Background: Adavosertib is a first-in-class, selective small-molecule inhibitor of Wee1. Olaparib is an inhibitor of poly(ADP-ribose) polymerase (PARP). Preclinical data suggest that adavosertib enhances the antitumor effect of PARP inhibitors., Objective: The safety, tolerability, and efficacy of adavosertib plus olaparib were evaluated in patients with refractory solid tumors to define the maximum tolerated dose (MTD) and recommended phase II dose (RP2D)., Patients and Methods: Eligible patients in part A (dose finding) had a refractory solid tumor for which there is no established treatment and had received ≥ 1 prior course of systemic therapy; in part B (dose expansion), patients had platinum-sensitive extensive-stage or relapsed small-cell lung cancer (SCLC). Patients received adavosertib [once (qd) or twice daily (bid)] for 3 consecutive days with 4 days off treatment (3/4), or 5 consecutive days with 2 days off (5/2), plus olaparib (bid) for 14 or 21 days of a 21-day cycle., Results: A total of 130 patients were enrolled in the study, 120 in part A and 10 in part B. The MTD for adavosertib bid was 175 mg (days 1-3, 8-10/21-day cycle) plus continuous olaparib 200 mg bid; the once-daily MTD (and RP2D) was adavosertib 200 mg (days 1-3, 8-10/21-day cycle) plus continuous olaparib 200 mg bid. In the MTD/RP2D cohort, one patient (7%) experienced a dose-limiting toxicity (DLT) of thrombocytopenia. The most common treatment-related adverse events (TRAEs) in the cohorts in which MTD/RP2D for bid dosing and RP2D for qd dosing were determined were fatigue (64.3% and 15.4%, respectively), diarrhea (42.9% and 30.8%), decreased appetite (35.7% and 23.1%), nausea (35.7% and 15.4%), and anemia (35.7% and 38.5%). In the SCLC dose-expansion cohort, TRAEs occurred in eight patients (88.9%), including thrombocytopenia (66.7%) and anemia (55.6%). In part A, objective response rate (ORR) was 14.8% [95% confidence interval (CI) 8.7-22.9] overall; for the cohorts in which MTD/RP2D for bid dosing and RP2D for qd dosing were determined, ORR was 30.8% (9.1-61.4) and 9.1% (0.2-41.3), respectively. ORR was 11.1% [95% CI 0.3-48.2; one partial response (PR)], disease control rate was 22.2% (2.8-60.0; one PR, one stable disease), and median progression-free survival was 1.5 months (1.3-4.2) in the SCLC dose-expansion cohort., Conclusions: Adverse events and DLTs observed in the bid MTD and once-daily MTD/RP2D dosing schedules were manageable and consistent with known adavosertib and olaparib safety profiles. Limited antitumor activity was observed with adavosertib plus olaparib combination therapy., Trial Registration: ClinicalTrials.gov, NCT02511795 (registration: 28 July 2015)., Competing Interests: Declarations Funding This study was funded by AstraZeneca and is part of an alliance between AstraZeneca and Merck Sharp & Dohme LLC, a subsidiary of Merck & Co, Inc, Rahway, NJ, USA, who are codeveloping olaparib. Disclosure of Potential Conflicts of Interests Erika P. Hamilton: Institutional funding (AstraZeneca, Pfizer, Genentech/Roche, Lilly, Puma Biotechnology, Daiichi Sankyo, Mersana, Boehringer Ingelheim, Cascadian Therapeutics, Hutchinson MediPharma, OncoMed, MedImmune, Stem CentRx, Curis, Verastem, Zymeworks, Syndax, Lycera, Rgenix, Millennium, TapImmune, BerGenBio, Medivation, Tesaro, Eisai, H3 Biomedicine, Radius Health, Acerta Pharma, Takeda, Macrogenics, Abbvie, Immunomedics, Fujifilm, eFFECTOR Therapeutics, Merus, Nucana, Regeneron, Leap Therapeutics, Taiho Pharmaceutical, EMD Serono, Clovis, CytomX, InventisBio, Novartis, Silverback, Black Diamond, ArQule, Sermonix Pharmaceuticals, Sutro Biopharma, Zenith Epigenetics, Arvinas, Torque Therapeutics, Harpoon Therapeutics, Fochon Pharmaceuticals, Orinove, Molecular Templates, Unum Therapeutics, Aravive, Dana Farber Cancer Institute, G1 Therapeutics, Karyopharm Therapeutics, and