Citations

Abstract Purpose: Prostate-specific membrane antigen (PSMA)–directed therapies provide meaningful clinical benefit in metastatic castration-resistant prostate cancer (mCRPC), yet responses remain limited, underscoring the need for additional biomarkers of PSMA expression heterogeneity. In this study, we explored the relationship between PSMA imaging and PSMA expression on circulating tumor cells (CTC) in our early-phase trial using a PSMA-targeted small molecule. Experimental Design: This phase I study investigated EC1169, a small molecule conjugated to a tubulysin analog warhead, and 99mTc–EC0652, a PSMA imaging agent. Part A (dose escalation) identified the recommended phase II dose. Part B (dose expansion) assessed radiographic progression-free survival (rPFS) as its primary endpoint. We enrolled and treated 103 patients with mCRPC. Most part B patients underwent 99mTc–EC0652 PSMA imaging. A CTC assay assessed PSMA-positive CTCs and their association with response and PSMA imaging. Results: 99mTc–EC0652 single-photon emission computed tomography (SPECT) imaging demonstrated increased sensitivity for detecting bone lesions compared with standard scans (computed tomography/bone scans). Using an optimized CTC assay, we observed that patients with a decrease in PSMA+ CTCs at baseline versus C3D1 displayed a longer rPFS (8 vs. 2.9 months; P = 0.04). Importantly, patients with predominantly PSMA-positive disease on 99mTc–EC0652 imaging also harbored PSMA-negative CTCs, with a subset displaying neuroendocrine prostate cancer–like morphology. Conclusions: Although EC1169 showed limited activity, CTC and imaging analyses showed significant heterogeneity in PSMA expression on CTCs in patients with predominantly PSMA-positive lesions on SPECT. Our study highlights the importance of assessing both PSMA-based CTC and imaging assays in future validation trials.

8501 Background: TROPION-Lung02 (NCT04526691) evaluated the TROP2-directed antibody-drug conjugate (ADC) Dato-DXd plus pembro combination with or without Pt-CT in aNSCLC. Here we report primary analyses of pts receiving combination therapy in the 1L setting. Methods: Pts across 6 cohorts were dosed with Dato-DXd (4 or 6 mg/kg) plus pembro 200 mg alone (doublet) or with pembro plus Pt-CT (triplet; cisplatin 75 mg/m 2 or carboplatin AUC 5) Q3W. PD-L1 expression (tumor proportion score) was assessed locally by immunohistochemistry (22C3 assay). Primary objectives were safety and tolerability; efficacy was a secondary objective. Results: As of Apr 29, 2024, 96 pts received either the doublet (n=42) or triplet (n=54) combination as 1L therapy; 29% and 15% of pts were ongoing, respectively. Median ages were 65 (doublet) and 64 years (triplet). Median treatment durations were 9.7 and 5.8 months, respectively. Stomatitis (doublet, 57%; triplet, 33%) and nausea (doublet, 42%; triplet, 48%), primarily Gr 1–2, were the most common adverse events (AEs) across both regimens. Treatment related serious AEs occurred in 5 (12%) and 12 (22%) pts in each cohort and no deaths related to study drug were seen. Efficacy outcomes, including by histology, are summarized in the Table. Biomarker analyses, including efficacy by PD-L1 status, will be presented. Conclusions: In this largest data set to date evaluating an ADC combined with an anti-PD-1/L1 agent in the 1L setting, the combination of Dato-DXd plus pembro treatment both with and without Pt-CT elicited durable antitumor activity in pts with aNSCLC. Tolerability of the combinations was as expected, based on known profiles of the individual agents. Clinical trial information: NCT04526691 . All 1L (n=96) 1L, Nonsquamous (n=75) 1L, Squamous (n=21) Response, n (%) Doublet (n=42) Triplet (n=54) Doublet (n=33) Triplet (n=42) Doublet (n=9) Triplet (n=12) Confirmed objective response rate 23 (55) 30 (56) 17 (52) 24 (57) 6 (67) 6 (50) Complete response 1 (2) 2 (4) 1 (3) 2 (5) 0 0 Partial response 22 (52) 28 (52) 16 (49) 22 (52) 6 (67) 6 (50) Stable disease 14 (33) 18 (33) 12 (36) 14 (33) 2 (22) 4 (33) Progressive disease 3 (7) 2 (4) 3 (9) 1 (2) 0 1 (8) Disease control rate a 37 (88) 48 (89) 29 (88) 38 (91) 8 (89) 10 (83) Median duration of response, mo (95% CI) 20.1 (9.7–NE) 13.7 (5.7–NE) 24.9 (9.7–NE) 18.0 (8.0–NE) 12.0 (5.5–NE) 5.5 (4.1–NE) Median PFS, mo (95% CI) 11.2 b (8.2–21.3) 6.8 c (5.5–11.1) 11.2 (6.1–21.3) 10.8 (5.5–17.3) 10.2 (0.4–NE) 6.7 (1.0–8.2) a Proportion of pts with confirmed CR + PR + SD at 12 wks. b.c Median (95% CI) PFS follow-up, mo: b 17.3 (11.3−26.8); c 23.5 (17.3−27.9). mo, months; NE, not evaluable.

