Background: Both innate and adaptive immune responses are important components of anticancer immunity. The CD47-SIRPα interaction could represent an important pathway used by tumour cells to evade immune surveillance. We aimed to evaluate the safety, pharmacokinetics, pharmacodynamics, and anticancer activity of evorpacept (also known as ALX148), a high-affinity CD47-blocking protein with an inactive IgG Fc region in patients with solid tumours., Methods: We did a first-in-human, open-label, multicentre, phase 1 dose-escalation and dose-expansion study at nine hospitals and one clinic in the USA and Korea. Eligible patients for the dose-escalation and safety lead-in phases were aged 18 years or older with histological or cytological diagnosis of advanced or metastatic solid tumours with no available standard therapy, measurable or unmeasurable disease according to the Response Evaluation Criteria in Solid Tumors version 1.1, and an Eastern Cooperative Oncology Group performance status score of 0 or 1. In the dose-escalation phase, which used a 3 + 3 design, patients received intravenous evorpacept at either 0·3, 1, 3, or 10 mg/kg once per week in 21-day cycles, or 30 mg/kg once every other week in 28-day cycles. In the safety lead-in phase, patients were given the maximum tolerable dose of evorpacept from the dose-escalation phase plus either intravenous pembrolizumab (200 mg administered once every 3 weeks) or intravenous trastuzumab (8 mg/kg loading dose followed by 6 mg/kg once every 3 weeks). In the dose-expansion phase, additional patients aged 18 years or older with second-line or later-line advanced malignancies were enrolled into three parallel cohorts: those with head and neck squamous cell carcinoma (HNSCC) and those with non-small-cell lung cancer (NSCLC) were given the maximum tolerated dose of evorpacept plus intravenous pembrolizumab (200 mg administered once every 3 weeks), and patients with HER2-positive gastric or gastroesophageal junction cancer were given the maximum tolerated dose of evorpacept plus intravenous trastuzumab (8 mg/kg loading dose followed by 6 mg/kg once every 3 weeks) until disease progression, voluntary withdrawal from the study, or unacceptable toxicity. The primary endpoint was the maximum tolerated dose of evorpacept administered as a single agent and in combination with pembrolizumab or trastuzumab, measured by the occurrence of dose-limiting toxicities during the first cycle, and was assessed in all patients who had received at least one dose of evorpacept. Secondary outcomes included the safety, tolerability, and antitumour activity of evorpacept, alone or in combination with pembrolizumab or trastuzumab. The primary outcome, safety, and tolerability were assessed in all patients who had received at least one dose of evorpacept, and antitumour activity was assessed in those who recieved at least one dose of study treatment and underwent at least one post-baseline tumor assessment. This trial is registered with ClinicalTrials.gov, NCT03013218., Findings: Between March 6, 2017, and Feb 21, 2019, 110 patients received single-agent evorpacept (n=28), evorpacept plus pembrolizumab (n=52), or evorpacept plus trastuzumab (n=30), and were included in the safety analysis. Median follow-up was 29·1 months (95% CI not calculable [NC]-NC) in the single-agent cohort, 27·0 months (25·1-28·8) in the evorpacept plus pembrolizumab cohort, and 32·7 months (27·0-32·7) in the evorpacept plus trastuzumab cohort. Two (7%) dose-limiting toxicities in the first cycle were reported in patients who received single-agent evorpacept; neutropenia with an associated infection in one patient with gastroesophageal junction cancer who received 3 mg/kg once per week, and thrombocytopenia with associated bleeding in one patient with pancreatic cancer who received 30 mg/kg once every other week. No maximum tolerated dose was reached; the maximum administered doses were 10 mg/kg once per week or 30 mg/kg once every other week. The 10 mg/kg once per week dose was used in the expansion cohorts in combination with pembrolizumab or trastuzumab. The most common grade 3 or worse treatment-related adverse events were thrombocytopenia with single-agent evorpacept (two [7%] patients) and evorpacept plus pembrolizumab (two [4%]), and thrombocytopenia (two [7%]) and neutropenia (two [7%]) with evorpacept plus trastuzumab. In patients who received single-agent evorpacept, four treatment-related serious adverse events were reported. Five serious treatment-related adverse events related to evorpacept plus pembrolizumab were reported, and one serious adverse event related to evorpacept plus trastuzumab was reported. In response-evaluable patients in the dose-escalation phase (n=15) receiving single-agent evorpacept once per week, four (27%) had a best overall response of stable disease (two received 0·3 mg/kg, one received 3 mg/kg, and one received 10 mg/kg); in the 11 patients who received single-agent evorpacept at the highest dose of 30 mg/kg once every other week, two (18%) had stable disease. In the dose-expansion cohort, overall responses were recorded in four (20·0%; 95% CI 5·7-43·7) of 20 patients with HNSCC who received evorpacept plus pembrolizumab, in one (5·0%; 0·1-24·9) of 20 patients with NSCLC who received evorpacept plus pembrolizumab, and in four (21·1%; 6·1-45·6) of 19 patients with gastric or