Compugen) and non-financial support (Amgen, Bayer, Bristol-Myers Squibb, Genzyme, Helsinn Therapeutics, HERON, Lexicon, Medivation, Merck, Novartis, Roche, Sysmex, Guardant Health, Foundation Medicine, Deciphera, and NanoString). Gerald S. Falchook: Royalties, self (Wolters Kluwer); advisory role, institution (AbbVie, Fujifilm, Silicon, Navire, Turning Point, Predicine, Inspirna, Regeneron, Jubilant, BostonGene, Teon, Merck, Sanofi, and BridgeBio); advisory role, self (EMD Serono); speaker’s honorarium for CME (Total Health Conferencing and Rocky Mountain Oncology Society); travel, self, for work and/or research related to institution [Amgen, Bristol-Myers Squibb, EMD Serono, Fujifilm, Millennium, Sarah Cannon Research Institute (employer), and Synthorx/Sanofi]; research funding, to institution (3-V Biosciences, Abbisko, Abbvie, ABL Bio, ADC Therapeutics, Accutar, Agenus, Aileron, American Society of Clinical Oncology, Amgen, ARMO/Eli Lilly, Artios, AstraZeneca, Bayer, BeiGene, Bioatla, Bioinvent, Biothera, Bicycle, Black Diamond, Boehringer Ingelheim, Celgene, Celldex, Ciclomed, Curegenix, Curis, Cyteir, Daiichi, DelMar, eFFECTOR, Eli Lilly, EMD Serono, Epizyme, Erasca, Exelixis, Freenome, Fujifilm, Genmab, GlaxoSmithKline, Hutchison MediPharma, IGM Biosciences, Ignyta, Immunitas, ImmunoGen/MacroGenics, Incyte, Jacobio, Jazz, Jounce, Jubilant, Kolltan, Loxo/Bayer, MedImmune, Merck, Metabomed, Millennium, Mirati, miRNA Therapeutics, Molecular Templates, National Institutes of Health, Navire/BridgeBio, NGM Bio, NiKang, Novartis, OncoMed, Oncorus, Oncothyreon, Poseida, Precision Oncology, Prelude, PureTech, Pyramid, RasCal, Regeneron, Relay, Rgenix, Ribon, Samumed, Sapience, Seagen, Silicon, Simcha, Sirnaomics, Strategia, Syndax, Synthorx/Sanofi, Taiho, Takeda, Tallac, Tarveda, Teneobio, Tesaro, Tocagen, Turning Point, U.T. MD Anderson Cancer Center, Vegenics, Xencor, and Zhuhai Yufan). Judy S. Wang: Consulting fees (Janssen, Stemline/Menarini, BioNTech, and Kanaph Therapeutics); honoraria for speakers’ bureau (AstraZeneca and Eisai); research funding to institution only (Syndax, AstraZeneca, Phoenix, Accutar, Celgene, BMS, Tenebio, LOXO, Nurix, Step Pharma, BioNTech, Moderna, Merck, Compugen, Boehringer Ingelheim, Kymab, Sanofi, Bicycle, Cyteir, Daiichi Sankyo, Xencor, Ribon, Klus Pharma, Artios, Genentech, Treadwell, MabSpace, IGM, Immunogen, PureTech, Erasca, Bayer, 7.8 Pharma, NGMBio, Zymeworks, Immuno-Onc, Astellas, Immunitas, Prelude, Blueprint, Beigene, Pionyr, Agenus, Adagene, Macrogenics, Jazz, StingThera, Coherus, Takeda, Taiho, Jounce, Evelo, Vedanta, Stemline, GSK, Janssen, Black Diamond, Vigeo, Relay, and Jacobio). Siqing Fu: Clinical trial research support/grant funding, institution {National Institutes of Health [NIH]/National Cancer Institute [NCI] P30CA016672 – Core Grant [Cancer Center Support Grant (CCSG) Shared Resources]; Abbisko; BeiGene; BioAtla, LLC.; Boehringer Ingelheim; CUE Biopharma, Inc.; Eli Lilly & Co.; Exelisis; Greenfire Bio, Inc.; Hookipa Biotech; IMV, Inc.; Innovent Biologics, Co., Ltd.; K-Group Beta; Lyvgen Biopharm, Co., Ltd.; MacroGenics; MediLink Therapeutics, Co. Ltd.; Millennium Pharmaceuticals, Inc.; Nerviano Medical Sciences; NeuPharma, Inc.; NextCure, Inc.; Ningbo NewBay Technology Development Co., Ltd.; Novartis; NovoCure; Nykode Therapeutics AS.; Parexel International, LLC; Pionyr Immunotherapeutics, Inc.; PureTech Health, LLC; Sellas Life Sciences Group; Soricimed Biopharma, Inc.; SQZ Biotechnologies; Sumitomo Dainippon; Taiho Oncology and NCCN; Treadwell Therapeutics; Turnstone Biologics; Tyligand Bioscience, Ltd.; and Virogin Biotech, Ltd}. Amit M. Oza: principal investigator (PI) and steering committees (AstraZeneca, GSK, and Clovis); advisory boards (AstraZeneca, Morphosys); CEO [Ozmosis Research (uncompensated)], institutional support for clinical trials (AstraZeneca, GSK, Esperas