An experimental model of postnatal jaundice in the suckling rat. Suppression of induced hyperbilirubinemia by Sn-protoporphyrin

Efficacy and Safety Concerns with Sn-Mesoporphyrin as an Adjunct Therapy in Neonatal Hyperbilirubinemia: A Literature Review

Neonatal hyperbilirubinemia is a frequently observed clinical situation that, sometimes, may result in complications ranging from mild neurodevelopment impairment to serious outcome of kernicterus. The rationale logic of heme oxygenase enzyme inhibition to lower bilirubin levels is intriguing. In compliance with that rationale, metalloporphyrin was discovered. After successful results in in-vitro and animal studies, tin mesoporphyrin is now under phase II clinical trial to test for preventive and therapeutic efficacy in unconjugated hyperbilirubinemia. This review evaluates in-vitro studies, animal studies, and clinical trials for the efficacy and safety of tin analogues of metalloporphyrin. Few alternatives to metalloporphyrins are also available, synchronizing with the same rationale logic of inhibition of bilirubin production, which need further research.

Animal Kernicterus Models: Progress and Challenges

Kernicterus is a leading cause of neonatal death throughout the world, especially in low-middle-income countries. It is developed by an unconjugated hyperbilirubinemia in the blood and brain tissue, triggering pathological processes that spawn neurotoxicity and neurodegeneration. However, the biological mechanism (s) of bilirubin-induced neurotoxicity and Kernicterus development remain to be well elucidated. Likewise, a practical therapeutic approach for human Kernicterus has yet to be found. Undoubtedly, animal models of Kernicterus can be helpful in the identification of underlying biological processes of hyperbilirubinemia evolution to Kernicterus, as well as the evaluation of various treatments efficacy in preclinical studies. More importantly, establishing an animal model that can mimic the Kernicterus and its behavioral, neuro-histological, and hematological manifestations is a severe priority in preclinical studies. So far, several Kernicterus animal models have been established that could partially mimic one or more clinical and paraclinical signs of human Kernicterus. The present study aimed to review all methods modeling Kernicterus with a focus on their potentials and shortcomings and subsequently provide the optimal methods for an ideal Kernicterus animal model.

Chronic Intermittent Hypoxia Causes Lipid Peroxidation and Altered Phase 1 Drug Metabolizing Enzymes in the Neonatal Rat Liver

Critically ill preterm neonates requiring oxygen therapy often experience frequent apneas with intermittent hypoxia (IH). IH-induced oxidative stress causes lipid peroxidation, which targets the liver and contributes to toxic drug reactions. We tested the hypothesis that incremental IH episodes induce oxidative damage in the neonatal liver and alter the expression of genes that regulate drug metabolism. Newborn rats were exposed to increasing IH episodes (12% O₂) during hyperoxia (50% O₂), or placed in room air (RA) until postnatal day 21 (P21) for recovery from IH (IHR). RA littermates served as controls, and pups exposed to 50% O₂ served as hyperoxia controls. Hepatic histopathology, biomarkers of oxidative stress and oxidative DNA damage, antioxidants, and expression of genes that regulate drug metabolism were assessed. Oxidative stress and DNA damage, evidenced by 8-isoprostaglandin F_2α (8-isoPGF_2α) and 8-hydroxy-2'-deoxyguanosine (8-OH-dG), respectively, increased as a function of IH episodes, and was associated with decreased superoxide dismutase (SOD) and increased catalase activities. Pathological changes including cellular swelling, steatosis, necrosis, and focal sinusoid congestion were seen in IH, but not in IHR. Similarly, IH was associated with upregulation of several genes involved in DNA repair, which were downregulated during IHR. Of the genes involved in drug metabolism, aldehyde dehydrogenases (involved in lipid peroxidation) and cytochrome P450 (CYP) genes of the 2C family (involved in oxidative stress) were robustly upregulated both in IH and in IHR. Hepatic oxidative stress and lipid peroxidation occurring in response to chronic IH have implications for preterm infants, and may explain, in part, the pharmacokinetic variations and drug toxicities in this vulnerable population.

Metalloporphyrins - an update

Metalloporphyrins are structural analogs of heme and their potential use in the management of neonatal hyperbilirubinemia has been the subject of considerable research for more than three decades. The pharmacological basis for using this class of compounds to control bilirubin levels is the targeted blockade of bilirubin production through the competitive inhibition of heme oxygenase (HO), the rate-limiting enzyme in the bilirubin production pathway. Ongoing research continues in the pursuit of identifying ideal metalloporphyrins, which are safe and effective, by defining therapeutic windows and targeted interventions for the treatment of excessive neonatal hyperbilirubinemia.

