Protoporphyrin IX Fluorescence Photobleaching and the Response of Rat Barrett's Esophagus Following 5-aminolevulinic Acid Photodynamic Therapy

Abstract

Barrett's esophagus (BE) can experimentally be treated with 5-aminolevulinic acid-based photodynamic therapy (ALA-PDT), in which ALA, the precursor of the endogenous photosensitizer protoporphyrin IX (PpIX) and subsequent irradiation with laser light are applied to destroy the (pre)malignant tissue. Accurate dosimetry is critical for successful ALA-PDT. Here, in vivo dosimetry and kinetics of PpIX fluorescence photobleaching were studied in a rat model of BE. The fluence and fluence rate were standardized in vivo and PpIX fluorescence was measured simultaneously at the esophageal wall during ALA-PDT and plotted against the delivered fluence rather than time. Rats with BE were administered 200 mg kg(-1) ALA (n = 17) or served as control (n = 4). Animals were irradiated with 633 nm laser light at a measured fluence rate of 75 mW cm(-2) and a fluence of 54 J cm(-2). Large differences were observed in the kinetics of PpIX fluorescence photobleaching in different animals. High PpIX fluorescence photobleaching rates corresponded with tissue ablation, whereas low rates corresponded with no damage to the epithelium. Attempts to influence tissue oxygenation by varying balloon pressure and ventilation were shown not to be directly responsible for the differences in effect. In conclusion, in vivo dosimetry is feasible in heterogeneous conditions such as BE, and PpIX fluorescence photobleaching is useful to predict the tissue response to ALA-PDT.

Protoporphyrin IX fluorescence photobleaching increases with the use of fractionated irradiation in the esophagus

“…Eventually this information could be useful to impact the treatment efficacy, since the rate of photobleaching. 1,6 is expected to be correlated to the treatment effect.…”

“…Figure 1 shows the spectrum of PpIX both before and after 10-J/cm 2 irradiation to the solution. Measuring fluorescence from PpIX is a well-developed method for dosimetric measurements, 1,6,[12][13][14] and in this paper, the small fiber-optic-based system was used to produce real-time "microsampling" of photosensitizer tissue fluorescence 11,15,16 in a manner that is not highly affected by the tissue optical properties.…”

“…3 It is well known that PpIX is photobleached quickly, 4,5 yet it is also well established that there is a later phase during PDT when the bleaching rate is substantially lower than the initial rate, which is presumably due to the absence of oxygen. 1,6 The bleaching rate also appears to have some information about the photochemical deposition, which could be exploited. Independent of the real underlying physical chemistry, a system for real-time control of bleaching is essential to providing some intelligent and dynamic feedback control of the PDT process in the esophagus.…”

Tumor Microenvironment as a Determinant of Photodynamic Therapy Resistance

“…When the damage is due primarily to singlet oxygen reactions, and the photobleaching is due to singlet oxygen reactions with the photosensitizer in its ground state, the local dose is related quantitatively to the local bleaching [129]. Fluorescence photobleaching of PpIX has been shown to correlate with skin reactions in preclinical models [130,131], and has been used clinically to limit pain in PDT [132].…”

Quantitative evaluation ofin vivovital-dye fluorescence endoscopic imaging for the detection of Barrett’s-associated neoplasia

“…Ingestible agents such as 5-aminolevulinic acid have been shown to identify neoplastic lesions in vivo; 20,21 however, current research prioritizes its use with photodynamic therapy. 22,23 A variety of biomarkers have been identified to assess the presence of disease ex vivo through histologic staining; the cytosponge technique has been shown to aid in identifying those biomarkers postprocedure. 24,25 However, being able to identify and localize these biomarkers at the point of surveillance may help identify lesions in vivo.…”