PARP Inhibitor Activity Correlates with SLFN11 Expression and Demonstrates Synergy with Temozolomide in Small Cell Lung Cancer

Abstract

Purpose PARP inhibitors (PARPi) are a novel class of small molecule therapeutics for small cell lung cancer (SCLC). Identification of predictors of response would advance our understanding, and guide clinical application, of this therapeutic strategy. Experimental Design Efficacy of PARP inhibitors olaparib, rucaparib, and veliparib, as well as etoposide and cisplatin in SCLC cell lines, and gene expression correlates, were analyzed using public datasets. HRD genomic scar scores were calculated from Affymetrix SNP 6.0 arrays. In vitro talazoparib efficacy was measured by cell viability assays. For functional studies, CRISPR-Cas9 and shRNA were used for genomic editing and transcript knockdown, respectively. Protein levels were assessed by immunoblotting and immunohistochemistry (IHC). Quantitative synergy of talazoparib and temozolomide were determined in vitro. In vivo efficacy of talazoparib, temozolomide, and the combination was assessed in patient-derived xenograft (PDX) models. Results We identified SLFN11, but not HRD genomic scars, as a consistent correlate of response to all three PARPi assessed, with loss of SLFN11 conferring resistance to PARPi. We confirmed these findings in vivo across multiple PDX and defined IHC staining for SLFN11 as a predictor of talazoparib response. As temozolomide has activity in SCLC, we investigated combination therapy with talazoparib and found marked synergy in vitro and efficacy in vivo, which did not solely depend on SLFN11 or MGMT status. Conclusions SLFN11 is a relevant predictive biomarker of sensitivity to PARP inhibitor monotherapy in SCLC and we identify combinatorial therapy with TMZ as a particularly promising therapeutic strategy that warrants further clinical investigation.

Targeting DNA damage in SCLC

“…As already indicated, SCLC exhibits high levels of PARP1 expression and there are preclinical data to support PARP1 inhibition for clinical evaluation as a monotherapy and in combination with DNA damaging agents [36,41,42]. In tumour models PARP inhibitors synergise with agents that increase the prevalence of SSBs such as temozolomide [43,44]. In addition, in preclinical SCLC xenografts Byers et al demonstrated single agent activity of olaparib, which was further increased when combined with cisplatin and etoposide or irinotecan [45].…”

“…Olaparib is the most extensively investigated PARPi and is approved by the US FDA for use in pretreated advanced germline BRCA mutated ovarian cancer [40]. As already indicated, SCLC exhibits high levels of PARP1 expression and there are preclinical data to support PARP1 inhibition for clinical evaluation as a monotherapy and in combination with DNA damaging agents [36,41,42]. In tumour models PARP inhibitors synergise with agents that increase the prevalence of SSBs such as temozolomide [43,44].…”

“…The precise mechanism of action of PARP inhibition in SCLC is not well understood. However, non-HRR dependent mechanisms of PARP inhibitor sensitivity have recently been recognised and to date candidate biomarkers for PARPi sensitivity in SCLC identified have included a 17 DNA repair protein score [42] and SLFN11 expression [44]. Several PARP inhibitors have now entered clinical testing in patients with SCLC.…”

“…SLFN11 is actively recruited to sites of DNA damage, inhibiting HR respectively) [54] and activating a cellular replication-stress response [55,56]. SLFN11 suppression has been associated with chemoresistance in SCLC models [57] and identified as a biomarker of PARP inhibitor response in SCLC PDX [44]. Circulating tumour cell enumeration was undertaken at baseline and after 1 cycle of chemotherapy.…”

“…A novel 'DNA repair score' consisting of 17 DNA repair proteins, applied to SCLC cell lines and xenografts established that baseline activation of the PI3K/mTOR pathway is associated with resistance to the PARP inhibitor BMN673 [42]. Another biomarker, SLFN11 expression, is associated with PARP inhibitor sensitivity in SCLC cell lines and PDX models [44]. In addition, high levels of SLFN11 expression (Hscore > = 1) were associated with a trend toward improved OS and favourable tumour responses in patients with recurrent SCLC that received TMZ and Veliparib as second line regiment, but not temozolomide plus placebo in a randomised phase II clinical trial [99] highlighting evident biomarkers that could guide clinical decision making.…”