Biomarkers for Systemic Therapy in Metastatic Breast Cancer Guideline Update

An interview with Dr. Lynn Henry from the University of Michigan in Ann Arbor, MI, lead author on "Biomarkers for Systemic Therapy in Metastatic Breast Cancer: ASCO Guideline Update." Dr. Henry reviews new biomarkers for the purposes of making treatment decisions for triple-negative breast cancer, and hormone receptor-positive, HER2-negative breast cancer, as well as tumor agnostic tumor biomarkers. Specific biomarkers addressed in this conversation include PIK3CA, ESR1, BRCA 1/2, PALB2, HRD, PD-L1, dMMR/MSI-H, TMB, NTRK, ctDNA, and CTCs. Read the full guideline at www.asco.org/breast-cancer-guidelines.

TRANSCRIPT

Brittany Harvey: Hello and welcome to the ASCO Guidelines Podcast series, brought to you by the ASCO Podcast Network, a collection of nine programs, covering a range of educational and scientific content and offering enriching insight into the world of cancer care. You can find all the shows including this one at asco.org/podcasts.

My name is Brittany Harvey and today I'm interviewing Dr. Lynn Henry from the University of Michigan in Ann Arbor, Michigan, lead author on 'Biomarkers for Systemic Therapy in Metastatic Breast Cancer: ASCO Guideline Update'.

Thank you for being here, Dr. Henry.

Dr. Lynn Henry: Thank you very much for inviting me to participate.

Brittany Harvey: First, I'd like to note that ASCO takes great care in the development of its guidelines and ensuring that the ASCO conflict of interest policy is followed for each guideline.

The full conflict of interest information for this guideline panel is available online with the publication of the guideline in the Journal of Clinical Oncology.

Dr. Henry, do you have any relevant disclosures that are related to this guideline topic?

Dr. Lynn Henry: No, I do not.

Brittany Harvey: Great! Thank you. Then let's talk about the scope of this guideline. So, what prompted this update to the guideline on the use of biomarkers to guide decisions on systemic therapy for patients with metastatic breast cancer, which was last updated in 2015? And what is the scope of this guideline update?

Dr. Lynn Henry: Yes, so a lot has happened in the past six or seven years that influence how we treat metastatic breast cancer. And there are many new drugs that have been approved by the FDA during that time.

When we reviewed the prior guideline and the new treatment landscape, we realized that while much of what was included in the old guideline was still relevant, there were quite a number of new biomarkers related to new drugs that needed to be included.

The newly recommended biomarkers are primarily applicable to making decisions about treatment of estrogen receptor, progesterone receptor, and HER2-negative breast cancer, also called triple-negative breast cancer, as well as for treatment of hormone receptor-positive HER2-negative breast cancer. And finally, there are now some tumor biomarkers that can be tested for that are tumor agnostic, and these were included as well.

Brittany Harvey: Great! So, then let's discuss the updated guideline recommendations based off these new biomarkers for our listeners. The guideline reviews testing for several different biomarkers. So, I would like to review each of the biomarkers that the panel assessed. Let's start with what is the role of PIK3CA mutation testing?

Dr. Lynn Henry: Yeah! So, PIK3CA activating mutations are commonly found in tumors that are hormone receptor-positive and HER2-negative. Based on the results of the SOLAR-1 trial, patients whose tumors have an activating PIK3CA mutation had improved progression-free survival when treated with the PI3 kinase inhibitor alpelisib plus fulvestrant compared to fulvestrant alone.

This improvement was not seen in patients whose cancers didn't have a mutation. So, therefore, this provided the evidence for the clinical utility of evaluating tumors for the somatic PIK3CA mutations.

Testing of either a tumor specimen or plasma to look for PIK3CA mutations can be performed. However, it's important that if the plasma is tested, and no PIK3CA mutations are identified in the circulating tumor DNA, then our tumor specimen should really be tested if possible, because of the possibility of a false negative finding in the plasma.

Also, since these mutations can be acquired over time, a more recent specimen should be tested if possible, as opposed to testing the primary tumor.

Finally, in the SOLAR-1 trial, a patient's tumor had to have one of the 11 pre-specified PIK3CA mutations in exon 7, 9, or 20. And therefore, when mutations are identified using next-generation sequencing, it is important to confirm that the identified mutation is one of those 11 activating mutations and not a different one that may not convey benefit from treatment with a PI3 kinase inhibitor.

Brittany Harvey: Great! I appreciate you're reviewing that recommendation, as well as the clinical utility of it and the evidence behind it. So, then following those recommendations, what is the role of testing for ESR1 mutations?

