The metalloporphyrin zinc protoporphyrin (ZnPP) and its synthetic analogs are being studied as a potential drugs for the prevention of excessive hyperbilirubinemia and for elucidating a potential physiologic role for carbon monoxide (CO). Some investigators have reported on photoreactive properties whereas others have found no evidence for this phenomenon. If ZnPP is to be used as a drug, photoractive properties could cause potentially serious side effects. We tested the hypothesis that the presence of synthetic- or natural(bile acids) surfactants could cause ZnPP to exhibit photoreactivity in vitro. Fifty μM of each ZnPP, chromium mesoporphyrin (CrMP), and tin mesoporphyrin (SnMP, known to be photoreactive) were incubated for 15 min. at 37°C with 44 mM of each of 7 synthetic- and 11 natural surfactants in a sealed vial with histidine in the presence of white light (20μW/cm2). CO, a product of histidine photodegradation was measured in the vial headspace by gas chromatography. These data (mean±SD) are consistent with the observation that ZnPP is not photosensitizing in vivo [Dev Pharmacol Ther 1990;14:187-92], but can exhibit photoreactivity under in vitro conditions, when incubated with synthetic surfactants. As expected, SnMP was uniformly photoreactive in the presence of no- and all surfactants. In contrast, CrMP was nonreactive with any surfactant. Besides explaining observed difference in the behavior of metalloporphyrins in vitro and in vivo, these results could give us the basis for strategies to minimize photosensitivity for particular compounds. Table

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