Guide mapping for effective superficial photothermal coagulation of the esophagus using computer simulations with ex vivo sheep model validation study

OBJECTIVES

The transfer and widespread acceptance of laser-induced thermal therapy into gastroenterology remain a topic of interest. However, a practical approach to the quantitative effect of photothermal injury in the esophagus needs further investigation. Here, we aim to perform computer simulations that simulate laser scanning and calculate the laser-induced thermal damage area. The simulation engine offers the results in a guide map for laser coagulation with a well-confined therapeutic area according to laser irradiance and surface scanning speed. The study also presents validation experiments that include histology analyses in an ex vivo sheep esophagus model.

METHODS

The simulation engine was developed based on the Monte-Carlo method and the Arrhenius damage integral. The computational model mimicked laser scanning by shifting the position of the calculated heat source in the grating system along the axis to be scanned. The performance of the simulations was tested in an ex vivo sheep esophagus model at a laser wavelength of 1505 nm. Histological analysis, hematoxylin-eosin staining, light microscope imaging, and block-face scanning electron microscopy were used to assess thermal damage to the tissue model.

RESULTS

The developed simulation engine estimated the photothermal coagulation area for a surface scanning speed range of 0.5-8 mm/second and laser power of up to 0.5 W at a 0.9-nm laser diameter in a tissue model with a volume of 4 × 4 × 4 mm³ . For example, the optimum laser irradiation for effective photothermal coagulation in the mucosa and superficial submucosa depths was estimated to be between 16.4 and 31.8 W/cm² , 23.2 and 38.1 W/cm² at 0.5 and 1 mm/second, respectively. The computational results, summarized as a guide map, were directly compared with the results of ex vivo tissue experiments. In addition, it was pointed out that the comparative theoretical and experimental data overlap significantly in terms of energy density.

CONCLUSIONS

Our results suggest that the developed simulation approach could be a seed algorithm for further preclinical and clinical trials and a complementary tool to the laser-induced photothermal coagulation technique for superficial treatments in the gastrointestinal tract. In future preclinical studies, it is thought that the simulation engine can be enriched by combining it with an in vivo model for different laser wavelengths.

mTHPC-Loaded Extracellular Vesicles Significantly Improve mTHPC Diffusion and Photodynamic Activity in Preclinical Models

Extracellular vesicles (EVs), derived from the cell, display a phospholipid bilayer membrane that protects the cargo molecules from degradation and contributes to increasing their stability in the bloodstream and tumor targeting. EVs are interesting in regard to the delivery of photosensitizers (PSs) used in the photodynamic therapy (PDT), as they allow us to overcome the limitations observed with liposomes. In fact, liposomal formulation of meta-tetra(hydroxyphenyl)chlorin (mTHPC) (Foslip^®), one of the most potent clinically approved PSs, is rapidly destroyed in circulation, thus decreasing in vivo PDT efficacy. mTHPC-EV uptake was evaluated in vitro in a 3D human colon HT-29 microtumor and in vivo study was performed in HT-29 xenografted mice. The obtained data were compared with Foslip^®. After intravenous injection of the mTHPC formulations, biodistribution, pharmacokinetics and PDT-induced tumor regrowth were evaluated. In a 3D model of cells, mTHPC-EV uptake featured a deeper penetration after 24h incubation compared to liposomal mTHPC. In vivo results showed a considerable improvement of 33% tumor cure with PDT treatment applied 24h after injection, while 0% was observed after Foslip^®/PDT. Moreover, 47 days were required to obtain ten times the initial tumor volume after mTHPC-EVs/PDT compared to 30 days for liposomal mTHPC. In conclusion, compared to Foslip^®, mTHPC-EVs improved mTHPC biodistribution and PDT efficacy in vivo. We deduced that a major determinant factor for the improved in vivo PDT efficacy is the deep mTHPC intratumor penetration.

