NEW YORK – In the wake of a recent study, researchers are hoping to validate the ability of a relatively straightforward and easy-to-measure protein, called Major Histocompatibility Complex Class II, or MHC-II, to predict immunotherapy response when expressed on the surface of breast cancer cells.
The study, published Tuesday in Clinical Cancer Research, comes at a time when improving patient selection for immunotherapy treatment — particularly immunotherapy in the neoadjuvant and adjuvant settings for early-stage triple-negative breast cancer (TNBC) — has been top of mind for many in the field. Several weeks ago, Merck presented updated results from its Keynote-522 trial showing that adding pembrolizumab (Keytruda) to chemotherapy in a pre-surgery setting, then continuing on with post-surgery pembrolizumab monotherapy improved event-free survival for patients with high-risk, early-stage triple-negative breast cancer versus neoadjuvant chemo alone followed by adjuvant placebo. The US Food and Drug Administration granted full regulatory approval to the immunotherapy regimen shortly thereafter.
However, oncologists have questioned whether the extensive — and sure to be expensive — treatment is truly necessary in this TNBC setting, particularly since the three-year event-free survival rate in the comparator arm was around 77 percent, suggesting that early-stage TNBC patients could hope to fare relatively well on just neoadjuvant chemo and no adjuvant immunotherapy. Although some have raised the need to use biomarkers to identify best responders to the pembrolizumab-based regimen, attempts at stratification using standard biomarkers like PD-L1 weren't all too telling in Keynote-522.
In the recent Clinical Cancer Research study, researchers, including Justin Balko of the Vanderbilt University Medical Center, investigated whether MHC-II expression on tumor cells could predict neoadjuvant immunotherapy benefit among early-stage TNBC and high-risk ER-positive, HER2-negative breast cancer patients. The biomarker has previously demonstrated utility in predicting immunotherapy response among melanoma and Hodgkin's lymphoma patients.
After completing the latest study, despite it being an early and retrospective analysis, Balko is confident that the MHC-II expression may have similar utility in early-stage breast cancer. "This is our first opportunity to [look at samples from] some of the smaller phase clinical trials and ask, 'Does this actually work in breast cancer and in the early setting?,' and it really seems like it does," Balko said, adding that the biomarker will have to be validated in prospective and larger Phase III trials.
"An optimal biomarker in this setting is one that specifically identifies patients who require the addition of immunotherapy to neoadjuvant chemotherapy to achieve pathological complete response, while not identifying patients who have high pathological complete response rates to chemotherapy alone," Balko and co-authors wrote. They designed their study not only to assess how well MHC-II expression could predict patients' responses to immunotherapy, but also to rule out the possibility that MHC-II could predict patients' likelihood of deriving benefit from chemotherapy alone.
Among all the samples evaluated, 12 percent met the cutoff for positivity. In TNBC specifically, 15 percent were MHC-II positive, while 10 percent of HR-positive tumors met the positivity threshold. The researchers reported that MHC-II positivity appeared to be identifying those more likely to achieve pathologic complete response with an immunotherapy-based regimen. Specifically, among patients treated with neoadjuvant chemotherapy plus durvalumab and with MHC-II-positive tumors, the pathological complete response rate was 67 percent versus 36 percent for those without the predefined level of MHC-II expression.
Through analysis of additional samples collected from high-risk HER2-negative breast cancer patients within the I-SPY2 trial, researchers found that MHC-II expression was significantly higher in patients achieving pathological complete response after treatment with pembrolizumab plus neoadjuvant chemotherapy, but the biomarker was not predictive of pathological complete response in the cohort of patients who received neoadjuvant chemotherapy alone. Across each of the trials included in the retrospective analysis, patients who had MHC-II expression in at least 5 percent of tumor cells had higher rates of pathological complete response when their treatment contained a checkpoint inhibitor, while the same was not true of patients treated with chemo alone.
