The epidermal growth factor receptors, EGFR (erbB-1) and HER2 (erbB-2), and their down stream signaling events promote tumor growth and survival in a variety of epithelial tumors. Current treatment strategies used to target these receptors include mAbs directed against their extracellular domains and small molecule inhibitors of their tyrosine kinase activities. The efficacy of these types of therapeutics is increasingly being found to be contingent on the genotype of the targeted cancer. In vitro evaluation of mAbs is reliant on selection of tumor cell lines driven by the specific antigen proteins of interest and their associated pathways. We selected a panel of eight human tumor cell lines, six breast cancer (BT474, SK-BR-3, MDA-MB-453, MDA-MB-231, MDA-MB-468 and MCF-7), one colon (SW48) and one skin (A431) with varying levels of HER2, EGFR and EGFR/HER2 expression to evaluate the mAbs cetuximab and trastuzumab for antiproliferative effects, ability to induce apoptosis and the ability to block cell cycle. These findings were compared to results obtained after treatment with lapatinib, erlotinib, Ly294002 and Tamoxifen. The BT474 and SK-BR-3 breast cancer cell lines with HER2 amplified expression demonstrated cytostatic antiproliferative and G1/S cell cycle block sensitivity to the anti-HER2 antibody trastuzumab. The more prominent inhibitory effects observed in the BT474 cell line were further confirmed by measurable induced decreased levels of phospho-Akt and increased levels of the CDK2 inhibitor p27Kip1 following treatment with trastuzumab, lapatinib and Ly294002. The SW48 colon and A431 skin cancer cell lines with EGFR mutations and EGFR over expression, respectively, demonstrated cytostatic antiproliferative sensitivity to the anti-EGFR antibody cetuximab. This in vitro cell-based multiplexed approach for assessment of oncology therapeutics based on genetically directed cell line selection, demarcation of cytostatic and cytotoxic cell proliferation inhibition and assessment of receptor down stream signaling biomarkers offers greater predictive efficacious value and can be used to identify optimal candidates for combination therapies. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P6-14-17.