The protective properties of synthetic porphyrin tin complexes in toxic hyperbilirubinemia

Tin complexes of meso-substituted synthetic porphyrins, namely Sn4+-meso-tetraphenyl- porphyrin ( Sn - TPP ) and Sn4+-meso-tetrakis(N-methyl-3-pyridyl)porphyrin tetratosylate ( Sn - TMe -3- PyP ), efficiently decrease the serum bilirubin level when injected subcutaneously at a dose of 100 μM kg−1body weight into mice. These compounds are active during hyperbilirubinemia, induced by phenylhydrazine, hemin and tetrachloromethane, and also during autoimmune hemolytic anemia. In the latter case a decrease in serum bilirubin content was observed, as well as a decrease in the amount of blood reticulocytes which reflects a milder course of the disease. The Sn complexes under study induce, in vivo, cytochrome P-450, inhibit microsomal heme oxygenase and decrease the intensity of lipid peroxidation. At the same time, in vitro the hepatic and splenic heme oxygenase activity is blocked only when a 0.1 μM concentration of Sn - TMe -3- PyP or Sn -protoporphyrin IX is added to the incubation mixture. Sn - TPP does not affect the activity of this enzyme in vitro.

Targeted suppression of heme oxygenase-1 by small interference RNAs inhibits the production of bilirubin in neonatal rat with hyperbilirubinemia

Background

Excessive accumulation of bilirubin contributes to neonatal hyperbilirubinemia in rats. Heme oxygenase (HO) is one of the rate-limiting enzymes in catabolizing heme to bilirubin. In the present study, we investigated whether suppression of rat HO-1 (rHO-1) expression by small interference RNAs (siRNAs) reduces bilirubin levels in hyperbilirubinemic rats.

Results

Four pairs of siRNA targeting rHO-1 mRNA were introduced into BRL cells and compared for their inhibitory effect on the expression of rHO-1 gene and production of rHO-1 protein. The siRNA exhibiting the most potent effect on HO-1 expression and activity was then administered intraperitoneally to 7 to 9-day-old rats with hyperbilirubinemia. The siRNA distributed mostly in the liver and spleen of neonatal rat. Serum bilirubin levels and hepatic HO-1 expression were further evaluated. Systemic treatment of siRNA targeting rHO-1 reduced hepatic HO-1 expression and decreased the serum bilirubin levels in a time- and dose-dependent manner, and siRNA decreased the indirect bilirubin levels more effectively than Sn-protoporphyrin (SnPP), an HO-1 inhibitor.

Conclusion

siRNA targeting rHO-l attenuates hepatic HO-1 expression and serum bilirubin levels. Thus this study provides a novel therapeutic rationale for the prevention and treatment of neonatal hyperbilirubinemia.

Control of severe hyperbilirubinemia in full-term newborns with the inhibitor of bilirubin production Sn-mesoporphyrin

OBJECTIVE

To assess the efficacy of Sn-mesoporphyrin (SnMP), a potent inhibitor of bilirubin production, in: a) moderating the need for phototherapy (PT) in full-term breastfed infants with plasma bilirubin concentrations (PBC) of >/=256.5 micromol/L and </=307.8 micromol/L (>/=15 mg/dL and </=18 mg/dL, respectively) that were reached between >/=48 and </=96 hours of age; b) diminishing the time required for the PBC to decline to </=222.3 micromol/L (</=13 mg/dL) (closure of the case); c) decreasing the number of bilirubin determinations required for monitoring hyperbilirubinemia.

STUDY PARTICIPANTS

Healthy full-term breastfed infants with a PBC between >/=256.5 micromol/L and </=307.8 micromol/L (>/=15 mg/dL and </=18 mg/dL, respectively) reached between 48 to 96 hours of age.

DESIGN/METHODS

After obtaining informed consent from the parents, infants were randomized to either the SnMP (6.0 micromol/kg birth weight, single dose, intramuscular) group or the control group. The infants' PBCs were followed by daily measurements either in the hospital or at discharge as outpatients until the hyperbilirubinemia had subsided (PBC </=222.3 micromol/L [13mg/dL]). The total number of newborns enrolled in the study was 84; the SnMP group comprised 40 infants; the control group comprised 44 infants. The groups were similar in sex ratio, birth weight, gestational age, PBC, and age at enrollment. All infants were breastfed. Phototherapy was initiated at a PBC of 333.5 micromol/dL (19.5 mg/dL).

RESULTS

SnMP entirely eliminated the need for supplemental PT to control hyperbilirubinemia; in contrast, of the 44 control infants, 12 required treatment with PT (27%) when their PBC reached or exceeded the level (333.5 micromol/dL; 19.5 mg/dL) at which time the use of PT was dictated by hospital guidelines. None of the 40 SnMP-treated infants reached a PBC of 19.5 mg/dL. SnMP also markedly diminished the median hours to case closure (SnMP: median, 86.5 hours; minimum/maximum, 24/216 hours; controls: median, 120 hours; minimum/maximum, 72/336 hours); and significantly reduced the number of bilirubin determinations required for clinical monitoring of the infants (SnMP: median, 3; minimum/maximum, 1/9; controls: median, 5; minimum/maximum, 3/11). No adverse effects of SnMP use were observed.

