Pyridoxalated hemoglobin polyoxyethylene: a nitric oxide scavenger with antioxidant activity for the treatment of nitric oxide-induced shock

Hemoglobins modified for therapeutic use as either hemoglobin-based oxygen carriers or scavengers of nitric oxide are currently being evaluated in clinical trials. One such product, pyridoxalated hemoglobin polyoxyethylene conjugate (PHP), is a human-derived and chemically modified hemoglobin that has yielded promising results in Phase II clinical trials, and is entering a pivotal Phase III clinical trial for the treatment of shock associated with systemic inflammatory response syndrome (SIRS). Shock associated with SIRS is a NO-induced shock. PHP, a new mechanism-based therapy, has been demonstrated in clinical trials to have the expected hemodynamic activity of raising blood pressure and reducing catecholamine use, consistent with its mechanism of action as a NO scavenger. PHP is conjugated with polyoxyethylene, which results in a surface-decorated molecule with enhanced circulation time and stability as well as in attachment of soluble red blood cell enzymes, including catalase and superoxide dismutase. PHP thus contains an antioxidant profile similar to the intact red blood cell and is therefore resistant to both initial oxidative modification by oxidants such as hydrogen peroxide and subsequent ferrylhemoglobin formation. These studies suggest both that the redox activity of modified hemoglobins can be attenuated and that modified hemoglobins containing endogenous antioxidants, such as PHP, may have reduced pro-oxidant potential. These antioxidant properties, in addition to the NO-scavenging properties, may allow the use of PHP in other indications in which excess NO, superoxide, or hydrogen peroxide is involved, including ischemia-reperfusion injury and hemorrhagic shock.

Nitric oxide and peroxynitrite production in ocular inflammation

Recent studies have implicated nitric oxide and peroxynitrite in the pathogenesis of many diseases, such as septic shock, arthritis, lung disease, and atherosclerosis. Nitric oxide (.NO) exerts many diverse effects on vascular tone, affecting neurotransmission and cellular cytotoxicity/communication. Our laboratory and others have documented a proinflammatory role for .NO in ocular inflammation. Uveitis, which is an inflammation of the highly vascular uveal tract in the eye, is a debilitating condition that can lead to visual impairment and blindness. It is characterized by acute, recurrent, or persistent inflammation with disruption of the blood-aqueous barrier and is accompanied by protein leakage and leukocyte infiltration into the aqueous humor and anterior chamber. Systemic injection of endotoxin into mice and rats, or intraocular injection of endotoxin into mice, rats, and rabbits induces acute uveitis, which clinically and histologically resembles acute anterior uveitis in humans. These models facilitate the study of pathogenic mechanisms that contribute to ocular inflammation. In addition to .NO, superoxide anion radicals (O2.-), and peroxynitrite (ONOO-), the products of the reaction between .NO and O2.-, are also implicated in uveitis. The role of peroxynitrite in ocular inflammation is still largely unknown. Characterization of the roles of these important uveitic mediators in the ocular inflammatory response will provide information critical to the understanding of the pathogenesis of intraocular inflammation so that more effective therapeutic intervention(s) can be developed.

Autoxidation of pyridoxalated hemoglobin polyoxyethylene conjugate

Hemoglobin-based therapeutics are currently in clinical trials in the United States and abroad as blood replacement solutions, nitric oxide scavengers, and radiation sensitizers. The potency of the therapeutics may be influenced by the oxidation state of the iron in the heme moiety. The oxidation state is dependent upon the physical environment of the molecule and is influenced by parameters such as the chemical nature of the hemoglobin therapeutic and its formulation. Pyridoxalated hemoglobin polyoxyethylene conjugate (PHP) is one such compound currently in clinical trials in the U.S. for treatment of nitric oxide-dependent, volume refractory shock. The autoxidation rates for PHP have been determined over a range of temperatures. The oxidation events were shown to be biphasic and were similar to those observed for purified human hemoglobin (HbAo). The initial fast oxidation events were modeled with first order rate constants at 37 degrees C and determined to be 0.022 hr-1 and 0.025 hr-1 for PHP and HbAo, respectively. The autoxidation of PHP was shown to be independent of concentration from approximately 5 to 100 mg/mL.

Production, purification, and characterization of recombinant human hemoglobin rainier expressed in Saccharomyces cerevisiae

Hemoglobin Rainier is a naturally occurring hemoglobin variant in which the beta 145 tyrosine is substituted with cysteine. The alpha and beta Rainier globin cDNAs were cloned in a high copy number vector and expressed in Saccharomyces cerevisiae under the control of galactose-regulated hybrid promoters. Using this system, we have expressed individual alpha and beta Rainier globin chains. Coexpression of both alpha and beta Rainier cDNAs resulted in the production of a functional hemoglobin molecule. Purification of the recombinant protein was accomplished by ion exchange chromatography. The N-termini of the alpha and beta chains were correctly processed, and the molecular mass, as determined by mass spectrometry, indicated amino acid composition identical to that of natural hemoglobin Rainier. The chromatographic properties of the recombinant hemoglobin Rainier were similar to human-derived hemoglobin A0. The purified recombinant hemoglobin molecule was shown to have an elevated oxygen affinity and a reduced cooperativity as previously reported for natural hemoglobin Rainier. Production of recombinant hemoglobin and especially hemoglobin variants like hemoglobin Rainier has the potential to facilitate use of hemoglobin as a blood substitute as well as in specific applications, such as for use as a therapeutic agent in the treatment of hypotension associated with septic shock.

Differential proliferative responses of Syrian hamster embryo fibroblasts to paraquat-generated superoxide radicals depending on tumor suppressor gene function

Oxygen radicals have been widely implicated in neoplastic transformation; however, little is known regarding their mode of action. In an attempt to delineate potential mechanisms of action, an analysis of superoxide effects on cell growth was studied in normal and two nontumorigenic, immortal cell lines derived from normal Syrian hamster embryo (SHE) fibroblasts. The two immortal cell lines differed in their ability to suppress tumorigenicity of tumor cells in cell hybrids. One cell line suppressed tumorigenicity (sup+), while a second clone was unable to suppress tumorigenicity (sup-). Paraquat was used to generate superoxide through its capacity to be reduced by NAD(P)H and to generate superoxide radicals. The growth response of the various cell types was measured by colony-forming ability as well as by tritiated thymidine incorporation using autoradiography. At low paraquat concentrations (25 microM), primary SHE cells and two sup+ clones showed up to a 40% enhancement in colony formation, while two sup- clones showed no increase. Toxicity was observed at high doses, starting at approximately 100 microM paraquat. Since oxygen radicals are also mutagenic, primary SHE cells were examined for chromosomal aberrations. Chromatid gaps and breaks were induced at all concentrations of paraquat used. Thus, superoxide not only causes cellular toxicity at high doses but at low doses enhances cell growth of certain cells (primary SHE cells and sup+ cells) but not others (sup- cells). Therefore, differing responses of cells at different stages of neoplastic progression must be considered in understanding oxygen radical effects in growth control and carcinogenesis.