PERCIST Evaluation of Trastuzumab Deruxtecan in the Treatment of HER2 Low Breast Cancer

The development of trastuzumab deruxtecan (T-DXd), an anti-HER2 conjugated antibody, has changed the therapeutic landscape of breast cancer. Anti-HER2 molecules were previously used exclusively in cases of HER2-positive breast cancer (IHC HER2 3+ or HER2 2+ with amplified FISH/SISH).

Since a couple of years, T-DXd is becoming a reference treatment in patients with low HER2 breast cancer (IHC 1+ or 2+ with non-amplified FISH/SISH) after hormone therapy and at least one line of chemotherapy. The results of the DB04 trial revealed that T-DXd increased progression-free survival and overall survival compared to investigator's choice chemotherapy, in HER2 low breast cancers patients after one or two previous lines of chemotherapy (Modi, S. et al., 2022).

In several trials, T-DXd validation was done thanks to radiological evaluation based on conventional imaging, computed tomography scan (CT-scan) with or without contrast dye injection and bone scintigraphy.

18F-Fluorodeoxyglucose positron emission tomography coupled with computed tomography (18F-FDG PET-CT) represents a major tool for diagnosis, staging, and therapeutic follow-up in oncology.

It was demonstrated that 18F-FDG PET-CT is more sensitive in detecting metastatic disease progression in HER2-positive breast cancer patients treated with trastuzumab and pertuzumab or trastuzumab and lapatinib (Ma, G. et al., 2023 et Lin, N. U. et al., 2015). Moreover, a prospective study investigated the correlation between the response rate defined by 18F-FDG PET-CT and thoraco-abdomino-pelvic CT-scan (TAP CT-scan) and the progression-free survival of patients with breast cancer. PET evaluation allowed better identification of responders versus non-responders, with a significant correlation with progression-free survival (Vogsen, M. et al., 2023).

With this study, the investigators aim to determine the role of 18F-FDG PET-CT in evaluating T-DXd treatment in patients with metastatic low HER2 breast cancer. 18F-FDG PET-CT could be an earlier evaluation tool than CT-scan in identifying non-responsive patients; and thus, avoid continuing an expensive treatment with an unfavorable toxicity profile, which could worsen both patient prognosis and quality of life.