We present a method for tissue fluorescence quantification in situ using a handheld fiber optic probe that measures both the fluorescence and diffuse reflectance spectra. A simplified method to decouple the fluorescence spectrum from distorting effects of the tissue optical absorption and scattering is developed, with the objective of accurately quantifying the fluorescence in absolute units. The primary motivation is measurement of 5-aminolevulinic acid-induced protoporphyrin IX (ALA-PpIX) concentration in tissue during fluorescence-guided resection of malignant brain tumors. This technique is validated in phantoms and ex vivo mouse tissues, and tested in vivo in a rabbit brain tumor model using ALA-PpIX fluorescence contrast.
As part of an ongoing program to develop two-photon (2-) photodynamic therapy (PDT) for treatment of wet-form age-related macular degeneration (AMD) and other vascular pathologies, we have evaluated the reciprocity of drug-light doses in focal-PDT. We targeted individual arteries in a murine window chamber model, using primarily the clinical photosensitizer Visudyne/liposomal-verteporfin. Shortly after administration of the photosensitizer, a small region including an arteriole was selected and irradiated with varying light doses. Targeted and nearby vessels were observed for a maximum of 17 to 25 h to assess vascular shutdown, tapering, and dye leakage/occlusion. For a given end-point metric, there was reciprocity between the drug and light doses, i.e., the response correlated with the drug-light product (DLP). These results provide the first quantification of photosensitizer and light dose relationships for localized irradiation of a single blood vessel and are compared to the DLP required for vessel closure between 1- and 2- activation, between focal and broad-beam irradiation, and between verteporfin and a porphyrin dimer with high 2- cross section. Demonstration of reciprocity over a wide range of DLP is important for further development of focal PDT treatments, such as the targeting of feeder vessels in 2- PDT of AMD.
Intravital imaging using confocal microscopy facilitates high-resolution studies of cellular and molecular events in vivo. We use this, complemented by Doppler optical coherence tomography (OCT), to assess blood flow in a mouse dorsal skin-fold window chamber model to image the response of individual blood vessels to localized photodynamic therapy (PDT). Specific fluorescent cell markers were used to assess the effect on the vascular endothelial cell lining of the treated vessels. A fluorescently tagged antibody against an endothelial transmembrane glycoprotein (CD31) was used to image endothelial cell integrity in the targeted blood vessel. A cell permeability (viability) indicator, SYTOX Orange, was also used to further assess damage to endothelial cells. A fluorescently labeled anti-CD41 antibody that binds to platelets was used to confirm platelet aggregation in the treated vessel. These optical techniques enable dynamic assessment of responses to PDT in vivo, at both the vascular endothelial cell and whole vessel levels.
Two-photon (2-γ) photodynamic therapy (PDT) as opposed to "standard" one-photon (1-γ) PDT with Visudyne has recently been suggested as a targeted treatment alternative for wet-form age-related macular degeneration (AMD) and other neovascular diseases. AMD is a major cause of severe vision loss in the older population. It occurs due to growth of new leaky blood vessels (neovasculature) from the choriocapillaris, which results in destruction of photoreceptors in the fovea and loss of central vision. Damage outside the diseased region is always a concern, due to photosensitizer accumulation and its 1-γ excitation. Highly targeted 2-γ excitation, due to its non-linear intensity dependence, intrinsically avoids out-of-focus damage to healthy tissues and so could be valuable for wet-AMD. We have previously developed a quantitative approach for comparing the 2-γ efficacy of photosensitizers in vitro. In this study, we report further the development of ex vivo and in vivo techniques. A mouse mesenteric vessel has been investigated as the ex vivo model of neovasculature. For the in vivo studies, we have explored a mouse dorsal skin-fold window chamber model. Two-photon PDT is delivered using tightly focused ~300 fs laser pulses from a Ti:sapphire laser operating at 850 nm with 90 MHz pulse repetition rate. Confocal microscopy coupled to the laser was used to visualize the vessel's/cell's response before, during and after the treatment. We are able to demonstrate quantitative biological techniques to evaluate efficacy of 2-γ PDT photosensitizers in vivo.
Photodynamic therapy (PDT) using verteporfin is widely used for treatment of age related macular degeneration (AMD).