TPS3164 Background: TUB-030 is a novel antibody-drug conjugate (ADC) targeting 5T4, an oncofetal antigen expressed in various solid tumors with limited expression in healthy tissues. TUB-030 leverages optimized biophysical properties, an effector-silenced antibody, and an exatecan payload to maximize the therapeutic index and minimize off-target toxicities. Preclinical studies demonstrated potent anti-tumor activity, including long-lasting tumor regression at doses as low as 1 mg/kg and durable responses even in tumors with low 5T4 expression. Methods: 5-STAR 1-01 is a multicenter, first-in-human dose escalation and dose optimization Phase I/IIa clinical trial designed to investigate safety, tolerability, pharmacokinetics (PK), and efficacy of the anti-5T4 ADC TUB-030 in patients with advanced and metastatic solid tumors. Eligible patients have one of the following tumor types: head and neck squamous cell carcinomas (HNSCC), non-small-cell lung cancer (NSCLC), small cell lung cancer, pleural mesothelioma, triple-negative breast cancer, HR+/HER2- breast cancer, esophageal cancer, gastric cancer, pancreatic adenocarcinoma, colorectal cancer, bladder cancer, prostate cancer, cervical cancer, osteosarcoma, or soft tissue sarcomas and must have exhausted available standard-of-care therapies. Phase I is an open-label, single-arm dose escalation trial, with administration every 21 days. Dose escalation follows an accelerated titration design (ATD) transitioning to Bayesian optimal interval (BOIN) upon predefined toxicity thresholds. Backfill cohorts are planned in NSCLC and HNSCC to further evaluate the safety and efficacy profile at, or near, the maximum tolerated dose (MTD). Primary endpoints include safety and tolerability of TUB-030 as monotherapy, determination of the MTD and the recommended phase II doses; secondary endpoints assess pharmacokinetics, immunogenicity, and preliminary clinical activity using RECIST v1.1 criteria. Exploratory endpoints include analysis of circulating tumor DNA. In phase IIa, dose-optimization will evaluate two dose levels in select indications in order to identify the optimal dose for further development. Enrollment of approximately 130 patients across the US and Canada is planned, with dose escalation currently underway. This study investigates TUB-030, a novel 5T4 targeted ADC as a therapy for advanced/metastatic solid tumors. Clinical trial information: NCT06657222 .

8617 Background: Dato-DXd is an antibody-drug conjugate (ADC) composed of an anti-TROP2 monoclonal antibody linked to a potent topoisomerase I inhibitor payload. We report updated findings from TROPION-Lung02 (NCT04526691) evaluating Dato-DXd plus pembro with or without Pt-CT as 1L therapy for aNSCLC. Methods: Patients (Pts) received Dato-DXd (4 or 6 mg/kg) plus pembro 200 mg alone (doublet) or with Pt-CT (triplet; cisplatin 75 mg/m2 or carboplatin AUC 5) Q3W across 6 cohorts. Pts in the expansion stage were primarily treatment (tx)-naïve. PD-L1 expression was evaluated by tumor proportion score (TPS < 50% or ≥50%) using immunohistochemistry (22C3 assay). The primary objectives were safety and tolerability, with efficacy a secondary objective. Results: 96 pts were treated with doublet (n = 42) or triplet (n = 54) therapy in the 1L setting. At data cutoff (Oct 31, 2023), 45% and 20% of pts receiving doublet or triplet tx were ongoing. Median age was 66 years in both subgroups. Median treatment durations were 6.6 and 5.8 