Selection of Metalloporphyrin Heme Oxygenase Inhibitors Based on Potency and Photoreactivity

Abstract

The heme oxygenase inhibitor, tin protoporphyrin, is being studied for the prevention of neonatal jaundice. This potential drug, however, is also a photosensitizer that could cause serious and unknown side effects when administered to newborns. Therefore, we have developed in vitro and in vivo procedures for the screening and further characterization of potentially safe heme oxygenase inhibitors. The ideal inhibitor: 1) contains a biocompatible metal, 2) is not degraded in tissues, 3) is a highly potent inhibitor of heme oxygenase, and 4) does not participate in photochemical reactions. Proto- and mesoporphyrin derivatives with the tin, zinc, manganese, chromium, nickel, and magnesium were screened in vitro for suitability. Chromium protoporphyrin and mesoporphyrin were further studied in vitro and in vivo and were found to meet the ideal criteria. Chromium mesoporphyrin appeared to be the most potent in vitro inhibitor of adult Wistar rat tissue heme oxygenase. Four mumol of chromium protoporphyrin or chromium mesoporphyrin/kg body weight, administered intraperitoneally to adult male Wistar rats given a heme load through intraperitoneal administration of 30 mumol heme/kg body weight, caused significant suppression of hemolysis-induced increase in carbon monoxide production to 72 and 44% of control, respectively, 5.5 h after treatment. At t = 6 h, the tissue heme oxygenase activity, measured in vitro, was significantly reduced to 33 and < 5% in liver and to 22 and < 5% in spleen after the administration of chromium protoporphyrin and mesoporphyrin, respectively, but was not reduced in brain. The results show that there exist effective metalloporphyrin heme oxygenase inhibitors without photosensitizing properties.

Effect of light exposure on metalloporphyrin-treated newborn mice

“…These results confirm earlier in vitro observations describing CrMP as not photoreactive (20). Although we found no photosensitizing effects of CrMP in vivo, its safe use in neonates is concerning because of its chemical toxicity, which (25) demonstrated that 40 µmol CrMP/ kg intravenously decreases mean arterial pressure, sinusoidal diameter, and blood flow in rats.…”

“…Both are potent inhibitors of HO, are orally absorbable, and have negligible effects on nitric oxide synthase and soluble guanylyl cyclase (10). Moreover, CrMP does not affect brain or spleen HO activity 24 h after oral administration (unpublished data), decreases liver HO activity in a hemolytic model (unpublished data), and is not photoreactive (20). We have shown that ZnBG is highly potent and fast-acting with a short duration of action, minimally increases HO-1 gene expression and protein levels, and decreases HO activity and bilirubin production in a hemolytic mouse model (19,21).…”

In vitro inhibition of heme oxygenase isoenzymes by metalloporphyrins

“…The concentration was adjusted to 1.2 mM. 18 Concentrations were then varied through appropriate dilutions with buffer. All experiments were conducted under subdued light.…”

“…17 Natural and synthetic derivatives of heme (iron protoporphyrin) or metalloporphyrins (Mps) have been found to inhibit competitively in vitro and in vivo HO activity. 18 In vivo administration of Mps results in the suppression of plasma bilirubin, 19 total body CO excretion 20 and biliary heme excretion. 17,21 As a result, these compounds have potential use for the prevention of neonatal jaundice.…”

“…18,20,[22][23][24] These include the metal-free (Mf) and metal-containing porphyrins with ring modifications to yield deutero-, meso-, proto-and bis-glycol porphyrins ( Figure 1). However, no comprehensive or systematic study has yet been reported that examines whether the two major HO isoenzymes are differentially sensitive to inhibition by Mps.…”