By David N. Leff
When a normal, healthy cell finds itself transformed into a malignant one, it leaves home, and disguises its former identity.
"Cancer cells grow out of control," explained structural biologist Michael Finnin, "so they're no longer cytodifferentiated to muscle cells, or bone cells or anything like that. They are without a set life span or a purpose. Transformed cells don't have these controls any more, and that leads to cancer."
Restoring that lost cytodifferentiation pattern - rather than cutting, burning or poisoning malignant tumors with surgery, radiation or chemotherapy - is a trendy new approach among research oncologists. One leading player in this arena is the Memorial Sloan-Kettering Cancer Center in New York. Finnin is a Howard Hughes postdoctoral X-ray crystallographer at the center.
He is lead author of an article in today's Nature, dated Sept. 9, 1999, which reports structural analysis of a promising new anti-tumor compound called SAHA. The paper's title is: "Structures of a histone deacetylase homologue bound to the TSA and SAHA inhibitors."
"What agents such as SAHA do," Finnin told BioWorld Today, "is tell the transformed cells to differentiate and became once again a certain set of cells. Then they can have a programmed existence, and live and die like normal cells."
SAHA stands for "suberoylanilide hydroxamic acid." Cell biologist Victoria Richon, a co-author of the Nature paper, told BioWorld Today, "SAHA was discovered in our laboratory, and synthesized by Ronald Breslow, a chemist at Columbia University. He, Paul Marks, Richard Rifkind and myself [all co-authors] were working on the concept of developing agents that induce differentiation of transformed cells. (Marks directs the Developmental Cell Biology Program at Sloan-Kettering, and is the center's president.)
Richon outlined the basic premise of their strategy: "Most transformed cells are blocked in their cytodifferentiation pattern, but when they're transformed you know they are breast carcinoma or bladder carcinoma, for example. So you're going to induce redifferentiation along the pathway that's been blocked."
As for how SAHA does what it does, Richon went on: "We're working very hard on trying to understand the drug's mechanism. We had a big breakthrough when we discovered that one of SAHA's cellular targets is an enzyme called histone deacetylase. That's certainly one of the things we're working on right now, in trying to understand how inhibition of human histone deacetylase leads to either induction of differentiation, or cell growth arrest or apoptosis of cells in culture."
It's A Wrap For Nuclear DNA
Histones are proteins in the nuclei of cells. They serve as cores or spools, around which the cell's long DNA chains are wound. For a gene in that chain to be expressed, the DNA must be unwrapped from its histone core. The histone deacetylase enzyme keeps it tightly wound. So when SAHA blocks that enzyme, it lets the DNA unwind, thus making genes accessible to the cellular switches that turn them on.
"What SAHA does," Finnin pointed out, "is bind to the active binding site of histone deacetylase, and prevent it from functioning." He did most of the work involving the crystallography and biochemistry of the drug binding, as depicted in Nature. The center's scientists acclaim this as "the first-ever 'snapshot' of a new drug interacting with its cellular target."
"It's an atomic-resolution model," Finnin observed. "We have a picture - a map - of how SAHA interacts with its target enzyme. So we can see one of its key chemical groups, hydroxamic acid, binding to an active-site metal, zinc. That is probably the business end of the enzyme. The structure of the active site is a long tube, slightly pinched in the middle. The structure of the drug is a long extended molecule, which fits right into the active site tube and plugs it up, preventing the histone deacetylase enzyme from working."
As a matter of fact, that X-ray crystallographic photo lineup didn't capture the atomic structure of human histone deacetylase, but of a cut-out - a surrogate molecule. "We solved the drug's binding not exactly to the human enzyme," Finnin allowed, "but to analogues in bacterial histone deacetylase. The real enzyme," he added, "is extremely difficult to overexpress and purify, and have enough quantities to do X-ray crystallography. We would have needed maybe 1,000-fold of what we could get of the real histone deacetylase."
Whereupon, he and his co-authors hunted in databases for an analogue, perhaps yeast cells. Instead, they hit upon a far farther-out source, the hyperthermophilic bacterium Aquifex aeolicus, which shares 35.2 percent identity with the human enzyme.
"I guess the fact that this thermophile is living in such high temperatures - 95 degrees Celsius," Finnin surmised, "maybe had adapted its enzyme to be more stable, so we could get a lot of quantities of it that we could work with. Then the Marks group gave us information on the SAHA drug, and helped us assaying the thermophile enzyme, to make sure it was similar enough to the real histone deacetylase that it would be relevant."
Putting SAHA To The Anti-Tumor Tests
Meanwhile, the cell biologists were running preclinical in vitro and in vivo tests of SAHA's tumor-blocking potential. "One of the possible systems that we've studied," Richon recounted, "is acute promyelocytic leukemia, in which histone deacetylases are recruited to the oncogenic transcription factor that causes the leukemia. We've also looked at other models of cancer as well - prostate cancer in nude mice and, with SAHA again alone as therapy, at carcinogen-induced mammary tumors in rats. In both cases we found that this drug inhibits the growth of tumors."
Now Sloan-Kettering is gearing up for Phase I clinical trials of SAHA in patients with advanced breast, lung and prostate cancers. "We're hoping to begin this study before the end of the year," Richon said. "It will be strictly Phase I for solid tumors initially. This type of compound has never been given before," she pointed out, "so we want to start off administering SAHA by intravenous injection, in order to control the toxicity. However," she concluded, "we hope to move on into oral dosing."