Abstract Background: PolyADP-ribose polymerase (PARP) inhibitors are a novel promising strategy toward triple-negative breast cancer (TNBC), which often shows genomic instability or BRCA mutations. However, clinical results are controversial, and no benefits were demonstrated in case of wild type BRCA, possibly due to poor bioavailability, and inadequate nuclear delivery. Nanotechnology could overcome these major limitations. The aim of this study was to assess the anticancer efficacy of H-Ferritin nanoformulated Olaparib (HOla) vs. free Olaparib (Ola) on BRCA-mutated and non mutated TNBC cells. Methods: BRCA-mutated HCC1937 cells and BRCA-wild type MDA MB-231 and MDA MB-468 cells were treated with HOla or free Ola in vitro. Active targeting and binding capability of HOla toward transferrin receptor 1 (TfR-1), over-expressed on TNBC cells, was assessed by flow cytometry. Internalization and intracellular localization of Ola and HOla was assessed by confocal microscopy. Anticancer efficacy was assessed by administration of increasing doses of HOla or Ola, comparing cell viability, cell cycle, cell death, PARP-1 cleavage and DNA damage. Finally, anti-PARP efficacy and proportion of drug in the nuclear compartment were compared between treatments. Results: All TNBC cell lines over-expressed TfR-1 and were succesfully recognized by HOla. Confocal microscopy showed a fast internalization of nanoparticles into cells, with intracellular persistence up to 48h. A marked increase in nuclear concentration of drug was observed with HOla compared to Ola, due to a strongly improved nuclear delivery by H-Ferritin mediated by a self-triggered mechanism. No significant antiproliferative effect was demonstrated with Ola at 10 nM, 50 nM or 100 nM. Conversely, HOla at 50 nM and 100 nM showed a 1000-fold higher anticancer activity in all TNBC cell lines. A possible contribution in cytotoxicity by H-Ferritin nanovector itself was excluded treating cells with void nanoparticles. Proportions of cell cycle arrest in G2/M, cell death, cleaved PARP-1 and DNA damage in terms of phosphorylated histone H2A.X were higher in HOla treated samples than in ones treated with free Ola. Conclusions:Our findings suggest that nanoformulation of Ola strongly enhances cytotoxic efficacy of PARP inhibition as a stand-alone therapy, on both BRCA-mutated and wild type TNBCs allowing a targeted delivery into TNBC cells and a prompt homing into the nuclear compartment. Citation Format: Mazzucchelli S, Truffi M, Sorrentino L, Bellini M, Rizzuto MA, Ottria R, Ciuffreda P, Prosperi D, Corsi F. Olaparib nanoformulation in H-ferritin as a promising option for both BRCA-mutated and sporadic triple negative breast cancer: An in vitro study [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-10-13.