Due to non-perfect selectivity of the drug accumulation in the neovasculature some collateral damage to healthy tissue
arises during the treatment. Damage to healthy structures in the eye is always a concern because of a high probability of
reducing visual acuity. Two-photon (2-&ggr;) photodynamic therapy potentially offers much higher treatment selectivity than
its one-photon (1-&ggr;) counterpart. By utilizing focused light for 2-&ggr; excitation, treatment volumes on the order of
microliters can be achieved thus maximizing localized insult to abnormal blood vessels and sparing healthy tissue. We
propose that 2-&ggr; photodynamic therapy will be valuable in the treatment of choroidal neovascularization secondary to
age related macular degeneration as well as other conditions. To ascertain feasibility of 2-&ggr; photodynamic therapy we
measured 2-&ggr; spectrum and cross sections of verteporfin (80 GM at 940 nm, 1 GM = 10-50 cm4s/photon), chlorin e6 (14
GM at 800 nm) and tetrasulfonated aluminum phthalocyanine (140 GM at 900 nm) and investigated their in vitro
efficiency under 2-&ggr; excitation. Only verteporfin demonstrated cell kill under the used irradiation parameters (average
light intensity 9.1 mW, wavelength 850 nm, total light dose 6900 J/cm2). Dorsal skinfold window chamber model in
mouse was used to test efficiency of 2-&ggr; PDT with verteporfin in vivo. Although we were able to induce photodynamic
damage to a blood vessel using 1-&ggr; excitation, 2-&ggr; excitation resulted in no visible damage to irradiated blood vessel. The most probable reason is low efficiency of verteporfin as a 2-&ggr; photosensitizer. We also report 2-&ggr; spectrum of new
photosensitizer, HCC4 (4300 GM at 830 nm), specifically designed for efficient 2-&ggr; excitation.
Age related macular degeneration (AMD) is a major cause of severe vision loss in the older population, due to ingrowth
of new leaky blood vessels (neovasculature) from the choriocapillaris, which results in destruction of photoreceptors in
the fovea and loss of central vision. "Standard" one-photon (1-γ) photodynamic therapy (PDT) using Visudyne(R) is an
approved method of AMD treatment but has the potential to damage healthy tissues lying above and below the
neovasculature due to photosensitizer accumulation and its wide-beam 1-γ excitation. Highly-targeted two-photon (2-γ)
excitation may avoid this, since, due to its non-linear intensity dependence, the probability of 2-γ excitation is greatest in
the focal plane, which intrinsically avoids out-of-focus damage to healthy tissues.
The aim of the present study is to evaluate the 2-γ efficiency of Visudyne and to compare it to the archetypal
photosensitizer Photofrin(R). Since neovascular endothelium is targeted in AMD, an endothelial cell line (YPEN-1) was
selected as the in vitro model. 2-γ PDT was delivered using tightly focused ~300 fs laser pulses from a Ti:sapphire laser
operating at 850 nm with 90 MHz pulse repetition rate. An assay was developed for quantification of the cellular damage
using the permeability stain Hoechst 33258 and the viability stain SYTOX. Visudyne (LD50= dose to kill 50% of cells:
500 J/cm2, 10 M, 7.2 μg/ml) was about an order of magnitude more effective than Photofrin (LD50 : 7500 J/cm2, ~42 μM, 25 μg/ml). We also demonstrate for the first time the quadratic dependence of the cellular response to 2-γ PDT.
This in vitro work will lead to the design of optimized in vivo studies in animal models of AMD.
Photodynamic therapy (PDT) based on simultaneous two-photon (2-γ) excitation has a potential advantage of highly targeted treatment by means of nonlinear localized photosensitizer excitation. One of the possible applications of 2-γ PDT is a treatment of exodus age-related macular degeneration where highly targeted excitation of photosensitizer in neovasculature is vital for reducing collateral damage to healthy surrounding tissue. To investigate effect of 2-γ PDT Photofrin was used as an archetypal photosensitizer. First, 2-γ absorption properties of Photofrin in the 750 - 900 nm excitation wavelength range were investigated. It was shown that above 800 nm 2-γ interaction was dominant mode of excitation. The 2-γ cross section of Photofrin was rather small and varied between 5 and 10 GM (1 GM = 10-50 cm4s/photon) in this wavelength range. Next, endothelial cells treated with Photofrin were used to model initial effect of 2-γ PDT on neovasculature. Ultrashort laser pulses provided by mode-locked Ti:sapphire laser (pulse duration at the sample 300 fs, repetition rate 90 MHz, mean laser power 10 mW, excitation wavelength 850 nm) were used for the excitation of the photosensitizer. Before 2-γ excitation of the Photofrin cells formed a single continuous sheet at the bottom of the well. The tightly focused laser light was scanned repeatedly over the cell layer. After irradiation the cell layer of the control cells stayed intact while cells treated with photofrin became clearly disrupted. The light doses required were high (6300 Jcm(-2) for ~ 50% killing), but 2-γ cytotoxicity was unequivocally demonstrated.