Abstract 1910: Acquired resistance to HER2 targeted therapy: A common mechanism

Abstract

Abstract There are currently two FDA approved targeted therapies being utilized in the clinic to treat Her2+ breast cancer, trastuzumab and lapatinib. Acquired resistance has become a major concern and several mechanisms for resistance have been proposed. The upregulation of a truncated transcriptional variant of Darpp-32, known as t-Darpp, is a characteristic shared by several independently developed trastuzumab-resistant cell lines. Moreover, exogenous t-Darpp overexpression is sufficient for conferring trastuzumab resistance. No consistent mechanism, however, has been identified for lapatinib resistant cells. By inhibiting signaling through Her2+ pathways such as PI3K/Akt and Erk, lapatinib induces a strong apoptotic response by down-regulating survivin and inducing BIM. Darpp-32 and t-Darpp both have been linked to apoptosis, with t-Darpp identified as having anti-apoptotic properties through its regulation of Bcl-2. The purpose of the current study was to determine if t-Darpp or other dysregulated apoptosis proteins play a role in lapatinib resistance. Lapatinib-resistant SKBR3 cells (Her2+, ER-) were selected by continuous exposure to increasing concentrations of lapatinib (0.3-2.0µM) over the course of one year. Using Sulforhodamine B and colony forming assays, we found that individual clones and pools of SK/LapR cells were, on average, 50-fold resistant to lapatinib, relative to parental SKBR3 cells. This resistance phenotype was stable even after cells were maintained for 1 month outside of drug. As observed in trastuzumab resistance and similar to previous reports of lapatinib-resistant cells, we found that SK/LapR cells had sustained PI3K/Akt signaling in the presence of lapatinib, whereas parental cells did not. Additionally, survivin inactivation and BIM induction by lapatinib seemed to be partially impaired in SK/LapR cells. Finally, SK/LapR cells appeared to down-regulate Darpp-32 and up-regulate t-Darpp, similar to what we have reported in BT474 cells selected for resistance to trastuzumab. Although t-Darpp overexpression is sufficient to confer resistance to trastuzumab, this does not appear to be the case for lapatinib resistance – SK.tDp cells transfected with a t-Darpp cDNA are not resistant to lapatinib. Experiments are currently underway to determine if the t-Darpp overexpression that we observe in SK/LapR cells is required for lapatinib resistance. Identifying a shared mechanism of resistance to lapatinib and trastuzumab would provide a novel therapeutic target that could be widely used among Her2+ breast cancer patients to ameliorate acquired drug resistance. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1910. doi:1538-7445.AM2012-1910