To demonstrate efficacy for many cancer therapeutics in in vitro cell proliferation assays, testing in longer cell culture systems may be necessary. It is noted that inhibition of many epigenetic targets results in a significantly delayed drug response that is not revealed by conventional, 72 hour cell proliferation assays. OncoPanel™ from Eurofins Pharma Discovery Services is a large collection of cancer cell lines with broad cancer type and subtype representation and extensive genomic characterization. Assay conditions were developed to yield robust cell line profiling data for therapeutics that require an extended assay duration. Using validated seeding densities, we demonstrated that exponential growth were achieved in more than 280 cell lines over the course of 10-days, two-dimensional growth assays. Using a PARP-selective inhibitorr, Olaparib, and an epigenetic target EZH2-selective inhibitor, GSK343, we demonstrate the advantages of cell line profiling under long-term assay conditions with OncoPanel™ LT when drug response reflects a protracted mechanism of action.. The significant biomarkers identified from the genomic analysis of the data substantiate the relevance of the long term cell profiling.