A natural antibacterial, anticancer drug, calicheamicin by name, is 5,000 times as potent as adriamycin, a frontline antibiotic and anticancer drug. However, calicheamicin can't be used clinically - with one exception - because it's too toxic.
"Calicheamicin is probably the most potent cancer agent ever discovered," observed biochemist and molecular biologist Chris Farnet, president and CEO of Ecopia Biosciences Inc. in Montreal. "Its root word, caliche," he surmised, "comes from the Spanish word for clay. Its particular microorganism, Micromonospora echinospora ssp. calichensis, was unearthed from the clayey soil of Texas in the mid-1960s by scientists at American Cyanamid Lederle, now Wyeth.
"It really captured the fascination of chemists for many years," Farnet continued, "when the full chemical synthesis of calicheamicin was finally achieved in 1993 by K.C. Nicolau at the Scripps Research Institute in La Jolla, Calif. Its Micromonospora microbial origin was shown to be a member of a unique molecular-architecture family called the enediynes, which has a very complicated structure we call its warhead.'
"It belongs to the bacterial family of Actinomycetes, which are the most prolific producers of antibiotics and anticancer agents - notably erythromycin, tetracycline, vancomycin, streptomycin, doxorubicin - and the edeiynes, which are classified as antitumor antibiotics. They're DNA-reactive agents," he went on. "One portion of the molecule targets DNA. That allows proximity of the warhead against the product's genomes. When the warhead's ring structure comes into contact with the double helix, it causes double-strand breaks that are lethal to the cell, hence toxic. When the warhead touches the DNA, it explodes. It's really a diabolical molecular machine." He noted that "interestingly, calicheamicin polyketide synthase [PKS], 1,919 amino acids long, shows remarkable similarity (56 percent identity and 67 percent similarity overall) in both sequence and organization to an enediyne PKS from the 85-kilobase C-1027 biosynthetic gene locus."
Two Science Articles Present Data As One
Farnet is senior author of a paper in today's issue of Science, dated Aug. 16, 2002, and titled: "The calicheamicin gene cluster and its iterative type 1 enediyne PKS." An accompanying back-to-back article in the same Science bears the title: "Biosynthesis of the enediyne antitumor antibiotic C-1027." Its senior author is pharmaceutical scientist Ben Shen at the University of Wisconsin at Madison. Shen sits on Ecopia's scientific advisory board.
"We've found the genetic raw material to produce these compounds," Shen said. "With the genes in hand, we can take them apart and put them back together and that will allow improvements and development of new compounds. These are tamable microbes, and that lays the groundwork for the amount of the molecule that you can produce through fermentation. Through genetic engineering," he added, "we can also now alter the enediynes' reactivity, and address issues of metabolism and production."
Farnet digressed in order to parse his company's name: "Ecopia is a fusion of eco,' meaning from nature, and copia,' abundance - abundance from nature.
"The really novel aspect of our two papers in Science," he commented, "is that for many years it was debated how these complex warhead structures would be synthesized in nature. Together with our collaborators at the University of Wisconsin, we used genomics to discover and analyze the genetic blueprint for calicheamicin from a microorganism that naturally produces the compound. The work presented in the two Science articles shows just how good nature is at making complex bioactive molecules. We found an entirely new class of polyketide synthase in both loci. PKSs are enzymes well known to be involved in biosynthesis of other natural products - such as a number of antibiotics. But this PKS is different from all the others. It appears that this one simple enzyme is able to do most of the work in constructing the warhead. This discovery will open up new avenues for synthesizing new enediyne compounds. It's the first glimpse of nature's blueprint for making such very complex and potent drug-like molecules."
Farnet pointed out, "The C-1027 antibiotic, produced by Streptomyces globisporus, is also a member of the same family as enediynes. It has the same warhead structure in common with calicheamicin. Previously, it was thought that C-1207 and calicheamicin represented two very distinct types of enediyne warhead. But what our two papers show is that the enzymatic machinery behind making this warhead is the same."
He made the added point that "calicheamicin, while too toxic as such - and the C-1027 likewise - can both be domesticated somehow by conjugation with antibodies. It's like creating a smart bomb," he said. "They're too toxic for systemic use because they will get into any cell and break its DNA. But with an antibody, you can target it to a particular subset of cells. So it can be a very effective anticancer agent, as seen with the anti-leukemia drug Mylotarg [Wyeth]."
One Approved Compound Eludes Rule
The chemical name for Mylotarg is gemtuzumah ozogamicin. It's composed of a recombinant humanized antibody conjugated with calicheamicin. Its antibody portion binds specifically to the CD33 antigen found on the surface of leukemic cells in more than 80 percent of patients with acute myeloid leukemia.
"Being able to create safe antineoplastic drugs by this strategy," Farnet allowed, "bodes well for finding new enediynes that possess better properties. So this development gives us the ability to look for new systems that might be more amenable to developing into therapeutics. Because there's still a lot that's not known about the potency and targeting of enediynes, it doesn't mean there's an imminent advance in converting these anticancer agents.
"What we're doing right now," he recounted, "is looking for new classes of antibiotics and antifungal agents from Actinomycetes - this very natural product source, from these soil-dwelling microorganisms. We scan the genomes of organisms that come from the soil, and use them to predict which of them are able to produce new natural products - new antibiotics, new anticancer agents - and turn them on that way. Genomics is much more sensitive than just growing the organism on a broth and testing for activity.
"The company," Farnet concluded, "has filed for patent protection on the new class of polyketide synthase, its related genes and other genes and proteins involved in enediyne production," with Farnet as an inventor.