Sling Therapeutics is developing linsitinib, an experimental oral drug for adults with moderate to severe active thyroid eye disease, an autoimmune condition that can push the eyes forward and impair vision. The company closed a $123 million Series C to advance the drug through a global Phase 3 study. Linsitinib is not approved; trial participants take it as a twice-daily pill, a format Sling is testing as an alternative to infused biologic treatment.
Thyroid eye disease often occurs in people with Graves’ disease and hyperthyroidism. Dysfunction in the IGF-1R signaling pathway can cause inflammation and tissue growth behind the eyes, producing pain, redness, double vision and proptosis, or eye bulging. Sling’s intended treatment workflow centers on oral dosing rather than the lengthy infusion regimens or orbital surgery the company identifies as current options.
Linsitinib inhibits IGF-1R, the same target class used by approved treatments for the disease. In company-reported topline results from the earlier LIDS study, 52% of patients receiving the 150-milligram dose twice daily qualified as proptosis responders after 24 weeks, meaning the affected eye moved back by at least two millimeters. That randomized study enrolled 90 patients, but its results remain prior-stage evidence rather than Phase 3 validation or regulatory approval.
The new ORBIT trial is expected to enroll about 130 adults and randomly assign them to 150 milligrams of linsitinib or placebo twice daily for 24 weeks. Its main test is the share of patients achieving at least a two-millimeter reduction in proptosis without corresponding deterioration in the other eye. Secondary measures include disease activity, average change in eye bulging and quality of life, followed by an optional 24-week open-label extension.
Forbion led the Series C, with existing investor TPG Life Sciences Innovations and new investor Sectoral Asset Management participating. Forbion partner Regina Salvat and Sectoral’s François Beaubien joined Sling’s board. The company said the proceeds will support continued clinical development of linsitinib, which has also received Fast Track designation from the Food and Drug Administration.
Sling is concentrating the new capital on a single late-stage program whose proposed convenience still depends on clinical performance. The consequential test is whether ORBIT can reproduce the earlier proptosis response in a larger pivotal study while supporting the safety profile needed for an oral drug to become a practical alternative to infusion-based care.