The drug G202 is chemically derived from a weed called Thapsia garganica that grows naturally in the Mediterranean region. The plant makes a product, dubbed thapsigargin (see the structure, that since the time of ancient Greece has been known to be toxic to animals. In Arab caravans, the plant was known as the "death carrot" because it would kill camels if they ate it, the researchers noted.
Thapsigargin is,
(3S,3aR,4S,6S,6aR,7S,8S,9bS)-6-(acetyloxy)-4-(butyryloxy)-3,3a-dihydroxy-3,6,9-trimethyl-8-{[(2Z)-2-methylbut-2-enoyl]oxy}-2-oxo-2,3,3a,4,5,6,6a,7, 8, 9b-decahydro azuleno[4,5-b]furan-7-yl octanoate)
"Our goal was to try to re-engineer this very toxic natural plant product into a drug we might use to treat human cancer," says lead study author Samuel Denmeade, M.D., professor of oncology, urology, pharmacology and molecular sciences. "We achieved this by creating a format that requires modification by cells to release the active drug."
By disassembling thapsigargin and chemically modifying it, the researchers created a form that Denmeade likens to a hand grenade with an intact pin. [Thapsigargin prodrug G-202, is a cytotoxic analog of thapsigargin, 8-O-(12Aminododecanoyl)-8-O debutanoylthapsigargin (12-ADT) linked, via a carboxyl group, to the targeting peptide containing aspartic acid].
The drug can be injected and can travel through the bloodstream until it finds the site of cancer cells and hits a protein called prostate-specific membrane antigen (PSMA). PSMA is released by cells lining tumors of the prostate and other areas, and in effect "pulls the pin" on G202, releasing cell-killing agents into the tumor and the blood vessels that feed it, as well as to other cells in the vicinity. Specifically, G202 blocks the function of a protein called the SERCA pump, a housekeeping protein necessary for cell survival that keeps the level of calcium in the cell at the correct level, the researchers report.
"The exciting thing is that the cancer itself is activating its own demise," says senior study author John Isaacs, Ph.D., professor of oncology, urology, chemical and biomedical engineering at Johns Hopkins.
Because the drug is targeted to the SERCA pump, which all cells need to stay alive, researchers say it will be difficult for tumor cells to become resistant to the drug, because they cannot stop making the protein.
