This study represents the largest breast cancer adjuvant vaccine trial conducted to date and the only 1 using the E75 + GM-CSF formulation. We previously reported the initial results of these exploratory E75 vaccine trials, demonstrating the vaccine to be safe and effective in eliciting HER2-specific immunity.2
Those early results suggested that the vaccine may have clinical efficacy in preventing or delaying disease recurrence in patients at high risk for relapse. The findings in this 24-month landmark analysis provide important information regarding which patients may benefit most from vaccination and have served to identify the population of patients to be targeted for enrollment onto a phase 3 trial. On the basis of these data, such a phase 3 trial investigating the efficacy of E75 in the adjuvant setting is warranted.
An important result of the current analysis is that it has further elucidated the population of patients that may benefit the most from E75 vaccination. Interestingly, despite the finding that this vaccine targets HER2, our previously reported data suggest that patients whose tumors have low HER2 expression mount a more robust immunologic response after vaccination than do patients whose tumors overexpress HER2. Patients with HER2 low-expressing tumors demonstrated significantly larger delayed-type hypersensitivity responses after completion of the vaccination series as well as more sustained long-term E75-specific CTL responses.5
This 24-month analysis provides further evidence suggesting that patients whose tumors have low HER2 expression benefit from vaccination, showing a statistically significant improvement in DFS rates (P
= .04) compared with controls. These results suggest that the E75 vaccine could represent targeted therapy for patients whose tumors have some degree of HER2 expression but who do not meet current clinical criteria to receive trastuzumab.
We have previously hypothesized that patients with HER2-overexpressing tumors have some element of immunologic tolerance.5
However, despite the finding that these patients do not mount as robust an immune response as do patients with low HER2-expressing tumors, the vaccine still augments their HER2-specific immunity. It appears that patients with HER2-overexpressing tumors benefit from vaccination in addition to trastuzumab; we did not observe any disease recurrences in 11 patients who received both compared with recurrences in almost 20% of patients with HER2-overexpressing tumors vaccinated in the absence of trastuzumab. Because only 3 patients with HER2-overexpressing tumors received trastuzumab without vaccination, we are limited in the conclusions that can be drawn regarding the benefit of adding vaccination to standard of care trastuzumab therapy. However, data from the combined analysis of the National Surgical Adjuvant Breast and Bowel Project B-31 and North Central Cancer Treatment Group N9831 trials can be used to put our data into context. These trials evaluated the efficacy of trastuzumab in the adjuvant setting. At 3 years, the DFS in patients with HER2-overexpressing tumors who received trastuzumab was 87%.7
The number of patients in our trial is small; therefore, the data must be interpreted with caution. However, taken together with the results of the adjuvant trastuzumab trials, these data suggest that patients with HER2-overexpressing tumors who receive trastuzumab might derive additional benefit from vaccination. These findings are consistent with preclinical data published by our group and others demonstrating that treatment with trastuzumab enhanced the sensitivity of HER2-expressing tumor targets to HER2-specific CTLs.8–10
These findings are also consistent with previously published data from these vaccine trials demonstrating that patients who received the vaccine after trastuzumab were more likely to maintain long-term specific immunity, as quantified by the percentage of E75-specific CTLs.5
We are currently conducting a randomized phase 2 trial (NCT00524277; principal investigator, E.A.M.) investigating GP2, another HER2-derived HLA class I peptide that can stimulate the immune system. Because trastuzumab is now the standard of care for patients with HER2-overexpressing breast tumors, all patients with HER2-overexpressing tumors in that trial are receiving trastuzumab before vaccination. We anticipate that data from that trial will yield additional insight into this combination immunotherapeutic strategy of trastuzumab plus a CTL-eliciting vaccine.
