Potency of Ovarian-Cancer Drug Magnified Many Times When Delivered to Cells Via Liposomes

BUFFALO, N.Y. -- A drug-delivery system that some pharmaceutical scientists had all but written off has been found to boost greatly the efficacy of a promising ovarian cancer treatment, according to a paper published by University at Buffalo scientists in the current issue of Pharmaceutical Research.

Cell-culture studies conducted by them showed that the potency of an ovarian-cancer drug increased by from 22 to 570 times when encapsulated in liposomes.

To trigger release of the drug from tiny lipid particles, the system exploits the increased acidity of compartments within cells, which form naturally as they take in nutrients.

The results provide some initial evidence that an ovarian-cancer treatment could eventually be formulated to be injected directly into the peritoneal cavity.

"Our liposome-delivery system keeps a drug out of the circulation, where it can become toxic to normal cells, and releases it only when it's inside cancer cells," said Robert M. Straubinger, Ph.D., UB assistant professor of pharmaceutics and principal investigator.

Liposomes -- microscopic lipid balloons that can be filled with a drug and targeted to specific tumor sites -- were first considered an ideal delivery system for getting drugs directly to cancer cells. However, problems with stability, significant cost and a poor understanding of how they could be used therapeutically caused many scientists to become increasingly skeptical, Straubinger explained.

Results of the UB cell-culture studies demonstrate that by customizing both the liposome and the drug it carries and then applying it to the "right" disease, scientists could make very stable delivery systems that are even effective against some drug-resistant strains of ovarian cancer.

"If the system succeeds in animal-tumor models, this could move relatively quickly to clinical trials," said Straubinger.

The research demonstrates that when tested against human ovarian-cancer cell lines, including drug-resistant ones, a drug called PALA encapsulated in liposomes became from 22 to 570 times more potent than it was when not encapsulated.

"PALA may be the most nearly ideal liposome-dependent agent yet described," he said.

The work follows up on leads, established by Timothy Heath at the University of Wisconsin at Madison, that showed that encapsulation of PALA in liposomes could improve the drug's delivery into cultured cells that were used as test targets.

One of the first "rationally designed drugs," where new molecules are tailored to "fit" a specific target, PALA is an extremely potent agent once inside ovarian cancer cells. The drug is most toxic to cells when they are dividing. In earlier in vitro experiments, Straubinger explained, researchers found that the problem was getting it inside the cell.

"PALA gets into cancer cells by a process that could be described as cellular drinking, where the cells drink up bits of the fluid around them," he said. "The fluid the cell consumes is taken in by small vesicles that form at the cell's surface, and are then carried into the cell. When these vesicles become acidic, the PALA is able to slip into the cytoplasm of the cell and bind to the target enzyme."

Because this process only lets in small amounts of fluid, very little PALA was found to penetrate cancer cells when it was administered intravenously. The drug also does not cross membranes easily, which, Straubinger said, hampers its ability to escape the small vesicles once inside the cancer cell.

The liposome-encapsulated PALA avoids both of those problems.

Straubinger explained that many more molecules of PALA can be packed inside liposomes than can be delivered to cells as free drug.

"The drug within liposomes is thousands of times more concentrated than it could ever be in the body as free drug," said Straubinger.

In addition, liposomes are taken up by the same mechanism with which cells consume certain nutrients, by binding to a cell's surface receptors and becoming engulfed in small vesicles.

Straubinger explained that liposomes need to be highly charged so that a drug won't leak out before it gets into cells, and to promote the binding of liposomes to target cells. But once the liposome is inside the tumor cell, it must be able to release the drug so that it can go to work, blocking the cell's unregulated division.

"The vesicles that form when liposomes bind to cell receptors become progressively more acidic," said Straubinger. "As they do, the drug loses some of its electrical charge, which enables it to slip across membranes more easily to the cytoplasm."

To target tumor cells, the UB group first used antibodies with liposomes. While this is a very selective way to target tumor cells, Straubinger noted that antibodies are very difficult to produce for human use.

"And manufacturing a product that included antibodies, liposomes and a new drug could be a regulatory nightmare," he said. "We have engineered a liposome that is extremely stable and has a natural target mechanism."

The liposome's ability to be effective with drug-resistant tumor cells is also important. According to Straubinger, drug resistance is a major issue in treating ovarian cancer.

"Detection usually occurs at a fairly advanced stage," he said. "Initial treatment with surgery and drugs often puts the cancer into remission, and then drug-resistant tumors grow back over a period of months or years."

Destroying these hardy survivors requires a potent drug, Straubinger explained, but the more potent a cancer treatment is, the more damage it may do to normal tissue.

For this reason, scientists have been interested in injecting ovarian-cancer drugs directly into the peritoneal cavity. Straubinger noted, however, that the whole area is so richly supplied with blood vessels, most drugs are cleared from it before they have a chance to be effective.

"The idea is that since liposomes are particles much larger than drugs, they are cleared by a much slower mechanism," he said. "Even if the liposomes do drain into the bloodstream, the drug is still trapped inside the particle. The acid-release mechanism should only occur within cells, so the drug will not be available to damage normal cells."

Straubinger noted that because the PALA formulations his group used are similar to some already under investigation in human trials, he is optimistic that the liposome-encapsulated drug should be able to be applied in vivo fairly easily.

"The problem is testing whether the concept of tumor targeting and acid-triggered drug release actually can occur in the much more complicated environment of real ovarian tumors," said Straubinger.

The next step is to test whether the approach works in the much more complex environment in the body, first applying it in special immune-compromised mice that have human ovarian tumors, and then in humans.

Co-authors on the paper are Amarnath Sharma, a doctoral candidate at UB, and Ninfa L. Straubinger, a research support specialist.