Poster Presentation 28th Lorne Cancer Conference 2016

Radioprotection by the combination of DNA binding antioxidants and aminothiol radical scavengers (#253)

Jai Smith 1 2 , Andrea Smith 1 , Colin Skene 3 , Jonathan M White 3 , Pavel Lobachevsky 1 , Roger F Martin 1 2
  1. Molecular Radiation Biology Laboratory, Peter MacCallum Cancer Centre, East Melbourne, Vic, Australia
  2. The Sir Peter MacCallum Department of Oncology, University of Melbourne, Melbourne, Vic, Australia
  3. School of Chemistry, University of Melbourne and Bio21 Institute, Melbourne, VIC, Australia

The unavoidable irradiation of normal tissue during cancer radiotherapy can lead to serious dose limiting side effects.  For example, radiation-induced oral mucositis during head and neck radiotherapy is still a problem even with modern dose delivery technology, and can lead to treatment interruptions.  One strategy for ameliorating such problems is pharmacological radioprotection of “at risk” normal tissues.

The first clinically approved radioprotector was amifostine, a prodrug of the aminothiol radical scavenger WR1065. M2PB, an analogue of methylproamine, represents a new class of radioprotector that protects via a unique mechanism, involving electron transfer from the minor groove bound ligand to transient radiation induced oxidising species on the DNA.  Additive radioprotection by M2PB in combination with amifostine (or WR1065) has been observed for both in vitro and in vivo endpoints, indicating complementary mechanisms of radioprotection.

In vitro clonogenic survival studies indicated near-additive radioprotection for the combination (DMF=4.4), compared to either WR1065 (DMF=2.5) or M2PB (DMF=2.8) alone. Similar results have been obtained for WR1065 in combination with 2PH and methylproamine (two analogues of M2PB).  In vivo additive radioprotection in mice was also observed using the Withers micro colony survival assay when M2PB (DMF=1.24) and amifostine (DMF=1.26) were combined (DMF=1.75).

Further mechanistic studies involve the radiation induced mutagenesis endpoint.  The frequency of radiation induced mutations at the HPRT locus, following 6 Gy g-radiation, was 65.4 mutants/106 cells.  Protection of radiation induced mutations was observed for WR1065 (24.5 mutants/106 cells) and M2PB (23.5 mutants/106 cells). Interestingly, an additional reduction was observed when the two radioprotectors were used in combination (10.0 mutants/106 cells).

These results demonstrate additive radioprotection by the combination of DNA binding antioxidants and aminothiol radical scavengers in in vitro and in vivo endpoints.  Additionally, radioprotection appears to be accompanied by a corresponding reduction in radiation induced mutations, an important clinical consideration.