Nucleoside analogues 15, 16, 19, 20, 22-b and 23 were evaluated in cytotoxicity assays against human pancreatic cancer (PANC-1) and human primary glioblastoma (U87-MG) cells and their CC50 values compared to commercial arabinocytidine (Ara-C) and gemcitabine (Table ). Only analogue 19 displayed measurable cytotoxicity, with a CC50 value of 0.59 μM in U87-MG cells and 12 μM in PANC-1 cells (Entry 3, Table ); all other analogues had CC50 values >200 μM. The data for 19 were not immediately comparable to Ara-C or gemcitabine, as both displayed superior CC50 values in the cell lines evaluated (Entries 1 and 2, Table ). These results suggest that a thioarabino chemotype may