months. Across doublet and triplet tx, the most common any-grade (Gr) treatment-emergent adverse events (TEAEs) were stomatitis (57%, 33%) and nausea (40%, 46%), mainly Gr 1–2. Gr ≥3 TEAEs were seen in 57% and 76% of pts; serious TEAEs in 38% and 44%. TEAEs associated with Dato-DXd discontinuation occurred in 29% and 39% of pts receiving doublet or triplet tx. TEAEs associated with death were seen in 2% and 9%, none considered related to study drug. Efficacy outcomes, including ORR and preliminary PFS, in the 1L setting overall and by PD-L1 status are summarized in the Table. Conclusions: As 1L therapy for aNSCLC, Dato-DXd plus pembro with or without Pt-CT continues to demonstrate durable antitumor activity. Efficacy was observed regardless of PD-L1 expression. Tolerability of the combinations was as expected based on known profiles of the individual agents, with no new safety signals observed. To date, this is the largest dataset reported for any ADC combined with an anti-PD-1/L1 agent in the 1L setting for aNSCLC. Clinical trial information: NCT04526691 . [Table: see text]

TPS3173 Background: ELU001, pasifolate exatecan, is an ultra-small nanoparticle drug conjugate, known as a CDC (~6 nm), designed to target and penetrate into solid tumors. Due to its size, ELU001 exhibits rapid renal elimination which is expected to reduce or eliminate the toxicities associated with many antibody drug conjugates. ELU001 has ~13 folic acid targeting moieties and ~21 cathepsin-B cleavable exatecan topoisomerase-1 inhibitor payloads covalently bound to short polyethylene glycol chains surrounding a silica core, the C’Dot. ELU001 binds to cancer cells expressing FRα and is internalized into the lysosome where it releases the exatecan payload. ELU001’s dose escalation study reported at ESMO Congress 2023 (1), examined weekly, bi-weekly (Q2W), and tri-weekly schedules over a range of doses in 42 patients. The study revealed that ELU001 could be administered safely and identified a dose level and schedule for the Expansion Studies described below. 22 of the patients with ovarian or endometrial cancers had >1 post-baseline scan(s), of these 2 had a PR and 19 had SD by RECIST v1.1 criteria. ELU001 demonstrated activity against tumors across a range of FRα expression levels, from low to high, retrospectively assessed using the VENTANA FOLR1 RxDx assay and the PS2+ scoring system. Methods: The current multicenter, Tumor Group Expansion open-label phase 1/2 clinical trial investigates ELU001 in patients with ovarian and endometrial cancers and is enrolling three cohorts; 1) ovarian cancer with high FRα expression, 2) ovarian cancer with moderate and low expression and 3) endometrial cancer with low to high expression with assessments based upon the PS2+ scoring system (high = ≥ 75%, moderate = ≥ 50% to < 75%, low = 25% to < 50%, negative = <25% of tumor cells with 2+ or 3+ staining intensity). An initial exploratory dose of 2.0 mg/m2 Q2W was employed in all three cohorts and a second dose of 1.7 mg/m2 Q2W will be studied in the ovarian cancer cohorts. Patients will ultimately be randomized to one of two or more doses to optimize dosing for registration. Primary endpoint: Objective Response Rate (ORR); secondary endpoints include DOR, PFS, TFST, PFS2, OS, adverse events, PK, and ADA assessments. Approximately 20 patients per tumor group will be recruited in the U.S. 1. Ann. Oncol. (2023) 34 (suppl_2): S458-S497. Clinical trial information: NCT05001282 .