Previous data reporting on the recurrences seen in these trials demonstrated that overall, vaccinated patients were less likely than controls to have recurrences and did not have recurrence of bone-only disease compared with controls, of whom 50% had bone-only disease.11
These data suggest that patients with biologically less aggressive disease may respond better to vaccination. Because tumor grade is a marker of disease aggressiveness, we compared DFS by grade. Patients with low-grade tumors who were vaccinated were more likely to have T1 tumors but were less likely to have ER+
disease than were controls. These are competing risk factors, as it is known that smaller tumors are less likely to recur; however, ER−
tumors are more likely to recur, in part because these patients do not receive the benefit of hormonal therapy.12–14
Regardless, among patients who had low-grade tumors, those who were vaccinated achieved a significant improvement in DFS over unvaccinated controls.
When evaluating these data, an important consideration is that these trials began as phase 1 trials designed to determine the safety and optimal dose/schedule of the vaccine; therefore, not all patients received the optimal dose. This is true for the overall DFS analysis as well as for subset analyses. We previously reported that patients who received the optimal dose of E75 had similar toxicity and enhanced immune responses compared with patients who received a lower dose. Importantly, there was a trend toward decreased recurrences in optimally dosed patients.6
Data presented in the current report confirm that finding. It also appears that incorporating a booster inoculation will be important in the E75 vaccine strategy. The decrease in E75-specific CTLs that we demonstrated in vaccinated patients in this trial is consistent with what is known about T-cell biology and response to antigenic stimulation. Immediately after stimulation, CTLs undergo a dramatic expansion phase, after which they contract and plateau during the death phase, when many CTLs undergo apoptosis. The remaining cells are antigen-specific central and effector memory CTLs that are critical for the maintenance of active immunologic memory. Levels of antigen-specific memory CTL have been observed to decline in this and other vaccine studies.3,15–17
Importantly, booster inoculations are effective in restimulating E75-specific immunity,3
and data in this report suggest that this restimulation may contribute to the prevention of disease recurrence.
On the basis of the data obtained in these exploratory trials, a phase 3 trial evaluating efficacy of the vaccine is warranted. In designing that trial, we used data from this study to define the appropriate patient population to enroll. The planned trial will enroll lymph node-positive patients, as they are at higher risk for disease recurrence. Only patients with HER2 low-expressing tumors defined as IHC 1+ or 2+ or FISH <2.2 but >1.4 will be eligible, as these patients have the most robust response to vaccination. All patients will receive the optimal dose plus booster inoculations. Importantly, HLA-A2/A3+ patients who meet eligibility criteria will be randomized to 1 of 2 arms: E75 + GM-CSF or GM-CSF alone. This double-blind trial design will address weaknesses of our phase 1–2 trials. First, our initial trials vaccinated all HLA-A2/A3+ patients and used HLA-A2/A3− patients as controls. It is unknown whether HLA status itself had a prognostic effect in these patients; however, direct comparisons between the vaccinated and control groups did not reveal significant clinically relevant differences in prognostic factors. Second, the completed trials had no immunoadjuvant-only arm; therefore, one could question whether the responses seen were because of GM-CSF alone, although this is unlikely based on our current randomized phase 2 GP2 trial with a GM-CSF–only arm.
By using the criteria proposed for the phase 3 trial, we identified 18 vaccinated and 27 control patients from the current cohort who were lymph node positive, had HER2 low-expressing tumors, and received optimal doses of the vaccine. There were no differences with respect to clinicopathologic characteristics between the 2 groups. Among these patients, the 24-month DFS rate was 100% for the vaccinated patients and 77.8% for controls (P = .04). There are limitations to this analysis. Specifically, unlike the strategy in the proposed phase 3 trial, the controls in these trials were not HLA-A2/A3+ and did not receive GM-CSF alone. Nevertheless, the significant difference in DFS noted between vaccinated and control patients suggests that the patient population we identified based on analysis of our phase 1–2 trials is the appropriate population to study in the next trial.
In summary, these analyses have demonstrated that the E75 vaccine may have clinical efficacy when optimally dosed and administered to appropriate patients. A phase 3 trial targeting the specific population of patients with lymph node-positive, HER2 low-expressing tumors is warranted.