Far-red-absorbing photosensitizers: their use in the photodynamic therapy of tumours

Abstract

Light in the 600–1000 nm spectral region is scattered to a relatively small extent by most mammalian tissues and is poorly absorbed by endogenous chromophores such as melanin, cytochromes and haemoglobin. As a consequence, red light processes a high penetration power into human tissues and can be selectively absorbed by photosensitizing agents ( e.g. porphyrins, chlorins, phthalocyanines, naphthalocyanines) localized in predetermined sites of the organism. Recently developed procedures allow for the specific loading of tumour tissues by several red-light-absorbing photosensitizers; this property is the basis of a novel phototherapeutic modality for the treatment of a variety of solid tumours. The efficacy of the light plus photosensitizer combination in inducing tumour regression (the technique is often defined as “photodynamic therapy”) is dependent on the photophysical properties of the photosensitizer and its affinity for malignant tissues.