Researchers at Kolkata’s Indian Association for the Cultivation of Science (IACS) have synthesised a novel compound that shows potent anticancer activity. The porphyrin compound selectively targets and binds to cellular topoisomerase 1 enzyme (which is essential for maintenance of DNA architecture in the cells) found in cancer cells. Since Top1 enzyme is essential for cell replication and transcription from DNA to RNA, inhibition of its activity leads to DNA torsional strain, overproduction of reactive oxygen species, degradation of the DNA and ultimately cell death.
Though the currently available drug and its derivatives and the compound synthesised by IACS researchers target Top1 enzyme, the pathways are very different, which was validated in both in vitro and ex vivo cellular studies. As a result, even when chemoresistance to currently available drug (camptothecin) sets in, the new compound could be used for effective killing of cancer cells.
“We used knocked-down breast cancer cells where the amount of Top1 enzyme in the cells was less. When challenged with our compound the cytotoxicity was less. This confirms that Top1 in the cell is the target of the compound and overexpression of Top1 by cancer cells will attract more of the compound leading to cell death,” says Prof. Benu Brata Das from the Department of Biological Chemistry at IACS and corresponding author of a paper published in Journal of Medicinal Chemistry.
Before a cell replicates, the Top1 enzyme binds to the DNA, cuts it and unwinds the DNA before rejoining the cut DNA. While the currently available drug inhibits the Top1 enzyme after the DNA has been cut, the porphyrin compound comes into play much earlier by inhibiting the Top1 enzyme from even binding to the DNA.
“Cellular experiments show that the compound acts well before the enzyme can cut the DNA [which is when the currently available drug begins to act]. This gives us confidence that our compound may be effective in patients who are chemoresistant to the existing drug,” Prof. Das says.
Cellular studies using breast cancer cells showed that the compound induced 3-5 fold increase in reactive oxygen species (ROS) in cells. Elevated levels of ROS in a cell can lead to DNA strand breakage. “The breast cancer cells had about 8-fold increase in DNA strand breaks due to elevated ROS resulting in about 7-fold increase in cell death,” says Subhendu Kumar Das from IACS and first author of the paper. “There were elevated ROS levels when we increased the duration of treatment.”
“Porphyrins are well known to produce increased ROS inside cells. But so far only the cationic and anionic porphyrins have been studied. This is the first time a neutral porphyrin is shown to produce elevated ROS,” says Dr. Harapriya Rath who synthesised the compound. She is from the Department of Inorganic Chemistry at IACS and the other corresponding author.
The compound showed potent cytotoxicity when tested on different cancer cell lines — cervical cancer, ovarian cancer, and colon cancer — while it showed “markedly reduced or no toxicity” in the case of human embryonic kidney cell lines and mouse embryonic fibroblasts.
While the synthesised compound can also be taken up by healthy cells, the uptake is more in the case of cancer cells. “Since the rate of proliferation is higher in the case of cancer cells, the chances of drug uptake are higher,” says Prof. Das. “Studies show that our compound is a potential anticancer agent. More studies are needed to confirm the lab results,” he says.