Niraparib Maintenance Treatment in mCRC With a Partial o Complete Response After Oxaliplatin-based Induction Therapy

Inclusion Criteria:

* Signed informed consent
* Age ≥ 18 years at time of study entry
* Histologically confirmed diagnosis of Stage IV colorectal adenocarcinoma
* Disease evaluation with proven Complete Response or Partial Response according to RECIST v 1.1 after 4 months of oxaliplatin-based doublets or triplets with or without anti-VEGF or anti-EGFR agents
* Availability of Formalin-Fixed Paraffin-Embedded tumor tissue
* Treatment with Niraparib must be started after at least 2 weeks the end of platinum-based induction therapy up to 6 weeks
* Eastern Cooperative Oncology Group Performance Status \< 1
* Adequate normal organ and marrow function as follow: Haemoglobin≥9.0 g/dL; b. Absolute neutrophil count ≥1.5 × 109 /L; Platelet count ≥100 × 109/L; Serum bilirubin ≤1.5 x Upper Limit of Normal This will not apply to patients with Gilbert's syndrome; Calculated creatinine CL\>40 mL/min by the Cockcroft-Gault formula; Aspartate aminotransferase /Alanine Aminotransferase ≤2.5 x Upper Limit of Normal o ≤ 5 x Upper Limit of Normal in the presence of liver metastases
* Willing and able to comply with all of the requirements and visits in the protocol
* Participant must agree not to donate blood during the study or for 90 days after the last dose of Niraparib
* Fertile women must have a negative urine or serum pregnancy test within 7 days prior to taking study treatment
* Fertile women must use highly effective contraception, starting with the screening visit through 6 months post last dose of Niraparib.
* Male participant agrees to use an adequate method of contraception starting with the first dose of study treatment through 90 days after the last dose of study treatment.
* Male participant must not donate sperm for 90 days after last dose of Niraparib.

Exclusion Criteria:

* Prior adjuvant treatment for stage II/III colorectal cancer ending within 12 months before the start of induction treatment
* Patients with Microsatellite instability high or DNA mismatch repair deficiency DSBs DNA double-strand breaks are not allowed
* Any systemic disease that, in the opinion of the Investigator, it is not compatible with the protocol
* Major surgery ≤ 3 weeks prior to initiating Niraparib
* Participation in another interventional clinical trial
* Radiation therapy encompassing \>20% of the bone marrow within 2 weeks; or any radiation therapy \< 1 week prior to taking Niraparib
* Hypersensitivity to Niraparib components or excipients
* Transfusion (platelets or red blood cells) ≤ 4 weeks prior to initiating Niraparib
* Colony stimulating factors (e.g., granulocyte colony-stimulating factor, granulocyte macrophage colony stimulating factor, or recombinant erythropoietin) within 4 weeks prior Niraparib
* Any known Grade 3 or 4 anemia, neutropenia or thrombocytopenia due to prior chemotherapy that persisted \> 4 weeks
* History of myelodysplastic syndrome, acute myeloid leukemia or Prior Reversible Encephalopathy Syndrome
* Serious, uncontrolled medical disorder, nonmalignant systemic disease, or active, uncontrolled infection.
* Diagnosis, detection, or treatment of another type of cancer ≤ 2 years prior to initiating Niraparib (except basal or squamous cell carcinoma of the skin and cervical cancer in situ that has been definitively treated)
* Active infection with Human Hepatitis B Virus or Hepatitis C Virus
* Pregnancy or breastfeeding
* Any impairment of gastrointestinal function or disease that may significantly impair the absorption of oral drugs, malabsorption syndrome, bowel obstruction, or inability to swallow tablets).