natural product – Drug Discovery Opinion

New Tools for Characterizing Natural Products

Researchers at the University of California in San Diego have developed computational tools that will allow scientists to quickly and easily determine whether newly isolated nonribosomal peptides (NRPs) are novel or already known. NRPs are a diverse family of secondary metabolites produced by microorganisms such as bacteria and fungi and have a wide range of biological activities: examples of NRPs in clinical use include the antibiotics, penicillin, cephalosprorin and vancomycin; the immunosuppressant, cyclosporine; and the cytostatic drug, bleomycin. NRPs, which are produced independently of mRNA by nonribosomal peptide synthetases, often have cyclic and/or branched structures and may contain non-proteinogenic amino acids as well as numerous other chemical modifications. The lack of genomic DNA information and complex chemical structures have made it very difficult, time-consuming and costly to determine the structure of NPRs, but the new algorithms should replace manual annotations and allow much more rapid characterization. The algorithms are able to calculate the chemical structure of a newly isolated cyclic NRP from fragments generated by mass spectrometry and can be used alongside ‘dereplication’ tools that can calculate the mass spectrometry signature of known NRPs to determine if the newly isolated NRP has already been described.

The study is published in full in the journal Nature Methods, and the web-based tools for sequencing NRPs are available (at no cost to researchers at non-profit organizations) at: bix.ucsd.edu/nrp.

New Lead for Autoimmune Disease

A major challenge in treating autoimmune disease is to prevent tissue damage without generalised immunosuppression. US researchers now suggest that selective damping down of the T_H17 response using compounds such as halofuginone may provide an answer to this challenge.

Writing in the June 5^th edition of the journal Science, they show that halofuginone specifically inhibits the development of T_H17 cells which are believed to play a key role in tissue injury in autoimmune diseases such as inflammatory bowel disease, multiple sclerosis, type 1 diabetes, eczema and psoriasis. When halofuginone was added to cultures of naïve mouse CD4⁺ T-cells containing cytokines that would normally induce differentiation into T_H17 cells, the number of T_H17 cells – but not T_H1, T_H2 or T regulatory cells – was substantially reduced. In cultured human CD4⁺ T-cells, halofuginone also selectively suppressed levels of IL-17, the main cytokine produced by T_H17 cells. In mice with experimental autoimmune encephalitis (EAE), an artificially-induced immune disease resembling multiple sclerosis and marked by infiltration of T_H17 cells into the central nervous system, treatment with low doses of halofuginone significantly reduced both the development of EAE and its severity.

To understand how halofuginone works, the researchers looked at alterations in gene expression in response to drug treatment and found that a cytoprotective signalling pathway, the amino acid starvation response (AAR), was activated. Inhibition of T_H17 differentiation by halofuginone could be overcome by the addition of excess amino acids and was mimicked by AAR activation in response to selective amino acid depletion.

Halofuginone is a synthetic analogue of febrifugine, the active principal of the Chinese herb, chang shan (Dichroa febrifuga), which has been used to treat fever and malaria for more than 2000 years. Febrifugine itself causes severe emesis and gastrointestinal irritation and, in the 1960s, a number of analogues – including halofuginone – were synthesized by U.S. Army scientists looking for novel antimalarials. Halofuginone also inhibits synthesis of collagenase and collagen type 1 and underwent clinical trials for the treatment of scleroderma, a chronic, autoimmune condition of the connective tissue. In animal husbandry, halofuginone (as Stenerol®) is used prophylactically to control coccidial infection in poultry flocks.

First Total Synthesis of (+)-11,11′-Dideoxyverticillin A

shiraia bambusicola Although it is ten years since the anti-cancer properties of (+)-11,11′-dideoxyverticillin A were first described, it is only now that the first total synthesis has been reported. The compound was originally isolated from the mycelium of a marine-derived fungus of the genus Penicillium and has since been found in several other fungal species including Shiraia bambusicola, a parasitic fungus which grows on some species of bamboo.

