Abstract 386: Addition of mTOR inhibition to HER2 blockade results in improved antitumor activity in a preclinical model of uterine serous carcinoma with HER2 gene amplification and a gain of function mutation in PIK3CA
Authors: Hernandez Silvia F. , Chisholm Sarah , Borger Darrell , Foster Rosemary , Rueda Bo R. , Growdon Whitfield B. 
Published: July 2016

Abstract
Subsets of uterine serous carcinomas (USC) harbor simultaneous HER2 (ERBB2) over-expression and gain of function mutations in PIK3CA that may play a role in resistance to single agent anti-HER2 therapies. Targeting the mammalian target of rapamycin (mTOR) may be a promising option to heighten anti-tumor response and restore sensitivity to anti-HER2 therapies.
HER2 gene amplified cell lines with (ARK1, E542K) and without (ARK2) PIK3CA gene mutations were divided into four arm cohorts with equivalent tumor volumes and treated with either vehicle, ridaforolimus (1 mg/kg, lapatinib (150 mg/kg) and trastuzumab (10mg/kg) or the combination of lapatinib/trastuzumab and ridaforolimus. At day 22, tumors were collected to measure downstream target proteins and calculate the proliferation and apoptosis index.
In the ARK1 USC xenograft cohort, inhibition of tumor growth was observed following treatment with ridaforolimus compared to vehicle (P&lt;0.05) and this was not statistically different from the lapatinib/trastuzumab group. However, when lapatinib/trastuzumab was combined with ridaforolimus, improved anti-tumor activity was observed compared to all other arms (P&lt;0.01). Similarly, the treatment of ARK2 mice with ridaforolimus slowed tumor growth (P&lt;0.05) compared to the vehicle. Both the lapatinib/trastuzumab and lapatinib/trastuzumab/ridaforolimus arms induced reductions in tumor sizes compared to the vehicle and ridaforolimus arms (P&lt;0.01), but ridaforolimus failed to add any additional anti-tumor activity.
Analysis of the treatment effects revealed the lapatinib/trastuzumab/ridaforolimus therapy was required to decrease Ki67 positive cells in ARK1 xenografts compared to the vehicle (P&lt;0.01) however in ARK2 xenografts, lapatinib/trastuzumab was sufficient to decrease Ki67 positive cells with or without ridaforolimus. Similarly, differential TUNEL staining was observed in the two cell lines with ridaforolimus being required to increase the percentage of apoptotic cells in ARK1, while the apoptotic index was solely dependent of lapatinib/trastuzumab administration in ARK2. In ARK1, we observed that pS6 expression was reduced (p&lt;0.01) only in the arms where ridaforalimus was administered but in ARK2, its expression was again independent of mTOR inhibition.
These data suggest that USCs harboring gain of function PIK3CA mutations are relatively insensitive to anti-HER2 therapies. The addition of downstream mTOR inhibition to HER2 blockade increased the anti-tumor activity only in the xenograft model with PIK3CA mutation and provides rationale for clinical trials in HER2 positive endometrial cancers based on molecular genotype.
Citation Format: Silvia F. Hernandez, Sarah Chisholm, Darrell Borger, Rosemary Foster, Bo R. Rueda, Whitfield B. Growdon. Addition of mTOR inhibition to HER2 blockade results in improved antitumor activity in a preclinical model of uterine serous carcinoma with HER2 gene amplification and a gain of function mutation in PIK3CA. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 386.