CONCLUSION

A single dose of SnMP proved effective in controlling severe hyperbilirubinemia in full-term breastfed newborns with high bilirubin levels between 48 and 96 hours. In addition, SnMP eliminated the need for PT and reduced the use of medical resources in the clinical treatment of this problem as well as the related, important and painful, emotional costs for both mothers and infants.

Control of hyperbilirubinemia in glucose-6-phosphate dehydrogenase-deficient newborns using an inhibitor of bilirubin production, Sn-mesoporphyrin

BACKGROUND

Hyperbilirubinemia in new-borns with glucose-6-phosphate dehydrogenase (G6PD) deficiency is a serious clinical problem because of the severity and unpredictability of its course. An innovative approach to this problem is suggested by previous experience with Sn-mesoporphyrin (SnMP), a potent inhibitor of bilirubin production, in moderating neonatal hyperbilirubinemia caused by ABO incompatibility, immaturity, and unspecified mechanisms.

OBJECTIVE

To compare the effectiveness of the preventive and therapeutic uses of SnMP in ameliorating the course of bilirubinemia of G6PD-deficient neonates.

METHODS

Neonates born at the Metera Maternity Hospital, Athens, Greece, and found to be G6PD-deficient by cord blood testing were stratified by sex and gestational age (210-265 days and >265 days) and randomized in pairs to receive SnMP (6 micromol/kg birth weight, intramuscularly) either on the first day of life (preventive use) or if and when the plasma bilirubin concentration (PBC) level reached an age-specific threshold level for intervention (therapeutic use). In the case of failure of SnMP to control the rise of PBC levels, the protocol defined precisely the threshold PBC levels for switchover to phototherapy (PT) and, if necessary, exchange transfusion. PBC was measured daily until a declining value was obtained and the case was closed.

RESULTS

A total of 86 G6PD-deficient neonates were randomized: 42 in the preventive arm and 44 in the therapeutic arm. Of the latter, 20 (45%) reached PBC levels requiring therapeutic intervention and thus received SnMP. Regardless of the trial arm, none of the 86 neonates required PT, whereas in a previous study in the same population, 33% of G6PD-deficient neonates required PT. In the intrapair sequential analysis, the favored arm was decided on the criterion of the age at closure of the case being shorter by at least 1 day. After plotting 30 untied pairs in the sequential analysis graph, the preventive use of SnMP proved to be the favored arm, and the trial was stopped. At this point, there were 2 unpaired neonates, 12 tied pairs, 22 pairs in which the preventive use of SnMP was favored and 8 pairs in which the therapeutic use of SnMP was favored. In the group analysis, infants in the preventive group, compared with those in the therapeutic group, had a lower maximum PBC level (8.2 +/- 3.1 and 10.9 +/- 2.8 mg/dL, respectively), which was reached at an earlier age (63.5 +/- 34.8 and 82.2 +/- 24.7 hours, respectively) as well as a lower closing PBC level (7.2 +/- 2.9 and 9.6 +/- 2.5 mg/dL, respectively) and an earlier age at closing (89.1 +/- 35.6 and 110.8 +/- 23.6 hours, respectively). Moreover, a PBC level of >/=8.0 mg/dL, a level at which jaundice is clearly visible, was not reached by 52% of the neonates in the preventive arm and 16% of the neonates in the therapeutic arm.

CONCLUSIONS

In G6PD-deficient neonates, a single dose of SnMP administered preventively or therapeutically entirely supplanted the need for PT to control hyperbilirubinemia. The preventive use of SnMP offers practical advantages in populations with a high enough prevalence of G6PD deficiency to justify cord blood screening.

Synergistic effect of retinoic acid on Sn-PP mediated suppression of heme oxygenase activity in vivo in rats

The present investigation is designed to probe the influence of excess retinoic acid (50,000 I.U.) and Sn-protoporphyrin (50 mumol/kg bw) along with retinoic acid on the activity patterns of the rate-limiting enzyme, heme oxygenase, of the heme catabolic pathway in the liver, spleen, kidney, brain, heart, and lung of male Wistar rats. Our results are noteworthy as SnPP is being used for the amelioration and management of hyperbilirubinemia, and they emphasize that the combined dosing of retinoic acid and SnPP attenuates the suppression of the activity of HMOX, thereby decreasing plasma bilirubin levels. The features of action of retinoic acid and SnPP together in vivo, i.e., a substantial suppression of the formation of a potentially neurotoxic metabolite, bilirubin, and the enhancement of disposal of the untransformed substrate (heme) of the enzyme that is inhibited, define some of the prerequisites of a therapeutically useful formulation.

