Targeting DNA Topoisomerase I for the Treatment of Cancer: Past, Present and Future.

As an enzyme that relaxes torsionally strained DNA, TOP1 is present in all nucleated human somatic cells. Even though this ubiquity makes TOP1 an unlikely anticancer drug target, six FDA-approved antineoplastic treatments, including two approved in the past five years, and a variety of experimental agents inhibit the TOP1 catalytic cycle. To provide insight into the continuing effort to develop TOP1-directed agents, here we briefly review the biology of TOP1, the cellular effects of stabilizing TOP1-DNA covalent complexes, mechanisms of resistance to TOP1 poisons, and strategies to overcome this resistance before describing efforts to develop TOP1 catalytic inhibitors as well as an exciting new generation of tumor targeting nanoparticles and antibody-drug conjugates that deliver TOP1-directed agents to cancers at high concentrations while sparing normal tissues. When paired with inhibitors of DNA damage response pathways, epigenetic therapies, or immune modulators, these new TOP1-directed agents promise to improve the therapy of a wide range of solid tumors.