Identification of novel drug-specific PARP inhibitor resistance mechanisms in ovarian cancer-implications for clinical practice.

BACKGROUND: Maintenance PARP inhibitor (olaparib or niraparib) treatment is commonly prescribed following carboplatin/paclitaxel chemotherapy in ovarian cancer patients, but response is compromised by adaptive drug resistance [1]. We have shown that P-gp/ABCB1 influences resistance to paclitaxel and olaparib, but similar niraparib resistance mechanisms have not been described [2, 3].

METHODS: We used qRT-PCR, Western blot, RNASeq and LC-MS/MS proteomics analysis to compare drug transporter expression in sensitive and resistant immortalised and primary patient-derived cell lines. ABCB1 and ABCG2 expression was modified by shRNA-mediated knockdown and heterologous expression, with chemosensitivity changes assessed by MTT and clonogenic assays. Substrate specificity of P-gp and BCRP was assessed by efflux assays in polarised cells.

RESULTS: P-gp/ABCB1 expression was not increased in A2780nirapR cells, which alternatively up-regulated BCRP/ABCG2. ABCG2 was consistently induced in niraparib-resistant patients, but ABCB1 only in patients pre-treated with paclitaxel. shABCG2 re-sensitised A2780nirapR cells, while heterologous expression in A2780 cells induced drug resistance. Efflux assays confirmed that olaparib and niraparib are both P-gp and BCRP substrates, suggesting that resistance results from transcriptional regulation of efflux transporters not substrate specificity.

CONCLUSIONS: Treatment-induced BCRP/ABCG2 induction is a novel clinically relevant niraparib resistance biomarker. Routine inclusion of paclitaxel in first-line chemotherapy regimens may promote efflux transporter-mediated resistance, compromising response to PARPi maintenance treatment.