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A novel intratumoral therapy is showing promise in hard-to-treat melanoma. STX-001, an mRNA-based drug that instructs tumors to produce immune-activating IL-12, showed durable responses in patients whose cancer had stopped responding to PD-1 immunotherapy. Notably, tumor shrinkage was observed not just at the injection site but in distant, untreated lesions — suggesting the treatment is training the immune system to fight cancer system-wide.
A novel intratumoral therapy is showing promise in hard-to-treat melanoma. STX-001 — a lipid nanoparticle carrying self-replicating mRNA that encodes the immune-activating protein IL-12 — demonstrated durable tumor regression in both injected and distant, non-injected lesions among patients with advanced solid tumors, including those resistant to PD-1 checkpoint inhibitors. The findings come from a phase 1/2 first-in-human trial presented at the 2026 SITC Annual Meeting.
The 100-µg dose emerged as the sweet spot for efficacy and tolerability. In the melanoma expansion cohort at that dose, the confirmed objective response rate hit 33%, clearing the trial's predefined success bar. Nearly half of all enrolled patients (48.7%) remained on study for at least 6 months, with a median response duration of 8.7 months. Translational data revealed that responders showed robust increases in CD8+ T cells and interferon-γ signaling — both at the injection site and in distant tumors — suggesting the drug is priming the immune system to attack cancer more broadly.
By the Numbers:
40% of evaluable patients experienced tumor regression
Why it matters: For melanoma patients who've exhausted PD-1 and CTLA-4 options, treatment choices are slim. STX-001's ability to trigger systemic immune responses — including shrinking tumors it never directly touched — could represent a meaningful new option for this difficult-to-treat population.
