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Purdue 'stealth compounds' attack cancer cells

Purdue University

Imagine ordering a part to repair your car, and having the new part delivered in pieces you must first assemble.

A similar situation often occurs in treating cancer, because the components needed to put the brakes on the cells' abnormal growth can be readily delivered through the cell membrane only in pieces that then must be assembled by the cell.

Scientists at Purdue University have developed a method for getting these compounds, called nucleotides, into tumor cells -- already assembled.

The method may lead to the development of new, more powerful treatments that have fewer side effects and are less likely to produce drug resistance in patients being treated for cancer and certain viruses such as HIV, says Richard Borch, principal investigator of the study who is the Lilly Distinguished Professor of Medicinal Chemistry and Molecular Pharmacology at Purdue.

"Potentially this system will work for all types of cancer, and it may prove useful in treating cancers that have been resistant to other treatments, such as pancreatic cancer," Borch says.

Nucleotides, which act as building blocks for the molecules that make up DNA and RNA, also carry out several essential functions needed for cell replication. By delivering specific forms of nucleotides to a cell, scientists can throw a chemical wrench into the cell's machinery to block the replication of viruses and cancer cells.

"Nucleotides can be used in a number of ways to inhibit a specific critical pathway that a cell requires to proliferate," Borch says. "The problem was, we couldn't deliver nucleotides directly into a cell because they carry a negative charge that prevents them from crossing the cell's membrane."

Many current therapies instead deliver precursor compounds that the cell then uses to "build" nucleotides.

Borch and his group have developed a way to hide the negative charge, creating a "stealth compound" with specially designed nucleotides that can cross the cell membrane undetected. The method works by adding another chemical component to the charged phosphate of the nucleotide. Once the compound enters a cell, enzymes break it apart, releasing the nucleotides into the cell.

Because the enzymes needed to break the compound apart are found only in cancer cells, the new method may allow researchers to develop drug therapies with fewer side effects, Borch says.

"Though a stealth compound is capable of entering normal cells, without the particular enzymes needed to break it down, the compound cannot release its nucleotides and should have little effect on the cell's ability to function," he says.

The method, described in detail at the recent American Association for Cancer Research meeting in San Francisco, will allow scientists to build on many current treatments that now require a time-consuming process to form nucleotides from precursor compounds, called nucleosides.

"The problem with using nucleosides is that viruses and cancers get 'smart' and quit carrying out the conversion necessary to develop the nucleotides," Borch says. "When this happens, the patient will develop resistance."

So far, the new method has been tested using an established bioassay and a mechanism-based assay developed by Borch and his research group at Purdue's Cancer Center. The researchers now plan to conduct animal studies using anti-cancer compounds developed at the center.

Though pharmaceutical companies currently have several technologies available for carrying nucleotides into cells, those existing methods fall short in several ways, Borch says.

"With current technologies, the activation process takes place in the bloodstream, rather than inside the cell. This means that you lose a lot of the drug, making it difficult to deliver adequate amounts of the drug to the cell," he says.

"Also, the activation process is very slow. The use of these nucleotides requires a two-step process, and the existing technologies are so slow that the cell can't accumulate much nucleotide because it is breaking down the components almost as quickly as it processes them."

Borch says his group's new method has the advantage that there is no premature breakdown in the bloodstream, and it carries out the process very quickly.

Purdue has filed for a patent on the new technology.

Borch says he already has been contacted by several pharmaceutical companies interested in the new method.

"Companies are interested in the new technology because they already have potential compounds that could be delivered with this system," he says.


Borch's studies at Purdue are supported by the National Cancer Institute.

Related Web site: Purdue Cancer Center,

ABSTRACT The design of prodrugs that can release nucleotides intracellularly represents an attractive therapeutic approach to the treatment of cancer. Our strategy utilizes a phosphoramidate incorporating an ester that undergoes intracellular enzymatic hydrolysis and an amide that undergoes spontaneous P-N bond cleavage after liberation of the ester group. A series of 5-fluoro-2'-deoxyuridine haloethyl phosphoramidate analogues containing a benzotriazolyl or nitrofuryl ester moiety was prepared and evaluated for growth inhibitory activity against murine L1210 cells. The best compounds exhibited potent inhibition of cell growth (IC50 = 10-50 nM) that was reversed by the addition of 5mM thymidine. 31P NMR studies were carried out in order to determine the mechanism of activation of these compounds. The results show that initial cyclization of the haloethyl phosphoramidate anion is followed by non-selective nucleophilic attack of water at both carbon and phosphorus of the resulting aziridinium ion intermediate, suggesting that approximately 50% of the prodrug is converted to nucleotide. Further modification of the phosphoramidate moiety has provided a novel prodrug that undergoes rapid and quantitative conversion to the corresponding nucleotide. This compound is the most potent inhibitor in the series (IC50 = 2 nM). Supported by grants R01 CA34619 and T32 CA09634.

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