Spatial heterogeneity and temporal kinetics of photosensitizer (AlPcS2) concentration in murine tumors RIF-1 and MTG-B

Abstract

In this study we compared the photosensitizer concentration in two experimental murine tumors using an in situ fluorescence detection instrument to examine temporal and spatial variations, after intravenous versus intratumor injection. Also, the variations in the estimate as detected by large area sampling and micro-region sampling are compared, in order to determine what the inter-tissue and inter-animal variations are, and how the method of sampling affects this estimate. The latter study was carried out ex vivo in the same tumors, which had been harvested and frozen after in vivo measurements were made. The photosensitizer, disulphonated aluminum phthalocyanine (AlPcS2) was injected either intravenously (IV) or directly into the tumor (ITu), using two murine models, MTG-B (mammary adenocarcinoma) and RIF-1 (radiation-induced fibrosarcoma) grown subcutaneously on the flank. An in situ microsampling fluorescence probe was used to assess photosensitizer concentration, through real-time measurement of the remitted intensity. The photosensitizer concentration was evaluated at 8 time endpoints between 15 min and 48 h post-injection. Inter-tumor and intra-tumor variations were assessed by repeated samples from the tumor tissues. The average photosensitizer level reaches a peak between 3 to 6 h in both tumor and normal tissues using IV administration, but peaks within 1 h following ITu administration. MTG-B tumors demonstrated a factor of 2 higher uptake than RIF-1 tumors. The pharmacokinetic uptake rates of the RIF-1 tumor were 3 times faster than for MTG-B, while there was no statistical difference in their clearance rates. Preferential uptake of AlPcS2 by both tumors compared to contra-lateral flank subcutaneous normal tissue was documented, with ITu injection exceeding IV injection by a factor of 10 in the tumor to normal tissue ratio. Inter-animal standard deviation in the mean fluorescence was near 76% for both routes of administration, but estimates of the variation within tumor were near 16% standard deviation when a large sampling volume was used. In contrast, microscopic intra-tumor standard deviation in the mean estimate was near 76%, with IV injection, indicating that high heterogeneity exists in the photosensitizer concentration on a smaller distance scale. The inter-tumor variation was reduced by ITu injection, but at the expense of increasing intra-tumor variation.

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

“…The clinical dosimetry system in vivo was developed and demonstrated in previous studies. 11,15,[17][18][19] Briefly, the system uses blue light (405 nm wavelength) excitation and very small optical fibers (100 m core diameter) to limit the penetration and distance traveled by light that is detected. The light that is captured from the tissue must have originated within a few scattering distances (typically 100 to 300 m) of the fibers surface for significant probability of detection.…”

Analysis of sampling volume and tissue heterogeneity on the in vivo detection of fluorescence

“…20,21,23,27 The probe end was designed with 100-m-diam fibers to constrain the volume of tissue sampled to be smaller than the average scattering distance of tissue ͑typically 100 to 200 m͒. 20 The system has been tested in several phantoms and animal studies, and has been systematically compared to tissue extraction values, using the photosensitizer tetrasulphonated aluminum phthalocyanine.…”

“…[3][4][5] While in general, careful calibration can be used to interpret the signal and yield a semiquantitative measurement, the problem still remains that different approaches to sampling fluorescence yield vary in results, and cannot easily be compared. Fluorescence within tumors is known to be highly heterogeneous, with a variance that can be as high as 100% of the mean value, 6 and inspection of any fluorescence microscope image will reveal a microscopic-level heterogeneity that changes from near zero to extremely high values. [7][8][9] In this study, the problem of measuring fluorescence was examined in detail, using flow cytometry, fluorescence microscopy, and in vivo measurements with a fiber optic probe that was designed for microsampling of tissues.…”

“…In a recent series of studies, the development of an optimized fiber probe was outlined and tested in phantoms and animal tissues. 6,20,21 The design of the probe was developed around the idea that when the sampling volume is smaller in dimension than the average scattering distance in tissue ͑approximately 100 m͒, then the detected fluorescence signal was not highly affected by the tissue optical properties. 20,22 This design was coupled with the idea of using many small sampling fibers, spaced apart by enough distance to minimize cross talk between them.…”

Explicit dosimetry for 2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide-a-mediated photodynamic therapy: macroscopic singlet oxygen modeling

“…However, PDT dose has several problems: (1) The local concentration of photosensitizer varies from site to site in the tissue and from individual to individual. 6,7,25 (2) The penetration of light into the target depends on the optical properties of that tissue. (3) Tissue and blood oxygenation is a central component for PDT and also affects the tissue optical properties.…”