With rapid advances in our understanding of cancer, there is an expanding number of potential novel combination therapies, including novel-novel combinations. Identifying which combinations are appropriate and in which subpopulations are among the most difficult questions in medical research. We conducted a Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)-guided systematic review of trials of novel-novel combination therapies involving immunotherapies or molecular targeted therapies in advanced solid tumors. A MEDLINE search was conducted using a modified Cochrane Highly Sensitive Search Strategy for published clinical trials between July 1, 2017, and June 30, 2020, in the top-ranked medical and oncology journals. Trials were evaluated according to a criterion adapted from previously published Food and Drug Administration guidance and other key considerations in designing trials of combinations. This included the presence of a strong biological rationale, the use of a new established or emerging predictive biomarker prospectively incorporated into the clinical trial design, appropriate comparator arms of monotherapy or supportive external data sources and a primary endpoint demonstrating a clinically meaningful benefit. Of 32 identified trials, there were 11 (34%) trials of the novel-novel combination of anti-programmed death 1 (PD-1)/programmed death ligand 1 (PD-L1) and anti-cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) therapy, and 10 (31%) trials of anti-PD-1/PD-L1 and anti-vascular endothelial growth factor (VEGF) combination therapy. 20 (62.5%) trials were phase II trials, while 12 (37.5%) were phase III trials. Most (72%) trials lacked significant preclinical evidence supporting the development of the combination in the given indication. A majority of trials (69%) were conducted in biomarker unselected populations or used pre-existing biomarkers within the given indication for patient selection. Most studies (66%) were considered to have appropriate comparator arms or had supportive external data sources such as prior studies of monotherapy. All studies were evaluated as selecting a clinically meaningful primary endpoint. In conclusion, designing trials to evaluate novel-novel combination therapies presents numerous challenges to demonstrate efficacy in a comprehensive manner. A greater understanding of biological rationale for combinations and incorporating predictive biomarkers may improve effective evaluation of combination therapies. Innovative statistical methods and increasing use of external data to support combination approaches are potential strategies that may improve the efficiency of trial design. Designing trials to evaluate novel-novel combination therapies presents numerous challenges to demonstrate efficacy in a comprehensive manner. A greater understanding of biological rationale for combinations and incorporating predictive biomarkers may improve effective evaluation of combination therapies. Innovative statistical methods and increasing use of external data to support combination approaches are potential strategies that may improve the efficiency of trial design., Competing Interests: Competing interests: ACT reports consultant or advisory roles for Amgen. SJB reports research grants from Incyte, Eli Lilly, Novocure, GSK; consultant or advisory roles for Bayer and Novocure; and US patent for 'Combination therapies of EGFRvIII chimeric antigen receptors and PD-1 inhibitors'. PYW reports consultant or advisory roles for Agios, Astra Zeneca, Bayer, Boston Pharmaceuticals, CNS Pharmaceuticals, Elevate Bio Immunomic Therapeutics, Imvax, Karyopharm, Merck, Novartis, Nuvation Bio, Vascular Biogenics, VBI Vaccines, Voyager and QED; research funding from Agios, Astra Zeneca/Medimmune, Beigene, Celgene, Eli Lily, Genentech/Roche, Kazia, MediciNova, Merck, Novartis, Nuvation Bio, Oncoceutics, Vascular Biogenics and VBI Vaccines; is an editor for UpToDate and Elsevier. ML reports consultant or advisory roles for Tocagen, SQZ Technologies, VBI, InCephalo Therapeutics and Pyramid Bio; non-research consulting roles for Stryker; research support from Arbor, Bristol-Myers Squibb, Accuray, DNAtrix, Tocagen, Biohaven and Kyrin-Kyowa; and has patents for focused radiation+checkpoint inhibitors and local