Chemistry and biology of heme. Effect of metal salts, organometals, and metalloporphyrins on heme synthesis and catabolism, with special reference to clinical implications and interactions with cytochrome P-450

Although free porphyrins occur in nature in small quantities, no known function has been assigned to them. In contrast, heme and cobalamin, which are Fe and Co chelates of porphyrins or porphyrin derivatives, respectively, carry out crucial biological functions. Heme is the prosthetic group for a number of hemoproteins. These include myoglobin and hemoglobin, which carry out oxygen binding or transport; mitochondrial cytochromes aa3, b, c, and c3, which are important in transferring electrons; microsomal cytochrome P-450, which catalyzes mixed-function oxidations; catalase, which decomposes H2O2; peroxidase, which activates H2O2; and tryptophan pyrrolase, which catalyzes the oxidation of tryptophan. Recently, heme has also been shown to be the prosthetic group of prostaglandin and peroxide synthetase and indoleamine dioxygenase. The elegant studies of the biochemical pathway for the formation of heme demonstrated the arrangement in the porphyrin macrocycle of the carbon and nitrogen atoms originating from the eight glycine and the succinic acid molecule that are the precursors of porphyrins. There are eight enzymes involved in the synthesis of heme. The first and last three of these enzymes are localized in mitochondria, while the intermediate enzymes are localized in cytosol. The catalytic site of HMOX recognizes metalloporphyrins with central metal atoms other than iron; it favors some of these metalloporphyrins over heme as a potential substrate, sometimes by a large factor, permitting the synthetic heme analogue to serve as a potent competitive inhibitor of HMOX reaction. Since these synthetic metalloporphyrins do not bind molecular oxygen, they are not metabolically degraded by ring rupture and do not add to the body pool of bile pigment. One possible consequence of this competitive inhibition of heme degradation is suppression of bile pigment formation to such a degree that excessive plasma levels of bilirubin may be diminished. The studies of Drummond and Kappas (1981) and later studies in rats, mice, monkeys, and man, and also our studies have proved the latter phenomenon. The compound does not appear to affect the metabolic disposition of preformed bilirubin but inhibits biliary bilirubin excretion derived from the metabolism of endogenous or exogenous heme. Whether some of the effect of Sn-PP on naturally occurring or experimentally induced jaundice in animals reflects diversion of heme to nonheme to oxygenase-dependent pathways of heme metabolism, or whether a pathway which is normally latent becomes activated concurrent with HMOX inhibition is not known.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies with the haeme oxygenase inhibitor Sn-protoporphyrin in patients with primary biliary cirrhosis and idiopathic haemochromatosis

Sn(tin4+)-protoporphyrin, a potent competitive inhibitor of haeme oxygenase, the rate limiting enzyme in the degradation of haeme to bile pigments, was given intravenously to six patients with primary biliary cirrhosis and to four patients with idiopathic haemochromatosis. Serum bilirubin concentrations decreased in all patients after administration of 1-2 mumol/kg body weight of the metalloporphyrin, given in two doses eight to 24 hours apart. This reduction lasted approximately four to five days after injection of the compound. Excretion of endogenous haeme in bile increased (mean increase approximately two to threefold) in parallel with the decrease in serum bilirubin concentrations in both patient groups, and the highest biliary haeme concentrations were found during the first 48 hours after treatment. Sn-protoporphyrin was cleared rapidly from plasma with a half-life of 3.4 hours. Biliary bilirubin concentrations decreased (mean decrease, 49%) in the haemochromatosis patients after Sn-protoporphyrin administration. No decrease in biliary bilirubin concentrations could be detected in the primary biliary cirrhosis patients under the same conditions. Thus, Sn-protoporphyrin treatment resulted in a decrease in serum bilirubin concentrations and an increase in biliary haeme excretion in patients with haemochromatosis and primary biliary cirrhosis, as has previously been shown in normal subjects. The results indicate that the synthetic haeme analogue inhibits haeme oxidation activity in the two patient groups studied, as it does in normal people and in experimental animals. The lack of effect of Sn-protoporphyrin on biliary bilirubin excretion in primary biliary cirrhosis may be related to a differently affected hepatic clearance system or to a different distribution of tissue bilirubin pools in this condition.

Pharmacokinetics of tin-mesoporphyrin in man and the effects of tin-chelated porphyrins on hyperexcretion of heme pathway precursors in patients with acute inducible porphyria

Tin-mesoporphyrin shares many of the properties of its parent compound, tin-protoporphyrin. These include competitive inhibition of heme oxygenase, amelioration of jaundice and suppression of chemically induced hepatic porphyria. Tin-mesoporphyrin is cleared from the plasma of normal subjects with dose-dependent pharmacokinetics (T1/2 = 3.8 hr following i.v. administration of 1 mumole per kg body weight), and small amounts (less than 1% of administered dose) are excreted into the urine and feces. Intramuscular administration of tin-mesoporphyrin resulted, within 2 hr, in plasma concentrations identical to those obtained following i.v. administration, but the compound was not absorbed orally. The only dose-limiting side effect was transient cutaneous photosensitivity. High doses (1 mumole per kg body weight) of tin-mesoporphyrin resulted in significant decreases in plasma bilirubin concentrations at 24 and 48 h after treatment of normal subjects. Administration of both tin-protoporphyrin and tin-mesoporphyrin resulted in decreases in the urinary excretion of heme pathway intermediates in stable hyperexcreters with acute hepatic porphyria.

