How Low Can We Go? Metastatic Breast Cancer Enters a New Era of HER2 Classification

Erika P. Hamilton, MD, shares how multidisciplinary collaboration between pathologist and breast oncologists is key to staying abreast of not only the classification-actionable HER2 mutations in metastatic breast cancer but also the evolving definition of expression.

Data from DESTINY-Breast04 (NCT03734029) presented at the 2022 ASCO Annual Meeting thrust the topic of defining HER2-low metastatic breast cancer into the spotlight; however, the continuum of HER2 expression in breast cancer has been an evolving topic in the field for several years, according to Erika P. Hamilton, MD.

In a presentation during the 21st Annual International Breast Cancer Congress® West, Hamilton said that multidisciplinary collaboration between pathologist and breast oncologists is key to staying abreast of not only the classification-actionable HER2 mutations but also the evolving definition of expression.

Finding the Lowest Level of HER2

“What we traditionally classify as HER2-low is not HER2 0 and it is not what’s traditionally considered HER2 positive—[immunohistochemistry] IHC 3+. For patients [with HER2-low disease] they may have tumors that are 1+ or 2+ by IHC and [in situ hybridization] ISH negative,” said Hamilton, who is the director of the Breast and Gynecologic Cancer Research Program at Sarah Cannon Research Institute in Nashville, Tennessee.

Using this definition, the prevalence of HER2-low disease in patients with hormone receptor–positive breast cancer (n = 5563) was approximately 75%, with 27% of patients having HER2 IHC 1+ and 48% having IHC 2+ expression. In triple-negative breast cancer, Hamilton noted that HER2-low disease was less common, but still reported among approximately half of patients (n = 607).1

Investigators have challenged the rationale that patients must have IHC 3+ to derive benefit from HER2-targeted agents for several years. For example, in the phase 3 NASBP B-47 trial (NCT01275677), investigators evaluated the efficacy of adjuvant trastuzumab (Herceptin) and chemotherapy in patients with what was then defined as HER2-negative disease (IHC 1+ or 2+).2

NASBP B-47 did not show a clinical benefit in invasive disease-free survival with the addition of trastuzumab to chemotherapy for patients, with a hazard ratio of 0.98. “You cannot tell what color line that was supposed to be [on the Kaplan-Meier curve] because they’re laying right on top of each other,” Hamilton said, adding that despite these early misses with first-generation TKIs, the difference now is the availability of new agents.

The most prominent agent is the antibody-drug conjugate (ADC) fam-trastuzumab deruxtecan-nxki (Enhertu). “[This is] a humanized, anti-HER2 monoclonal antibody, same amino acid sequences as trastuzumab with a drug linker, and an exatecan derivative payload. The drug-to-antibody ratio is 8:1 and it is a topoisomerase I inhibitor payload with a short systemic half-life. [It’s] also highly membrane permeable, which enables a bystander effect that we think may be particularly important, [allowing it to have] such great activity even where HER2 [expression] may be low or heterogeneous,” Hamilton said.

Early efficacy with the agent was demonstrated in a phase 1b (NCT02564900) trial of 54 patients with HER2-low metastatic breast cancer.3 Treatment with trastuzumab deruxtecan at 5.4 or 6.4 mg/kg elicited an objective response rate (ORR) of 37% (95% CI, 24.3%-51.3%), with a disease control rate (DCR) of 87% and a median duration of response (DOR) of 10.4 months (95% CI, 8.8-not estimable [NE]). “What I think is even more impressive is the DCR approaching 90%, the DOR of at least 10 months, and progression-free survival [PFS] of almost a year,” Hamilton noted. The median PFS was 11.1 months (95% CI, 7.5-NE).3

To decipher the mechanism of action that allows for trastuzumab deruxtecan to elicit a benefit in this subgroup of patients, investigators examined the bystander effect of the ADC. The classic ADC mode of action involves the ADC binding to a HER2 receptor and being internalized into the cell by endocytosis, which is when the payload is released and has a cytotoxic effect.

The bystander killing effect, on the other hand, involves the release of the drug payload from the antibody after antigen binding prior to internalization.1 “[This results in] the drug payload in the intracellular space due to a high drug membrane permeability and, as you can imagine, if you have HER2 expression on some cells, and maybe not as much on cells around them, this is a way to target all of those cells,” Hamilton explained.

