Researchers at the Indian Institute of Science, Bengaluru have synthesised a small molecule that shows a degree of promise as an anticancer agent. In particular, the inhibitor was effective against leukaemia. The work was done in collaboration with researchers from the University of Mysore.
The molecule (benzothiazole derivative), codenamed 5g, was found to be effective in inhibiting cell proliferation in both leukaemia and breast cancer cell lines. This was achieved by arresting a particular phase (G2/M) of the cell cycle, thereby preventing cancer cells from dividing and growing in number. In the case of mouse models, the 5g molecule was able to arrest tumour growth without causing significant side effects.
The inhibitor was able to arrest the cancer cells from proliferating by elevating the levels of intracellular reactive oxygen species (ROS), which, in turn, causes DNA damage by breaking the DNA’s double-strands. The molecule also activated the cell death pathway when higher concentration was used. However, the molecule did not cause any damage to normal blood cells. The results were published in the journal Scientific Reports.
“Depending on the dosage, the molecule can either kill or cause DNA damage thus arresting normal cell cycle , or allow the cells to repair the DNA double-strand breaks and revert back to normal cell cycle [at lower concentrations],” says Dr. Sathees C. Raghavan from the Department of Biochemistry at IISc and the corresponding author of the paper.
“At this point we don’t know how exactly the 5g molecule is inducing ROS inside the cells. However, it is well established that elevated levels of ROS damages the DNA,” says Dr. Mahesh Hegde from the Department of Biochemistry at IISc and the first author of the paper; he is currently a Postdoc Research Associate at the University of Massachusetts Medical School, Worcester, Massachusetts, U.S.
Safety of the molecule
At a dosage of 50 micromolar, about 70% of leukaemia cells were killed, compared with 25% of normal blood cells. This suggests that the 5g molecule could be “less toxic” to normal cells compared with cancer cells. “25% cell death was observed when we cultured normal cells in the lab. However, animal studies did not show significant changes in blood parameters, kidney function and liver function tests,” clarifies Dr. Hegde.
Even when the dosage was reduced to 10 micromolar, the molecule was able to arrest the cell cycle particularly after 36 hours of treatment. However, at the end of 48 hours, the cells were either dead or repaired their DNA damage and proceeded with normal cell cycle of division and proliferation.
A majority of the cancer cells were killed but some reverted back to normal cell cycle. The reason for this is not known.
“Although the molecule is good, we are trying to synthesise derivatives so that they are effective even at a lower dosage. Right now, relatively high concentration of about 10 micromolar is required to kill leukaemia cells,” says Dr. Raghavan. “In the case of non-leukaemia cells, even higher concentration (10-30 micromolar) is required.”
In mouse models, the molecule was able to arrest cancer cells’ cell cycle progression when 60 and 120 mg per kg of body weight dosages were used. Also, “significant” reduction in tumour volume and “moderate” increase in life span were observed when treated with 60 mg per kg of body weight for 14 days. The molecule was able to reduce the tumour burden by arresting the cell cycle than by causing cell death, the researchers found.
Since on its own the molecule did not bring about cell death in mouse models, it cannot be used as a standalone therapy. “From a clinical point of view, there is certainty when there is cell death. When cancer cells are not killed, there is a possibility that the arrested cells may revert back to normal cell cycle progression and that might lead to tumour relapse,” says Supriya V. Vartak from the Department of Biochemistry at IISc and one of the authors of the paper.
“This is a good proof-of-concept of G2/M cell cycle inhibitor. We feel there is scope for synthesising derivatives to get a potent chemotherapeutic agent,” says Ms. Vartak.