Inhibition of human adult and fetal heme oxygenase by new synthetic heme analogues

Heme oxygenase (HO) is the rate-limiting enzyme for heme degradation, and elevated levels of HO may be associated with a variety of pathologic disturbances. A limited number of HO inhibitors such as the metalloporphyrins have been proposed as possible chemotherapeutic agents for the treatment of hyperbilirubinemia. We undertook the study of various natural newly synthesized heme analogues as possible inhibitors of HO in human adult and fetal liver microsomes. We investigated two compounds with substitutions at the 2 and 4 position of the porphyrin ring, iron deuteroporphyrin 2,4 disulfonic (1a) and iron deuteroporphyrin 2,4 bis glycol (1b), and two compounds with substitutions of aromatic groups on the methene bridges of the porphyrin molecule, meso-tetra-4-carboxyphenyl-porphine (2a) and meso-tetra-4-sulfonatophenyl-porphine (2b). When these heme analogues were incubated in the reaction media in the presence of heme, two of the analogues (1a) and (1b) inhibited the conversion of heme to bilirubin. This inhibition was 97% and 65% respectively for (1a) and (1b) when both were present in 30 microM concentrations. Both of these compounds exhibited competitive type inhibition. The kI for the more potent inhibitor, (1b), was determined to be 0.30 microM. Porphyrins with aromatic substitutions at the methene bridges (2a, 2b) did not inhibit the conversion of heme to bilirubin, even at relatively high concentrations. Furthermore, the specific activity of HO was significantly greater (5X) in fetal microsomes as contrasted with adult microsomes as contrasted with adult microsomes. Even though fetal microsomes had greater HO activity, 5 microM of compound (1b) caused a similar degree of inhibition in both adult and fetal preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Sn-protoporphyrin lowers serum bilirubin levels, decreases biliary bilirubin output, enhances biliary heme excretion and potently inhibits hepatic heme oxygenase activity in normal human subjects

Sn-protoporphyrin, a potent competitive inhibitor of heme oxygenase, the rate-limiting enzyme in the degradation of heme to bile pigment, was administered to 10 normal volunteers: 8 males and 2 females. A significant decrease in the levels of serum (mean decrease; 38%) and biliary bilirubin (mean decrease: 47%) was demonstrated in all 10 subjects. The decrease in these parameters lasted for a minimum of 4 days after administration of the metalloporphyrin. Sn-protoporphyrin also facilitated the excretion of endogenous heme in bile during the 24- to 48-hr period following administration of the compound. The metalloporphyrin was rapidly cleared from plasma with a half-life of 3.4 hr. A small amount (3 to 6%) of Sn-protoporphyrin was excreted in both urine and bile. The activity of microsomal heme oxygenase was measured in five human liver samples freshly obtained at the time of cholecystectomy and varied from approximately 2 to 4 nmoles bilirubin formed per mg protein per hour. The addition of Sn-protoporphyrin to microsomal heme oxygenase isolated from these liver samples resulted in a dose-dependent decrease in the activity of this enzyme at concentrations of the metalloporphyrin in the range of 10(-7) to 10(-9) M. The findings reported here provide strong evidence that the suppression of biliary and serum bilirubin levels and the enhancement of heme excretion in the bile of the normal volunteers examined in this study were due to inhibition of physiological rates of heme oxidation activity by the synthetic heme analog, Sn-protoporphyrin.

Reduction of the C2 and C4 vinyl groups of Sn-protoporphyrin to form Sn-mesoporphyrin markedly enhances the ability of the metalloporphyrin to inhibit in vivo heme catabolism

