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TPS3157 Background: Kidney associated antigen 1 (KAAG1) is highly and selectively expressed on tumor cell surface, such as ovarian, prostate, and triple negative breast cancers (TNBC), and is rapidly internalized and co-localized with a lysosomal marker, making an ideal candidate for an antibody-drug conjugate (ADC) target. ADCT-901 is an ADC composed of a humanized monoclonal antibody IgG1 against KAAG1, conjugated through a cathepsin-cleavable linker to SG3199, a pyrrolobenzodiazepine (PBD) dimer cytotoxin. In mouse xenograft models of human-derived TNBC, ovarian, and renal cancers significant tumor reduction was observed after a single dose of ADCT-901, providing the rationale for clinical development of a PBD-based ADC to treat KAAG1 expressing tumors (Zammarchi et al, AACR 2019). This study aims to identify the recommended dose and schedule for expansion and to characterize safety and tolerability of ADCT-901 in patients (pts) with selected advanced solid tumors that generally express KAAG1. Methods: ADCT-901-101 is a phase 1, multicenter, 2-part, open-label study that will enroll ̃70 pts (NCT04972981). Part 1: pts will receive escalating doses of ADCT-901 guided by a 3+3 design (1st dose: 15 µg/kg every 3 weeks [Q3W]; highest dose: 290 µg/kg Q3W). Dose escalation will be evaluated by administering the lowest dose to first 3 pts, then increasing/decreasing the dose based on dose-limiting toxicity (DLT) experienced by pts. The dose and schedule of ADCT-901 identified in part 1 will be tested in part 2 to characterize safety, tolerability, and preliminary efficacy of ADCT-901. Primary endpoints include incidence of DLTs (part 1 only), frequency/severity of adverse events (AE) and serious AE, clinically significant changes in vitals, laboratory values, overall tolerability, and frequency of dose interruptions and reductions. Secondary endpoints include overall response rate, duration of response, progression-free and overall survival, pharmacokinetic parameters of ADCT-901 total antibody, PBD-conjugated antibody, unconjugated SG3199 in serum, and frequency of confirmed positive antidrug antibody responses. Exploratory endpoints include tumor modulation and potential pharmacodynamic changes. Key inclusion criteria: pathologic diagnosis of selected solid tumor (cholangiocarcinoma, renal cell carcinoma, ovarian/fallopian tube and prostate cancers, TNBC) locally advanced or metastatic at time of screening, pts refractory or intolerant to existing therapy, tissue biopsy or available tissue sample, ECOG of 0-2, and adequate organ function based on predefined laboratory parameters. Pts with symptomatic CNS metastases and clinically significant third space fluid accumulation will be excluded. The study opened for recruitment in September 2021; enrollment is ongoing. Funding: ADC Therapeutics; medical writing: CiTRUS Health Group. Clinical trial information: NCT04972981.

9029 Background: Identification of new regimens for patients (pts) with relapse/refractory NSCLC is an ongoing need. HDAC inhibitor monotherapy has cytostatic activity in pts with NSCLC. The HDAC6 inhibitor ACY-241 has immunomodulatory activity and may synergize with immune checkpoint inhibitors (ICI). This phase Ib study investigated ACY-241 + nivo in metastatic NSCLC. Methods: Previously platinum-treated, ICI-naïve pts with advanced NSCLC and ECOG PS ≤ 1 were eligible. In 3 + 3 dose-escalation, pts received ACY-241 (180, 360, or 480 mg PO; d 1-28) + nivo 240 mg on d 15 of cycle 1 and d 1,15 subsequently in 28-d cycles. Primary outcomes: safety and RP2D. Secondary outcome: antitumor activity. Exploratory outcome: biomarker analyses; peripheral blood samples and tumor biopsies were collected pre- and on-treatment. Results: As of Jan 17, 2019, 18 pts were treated. Median age 66 y; 56% male, 78% PS 1, 93% stage IV, 28% PD-L1–negative, and 89% adenocarcinoma (11% squamous). No DLTs were observed at ACY-241 180 or 360 mg, and 2 DLTs were observed at 480 mg (gr 3 nausea lasting > 72 h despite medication; gr 5 cardiac arrest); 360 mg was declared RP2D. Common all-grade AEs: fatigue (22%), arthralgia (17%), and cough (17%); 1 gr 3 cerebrovascular accident (related to brain metastasis unrelated to study drug) and 1 additional gr 5 AE (myasthenia gravis, possibly related to ACY-241 and nivo) occurred. Of 13 response-evaluable pts, 8 had clinical benefit (1 CR, 3 PR [1 PD-L1–negative], 1 unconfirmed PR [tumor shrinkage followed by new lesion], 3 SD); 5 had PD. At data cutoff, treatment was ongoing in 5 pts (12 – 30 cycles). Histone and tubulin acetylation levels were transiently increased after treatment. Circulating IL-2, IL-1β, MMP-9, and IL-10 levels increased, with undetectable TNF-α and IFN-γ modulation. Treatment-related changes in tumor-infiltrating Treg, MDSC, TCM, and macrophages varied between pts but revealed a trend of increased infiltrating cytotoxic T and NK cells. Conclusions: The ACY-241 + nivo combination is feasible and can be considered for further research as a potential NSCLC treatment option. Biomarker analyses may help identify an optimal population for the combination. Clinical trial information: NCT02635061.

2547Background: Citarinostat is an oral, selective histone deacetylase 6 inhibitor that inhibits growth of solid tumor cell lines and exhibits potent synergy with Pac in preclinical studies. We con...