The presence of the telomerase enzyme in the majority of cancers and its conferral of unrestricted proliferation to cancer cells presents a major dilemma in the treatment of cancer. Both radiation therapy and chemotherapy cause an increase in telomerase activity (TA) in cells comprising the tumor. When activated, the enzyme prevents the shortening of the telomeric ends of DNA and protects the chromosomal ends of DNA from deteriorating or combining with other chromosomes, by adding the TTAGGG sequences to the ends of chromosome's telomeres. Shortening of the telomeres causes the cancer cells to undergo various death modes or to become sensitive to various treatments and increases the probability of successful cancer therapy.
It may thus be appreciated that the activation of telomerase that occurs following chemo- or radio-therapy can alter the risk vs. benefit ratio of these treatments. That is, if the treatments result in the activation of telomerase in those cells that survived the primary treatment, they could become immortal due to the lengthening of the telomeres. Therefore, inhibiting the activation of TA would be a very useful approach in controlling tumor growth and many investigators are synthesizing and evaluating agents to inhibit TA for this purpose. Whereas telomerase activation and its effect on maintaining the length of the telomeres are protective to normal cells, both features are highly detrimental when attempting to induce lethality in cancer cells.
During normal cell division, when the cell reaches its finite life span, its telomeres are degraded. However, in human cancer cells, telomeres are continuously replenished by the human telomerase reverse transcriptase enzyme (hTERT), which is a subunit of telomerase. Thus, the activation of hTERT is problematic because it increases the resistance of cancer cells to various therapies. Many studies have shown the activation of hTERT as a factor in the treatment resistance to ionizing radiation; similarly, hTERT increases the resistance to chemotherapeutic agents. Therefore, numerous scientists are also investigating the use of telomerase- or hTERT-inhibiting agents to improve the efficacy of radiation or chemotherapy treatments. Among these drugs is tetra-(N-methyl-4-pyridyl)-porphyrin (TMPyP4) that binds to and stabilizes G-quadruplex (GQ) structures both in DNA and within the telomeres.
In DNA, the formation of a GQ structure in the promoter region of the c-myc oncoprotein is kinetically favored. Mutations occurring during chemo- or radio-therapy were shown to increase c-myc transcriptional activity 3-fold. Such over-expression results in gene amplification (e.g., multiple copies of c-myc), the formation of extra-chromosomal elements, chromosome breakage, deletions, increased aneuploidy, and polyploidization; all of these consequences demonstrate the instability caused to the genome by c-myc over-expression. Because TMPyP4 stabilizes the c-myc GQ, it can suppress its transcriptional activity and over expression. Of relevance is the fact that the hTERT gene, which encodes the catalytic subunit of telomerase, is also transcriptionally regulated by c-myc. Therefore, the stabilization of c-myc by TMPyP4 would additionally cause a decrease in hTERT levels and a reduction in telomerase activation (Papanikolaou V. et al., 2011, Int J Radiat Biol 87, 609-621), increased hTERT and TA was shown after irradiation of breast cancer cells and resulted in increased survival of the cells. This study showed that the HER2 receptor mediated hTERT expression through the sequential induction of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and c-myc.
It may therefore be hypothesized that the radiation-induced mutations increased c-myc transcriptional activity and resulted in its over-expression, thereby increasing hTERT and telomerase activation as is discussed later. Therefore, the stabilization of GQ structures at the c-myc promoter locus could offer a very important contribution to cancer treatment.
Although several telomerase-inhibiting drugs have been synthesized, experimental results using said drugs have indicated that they are of limited efficacy. In this regard, the present inventors have now found that this problem is at least in part due to the rapid clearance of hydrophilic agents (for example the aforementioned TMPyP4) from the tumor. It appears that inhibition of the telomerase enzyme requires the continuous presence of the inhibitor in the tumor over the long term.
Various uses of porphyrins in treatment of cancer have been disclosed in WO03/063757. Porphyrins, such as metalloporphyrins of indium, gadolinium, platinum, palladium and gold, have been implied as Auger emitters, useful in conjunction with radiotherapy, such as brachytherapy.
A large number of controlled-release drug delivery compositions and devices have been described in the prior art. One such prior art publication (U.S. Pat. No. 6,206,920) discloses and teaches an in-situ forming injectable implant composition comprising poly(lactic-co-glycolic acid) copolymer (PLGA) in a solvent which is glycofurol.
U.S. Pat. No. 5,366,734 describes the combination of polylactide and a pharmaceutically active ingredient for the continuous release thereof in the form of a solid implant.
WO2011/071970 describes a combination of PLGA and porphyrins for photodynamic therapy.
It may be appreciated from the foregoing that there is a pressing need for a long-term, controlled delivery system that is suitable for use with telomerase-inhibiting agents.