Sn (tin)-mesoporphyrin (Sn-protoporphyrin in which the vinyl groups at C2 and C4 have been reduced to ethyl groups) when incubated with rat splenic microsomal heme oxygenase proved to be a potent competitive inhibitor of enzyme activity in vitro, with a Ki of 0.014 microM. Sn-mesoporphyrin (1 mumol/kg body wt) also inhibited hepatic, renal, and splenic heme oxygenase activity in vivo in adult animals for extended periods of time. Sn-mesoporphyrin (1 mumol/kg body wt) prevented the transient increase in serum bilirubin 24 h after birth in the rat neonate and substantially reduced the levels of serum bilirubin in ALA (delta-aminolevulinic acid) induced hyperbilirubinemia in the 7-day-old suckling neonate. Tissue heme oxygenase activity was decreased in both animal models of hyperbilirubinemia. Sn-mesoporphyrin administration led to a prolonged increase in the heme saturation of hepatic tryptophan pyrrolase indicating an increase in the "heme pool" related to tryptophan pyrrolase and the compound also suppressed chemically induced hepatic porphyria. The administration of Sn-mesoporphyrin to bile duct-cannulated rats was followed by a prompt and sustained decrease in bilirubin output in bile. In addition the excretion of heme in bile was enhanced in these animals. These studies indicate that Sn-mesoporphyrin, like Sn-protoporphyrin, decreases serum bilirubin by inhibiting the production of bilirubin in vivo and its mode of action is through a sustained competitive inhibition of heme oxygenase. However, when a direct comparison of Sn-protoporphyrin and Sn-mesoporphyrin was made, these studies clearly established that the reduction of the C2 and C4 vinyl groups of the porphyrin macrocycle to ethyl groups increases the effectiveness of the Sn-mesoporphyrin derivative 10-fold or more as compared with Sn-protoporphyrin in inhibiting heme catabolism in the animal model systems examined. Thus alterations in the side chain substituents as well as of the central metal atom can influence in a significant manner the potency of the resultant synthetic heme analog as an agent capable of inhibiting heme degradation in vivo.

Dual control mechanism for heme oxygenase: tin(IV)-protoporphyrin potently inhibits enzyme activity while markedly increasing content of enzyme protein in liver

Tin(IV)-protoporphyrin (Sn-protoporphyrin) potently inhibits heme degradation to bile pigments in vitro and in vivo, a property that confers upon this synthetic compound the ability to suppress a variety of experimentally induced and naturally occurring forms of jaundice in animals and humans. Utilizing rat liver heme oxygenase purified to homogeneity together with appropriate immunoquantitation techniques, we have demonstrated that Sn-protoporphyrin possesses the additional property of potently inducing the synthesis of heme oxygenase protein in liver cells while, concurrently, completely inhibiting the activity of the newly formed enzyme. Substitution of tin for the central iron atom of heme thus leads to the formation of a synthetic heme analogue that regulates heme oxygenase by a dual mechanism, which involves competitive inhibition of the enzyme for the natural substrate heme and simultaneous enhancement of new enzyme synthesis. Cobaltic(III)-protoporphyrin (Co-protoporphyrin) also inhibits heme oxygenase activity in vitro, but unlike Sn-protoporphyrin it greatly enhances the activity of the enzyme in the whole animal. Co-protoporphyrin also acts as an in vivo inhibitor of heme oxygenase; however, its inducing effect on heme oxygenase synthesis is so pronounced as to prevail in vivo over its inhibitory effect on the enzyme. These studies show that certain synthetic heme analogues possess the ability to simultaneously inhibit as well as induce the enzyme heme oxygenase in liver. The net balance between these two actions, as reflected in the rate of heme oxidation activity in the whole animal, appears to be influenced by the nature of the central metal atom of the synthetic metalloporphyrin.

Sn-protoporphyrin inhibition of fetal and neonatal brain heme oxygenase. Transplacental passage of the metalloporphyrin and prenatal suppression of hyperbilirubinemia in the newborn animal

Sn(tin)-protoporphyrin, a potent competitive inhibitor of heme oxygenase, can suppress hyperbilirubinemia in animal neonates and significantly reduce plasma bilirubin levels in animals and man. To further explore the biological actions and metabolic disposition of Sn-protoporphyrin, we have examined its effect in the suckling neonate when administered to the mother either 24-48 h before or immediately after birth. Sn-protoporphyrin, when administered before birth, crossed the placental membranes, inhibited fetal heme oxygenase, and suppressed the transient hyperbilirubinemia that occurs in the neonate after birth in a dose-dependent manner. Tissue heme oxygenase activity in the neonate was also lowered in a dose-dependent manner. The blood-brain barrier of the neonate was permeable to Sn-protoporphyrin for a period of between 20-28 d of postnatal life. Sn-protoporphyrin, however, was not retained in brain, but left the brain space with a t1/2 of 1.7 d. In addition, Sn-protoporphyrin administered once at birth to neonates inhibited brain heme oxygenase in a dose-dependent manner. The results of this study demonstrate that Sn-protoporphyrin can cross the placental membranes, inhibit tissue heme oxygenase activity in the fetus, and can also, following such prenatal treatment, suppress the hyperbilirubinemia of the newborn animal.

