Background: KB-0742 is a potent, and selective, oral inhibitor of CDK9 being evaluated in a phase I/II study in patients with transcriptionally addicted advanced solid tumors (NCT04718675). Interim data from the first 4 dose levels were presented previously noting manageable safety (MTD not reached), a 24-hour plasma half-life, linear PK, CDK9 target engagement in peripheral blood mononuclear cells (PBMCs), and anti-tumor activity in patients with transcription factor fusion (TFF) driven sarcomas. Here we present updated KB-0742 safety, pharmacokinetics (PK), pharmacodynamic data (PD) and anti-tumor activity for patients from the ongoing dose escalation through 5 dose levels and 60 mg expansion. Methods: Study objectives include evaluation of safety, tolerability, PK, PD, and identification of KB-0742 MTD and RP2D. KB-0742 is administered orally once daily for 3 consecutive days followed by 4 days off, weekly in 28-day cycles, until unacceptable toxicity or disease progression. Eligible patients were enrolled in 5 escalation cohorts (10, 20, 40, 60 and 80 mg) or 60 mg dose expansion. Eligibility criteria include age >18 years, relapsed or refractory solid tumors, and ECOG PS < 2. PD is assessed in (PBMCs) and tumor tissue from pre- and on-treatment biopsy samples. Results: As of January 4, 2024, 112 patients were enrolled, 42 in dose escalation and 70 in expansion. Patients received a median of 3 lines of prior therapy. The most common tumor types enrolled were soft tissue sarcoma (STS) (n=36; 18 TFF positive) and adenoid cystic carcinoma (ACC) (n=18). Treatment-emergent adverse events occurring in >15% of patients include nausea, vomiting, anemia, fatigue, diarrhea, and constipation; none assessed as grade 4 or 5. The most common reason for treatment discontinuation was disease progression (54.5%). Across 5 dose levels, PK remains linear with a terminal half-life of 24 hours. At 60 mg, evidence of target engagement was observed in post-treatment paired tumor tissue biopsies. Within STS, TFF positive patients displayed a trend towards improved outcomes vs. those without a TFF with a disease control rate (DCR) of 42.8% vs. 29.4%, and one partial response was observed in a patient with TFF positive myxoid liposarcoma at 60mg. The best observed response was durable stable disease (SD) yielding a DCR of 53.8% in ACC (n=18), and 83% in NSCLC (n=6). Two patients (MYCL1+ ovarian, NSCLC) with prolonged SD (>140 days) continue treatment on 60mg. Conclusions: KB-0742 treatment at 60 and 80 mg was well tolerated, with manageable toxicity. Achievement of long-term SD and some preliminary anti-tumor efficacy in highly pretreated patients motivates continued enrollment of patients with transcriptionally addicted tumors. Dose escalation and expansion in transcriptionally addicted (MYC amplification/overexpression) or TFF driven tumors continues. Clinical trial information: NCT04718675.