AIMS: Small-interfering RNA (siRNA)-based targeting of proprotein convertase subtilisin/kexin type 9 (PCSK9) represents a novel therapeutic approach that may provide a convenient, infrequent, and safe dosing schedule to robustly lower low-density lipoprotein cholesterol (LDL-C). Given the long duration of action, however, establishing safety in particular with respect to immunogenicity is of paramount importance. In earlier clinical studies of other RNA-targeted treatment approaches (antisense oligonucleotide therapy) immunological and haematological adverse effects, in particular thrombocytopenia and pro-inflammatory effects, have been reported. Here, we present the pre-specified safety analysis from ORION-1 evaluating platelets, immune cells, immunological markers, antidrug antibodies, and clinical immunogenicity adverse events (AEs) under PCSK9 siRNA treatment with inclisiran. METHODS AND RESULTS: The pre-specified safety analysis from ORION-1 was performed in six different inclisiran dosing regimens in patients at high risk of cardiovascular disease with elevated LDL-C levels. Patients received either a single dose (SD: 200 mg, n = 60; 300 mg, n = 62 or 500 mg, n = 66) or double-dose starting regimen (DD: 100 mg, n = 62; 200 mg, n = 63; or 300 mg, n = 61 on days 1 and 90) of inclisiran or placebo (SD: n = 65; DD: n = 62). The effects of inclisiran on haematological parameters including platelet counts, lymphocytes, and monocytes as well as on the immune markers interleukin 6 (IL-6) and tumour necrosis factor-α (TNF-α) were examined after 180 days. Immunogenicity was further evaluated by analysis of anti-drug-antibodies (ADAs) towards inclisiran in 6068 study samples and by careful analysis of immunogenicity AEs as part of the pharmacovigilance strategy. At day 180, no significant alterations of platelet counts were observed in any of the dosing groups (change from baseline, SD: 200 mg: 0.8%; 300 mg: -0.5%; 500 mg: -1.8%; DD: 100 mg: 1.3%; 200 mg: -0.5%; 300 mg: 1.0%; no significant difference for any group as compared with placebo). No significant effects on other immune cells, including leucocytes, monocytes, or neutrophils were detected. Notably, no significant increase of inflammatory biomarkers (IL-6 or TNF-α) with either the SD or DD regimen became evident. There was no evidence for immunogenicity based on ADA level analysis and careful review of clinical immunogenicity AEs in none of the treatment regimens. CONCLUSION: In this pre-specified safety analysis of ORION-1 for the siRNA therapeutic inclisiran, no adverse effects on measures of inflammation or immune activation nor adverse effects on platelets or clinical immunogenicity AEs were observed over at least 6-month treatment. These safety findings in the largest analysis of an RNAi study in humans to date provide strong reassurance about the safety of inclisiran and the potential of cardiovascular RNA-targeted therapies.