Validation of proliferation indices as surrogate endpoint biomarkers

During the 1990s, research interest in the use of chemopreventive agents to reverse human colon carcinogenesis increased exponentially. In parallel, there has been an increase in the need for putative surrogate endpoint biomarkers (SEBs) of cancer risk. Since the hallmark studies of Lipkin et al. and Terpstra et al., among others, the rate and patterns of rectal mucosal proliferation have been established as intermediate biomarker endpoints for colon cancer risk, modulated by potential chemopreventive agents including calcium, wheat bran fiber, and nutritional stress diets. Researchers rely heavily on these rectal mucosal proliferation indices as surrogate endpoints to evaluate the relative efficacy of various chemopreventive intervention strategies. Standardization through quality control/quality assurance (QC/QA) programs which continuously validate the accuracy, reproducibility, and variability of these indices is increasingly needed. Along with many others, we have attempted to validate 3H-thymidine and bromodeoxyuridine labeling indices in rectal mucosal biopsies as reliable SEBs. In this manuscript we outline a series of QC/QA steps that can be followed in the validation process for new as well as "old" biomarkers prior to their use as primary efficacy surrogate endpoints for chemopreventive agent intervention trials.