There are the biomarkers that we have been using for decades that the pathologist provides (ie, estrogen receptor [ER], progesterone receptor [PR], and HER2), and then we also have the biomarkers that are based on the use of next-generation sequencing (NGS), which provides data on 50 to 500 genes and can be used to detect somatic alterations within the tumor that are targetable. When we order a somatic panel, we also order germline testing to detect other biomarkers that can be targeted.

When we see a patient with metastatic breast cancer that is HR+/HER2-, CDK4/6 inhibitors are the standard of care in the first line. Once a patient progresses on a CDK4/6 inhibitor and endocrine therapy, the presence or absence of an actionable target influences what therapies are utilized. For tumors with BRCA1/2 or PALB2 mutations, we can use a PARP inhibitor. If the tumor has a PIK3CA mutation, we can use a PI3K/AKT inhibitor such as alpelisib. Capivasertib is another AKT inhibitor that is not yet approved by the US Food and Drug Administration, but data from the CAPitello-291 trial were interesting in that they indicated that capivasertib had activity not only in patients with alterations in PIK3CA/PTEN/AKT but also in all patients. Based on these data, I think that the consideration of capivasertib would be reasonable for a patient with a mutation in PIK3CA, PTEN, or AKT, but we need more information on how effective it is in patients without these alterations.

We also have elacestrant for tumors with ESR1 mutations based on the results of the EMERALD trial. I think that, in the future, patients who have an ESR1 mutation after receiving a CDK4/6 inhibitor will be treated with ESR1-targeted therapy in combination with other drugs. A phase 3 trial that is beginning soon is evaluating lasofoxifene plus abemaciclib in patients with an ESR1 mutation who have progressed on a CDK4/6 inhibitor. Finally, there are exciting new data on the drug alisertib, an AURKA inhibitor, demonstrating remarkable antitumor activity in patients who have progressed on a CDK4/6 inhibitor. 

For second-line therapy, after progression on first-line endocrine therapy with a CDK4/6 inhibitor, usually molecular testing of either the blood for circulating tumor DNA (ctDNA) or the tumor is indicated, primarily to look for activating mutations in PIK3CA that result in the activation of the PI3K/AKT/mTOR pathway, which, as Dr Goetz alluded to, occurs in approximately 40% of ER-positive/HER2- breast cancers. PIK3CA mutations tend to be truncal. You can test for them at the time of diagnosis, but you certainly should test for them at the time of progression after first-line endocrine therapy because, if the patient does have an activating PIK3CA mutation, treatment will generally include additional endocrine therapy, usually fulvestrant, in combination with a PIK3CA inhibitor, such as alpelisib.

Something that has changed in recent years is the availability of assays to detect ctDNA fragments in the blood, for both activating PIK3CA mutations and ESR1 mutations. Tumors that have these ESR mutations have ligand-independent activation of ER signaling, and the presence of ctDNA for ESR1 can identify patients who can benefit from elacestrant. So-called liquid biopsy is also relevant for germline mutations. The presence of a BRCA mutation in ctDNA identifies patients who may have a germline BRCA mutation that could benefit from a PARP inhibitor.

The demonstration that patients who have low levels of HER2 protein expression, and without gene amplification, can benefit from antibody-drug conjugates such as trastuzumab deruxtecan represents an important advance in biomarker testing. 

As drug development efforts continue and as our knowledge base expands, NGS is becoming increasingly important in determining the best systemic therapy options for patients, particularly after progression on a CDK4/6 inhibitor. Patients with PIK3CA mutations can be offered treatment with alpelisib and fulvestrant, while those with ESR1 mutations may benefit from elacestrant. The presence of a BRCA mutation identifies patients who may have a germline BRCA mutation that could benefit from a PARP inhibitor. NGS testing can also provide information on tumor mutational burden; if it is high, pembrolizumab monotherapy may be offered in heavily pretreated patients with metastatic breast cancer who have no other treatment options. In the absence of actionable mutations, treatment options include everolimus and exemestane or single-agent endocrine therapy.

It is important to consider obtaining a tissue or liquid biopsy at the time of disease progression to identify any new actionable mutations. Evaluation of ER, PR, and HER2 status should be requested, as these biomarkers may change over time, impacting treatment options. Patients with HER2-low (IHC 1+ or IHC 2+/FISH-) disease are eligible for treatment with the antibody-drug conjugate trastuzumab deruxtecan once the endocrine therapy options have been exhausted.

In the treatment of metastatic breast cancer, the serial measurement of ctDNA may be used to follow responses to targeted treatment; however, one should be cautious on making treatment decisions based soley on these measurements. In asymptomatic patients with stable disease (based on imaging), it is reasonable to continue with current therapy. Additional research is needed to determine the impact of changes in ctDNA on improving clincal outcomes.