Topoisomerase I inhibitors overview: Human DNA-topoisomerase I (topoI) is an essential and ubiquitous enzyme that is involved in the release of torsional forces generated during DNA transcription and replication. The identification of TopoI as a specific target of the anticancer drug camptothecin (CPT) led to the rapid development of its structure function and a large number of CPT analogues. Hycamtin (topotecan), Camptosar (irinotecan), and Onivyde (irinotecan liposome) are FDA-approved CPTs that are used to treat metastatic colorectal cancer (CRC) and pancreatic cancer in first line therapy; gastric, ovarian, and small cell lung cancer in second line therapy. Notably, approval of two antibody drug conjugates (ADCs), Trodelvi and Enhertu, with topoI inhibitors as the cytotoxic warheads has placed this class of drug front and center in future anticancer drug discovery efforts.
Three mechanisms have been proposed to define the CPT drug resistance mechanisms: the lack of accumulation of the drug due to ABC transporters/multi-drug resistance (MDR) genes, mutations in topoI that alter the topoI-DNA-CPT interaction, and the rate of ubiquitin proteasomal pathway (UPP) dependent degradation of topoI. However, now it is established that CPTs are not MDR substrates and topoI mutations are very rare in the patient population, making UPP-mediated degradation of topoI a key regulator of CPT response. One of the most remarkable phenomena observed in the cellular response to CPTs is degradation of topoI. Importantly, the cells that degrade topoI rapidly are resistant to irinotecan. The seminal work carried out at Dr. Bharti’s lab defined the precise mechanism of topoI degradation by UPP and identified the molecular determinants of this pathway. Briefly, a decade of work has demonstrated that: i) topoI associates with DNA-PK, and DNA-PKcs phosphorylates topoI at Serine 10; ii) Phosphorylated topoI is ubiquitinated by BRCA1; iii) cells with higher basal levels of topoI-pS10 degrade topoI rapidly and are resistant to irinotecan; iv) the higher basal level of topoI-pS10 is maintained by phosphatases that deregulate kinase cascade and maintain steady state DNA-PKcs activation. The pathway has since been validated and published at several levels, including the demonstration that CPT-resistant cells have a higher basal level of phosphorylated topoI-S10.