Cell-based screening is an important part of drug discovery and a variety of cellular effects can be measured to reflect the action of the target. However, the behavior of tumor cells grown in either monolayer or suspension cultures differ significantly from the in vivo setting, and in some cases these assays have limited predictive value for determining the preclinical efficacy of a compound. In comparison, 3-dimensional cultures are considered to be an improved in vitro model to simulate the dynamic microenvironment of a tumor. In glandular tissues such as the breast, the interaction between the epithelium and surrounding basement membrane (a specialized form of extracellular matrix) is critical as its dysregulation is often associated with tumor progression. To account for the role of these matrix macromolecules in our drug discovery models, we employed a reconstituted form of basement membrane to characterize HER2 signaling mechanisms in HER2+ breast cancer cells. HER2 is overexpressed in 20-25% of breast cancers and defines a sub-class of patients that benefit from the HER2 targeted therapy, trastuzumab (Herceptin®). HER2 signaling may be mediated by a combination of HER2 homodimers and heterodimers with other HER family receptors to activate a myriad of downstream signaling events. Within a 3-dimensional context, we use RNA interference and anti-neoplastic therapeutics, such as trastuzumab and pertuzumab (Omnitarg®), to demonstrate that HER3 inactivation potently inhibits heregulin-induced morphogenesis in doxycycline-inducible HER3 shRNA BT474 cells. Moreover, our assay methods reveal distinct phenotypes associated with the effects of trastuzumab versus pertuzumab and provide a novel approach for evaluating the mechanism of action of these compounds. Taken together, our results support the use of 3-dimensional culture as a more reliable predictor of in vivo efficacy and as a utilitarian system for characterizing novel inhibitors and targets.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA