Del-desiran is an experimental drug for myotonic dystrophy type 1 (DM1), a rare progressive disease with no available treatment options.[1] The drug works as an antibody-oligonucleotide conjugate that targets reducing the levels of a toxic mRNA (DMPK) that causes the disease.[1] In the Phase 1/2 MARINA trial, del-desiran successfully delivered to muscle tissue and achieved an approximately 40% reduction in DMPK mRNA.[1] The treatment resulted in improvements in myotonia (muscle spasms), muscle strength, mobility and quality of life in patients.[1] The drug showed an acceptable safety profile, with most adverse effects being mild or moderate in severity.[1] Long-term data from the MARINA-OLE extension study showed that treatment with 4 mg/kg del-desiran administered quarterly was safe and effective, with reversal of disease progression compared to the natural course of DM1.[2] A global phase 3 HARBOR trial is currently underway with more than 150 DM1 patients aged 16-65 years.[2]