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