Pharm, Clovis, Karyopharm, Ocellaris, Senhwa, Merck, ImmunoGen, Immunovaccine, Pfizer, Aravive, Mersana, Seagen, Alkermes, Verastem, Zentalis, Shuttck Labs, Sutro, NovoCure, Plexxikon, and Amgen). Esteban Rodrigo Imedio: Former employee (AstraZeneca). Sanjeev Kumar: Employee and stock ownership (AstraZeneca). Lone Ottesen: Former employee and stock ownership (AstraZeneca). Ganesh M. Mugundu: Former employee and stock ownership (AstraZeneca). Elza C. de Bruin: Employee and stock ownership (AstraZeneca). Mark J. O’Connor: Employee and stock ownership (AstraZeneca). Suzanne F. Jones: Consulting/advisory role (Amgen) – all payments to institution; stock ownership (HCA Healthcare). David R. Spigel: Institutional funding (AstraZeneca, Genentech/Roche, Novartis, Celgene, Bristol-Myers Squibb, Pfizer, Boehringer Ingelheim, Abbvie, Foundation Medicine, GlaxoSmithKline, Lilly, Merck, Moderna Therapeutics, Nektar, Takeda, Amgen, University of Texas Southwestern Medical Center – Simmons Cancer Center, G1 Therapeutics, Neon Therapeutics, Celldex, Clovis Oncology, Daiichi Sankyo, EMD Serono, Acerta Pharma, Oncogenex, Astellas Pharma, GRAIL, Transgene, Aeglea Biotherapeutics, Tesaro, Ipsen, ARMO BioSciences, and Millennium); consultation (AstraZeneca, TRM Oncology, Precision Oncology, Evelo Therapeutics, Illumina, PharmaMar, Genentech/Roche, Novartis, Celgene, Bristol-Myers Squibb, Pfizer, Boehringer Ingelheim, Abbvie, Foundation Medicine, GlaxoSmithKline, Lilly, and Merck); travel and expenses (AstraZeneca, Genzyme, Intuitive Surgical, Purdue Pharma, Spectrum Pharmaceuticals, Sysmex, EMD Serono, Genentech/Roche, Novartis, Celgene, Bristol-Myers Squibb, Pfizer, Boehringer Ingelheim, Abbvie, Foundation Medicine, GlaxoSmithKline, Lilly, and Merck). Bob T. Li: Uncompensated advisor and consultant (Amgen, AstraZeneca, Boehringer Ingelheim, Bolt Biotherapeutics, Daiichi Sankyo, Genentech, and Lilly); research grants, institution (Amgen, AstraZeneca, Bolt Biotherapeutics, Daiichi Sankyo, Genentech, Hengrui USA, and Lilly); academic travel support (Jiangsu Hengrui Medicine and MORE Health); inventor on two institutional patents at MSK (US62/685,057 and US62/514,661); intellectual property rights as a book author at Karger Publishers and Shanghai Jiao Tong University Press. Ethics Approval and Consent to Participate The institutional review boards of all participating sites approved the study, all patients provided written informed consent, and the study was conducted in accordance with the Declaration of Helsinki. Consent to Publish Not applicable. Availability of Data and Material Data underlying the findings described in this manuscript may be obtained in accordance with AstraZeneca’s data sharing policy described at https://astrazenecagrouptrials.pharmacm.com/ST/Submission/Disclosure. Data for studies directly listed on Vivli can be requested through Vivli at www.vivli.org. Data for studies not listed on Vivli could be requested through Vivli at https://vivli.org/members/enquiries-about-studies-not-listed-on-the-vivli-platform/. AstraZeneca Vivli member page is also available outlining further details: https://vivli.org/ourmember/astrazeneca/. Code Availability Not applicable. Author Contributions Writing – original draft; writing – review and editing: E.P. Hamilton, G.S. Falchook, J.S. Wang, S. Fu, A.M. Oza, E.R. Imedio, S. Kumar, L. Ottesen, G.M. Mugundu, E. de Bruin, M. J. O’Connor, S.F. Jones, D.R. Spigel, and B.T. Li. Investigation: E.P. Hamilton, G.S. Falchook, J.S. Wang, S. Fu, A.M. Oza, and B.T. Li. Methodology: E.R. Imedio, S. Kumar, G.M. Mugundu, S.F. Jones, and D.R. Spigel. Conceptualization: E.R. Imedio, S. Kumar, G.M. Mugundu, M. J. O’Connor, S.F. Jones, and D.R. Spigel. Supervision: E.R. Imedio, S. Kumar, and G.M. Mugundu. Project administration: E.R. Imedio, S. Kumar, and G.M. Mugundu., (© 2024. The Author(s).)