Background: PARP inhibitors (PARPi), as Olaparib (OLA), have changed the therapeutic landscape in HGSOC and TNBC. However, most patients will develop resistance to PARPi. Although resistance mechanisms are poorly understood, it is suggested that BRCA mutations reversions or cell cycle alterations could be involved. APR-246 is a new targeted agent that restores p53 in TP53-mutated cells. The aim of our study was to assess the potential synergism of the combination of carboplatin (carbo) plus APR-246 in both OLA-sensitive and OLA-resistant HGSOC and TNBC cell lines. Methods: We selected two HGSOC (PEO1-S and Kuramochi-S) and one TNBC TP53 mutant cell lines (MDA-MB-231-S). Resistance to OLA was generated by pulse doses with IC75 OLA for 24 hours and recovered for two weeks during a period of 6 months (PEO1-R) and to increased concentrations of OLA for 72 hours during a period of 1 year (Kuramochi-R, MDA-MB-231-R). Next-generation sequencing was performed in order to assess mutations. IC50 of APR-246 and Carbo and the combination of both (combo) were calculated for Kura-S/R, PEO1-S/R and 231-S/R cell lines. Synergism between both drugs was assessed with Chou-Talalay method for combination index (CI) at Fa50. DNA damage was assessed by gamma-H2AX expression by western-blot (WB). Results: 231-S harboured a TP53 mutation (mut) (missense R280K) and BRCA wildtype (wt), in PEO1-S BRCA2 mut p.Y1655* and TP53 mut was detectable. Kuramochi-S was TP53 mut (missense D816Y) and BRCA2 mut. BRCA and p53 mutation remained unchanged in the resistant lines generated.The mutation profile of each line and the IC50 of APR-246, carbo and combo are shown in the table. IC50 of carboplatin is superior in all 3 resistant cell lines when compared to their sensitive parental line, suggesting a potential cross-resistance to platinum in OLA-resistance. The combination of carbo and APR-246 is synergistic even in OLA-resistant cell lines, in fact in those cross-resistant lines values of synergism by Chou-Talalay are improved by the combo suggesting a potential role of this combination in PARPi resistant cell lines. WB showed an increased DNA damage with the combo when compared to either single agent, with an increase in H2AX in all lines either sensitive or resistant. Conclusions: The combination of APR246 and carboplatin is synergistic in all 3 studied cell lines but also in the OLA resistant ones. The combo increased its synergistic effect suggesting a potential role of combo in OLA-resistant tumors.