Following on from the paper on the anti-parasitic drug mebendazole (which I first discussed on this site a while ago) and the antacid cimetidine (paper not yet published), I've been working on another ReDO paper on the drug nitroglycerin. Like all the repurposed drugs we're looking at in the ReDO project this one is commonly used clinically for non-cancer uses, in this case it's a drug used to treat heart problems and blood pressure. Available as tablet you stick under the tongue, or a spray or even a transdermal patch, nitroglycerin is a drug that has been used for over a 100 years as a vasodilator - in other words it relaxes the blood vessels. It's partly this property that makes it interesting in terms of anti-cancer treatment.
Like other tissues, tumours need a blood supply for food and oxygen, and it is well-known that they release chemical signals that cause new blood vessels to form. This is the process called angiogenesis, and for many years scientists have been looking at ways to disrupt the process - with drugs like avastin (bevacizumab) developed to stop this happening. The idea is that with no blood supply tumours can't grow. However, even when angiogenesis does take place and tumours sprout the blood vessels they need, the vessels that are formed aren't normal. The blood supply is chaotic and the vessels are much leakier than normal. Back in the late 1980s some scientists started looking at how we could use this to our advantage. The idea is that you take advantage of the leakiness by using drugs that leak out into the tumours rather than spreading throughout the body (as normal chemo does). Hiroshi Maeda and his co-workers termed this the 'enhanced permeability and retention' (EPR) effect.
Nitroglycerin enters the picture as a way of making the leakiness worse by relaxing the blood vessels, thus encouraging large drug molecules to leak into the tumours. And once they've leaked out, the chaotic structure of the vessels means the drugs are retained in the tumour where they can have an effect. It is, in theory at least, a way of targeting anti-cancer drugs to the tumours and not to the rest of the body. It's an elegant idea and has lots of experimental evidence going for it. And the evidence includes some small trials in humans - primarily in lung and prostate cancers. There are more clinical trials on-going, and we can but hope that their results encourage more work in this area. To really work well we need to team up the nitroglycerin with some reworked chemotherapy drugs that are specifically designed to work with the EPR effect.