Free radicals as carcinogens and their quenchers as anticarcinogens.

An oxygen dependent signal was detected, late in the 1950s by electron spin resonance (ESR) in a saline solution of hematoporphyrin (Hp) excited by light. This signal expressed a free radical consisting of 'some kind of an association between Hp and oxygen', that Smaller et al. called 'oxyradical' (HpOO.). It soon opened a new level of understanding in carcinogenesis triggered by photodynamic substances, including Hp itself, polycyclic hydrocarbons (PCHs), as well as any carcinogen involving molecular species activated by radiation and/or metabolic reaction. Early in the 1960s, this prompted the discovery of benzo(a)pyrene (BP) photocarcinogenic enhancement (BP-PCE) in mice, probably due to an increase in free oxygen radical generation following correct light exposure. This assumption was confirmed in 1980 by the fact that mice orally loaded with antioxidants and radical quenchers, such as beta-carotene (BC) and cantaxanthin (CX), were protected against BP-PCE at 100% and against total BP carcinogenicity at more than 60%. These achievements were presented as the bases of the current explosion of interest in biology and medicine in building up the new field of chemoprevention against cancer and other chronic diseases by supplementation with antioxidant vitamins, retinoids and especially carotenoids and their synergistic association. The relevant findings of this research obtained in the last decade in in vitro and in vivo experiments as well as human interventions are reported and discussed with personal contributions.