Dr. Lynn Henry: At this time, there are insufficient data to support routine testing of metastatic hormone receptor-positive HER2-negative tumors for ESR1 mutations. However, the panel did note that there's a retrospective analysis of two different phases three trials that demonstrated that fulvestrant improved progression-free survival compared to the aromatase inhibitor exemestane in patients who had previously progressed on a non-steroidal AI and whose tumors had an ESR1 mutation.

Importantly, there are ongoing clinical trials addressing this issue, including the PADA-1 trial, which is evaluating the effect of the switch of fulvestrant from aromatase inhibitor therapy, versus remaining on that therapy when ESR1 mutations are detected in the blood. However, although preliminary findings were presented at a recent large breast cancer meeting, and were suggestive of a possible progression-free survival benefit from switching therapy, data have not yet been published, and therefore they were not included in this guideline.

Brittany Harvey: Great! So, we'll look forward to those updated data to potentially review that recommendation in the future. So, following those recommendations, what is the role of testing for germline BRCA 1 or 2 and PALB2 pathogenic mutations?

Dr. Lynn Henry: So, the answer for germline BRCA1 and BRCA2 mutations is relatively straightforward. Patients with metastatic HER2-negative breast cancer can be either hormone receptor-positive or negative, and who are candidates for treatment with a PARP inhibitor should undergo testing for germline BRCA1 and BRCA2 pathogenic or likely pathogenic mutations to determine whether they should receive treatment with a PARP inhibitor.

This recommendation is based on the results of two large randomized clinical trials comparing PARP inhibitor therapy to physician's choice chemotherapy, although notably, the chemotherapy options did not include taxanes, anthracyclines, or platinums.

In contrast, there remains insufficient evidence to support a recommendation either for or against testing for germline PALB2 pathogenic variant for the purpose of determining eligibility for treatment with a PARP inhibitor.

The panel did note, however, that there are small single-arm studies that show that there is high response rate to PARP inhibitors in patients with metastatic breast cancer and coding DNA repair defects, such as either germline PALB2 pathogenic variants or somatic BRCA1 or 2 mutations. It was also noted that it is likely that patients who harbored mutations in these genes will actually be identified through routine testing with panel testing for germline variants.

Brittany Harvey: Okay, understood. So, then following those recommendations, what is the role of testing tumors for homologous recombination deficiency?

Dr. Lynn Henry: So, although there are emerging data from other solid tumors to support the use of homologous recombination deficiency, or HRD testing to guide therapy, current data do not support the assessment of HRD in the management of metastatic breast cancer. Therefore, we did not recommend routine testing of tumors for HRD at this time.

Brittany Harvey: It's important to note where we both have evidence and where we don't have evidence. So, then what is the role of testing for expression of PD-L1 in metastatic breast cancer?

Dr. Lynn Henry: So, the panel recommends that patients who are candidates for treatment with immunotherapy, with either a PD1 or PD-L1 inhibitor, should undergo testing for expression of PD-L1 in the tumor and immune cells with an FDA-approved test.

At present in the United States, pembrolizumab is the only approved immunotherapy for the treatment of metastatic breast cancer, and it is given in combination with chemotherapy.

The FDA-approved test for this drug is the 22C3 assay which evaluates PD-L1 staining in the tumor and surrounding stroma to calculate a combined positive score or CPS, with positive considered to be a score of 10 or greater.

Of note, in other countries, there are different anti PD1 and PD-L1 antibodies that are approved for treatment, and each has been approved with its own companion diagnostic. So, it is important to make sure that you're using the right biomarker test, depending on which drug you are planning to use.

Brittany Harvey: Great! I appreciate you reviewing the test in addition to the role of the biomarker. So, then, following those recommendations, what is the role of testing for deficient mismatch repair microsatellite instability-high?

Dr. Lynn Henry: Similar to PD-L1 testing, it is recommended that patients with metastatic breast cancer who are candidates for a treatment regimen that includes an immune checkpoint inhibitor should undergo testing for deficient mismatch repair or microsatellite instability-high to determine eligibility for treatment with one of the drugs that is currently FDA approved, either dostarlimab or pembrolizumab.

In contrast to the PD-L1 data, however, there are no randomized studies that have been conducted specifically in patients with breast cancer addressing this question. The testing recommendation was therefore included in these guidelines because of the tumor agnostic FDA approval of these drugs.

In terms of which biomarker methodology to use, it was noted that, while the original studies assessed the deficient mismatch repair and MSI high using immunochemistry, and PCR respectively. The FDA has subsequently approved the next-generation sequencing platform to use in selecting candidates for these treatments. And so, therefore, there are a number of different tests that can be used.

Brittany Harvey: Thank you for reviewing those recommendations as well. So, then following, what is the role of testing for tumor mutational burden?

