Uterine serous carcinoma (USC) accounts for up to 40% of endometrial cancer-related deaths. In contrast to the more common endometrioid histology, USC is more likely to present in advanced stage and carries a worse prognosis. USC mimics the most common serous carcinoma of the ovary and has high probability for nodal and intra-peritoneal spread. Furthermore, studies have indicated that USC harbors a high frequency of somatic TP53 mutations, germline BRCA1 mutations, and mutations within the Fanconi Anemia - BRCA pathway. Data is supportive of the USC association with hereditary breast and ovarian cancer, essentially harboring mutations in DNA repair genes. Approximately 5% of women with USC have germline mutations in 3 different tumor suppressor genes including BRCA1, CHEK2, and TP53.(1,2) The Cancer Genome Atlas Research network reported 4 groups of endometrial tumors based on integrated genomic data - including a novel POLE subtype in 10% endometrial tumors. Patients with uterine serous cancers shared many similar characteristics with basal-like breast and high grade serous ovarian cancers, suggesting a correlation with "BRCAness".(3)

Given the "BRCAness" of USC, recent multicenter prospective cohort study of 1083 women with BRCA1 and BRCA2 mutations who underwent risk reducing salpingo-oophorectomies (RRSO) without hysterectomy were noted to have increased serous/serous-like endometrial carcinoma if they harbored BRCA1 mutations.(4) The study recommended to consider this risk of uterine cancer when discussing the advantages and risks of hysterectomy at the time of RRSO in BRCA1 women. This further supports the high rates of mutations noted among USC patients. A recent systematic review and meta-analysis support the view that USC is a component of BRCA 1/2 -associated tumors. Furthermore, this analysis supports that women with USC should be offered screening for germline mutations when there is a positive family history of malignancies associated with hereditary breast and ovarian cancer syndrome(HBOCS).(5) Furthermore, the Cancer Genome Atlas Research network reported 4 groups of endometrial tumors based on integrated genomic data - including a novel POLE subtype in 10% endometrial tumors. Patients with uterine serous cancers shared many similar characteristics with basal-like breast and high grade serous ovarian cancers, suggesting a correlation with "BRCAness".(3)

Poly(ADP-ribose) polymerases (PARP1 and PARP2) play an important role in DNA repair. Upon formation of DNA breaks, PARP binds at the end of broken DNA strands helping in DNA repair of damage. The hypothesis is that treatment with PARP inhibitors will allow the killing of a subset of cancer cells with deficiencies in DNA repair pathways. For example, a tumor harboring a BRCA or Homologous Recombination gene mutation will have selective blockage by PARP inhibitors in order to maintain genomic integrity. Furthermore, the data in serous ovarian cancer has indicated that tumors arising in a non-BRCA patient that has a homologous recombination deficiency could also enhance tumor cell sensitivity to PARP inhibitors.

PARP inhibitors (PARPi) are synthetically lethal to tumor cells with homologous recombination deficiency (HRD). HRD leads to common phenotype of genome-wide loss of heterozygosity (LOH). Recent analysis of the ARIEL2 part 1 in platinum sensitive ovarian cancer trial found that patients with germline or somatic BRCA mutation or wild-type BRCA with high LOH had longer progression-free survival and improved responses with rucaparib treatment than did patients with wild-type BRCA and low LOH.(7)

The rationale for this current trial is based on significant clinical and genomic similarities of USC and epithelial ovarian carcinomas.(1,2,4) Currently the treatment for stage III and IV USC yields approximately 20-30% survival at 2 years and 10%-20% survival at 3-5 years post diagnosis with current standard therapy of chemotherapy +/-radiation depending on the sites of the disease at surgical staging/debulking. Furthermore, there is no successful second line therapy for patients with recurrent USC and no available clinical trials for patients with recurrent disease. Given the most recent findings of the multi-national, Phase 3 NOVA trial in women with platinum sensitive, recurrent ovarian cancer, Niraparib significantly prolonged the median progression-free survival - irrespective of the presence or absence of a germline BRCA mutation or the presence/absence of a homologous recombinant deficiency.(6) The investigators hypothesize that patients receiving Niraparib maintenance in addition to standard therapy for USC may lead to improved progression free survival in women with suboptimally debulked stage III, stage IV, and platinum-sensitive recurrent USC. The investigators hypothesize that this treatment will be well tolerated in this group of patients.