December 1, 2006 (Vol. 26, No. 21)
Susan Aldridge, Ph.D.
Dipping into the Sea to Discover Therapies for Cancer and Infections
Natural products have long been a major source of medicines. In recent years, however, they’ve had to compete with synthetic chemicals, especially those from combinatorial chemistry. It has been suggested, though, that dropping natural-product programs has contributed to big pharma’s decline in productivity in recent years.
At “Bioprosp 2006,” the third international conference on marine bioprospecting in subarctic oceans, which took place in Tromso, Norway, delegates argued that natural products, especially those from marine sources, are ready to make a real impact in drug discovery and development.
The sea’s potential as a resource is largely untapped. So far, there are only four marine-based drugs on the market and about 40 in clinical/preclinical development.
Norway, with its long tradition of excellence in marine science, wants to play a global role in developing products from marine bioprospecting and has launched several initiatives to achieve this. One initiative is Mareano, an integrated mapping program for the Norwegian seas and coastal areas. Ole Jorgen Lonne, Ph.D., who heads the initiative, says the Barents Sea is especially important in terms of bioprospecting with over 3,000 species associated with the sea floor. “We probably know more about the Moon than we do about the bottom of the sea,” says Dr. Lonne.
Establishment of a Repository
The first requirement for any natural resource discovered in this way is the establishment of some kind of repository for R&D. David Newman, Ph.D., acting chief of the open and active repository programs at NCI’s Natural Products Branch, shared some examples of good practice. Such repositories need to be integrated at all levels—collection, processing, taxonomy, assay, and any other data.
The NCI repository, consisting of about 22,000 samples, is stored on 96-well plates, which are available free of charge to any researcher, so long as they sign a letter that is intended to protect the interests of the country of origin of the sample. NCI screens samples against its standard 60 cancer cell lines, but Dr. Newman would like to see more chemists getting involved in assessing them.
Though NCI does not do DNA work on natural product samples, the genomic aspect is becoming increasingly important. Nils Peder Willasen, Ph.D., and his team at the University of Tromso have sequenced and annotated what will likely be the first fish pathogen genome. Vibrio salmonicida is a cold-adapted halophilic bacterium that is responsible for a major fish disease. “Vibrio salmonicida is a model system for fish pathogens,” said Dr. Willasen. “It is also a source of unique enzymes and proteins.”
The Vibrio salmonicida genome will shed new light on virulence mechanisms. The team is also mining it for cold-resistant enzymes, having so far cloned, expressed, and solved the 3-D structure of a catalase and an endonuclease. They have also identified a potential fish vaccine from this work and four new antimicrobial targets from elucidation of the bacterium’s quorum sensing pathways.
Looking at the warmer seas of Fiji, Marcel Jaspars, Ph.D., professor of organic chemistry at the University of Aberdeen, Scotland is collaborating with the Institute of Life Sciences in Scotland and in Luxembourg. They have set up a screening assay program to assess activity against nuclear factor kB (NF-kB), which is a family of transcription factors involved in many diseases, including cancer and inflammation, and thus a major drug discovery target.
Out of a total of 224 crude extracts from algae, sea squirts, soft corals, echinoderms, and sponges, around nine percent showed strong NF-kB activity. The compounds responsible have been isolated, structurally characterized, and their mechanisms of action on the NF-kB pathway studied.
Dr. Jaspars also worked with a team at Dundee looking at neglected diseases, such as sleeping sickness, and found marine extracts to be a richer source of hits than a library of drug-like compounds (6.8% versus 0.8%). “This shows the power of natural products in preliminary screening,” states Dr. Jaspars.
The Sea’s Arsenal Against Infections
A major therapeutic area for marine natural products is infection. Tor Haug, Ph.D., associate professor at the Norwegian College of Fishery Science, Tromso, has isolated novel antimicrobial peptides (AMPs) from the blood of the small spider crab Hyas araneus and the green sea urchin Strongylocentrotus droebachiensis. Some of these AMPs contain modified amino acids that may protect them from degradation.
“Antimicrobial peptides are a new class of drugs,” says Dr. Haug. “They have a broad spectrum of activity against fungi, bacteria, and viruses. Their target is the bacterial membrane. They cause rapid lysis of the microbe and only rarely induce resistance.” He added that in terrestrial sources, there is a high rate of rediscovery of antibiotic compounds, which means the rate of novel compound discovery has been going down. In marine resources, the reverse is true. Also, marine compounds often contain halogen atoms, which terrestrial compounds rarely do, and this in itself may confer novelty.
Inge Nilsen, Ph.D., of the Norwegian Institute of Fishery Science, Tromso, has also been looking at new antibacterial weapons from the sea. Lysozyme is an enzyme with potent antibacterial properties that is present in all body fluids. It has recently been discovered that bacteria, mainly gram negatives, produce specific lysozyme inhibitors that disable this natural defense mechanism.
