Chemosensitivity-Resistance Assay as Functional Profiling

Modern cancer research can be divided into three principal disciplines based upon methodology:

1.     Genomic — the analysis of DNA sequences, single nucleotide polymorphisms (SNPs), amplifications and mutations to develop prognostic and, to a limited degree, predictive information on cancer patient outcome.

2.     Proteomic — the study of proteins, largely at the level of phosphoprotein expressions.

3.     Functional — the study of human tumor explants isolated from patients to examine the effects of growth factor withdrawal, signal transduction inhibition and cytotoxic insult on cancer cell viability.

Contrary to analyte-based genomic and proteomic methodologies that yield static measures of gene or protein expression, functional profiling provides a window on the complexity of cellular biology in real-time, gauging tumor cell response to chemotherapies in a laboratory platform. By examining drug induced cell death, functional analyses measure the cumulative result of all of a cell’s mechanisms of resistance and response acting in concert. Thus, functional profiling most closely approximates the cancer phenotype.  Insights gained can determine which drugs, signal transduction inhibitors, or growth factor inhibitors induce programmed cell death in individual patients’ tumors. Functional profiling is the most clinically validated technique available today to predict patient response to drugs and targeted agents.

Chemosensitivity Testing That Makes Sense

Much of the controversy that has surrounded chemosensitivity-resistance assays (CSRA), reflects the fact that the majority of these tests were developed based on the erroneous belief that cancer was driven by its proliferative capacity and that the most active drugs could be chosen based upon their capacity to inhibit cancer cell growth. This led to a long series of unsuccessful attempts to predict clinical response based on cell proliferation endpoints.  Since the 1980’s we have come to realize that cancer represents a dysregulation of cell death and that effective drugs must kill cells outright (not inhibit their growth) in order to provide clinical response to patients.

The Ex Vivo Analysis of Programmed Cell Death (EVA-PCD) ® assay developed by Rational Therapeutics pioneered the application of drug induced cell death for the prediction of clinical response in cancer patients.  The EVA-PCD® assay was the first to incorporate this new understanding of cancer biology. By expanding the application of the EVA-PCD® platform to targeted therapies, RTI is now exploring new classes of compounds that function by inhibiting survival signals in cancer cells.  Many signaling pathways like the epidermal growth factor receptor (EGFr) have extracellular domains that function as cellular switches activating downstream phosphorylations following receptor ligation by proteins like EGF, amphiregulin and TGF alpha.

These mitogen activated protein kinases (MAPK) induce additional cascades of phosphorylations ultimately signaling transcription factors at the level of DNA. While these phenomena were originally thought to represent mitotic events, it is now recognized that most cells are not actively dividing, yet require all of these signaling pathway activations to remain alive. Thus, what was once described as growth factors are more likely better described as anti-death factors.

If indeed cancer doesn’t grow too much but dies too little, it is evident that effective therapies induce cell death, not growth inhibition in the patient.  This is why it is critical to apply lab analyses that measure cell death. Furthermore, as most of the signals for cell survival emanate from the extracellular environment, it is clear that cancer cells must be maintained in their native state to provide clinically relevant information. This is the basis of RTI’s human tumor microspheroid assay platform.