Exhibit 99.1

Title: Results of an ongoing Phase 1/1b study of SBT6050 as monotherapy and combined with pembrolizumab in patients with advanced HER2-expressing or amplified solid tumors

Samuel J. Klempner¹, Muralidhar Beeram², Dhanusha Sabanathan³, Arlene Chan⁴, Erika Hamilton⁵, Sherene Loi⁶, Do-Youn Oh⁷, Leisha A. Emens⁸, Amita Patnaik², Jeong Eun Kim⁹, Yeon Hee Park¹⁰, Valerie Odegard¹¹, Sue Hamke¹¹, Graham Jang¹¹, Celine Jacquemont¹¹, Naomi Hunder¹¹, Sarina Piha-Paul¹²

¹Massachusetts General Hospital, Boston, MA, USA; ²The Start Center for Cancer Care, San Antonio, TX, USA; ³Macquarie University Hospital Clinical Trials Unit, Sydney, NSW, Australia; ⁴Breast Cancer Research Centre - WA, Nedlands, WA, Australia; ⁵Sarah Cannon Research Institute/Tennessee Oncology, Nashville, TN, USA; ⁶Peter MacCallum Cancer Centre, Melbourne, VIC, Australia; ⁷Seoul National University Hospital, Seoul, South Korea; ⁸University of Pittsburgh Medical Center Hillman Cancer Center, Pittsburgh, PA, USA; ⁹Asan Medical Center, Seoul, South Korea; ¹⁰Samsung Medical Center, Seoul, South Korea; ¹¹Silverback Therapeutics, Inc., Seattle, WA, USA; ¹²MD Anderson Cancer Center, Houston, TX, USA

Background

SBT6050 comprises a TLR8 agonist linker-payload conjugated to a HER2-directed antibody and is designed to activate myeloid cells in tumors expressing moderate to high levels of HER2. TLR8 is expressed in myeloid cell types prevalent in human tumors and TLR8 agonism can activate a broad spectrum of anti-tumor immune mechanisms.

Methods

This FIH, open-label, multicenter, dose-escalation and expansion study is evaluating SBT6050 alone and in combination with pembrolizumab (NCT04460456). Pts have HER2-expressing or -amplified solid tumors with progression following therapies known to confer clinical benefit. A single-agent dose-escalation (P1) is followed by tumor-specific expansion cohorts at the RP2D (P2). A pembrolizumab combination dose-escalation (P3) is followed by an expansion cohort at the RP2D (P4).

Results

As of 4 April 2021, 18 pts across multiple tumor types were treated at 4 dose levels (P1, n=14; P3, n=4). Dose levels of SBT6050 ranging from 0.15 mg/kg to 0.6 mg/kg, the predicted RP2D based on preclinical studies, were each pharmacologically active and demonstrated a greater than 70-fold difference in SBT6050 serum exposure. Induction of blood-based biomarkers associated with myeloid cell and NK or T lymphocyte activation, consistent with SBT6050 MOA, were observed at each dose level, generally increasing in a dose-dependent manner. The most frequent (>25%) related TEAEs were chills, diarrhea, fatigue, hypotension, injection site reaction, nausea, pyrexia, and vomiting. Dose levels >0.6 mg/kg were evaluated to assess the upper limits of the dose range; G3 DLTs were observed in P1 at 1.2 mg/kg Q2 wks. In response-evaluable pts (N=14), best overall response was PR (n=1), SD (n=3), and PD (n=9).

Conclusions

Based on preliminary safety data, SBT6050 given alone or in combination with pembrolizumab has a manageable safety profile. Related TEAEs are consistent with immune activation. Evidence of pharmacological activity was observed at each dose level evaluated. A single-agent dose of 0.6 mg/kg Q2 wks had a tolerable safety profile and had drug exposure and pharmacodynamic activity similar to preclinical doses associated with activity. P1 and P3 are ongoing.