Study of porphyrin fluorescence in tissue samples of tumour-bearing mice

In vivo tumor detection in small animals by hematoporphyrin-mediated fluorescence imaging

OBJECTIVE

Noninvasive in vivo imaging of human tumors implanted in mice provides a reliable and economic tool for the investigation of tumor progression and metastasis and of the effectiveness of the antiblastic drugs on them. The purpose of this study is to report on the performance achievable by the well-known and extensively investigated HP-FRI (HematoPorphyrin (HP)-mediated Fluorescence Reflectance Imaging) when a high-quality image-acquisition device is used.

BACKGROUND DATA

Previous articles of ours showed that HP-FRI still represents a useful, simple and reliable optical imaging technique to detect surface tumors. Therefore, it is particularly suitable to be used in combination with other imaging modalities in a multimodal imaging system endowed with diagnostic capabilities much better than each separate modality.

MATERIALS AND METHODS

Six-week-old Crl:CD-1 nude mice were subcutaneously inoculated with tumor cells. Tumor-bearing mice were irradiated in vivo by a frequency-doubled pulsed Nd:YAG laser (lambda = 532 nm). A cooled CCD digital camera recorded fluorescence light emitted by HP injected in mice through a cut-on long-wavelength pass filter.

RESULTS

The system we developed allows in vivo imaging of surface tumors on small animals with a large field of view, high photometric sensitivity, adequate space resolution, and short measurement time. The estimated spatial resolution is 730 microm for a fluorescence source placed about 0.5 mm under the mouse skin. The first exploration of the capabilities of this HP-FRI setup on few mice shows that it allows the detection of (a) both types of investigated tumors, (b) early stage and late stage but visually unrecognizable tumors, (c) the gross structure of tumors, and (d) the discrimination of necrotic and nonnecrotic tumor regions.

Study of the efficacy and mechanism of ALA-mediated photodynamic therapy on human hepatocellular carcinoma cell

OBJECTIVES

To examine the efficacy and mechanism of delta- or 5-aminolevulinic acid (ALA)-mediated photodynamic therapy (PDT) on a human hepatocellular carcinoma cell line.

MATERIALS AND METHODS

The optimal uptake of photosensitizer ALA in HepG2 (p53 wild) cells was investigated by means of spectrometric measurement. Cell viability was determined by trypan blue exclusion assay. Morphological apoptotic changes in HepG2 cells before and after ALA-mediated PDT were determined by microscopic examination. Detection of apoptotic bodies was examined by DAPI staining. The changes in p53 expression were revealed by the immunostaining method.

RESULTS

ALA/protoporphyrin IX (PpIX) was mainly located in the cytoplasm of HepG2 cells. The maximal cellular uptake occurred after 18 h in vitro incubation. The photocytotoxic assay showed that ALA PDT induced 80% killing at 2 mM drug dose and 2 J/cm2 light intensity. Up to 70% of cells showed membrane blebbing and positive DAPI staining, indicating that ALA-PDT-mediated cell death was predominantly via apoptosis. In addition, p53 was upregulated after treatment, implying that p53 might evoke apoptotic cell death.

CONCLUSIONS

HepG2 cell line is sensitive to ALA-mediated PDT. ALA-PDT induces apoptosis in the HepG2 cell line that may be mediated by a p53-dependent pathway.

pH dependent uptake of porphyrin-type photosensitizers by solid tumor cells in vitro is not induced by modification of transmembrane potential

The uptake of HpIX, TPPS2a and mTHPC by WiDr, THX cells and skin fibroblasts at pH 7.4 and 6.8 was compared. In the absence of serum, the uptake of HpIX was higher at lower pH. The difference was significant in WiDr cells (P < 0.01) and skin fibroblasts (P < 0.05). TPPS2a nor mTHPC showed any pH dependent uptake. Lowering the extracellular pH resulted in a significant depolarization (3-8 mV) of the cells. Application of tetraethylammonium chloride did not affect the cellular uptake of any of the photosensitizers. We conclude that the pH dependent uptake of photosensitizers is not mainly related to altered transmembrane potential.

Determination of the modulation transfer function for a time-gated fluorescence imaging system

The use of fluorescence for cancer detection is currently under investigation. Presently, steady-state fluorescence detection methods are in use, but have limitations due to poor contrast between the fluorescence of the tumor and background autofluorescence. Improved contrast can be obtained with time-resolved techniques because of the differing lifetimes between autofluorescence and exogenous photosensitizers that selectively accumulate within tumor tissue. An imaging system is constructed using a fast-gated (200-ps) charge-coupled device (CCD) camera and a pulsed 635-nm laser diode. To characterize the ability of the system to transfer object contrast to an image, the modulation transfer function (MTF) of the system is acquired by employing an extended knife-edge technique. A knife-edge target is assembled by drilling a rectangular well into a block of polymethyl methacrylate (PMMA). The imaging system records images of the photosensitizer, chloroaluminum phthalocyanine tetrasulfonate (AlPcTS), within the well. AlPcTS was chosen to test the system because of its strong absorption of 635-nm, high fluorescence yield, and relatively long fluorescence lifetime (approximately 7.5 ns). The results show that the system is capable of resolving 10(-4) M AlPcTS fluorescence as small as 1 mm. The findings of this study contribute to the development of a time-gated imaging system using fluorescence lifetimes.

