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the peptide keeps the toxin inactive.
But when G-202 encounters a cell with the enzyme PSMA (prostate-specific membrane antigen) on its surface, the enzyme clips the peptide off the toxic 12ADT molecule. The toxin, now insoluble in water but highly soluble in fats, can sink through the lipid-rich outer membrane of a nearby cancer cell and start its deadly work.
Dionne says G-202 is similar to the new class of targeted cancer drugs called antibody-drug conjugates, which are also designed to deliver a toxin only to particular tumor cells. These drugs combine a toxin with an antibody engineered to bind to a specific molecule on the surface of a cancer cell. However, antibodies are large proteins whose unique amino acid sequences and folding patterns can help them fit selectively only with certain cell surface receptors.
By contrast, G-202‘s targeting agent, the small peptide, could theoretically be clipped away from its toxic payload by enzymes other than PSMA, Dionne says. If that happened, the toxin could be released in the vicinity of normal cells rather than tumor cells. This is one of the reasons why GenSpera has spent years on preclinical testing and animal studies before launching its first clinical trial of G-202 in humans in 2011, he says.
Dionne says the company also took a close look at the cardiovascular effects of the experimental drug because of its molecular mechanism of action. Thapsigargin works by disrupting a molecular pump that regulates the level of calcium ions inside cells—and ion exchanges are central to the functioning of the cardiovascular system, Dionne says. Once inside any cell, whether a tumor cell or a normal one, thapsigargin causes a damaging influx of calcium, which starts a process of programmed cell death called apoptosis.
GenSpera also had to watch out for off-target effects of G-202 for another reason. Although the target enzyme PSMA appears in a higher concentration on cells of the prostate than on many other cells, it has been found as well in certain normal tissues, such as sections of the brain, small intestine, and kidney. Dionne says G-202 can’t pass through the blood-brain barrier. Studies of the drug’s action in animals and in humans so far have not turned up significant damage to the other normal cells bearing the PSMA enzyme, he added.
However, the discovery of enriched PSMA levels in yet another type of tissue has expanded the possible uses of G-202. The enzyme is more abundant on the cells of blood vessels that bring nutrients and oxygen to solid tumors than it is in normal blood vessels. Based on this finding, … Next Page »