Invisible Army Rouses Home Guard

Writing this blog – perhaps any blog – is an odd pastime because you never really know who, if anyone, reads it or what they get out of it. Regardless of that, one person that it certainly helps is me. That is, trying to make sense of the latest cancer news is one of the best possible exercises for making you think clearly – well, as clearly as I can manage!

But over the years one other rather comforting thing has emerged: more and more often I sit down to write a story about a recent bit of science only to remember that it picks up a thread from a piece I wrote months or sometimes years ago. And that’s really cheering because it’s a kind of marker for progression – another small step forward.

Thus it was with this week’s headline news that a ‘cancer vaccine’ might be on the way. In fact this development takes up more than one strand because it’s about immunotherapy – the latest craze – that we’ve broadly explained in Self Help Part-1Gosh! Wonderful GOSH and Blowing-up Cancer and it uses artificial nanoparticles that we met in Taking a Swiss Army Knife to Cancer.

Arming the troops

What Lena Kranz and her friends from various centres in Germany described is yet another twist on the idea of giving our inbuilt defence – i.e. the immune system – a helping hand to tackle tumours. They made small sacs of lipid called nanoparticles (they’re so small you could get 300 in the width of a human hair), loaded them with bits of RNA and injected them into mice. This invisible army of fatty blobs was swept around the circulatory system whereupon two very surprising things happened. The first was that, with a little bit of fiddling (trying different proportions of lipid and RNA), the nanoparticles were taken up by two types of immune cells, with very little appearing in any other cells. This rather fortuitous result is really important because it means that the therapeutic agent (nanoparticles) don’t need to be directly targetted to a tumour cell – thus avoiding one of the perpetual problems of therapy.

The second event that was not at all a ‘gimme’ was that the immune cells (dendritic cells and macrophages) were stimulated to make interferon and they also used the RNA from the nanoparticles as if it was their own to make the encoded proteins – a set of tumour antigens (tumour antigens are proteins made by tumour cells that can be useful in identifying the cells. A large number of have now been found: one group of tumour antigens includes HER2 that we met as a drug target in Where’s That Tumour?)

The interferon was released into the tumour environment in two waves, bringing about the ‘priming’ of T lymphocytes so that, interacting via tumour antigens, they can kill target cells. By contrast with taking cells from the host and carrying out genetic engineering in the lab (Gosh! Wonderful GOSH), this approach is a sort of internal re-wiring achieved by giving a sub-set of immune system cells a bit of genetic code (in the form of RNA).

TAgs RNA Nano picNanoparticle cancer vaccine. Tiny particles (made of lipids) carry RNA into cells of the immune system (dendritic cells and macrophages) in mice. A sub-set of these cells releases a chemical signal (interferon) that promotes the activation of T lymphocytes. The imported RNA is translated into proteins (tumour antigens) – that are presented to T cells. A second wave of interferon (released from macrophages) completes T cell priming so that they are able to attack tumour cells by recognizing antigens on their surface (Kranz et al. 2016; De Vries and Figdor, 2016).

So far Kranz et al. have only tried this method in three patients with melanoma. All three made interferon and developed strong T-cell responses. As with all other immunotherapies, therefore, it is early days but the fact that widely differing strategies give a strong boost to the immune system is hugely encouraging.

Other ‘cancer vaccines’

As a footnote we might add that there are several ‘cancer vaccines’ approved by the US Food and Drug Administration (FDA). These include vaccines against hepatitis B virus and human papillomavirus, along with sipuleucel-T (for the treatment of prostate cancer), and the first oncolytic virus therapy, talimogene laherparepvec (T-VEC, or Imlygic®) for the treatment of some patients with metastatic melanoma.

How was it for you?

As we began by pointing out how good writing these pieces to clarify science is for me, the question for those dear readers who’ve made it to the end is: ‘How did I do?’

References

Kranz, L.M. et al. (2016). Systemic RNA delivery to dendritic cells exploits antiviral defence for cancer immunotherapy. Nature (2016) doi:10.1038/nature18300.

De Vries, J. and Figdor, C. (2016). Immunotherapy: Cancer vaccine triggers antiviral-type defences.Nature (2016) doi:10.1038/nature18443.

 

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Taking a Swiss Army Knife to Cancer

Murder is easy. You just need a weapon and a victim. And, I guess the police would add, opportunity. I hasten to point out that’s an observational note rather than an autobiographical aside. It’s relevant here because treating cancer is intentional homicide on a grand scale – the slaughter of millions of tumour cells. For individuals we cannot say whether the perfect murder is possible – how would we know – but on the mass scale history shows that even the most efficient machines for genocide have been, fortunately, less than perfect. In other words through incredible adaptability, ingenuity, determination and sheer will power, some folk will survive even the most extreme efforts of their fellow men to exterminate them. Cancers tend to mirror their carriers. With only rare exceptions, whatever we throw at them in attempts to eliminate their unwelcome presence, some of the little blighters will dodge the bullets, take a deep breath and start reproducing again. ‘What doesn’t kill you makes you stronger’ as the American chanteuse Kelly Clarkson has it – though to be fair I think she pinched the line from Nietzsche.