Dr. Henry Lynn: So, tumor mutational burden describes the quantity of somatic mutations in the tumor. Similar to the biomarkers we were previously discussing, there are minimal data specifically in metastatic breast cancer to support the assessment of tumor mutational burden for making treatment decisions.

However, the testing recommendation was again included in the guidelines because of the tumor agnostic FDA approval of the drug pembrolizumab in the setting of high TMB. And also there is one single arm phase two trial that looked at this specifically.

Importantly, the panel noted that there are a variety of factors that influence assessment of TMB. These include sample type, pre-analytical factors so how the sample was handled, the size of the panel and mutations that are tested, depths of the sequencing, type of the mutations that are included on the panel, and cut point variables.

So, in particular, assessment of TMB in cell-free DNA assays such as circulating tumor DNA is an area of evolving evidence. There are therefore very important caveats to be aware of when selecting a TMB assay and assessing the results, many of which are outlined in the guideline manuscript itself, and different assays can yield different results for the same tissue specimen. It is therefore very important to use the approved companion assay and the approved cut point when making decisions regarding a specific treatment.

Brittany Harvey: Absolutely. I appreciate your reviewing those details. So, then what is the role of testing for neurotrophic tyrosine receptor kinase?

Dr. Lynn Henry: So, I'm going to abbreviate that to NTRK. So, NTRK fusions are rare in metastatic breast cancer. One study said 0.39% of all breast cancers have NTRK fusions.

So, as with the above biomarkers, the NTRK testing recommendation is based on the results of phase 1 and phase 2 studies that were identified by the panel evaluating the efficacy and safety of these inhibitors for the treatment of advanced solid tumors with NTRK gene fusions, noting that there are only minimal data available that are specific to metastatic breast cancer.

Brittany Harvey: Understood. Some of these are very rare in metastatic breast cancer. So, then, following that recommendation, what is the role of using circulating tumor DNA?

Dr. Lynn Henry: So, for circulating tumor DNA, although the ctDNA technology holds promise in metastatic disease, for its ability to potentially identify tumor-specific mutations that are shed into the blood and that may be targetable, to date, neither the measurement of changes in ctDNA as a marker of treatment responsiveness nor identification of specific mutations in the blood to direct therapy has actually been prospectively shown to improve patient outcomes compared to standard imaging-based detection of tumor progression.

Therefore, at present, the guideline does not recommend routine assessment of ctDNA for monitoring response to therapy among patients with metastatic breast cancer, although many studies are underway evaluating this question.

Brittany Harvey: Understood. Then the last biomarker that the panel assessed in this guideline update, what is the role of using circulating tumor cells?

Dr. Lynn Henry: Similar to circulating tumor DNA, there are insufficient data to recommend routine use of circulating tumor cells to monitor response to therapy among patients with metastatic breast cancer.

To date, studies that have examined the clinical utility of this marker to determine the optimal time for treatment change have not led to improvements in outcomes in metastatic breast cancer.

Brittany Harvey: Great! Well, thank you for reviewing all of these recommendations. The panel certainly took on a lot of biomarkers and performed a critical review of all the evidence to make recommendations in this setting. So, in your view, Dr. Henry, what is the importance of this guideline update and what should clinicians know as they implement these updated recommendations?

Dr. Lynn Henry: Yeah, that's an excellent question. So, this guideline addresses the key questions that we face, as we're making decisions about how best to treat patients with metastatic breast cancer.

Importantly, the guideline highlights the current state of the science, with a focus on the available published data from randomized clinical trials. It also discusses the limitations of our current knowledge, as well as key considerations for different biomarkers.

Of course, we recognize that there are new data emerging on a regular basis. And the panel therefore also highlighted where data are anticipated but not yet available, as well as key questions which we hope will be able to be addressed in the more distant future.

Brittany Harvey: And then finally, how will these guideline recommendations affect patients with metastatic breast cancer?

Dr. Lynn Henry: Yeah, so really, that is the bottom line, isn't it? So, ideally, this guideline will enable the dissemination of best practices in terms of biomarker selection and analysis to guide clinicians as they are making treatment decisions in conjunction with patients.

Treatment of metastatic breast cancer has become more complex, with regimen selection affected by both inherited germline genetics and somatic changes in the cancer that can evolve over time. The assessment of relevant biomarkers should allow patients to receive the optimal therapies that are most likely to be effective based on the individual characteristics of their cancers.

Brittany Harvey: Well, I want to thank you so much for reviewing this guideline with me today, and all of the recommendations and our gaps in evidence, for our listeners. Thank you for your work on this guideline update and thank you for your time today, Dr. Henry.

Dr. Lynn Henry: Thank you so much!

Brittany Harvey: And thank you to all of our listeners for tuning in to the ASCO Guidelines podcast series. To read the full guideline, go to www.asco.org/breast-cancer-guidelines.

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