Abstract P5-01-06: 18F-radiolabeled PARP-1 inhibitor uptake as a marker of PARP-1 activity in breast cancer

Abstract

Objectives: The nuclear enzyme PARP-1 plays a central role in sensing DNA damage and facilitating repair. Tumors with BRCA1/2 mutations are highly dependent on PARP-1 as an alternative mechanism for DNA repair, and PARP inhibitors generate synthetic lethality in tumors with BRCA mutations, resulting in cell cycle arrest and apoptosis. Zhou et al. recently synthesized an 18F-labeled PARP-1 inhibitor (18F-FluorThanatrace) for PET, and demonstrated high specific tracer uptake in a xenograft model of breast cancer (Zhou, Bioorg Med Chem, 22:1700, 2014). The current study seeks to quantify the relationship between 18F-FluorThanatrace binding (both in vitro and on PET imaging of human tumor xenografts) and the level of constitutively active PARP-1, using multiple human breast cancer cell lines, including a BRCA1 defective line. Methods: BRCA1 defective HCC1937, triple negative MDA-MB-231, and luminal A MCF-7 human breast cancer lines were assessed for constitutive PARP-1 activity via a chemiluminescent ELISA assay for PAR and by Western blot. The same cell lines were incubated with 18F-FluorThanatrace over various time increments, and tracer uptake was assayed via a gamma counter. Specificity of tracer binding was verified via co-incubation with competitive inhibitor Olaparib, and specific tracer uptake was calculated as the difference between uptake with and without Olaparib. Specific tracer uptake was compared to levels of constitutive PARP-1 activity in all cell lines. In addition, HCC1937 and MDA-MB-231 xenograft tumor models were imaged via 18F-FluorThanatrace-PET/CT, and PET uptake was correlated with PARP-1 activity. Results: BRCA1-defective HCC1937 had higher constitutive PARP-1 activity than cell lines with intact BRCA1. In vitro levels of 18F-FluorThanatrace uptake correlated with constitutive PARP-1 activity across cell lines. In addition, 18F-FluorThanatrace measured by PET in xenograft breast cancer tumor models correlated with constitutive PARP-1 activity. Conclusions: Tumor uptake of 18F-FluorThanatrace, both in vitro and on PET imaging of xenograft tumor models, quantitatively reflects differences in PARP-1 activity across different breast cancer cell lines, including BRCA1 defective. This motivates further studies of 18F-FluorThanatrace as an in vivo measure of PARP-1 activity and possibly as a predictive marker for PARP-1 therapy in patients, including those with BRCA1/2 mutations. Citation Format: Edmonds CE, Lieberman BP, Xu K, Zeng C, Makvandi M, Li S, Hou C, Lee H, Greenberg RA, Mankoff DA, Mach RH. 18F-radiolabeled PARP-1 inhibitor uptake as a marker of PARP-1 activity in breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-01-06.

PARP-1 Expression Quantified by [18F]FluorThanatrace: A Biomarker of Response to PARP Inhibition Adjuvant to Radiation Therapy

“…21 We have previously shown that these radiotracers bind to PARP in vivo by using breast cancer tumor models and can be competitively blocked by using a clinical PARP inhibitor. 22,23 In this study, we explored a PARP-1 PET imaging agent [ The cell lines used in this study were selected based on the genetic profile as either ovarian cancer BRCA1 mutant (SNU-251) or BRCA1 functional-wild type (SKOV3). We hypothesize that [ 18 F]FTT uptake in vitro will correspond to PARP-1 expression, and ovarian cancer cells with higher PARP-1 expression will be more sensitive to PARP inhibition adjuvant to radiation therapy.…”

[18F]AZD2461, an Insight on Difference in PARP Binding Profiles for DNA Damage Response PET Imaging

“…18 F-radiolabelling of neutral (hetero)arenes often requires a robust synthetic approach in order to secure radiosynthesis. Most studies rely on aliphatic radiolabelling or the use of radiolabelled prosthetic groups to produce radiotracers in reasonable activity yields (AY) and molar activities (A m ) [21,23]. However, as previously demonstrated with [ 18 F]olaparib, in this study, we were able to efficiently radiolabel [ 18 F]AZD2461 using the Cu-mediated radiofluorination method, originally developed by Tredwell et al [31].…”

“…Taken together, this suggests the ability of AZD2461 to bind to alternative targets in addition to those engaged by olaparib, although the precise targets have not been yet identified. While many studies describe the use of labelled inhibitors or associated variants [20,23,24,27,33], our study emphasises the critical evaluation of different PARP isoforms or alternative binding epitopes for PET imaging of PARP expression. Reilly and co-workers have shown the use of an 18 F-labelled variant of AZD2461 [27].…”

“…To date, together with the rucaparib-related tracer [ 18 F]FTT, it is one of the few labelled PARP inhibitors whose structure remains closely related to the native inhibitor that has been investigated in clinical trials [20,22]. While other labelled compounds have been investigated, such as [ 18 F]BO or [ 18 F]PARPi-FL, few radiotracers have been studied for their PARP PET imaging potential with a view to assess a different PARP isoform interaction profile [23][24][25]. A radiolabelled PARP inhibitor that possesses a different PARP inhibitory profile may allow for a better understanding of PARP expression in vivo.…”

[18F]F-Poly(ADP-Ribose) Polymerase Inhibitor Radiotracers for Imaging PARP Expression and Their Potential Clinical Applications in Oncology

“…Zhou et al showed that among the Rucaparib-based tracers that they had developed and investigated, [ 18 F]FTT outperformed others and showed the greatest potential for clinical translation [67]. In both in vitro and preclinical xenograft breast and ovarian cancer mouse models, this and other groups showed a direct correlation between [ 18 F]FTT uptake, PARP1 expression, and competitive inhibition with Olaparib co-administration [67][68][69]. Effron et al also evaluated PARP expression using [ 18 F]FTT post-adjuvant PARP inhibitor therapy [70].…”