Monday, May 14, 2012

11 May 2011 – Uncommon scents


Fascinating news this week out of Vanderbilt University.  In a report published this week in the Proceedings of the National Academy of Sciences, Vanderbilt professor of biological sciences and pharmacology Lawrence Zwiebel and his laboratory team are credited with discovering a compound that may prove thousands of times more effective than contemporary chemicals at repelling mosquitoes and other pernicious insects.(Note A)

The state of the art in bug repellents today is DEET (N,N-Diethyl-3-methylbenzamide), a yellowish oil developed by the US Army after its experience with malaria in jungle warfare during the Second World War.  It entered civilian use in 1946 and is the key active ingredient in most insect repellent sprays.  DEET works primarily by activating a mosquito’s olfactory cells that are optimized for detecting 1-octen-3-ol, a volatile organic compound present in human breath and sweat; it also works because it activates some of the olfactory receptor neurons that are also activated by natural repellents, e.g., eucalyptus oil.  Basically, it’s an odour that mosquitoes just don’t like.  It’s not without problems; for example, DEET is a mild fish toxin and a skin irritant, and it’s recently been discovered to be a mild acetylcholinesterase inhibitor, similar to carbamate toxins and insecticides (or organophosphorous nerve agents).  Health Canada prohibits sale of DEET in concentrations higher than 30%, and has recommended against using concentrations higher than 10% on children.

What makes the new compound so much more effective is the way it impacts a deeper layer of the mosquito’s olfactory system.  Mosquitoes have an array of exquisitely sensitive olfactory cells called odorant receptors (ORs) located on their antennae, and it has been known for some time that different ORs are activated by different chemicals.  This enables the mosquito to find its primary food, i.e., blood, by looking for and following the wide array of different volatile organic compounds emitted by mammals.  In order to attack the mosquito through its ORs, it would be necessary to target all of the different receptor functions, which would mean finding a mixture of chemical compounds that covers every possible chemical target - and there are a lot of them.  Obviously, this would be impractical.  The vulnerability of the mosquito olfactory system lies in how the activation messages from individual ORs are transmitted to the mosquito’s brain.  Each OR complex is connected to the brain via a special cell called an OR co-receptor (ORCO) which receives an activation message from the OR and converts it into a general signal to the brain.  Zwiebel’s team has been trying to find a chemical compound that directly activates the ORCOs instead of trying to target the individual ORs - and it looks like they’ve found one.  Dubbing it “Vanderbilt University Allosteric Agonist” or VUAA1, the chemical simultaneously “turns on” all of a mosquito’s ORCOs at once, overloading its olfactory system and preventing it from receiving any of the olfactory cues it needs to find food.

What’s nearly as interesting as the discovery is how they did it.  Over the past ten years, the chemical and pharmaceutical research and development industries have been revolutionized by computerized microsynthesis.  Instead of each synthetic operation and all of the subsequent testing for a new compound being performed by a human in a fume hood with Bunsen burners, Florence flasks and pipettes, the operations are now all carried on in miniaturized, computerized laboratories operated by complex robotics.  New chemicals can be synthesized in microlitre quantities (amounts far too small for a human to manipulate) and tested against arrays of other chemicals or biological samples much more rapidly and comprehensively than ever before.  This makes it possible to take a “broad-brush” approach to synthesis and analysis.  Rather than try to guess which chemicals might do the job, Zweibel’s team, using Vanderbilt’s high-throughput screening facility (originally installed for drug research), decided to “throw the kitchen sink” at the problem.  They ran computerized test runs using the full array of 118,000 small molecules normally employed in drug testing against genetically engineered human embryonic kidney cells that had been modified to include mosquito ORs. The computerized test runs found numerous chemicals that activated one or more individual Ors - and also identified a compound that activated them all simultaneously.  This is the compound that became VUAA1.  The researchers note that it could serve as the basis for similar chemicals promising broad-spectrum control options for all manner of scent-oriented pernicious nasties.(Note D)

The project, which was originally designed to identify new solutions to the ages-old problem of malaria transmissions, was funded by the National Institutes of Health using money provided from the Bill and Melinda Gates foundation, and it offers a potentially fantastic solution to malaria-stricken nations.  Previously, the only solution to the anopheles mosquito was to kill it, and the importance of programmes aimed at controlling mosquito populations has been underscored by the disastrous EPA decision in the early 1970s to ban DDT (1,1,1-trichloro-2,2-di(4-chlorophenyl)ethane) and to restrict funding to developing nations that continued to use it.  The UN has placed annual malarial infections at 20M+, and deaths at ca. 850,000, 90% in Africa, and mostly among children under 5; other credible estimates are higher.  If VUAA1 proves to be as effective as this new research is suggesting, it might be possible to simply neutralize mosquitoes as disease vectors without having to resort to insecticides - even largely harmless ones, like DDT - to kill them.

The next step, according to the project’s scientists, is to try to modify the compound by eliminating parts of the molecule that don’t contribute to its activity.  Once that’s done, they have to test the resulting, trimmed-down compound for toxicity - especially mammalian toxicity.  This is a fairly important consideration; one of the problems with identifying good, broad-spectrum insecticidal compounds is ensuring that they don’t kill us, our animals, or our plants.  Earlier this week, news reports out of New Zealand alleged that the deaths of 7 tourists since January at the “Downtown Inn” in Chiang Mai, Thailand, were the result of poisoning by chlorpyrifos, an organophosphorous insecticide commonly used to kill bedbugs.(Note B)  This is an old, old story; after the DDT ban in the US, farmers switched to organophosphorous insecticides like malathion (S-1,2-bis(ethoxycarbony)ethyl O, O-dimethyl phosphorodithioate ), which is as effective an insecticide as DDT, but which has much higher mammalian toxicity.  While there is no record of any fatalities resulting from DDT use in the US or elsewhere, the human health impact of malathion is significant.  When Pakistan began spraying with malathion in the late 1970s after the DDT ban, for example, 2800 out of 7500 spray men were poisoned, and 5 of them died.(Note C)

Most of chemistry is common sense, and it makes good sense to use the least hazardous chemical necessary to get the job done.  It also makes sense to deter if you can, and destroy only if you must.  If VUAA1 pans out, it will be one of the most significant advances in insect control technology in a generation.  Here’s hoping.




C) W.N. Aldridge, J.W. Miles, D.L. Mount and R.D. Verschoyle, “Malathion Not as Safe as Believed - 5 Die - 2,800 Poisoned”, Archives in Toxicology, 42:95-106, 1979.

D) I’d be remiss if I failed to note that, from a defence perspective, computerized microsynthesis and analysis setups are also ideal for rapidly identifying supertoxic compounds in small, easy-to-conceal laboratory settings.  The future of chemical warfare (and especially chemical terrorism, if our enemies decide to get serious about it) is not in huge plants producing tons of mustard or nerve agent; it’s in small-scale custom synthesis facilities capable of fast-turnaroud computerized batch production.  And the future of identifying new toxic chemicals for warfare or terrorist use lies in microsynthesis.