Strand Therapeutics Doses First Patients with STX-003, a Systemic Programmable mRNA Therapy Designed for Tumor-Selective Activation

First-in-human study evaluates Strand’s ability to unlock the potential of systemic mRNA-encoded medicines with an IL-12 payload through programmable, tumor-selective expression.

BOSTON--(BUSINESS WIRE)--Strand Therapeutics, a clinical-stage biotechnology company pioneering programmable mRNA therapeutics, today announced that the first patients have been dosed in ASCENT-1, the Phase 1 clinical trial of STX-003. STX-003 is an intravenously delivered self-replicating mRNA therapy designed to selectively express IL-12 within tumors while restricting expression in healthy tissue. The milestone marks Strand’s expansion into systemically delivered programmable mRNA medicines and is the first clinical evaluation of its proprietary SignalLock genetic circuitry designed to control where therapeutic payloads are expressed in the body.

STX-003 expresses interleukin-12 (IL-12), a potent immune-activating protein with well-documented anti-tumor activity that has historically been limited as a systemic therapy by its narrow therapeutic window. Strand’s programmable approach is designed to separate where a medicine goes from where it is active. SignalPath delivery technology carries the mRNA to tumor tissue, while SignalLock genetic circuitry reads endogenous microRNA signatures within cells, permitting IL-12 expression in tumor cells while restricting expression in healthy tissue. This ability to control expression at the cellular level rather than relying on delivery alone is a foundational capability of Strand’s platform.

STX-003 represents the first systemic clinical application of Strand’s broader vision for programmable mRNA medicines: therapies engineered not only to encode a therapeutic payload, but also to control where, when and for how long that payload is expressed. If validated clinically, this approach could expand the therapeutic reach of mRNA beyond what can be achieved through delivery alone and enable potent biology that has historically been constrained by systemic toxicity.

“This milestone represents an important step toward what we believe is the next frontier for mRNA medicines as we program not just what a medicine expresses, but where it is active,” said Jake Becraft, PhD, Co-founder and Chief Executive Officer of Strand Therapeutics. “STX-003 brings together the core capabilities of our platform to address the challenge of systemically delivering mRNA beyond the liver while controlling therapeutic activity at the cellular tissue level, a problem the field has been dealing with for years. If successful, this approach could unlock an entirely new class of programmable medicines and therapeutic payloads that have historically been limited by systemic toxicity.”

“The central challenge in systemic genetic medicine is not simply getting a therapeutic into the body; it is achieving precise control over where that therapeutic becomes active,” said Tasuku Kitada, PhD, MBA, Co-founder, President and Head of R&D at Strand Therapeutics. “SignalLock was designed to provide that additional layer of control by enabling the medicine itself to interpret the biology of the cell. STX-003 is our first opportunity to test that capability systemically in patients and an important step toward realizing the broader potential of programmable mRNA medicines.”

STX-003 builds on Strand’s clinical experience with STX-001, its intratumorally delivered self-replicating mRNA therapy expressing IL-12. In patients with advanced solid tumors who had progressed on prior treatments, STX-001 has demonstrated preliminary evidence of systemic immune activation and anti-tumor activity, including responses in tumors that were never directly injected. These findings provide early clinical validation of Strand’s self-replicating mRNA approach and the potential for locally expressed IL-12 to drive systemic anti-tumor activity. STX-003 is designed to take the next significant step: bringing that biology to tumors throughout the body through systemic administration while using SignalLock circuitry to restrict where IL-12 is expressed.

“Our experience with STX-001 has given us important clinical evidence that sustained, local IL-12 expression can generate systemic immune and anti-tumor activity,” said Jason Luke, MD, Chief Medical Officer at Strand Therapeutics. “STX-003 is designed to build directly on those learnings by enabling systemic administration while using programmable circuitry to control where IL-12 is expressed. ASCENT-1 will allow us to evaluate the safety, pharmacology and biological activity of this approach as we work to establish an optimized dose and development path for STX-003.”

ASCENT-1 is a multiple-ascending-dose study evaluating IV STX-003. The Phase 1 portion is enrolling adults with measurable, locally advanced or metastatic solid tumors for which an anti-PD-1 or anti-PD-L1 therapy is approved and who lack curative options. The trial is open at three sites in the United States and one in Australia. Additional information is available at ClinicalTrials.gov under identifier NCT07751042.

“Patients with advanced solid tumors that progress after checkpoint inhibitors need new options,” said Justin Moser, MD, from HonorHealth Research Institute, an investigator on the trial. “This study will test whether programmable RNA can concentrate IL-12 activity inside tumors while keeping toxicity within an acceptable range, which is the problem that has held this biology back.”

STX-003 advanced rapidly from investigational new drug (IND) clearance into clinical development, with the first patient dosed approximately eight weeks after clearance. ASCENT-1 is now actively progressing through dose escalation, evaluating safety, tolerability, pharmacokinetics, pharmacodynamics and preliminary anti-tumor activity. Strand expects initial development to focus on non-small cell lung cancer (NSCLC) and cutaneous melanoma, with the study designed to inform dose optimization and subsequent clinical development.

About Strand Therapeutics

Strand Therapeutics is a clinical-stage biotechnology company pioneering programmable mRNA therapeutics to realize the full potential of genetic medicines. Strand's proprietary Signal Stack platform, which includes EverScript circular RNA, AmpliScript self-replicating RNA, SignalPath targeted delivery and logic-driven SignalLock genetic circuits, provides multiple approaches for controlling therapeutic activity across tissues and cell types to address foundational limits in targeting, durability and safety.

The company is advancing a pipeline focused on oncology and autoimmune disease. Its lead clinical program, STX-003, is a systemically delivered self-replicating mRNA therapy expressing IL-12 and leveraging Strand’s SignalLock circuitry to enable tumor-selective therapeutic activity. Strand is also advancing STX-005, an in vivo CAR-T program designed to combine targeted delivery with controlled, durable and re-dosable CAR expression. Strand’s first clinical program, STX-001, provides preliminary clinical evidence of systemic immune activation and anti-tumor activity from locally expressed IL-12 and helped establish the foundation for STX-003. Founded by MIT synthetic biology pioneers, Strand has raised over $250 million and is headquartered in Boston, MA. Learn more at https://www.strandtx.com, and follow Strand on LinkedIn and X at @strandtx.