9004 Background: Despite advances in first-line (1L) immunotherapy ± CT, most patients (pts) with aNSCLC experience disease progression, necessitating novel strategies. Dato-DXd is an antibody drug conjugate (ADC) composed of a humanized anti-TROP2 IgG1 monoclonal antibody covalently linked to a topoisomerase I inhibitor payload via a plasma-stable tetrapeptide-based cleavable linker. Dato-DXd had encouraging efficacy and manageable safety in heavily pretreated aNSCLC. Dato-DXd + immunotherapy yielded greater preclinical activity than either agent alone. Methods: TROPION-Lung02 (NCT04526691) is a phase 1b, global, dose-escalation and -expansion study evaluating Dato-DXd (4 or 6 mg/kg) + pembro 200 mg ± Pt-CT (cisplatin 75 mg/m2 or carboplatin AUC 5) every 21 days across 6 cohorts. Pts in escalation may have received ≤2 prior lines of therapy for aNSCLC. Pts in expansion were primarily treatment (tx) naive (pts receiving Dato-DXd + pembro may have ≤1 prior Pt-based tx). The primary objective is to assess safety and tolerability, including dose-limiting toxicities (DLTs). Secondary objectives include evaluation of efficacy, pharmacokinetics, and immunogenicity. Results: As of the Oct 31, 2022, data cutoff (DCO), 120 pts were treated. All cohorts met DLT criteria to move to escalation. Median age was 65 years. PD-L1 expression was <1%, 1%-49%, and ≥50% in 40%, 33%, and 26% of pts, respectively. Median tx duration was 4.6 mo, with 55% receiving tx at DCO. The most frequent any-grade (gr) tx-emergent adverse events (TEAEs) were nausea (45%) and stomatitis (45%). Gr ≥3 TEAEs occurred in 61% of pts; most frequent were neutrophil count decreased (8%) and amylase increased (8%). TEAEs that were serious, associated with discontinuation, or associated with death occurred in 31%, 24% (16% associated with Dato-DXd), and 6% of pts, respectively. Dose reductions due to TEAEs associated with Dato-DXd occurred in 17%. Drug-related interstitial lung disease occurred in 12 pts (10%; 9 gr 1/2; 3 gr 3). The objective response rate (ORR) with 1L Dato-DXd + pembro doublet and Dato-DXd + pembro + Pt-CT triplet tx was 60% (95% CI, 36%-81%; 12 [2 unconfirmed]/20) and 55% (95% CI, 39%-70%; 23 [5 unconfirmed]/42), respectively. The ORR for any line of therapy was 38% (95% CI, 25%-54%; 18 [2 unconfirmed]/47) with doublet and 47% (95% CI, 34%-60%; 28 [5 unconfirmed]/60) with triplet tx. In both subsets, the disease control rate was 85% and median duration of response was not reached. Responses were observed in all 3 PD-L1 expression level subgroups. Although immature, median progression-free survival (95% CI) was 10.8 (8.3-15.2) and 7.8 mo (5.5-NE) with doublet and triplet tx, respectively. Conclusions: Dato-DXd + pembro ± Pt-CT demonstrated tolerable safety with notable 1L activity in this first and largest dataset with an ADC + immunotherapy ± Pt-CT in aNSCLC pts. Clinical trial information: NCT04526691 .

TPS3159 Background: ELU001 is a novel first-in-class nanoparticle drug conjugate that consists of ~13 folic acid targeting moieties and a payload of ~22 molecules of the topoisomerase-1 inhibitor, exatecan. Folic acid and exatecan are covalently bound by non-cleavable and cathepsin-B cleavable linkers, respectively, to short polyethylene glycol chains which surround the C’Dot’s silica core. CDCs are very small (~6 nm), allowing for more efficient penetration into solid tumors compared to ADCs and a rapid elimination by the kidneys, which is expected to lead to less toxicity than ADCs that have a longer half-life in circulation. ELU001’s high avidity is designed to promote binding to FRα on the surface of cancer cells that express any level of antigen (low to high). ELU001 then internalizes into the tumor cell and traffics to the lysosome where enzymatic cleavage releases the exatecan payload. ELU001 has demonstrated efficacy in preclinical models with low, moderate and high levels of FRα. The first-in-human trial, ELU-FRα-1, is currently recruiting patients with ovarian, endometrial, colorectal, gastric, gastroesophageal junction, triple negative breast, or non-small cell lung cancers, or cholangiocarcinoma cancers, and, in the opinion of the Investigator, have no meaningful life-prolonging therapy option available. Methods: This is a Phase 1 / 2 multicenter, open label clinical trial with two parts: Part 1 Dose Escalation and Part 2 Tumor Group Expansion Cohort(s). In Part 1, patients with cancer types with a high likelihood of having FRα overexpressing tumors, (ovarian, endometrial, colorectal, gastric, gastroesophageal junction, triple negative breast, or non-small cell lung cancers, or cholangiocarcinoma) will be enrolled. FRα expression is determined retrospectively. ELU001 is administered either weekly (QW) (once a week for 3 weeks, 1 week rest), every other week (Q2W, with no rest between cycles), or every three weeks (Q3W). Initial Part 2 expansion cohorts are planned in ovarian cancer with high FRα expression, ovarian cancer with moderate or low FRα expression, and endometrial cancer with any positive FRα expression. Here, FRα will be prospectively determined. Other cancer types may be assessed in the future. The primary objective for Part 1 is to identify the MTD/RP2D. The primary objective for Part 2 is to determine the ORR. Secondary objectives include DOR, PFS, TFST, PFS2, OS, frequency, severity and tolerability of adverse events, PK, ADA, and FRα expression assessments. Part 1 Dose Escalation will recruit about 2-5 patients at each dose for each regimen (QW; Q2W; Q3W). Part 2 Stage 1 will recruit about 15 patients per tumor group expansion cohort and is expected to begin recruitment in the second half of 2023. The study is enrolling in Part 1 Q2W Cohort 301 and Q3W Cohort A in the US. Clinical trial information: NCT05001282 .