Sn-protoporphyrin suppresses chemically induced experimental hepatic porphyria. Potential clinical implications

The ability of Sn(tin)-protoporphyrin to inhibit the induction of hepatic delta-aminolevulinate (ALA) synthase by allylisopropyl acetamide (AIA) was examined in the adult rat. Doses of Sn-protoporphyrin of 1, 10, and 50 mumol/kg body wt resulted in decreases in AIA-induced hepatic ALA-synthase activity of 32, 52, and 60%, respectively, compared with rats treated with AIA alone; inhibition of ALA-synthase was not a direct effect of Sn-protoporphyrin. This inhibition of the enzyme activity in liver was reflected in concurrent decreases in urinary excretion of ALA and porphobilinogen (PBG). The increased urinary excretion of ALA and PBG observed following AIA treatment was reduced by the lowest dose of Sn-protoporphyrin (1 mumol/kg body wt) and abolished completely by the higher doses of the metalloporphyrin (10 and 50 mumol/kg body wt). These findings in a rat model of hepatic porphyria suggest that Sn-protoporphyrin may be useful in the treatment of acute exacerbations of "inducible" hepatic porphyrias in man, especially since Sn-protoporphyrin, unlike hematin which is presently used for this purpose, is neither degraded by nor induces the activity of heme oxygenase.

Long-term administration of massive doses of Sn-protoporphyrin in anemic mutant mice (sphha/sphha)

The effects of long-term administration of very large doses of Sn-protoporphyrin on hematological indices, histological changes, plasma bilirubin levels, tissue heme oxygenase activity, and activities of heme biosynthetic enzymes, were examined in genetically anemic mutant mice with hemolytic anemia (sphha/sphha). Long-term weekly treatment with Sn-protoporphyrin (100 mumol/kg body weight for 32 wk) did not alter hematological indices, histological findings, or enzyme activities related to heme biosynthesis, even though it resulted in sustained decreases in microsomal heme oxygenase activity in the liver, kidney, and spleen, and a prolonged decrease in plasma bilirubin concentration. Inhibition of heme oxygenase did not alter the level of cytochrome P-450 in the liver and the kidney. The results indicate that long-term treatment with massive doses of Sn-protoporphyrin suppresses bilirubin formation but does not produce significant histopathological changes or appreciably interfere with heme synthesis, in this strain of genetically anemic mice. These findings provide further support for the idea that suppression of heme degradation to bile pigment by the inhibition of heme oxygenase may prove useful to the prevention of severe hyperbilirubinemia in humans.

Studies on the mechanism of Sn-protoporphyrin suppression of hyperbilirubinemia. Inhibition of heme oxidation and bilirubin production

The synthetic heme analogue Sn-protoporphyrin is a potent competitive inhibitor of heme oxygenase, the rate-limiting enzyme in heme degradation to bile pigment, and can entirely suppress hyperbilirubinemia in neonatal animals and significantly reduce plasma bilirubin levels in a variety of circumstances in experimental animals and man. To further explore the mechanism by which this metalloporphyrin reduces bilirubin levels in vivo, we have examined its effects on bilirubin production in bile duct-cannulated rats, in which bilirubin derived from heme catabolism is known to be rapidly excreted in bile. The administration of Sn-protoporphyrin (10-50 mumol/kg body weight) was followed by prompt (within approximately 1 h) and sustained (up to at least 18 h) decreases in bilirubin output, to levels 25-30 percent below the levels of bilirubin output in control bile fistula animals. The metalloporphyrin had no effect on bile flow or the biliary output of bile acids. Infusions of heme, which is taken up primarily in hepatocytes, or of heat-damaged erythrocytes, which are taken up in reticuloendothelial cells, resulted in marked increases in bilirubin output in bile in control animals; these increases were completely prevented or substantially diminished by Sn-protoporphyrin administration. By contrast, the metalloporphyrin did not alter the high levels of bilirubin in plasma and bile that were achieved in separate experiments by the constant (16 h) infusion of unconjugated bilirubin to bile duct-cannulated rats. Thus, Sn-protoporphyrin exerts no major effects on the metabolic disposition of preformed bilirubin. Heme oxygenase activities were markedly decreased in microsomal preparations from liver, spleen, and kidneys in these experiments, to a degree comparable to the decreases we have observed in the intact rat. We also demonstrated that a substantial proportion (19-35%) of a dose of Sn-protoporphyrin is promptly excreted in bile and that the time course of biliary excretion of this compound more closely reflects plasma concentrations of the metalloporphyrin, which decline rapidly, rather than concentrations in liver, which are considerably more persistent. These results indicate that Sn-protoporphyrin substantially reduces the in vivo production of bilirubin from the degradation of endogenous as well as exogenous heme in the rat. Moreover, this inhibitory effect of the synthetic metalloporphyrin on bilirubin production occurs in both hepatocytes and reticuloendothelial cells, which are the major tissue sites for bilirubin formation. In other studies, we have established that heme oxygenase blockade by Sn-protoporphyrin leads to a marked and rapid excretion of heme into bile presumably because the synthetic metabolism to bile pigment and making it available for excretion via the biliary system in to the gut, These studies strongly suggest that Sn-protoporphyrin diminishes hyperbilirubinemia in animals and man by inhibiting the production of the bile pigment in vivo, and that its principal mode of action involves a potent and sustained competitive inhibition of heme oxygenase.

