EA1181, also known as CompassHER2, clinical trial basically enrolled patients with HER2+ breast cancer who had T1, T2, T3 disease as well as CN, clinical node, positive disease, those patients who were eligible, also node negative patients if they had tumours greater than 2cm were eligible. The patients were registered to receive a standardised therapy of trastuzumab plus pertuzumab in addition to a taxane, which could be paclitaxel, docetaxel or nab-paclitaxel. The patients then underwent surgery and patients with and without pCR were then analysed for the amount of tumour-infiltrating lymphocytes in this particular study.

The primary goal of the trial, however, was to identify the impact of pCR in terms of recurrence free survival at three years. One of the secondary endpoints, which is what we were looking at, is the development of pCR and factors associated with pCR with a particular focus on tumour-infiltrating lymphocytes. As you know, tumour-infiltrating lymphocytes are an important parameter for assessing immune response within a tumour. There’s plenty of data within triple-negative breast cancer, some of it generated from our group, which suggests that patients with high levels of tumour-infiltrating lymphocytes have an excellent prognosis. The data for HER2+ tumours, however, is much more controversial. There are certain studies that have shown that tumour-infiltrating lymphocytes are associated with good prognosis, as well as with pCR, but other studies, particularly clinical trials such as NeoSphere, TRAIN-2 and TRYPHAENA, have failed to establish significant association.

So, given the fact that we had a very large study of more than 2,000 patients in the parental study, we sought to identify what, if any, the role of tumour-infiltrating lymphocytes was in this cohort. We had 1,328 patients on whom we could analyse tumour-infiltrating lymphocytes and, basically, the analysis was performed as usual using the TILs working group guidelines first described by Roberto Salgado et al., of course I am part of that guideline, and then two observers looked at it and we had initially planned to look at two cut-offs of 10% and 60%. However, looking at the data, there were very few patients with TILs above 60% so the primary analysis was restricted to TILs greater than 10%.  We also did an exploratory analysis of TILs as a continuous variable and the reasons for that will be apparent very soon.

So, what did the primary analysis show? The primary analysis showed that TILs, by and large, were much lower in frequency in HER2+ breast cancer as opposed to triple-negative breast cancer. The median levels of TILs in this population was closer to 10% while typically in triple-negative breast cancer we have TILs that are between 15-25%, depending on the study that you look at. So that was the first important point.

The second important finding was when we looked at TILs at the 10% cut-off, TILs were an important prognostic parameter associated with pCR in HER2+/ER+ breast cancer. However, the 10% cut-off was not significant for ER-/HER2+ patients. So that was somewhat of a surprise and that’s the reason we also did a continuous variable analysis. The continuous analysis of TILs showed that it was significant, both in ER+ as well as ER- subsets of HER2+ tumours.

So what does this mean? Well, we do not have good prognostic parameters for ER+/HER2+ disease and TILs certainly seem to come up as a very good parameter that is associated with pathologic complete response. In ER-/HER2+ disease we have other parameters such as HER2DX that can be used and, in fact, in this particular study we did a subset analysis of HER2DX. We had almost 450+ patients on whom we had HER2DX data. In this we did a multivariable analysis and in the multivariable analysis, both in ER+ and in ER- HER2+ disease, TILs were not significant.

However, it must be noted that in ER+ disease TILS approached significance with a p-value of 0.08 so if we increase the number of patients in the cohort, which we are planning to do, we may have a level that is significant in the independent multivariable analysis.

So, in terms of conclusions, we can certainly say a few things. First, that TILs are much more infrequent in HER2+ disease. Second, they seem to have a prognostic association with pCR at the 10% cut-off, particularly in HER2+/ER+ disease, as opposed to that in HER2+/ER- disease the association was much weaker and we did not reach statistical significance at the 10% cut-off, although it remained significant as a continuous variable.

So all this basically means that we have multiple tools now to prognosticate patients with HER2+ disease and we can start using TILs as an additional tool within that armamentarium to predict outcomes.

Lastly, we still are following the patients up, that the primary goal of the trial, as I previously said, was a three-year recurrence-free survival analysis for patients who develop pCR. That analysis is ongoing and we will have that data and the correlations with all the other parameters that we have will be reported at a later date.

What is the clinical significance of these results?

A couple of points to be noted in relation to EA1181. First, the pCR rate in this particular regimen was a little lower than what we are now coming to expect with the HER2+ positive therapies, particularly with T-DXd and whatnot. But what is also noted in this particular trial is that the toxicity was also lower.

The second point of importance is we have never really assessed what pCR, that is pathologic complete response following neoadjuvant chemotherapy, is in terms of long-term benefit for patients. We have presumed that it is similar to triple-negative breast cancer. So EA1181 is the largest neoadjuvant HER2 trial that is assessing outcomes for patients with pCR and, as I said previously, three-year recurrence free survival is the primary endpoint. The secondary endpoints of pCR and here, as we already mentioned there was a strong association in HER2+/ER+ tumour with a 10% cut-off, although the similar association was not seen in HER2+/ER- patients. TILs continue to be a significant parameter as a continuous variable.

So what changes on Monday? Well, we can start using TILs as a prognostic parameter in HER2+ disease and based on this largest trial of neoadjuvant anti-HER2 therapy.