Differences in Sensitivity to New Therapies Between Primary and Metastatic Breast Cancer: A Need to Stratify the Tumor Response?

Abstract Objective: We compared therapeutic response of Varlitinib + Capecitabine (VC) versus Lapatinib + Capecitabine (LC) in patients with human epidermal growth factor receptor 2-positive metastatic breast cancer after trastuzumab therapy by assessing changes in target lesion (TL) diameter and volume per location.Methods: We retrospectively analyzed the CT data of the ASLAN001-003 study (NCT02338245). We analyzed TL size and number at each location focusing on therapeutic response from baseline to Week 12. We used TL diameter and volume to conduct an inter-arm comparison of the response according to: RECIST 1.1; stratified per TL location and; considering TLs independently. Multiple pairwise intra-arm comparisons of therapeutic responses were performed. Considering TL independently, weighted models were designed by adding weighted mean TL responses grouped by location. A sensitivity analysis tested the robustness of results.Results: We evaluated 42 patients (88 TL) and 35 patients (74 TL) respectively at baseline and Week 12.We found reductions in breast TL burden in the VC arm compared to the LC arm (p=0.002 (diameter), p<0.001 (volume)). Responses and TL sizes at baseline were not correlated.Explained variabilities of volume change per TL location, patient and patient:TL interaction were 36%, 10% and 4% (VC), and 13%, 1% and 23%, (LC).A test of inter-arm difference of responses yielded p=0.07 (diameter), and p<0.001 (volume).The sensitivity analysis confirmed our findings.Conclusions: The therapeutic responses differed across locations; differential responses were drug‑dependent. Stratified analysis provides better drug comparisons and is a powerful tool to understand TL heterogeneity.