Lapatinib is active in patients with HER2-amplified breast tumors expressing p95HER2.

Abstract

Abstract Abstract #708 Background: A subgroup of HER2 overexpressing tumors also express p95HER2, a truncated fragment of the HER2 receptor that lacks the extracellular domain and retains kinase activity. Tumors expressing p95HER2 have a worse clinical outcome and are resistant to trastuzumab therapy (Scaltriti M. et al, JNCI 2007). The hypothesis that lapatinib, a small molecule inhibitor that targets the intracellular kinase domain of EGFR and HER2, would be active in these tumors was tested by correlating p95HER2 expression with clinical response to lapatinib in EGF20009, a lapatinib monotherapy study
 Methods: Archival tumor blocks were obtained from patients (pts) participating in study EGF20009. In this study, pts with untreated locally advanced or metastatic HER2 amplified breast tumors either received lapatinib 1500mg daily or 500mg BID (H.L. Gomez et al, J Clin Oncol 2008). Pre-treatment samples were analyzed for p95HER2 expression by immunofluorescence using 2 antibodies against HER2, binding to the intracellular and extracellular domain, respectively, plus a pan-cytokeratin antibody. Tumors were scored as p95HER2 positive (p95HER2+) [if any cytoplasmic staining was detected with the intracellular HER2 antibody and this staining co-localized with the pan-cytokeratin antibody], negative (p95HER2-) [no intracellular HER2 cytoplasmic staining) or non-evaluable (N/E) [no cytokeratin or HER2 staining by immunofluorescence]. The presence of p95HER2 was then correlated with response rate (RR), stable disease for over 100 days (SD) and progression-free survival (PFS) to lapatinib. A Cox-proportional hazards model was used to compare the groups. In order to correlate the presence of p95HER2 with HER2 extracellular domain in the serum, a quantitative determination of the 105 kD ECD level in serum (Oncogene Science HER2/neu Microtiter ELISA) has also been performed.
 Results: 68/105 tumors were evaluable for p95HER2 expression. Fourteen were p95HER2+ (20.5%) and 54 were p95HER2- (79.5%). The observed percentage of p95HER2+ tumors is concordant with published reports. Among the 14 pts with p95HER2+ tumors, 7 responded (RR: 50%) and 1 had SD for > 100 days for an overall clinical benefit rate of 57% (RR+SD). In the p95HER2- population (N=54), 24 pts responded (RR: 44.4%) and 16 pts had SD >100 days for an overall clinical benefit rate of 56% (RR+SD). PFS was similar in both groups, with a median PFS of 24 wks in pts with p95HER2+ and p95HER2- tumors (p=0.97, HR 1.1). These findings compare with the median PFS of 20.7 wks for the entire population in the trial (N=138pts). Further correlation of p95 status and HER2 ECD status of the tumors will be presented.
 Conclusion: These data demonstrate that lapatinib has similar activity in pts with p95HER2+ or p95HER2-, HER2 amplified breast tumors. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 708.