oligonucleotidebased pharmaceuticals nonclinical and clinical safety signals and nonclinical testing strategies antisense oligonucleotides short interfering rnas sirnas and aptamers are oligonucleotidebased pharmaceuticals with a promising role in targeted therapies currently five oligonucleotidebased pharmaceuticals have achieved marketing authorization in europe or usa and many more are undergoing clinical testing however several safety concerns have been raised in nonclinical and clinical studies oligonucleotides share properties with both chemical and biological pharmaceuticals and therefore they pose challenges also from the regulatory point of view we have analyzed the safety data of oligonucleotides and evaluated the applicability of current nonclinical toxicological guidelines for assessing the safety of oligonucleotidebased pharmaceuticals oligonucleotidebased pharmaceuticals display a similar toxicological profile exerting adverse effects on liver and kidney evoking hematological alterations as well as causing immunostimulation and prolonging the coagulation time it is possible to extrapolate some of these effects from nonclinical studies to humans however evaluation strategies for genotoxicity testing of nonnatural oligonucleotides should be revised additionally the selective use of surrogates and prediction of clinical endpoints for nonclinically observed immunostimulation is complicated by its multiple potential manifestations demanding improvements in the testing strategies utilizing more relevant and mechanisticbased approaches and taking better account of species differences could possibly improve the prediction of relevant immunologicalproinflammatory effects in humans