The development of agents that sensitize tumours to existing therapeutics is an attractive goal in cancer therapy. We previously used cells over-expressing the multidrug resistance protein MRP4 to screen for compounds that sensitize to the purine analogue 6-mercaptopurine (6-MP), an MRP4 substrate. In addition to MRP4 inhibitors, we identified a compound (Hit 52) that inhibits nucleotide phosphatases, enzymes that oppose the incorporation of thiopurine compounds into DNA and RNA. Notably, this compound also inhibits the growth of cultured neuroblastoma cells as a single agent.
Focused library screening around Hit 52 identified a range of compounds with >95% homology. The majority of these “Hit 52 series” compounds also inhibit nucleotide phosphatases in enzyme assays, sensitize to 6-MP and inhibit neuroblastoma cell growth in culture. Notably however, inactivate and partially active analogues were also identified. This family of compounds should enable subsequent structure-activity studies.
We have conducted preliminary studies of the Hit 52 series using the TH-MYCN transgenic mouse model of neuroblastoma. Consistent with our in vitro findings, these compounds delayed tumour growth when used as single agents immediately post-weaning and prior to overt tumour development and sensitized established tumours to 6-MP, substantially prolonging survival. Surprisingly, these compounds also sensitized established tumours to the DNA crosslinking agent cisplatin, a key chemotherapeutic agent in neuroblastoma treatment, but did not sensitize to the topoisomerase I inhibitor irinotecan and the microtubule inhibitor vincristine.
We are currently studying the mechanisms of action of these compounds in more detail, investigating the basis of cisplatin sensitization and determining their efficacy in xenograft models of neuroblastoma.