Building on the results observed in the phase 1b trial, investigators leveraged the bystander killing effect in the phase 3 DESTINY-Breast04 trial. Patients with HER2-low metastatic breast cancer who had received 1 to 2 prior lines of chemotherapy in the metastatic setting were randomly assigned 2:1 to trastuzumab deruxtecan at 5.4 mg/kg every 3 weeks or physician’s choice of chemotherapy (TPC).4,5

Patients with hormone receptor–positive disease must have been considered refractory to endocrine therapy and served as the population for the primary end point of PFS. Among these patients (n = 499), 66% had received 3 or more prior treatments for metastatic disease, including 40% of patients having received 2 prior lines of chemotherapy and 70.5% having received a prior CDK4/6 inhibitor.4,5

The median PFS in the hormone receptor–positive cohort was 10.1 months with trastuzumab deruxtecan vs 5.4 months with TPC (HR, 0.51; 95% CI, 0.40-0.64; P < .0001). Among all treated patients, the median PFS was 9.9 months vs 5.1 months, respectively (HR, 0.50; 95% CI, 0.40-0.63; P < .0001).

The median overall survival (OS) in the primary efficacy cohort was 23.9 months with trastuzumab deruxtecan compared with 17.5 months with TPC (HR, 0.64; 95% CI, 0.40-0.86; P = .003). The median OS among all patients was 23.4 months vs 16.8 months, respectively (HR, 0.64; 95% CI, 0.49-0.84; P = .001).4,5

An exploratory analysis was conducted for patients with hormone receptor–negative disease as there were only 60 patients who met the criteria, Hamilton said. “[Despite the] very small numbers, it looks no worse, with a PFS of 2.9 months [with TPC] increasing to 8.5 months [with trastuzumab deruxtecan]. The PFS for more aggressive triple-negative patients in general is shorter, but the spread is still the same, with an increased magnitude of benefit for trastuzumab deruxtecan.” The hazard ratio for PFS was 0.46. Hamilton noted that OS data for these patients were also encouraging; the median OS was 18.2 months with trastuzumab deruxtecan vs 8.3 months with TPC (HR, 0.48).4,5

“These results really will establish trastuzumab deruxtecan as a new standard of care for HER2-low metastatic breast cancer and that really encompasses both hormone receptor–positive [disease] as well as patients we would typically consider triple-negative,” Hamilton said, adding that the safety data in the trial revealed no new toxicity signals.

“That [represents] an additional 50% of patients with metastatic breast cancer in addition to the already 15% to 20% with HER2-positive disease who will likely derive benefit from this therapy,” Hamilton said. In July 2022, the supplemental biologics application for trastuzumab deruxtecan for patients with unresectable or metastatic HER2-low breast cancer who have received at least 1 prior line of therapy was granted priority review.6

Hamilton emphasized that these improvements point toward the future of collaborative approaches of interpreting HER2 expression results. “I think a very important point is that it we need to talk to our pathologists and let them know that an IHC of 1+ or 2+ is now actionable,” she said. “IHC was never intended or built to find patients [with] HER2-low disease; it was built to find patients who were HER2 high [expressors]. Many pathologists will tell you that they quickly look at a slide and pull it off if they don’t see that the patient is HER2 positive. [As a result] we may be getting a lot of 0 results that aren’t actually 0s. They just know that they’re not positive. If you move around with a little bit of tumor heterogeneity, I think that we’re probably going to see even more patients who have at least a IHC 1+.”

Hamilton added that the medical oncologist also has a role in changing the way HER2 status is communicated. “We must make sure that we change the wording in our notes now that this is actionable. Even my notes up until several years ago, when we started having [trastuzumab deruxtecan] in the clinic, would say HER2 negative by ISH. That no longer tells the whole story. We need to make sure that we start capturing our patients who have IHC 1+ or 2+ and are HER2 low.”

Diving even further into HER2-low disease, Hamilton noted that the DAISY trial (NCT04132960) is evaluating trastuzumab deruxtecan in patients with HER2-low and HER2-null (IHC 0+) metastatic breast cancer.7

Among the 37 patients in the IHC 0+ cohort, the ORR was 30% with a median DOR of 6.8 months. The median PFS was 4.2 months in this patient population.

With the benefit seen here, Hamilton said that a shift in HER2 with an IHC of 0+ may no longer be appropriate terminology, as it may be separate from HER2-negative disease. “Another way to say this is HER2-null, and I would encourage us all to maybe start talking about HER2 with a little bit different terminology: HER2 positive or amplified, HER2 low, and then HER2 0 or null. I don’t think we’re going to know what HER2 negative really means anymore.”

Expanding on that, Hamilton said IHC 0 has imperfect connotations. “If we talk to our pathology colleagues, HER2 IHC 0 does not mean there’s not any HER2; you can actually have thousands of copies of HER2 on the cell and still be classified as a 0,” she said, noting that challenging these standards will rely on more results in patients with IHC 0 disease.

Although trastuzumab deruxtecan may be at the forefront of these conversations, other agents are making their mark in the HER2 landscape. These include trastuzumab duocarmazine (SYD985) and zenocutuzumab, a novel HER2/HER3 bispecific antibody.

