XDH and prostate cancer: A series of compounds incorporating the thieno[2,3-b]thiophene moiety (Figure 8) were synthesized and tested against several enzymes by Mabkhot et al. Three of these compounds demonstrated to be selective XO inhibitors, being determined for the most potent one (Figure 8) an IC50 value of 14.4 μM. This compound was also found to be noncytotoxic in a human prostate cancer cell line [30].