chemotherapy+checkpoint inhibitors. MP reports consultant or advisory roles for non-financial support from Roche, personal fees and non-financial support from Bayer, personal fees from Novartis, personal fees from Apogenix, non-financial support from Pfizer, personal fees from Affiris outside the submitted work. In addition, MP has a patent EP2753315B1 licensed to Bayer, a patent EP2800580B1 issued, a patent US20180155403A1 pending, a patent US20180246118A1 pending, a patent US20170254803A1 pending, a patent WO2018146010A1 licensed to Bayer, a patent WO2019101643A1 licensed to Bayer, a patent WO2019101647A1 licensed to Bayer, a patent WO2019101641A1 licensed to Bayer, and a patent WO2019101642A1 licensed to Bayer. HC reports consultant or advisory roles for Best Doctors/Teladoc, Karyopharm Therapeutics, Private Health, Orbus, Bayer, Forma Therapeutics and Adastra Pharmaceuticals; research funding (Inst) from Newlink Genetics, Plexxikon, Kadmon, Orbus, Merck, DNATrix, Abbvie, Beigene, Forma Therapeutics, GCAR, Array BioPharma, Karyopharm, Nuvation Bio, Bayer and Bristol Meyer Squibb. DMA reports stock and other ownership interests in Diverse Biotech; consulting or advisory roles for Istari Oncology and Jackson Laboratory for Genomic Medicine; patents, royalties, other intellectual property for 'Methods for predicting tumor response to immunotherapy, U.S. Provisional application no. 62/787' and 'Methods for predicting tumor response to immunotherapy, U.S. Provisional application no. 62/620,577'; and expert testimony for Tanoury, Nauts, McKinney & Gabarino, PLLC. WW reports consultant or advisory roles (Inst) for Agios, Bayer, MSD, Novartis and Roche; research funding (Inst) from Apogenix, Boehringer Ingelheim, Pfizer and Roche. SMC reports research funding (Inst) from Agios. EG reports consultant or advisory roles (Inst) for MedImmune, Inc., Agios Pharmaceuticals and Karyopharm; consultant or advisory roles for Gradalis, Inc. and Kiyatec, Inc.; research funding (Inst) from MedImmune, Inc., Tracon, Genentech and Bristol-Myers Squibb. MPM reports consultant or advisory roles for Zap, Mevion, Karyopharm, Tocagen and Astra-Zeneca; and Board of Directors options from Oncoceutics. ABH reports consultant or advisory roles for Caris Life Sciences and WCG Oncology; royalties on licensed intellectual property from Celldex Therapeutics and DNAtrix; research funding from Celularity, Carthera, Codiak, and Moleculin. DAR reports consultant or advisory roles for Abbvie, Advantagene, Agenus, Amgen, Bayer, Bristol-Myers Squibb, Celldex, DelMar, EMD Serono, Genentech/Roche, Imvax, Inovio, Medicenna Biopharma, Inc., Merck, Merck KGaA, Monteris, Novocure, Oncorus, Oxigene, Regeneron, Stemline, Sumitono Dainippon Pharma and Taiho Oncology, Inc.; honoraria from Abbvie, Advantagene, Agenus, Bristol-Myers Squibb, Celldex, EMD Serono, Genentech/Roche, Imvax, Inovio, Medicenna Biopharma, Inc., Merck, Merck KGaA, Monteris, Novocure, Oncorus, Oxigene, Regeneron, Stemline, Sumitono Dainippon Pharma and Taiho Oncology, Inc.; research funding (Inst) from Acerta Phamaceuticals, Agenus, Celldex, EMD Serono, Incyte, Inovio, Omniox and Tragara. SS reports consultant or advisory roles for Merck and Boehringer Ingelheim; research funding from Merck, Bristol-Myers Squibb, and Brooklyn ImmunoTherapeutics. RJS reports research funding (Inst) from Bayer, Astra Zeneca, Pfizer, Astella, Roche, Merck, Abbvie and Amgen. SJA reports consultant or advisory roles for Achilles, Amgen, AstraZeneca, Bristol-Myers Squibb, Caris Life Sciences, CBMG, Celsius Therapeutics, G1 Therapeutics, GlaxoSmithKline, Memgen, Merck, Nektar, RAPT Therapeutics, Venn, Glympse and Samyang; data review committee for EMD Serano. DB is co-owner of Berry Consultants, LLC, a company that designs adaptive Bayesian clinical trials for pharmaceutical and medical device companies, NIH cooperative groups, patient advocacy groups, and international consortia. MK reports consultant or advisory roles for Janssen, AbbVie, Ipsen, Pfizer Roche, and Jackson Laboratory for Genomic Medicine; research funding from AbbVie, Bristol-Myers Squibb, and Specialized Therapeutics. AM, SK and VKP report no conflicts of interest., (© Author(s) (or their employer(s)) 2021. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)