Similarly, Balko and colleagues found that MHC-II expression was predictive of improved event-free survival with pembrolizumab plus neoadjuvant chemotherapy among patients with high-risk, HER2-negative tumors, though the same was not true of patients treated with chemotherapy alone.
In Balko's view, the key differentiator that gives MHC-II an edge over biomarkers like PD-L1 expression or tumor-infiltrating lymphocyte counts is its specificity to immunotherapy response. Other biomarkers, including PD-L1, have been shown to predict benefit to immunotherapy and chemo alike, complicating their use guiding immunotherapy.
"PD-L1 is a poor biomarker, but it's kind of the only one we have had [for immunotherapy in breast cancer]," Balko said, explaining that this new biomarker could stand to change that.
The biological reasons why MHC-II predicts patients' response to immunotherapy but not chemotherapy is still murky, though researchers have theorized it may be due to inflammatory signals, such as interferons in the tumor microenvironment. The factors driving tumor cell-specific MHC-II expression and the association with immunotherapy benefit is an "important subject of study," wrote the Clinical Cancer Research authors.
The immediate priority, according to Balko, should be to validate MHC-II expression prospectively as an immunotherapy-predictive biomarker. The underlying biological mechanism is secondary, he suggested, as long as the biomarker works.
Further validation
Balko's group aims to take the latest findings in prospective validation trials. They are also trying to expand their findings into the adjuvant setting.
While the details are still evolving, Balko and fellow researchers are looking at several large ongoing consortium trials that are evaluating adjuvant immunotherapy in breast cancer patients who did not achieve a complete response to neoadjuvant immunotherapy. Balko noted that another large-scale immunotherapy trial in high-risk HR-positive breast cancer, which has yet to officially launch, might integrate the MHC-II marker into its design. "It would be years before that trial reads out, but it definitely puts us into a much more reasonable timeline where we can try to validate this biomarker and bring it to patients," he said.
Simultaneously, Balko and colleagues are building out a more comprehensive assay to measure MHC-II expression. Right now, a biomarker that relies on immunohistochemistry is simple for pathologists to score. "But we'd like to do it a little bit sharper than that," Balko said.
By "a bit sharper," he explained that he hopes to localize the assay to identify MHC-II expression just on tumor cells and incorporate other biomarkers into the same test. The biomarker pan-cytokeratin, for example, could localize the assay's findings just to tumor cells and not the surrounding healthy tissue, since cytokeratin only shows up on epithelial tumor cells.
A more comprehensive version of the panel might include biomarkers like PD-L1 expression, which doctors may still want to know, even though this was not an effective biomarker for predicting immunotherapy response in early-stage breast cancer, as Balko and colleagues pointed out in their study. "PD-L1 still seems to come up as the only biomarker we look at in all of these clinical trials, and that's just out of habit" he said. "By including that in a panel, that will give us more information while serving as somewhat of a negative control."
Bringing pharma on board
As Balko and colleagues pursue validation studies to develop MHC-II expression into a predictive tool, they may find it challenging to engage pharmaceutical partners among companies that are already marketing highly profitable immunotherapies in early-stage breast cancer indications unencumbered by a biomarker.
"We have to be able to access those larger, Phase III trials, and often, those are led by pharma," Balko said. "For instance, Merck is not very interested in finding biomarkers with their [Keynote-522] trial … because they just got approval in all patients [with high-risk, early-stage TNBC]."
Investigators involved in the Keynoted-522 trial previously indicated that identifying biomarkers would be a priority as the trial continues but haven't provided details.
Recognizing the possibility that drugmakers may be hesitant to voluntarily cut into a profitable indication with a biomarker, Balko suggested evaluating MHC-II expression in indications that have not yet seen immunotherapy approvals. "This is something we're trying to work on as much as we can … and in newer indications coming down the line," Balko said. "But of course, in order to become a part of that conversation, we need to publish the results that we get to help guide pharma to say, 'Hey, this is worth the risk.'"