KEYWORDS: Luminescence, Positron emission tomography, Cameras, In vivo imaging, Molecules, Signal to noise ratio, Proteins, In vitro testing, Image segmentation, Signal detection
Multimodal imaging is quickly becoming a standard in pre-clinical studies, and new developments have already confirmed the strength of acquiring and analyzing parallel information obtained in a single imaging session. One such application is the introduction of an internal reference moiety (e.g. radioisotope) to an activatable fluorescent probe. One of the limitations of this approach consists of working at concentrations which are within the overlapping range of sensitivities of each modality. In the case of PET/Fluorescence imaging, this range is in the order of 10-9 nM. Working in epi-illumination fluorescence imaging at such concentrations remains challenging. Here, we present in vitro and in vivo detection limits of a new fluorescent compound.
Shigeki Ito, Tsuneyuki Nagae, Shin Ishimaru, Sara Chau, Triet Dang, Leslie Anne Sabiniano, Mizuho Zempo, Mark Booth, Lih-Huei Liaw, Tatiana Krasieva, Bruce Tromberg, Johan van Lier, Michael Berns, Samuel Wilson
Initial hyperplasia is major cause of restenosis after vascular interventions for arterial occlusive disease. We reported that a fluorescent probe, Texas Red, conjugated to the scavenger-receptor ligand, maleylated bovine serum albumin accumulated almost exclusively in the injured, hyperplastic sites. The purpose of this study is to test the feasibility of enhanced drug delivery to the hyperplastic lesion by targeting the scavenger-receptors.
Red blood cells from BALB/c mice were labeled with short-lived (gamma) -emitting 99mTc and injected in EMT-6 tumor-bearing animals as a probe for the blood circulation. 99mTc-concentrations in light-exposed versus non-treated tumors were established for different photosensitizers at various dye doses and time intervals post-photodynamic therapy. At minimal dye doses for tumor cure, Photofrin IITM induced a doubling of the tumor radioactivity within the first hour post-PDT, followed by a marked decrease in the 99mTc-concentration at 24 hours post PDT. This pattern is characteristic of extensive hemorrhage followed by occlusion of the blood vessels. Under similar conditions the mono- and tetrasulfonated zinc phthalocyanines induced tumor response without prolonged effects on the tumor radioactivity levels. Accordingly, with the latter sensitizers, indirect cell kill via obstruction of the blood supply does not appear to constitute a major factor in eliciting a tumor response.
For a number of well-defined reasons, research into the development of alternative photosensitizers to PhotofrinTM, for use in Photodynamic Therapy, continues. The efforts described in this paper cover the study of a number of sensitizers absorbing at different wavelengths, which may have selective applications in the field of photomedicine. In one approach, the use of tetraazoporphyrins (porphyrazines) as sensitizers is studied. Although related tetramacrocyclic systems such as porphyrins and phthalocyanines have been, and continue to be, well studied in this field, porphyrazines have, to-date, received little attention. In a second study, purpurins are examined as possible alternatives to PhotofrinTM. In a recent study, tissue distribution properties of tin etiopurpurin dichloride, as a function of time, were examined. Results indicate that this purpurin continues to show promise for applications in PDT.
Among the water-soluble sulfonated metallo phthalocyanine (M-PcS), the monoand
disulfonated derivatives exhibit the highest photodynamic activity. The
effect of two further modifications of the M-PcS2 structure was studied using V-79
Chinese hamster cells. The addition of axial ligands onto the central metal ion
of Si-PcS12 did not interfere with cell uptake and appeared to reduce intracellularaggregation.
Combined with diminished in vitro phototoxicity these data
suggest that the tendency to form aggregates promotes targeting of the dye with
vital cell constituent. Addition of benzyl rings onto the mäcrocycle was evaluated
as a second structural modification. The resulting sulfonated metallo naphthalocyanines
(Zn-NcS) were 1-2 orders of magnitude less photoactive in vitro as the
corresponding phthalocyanine dyes.
Metallophthalocyanines and naphthalocyanines are under intensive study as second generation photosensitizers for PDT. They have attractive photophysical and chemical properties including strong absorption maxima at wavelengths where tissues provide optimal light transmissions, good capacity to generate singlet oxygen and facile chemical accessibility. Their photophysical properties are mainly determined by the nature of the central metal ion whereas ring substituents and axial ligands on the metal ion modulate solubility, tendencies to aggregate or associate with biomolecules, cell penetrating properties and the pharmacokinetics of the dyes. The availability of series of differently substituted metallophtha- locyanines and naphthalocyanines provides for the opportunity to assess structure- activity relationships with various parameters of importance in the overall out- come of PDT, an approach which could guide future syntheses of improved photosen- sitizers for the photodynamic therapy of cancer.
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