Wednesday, July 11, 2007

Oxepane Nucleic Acids - Part I

The chemistry you start your career working with tends to stick to you. Stuff you work on later seems much easier to shake off. Anyway, I started as a nucleoside/oligonucleotide chemists and although what I do now is miles away from this area every time my eyes wander over a graphical abstract with a nucleoside I stop. I just can't help it. It happened again the other day. Oxepan Nucleic Acids (ONA). Can you believe that it hasn't been made before. Apparently, no one has gone beyond the six membered ring until now. Now ONA is not a great nucleoside analogue. The T15 and A15 ONA oligonucleotides (ON) have affinities less than 5 oC towards DNA, a very low affinity towards itself (ONA T15 + ONA A15 = 12 oC) and a similar Tm towards RNA. In other words ONA is not suitable for antisense purposes due to the very low Tm. However, ONA is very stable towards nucleases and importantly activates RNase H. Now before I continue I should explain what Tm, antisense and RNase H is to the uninitiated. Firstly, Tm is the temperature at which 50 % of a duplex has denatured, ie. high Tm = stable duplex. Secondly, antisense is a different approach to drug development targeting RNA rather than proteins. The idea is to knock the RNA out before it gets translated into protein (See figure). This is achieved by synthesising an antisense ON that is complementary to your RNA target. The mechanism of action for antisense is either:
(a) Inhibit the translation to protein by physically blocking the RNA strand making it impossible for ribosomes to translate it
(b) Activate the enzyme RNase H that specifically targets DNA-RNA duplexes and only degrades the RNA strand.
A lot of people in the field believe that antisense can only work effectively with RNase H activation and I tend to agree. The cell is amazingly efficient at making RNA and translating it to protein so if you have to get stoichiometric amounts of antisense ON to RNA into the cell you are likely to have a problem. The beauty with RNase H activation is that the system is catalytic. In other words the antisense ON gets released after RNA degradation and moves on to the next victim. The problem is that you cannot use regular DNA for antisense purposes as it has a very short half life in serum (~15 minutes). So you have to devise an analogue that is stable in serum, has high affinity towards RNA and activates RNase H. Now obviously this is no easy feat so why bother? The (theoretical) advantages when compared to traditional protein targeting drugs are:
(a) Complete selectivity only for the intended target
(b) You can target anything involving RNA
(c) The chemistry is the same every time. You just have to figure out what the sequence of your target is and synthesise the required ON
(d) Getting drugs to market is rapid because drug development is significantly faster
Obviously, things are much more complicated than this. Antisense was the big thing in the 80s. It was going to cure everything within the next decade but the reality is that only one product has made it to market. It's an ON called Vitravene (ISIS Pharmaceuticals) that prevents AIDS patients from going blind by targeting cytomegalovirus retinitis. That said a lot of advances have been made and there are numerous antisense ON in late stage clinical trials. Anyway, after this super condensed course in antisense ON I think we are ready for the actual paper. I'll let you off the hook for now. The next post should be up in a couple of days. D!


Dan said...

The DNA helix in your figure is the incorrect enantiomer (left-twist). It should be the mirror image of what you have.

Daniel Sejer said...

Hmmmmm well if you really want to pick on details I don't think that cells, DNA, RNA, ribosomes or proteins really look like that either. It was just intended to be a simple way of illustrating something exceedingly complex. D!

taitauwai said...

Welcome back! Shame to admit, I don't really understand ONA. I am struggling to understand it. Thanks for the info, though. :)

آب said...

Hi Daniel,

Nice blog you have... very well explained, I really enjoy it! I came across your blog searching for my project on google... and guess what is my phd project? :-)
I am making the next generation of oxepane ONs!

Cheers from Montreal