The malfunctioning of tumor necrosis factor (TNF-alpha, TNF-a) has been linked to tumor growth, diabetes, and auto-inflammatory diseases such as rheumatoid arthritis and Crohn’s disease. Finding small molecules that can control its behavior could lead to the development of novel products for a wide range of health problems including cancer.
Identify Potent Inhibitors of TNF-a
A team led by Dik-Lung Ma of the Hong Kong Baptist University and Chung-Hang Leung of The University of Hong Kong, and co-workers from the University of California, San Diego, and The Scripps Research Institute, La Jolla, have used virtual screening to identify potent inhibitors of TNF-a.
Currently, TNF-a is targeted by synthetic antibodies, such as etanercept and infliximab. These are approved for treating inflammatory diseases but cause side-effects by weakening the immune system, leading to opportunistic infections. However, identifying small molecules to replace these treatments has proved difficult, with just two emerging previously: a polysulfonated naphthylurea suramin and an indole-linked chromone (SPD304), neither of which is without problems such as low potency, poor selectivity, and side-effects.
Identify New Leads
To identify new leads, the team used the reported co-crystal structure of TNF-a with SPD304. This compound fits the protein’s binding pocket well, so screening for compounds with a similar profile but without its toxic moiety could offer up useful candidates.
The researchers screened 20 000 compounds from a natural product type library and investigated further the 16 most promising. Two leads emerged. One a pyrazole-linked quinuclidine and the other an indolo-[2,3-a]quinolizidine. These two compounds can directly target human TNF-a and are at least as potent as previous compounds. The indoloquinolizidine has an IC50 (half maximal inhibitory concentration) of 10 micromolar and the quinuclidine 5 micromolar compared the lower IC50 of 3 micromolar for SPD304 itself.
Hit-to-Lead Optimization
“We are currently pursuing hit-to-lead optimization using a combination of computational, analytical and synthetic techniques to generate more potent anti-TNF analogues for in vitro testing,” team member Daniel Chan told ChemistryViews. “Once more potent inhibitors of TNF-a have been identified using our enzyme and cellular assays, the next step will involve in vivo animal studies to confirm the safety and efficacy of the drugs.”
The team expects their two compounds to be much less toxic than the first two TNF-a inhibitors. The researchers are currently using a computer-based hit-to-lead optimization technique to produce additional analogues for in vitro testing.
There Remains a Lot More Work To Do – F. Driss, University of Paris
Fathi Driss of University of Paris, France, agrees that finding small molecules for modulating the activity of TNF-a could help avoid the immunological side-effects seen with antibody therapies. “Only small molecules would exhibit some efficacy to modulate the harmful effects of TNF-a without a wide range of side effects,” he explains. Screening likely candidates is one approach, but Driss believes that a lot more effort will be required before a product based on small molecule chemistry becomes available. “My feeling is that there remains a lot more work to do,” he says, “I also have some concerns about bioavailability and metabolic pathways that may lead to unidentified metabolites.”
The Driss team is pursuing naturally occurring compounds for controlling TNF-a via a different route in which extracts of common dietary substances are tested. Recently, he and his team reported positive data on punicic acid from pomegranate seed oil.
References
- Structure-Based Discovery of Natural-Product-like TNF-alpha Inhibitors
D. Shiu-Hin Chan, H.-M. Lee, F. Yang, C.-M. Che, C. C. L. Wong, R. Abagyan, C.-H. Leung, D.-L. Ma,
Angew. Chem. Int. Ed. 2010, 49 (16), 2860 – 2864. - Punicic Acid, a Conjugated Linolenic Acid, Inhibits TNFα-Induced Neutrophil Hyperactivation and Protects from Experimental Colon Inflammation in Rats
T. Boussetta, H. Raad, P. Lettéron, M.-A. Gougerot-Pocidalo, J.-C. Marie, F. Driss, J. El-Benna
Plos One 2009, 4, e6458.
