TwoStep Therapeutics closed an oversubscribed $62.5 million Series A on September 10, 2026, with concurrent FDA clearance of its investigational new drug application for TS-104 1. The San Carlos company intends to advance the peptide-drug conjugate into a first-in-human Phase 1 trial later this year. No randomized controlled trial data exists for the candidate.
Pfizer Ventures and M Ventures, the venture arm of Merck KGaA, led the financing 1. Insight Partners and Medical Excellence Capital also participated 2. The pairing places a North American pharmaceutical venture arm alongside a European one to fund a single modality. This capital structure reflects distinct commercial traditions converging on a specific candidate without implying agreement on regulatory strategy.
TS-104 uses a knottin peptide to bind five tumor-associated integrins in their active conformation. The construct delivers monomethyl auristatin E as the cytotoxic payload. Preclinical pharmacokinetics and receptor affinity measurements supported the FDA clearance 3. The reduced molecular weight, relative to a conventional antibody-drug conjugate, is intended to alter tissue penetration 3.
These parameters derive from in vivo cohort models and cellular assays. No human adverse event profile exists. The primary endpoint for the planned Phase 1 protocol has not been registered in a public trial database. Half-life estimates and dose-limiting toxicity thresholds for TS-104 in patients remain undefined. Mechanism does not substitute for clinical evidence. The absence of a human safety record places this candidate at the earliest stage of clinical translation.
Industry interest in the modality extends beyond this single candidate, with broader forecasts suggesting peptide drug conjugates to hit $15b by 2035.
Regulatory Scope and Evidence Boundaries
The $62.5 million figure and the investor roster rely on trade reporting and registry trackers rather than primary regulatory financial disclosures 1 2. The funding and the regulatory clearance constitute the only verifiable milestones for this candidate at present.
The FDA investigational new drug clearance permits clinical testing under a specific protocol. This pathway is separate from the regulatory environment governing unapproved peptides. The broader sector currently faces enforcement actions, including instances where eli lilly sues six sellers over unapproved retatrutide and cases in which eli lilly targets illicit market for unapproved weight-loss peptides. Those actions concern unauthorized distribution. They do not intersect with the clinical development pathway TwoStep is pursuing for TS-104.
Conflating these categories obscures the evidentiary gap between a funded preclinical candidate and an approved therapeutic.
Planned Clinical Timeline
Patient enrollment for the initial Phase 1 safety and dosing cohort is scheduled for later in 2026 3. The cohort size, dosing schedule, and statistically significant thresholds for dose-limiting toxicities are not yet published. The distance between this scheduled enrollment and a usable treatment requires completion of the full clinical trial sequence.
