Grape seed procyanidin extract (GSE) has been reported to exert antineoplastic properties via the inhibition of cyclooxygenase-2 (COX-2)/prostaglandin E 2 (PGE 2 ) eicosanoid pathways. In addition, ample data link carcinogenesis to inflammatory events involving other major eicosanoid metabolic pathways, including prostacyclin (PGI 2 ) and 15-hydroxyeicosatetraenoic acid (15-HETE). We therefore evaluated the effects of GSE on prostacyclin synthase (PTGIS)/PGI 2 and 15-lipoxigenase-2 (15-LOX-2)/15-HETE productions by human lung premalignant and malignant cells and correlated the findings with antiproliferative or proapoptotic effects of GSE. The effects of GSE on PGI 2 and 15-HETE productions by human bronchoalveolar lavage (BAL) cells ex vivo were also determined. We further evaluated the bioactivity of oral administration of leucoselect phytosome (a standardized GSE) in the lungs of subjects participating in a lung cancer chemoprevention trial, by comparing the antiproliferative effects of coculturing matched pre-versus posttreatment BAL fluids with lung premalignant and malignant cells. GSE significantly increased PGI 2 (as measured by 6-keto PGF1a) and 15-HETE productions by these cells. Transfections of PTGIS or 15-LOX-2-specific siRNA partially abrogated the antiproliferative or proapoptotic effects of GSE in lung premalignant and malignant cells, respectively. GSE also increased PTGIS and inhibition of caspase-3, and transfection of 15-LOX-2 siRNA abrogated the GSEinduced apoptosis in A549 cells. In addition, culture supernatants from ex vivo GSE-treated baseline BAL cells, as well as BAL fluids from subjects treated with leucoselect phytosome, significantly decreased proliferations of lung premalignant and malignant cells. Our findings support the continued investigation of GSE as an anti-neoplastic and chemopreventive agent against lung cancer. Cancer Prev Res; 9(12); 925-32. Ó2016 AACR.