Bezafibrate derivatives as potent effectors of hemoglobin

Slow dynamics measured by phosphorescence lifetime reveals global conformational changes in human adult hemoglobin induced by allosteric effectors

The mechanism underlying allostery in hemoglobin (Hb) is still not completely understood. Various models describing the action of allosteric effectors on Hb function have been published in the literature. It has also been reported that some allosteric effectors-such as chloride ions, inositol hexaphosphate, 2,3-diphospho-glycerate and bezafibrate-considerably lower the oxygen affinity of Hb. In this context, an important question is the extent to which these changes influence the conformational dynamics of the protein. Earlier, we elaborated a challenging method based on phosphorescence quenching, which makes characterizing protein-internal dynamics possible in the ms time range. The experimental technique involves phosphorescence lifetime measurements in thermal equilibrium at varied temperatures from 10 K up to 273 K, based on the signal of Zn-protoporphyrin substituted for the heme in the β-subunits of Hb. The thermal activation of protein dynamics was observed by the enhancement of phosphorescence quenching attributed to O2 diffusion. It was shown that the thermal activation of protein matrix dynamics was clearly distinguishable from the dynamic activation of the aqueous solvent, and was therefore highly specific for the protein. In the present work, the same method was used to study the changes in the parameters of the dynamic activation of human HbA induced by binding allosteric effectors. We interpreted the phenomenon as phase transition between two states. The fitting of this model to lifetime data yielded the change of energy and entropy in the activation process and the quenching rate in the dynamically activated state. The fitted parameters were particularly sensitive to the presence of allosteric effectors and could be interpreted in line with results from earlier experimental studies. The results suggest that allosteric effectors are tightly coupled to the dynamics of the whole protein, and thus underline the importance of global dynamics in the regulation of Hb function.

Dissimilar flexibility of α and β subunits of human adult hemoglobin influences the protein dynamics and its alteration induced by allosteric effectors

The general question by what mechanism an "effector" molecule and the hemes of hemoglobin interact over widely separated intramolecular distances to change the oxygen affinity has been extensively investigated, and still has remained of central interest. In the present work we were interested in clarifying the general role of the protein matrix and its dynamics in the regulation of human adult hemoglobin (HbA). We used a spectroscopy approach that yields the compressibility (κ) of the protein matrix around the hemes of the subunits in HbA and studied how the binding of heterotropic allosteric effectors modify this parameter. κ is directly related to the variance of volume fluctuation, therefore it characterizes the molecular dynamics of the protein structure. For the experiments the heme groups either in the α or in the β subunits of HbA were replaced by fluorescent Zn-protoporphyrinIX, and series of fluorescence line narrowed spectra were measured at varied pressures. The evaluation of the spectra yielded the compressibility that showed significant dynamic asymmetry between the subunits: κ of the α subunit was 0.17±0.05/GPa, while for the β subunit it was much higher, 0.36±0.07/GPa. The heterotropic effectors, chloride ions, inositol hexaphosphate and bezafibrate did not cause significant changes in κ of the α subunits, while in the β subunits the effectors lead to a significant reduction down to 0.15±0.04/GPa. We relate our results to structural data, to results of recent functional studies and to those of molecular dynamics simulations, and find good agreements. The observed asymmetry in the flexibility suggests a distinct role of the subunits in the regulation of Hb that results in the observed changes of the oxygen binding capability.

Aryloxyalkanoic Acids as Non-Covalent Modifiers of the Allosteric Properties of Hemoglobin

Hemoglobin (Hb) modifiers that stereospecifically inhibit sickle hemoglobin polymer formation and/or allosterically increase Hb affinity for oxygen have been shown to prevent the primary pathophysiology of sickle cell disease (SCD), specifically, Hb polymerization and red blood cell sickling. Several such compounds are currently being clinically studied for the treatment of SCD. Based on the previously reported non-covalent Hb binding characteristics of substituted aryloxyalkanoic acids that exhibited antisickling properties, we designed, synthesized and evaluated 18 new compounds (KAUS II series) for enhanced antisickling activities. Surprisingly, select test compounds showed no antisickling effects or promoted erythrocyte sickling. Additionally, the compounds showed no significant effect on Hb oxygen affinity (or in some cases, even decreased the affinity for oxygen). The X-ray structure of deoxygenated Hb in complex with a prototype compound, KAUS-23, revealed that the effector bound in the central water cavity of the protein, providing atomic level explanations for the observed functional and biological activities. Although the structural modification did not lead to the anticipated biological effects, the findings provide important direction for designing candidate antisickling agents, as well as a framework for novel Hb allosteric effectors that conversely, decrease the protein affinity for oxygen for potential therapeutic use for hypoxic- and/or ischemic-related diseases.

