Sling Therapeutics has closed a $123 million series C round to advance its oral therapy for thyroid eye disease (TED) through a phase 3 trial and hopefully on to commercialization.
The first patients have already been dosed with the small molecule, called linsitinib, in the late-stage study that is aiming for 120 participants. The trial will test linsitinib's ability to reduce proptosis—bulging of the eyeball—over 24 weeks.
The financing was led by Forbion and included involved existing backer TPG Life Sciences Innovations alongside new investor Sectoral Asset Management.
Linsitinib is a tyrosine kinase inhibitor (TKI) designed to block the insulin-like growth factor 1 receptor (IGF-1R). This receptor has been shown to talk to other receptors on the surface of cells behind the eye as well as immune cells, and its overexpression or activation in these cells is what drives the inflammation, swelling and tissue damage characteristic of TED.
Linsitinib was originally developed by Astellas as a cancer treatment, but Sling licensed the therapy in 2022, while securing a $35 million series A to run a phase 2b trial of the drug in TED.
The FDA has approved multiple other IGF-1R inhibitors to treat TED, but these drugs have different mechanisms of action and routes of administration than linsitinib. Amgen’s blockbuster Tepezza and Viridian’s Lumvoa are monoclonal antibody therapies that bind to the IGF-1R receptor directly and are administered intravenously. Amgen began a phase 3 trial of a subcutaneous formulation of Tepezza last year, positioning it to potentially free patients from lengthy infusions in the future.
But Sling's option is taken orally and acts by affecting the phosphorylation of IGF-1R, with the aim of providing a safer and more durable clinical profile, Sling CEO Ryan Zeidan, Ph.D., explained.
More recently, ex-Pfizer chemist Robert Hoffman co-founded Khartis Therapeutics in 2024 to push its own oral IGF-1R inhibitor for TED into clinical trials. But Sling remains ahead, having posted positive data from its phase 2b/3 trial of linsitinib in TED back in early 2025.
That 90-person study showed that a high dose of linsitinib was able to induce a big enough reduction in eye bulging to meet the response criteria in 52% of patients.
“We do think [linsitinib] will unlock a new patient population that has declined monoclonal antibody therapy treatments, and also patients who ... may prefer an oral [treatment],” Zeidan told Fierce in an interview Tuesday. “We see this as both a market growth opportunity and an opportunity to give a new therapeutic option to patients who would take an antibody.”
“We do believe that IGF-1R is going to continue to be the mechanism that has proven efficacy in these patients, and that physicians are comfortable with,” the CEO added. “We see a great opportunity here to partner with patients and physicians to give them another treatment option that is really compelling.”