The use of chloroaluminium phthalocyanine tetrasulfonate (AlPcTS) for time-delayed fluorescence imaging

Phthalocyanine derivatives are currently under investigation for use in photodynamic therapy, which is a promising cancer treatment. These materials, which display preferential uptake in cancerous cells, also exhibit high fluorescence yields and can be used for tumour detection. Problems with steady-state fluorescence techniques such as excitation scatter and background autofluorescence can be eliminated by using time-resolved imaging techniques without the need for filters. A tissue phantom was assembled to test a constructed time-gated imaging system by drilling 36 wells of varying diameter and depth (10 mm to 1 mm) into a block of polymethyl methacrylate (PMMA). The system was used to record images of chloroaluminium phthalocyanine tetrasulfonate (AlPcTS) at differing concentrations in neat aqueous solvent and cell suspensions within the wells. A mixture of Intralipid (to mimic tissue scatter) and Evan's blue (to mimic tissue absorption) of depths ranging from 1 mm to 10 mm was placed on top of the PMMA block. The ensuing images were analysed using signal-to-noise ratios and contrast-detail curves. The results indicate that the time-gated imaging system can prevent background excitation scatter from distorting the fluorescence signal from a longer-lived photosensitizer without the need for filters.

Photocytotoxic and DNA damaging effect of temoporfin (mTHPC) and merocyanine 540 (MC540) on nasopharyngeal carcinoma cell

Photodynamic therapy (PDT) is a new approach to cancer treatment for a variety of malignant tumors. In this study, two clinical photosensitizers, Temoporfin (meta-tetra-hydroxyl-phenyl-chlorin; mTHPC) and merocyanine 540 (MC540), were selected to explore for their photocytotoxic and genotoxic effects on nasopharyngeal carcinoma cells (NPC/HK1 and CNE2). Results of tetrazolium reduction assay showed that 80% cell killing were achieved for both cell lines at 0.4 microg/ml mTHPC for 24 h incubation and then with 40 kJ/m2 light irradiation, whereas 40 microg/ml MC540 with 50 kJ/m2 light dosage was required to attain the same level of phototoxicity for NPC/HK1. On the contrary, NPC/CNE2 was quite resistant to MC540. Hence, mTHPC-mediated PDT exerted a more potent effect than MC540-mediated PDT, even though the molar extinction coefficient of the main absorption peak for MC540 is much higher than that of mTHPC. Dark cytotoxicity remained negligible for both sensitizers. Comet assay was used to evaluate the DNA strand break and potential genotoxic effect induced by mTHPC and MC540 on the NPC cells. No DNA strand break was detected in the absence of light, and under sublethal treatment (LD25) for either sensitizer-loaded cells. Confocal laser scanning microscopy showed that mTHPC and MC540 localized in the cytoplasm but not in the nucleus of the tumor cells, which provided evidence for undetectable DNA damage under dark and low photodynamic dose.

Synthesis and estrogen receptor binding affinity of a porphyrin-estradiol conjugate for targeted photodynamic therapy of cancer

A tetraphenylporphyrin-C11-beta-estradiol conjugate has been synthesized. Competitive binding assay of the conjugate with estrogen receptor (ER)-ligand-binding domain showed that the conjugate binds specifically to the protein with high affinity. Potential use of this conjugate to selectively deliver cytotoxic porphyrins to ER-positive cells in various carcinomas is discussed.

Tumour visualization in a murine model by time-delayed fluorescence of sulphonated aluminium phthalocyanine

Mice bearing the MS-2 fibrosarcoma were administered 0.25, 0.5 or 1 mg kg(-1) body weight (b.w.) of sulphonated aluminium phthalocyanine (AlS(2)Pc) (with average degree of sulphonation of 2.1), and time-gated fluorescence images were acquired up to 6 h after the injection. Different excitation wavelengths (610, 650 and 670 nm) were tested. Red light excitation and 3 ns delayed detection allow one to minimize natural fluorescence and scattered laser light, respectively. The best conditions for tumour detection are reached under either 650 or 670 nm Excitation, 2-4 h after the administration of either 0.5 or 1 mg kg(-1) b.w. of AlS(2)Pc. In these situations, the average fluorescence contrast between tumour area and surrounding healthy tissue is > 2, providing a clear identification of the pathological region. However, tumour localization is possible even after the injection of 0.25 mg kg(-1) b.w. of sensitizer. In conclusion, under low power excitation (< 100MuW cm(-2)), the technique allows real time detection of an intradermal tumour with good contrast.