Design, Synthesis, and Investigation of Novel Nitric Oxide (NO)-Releasing Prodrugs as Drug Candidates for the Treatment of Ischemic Disorders: Insights into NO-Releasing Prodrug Biotransformation and Hemoglobin-NO Biochemistry

We have developed novel nitric oxide (NO)-releasing prodrugs of efaproxiral (RSR13) for their potential therapeutic applications in a variety of diseases with underlying ischemia. RSR13 is an allosteric effector of hemoglobin (Hb) that decreases the protein's affinity for oxygen, thereby increasing tissue oxygenation. NO, because of its vasodilatory property, in the form of ester prodrugs has been found to be useful in managing several cardiovascular diseases by increasing blood flow and oxygenation in ischemic tissues. We synthesized three NO-donor ester derivatives of RSR13 (DD-1, DD-2, and DD-3) by attaching the NO-releasing moieties nitrooxyethyl, nitrooxypropyl, and 1-(pyrrolidin-1-yl)diazen-1-ium-1,2-diolate, respectively, to the carboxylate of RSR13. In vitro studies demonstrated that the compounds released NO in a time-dependent manner upon being incubated with l-cysteine (1.8-9.3%) or human serum (2.3-52.5%) and also reduced the affinity of Hb for oxygen in whole blood (ΔP50 of 4.9-21.7 mmHg vs ΔP50 of 25.4-32.1 mmHg for RSR13). Crystallographic studies showed RSR13, the hydrolysis product of the reaction between DD-1 and deoxygenated Hb, bound to the central water cavity of Hb. Also, the hydrolysis product, NO, was observed exclusively bound to the two α hemes, the first such HbNO structure to be reported, capturing the previously proposed physiological bis-ligated nitrosylHb species. Finally, nitrate was observed bound to βHis97. Ultraperformance liquid chromatography-mass spectrometry analysis of the compounds incubated with matrices used for the various studies demonstrated the presence of the predicted reaction products. Our findings, beyond the potential therapeutic application, provide valuable insights into the biotransformation of NO-releasing prodrugs and their mechanism of action and into hemoglobin-NO biochemistry at the molecular level.

Less known pathophysiological mechanisms of anemia in patients with diabetic nephropathy

Diabetes mellitus (DM) is currently considered a modern global epidemic, and diabetic nephropathy (DN) is the most common cause of chronic kidney disease (CKD). Anemia is one of the most significant complications of CKD, and it is mainly attributed to insufficient erythropoietin (EPO) production. However, anemia develops earlier in the course of CKD among patients with DM, and the severity of anemia tends to be more marked in these patients compared to nondiabetic subjects, regardless of the stage of CKD. In this review, we focus on the "less known" complex interacting mechanisms which are involved in the pathophysiology of anemia associated with DN. Although the major cause of anemia in DN is considered to be an inappropriate response of the plasma EPO concentration to anemia, several other possible mechanisms have been suggested. Glomerular hyperfiltration, proteinuria, renal tubular dysfunction and interstitial fibrosis are among the main culprits. On the other hand, systemic effects such as chronic inflammation, autonomic neuropathy and the renin-angiotensin system are also involved. Finally, several medications are considered to aggravate anemia associated with DN. Since anemia is an important predictor of quality of life and is implicated in the increased burden of cardiovascular morbidity and mortality, further research is required to elucidate its pathogenesis in diabetic patients.

Electrochemical investigation of the effect of some organic phosphates on haemoglobin

The effects of DPG,IHP,GTP,GDP and GMP on the structure and stability of haemoglobin were electrochemically investigated with an iodide-modified silver electrode in 0.01 M KNO 3 at pH 7.0.Anodic and cathodic peaks of haemoglobin were observed at 250 mV and 12 mV with a formal potential value of 133 mV vs.Ag/AgCl.The effects of different concentrations of DPG,IHP,GTP,GDP and GMP on the anaerobic redox reaction were determined. The results showed that DPG and IHP can lead to a positive shift in the reduction peak of haemoglobin,indicating that the oxidation peak shift of haemoglobin was small as a result of stabilization of the reduced state and destabilization of the R-like state of haemoglobin.GTP elicited a more positive shift in the cathodic and anodic peaks of haemoglobin at a higher concentration,signifying that it has a low-affinity binding site on haemoglobin.The positive shift of the cathodic and anodic peaks revealed a slight variation in the structure and indicated the unfolding of haemoglobin in the presence of high concentrations of GTP.Our study also showed that GDP and GMP did not cause significant shift the cathodic and anodic peaks of haemoglobin even at high concentrations,refuting the existence of specific GDP-and GMP-binding sites on the protein.Moreover,the iodide-modified silver electrode method proved to be easy and useful in investigating the effects of ligands or other effectors on haemoglobin in solution.

Allosteric Effectors Influence the Tetramer Stability of Both R- and T-states of Hemoglobin A

The contribution of heterotropic effectors to hemoglobin allostery is still not completely understood. With the recently proposed global allostery model, this question acquires crucial significance, because it relates tertiary conformational changes to effector binding in both the R- and T-states. In this context, an important question is how far the induced conformational changes propagate from the binding site(s) of the allosteric effectors. We present a study in which we monitored the interdimeric interface when the effectors such as Cl-, 2,3-diphosphoglycerate, inositol hexaphosphate, and bezafibrate were bound. We studied oxy-Hb and a hybrid form (alphaFeO2)2-(betaZn)2 as the T-state analogue by monitoring heme absorption and Trp intrinsic fluorescence under hydrostatic pressure. We observed a pressure-dependent change in the intrinsic fluorescence, which we attribute to a pressure-induced tetramer to dimer transition with characteristic pressures in the 70-200-megapascal range. The transition is sensitive to the binding of allosteric effectors. We fitted the data with a simple model for the tetramer-dimer transition and determined the dissociation constants at atmospheric pressure. In the R-state, we observed a stabilizing effect by the allosteric effectors, although in the T-analogue a stronger destabilizing effect was seen. The order of efficiency was the same in both states, but with the opposite trend as inositol hexaphosphate > 2,3-diphosphoglycerate > