The design, synthesis and biological activity of a series of non-planar dihydro-β-carboline and β-carboline-based derivatives of the toxic anticancer agent fascaplysin is presented. We show these compounds to be selective inhibitors of CDK4 over CDK2 with an IC₅₀ (CDK4-cyclin D1) = 11 µmol for the best compound in the series 4d. The crystallographic analysis of some of the compounds synthesised (3b/d and 4a–d) was carried out, in an effort to estimate the structural similarities between the designed inhibitors and the model compound fascaplysin.