The liver excretes large amounts of heme into bile when heme oxygenase is inhibited competitively by Sn-protoporphyrin

TinIV-protoporphyrin IX (Sn-protoporphyrin) potently inhibits heme degradation to bilirubin in vitro and in vivo, and it completely suppresses neonatal hyperbilirubinemia in experimental animals, including primates. It also reduces plasma bilirubin levels in certain naturally occurring or induced forms of jaundice in animals and man. We have examined in this study the fate of that fraction of heme whose degradation to bile pigment is inhibited in vivo by administration of this heme oxygenase (EC 1.14.99.3) inhibitor. In bile-duct-cannulated rats, infused exogenous heme is rapidly converted to biliary bilirubin; a small amount of the infused heme is excreted into bile as well. Sn-protoporphyrin, administered with the exogenous heme, markedly increased (3- to 4-fold) the amount of heme excreted into bile and greatly diminished biliary output of bilirubin. The increase in biliary heme output exceeded the decrease in bilirubin excretion elicited by the inhibitor metalloporphyrin. In the same experimental model, Sn-protoporphyrin substantially decreased the conversion of heme, derived from heat-damaged erythrocytes, to biliary bilirubin. This decrease in biliary bilirubin output was accounted for entirely by a prompt and marked increase in biliary excretion of unmetabolized heme. The enhanced biliary excretion of unmetabolized heme following administration of Sn-protoporphyrin is a newly defined and biologically important response associated with use of this synthetic heme analogue. The features of the action of this compound in vivo--suppression of formation of the potentially neurotoxic metabolite, bilirubin; enhancement of disposal of the untransformed substrate (heme) of the enzyme that it inhibits; and its own elimination without metabolic alteration--define some of the characteristics of a therapeutically useful chemical.

Sn-protoporphyrin rapidly and markedly enhances the heme saturation of hepatic tryptophan pyrrolase. Evidence that this synthetic metalloporphyrin increases the functional content of heme in the liver

Sn-protoporphyrin is a potent competitive inhibitor of heme oxygenase, the rate-limiting enzyme in heme degradation to bile pigment, and has been successfully utilized to suppress hyperbilirubinemia in a variety of experimental and naturally occurring forms of jaundice in animals and man. The compound is presumed to act in vivo primarily by inhibiting heme oxidation; thus it would be reasonable to expect that preservation of some functional moiety of cellular heme from degradation by heme oxygenase would occur after Sn-protoporphyrin administration. We have examined this question in liver by studying the heme saturation of tryptophan pyrrolase, the heme-dependent enzyme which controls the first and rate-limiting step in the catabolism of L-tryptophan. Sn-protoporphyrin, in doses (10 mumol/kg body wt) which entirely suppress neonatal hyperbilirubinemia in the experimental animal, leads to a very rapid (approximately 30-60 min) increase in the heme saturation of tryptophan pyrrolase from normal levels of approximately 50-60% to nearly 100%. The effect peaks at 1-2 h and lasts for at least 12 h. Sn-protoporphyrin is also able to block the rapid and marked decline in heme saturation of tryptophan pyrrolase elicited by inorganic cobalt, a potent inducer of heme oxygenase in liver. These findings establish clearly that after the administration of Sn-protoporphyrin in the whole animal, a functionally active heme pool, the one related to tryptophan pyrrolase, is rapidly increased in liver, confirming that the metalloporphyrin inhibits the degradation of endogenous heme by heme oxygenase.

Control of heme and cytochrome P-450 metabolism by inorganic metals, organometals and synthetic metalloporphyrins

The heme-cytochrome P-450 complexes represent sensitive metabolic systems for examining the biological impact of metals on important cellular functions. Many metals, both in the inorganic form and bound to organic moieties, potently induce heme oxygenase, the rate limiting enzyme of heme degradation. The resulting increase in the rate of heme breakdown is reflected in a marked depression of cellular cytochrome P-450 content and impairment of the oxidative metabolism of natural and foreign chemicals dependent on this hemeprotein. Organometal complexes do not mimic in all their aspects the actions of the inorganic elements which they contain. For example, organotins, in contrast to inorganic tin, produce a prolonged induction response of heme oxygenase in the liver but not in the kidney. Co-protoporphyrin is a much more potent inducer of heme oxygenase in liver than is inorganic cobalt; and Sn-protoporphyrin inhibits heme oxygenase activity nearly completely, whereas inorganic tin is a powerful inducer of the renal enzyme. Contrasting effects on heme metabolism exist as well within the metalloporphyrin species as demonstrated by the effects in vivo of Co-protoporphyrin and Sn-protoporphyrin on heme oxygenase activity; the former induces the enzyme whereas the latter potently inhibits it. In vitro, however, both compounds competitively inhibit heme oxidation activity. These differences, among others which characterize metal actions in vivo and in vitro attest to the importance of pharmacokinetic, adaptive and other host factors in defining the responses of the heme-cytochrome P-450 systems to the impact of metals in the whole animal.