HEALTH AND WELL-BEING
LIGHTRELIEF A
Using light-activated drugs in cancer therapy helps make treatment more precise – while also reducing the debilitating side effects. Lou Reade reports
n emerging cancer treatment therapy uses light-activated drugs to target tumours with greater precision. The technique, photodynamic therapy (PDT), uses drugs that only become active when irradiated with light. Originally restricted to surface cancers –
as light will not penetrate far into the body – PDT is becoming more widely applicable as scientists discover new ways to perform ‘internal irradiation’, ranging from using fibre optic cables, to employing a fluorescent molecule as the light source. Rather than using a conventional anti-cancer drug, which is toxic to
both cancer cells and other body tissues, PDT uses a ‘photosensitiser’. This is a drug that is non-toxic under normal circumstances, but highly toxic when irradiated with light of a particular wavelength. If the light is restricted to the area of the cancer, this means that the toxic side effects of the drug are minimised. ‘The problem with many anti-cancer agents is that they are non-
selective – so have lots of side effects,’ says Cinzia Imberti, a postdoctoral fellow at Warwick University, UK. ‘A photosensitiser is inert under normal conditions, but toxic when you shine a light on it.’ Surprisingly, the drug itself is not the toxic agent. Instead, the
photosensitiser becomes a catalyst when it is activated, converting the oxygen in cells to a state called ‘singlet oxygen’. This is highly reactive – and toxic – so will kill cancer cells. ‘We harness something we all have – oxygen – and make it toxic to cells,’ she says. ‘By selecting where we shine the light, we reduce side effects.’
Early promise Despite the promise of PDT, it is still relatively small scale: there is only one photosensitiser approved for cancer treatment. The drug, Tookad, is a palladium complex used to treat prostate cancer. Photosensitisers are often based around a metal complex. Imberti
is part of a team that – like many others – is looking to develop new photosensitisers. The Warwick researchers recently published a study into an iridium-based complex, which they say has pinpoint accuracy when targeting cancer cells (Angew. Chemie, doi: 10.1002/anie.201813002). ‘By attaching our iridium compound to albumin – a protein in the blood
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