset of patients."
In the past, these patients have been categorized as exceptional responders, though the reason for their responses has remained a mystery. By examining the role of functional MAPK pathway activation, Kurzrock and colleagues hoped to shed light on the previously unanswered question of why this small subset of patients with RAS-mutated cancers responded to MEK inhibitors while others did not.
To address this question, the researchers used a functional cell-based assay to quantify MAPK activation in the presence of different RAS mutations. Specifically, Fore Bio generated aberrant HRAS, NRAS, and KRAS variants in cell-based assays, fluorescently tagged ERKK2, which travels from the cytoplasm to the cell nucleus once MAPK is activated, and performed high-throughput imaging analysis.
"What [Fore] did was create the mutation, transfect it into the reporter cells, and then transfect in also the [fluorescently tagged] ERKK2," Kurzrock explained. "Then, they would give [the assay] time to produce a protein, and then there would be a readout as to the degree to which the ERKK2 was activated."
By fluorescently tagging ERKK2, researchers could calculate the "nuclear-to-cytoplasm ratio," which they used to inform the MAPK activation score of the different RAS variants. The researchers then selected the mean MAPK activity score of the RAS variants — which was 0.8 — as the cutoff for high versus low MAPK activity.
"This [approach] is very usable, because you don't have to take every patient's sample and do this again," Kurzrock said, explaining that once researchers quantify the MAPK activation of a specific RAS variant, patients can be tested on a widely available next-generation sequencing test like Foundation Medicine's FoundationOne CDx to identify if they have a RAS variant.
If they do, one would only have to look up the assigned MAPK activity score for that variant to inform treatment decisions. "You need to know the mutations, but once you know the mutations, you can determine the activations," she said. "You don't have to determine the activations again."
In their published study, Kurzrock and colleagues tested out this very premise in a retrospective analysis of a cohort of 62 patients who had RAS alterations identified via FoundationOne CDx and received MEK inhibitors, including trametinib, cobimetinib (Genentech's Cotellic), selumetinib (AstraZeneca's Koselugo), and SpringWorks Therapeutics' investigational mirdametinib.
Among the patients with confirmed RAS mutations who received MEK inhibitors, the most common cancer types were gastrointestinal cancers — both colorectal and non-colorectal — hepatopancreatobiliary, and gynecological cancers. The most common RAS mutations variants observed were KRAS G12D, KRAS G12V, NRAS Q61R, and KRAS G13D, among others, and these variants ranged in MAPK activity scores, as assessed by Fore's functional analysis, from 0.52 to 1.22. Of the variants observed among the MEK inhibitor-treated patients, the one with the highest MAPK activity was NRAS Q61R, and the one with the lowest MAPK activity was NRAS Q61H.
Upon considering patients' clinical outcomes on MEK inhibitors alongside the MAPK activity of their unique RAS variants, researchers found that patients with high MAPK activity scores had better clinical outcomes than those with low scores. Specifically, patients whose RAS mutations had high MAPK activity experienced a median progression-free survival of five months and a median overall survival of 20 months versus 2.3 months and five months, respectively, for patients with low MAPK activity. The clinical benefit rates, combining patients who achieved stable disease, had partial response, or experienced complete remission six months after treatment, followed a similar pattern.
Because the analysis was retrospective, Kurzrock acknowledged that there is no way to know for sure whether other factors, such as patient characteristics or prior treatment, affected the outcomes. Still, her team conducted a multivariate analysis, which "showed that the MAPK activity score was independently significant of predicting outcome," Kurzrock said. "Doing a multivariate analysis isn't perfect, but in the context of a study where we looked at real-world data retrospectively, that was a step toward ruling out potential confounders. The next step would be a prospective study."
There is a significant unmet need when it comes to patients with RAS-mutated cancers, and Kurzrock and colleagues' study suggested that there could be an opportunity to treat at least some of these patients with drugs that are already developed and approved by the US Food and Drug Administration.
"The reason that we wanted to look at this is that there are now four FDA-approved MEK inhibitors," Kurzrock said, pointing out that there are significant opportunities in precision oncology in terms of expanding the indications of existing drugs. For example, although these MEK inhibitors were developed and approved for different indications — most commonly for BRAF-mutated cancers, as is the case with trametinib — Kurzrock and colleagues' biomarker research has demonstrated how these agents could also be an option for patients with RAS mutations, perhaps even in combination with other therapies.
For example, drugmakers have made progress when it comes to targeting a specific RAS mutation, KRAS G12C. "It should be of interest to determine if the recently demonstrated responses to specific compounds targeting KRAS G12C [mutations] can be further enhanced by combining these compounds with MEK inhibitors because KRAS G12C had high levels of MAPK activity in our assays," the researchers suggested.
Much of Kurzrock's research is focused on exploring combination treatment strategies to overcome resistance commonly seen with single-agent precision oncology approaches. She expects there will be opportunities to combine MEK inhibitors with other pathway inhibitors beyond those in development for KRAS G12C.
With regard to this present European Journal of Cancer paper, however, Kurzrock acknowledged that the findings need further validation in larger patient cohorts and ultimately in prospective clinical trials. According to Kurzrock, additional trials evaluating this approach, including in the context of combination treatment, are underway already.
In the meantime, she said, "it's all about getting the data out there." The "rumor" that patients with RAS mutations cannot benefit from MEK inhibitors, can be turned on its head, she said, if enough players in the precision oncology space access these data and use them to advance their own studies.
"I often view my role as trying to ignite some of these ideas," she said, explaining that if this admittedly small retrospective analysis spurs large prospective trials, MEK inhibitors could become a treatment option for patients with RAS-mutated cancers, who often lack treatment options beyond minimally effective systemic therapies. Kurzrock cited the example of gemcitabine, an FDA-approved chemotherapy for pancreatic cancer — tumors that tend to harbor RAS mutations — with a response rate of around 5 percent.
Kurzrock was careful to note that in the wake of this study, the field is far off from a regulatory label for trametinib for these specific RAS-mutated cancers. Much more work is needed. But if the data from the present study holds up in larger prospective trials, she said there is "the possibility of trametinib being either part of a combination [treatment] or … an additional biomarker where you use trametinib for specific, highly functional RAS mutants."