Enzastaurin, an acyclic bisindolymaleimide, is a potent and selective competitive inhibitor of protein kinase Cβ, which has been shown to inhibit cancer cell proliferation and angiogenesis in human cancer cell lines. Gemcitabine and pemetrexed are two cytotoxic drugs that are currently used in non-small cell lung cancer (NSCLC) therapy. In this study, we have investigated whether the addition of enzastaurin to gemcitabine or to pemetrexed is able to increase their antitumor activity to establish an effective schedule of combined treatment. The effects on cancer cell proliferation, cell cycle distribution, intracellular mitogenic and antiapoptotic signaling pathways, and induction of apoptosis were evaluated in three different combination sequences (concomitant treatment, sequential treatment with the cytotoxic drug followed by enzastaurin, or sequential treatment with enzastaurin followed by the cytotoxic drug) in a panel of human NSCLC cell lines. The combination of enzastaurin with either gemcitabine or pemetrexed caused different antiproliferative and proapoptotic effects depending on the treatment schedule. A synergistic antiproliferative and proapoptotic activity was only obtained when chemotherapy was followed by treatment with enzastaurin. These effects were accompanied by the arrest of the surviving cancer cells in the S phase, thus limiting their ability to proceed through the cell cycle, and by a maximum inhibition in the activated, phosphorylated forms of Akt and mitogen-activated protein kinase. In contrast, the concomitant treatments or the sequential treatments, in which enzastaurin was given before chemotherapy, resulted in significant antagonistic effects. [Mol Cancer Ther 2008;7(6):1698–707]