PARP inhibitor rucaparib induces changes in NAD levels in cells and liver tissues as assessed by MRS

Poly(adenosine diphosphate ribose) polymerases (PARPs) are multifunctional proteins which play a role in many cellular processes. Namely, PARP1 and PARP2 have been shown to be involved in DNA repair, and therefore are valid targets in cancer treatment with PARP inhibitors, such as rucaparib, currently in clinical trials. Proton magnetic resonance spectroscopy (H-1-MRS) was used to study the impact of rucaparib in vitro and ex vivo in liver tissue from mice, via quantitative analysis of nicotinamide adenosine diphosphate (NAD(+)) spectra, to assess the potential of MRS as a biomarker of the PARP inhibitor response. SW620 (colorectal) and A2780 (ovarian) cancer cell lines, and PARP1 wild-type (WT) and PARP1 knock-out (KO) mice, were treated with rucaparib, temozolomide (methylating agent) or a combination of both drugs. H-1-MRS spectra were obtained from perchloric acid extracts of tumour cells and mouse liver. Both cell lines showed an increase in NAD(+) levels following PARP inhibitor treatment in comparison with temozolomide treatment. Liver extracts from PARP1 WT mice showed a significant increase in NAD(+) levels after rucaparib treatment compared with untreated mouse liver, and a significant decrease in NAD(+) levels in the temozolomide-treated group. The combination of rucaparib and temozolomide did not prevent the NAD(+) depletion caused by temozolomide treatment. The H-1-MRS results show that NAD(+) levels can be used as a biomarker of PARP inhibitor and methylating agent treatments, and suggest that in vivo measurement of NAD(+) would be valuable.