“What do we have coming [with regard to] ongoing and planned trials enrolling [patients with] HER-low disease? [There is] a slew of ADCs and [ado-trastuzumab emtansine [Kadcyla]) and trastuzumab deruxtecan are not going to be the only ones at the party any longer. We also have novel HER2 TKIs as well as HER2 vaccines,” Hamilton said.

Identifying Actionable Targets in HER2-Mutant Disease

Hamilton noted that among patients with HER2-mutant disease, identifying and sorting targets that will respond to therapy requires a nuanced approach. “There are at least 36 unique HER2-activating mutations that have been identified and counting,” she said. “What makes it tricky for us, as oncologists in the clinic, is not all HER2 mutations are actionable. Trying to figure out which ones are, and which ones are not activating, and therefore, not actionable for our patient can be a little bit difficult.”

Hamilton noted that when discussing HER2, it is important to separate HER2 mutations from HER2 overexpression or amplification. “Amplification is where we talk about too much of a receptor driven by the HER2 mutation and it can either be activating or it cannot be activating,” she said. Amplifications can occur in several locations, including the transmembrane or extracellular kinase domain, and overexpression is present in approximately 15% to 20% of breast cancers. HER2 mutations, on the other hand, are rarer and are present in approximately 3% to 4% of HER2-nonamplified cancers.1

“Most of the mutations that we’re aware of occur in the kinase domain,” Hamilton said, adding that accounts for 70% of the mutations which are mostly in exons 19 and 20. An additional 20% of mutations are found in the extracellular domain. In terms of targetability, Hamilton noted that among the known HER2-activiating mutations, some of them are insensitive to or resistant to first- and second-generation TKIs, but that newer agents have been shown to be effective.

Hamilton shared data from a personalized medicine team at Sarah Cannon showing that the frequency of HER2-activiating mutations among 306 patients with HER2-mutant breast cancer was 48%. Across 2161 patients with HER2-mutant solid tumors, the frequency was 30%. Although the rates were higher in the breast cancer cohort, Hamilton noted that only half of patients have targetable markers, and sensitivity to anti-HER2 therapies vary across agents.1

“What are our options for cancers that have a HER2 mutation?” Hamilton asked. “What makes it tricky is that the [mutations] do not all [respond] the same to different TKIs. [The] mutations are not all created the same; some are sensitive to one TKI and may be resistant to another. This certainly becomes complex when you have an individual patient [whose tumor harbors a] HER2 mutation. First, you [must consider] if the HER2 mutation is actionable, and then what TKI may be most appropriate.”

Several studies are underway to establish a greater understanding of the mutations that have greater sensitivity to available and investigational therapies.


  1. Hamilton EP. What’s new for HER2-mutant and HER2-low metastatic breast cancer. Presented at: 21st Annual International Breast Cancer Congress® West. July 29-30, 2022; San Diego, CA.
  2. Fehrenbacher L, Cecchini RS, Geyer Jr CE, et al. NSABP B-47/NRG Oncology phase III randomized trial comparing adjuvant chemotherapy with or without trastuzumab in high-risk invasive breast cancer negative for HER2 by FISH and with IHC 1+ or 2+. J Clin Oncol. 2020;38(5):444-453. doi:10.1200/JCO.19.01455
  3. Modi S, Park H, Murthy RK, et al. Antitumor activity and safety of trastuzumab deruxtecan in patients with HER2-low-expressing advanced breast cancer: results from a phase Ib study. J Clin Oncol. 2020;38(17):1887-1896. doi:10.1200/JCO.19.02318
  4. Modi S, Jacot W, Yamashita T, et al. Trastuzumab deruxtecan (T-DXd) versus treatment of physician’s choice (TPC) in patients (pts) with HER2-low unresectable and/or metastatic breast cancer (mBC): results of DESTINY-Breast04, a randomized, phase 3 study. J Clin Oncol. 2022;40(suppl 17):LBA3. doi:10.1200/JCO.2022.40.17_suppl.LBA3
  5. Modi S, Jacot W, Yamashita T, et al; DESTINY-Breast04 Trial Investigators. Trastuzumab deruxtecan in previously treated HER2-low advanced breast cancer. N Engl J Med. 2022;387(1):9-20. doi:10.1056/NEJMoa2203690
  6. Enhertu granted priority review in the U.S. for patients with HER2 low metastatic breast cancer. News release. Daiichi Sankyo. July 25, 2022. Accessed July 25, 2022.
  7. Diéras V, Deluche E, Lusque A, et al. Trastuzumab deruxtecan (T-DXd) for advanced breast cancer patients (ABC), regardless HER2 status: a phase II study with biomarkers analysis (DAISY). Cancer Res. 2022;82(suppl 4):PD8-02. doi:10.1158/1538-7445.SABCS21-PD8-02