Interestingly, the IC50 values ranged from submicromolar activity (9b for Cal27 and 9d for Cal27 and HepG2) to more than 50 µM (18c-f). The compounds of type A exhibited very potent activity, with 9a and 9c showing low single digit micromolar activity, 9b exhibiting submicromolar activity against Cal27, and 9d demonstrating submicromolar activity against both cell lines. Compounds 9b and 9d both feature a cyclohexyl group at the R 1 residue, which seems to be preferred, since the slightly less potent compounds 9a and 9c contain benzyl groups at the R 1 position. Also, type B compounds showed single digit micromolar antiproliferative activity. Type B compounds differ from only one residue at the 1position of the tetrazole. Bulkier aliphatic residues (13d, cyclohexyl; 13f iso-octyl) seem to be preferred over smaller alkyl residues (13c, tert-butyl; 13e, cyclopentyl) and aromatic residues (13a, benzyl; 13b, 2-(indole-3-yl)-ethyl). The most potent compound in this series was 13f, bearing an iso-octyl residue at the R 1 position. Remarkably, the type C series exhibited weak double-digit micromolar activity (18a and 18b) or no detectable activity (18c-f) in the assay concentration range (IC50 > 50 µM).