Superficial photothermal laser ablation of ex vivo sheep esophagus using a cone-shaped optical fiber tip

Superficial photothermal laser ablation (SPLA) may be useful as a therapeutic approach producing a depth of injury that is sufficient to eliminate mucosal lesion but not deep enough to induce thermal effects in deeper tissue layers. The purpose of this preliminary study is twofold: (a) to describe design steps of a fiber probe capable of delivering a tightly focused laser beam, including Monte-Carlo-based simulations, and (b) to complete the initial testing of the probe in a sheep esophagus model, ex vivo. The cone-shaped (tapered) fiber tip was obtained by chemical etching of the optical fiber. A 1505 nm diode laser providing power up to 500 mW was operated in continuous wave. The successful SPLA of the sheep mucosa layer was demonstrated for various speed-power combinations, including 300 mW laser power at a surface scanning rate of 0.5 mm/s and 450 mW laser power at a surface scanning rate of 2.0 mm/s. Upon further development, this probe may be useful for endoscopic photothermal laser ablation of the mucosa layer using relatively low laser power.

Design features for optimization of tetrapyrrole macrocycles as antimicrobial and anticancer photosensitizers

Photodynamic therapy (PDT) uses non-toxic dyes called photosensitizers (PS) and harmless visible light that combine to form highly toxic reactive oxygen species that kill cells. Originally, a cancer therapy, PDT, now includes applications for infections. The most widely studied PS are tetrapyrrole macrocycles including porphyrins, chlorins, bacteriochlorins, and phthalocyanines. The present review covers the design features in PS that can work together to maximize the PDT activity for various disease targets. Photophysical and photochemical properties include the wavelength and size of the long-wavelength absorption peak (for good light penetration into tissue), the triplet quantum yield and lifetime, and the propensity to undergo type I (electron transfer) or type II (energy transfer) photochemical mechanisms. The central metal in the tetrapyrrole macrocycle has a strong influence on the PDT activity. Hydrophobicity and charge are important factors that govern interactions with various types of cells (cancer and microbial) in vitro and the pharmacokinetics and biodistribution in vivo. Hydrophobic structures tend to be water insoluble and require a drug delivery vehicle for maximal activity. Molecular asymmetry and amphiphilicity are also important for high activity. In vivo some structures possess the ability to selectively accumulate in tumors and to localize in the tumor microvasculature producing vascular shutdown after illumination.

A comprehensive ovine model of blood transfusion

BACKGROUND

The growing awareness of transfusion-associated morbidity and mortality necessitates investigations into the underlying mechanisms. Small animals have been the dominant transfusion model but have associated limitations. This study aimed to develop a comprehensive large animal (ovine) model of transfusion encompassing: blood collection, processing and storage, compatibility testing right through to post-transfusion outcomes.

MATERIALS AND METHODS

Two units of blood were collected from each of 12 adult male Merino sheep and processed into 24 ovine-packed red blood cell (PRBC) units. Baseline haematological parameters of ovine blood and PRBC cells were analysed. Biochemical changes in ovine PRBCs were characterized during the 42-day storage period. Immunological compatibility of the blood was confirmed with sera from potential recipient sheep, using a saline and albumin agglutination cross-match. Following confirmation of compatibility, each recipient sheep (n = 12) was transfused with two units of ovine PRBC.

RESULTS

Procedures for collecting, processing, cross-matching and transfusing ovine blood were established. Although ovine red blood cells are smaller and higher in number, their mean cell haemoglobin concentration is similar to human red blood cells. Ovine PRBC showed improved storage properties in saline-adenine-glucose-mannitol (SAG-M) compared with previous human PRBC studies. Seventy-six compatibility tests were performed and 17·1% were incompatible. Only cross-match compatible ovine PRBC were transfused and no adverse reactions were observed.

CONCLUSION

These findings demonstrate the utility of the ovine model for future blood transfusion studies and highlight the importance of compatibility testing in animal models involving homologous transfusions.

Temoporfin (Foscan®, 5,10,15,20-tetra(m-hydroxyphenyl)chlorin)--a second-generation photosensitizer

This review traces the development and study of the second-generation photosensitizer 5,10,15,20-tetra(m-hydroxyphenyl)chlorin through to its acceptance and clinical use in modern photodynamic (cancer) therapy. The literature has been covered up to early 2011.