Abstract 673: Novel anti-tumor activity of lapatinib derives from off-target up-regulation of TRAIL death receptors
Authors: Dolloff Nathan G. , Hart Lori S. , Mayes Patrick A. , Dicker David T. , Humphreys Robin C. , Gilmer Tona M. , El-Deiry Wafik S. 
Published: February 2011

Lapatinib, a HER2/EGFR inhibitor, is the most recently approved targeted therapy for metastatic breast cancer. Based on clinical evidence that lapatinib enhances efficacy of capecitabine in breast cancer patients, we hypothesized that lapatinib could augment the anti-cancer activity of agents in other tumor types such as colon cancer. Using in vitro cell death and apoptosis assays, we found lapatinib to have minimal effect on chemotherapy-induced cytotoxicity in colon cancer cell lines. However, elevated concentrations of lapatinib (5-10μM) significantly enhanced the pro-apoptotic effects of TRAIL and agonistic TRAIL receptor antibodies. In vivo, xenografted colon tumors from mice treated with lapatinib/TRAIL combinations exhibited more immunostaining for cleaved caspase-8, an apical caspase in the extrinsic cell death pathway, compared to tumors from mice treated with lapatinib or TRAIL alone. Furthermore, the combination therapy suppressed tumor growth, whereas single agent treatments had little effect. Mechanistically, lapatinib up-regulated proapoptotic TRAIL death receptors, DR4 and DR5, leading to more efficient induction of apoptosis in the presence of TRAIL and agonistic TRAIL receptor antibodies. Lapatinib had no appreciable effects on death receptor protein stability, and DR5 - but not DR4 - mRNA levels were increased, suggesting that lapatinib transcriptionally up-regulates DR5. Interestingly, other EGFR and HER2 inhibitors, individually or in combination, failed to increase death receptor expression and TRAIL sensitivity, suggesting that this novel effect resulted from off-target activity. In support of this hypothesis, lapatinib induced death receptor up-regulation in SW620 cells, which lack EGFR expression and detectable levels of HER2 signaling. Lower lapatinib concentrations (1μM), which completely inhibited EGFR and HER2, were not sufficient to induce death receptor expression and increase TRAIL sensitivity, further demonstrating the off-target nature of this effect. High concentrations of lapatinib increased phosphorylation of c-Jun, extracellular signal related kinase (ERK) and c-Jun N-terminal kinase (JNK). RNAi-mediated knockdown experiments proved that c-Jun, but not ERK or JNK, were required for lapatinib-induced DR5 up-regulation. We conclude that high concentrations of lapatinib, through an EGFR/HER2-indpendent mechanism, stimulate c-Jun phosphorylation/transcriptional activity leading to DR5 up-regulation and increased TRAIL sensitivity. From a clinical perspective, therapeutic benefit may be gained from this off-target activity by using high doses of lapatinib in combination with TRAIL-receptor-activating agents. Secondly, death receptor induction and signaling may serve as a biomarker of response following lapatinib treatment to identify patients that have responded most favorably to treatment.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 673.