Olaparib is a poly-adenosine diphosphate ribose polymerase (PARP) inhibitor, originally used for the maintenance treatment of women with platinum-sensitive relapsed breast cancer gene (BRCA)-mutated high grade serious epithelial ovarian, fallopian tube, or peritoneal cancer, who are in response to platinum-based chemotherapy. Over the last two years, several therapeutic indications have been added to the drug label, such as first-line platinum-sensitive BRCA-mutated high grade serious epithelial ovarian, fallopian tube, or peritoneal cancer, germline BRCA1/2-mutated, human epidermal growth factor 2 (HER2-)negative, locally advanced or metastatic breast cancer and BRCA1/2-mutated metastatic castration-resistant prostate cancer, who have progressed following prior therapy. Since olaparib is very expensive, this increase of treatment population will have a significant impact on health care expenditures. To keep healthcare affordable and accessible for all patients, innovative strategies are warranted to reduce the dose of expensive drugs, without reduction of efficacy. For olaparib, pharmacokinetic (PK) boosting can be applied. PK boosting is the lay term for administering a non-therapeutic active strong inhibitor of a metabolic enzyme, for example the cytochrome p450 enzyme 3A (CYP3A), together with a therapeutic drug that is metabolized by the same enzyme. Boosting thus increases the concentration of the therapeutic drug and allows lower doses to be administered to patients. Hence, coadministration of a reduced dose of olaparib with cobicistat, a non-therapeutic, strong inhibitor of the CYP3A can lead to equivalent exposure to olaparib. Furthermore, inhibition of CYP3A could lead to less PK variability since metabolic capacity is a prominent cause for (intra- and inter-individual) variability in systemic exposure. Predictable olaparib exposure will reduce the number of patients who are unintentionally under- or overtreated. Lastly, tumor tissue itself may express CYP3A as a detoxification or resistance mechanism. Theoretically, PK boosting may also overcome CYP3A-mediated drug resistance. The purpose of this study is to establish the efficacy, safety and feasibility of co-administering olaparib with the PK booster cobicistat with the aim to implement boosting approach for olaparib in routine practice. The study is subdivided in two parts. In part A of the study the equivalent exposure of boosted low dose olaparib is determined compared to the normal dose. In part B of the study, non-inferiority of the boosted olaparib regimen will be confirmed.