Telomeres are protective caps at the ends of human chromosomes. Telomeres 
shorten with each successive cell division in normal human cells whereas, in 
tumors, they are continuously elongated by human telomerase reverse 
transcriptase (hTERT). Telomerase is overexpressed in 80-95% of cancers and is 
present in very low levels or is almost undetectable in normal cells. Because 
telomerase plays a pivotal role in cancer cell growth it may serve as an ideal 
target for anticancer therapeutics. Inhibition of telomerase may lead to a 
decrease of telomere length resulting in cell senescence and apoptosis in 
telomerase positive tumors. Several strategies of telomerase inhibition are 
reviewed, including small molecule inhibitors, antisense oligonucleotides, 
immunotherapies and gene therapies, targeting the hTERT or the ribonucleoprotein 
subunit hTER. G-quadruplex stabilizers, tankyrase and HSP90 inhibitors targeting 
telomere and telomerase assembly, and T-oligo approach are also covered. Based 
on this review, the most promising current telomerase targeting therapeutics are 
the antisense oligonucleotide inhibitor GRN163L and immunotherapies that use 
dendritic cells (GRVAC1), hTERT peptide (GV1001) or cryptic peptides (Vx-001). 
Most of these agents have entered phase I and II clinical trials in patients 
with various tumors, and have shown good response rates as evidenced by a 
reduction in tumor cell growth, increased overall disease survival, disease 
stabilization in advanced staged tumors and complete/partial responses. Most 
therapeutics have shown to be more effective when used in combination with 
standard therapies, resulting in concomitant telomere shortening and tumor mass 
shrinkage, as well as preventing tumor relapse and resistance to single agent 
therapy.