Pancreatic cancer is highly lethal, with a mere 3% survival rate 5 years after diagnosis in England and Wales, and it is the fifth most common cause of cancer death (London School of Hygiene and Tropical Medicine, 2014 & Cancer Research UK, 2014). The reason for the poor prognosis of pancreatic cancer is largely due to the delay in its diagnosis. Symptoms present in a similar manner to other illnesses, and the location of the pancreas, deep inside the body, presents an obstacle for the identification of tumours. Additionally a particular difficulty in drug discovery for pancreatic cancer is that the standard drug used for its treatment, gemcitabine, develops resistance. In the past 40 years there has been no improvement on the 3% survival rate, compared to breast cancer survival rates which have increased from 50 – 80% since the 1970s (Pancreatic Cancer Research Fund, accessed 2016). Accordingly there is a great urgency that the unacceptable prognosis of pancreatic cancer be improved through identifying novel therapeutics for its treatment.
What are G-quadruplexes?
G-quadruplexes are guanine rich secondary structures of DNA. Four guanine bases can associate through Hoogsteen hydrogen bonding to form a square planar structure, known as a guanine tetrad. When 2 or more guanine tetrads stack they form a G-quadruplex, which is further stabilised by the presence of a cation, usually potassium. The cation sits in the central channel between each pair of tetrads. A schematic of the G-quadruplex structure (described above) is shown in Figure 1.
Stabilisation of G-quadruplex structures formed within telomeric DNA with the use of G-quadruplex-selective ligands have the capacity for inhibiting the enzyme telomerase (Haider et al., 2003). Telomerase is a highly attractive target for stop the indefinite division of cancerous cells. It is almost universal in all human cancer cell lines, and is found in over 85% of primary tumours (Kim et al., 1994). Telomerase is responsible for the rebuilding of telomeres, the capping ends of chromosomes (Collins & Mitchell, 2002). During normal cellular replication telomeres shorten. Once the critical limit of telomere length has been met cells are programmed for apoptosis. However, with upregulated telomerase, cancer cells have the ability to replicate indefinitely. Accordingly G-quadruplex stabilising ligands can potentially induce apoptosis in cancerous cells.
Liu et al. (2005) have shown that sub-cytotoxic doses of G-quadruplex targeting ligands TMPyP4 and telomestatin in MIA PaCa-2 cells, pancreatic cancer cells, are associated with an increase in senescent cells by 15 and 10% respectively, as determined by β-galactosidase staining at day 39 and 103 respectively. These results validate that growth inhibition of cells is induced by the progressive loss of telomere DNA. This consequently leads to cellular senescence and cell death.
More recently a great interest has been shown in the vast abundance of G-quadruplexes in the promotor regions of oncogenes. The Neidle group at UCL have designed a compound called MM41, a tetra-substituted naphthalene-diimide derivative, shown in Figure 2, which reduces the growth of pancreatic tumours by 80% in treated mice (Ohnmacht et al., 2015). MM41 was delivered by IV with twice-weekly dosage of 15 mg/kg. Tumour growth prevention by MM41 over a time course is displayed in Figure 3. MM41 binds strongly to G-quadruplexes encoded within the promoter sequences of BCL-2 and k-RAS genes. Blocking these genes, which are prominent in cancer and key to the survival and growth of cancer cells, induces cancer cell apoptosis.
Further refinements of MM41 are underway as it is not yet ready for human trials. A derivative compound of MM41, CM03, has shown to target a gene which is involved in cancer resistance (Neidle, accessed 2016). In gemcitabine-resistant MIA PaCa-2 cells CM03 is still potent, thus showing promise in overcoming the hurdle that drug resistance poses on pancreatic cancer drug discovery (Neidle group, data not published). A key consideration is to ensure that potential drugs for pancreatic cancer which designed to interact with G-quadruplexes are selective in targeting cancerous cells, and do not interfere with normal cellular function. From the ongoing trials undertaken by the Neidle group, progress has been made on the drug discovery front for pancreatic cancer in recent years, and advances to human trials may be on the horizon. Thus there is reason to be optimistic about the future of drug discovery for pancreatic cancer.
Blog written by Rachael Besser
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