Agent in Phase II Testing to Prevent Drug Resistance

HERSHEY, Pennsylvania—A phase II study is about to begin testing a novel approach to enhancing the effectiveness of chemotherapy, one that uses a new agent to deactivate a critical DNA repair mechanism in malignant cells. Shutting down this repair system appears to increase the killing power of currently available drugs that work by disrupting DNA.

“What we’re trying to do is breathe some new life into some relatively old agents while we wait for new agents that are being designed based on the molecular biology of cancer,” said Anthony E. Pegg, PhD, of Pennsylvania State University. Dr. Pegg’s research provided the rationale for the study and the agent to be used in the phase II studies.

The trial will be carried out at Case Western Reserve University, the University of Chicago, and Duke University. The study follows a successful phase I trial at the same three universities.

Dr. Pegg, professor of cellular and molecular biology at Penn State’s Hershey Medical Center, described the innovative effort at the Council for the Advancement of Science Writing annual seminar.

A number of DNA repair mechanisms exist to deal with damage caused by natural breaks in the DNA chain, radiation, and exogenous chemicals. Several of these pathways are quite general, while others are very specific, which is the case with the mechanism that Dr. Pegg and his colleagues elucidated. This specific pathway repairs adducts that occur in a small methyl group found within the DNA’s helical structure.

“The pathway is a unique reaction, which is brought about by a protein I call AGT [O⁶-alkylguanine-DNA alkyltrans-ferase], which repairs this DNA,” Dr. Pegg said. In the absence of this repair mechanism, some tumor cells express what is called the mer– phenotype. Such cells are very susceptible to certain cancer drugs, such as DNA methylating agents and DNA crosslinking agents. These drugs include carmustine (BCNU), lomustine (CeeNu), and dacarbazine.

“If you treat these tumors with these agents, they work very well,” Dr. Pegg said. “Unfortunately, very few primary tumors lack this repair protein, so most tumors are inherently resistant.”

He and his colleagues set out to develop an inhibitor of AGT that would convert mer+ tumor cells to mer^– cells so that the cancer drugs “would be more generally useful.” They named their inhibitor O⁶-benzyl-guanine. In the phase I clinical trials, researchers found that following an hour-long infusion of O⁶-benzylguanine, the repair mechanism activity “is lost completely and remains low for 20 hours or so,” Dr. Pegg said. “This is really quite encouraging.”

The inhibitor breaks down quickly into an active metabolite (O⁶-benzyl-8-oxoguanine), which appears in much larger amounts than the original agent “and is as good an inhibitor as the parent drug was,” he added.

Phase I data indicate that doses of the inhibiting agent up to 100 mg/m², a dose that completely inactivates the AGT protein, can be given safely.

“It doesn’t appear that inactivating this repair process, in this situation, has any toxicity on its own,” Dr. Pegg said. “However, when you combine the inhibitor with the agent BCNU, the myelosup-pression induced by BCNU is enhanced. The dose-limiting toxicity of these agents is myelosuppression.” As a result, the dose of BCNU given to patients is “approximately one quarter of that used in the current therapeutic regimens.”

Only further testing will reveal whether O6- benzylguanine is a sufficiently potent inhibitor to be clinically useful. “We think it is, but there is certainly plenty of opportunity to make a compound that is more active or that could be given more easily,” Dr. Pegg said.

He also questioned whether the targeted repair mechanism is the only one in tumor cells that can confer resistance to chemotherapy. “It is very hard to kill tumor cells selectively, and I think it is very unlikely that there aren’t other means of resistance to these agents,” he said. “But the key point is that this DNA repair protein is absolutely a primary means of resistance. If you don’t knock this DNA repair program out, you will never be able to exploit the full potential of these chemotherapy agents.”