Background Vulnerable plaques play an important role in the onset of sudden cardiac events and strokes. How to stabilize vulnerable plaques is still a challenge to medical science. Alprostadil is a biologically active substance with strong activity on vessel. Our study assessed the stabilizing effects of an alprostadil liposome microsphere preparation (ALMP) on vulnerable plaques in the brachiocephalic artery of apolipoprotein E (Apo E) knockout mice. Methods Seventy-two male Apo E-knockout mice were fed a high-fat diet beginning at eight weeks of age. At week 17, they were divided randomly into groups for treatment with a high dose (3.6 mu g . kg(-1) . d(-1)) or low dose (1.8 mu g . kg(-1) . d(-1)) of an ALMP, or 0.2 ml/d normal saline (control group). The drug was administered using a micro-capsule pump. Twenty weeks after drug administration, pathological changes in the vulnerable plaques within the brachiocephalic artery were assessed, and levels of anti-mouse monocyte/macrophage monoclonal antibody (MOMA-2) and superoxide anions in the plaques were detected using immunofluorescence. The soluble intercellular adhesion molecule-1 (ICAM-1) expression was measured by ELISA, and the expression of matrix metalloproteinase-9 (MMP-9) and CD40 mRNA was measured using RT-PCR. Thrombospindin-1 (TSP-1) expression was detected using Western blotting. Results Compared with the control group, ALMP treatment significantly reduced the plaque area in the brachiocephalic artery (P <0.01), significantly lowered the contents of the lipid core (P <0.01), significantly reduced the number of ruptured fibrous caps (P <0.05), and increased the thickness of the fibrous cap and significantly reduced the incidence of intra-plaque hemorrhage (P <0.05). ALMP treatment significantly reduced the expression of MOMA-2, superoxide anion, MMP-9, ICAM-1 and CD40 in the plaques (P <0.01), decreased plasma ICAM-1 expression (P <0.01), and increased the expression of TSP-1. Conclusions Treatment with ALMP can stabilize vulnerable plaques by inhibiting inflammation. Chin Med J 2012;125(24):4380-4385