Locked nucleic acid (LNA) is a novel high-affinity and biologically stable RNA analog in which the normally flexible ribose sugar ring is fixed in a rigid conformation through a methylene 2'-O, 4'-C linkage. This fixed conformation brings substantial advantages to the design of effective RNA binding drugs, and enables single-stranded LNA oligonucleotides, termed 'RNA antagonists', to have superior efficacies in vivo in downregulating target mRNA when compared to oligonucleotides based on other chemistries or to published short interfering RNA. The features that allow LNA to be a valuable drug platform include unprecedented RNA binding affinity, excellent specificity, resistance to enzymatic degradation, safety, and ease of manufacture. Santaris Pharma A/S holds worldwide rights to the application of LNA in therapeutics, and is engaged in the clinical development of a series of drug candidates against cancer and metabolic diseases. SPC-2996, the company's most advanced product in development, entered an international, open-label, multicenter, phase I/II clinical trial in patients with severe chronic lymphocytic leukemia in May 2005. This trial is the first clinical evaluation of LNA chemistry. Two other LNA compounds have completed good laboratory practice safety studies with satisfactory outcome, and are likely to commence undergoing clinical development by 2007.