PDE5A and colonic neoplasm: Because PDE5 and 10 levels are elevated in colon tumor cells compared with colonocytes and the possibility that SS inhibits colon tumor cell growth by a mechanism involving PDE5 and/or 10 inhibition [13, 14, 19–22], we hypothesized that SS can be chemically modified to improve colon tumor cell growth activity by enhancing PDE5 and 10 inhibitory activity.