Dr. Nilsen has found several novel lysozyme inhibitors among a range of marine bacteria and is now looking for molecules that could block them. These molecules would not, in themselves, be antibiotics but would have the power to restore lysozyme’s ability to kill bacteria.
Marine-derived enzymes have huge commercial potential. The director of the Norwegian Structural Biology Centre (NorStruct), Arne Smalas, Ph.D., described work with the cold-adapted enzymes. “The best approach is to do comparative studies,” stated Smalas. Therefore, 3-D structures of salmon versus bovine trypsin, cod versus human uracil DNA glycosylase and the same enzyme, as well as endonucleases from V. salmonicida versus V.cholerae have been compared. These studies have yielded important clues to the structural features that are linked to an enzyme’s temperature profile.
Geir Klinkenberg, Ph.D., head of the high-throughput screening facility, described ongoing bioprospecting work at the Norwegian University of Science and Technology. They are searching the bottom sediments and surface biolayer of the Trondheim fjord for polyunsaturated fatty acids from eukaryotic protists, carotenoids from colored bacteria, and antimicrobials from actinomycetes. In the latter, the discovery program has so far yielded 33 isolates with activity against resistant fungi and 100 with activity against multidrug resistant enterococci.
Of course, terrestrial actinomycetes are well-established as an antibiotic source. “We put a lot of effort into dereplicating at an early stage but we still find a lot of known compounds,” said Dr. Klinkenberg. However, three to six potentially interesting compounds have been identified, and 100 more isolates are being subjected to further characterization.
The Oncology Sector
Another major area for marine-base therapies is in cancer. Stein Ove Doskeland, Ph.D., of the University of Bergen, described work showing that marine microorganisms are an unexpectedly rich source of antitumor and thrombosis-modulating agents.
Organisms from the bottom of Baltic and Portuguese seawaters appear to produce many compounds with apoptosis-inducing activity. His team has found that okadaic acid causes death of cancer cells by apoptosis in a similar way to radiotherapy.
Novel cyclic-peptides from benthic, Baltic Sea cyanobacteria can stop the death of hepatocytes, which may be helpful in preventing liver disease.
Meanwhile, a compound called B59 induces atypical apoptosis. It also inhibits platelet aggregation. “Marine bioprospecting is not just trawling the waters for anticancer compounds,” says Dr. Doskeland. “It is also elucidating marine toxin pathways that could identify new targets. In other words, how does nature kill cells? We should let nature do the screening and use microbes that can be grown in vitro, starting with materials from eco-niches that have to withstand harsh conditions, such as benthic areas.”
The bioactives extracted from marine resources in Norway clearly have great potential. MabCent, a new initiative based in Tromso, seeks to realize their value. “It is difficult in marine biotechnology to take a molecule all the way to market,” says Trond Jorgensen, Ph.D., who will head MabCent once it is up and running. He is currently scientific manager of MarBank, the newly opened National Marine BioBank, and Marbio, the associated screening platform, both of which will become part of MabCent.
The initiative will operate as a consortium of a number of stakeholders in Norway. It is going to integrate discovery research on marine resources and provide an infrastructure of research vessels and platforms (MarBank, MarBio, SmallStruct, and NorStruct—the latter will work on structural aspects of molecules discovered). MabCent will also integrate high level expertise in academia and industrial partners, which include Tromso-based companies Lytix (www.lytixbiopharma.com), ProBio (), and Biotec Pharmacon (www.biotec.no)
MabCent is one of 14 new Centres for Research-based Innovation (CRIs), an initiative backed by 1,120 million Norwegian Kroner (approximately $183 million) from the Research Council of Norway over the next eight years. The aim is to boost the country’s R&D efforts and make Norway a stronger global-player in key business sectors, such as life science, materials, and IT.
The Spanish company PharmaMar (www.pharmar.com) is also an industrial partner in MabCent and a useful role model for those involved in early-stage research into marine natural resources. PharmaMar specializes in discovery and development of marine-derived anticancer drugs. Over two decades, the company has built up a unique collection of marine invertebrates and microorganisms and has discovered many novel compounds, some with entirely new modes of action against cancer cells. “PharmaMar is the leader in developing marine compounds in cancer,” said Simon Munt, Ph.D., medicinal chemistry manager for PharmaMar R&D.
PharmaMar has five compounds in clinical development, including its lead product Yondelis® for soft tissue sarcomas and ovarian cancer. Discovered in the colonial tunicate Ecteinascidia turbinata and now produced synthetically, Yondelis is currently in Phase III. It is being developed in collaboration with Johnson & Johnson Pharmaceutical R&D (www.jnjpharmarnd.com) and has been granted orphan drug designation from the European Commission and the FDA. Launch of Yondelis is expected in 2007.
In addition, Yondelis is being studied in a pivotal Phase III trial in ovarian cancer and in Phase II trials for prostate, breast, and pediatric cancers. PharmaMar hopes to become a profitable biopharmaceutical company with the launch of Yondelis in a second indication by 2008/9.