Optical modelling of the gastric tissue to optimize the phototherapy efficacy against H. pylori infection

In the framework of the increasing phenomenon of antibiotic-resistant infections, those caused by Helicobacter pylori (Hp) have a particular significance, being worldwide diffused and associated with severe pathologies, among which gastric cancer. To overcome antibiotic-resistance, we propose the use of photodynamic therapy (PDT), based upon cytotoxic species production following bacteria illumination with visible light. In the case of Hp infection, this is due to the presence of endogenous light absorbers (porphyrins). To perform intra-gastric PDT, we have recently designed an illuminating ingestible device equipped with LED sources, where a crucial parameter is the choice of the emission wavelengths optimizing Hp photo-killing. For this purpose, both porphyrin absorption peaks and optical penetration in tissues have to be carefully accounted. By merging data on Hp porphyrin absorption spectrum with those coming by a Monte-Carlo optical modelling for illumination of the stomach wall surface, we have calculated the relative Hp photo-killing efficacy in three representative tissue models in the 400-650nm range. Results indicate that in thicker models, the red wavelengths are ultimately more effective than the blue-shifted ones, due to the greater penetration length in tissues despite a minor porphyrin absorption.