Abstract Background: ELU001 is a novel, first-in-class, new molecular entity described as a C’Dot Drug Conjugate (CDC). ELU001 consists of ~13 folic acid targeting moieties and a payload of ~22 molecules of the topoisomerase-1 inhibitor, exatecan. Folic acid and exatecan are covalently bound by non-cleavable and cathepsin-B cleavable linkers, respectively, to short polyethylene glycol chains which surround the C’Dot’s silica core. CDCs are very small (~6 nm), allowing greater ability to penetrate more efficiently into solid tumors compared to ADCs. CDCs are rapidly eliminated by the kidneys, which is expected to lead to less toxicity than ADCs that have a longer half-life in circulation. ELU001’s high avidity is designed to promote binding to FRα on the surface on FRα overexpressing cancer cells with a wide range of antigen expression including high, moderate and low expressing tumor cells. Following antigen binding, ELU001 internalizes into the tumor cell, and traffics to the lysosome where enzymatic cleavage releases the exatecan payload. The first-in-human trial, ELU-FRα-1, is currently recruiting patients that have advanced, recurrent or refractory tumors associated with indications that are known to potentially overexpress FRα and have been shown to be topoisomerase 1 inhibitor-sensitive, and, in the opinion of the Investigator, have no satisfactory therapeutic options available. Methods: This is a Phase 1/2 multicenter, open label clinical trial with two parts: Part 1 Dose Escalation and Part 2 Tumor Group Expansion Cohort(s). Part 1 is a basket trial enrolling patients with cancer types with a high likelihood of having FRα overexpressing tumors, (specifically, ovarian, endometrial, colorectal, gastric, gastroesophageal junction, triple negative breast, or non-small cell lung cancers, or cholangiocarcinoma). Patients are receiving ELU001 weekly (QW) (once a week for 3 weeks, 1 week rest), every other week (Q2W, with no rest between cycles), or every three weeks (Q3W). Analysis of FRα expression will be retrospectively determined. Part 2 will use Simon’s Two-Stage design to evaluate 4-6 Expansion Cohorts, comprised of tumor histologies anticipated to have greatest activity to ELU001 treatment. FRα will be prospectively determined. The primary objective for Part 1 is to identify the MTD/RP2D. The primary objective for Part 2 is to determine the ORR. Secondary objectives include DOR, PFS, TFST, PFS2, OS, frequency, severity and tolerability of adverse events, PK, ADA, and FRα expression assessments. Part 1 Dose Escalation will recruit about 25 patients per dose regimen (QW; Q2W; Q3W). The first stage of Part 2 (Dose Expansion) will recruit about 15 patients per tumor group expansion cohort. The study is actively enrolling in the US and currently recruiting in Q2W Cohort 201 and Q3W Cohort A. QW Cohorts 1-3 and Q2W Cohort 101 are complete. Clinical trial information: NCT05001282. Citation Format: Wen Wee Ma, Anthony Tolcher, Cesar A. Perez, Douglass Orr, Erika Hamilton, Yujie Zhao, Yonina Murciano-Goroff, Carey Anders, Gregory P. Adams, Catherine W. Reddick, Eliel Bayever. ELU-FRα-1: a study to evaluate ELU001 in patients with solid tumors that overexpress folate receptor alpha (FRα) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT255.