gastroesophageal junction cancer who received evorpacept plus trastuzumab., Interpretation: The safety findings support the use of evorpacept in combination with pembrolizumab or trastuzumab for patients with advanced solid tumours. Preliminary antitumour activity results support future investigation of evorpacept combined with pembrolizumab or trastuzumab in patients with HNSCC, gastric or gastroesophageal junction cancer, and NSCLC., Funding: ALX Oncology., Competing Interests: Declaration of interests NJL reports research funding from ALX Oncology, Ascentage, BeiGene, Constellation Pharma, Forty Seven, Alpine, Merck, Pfizer, Regeneron, Apexian, Formation Biologics (Forbius), Symphogen, CytomX, InhibRx, Incyte, Jounce, Livzon, Northern Biologics, Innovent Biologics, Ikena, Odonate, Loxo, Alpine Biosciences, Mersana, Astellas, Celgene, Seagen, Samumed, Sapience Therapeutics, Epizyme, and Mersana; and personal fees from Innovent Biologics, all outside the submitted work. LQMC reports receiving research funding from ALX Oncology; participation on an advisory board for Merck; and receipt of medical writing services from Merck, all outside the submitted work. JFG reports research support from Bristol-Myers Squibb, Tesaro, Moderna, Blueprint, Jounce, Array, Merck, Adaptimmune, Novartis, and ALX Oncology; consulting fees from Genentech-Roche, Bristol-Myers Squibb, Takeda, Loxo-Lilly, Blueprint, Oncorus, Regeneron, Gilead, Moderna, AstraZeneca, EMD Serono, Pfizer, Novartis, Merck, GlydeBio, and Karyopharm; payment or honoraria from Pfizer for lectures, presentations, speakers bureaus, manuscript writing, or educational events; and having a spouse who owns stock and has other ownership interests in Ironwood Pharmaceuticals, all outside the submitted work. PL reports participation on advisory boards or data safety monitoring boards for AbbVie, Agios, Five Prime, GenMab, Halozyme, Roche-Genentech, Genentech, CytomX, Takeda, Cybrexa, Agenus, Tyme, IQVIA, TRIGR, Pfizer, ImmunoMet, Black Diamond, GlaxoSmithKline, QED Therapeutics, AstraZeneca, EMD Serono, Shattuck, Astellas, Salarius, Silverback, MacroGenics, Kyowa Kirin Pharmaceutical Development, Kineta, Zentalis Pharmaceuticals, Molecular Templates, ABL Bio, STCube Pharmaceuticals, Bayer, and I-Mab; and was a consultant for SOTIO, SK Life Science, and I-Mab, all outside the submitted work. K-WL reports research support to their institution from ALX Oncology, AstraZeneca, Ono Pharmaceutical, Merck Sharp & Dohme, Merck, Pfizer, BeiGene, Astellas Pharma, Zymeworks, Five Prime Therapeutics, Macrogenics, Seagen, Bolt Biotherapeutics, Trishula Therapeutics, Oncologie, Pharmacyclics, LSK BioPharma, MedPacto, Green Cross, ABLBIO, Y-BIOLOGICS, Genexine, Daiichi Sankyo, Taiho Pharmaceutical, InvetisBio, Leap Therapeutics; and personal fees from ISU ABXIS, Bayer, Daiichi Sankyo, Bristol-Myers Squibb (consultation), Ono Pharmaceutical, and Boryung Pharmaceutical (honorarium), all outside the submitted work. JL reports receiving consulting fees from Mirati, Seattle Genetics, Oncxerna, and AstraZeneca, all outside the submitted work. Y-JB reports receiving research support paid to their institution from AstraZeneca, Novartis, Genentech-Roche, Merck Sharp & Dohme, Bayer, Merck Serono, Bristol-Myers Squibb, GlaxoSmithKline, Pfizer, Eli Lilly, Curis, Taiho, Takeda, Ono Pharmaceutical, Daiichi Sankyo, Astellas, BeiGene, Green Cross, and Genexine; and consulting fees from AstraZeneca, Novartis, Genentech-Roche, Merck Sharp & Dohme, Bristol-Myers Squibb, Merck Serono, Daiichi-Sankyo, Astellas, BeiGene, Green Cross, Samyang Biopharm, Hanmi, and ALX Oncology, all outside the submitted work. FSH reports research support to their institution from ALX Oncology, Bristol-Myers Squibb, Novartis, and the National Institutes of Health; consulting fees from Bristol-Myers Squibb, Novartis, EMD Serono, Genentech, Sanofi, Bicara, Apricity, Surface, Compass, Aduro, Pionyr, 7 Hills Pharma, Checkpoint, Bioentre, Gossamer, Iovance, Trillium, Catalym, Immunocore, Amgen, and Rheos; and ownership of stock or stock options in Pionyr, Apricity, Bicara, Checkpoint, and Torque, all outside the submitted work. RS-D reports receiving grants or contracts through his institution from ALX Oncology outside the submitted work. PF reports employment at ALX Oncology, and owning stock or stock options in ALX Oncology. PS is employed by IDDI, and reports research support from ALX Oncology outside the submitted work. FJ reports having a consulting or advisory role at ALX Oncology. HIW reports employment at ALX Oncology, receiving support for travel expenses from ALX Oncology, and owning stock and having other ownership interests in ALX Oncology outside the submitted work. JP reports employment at ALX Oncology, owning stock and having other ownership interests in ALX Oncology, research funding from ALX Oncology, patents planned, issued, or pending at ALX Oncology, and reports a leadership or fiduciary role at Tallac Therapeutics, all outside the submitted work. SSR is employed by and is on the board of directors at ALX Oncology, and reports owning stock and having other ownership interests in ALX Oncology, all outside the submitted work. WAM reports receiving research funding from ALX Oncology outside the submitted work. All other authors declare no competing interests., (Copyright © 2021 Elsevier Ltd. All rights reserved.)