Chinese chemists have developed a pH-responsive system that could deliver anticancer drugs to the colon without being broken down by stomach acid.
 
Enbo Wang from Northeast Normal University, Changchun, China, and colleagues attached a polyoxometalate (POM), through a dative bond, to mesoporous silica spheres. POMs are metal-oxygen cluster compounds that show a wide range of antiviral and anticancer activities. Though they are typically easier and cheaper to synthesise than their small-molecule organic pharmaceutical counterparts, scientists have found it difficult to stabilise and selectively deliver them to their intended therapeutic targets.

Wang tested his silica-bound POM's stability under acidic, basic and neutral conditions, replicating the conditions that they would come across travelling through the human digestive system to get to their target. He found that the loading and release of the POM from the silica spheres is controlled by the pH value of the surrounding environment. A base triggers the dative bond to break, releasing the POM. Acidic conditions, such as those found in the stomach, and neutral conditions don't affect the bond, meaning that the POM will only be released once it has reached the basic conditions in the colon. The team also tested the POM's activity against cancer cells in vitro and found that incorporating the POM into silica spheres actually increased its activity.  

'The pH-responsive controlled-release principle is likely to be of benefit for a variety of transition metal-containing drugs,' says Wang. Eric Maatta, an expert in transition metal systems from Kansas State University, Manhattan, US, agrees. He says that 'the strategy will likely be applicable to the delivery of other classes of anti-tumor agents and enhances the prospects for POM-based therapies'.

David Parker

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