The expression of telomerase in approximately 85% of cancers and its absence in 
the majority of normal cells makes it an attractive target for cancer therapy. 
However the lag period between initiation of telomerase inhibition and growth 
arrest makes direct inhibition alone an insufficient method of treatment. 
However, telomerase inhibition has been shown to enhance cancer cell 
radiosensitivity. To investigate the strategy of simultaneously inhibiting 
telomerase while delivering targeted radionuclide therapy to cancer cells, 
123I-radiolabeled inhibitors of telomerase were synthesized and their effects on 
cancer cell survival studied. An 123I-labeled analogue of the telomerase 
inhibitor MST-312 inhibited telomerase with an IC50 of 1.58 μM (MST-312 IC50: 
0.23 μM). Clonogenic assays showed a dose dependant effect of 123I-MST-312 on 
cell survival in a telomerase positive cell line, MDA-MB-435.