16030 Background: Enzastaurin (LY317615.HCl), a selective inhibitor of protein kinase C beta II (PKCβII), leads to apoptosis and suppression of angiogenesis and represents a promising drug for clinical development. The effects of Enzastaurin were investigated utilizing the ovarian cancer cell line HEY and several chemoresistant subclones. Methods: The cell line HEY was used to develop subclones with selective resistance against cisplatin, etoposide, docetaxel, paclitaxel, gemcitabine, and pemetrexed. PKCβII expression was determined by immunoblotting and RT-PCR analyses. The platin-sensitive ovarian cell line IGROV1 with high expression of PKCβII served as control. All cell lines were exposed to 5 μM Enzastaurin for up to three hours. Immunoblotting analyses determined the expression of Enzastaurin targets PKCβII and glycogen synthase kinase 3β (GSK3β). Proliferation and apoptosis induction after Enzastaurin treatment was evaluated using MTT assay and DAPI staining. Results: All resistant cells showed a significantly higher expression of phosphorylated GSK3β than the parental cells. The highest expression level was detected in cisplatin- and paclitaxel-resistant cells. A decrease of phosphorylation occurred after exposure to Enzastaurin for 30 minutes, most obvious in parental, cisplatin- and gemcitabine-resistant cells. Proliferation and apoptosis assays demonstrated the paclitaxel and docetaxel resistant cells as the most sensitive cells. In contrast, cisplatin-resistant cells exhibited the highest resistance. Enzastaurin resistance also correlated with elevated PKCβII expression. Conclusion: The results indicate that ovarian cell lines with high expression of phosphorylated GSK3β display also a strong dephosphorylation. Though in these cell lines the inhibitory effect of Enzastaurin is most prominent, cell proliferation is hardly affected, nor apoptosis is accelerated. There is cross-resistance between platinum and Enzastaurin in some cell lines; however Enzastaurin demonstrated promising activity against taxane resistant cell lines. [Table: see text]