Multiple whammys

For cancer cells dodging extinction requires adaptability: using the flexibility of the human genetic code enshrined in DNA to change the pattern of gene expression or to develop new mutations that short-circuit the effects of drugs – finding many different ways to render useless drugs that worked initially. You might draw a parallel with the idea, correct as it turned out, of the prisoners who staged The Great Escape from Stalag Luft III in World War II that if they initially dug three tunnels simultaneously (Tom, Dick and Harry) the guards might find one but they’d probably not find all three.

An approach increasingly permeating cancer therapy is how to target several escape routes at once – can we at least give the tumour cell a serious headache in the hope that while it’s grappling with a molecular carpet bombing it might be more likely to drop dead. One way of doing this is simply to administer drug combinations and this has met with some success. However, for the most part, agents are not specific for tumour cells and their actions on normal cells give rise to the major problem of side effects.

Step forward Yongjun Liu and colleagues from Shandong University, Jinnan, People’s Republic of China and the sophisticated world of chemistry with efforts to fire a broad-shot that combines different ways of killing tumour cells with at least some degree of specific targeting.

Making the bullets

These chemists are clever chaps but, taken one step at a time, what they’ve done to make a very promising agent is simple. The game is molecular Lego – making a series of separate bits then hooking them together. The trendy name is click chemistry, a term coined in 1998 by Barry Sharpless and colleagues at The Scripps Research Institute, to describe reactions in which large, pre-formed molecules are linked to make even more complex multi-functional structures. You could describe proteins as a product of ‘click chemistry’ as cells join amino acid units to make huge chains – but you wouldn’t as it’s better to keep the name for synthetic reactions that make novel modules.

It might help to recall some school chemistry:

acid + base = salt + water (e.g., HCl + NaOH = NaCl + H2O)

Click chemistry is the same idea but the reactants are large molecules, rather than atoms of hydrogen, chlorine and sodium.

Anti-freeze to anti-cancer in a couple of clicks

The starting point here is remarkably familiar – it’s antifreeze, a chemical added to cooling systems to lower the freezing point of water (e.g., in motor engines). Antifreeze is ethylene glycol (two linked atoms of carbon with hydrogens: HO-CH2-CH2-OH): make a string of these molecules and you have a polymer – poly-ethylene glycol (PEG).

For click chemists it’s easy to tag things on to biologicial molecules, including PEG and most proteins. This study used biotin – a vitamin that works like a molecular glue by sticking strongly to another small molecule called avidin, found in egg white. Avidin can therefore be used to fish for anything tagged with biotin – it simply hooks two biotins together. The protein used here is an antibody that binds to a signaling molecule (VEGFR) present on the surface of most tumour cells and blood vessels. VEGFR helps tumour growth by providing a new blood supply – an effect blocked when the antibody binds to it.

Sounds familiar?

If chains of carbon atoms decorated with hydrogens seem familiar, so they should. They’re fats (the saturated fats you get in cream and butter are very similar to the chains of PEG). As anyone who’s done the washing up knows, fats and water don’t get on (which is why we have detergents). Put them in water and fats huddle together in blobs called micelles – sacs of fat. This gives them a useful property: if you mix something else in the water – a drug for instance – and then add PEG and separate the micelles that form, you’ve got drug trapped in a kind of carrier bag. Often called nanoparticles, these small, molecular bubbles made by chemists are packets of drug ready to be delivered.

Micelle Blog picA sac of poly-ethylene glycol (PEG) with entrapped drug (red dots) tagged in three different ways (Liu et al., 2014).

Addressing the parcel

To turn PEG into a parcel two chemical tricks are needed. The first is to tag PEG with biotin. Now the nanoparticles will pick up VEGFR antibody labeled with avidin – and the antibody label can target the micelles to tumour cells and blood vessels.

Exploding the package

The second trick is the addition of another polymer (a chain of histidine amino acids) that triggers the disassembly of the nanoparticles when they find themselves close to or inside tumour cells – a more acidic environment than the circulation.

Seeing the results

The final twist is to include another modified PEG – this with a chemical group that binds gadolinium when it’s added to the water. Gadolinium is an ion (Gd3+) which shows up brightly in MRI scans – the idea being to highlight where the nanoparticles end up after injection into animals.

Does it work?

These multicomponent nanoparticles resemble a Swiss Army knife – all sorts of gadgets sticking out all over the place: PEG to make sacs that contain a drug, biotin hooked to VEGFR antibody to home in on tumour cells, an acidity sensor so the thing falls apart and releases its content on arrival and a contrast enhancer that shows up where this is happening in an MRI scan.

Injected into mice with liver tumours, these multi-functional nanoparticles do indeed home in on the tumours and their surroundings and drastically reduce tumour growth when they carry the drug sorafenib. Sorafenib is the only agent that has been shown to affect liver cancers, although its effects are brief. Compared to sorafenib alone, these new nanoparticles are about three times more potent – presumably because of their targeted delivery.

Where are we?

This wonderfully clever chemistry will not cure liver cancer. A good result when it reaches human trials would be six months remission by comparison with the current average of two months from treatment with sorafenib alone. But what it does show is that hitting cancers hard in multiple ways at least slows them down. We can only hope that more potent drugs and further ingenuity will progressively extend this capacity. The end is not in sight but brilliant technical advances such as that from Yongjun Liu’s lab may be spotlighting the way ahead.

Reference

Yongjun Liu et al., (2014). Multifunctional pH-sensitive polymeric nanoparticles for theranostics evaluated experimentally in cancer. Nanoscale 6, 3231-3242.