(+)-11,11'-dideoxyverticillin A structure With ten rings and eight stereogenic centres, (+)-11,11′-dideoxyverticillin A is one of the most complex of a family of dimeric epidithiodiketopiperazine natural products. Writing in the journal Science, chemists at the Massachusetts Institute of Technology have now described an enantioselective 11-step synthesis, starting from the commercially available amino acids, tryptophan and alanine. The synthesis was designed to mimic a plausible biosynthetic pathway and, as well as providing ready access to (+)-11,11′-dideoxyverticillin A itself, should provide access to analogues which may have enhanced pharmacological activity. The elegant synthesis used a minimum of protecting groups and, by taking full advantage of the inherent reactivity of intermediates, offers insights into the natural biosynthetic pathways. (+)-11,11′-Dideoxyverticillin A inhibits the tyrosine kinase activity of the epidermal growth factor receptor with an IC₅₀ of 0.14nM, shows anti-angiogenic activity, and prevents the growth of several cancer cell lines.

Long Lasting Pain Relief

A new slow-release anaesthetic drug-delivery system could potentially revolutionize the treatment of pain during and after surgery, and may also have a large impact on chronic pain management. Researchers at Children’s Hospital Boston have developed a liposomal delivery system for the ultra-potent local anaesthetic, saxitoxin. Saxitoxin is one of a family of neurotoxic alkaloids produced by aquatic microorganisms which block voltage-gated sodium channels on nerve cells. In studies in rats, liposomal delivery of saxitoxin blocked sciatic nerve transmission without causing significant nerve or muscle damage. The team found that liposomes containing only saxitoxin produced nerve blocks that lasted for two days whilst liposomes containing saxitoxin together with dexamethasone – a steroid known to enhance the action of encapsulated anaesthetics – caused a nerve block that lasted for seven days without significant damage to surrounding nerves or muscle. Despite the extreme potency of saxitoxin, systemic toxicity occurred only with high loadings of dexamethasone which increased release of the anaesthetic. Previous attempts to develop slow-release anaesthetics have been limited by short duration of action, toxicity to the surrounding tissue and/or systemic toxicity. The team hope that eventually a single injection that could cause a nerve block lasting for weeks – or even months – could be used to manage chronic pain and are optimising a formulation that would be suitable for clinical trials.

The research is published in the April 13th online edition of PNAS.

Chemical from Chinese Medicinal Plant can Combat HIV

Telomeres are structures which protect chromosomes during cell division and become progressively shorter as a result; telomerase adds telomere DNA to the ends of chromosomes and increases the potential for cell division. Most non-dividing cells show little or no telomerase activity but telomerase is up-regulated in cells that need to divide repeatedly, such as cells of the immune system. CD8⁺ T-cells play a crucial role in controlling HIV infection and, in CD8⁺ T-cells responding to viral antigens, telomerase is up-regulated. During chronic HIV-1 infection, however, CD8⁺ T-cells lose their ability to up-regulate telomerase, leading to critically short telomeres and reduced antiviral activity. A study jointly carried out by scientists at UCLA and the Geron Corporation has shown that TAT2, a small molecule telomerase activator isolated from the Astragalus root, can prevent or slow telomere shortening and increase antiviral efficacy. CD8⁺ T-cells from HIV-infected donors treated with TAT2 showed increased proliferation and enhanced ability to fight HIV.

The study is published in the Nov. 15 edition of the Journal of Immunology.

Although telomerase is up-regulated in cancer cells, TAT2 did not enhance telomerase production by tumour cells. Astragalus is also used in traditional Chinese medicine without obvious side effects suggesting that TAT2 will not promote the development of tumours.

Modified Plant Extract Could Rival Viagra

Plant extracts have traditionally been used as aphrodisiacs and a recent report describes the synthesis of a modified natural product that could be as effective as sildenafil (Viagra®) for treating erectile dysfunction. Icariin, a component of Epimedium brevicornum (also known as horny goat weed) was found to inhibit phosphodiestersae-5 (PDE5), but with an IC₅₀ value almost 100-fold greater than that of sildenafil.
Icariin
The researchers then modified the structure of icariin to produce compound 5 which was essentially equipotent with sildenafil as an inhibitor of PDE5. Since compound 5 is more selective for PDE5 than sildenafil, which also inhibits phosphodiesterase-6 (PDE6) and cyclic adenosine monophosphate-phosphodiesterase (cAMP-PDE), compound 5 has the potential to cause fewer side effects than sildenafil.