Expanded Peptide Potential Revealed in Surprise Discovery
Johns Hopkins researchers aren't sure whether they would call their recent discovery a "happy accident," but they would at least chalk it up to a great bit of luck.
 Stephen J. Meltzer, M.D. (left), and John M. Abraham, Ph.D., of The Johns Hopkins University in Baltimore, made a surprising discovery while tagging peptides to see how well they bonded to colorectal cancer cells. The researchers found that the radioactive phosphorous 32P tag was actually being absorbed into the cancerous cells, suggesting 32P-labeled peptides can potentially be used both as highly effective radiation treatment delivery systems and as a method of detecting cancerous cells and growths in the colon. |
While tagging peptides to see how well they bonded to colorectal cancer cells, gastroenterology researchers at The Johns Hopkins School of Medicine were surprised to find that the radioactive phosphorous 32P tag was actually being absorbed into the cancerous cells. The surprising discovery suggests that such 32P-labeled peptides have the potential for use both as highly effective radiation treatment delivery systems and as a method of detecting cancerous cells and growths in the colon.
John M. Abraham, Ph.D., and Stephen J. Meltzer, M.D., both of the Department of Gastroenterology and Hepatology at The Johns Hopkins University, said they did not set out to test radiation cell-delivery systems.
"The original design of the experiments employed 32P merely as a tag that could easily be followed and quantified," said Dr. Abraham, an assistant professor of medicine. He added that 32P is primarily used to treat red-blood-cell polycythemia and platelet essential thrombocythemia. "It was a surprise when we discovered that the 32P was actually being incorporated into the cellular proteins. After that, we began to research whether this radioisotope might be used therapeutically."
Smaller is Better
The discovery raises the possibility that 32P or similarly tagged peptides could deliver radiation treatment directly into the "heart" of a cancerous cell, rather than bombarding it from the outside. This could be especially effective on thick colon cancer tumors, which are often more resistant to drug penetration. "In the world of molecule therapeutics for the treatment of tumors, the belief is that smaller is better—the smaller an agent is, the better it can penetrate the tumor," said Dr. Abraham. "An antibody molecule is gigantic compared to the tiny peptides we are using—our tiny peptides are less than 3 percent of the size of an antibody molecule."
The therapy might also mean patients would experience fewer side effects such as nausea, gastrointestinal bleeding and low blood cell counts. "Peptides penetrate into tumors more easily and are more rapidly excreted by the kidneys, in turn reducing systemic toxicity," said Dr. Meltzer, the Harry and Betty Myerberg/Thomas R. Hendrix Professor of Gastroenterology and director of the Early Detection Biomarkers Laboratory at Johns Hopkins. Because the peptide contains the human enzyme protein A substrate, the body's immune system may not attack it, Dr. Meltzer added.
"The initial experiments used peptides of many different sizes, and one of the first peptides that resulted in extremely strong results happened to be a decapeptide with 10 amino acids," said Dr. Abraham.
While generating additional small 32P-labeled peptides, the researchers found that "of the nine members of the decapeptide family described in our recent paper, peptide MA5 bound best and transferred more radioisotope to cell lines derived from the human colon adenocarcinomas," said Dr. Abraham. "The peptides vary from one another by one to three amino acids, and these slight changes can make a great difference in their cellular binding and transfer abilities."
"We think that this random generation strategy will give us an unlimited library of peptides to choose from in designing and matching individualized therapies," said Dr. Meltzer.
Imaging Possibility Investigated
"It is hoped that increasing the number of potential peptide candidates will facilitate more options regarding possible imaging and treatment opportunities," said Dr. Abraham.
The radiation emitted by the peptides also raises the possibility that they could be used to detect cancerous colon cells. Drs. Meltzer and Abraham are now working with Martin Pomper, M.D., Ph.D., of the Department of Radiology at Johns Hopkins, whose research includes small animal imaging in drug development. "We are essentially molecular biologists with a focus on the gastrointestinal tract," said Dr. Abraham. "It's important to collaborate with experts in the imaging field. The 32P radioisotope is not used in current imaging tests, so we are working with Dr. Pomper and his team to develop peptides tagged with radioisotopes suitable for imaging."
"Particularly in cases where a tumor has already been resected, this test promises to be highly sensitive in detecting recurrences early," said Dr. Meltzer. "However, it may also be sensitive in detecting primary cancers early. A third use, where it is anticipated to be highly effective, would be to detect and treat metastases. In addition, the same peptides may end up being used to deliver therapy such as radiation to the tumors, killing two birds with one stone."
According to Dr. Pomper, other radioisotopes being tested include radioiodine, technetium-99m and fluorine-18. Dr. Pomper's team has been working with Drs. Abraham and Meltzer only for a few months. If all goes as planned, the research will move from the current stage of chemical synthesis into in vitro testing and then to animal models. Depending on those results, the final step would be toxicity testing en route to a clinical trial, which, Dr. Pomper said, is probably a couple of years away. "We'll start with imaging and if it works we can then pursue therapy," said Dr. Pomper. He noted that one important aspect of the work will be "figuring out the mechanism by which these compounds bind."
Of course, at the end of any such testing comes the potentially long road of FDA approval. Dr. Abraham noted, "We are proposing this as a possible use for imaging and the treatment of colorectal cancer, which is a very prevalent and devastating type of cancer. The FDA has stated that they are trying to put certain therapeutic approaches on a 'fast-track' for approval, so this work might fit into that classification."
Individualized Therapy Possible
Perhaps the most exciting aspect of the peptide binding discovery, said the researchers, is the hope that similar processes could someday be used on a variety of human cancers. "Another advantage is potential individualized therapy, with a unique peptide tailored to each patient's individual tumor," said Dr. Meltzer.
Added Dr. Abraham: "Over 95 percent of all colon cancers are of the adenocarcinoma classification. Although it is too early to tell, we are hopeful that additional work will generate peptides that might be utilized against other types of cancer."
"This is potentially a big deal if we manage to get past all the aforementioned hurdles," said Dr. Meltzer. "It could revolutionize the staging, management and surveillance of colon cancer patients."
