Positron Emission Tomography Imaging of Poly–(Adenosine Diphosphate–Ribose) Polymerase 1 Expression in Breast Cancer : A Nonrandomized Clinical Trial

Complete loss of BRCA1 or BRCA2 function is associated with sensitivity to DNA damaging agents. However, not all BRCA1 and BRCA2 germline mutation-associated tumors respond. Herein we report analyses of 160 BRCA1 and BRCA2 germline mutation-associated breast and ovarian tumors. Retention of the normal BRCA1 or BRCA2 allele (absence of locus-specific loss of heterozygosity (LOH)) is observed in 7% of BRCA1 ovarian, 16% of BRCA2 ovarian, 10% of BRCA1 breast, and 46% of BRCA2 breast tumors. These tumors have equivalent homologous recombination deficiency scores to sporadic tumors, significantly lower than scores in tumors with locus-specific LOH (ovarian, P = 0.0004; breast P < 0.0001, two-tailed Student’s t-test). Absence of locus-specific LOH is associated with decreased overall survival in ovarian cancer patients treated with platinum chemotherapy (P = 0.01, log-rank test). Locus-specific LOH may be a clinically useful biomarker to predict primary resistance to DNA damaging agents in patients with germline BRCA1 and BRCA2 mutations.

BACKGROUND. Poly(ADP-ribose) polymerase (PARP) inhibitors are effective in a broad population of patients with ovarian cancer; however, resistance caused by low enzyme expression of the drug target PARP-1 remains to be clinically evaluated in this context. We hypothesize that PARP-1 expression is variable in ovarian cancer and can be quantified in primary and metastatic disease using a novel PET imaging agent. METHODS. We used a translational approach to describe the significance of PET imaging of PARP-1 in ovarian cancer. First, we produced PARP1 -KO ovarian cancer cell lines using CRISPR/Cas9 gene editing to test the loss of PARP-1 as a resistance mechanism to all clinically used PARP inhibitors. Next, we performed preclinical microPET imaging studies using ovarian cancer patient–derived xenografts in mouse models. Finally, in a phase I PET imaging clinical trial we explored PET imaging as a regional marker of PARP-1 expression in primary and metastatic disease through correlative tissue histology. RESULTS. We found that deletion of PARP1 causes resistance to all PARP inhibitors in vitro, and microPET imaging provides proof of concept as an approach to quantify PARP-1 in vivo. Clinically, we observed a spectrum of standard uptake values (SUVs) ranging from 2–12 for PARP-1 in tumors. In addition, we found a positive correlation between PET SUVs and fluorescent immunohistochemistry for PARP-1 ( r 2 = 0.60). CONCLUSION. This work confirms the translational potential of a PARP-1 PET imaging agent and supports future clinical trials to test PARP-1 expression as a method to stratify patients for PARP inhibitor therapy. TRIAL REGISTRATION. Clinicaltrials.gov NCT02637934. FUNDING. Research reported in this publication was supported by the Department of Defense OC160269, a Basser Center team science grant, NIH National Cancer Institute R01CA174904, a Department of Energy training grant DE-SC0012476, Abramson Cancer Center Radiation Oncology pilot grants, the Marsha Rivkin Foundation, Kaleidoscope of Hope Foundation, and Paul Calabresi K12 Career Development Award 5K12CA076931.

Despite the availability of PARP inhibitors for cancer therapy, a biomarker to clearly stratify patients for selection of this treatment remains lacking. Here we describe a radiotracer-based method that addresses this issue, using the novel compound [(125)I] KX1: as a PARP-1-selective radiotracer that can accurately measure PARP-1 expression in vitro and in vivo The pharmacologic properties of the PARP radiotracer [(125)I] KX1: was characterized in multiple cell lines where single-agent sensitivity was correlated with [(125)I] KX1: binding to PARP-1. In vivo evaluation of [(125)I] KX1: verified in vitro results, validating PARP radiotracers to define PARP-1 enzyme expression as an in vivo biomarker. Notably, PARP-1 expression as quantified by [(125)I] KX1: correlated positively with the cytotoxic sensitivity of cell lines evaluated with PARP inhibitors. Overall, our results defined a novel technology with the potential to serve as a companion diagnostic to identify patients most likely to respond therapeutically to a PARP inhibitor. Cancer Res; 76(15); 4516-24. ©2016 AACR.