Trastuzumab Deruxtecan (T-DXd): Tailoring Treatment and Companion Diagnostics (CDx) by Liquid Biopsy

Human epidermal growth factor receptor 2-positive breast cancer represents 15%-20% of breast malignancies. In addition to this subtype characterized by HER2 overexpression or amplification, at least another 35% of breast cancer expresses HER2 at lower but detectable levels. These HER2-low tumors were defined as HER2 1+ and 2+ scores by immunohistochemistry (IHC) in the absence of gene amplification by in situ hybridization (ISH). There is evidence that HER2 overexpression/amplification may decrease during treatment HER2 blockade (4), determining that tumors of the 15% of HER2-positive group merge into the 35% of HER2-low group. Unfortunately, it is presently unclear how many HER2 overexpressing/amplified tumors are bound to join the HER2-low group, at which time during progression, and whether this may coincide to some extent with HER2 blockade inefficacy.

Traditionally, HER2 status is assigned: (a) at one point only (diagnosis), once and forever; (b) in tumor tissue only; (c) on a binary (yes/no) scale; and (d) with the limited aim of assigning conventional anti-HER2 therapy only. However, it is becoming increasingly clear that HER2 functional expression is not binary, but distributed along a continuum and intrinsically bimodal, e.g. (over)expression and copy number variations must be co-factored. It also appears that novel agents, such as Trastuzumab-deruxtecan (t-Dxd), may not be 'adapted' to the existing diagnostic scenario, but they may need a dedicated companion diagnostic (CDx). Moreover, IHC or ISH tissue diagnostic scheme is no longer applicable to the variety of available clinical HER2 blockade regimens and is in need of replacement. In this regard, liquid biopsy, with a comprehensive pan HER2 'unity solution', may be useful to capture HER2 on the wane as well as resistance traits, enable reallocation of patients to different subtypes, and assign treatment in a potentially practice-changing setting.

Preliminary data regarding the progressive HER2 loss during HER2 blockade and the potential of liquid biopsy in the metastatic HER-positive setting derive from the LiqBreasTrack trial (5). This is a small, single-center study exploring the role of liquid biopsy (at different time point from time zero until disease progression) in 22 HER2-positive breast cancer patients, treated with Trastuzumab emtansine (T-DM1) as per standard of care in 2nd or more line of therapy. Tumor DNA and circulating tumor DNA were tested by two commercial ThermoFisher targeted next generation sequencing (NGS) panels spanning about 50 (essentially overlapping) genes: the Ion AmpliSeqTM Cancer Hotspot and the OncomineTM Pan-Cancer panels. This study was designed to determine the potential impact of liquid biopsy in: 1) circulant tumor DNA (ctDNA) anticipation of clinical progression; 2. identification of recurrent mutational patterns on progression (e.g. alterations associated with, and possibly causative of, pharmacological resistance to T-DM1 and HER2 blockade altogether). The LiqBreasTrack trial showed that only five patients were still belonging to the 'HER2 subtype' (as assessed in blood) before T-DM1 administration. HER2 counterselection went on in all five of them, and a reversal to a HER2 neutral blood status was completed in the three who went all the way through >10 T-DM1 infusions until progression, whereas it was detectable, but incomplete, in the remaining two patients who had to discontinue T-DM1 due to rapid progression. Moreover, the development of a series of specific molecular alterations, either de novo appeared or progressively accumulated in blood, was associated (only when present in blood) with short time to progression. This suggests an oncogenic replacement of HER2 addiction by 'bypass' cancer drivers under strong and rapid adaptive selection pressure. This pilot study, although small, suggests several hypothesis: 1) T-DM1 (but this may apply to Trastuzumab-deruxtecan as well) rapidly pushes most or all tumors toward a presumably irreversible HER2 neutral state that had not been achieved by previous therapies; 2) in at least some cases, HER2-low tumors emerging from extensive HER2 blockade acquire features (e.g. hormone addiction) reminiscent of the naïve population of HER2-low tumors; 3) replacement of dominant HER2 addition comes at the cost (for the tumor) of developing alternative vulnerabilities, most of which had not been previously detected in tumor tissues, e.g. blood only alterations are the optimal biomarkers for cancer progression.

In summary, the HER2 loss may be a frequent event. It also remains to be determined how this HER2 loss intertwines with escape from HER2 blockade and the acquisition of the (so far scanty) list of adaptive resistance traits, both 'direct' (HER2 mutations) and 'bypass' (PTEN loss-of-function, PIK3CA mutations, integrin activation, YAP1 and IGF1 activation). In summary, regardless of whether constitutive or adaptively induced, an 'HER2-low status' spans a heterogeneous group of tumors comprising naïve and heavily pre-treated patients as well. Thus, there is an area of unmet medical need that may account for as many as 50% of all breast cancers and may include tumors on which neither conventional HER2 blockade nor hormone suppression strategies can determine a really clinical impact.

Liquid biopsy may reflect the dynamic changes in breast cancer biology, discriminate shades and grades of HER2 addiction and may be useful to identify the ideal CDx for Trastuzumab deruxtecan, the future second line standard of care for HER2-positive metastatic breast cancer (2). Indeed, liquid biopsy represents a non-invasive, non-dichotomic, quantitative, longitudinal test suitable for Trastuzumab-deruxtecan response monitoring and capturing potential mechanisms of resistance.

Based on these data, the DIAMOND study is a pilot, prospective study designed to primarily assess the ability of blood-based liquid biopsy using NGS to monitor disease progression to Trastuzumab-deruxtecan (as second line treatment) in patients with histologically confirmed diagnosis of HER2-positive metastatic disease previously treated with one treatment line for metastatic disease including pertuzumab with Trastuzumab or Trastuzumab alone (associated to taxane). The primary objective of the trial is to dynamically assess circulating alterations associated with resistance to Trastuzumab-deruxtecan, including changes of the HER2 status by a novel approach named HER2-2D that simultaneously assesses HER2 amplification and (over)expression in blood. The primary endpoint is the concordance between the clinical evaluation (progressive disease versus not progressive disease) based on RECIST criteria and the evaluation of liquid biopsy, (appearance of a new mutation and/or increasing of an existing one). Secondary objectives include: 1) molecular stratification of patients (dynamic approach by liquid biopsy); 2) lead time (anticipation of progression) assessment by liquid biopsy; 2) HER2-2D: use of a non-invasive liquid biopsy method to non-invasively and objectively assess HER2 status and to select patients displaying a HER2-low status.

Trastuzumab Deruxtecan will be administered at the dose of 5.4 mg per kilogram by intravenous infusion every 3 weeks (standard recommended dose). Treatment will be maintained until disease progression, severe toxicity or patient refusal. Liquid biopsy will be performed at baseline and repeated at different time points and/or at progression disease.