TPS3158 Background: ELU001 is a novel, first-in-class, new molecular entity described as a C’Dot Drug Conjugate (CDC). ELU001 consists of ̃12 folic acid targeting moieties and ̃22 exatecan topoisomerase-1 inhibitor payloads on Cathepsin-B cleavable linkers covalently bound to the surface of each silicon core/polyethylene glycol C’Dot nanoparticle. CDCs are small in size (̃6 nm), have a greater ability to penetrate into and through tumors as compared to ADCs, and are rapidly eliminated by the kidneys. The rapid systemic elimination is expected to lead to less toxicity than is observed with targeting platforms like ADCs that have a longer half-life in circulation. ELU001’s high avidity is believed to promote internalization into FRα overexpressing cancer cells, selectively delivering it’s ̃22 molecules of payload. The first-in-human clinical trial, ELU-FRα-1, is currently recruiting patients that have advanced, recurrent or refractory FRα overexpressing tumors considered to be topoisomerase 1 inhibitor-sensitive based on the literature, and, in the opinion of the Investigator, have no satisfactory therapeutic options available. Methods: This is a Phase 1 / 2 multicenter, open label clinical trial with two parts: Part 1 Dose Escalation and Part 2 Tumor Group Expansion Cohort(s). In Part 1, patients with cancer types with a high likelihood of having FRα overexpressing tumors based on historical data, (specifically, ovarian, endometrial, colorectal, gastric, gastroesophageal junction, triple negative breast, or non-small cell lung cancers, or cholangiocarcinoma), will be enrolled to the study. Patients will receive ELU001 on a weekly dose regimen (QW) (once a week for 3 weeks, 1 week rest) or every other week dose regimen (Q2W, with no rest between cycles). Retrospective analysis of FRα expression will be determined. Part 2 uses a Simon’s Two-Stage design to evaluate 4-6 tumor group expansion cohorts, each consisting of patients with specific tumor types (to be identified based on emerging data) that overexpress FRα (prospectively determined prior to enrollment). The primary objective for Part 1 is to identify the MTD/RP2D and for Part 2 is ORR. Dose Escalation will recruit about 25 patients per dose regimen (QW or Q2W). Dose Expansion will recruit about 15 patients per tumor group expansion cohort. Secondary objectives are anti-tumor activity (DOR, PFS, TFST, PFS2, OS), frequency, severity and tolerability of adverse events, PK, ADA, and FRα expression assessments. The study is actively enrolling in the US – Cohorts 1-2 have been completed without DLT. Enrollment to Cohort 3 began in December 2021. Clinical trial information: NCT05001282 .

Background Availability of checkpoint inhibitors has created a paradigm shift in the management of patients with solid tumors. Despite this, most patients do not respond to immunotherapy, and there is considerable interest in developing combination therapies to improve response rates and outcomes. B7-H3 (CD276) is a member of the B7 family of cell surface molecules and provides an alternative immune checkpoint molecule to therapeutically target alone or in combination with programmed cell death-1 (PD-1)–targeted therapies. Enoblituzumab, an investigational anti-B7-H3 humanized monoclonal antibody, incorporates an immunoglobulin G1 fragment crystallizable (Fc) domain that enhances Fcγ receptor-mediated antibody-dependent cellular cytotoxicity. Coordinated engagement of innate and adaptive immunity by targeting distinct members of the B7 family (B7-H3 and PD-1) is hypothesized to provide greater antitumor activity than either agent alone. Methods In this phase I/II study, patients received intravenous enoblituzumab (3–15 mg/kg) weekly plus intravenous pembrolizumab (2 mg/kg) every 3 weeks during dose-escalation and cohort expansion. Expansion cohorts included non–small cell lung cancer (NSCLC; checkpoint inhibitor [CPI]–naïve and post-CPI, programmed death-ligand 1 [PD-L1] <1%), head and neck squamous cell carcinoma (HNSCC; CPI-naïve), urothelial cancer (post-CPI), and melanoma (post-CPI). Disease was assessed using Response Evaluation Criteria in Solid Tumors version 1.1 after 6 weeks and every 9 weeks thereafter. Safety and pharmacokinetic data were provided for all enrolled patients; efficacy data focused on HNSCC and NSCLC cohorts. Results Overall, 133 patients were enrolled and received ≥1 dose of study treatment. The maximum tolerated dose of enoblituzumab with pembrolizumab at 2 mg/kg was not reached. Intravenous enoblituzumab (15 mg/kg) every 3 weeks plus pembrolizumab (2 mg/kg) every 3 weeks was recommended for phase II evaluation. Treatment-related adverse events occurred in 116 patients (87.2%) and were grade ≥3 in 28.6%. One treatment-related death occurred (pneumonitis). Objective responses occurred in 6 of 18 (33.3% [95% CI 13.3 to 59.0]) patients with CPI-naïve HNSCC and in 5 of 14 (35.7% [95% CI 12.8 to 64.9]) patients with CPI-naïve NSCLC. Conclusions Checkpoint targeting with enoblituzumab and pembrolizumab demonstrated acceptable safety and antitumor activity in patients with CPI-naïve HNSCC and NSCLC. Trial registration number NCT02475213.