BACKGROUND: Although a recent clinical trial demonstrated that alirocumab, a proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitor, significantly reduces the incidence of acute coronary events, the impact of alirocumab on plaque stabilization remains uncertain. The Efficacy of ALirocumab for Thin-cap fibroatheroma in patients with coronary Artery disease estImated by optical coherence tomogRaphy (ALTAIR) study will investigate the effect of alirocumab on thin-cap fibroatheroma (TCFA) in Japanese patients who underwent recent percutaneous coronary intervention (PCI). METHODS AND DESIGN: ALTAIR is a phase IV, open-label, randomized, parallel-group, single-center study involving blinded optical coherence tomography (OCT) image analysis in Japanese adults hospitalized for PCI and having suboptimal control of low-density lipoprotein cholesterol (LDL-C) levels (>70mg/dL) despite statin therapy. Patients will be randomized (1:1) to the alirocumab arm (alirocumab 75mg every 2 weeks added to rosuvastatin 10mg/day) or the standard-of-care arm (rosuvastatin 10mg/day, with initiation and/or dose adjustment of non-statin lipid-lowering to achieve an LDL-C target of <70mg/dL). OCT imaging will be conducted at baseline and at week 36 (post-treatment). The primary objective is to compare the alirocumab and standard-of-care arms regarding the change in TCFA fibrous-cap thickness after 9 months of treatment. CONCLUSION: The outcomes of ALTAIR (ClinicalTrials.gov identifier: NCT03552432) will provide insights into the effect of alirocumab on plaque vulnerability following PCI in patients with suboptimal LDL-C control despite stable statin therapy.