Thalidomide is clinically useful in a number of cancers. Antitumor activity may be related to a number of known properties, including anti-tumor necrosis factor (TNF)-alpha and T-cell costimulatory and antiangiogenic activities. However, it may also involve direct antitumor effects. A series of second generation thalidomide analogues have been separated into two distinct groups of compounds, each with enhanced therapeutic potential, i.e., SelCIDs, which are phosphodiesterase (PDE) type IV inhibitors, and IMiDs, which have unknown mechanism(s) of action. We report here our efforts to determine direct antitumor effects of thalidomide and compounds from both groups. We found that one of the SelCID analogues (SelCID-3) was consistently effective at reducing tumor cell viability in a variety of solid tumor lines but had no effect on non-neoplastic cells. The antitumor activity was independent of known PDE4 inhibitory activity and did not involve cAMP elevation. Growth arrest was preceded by the early induction of G(2)-M cell cycle arrest, which led to caspase 3 mediated apoptosis. This was associated with increased expression of pro-apoptotic proteins and decreased expression of antiapoptotic bcl-2. Furthermore, extensive apoptosis in vivo was detected during SelCID-3-mediated inhibition of tumor growth in a murine xenotransplantation cancer model. Our results suggest that SelCID-3 represents a novel antitumor agent distinct from thalidomide and from previously characterized analogues with therapeutic potential against a range of solid tumors. This effect appears to be mediated via alterations in the expression of bcl-2 family proteins.