Methods are provided herein for treatment of myotonic dystrophy and other toxic RNA diseases in a subject. In some examples, the method comprises administration of a compound that binds a nucleotide repeat expansion in a ribonucleic acid molecule, thereby treating the disease. In additional examples, the method comprises administration of a compound that disrupts binding of muscleblind-like proteins to an RNA nucleotide repeat expansion. Compounds for use in the disclosed method include pentamidine or heptamidine or derivatives thereof. Representative compounds are described herein.