Photodynamic therapy (PDT) is the use of drugs (photosensitizers) that are activated by visible or near infrared light to
produce specific biological effects in cells or tissues [1]. The basic steps in a PDT treatment are application of the photosensitizer
(systemically or topically), a time interval to allow for photosensitizer accumulation in the target diseased tissue or cells,
and illumination of the target area or volume with light of an appropriate wavelength to activate the sensitizer. PDT is a
highly multidisciplinary topic, involving optical biophysics and bioengineering, synthetic chemistry, pharmacology, photophysics
and photochemistry, photobiology, and different clinical specialties. The main emphasis in this chapter is on those aspects
of greatest interest to specialists in biophotonics.
The term “photodynamic” was first coined a century ago with the observation that light and an acridine dye in the presence
of oxygen could kill microorganisms [2]. At around the same time, light therapy was being used in patients but without any
administered photosensitizing agent. The first reported clinical use of PDT was in 1904 when eosin was applied locally to
a tumor on the lip and exposed to light. There was limited follow up to this early work, probably due to the lack of potent
photosensitizers and of suitable light generation/delivery technologies. The modern era of PDT started with the discovery
of hematoporphyrin derivative (HpD) in the 1950s/1960s, and this was first used in patients in the 1970s to treat bladder
cancer. Substantial preclinical and clinical studies were started in the late 1970s and the first government approval for
PDT was in 1993, with a purified version of HpD (Photofrin R).