Multiphoton versus single photon excitation of photosensitizers for laser-induced fluorescence diagnosis and photodynamic therapy of cancer cells

摘要

In laser-induced fluorescence diagnosis and photodynamic therapy of cancer the applied photosensitizers (PS) are often covalently derivatized with macromolecules to improve their selective accumulation in the cancerous tissue, while maintaining its single-photon excited photophysical properties. In this contribution methoxy-polyethylene glycol (MPEG, MW approx 5 kDa) and human serum albumin (HSA, MW -60 kDa) are used as PS carriers. Multiphoton (MP) excitation of the PS is favorable as compared to single-photon excitation because the penetration depth of the laser light is improved (>5 mm) due to the longer wavelength of the approx 200 fs laser pulses used in this case (700-1050 nm). In this study cotton fibers and silica gel beads (<20 mu m) were stained with various PS and multiphoton-induced fluorescence was detected with a MP laser scanning microscope. The slopes of the log-log plots of the detected fluorescence intensity versus the laser excitation intensity vary between 1.8-2.6 for the various PS investigated. The excitation wavelength dependence of the MP-induced fluoresence indicates that the excitation cross section maxima can be shifted substantially relative to twice the wavelength of the one-photon absorption maxima. Some PS (photofrin II, purpurin, mTHPC-[MPEG]_2 and diaminoanthra-quinone) do not exhibit multiphoton-induced fluorescence. Some derivatized PS (sulforhodamine B, erythrosin B, purpurin) exhibit MP-induced fluorescence, although no single-photon absorption band exists in the spectral region around half the excitation wavelength.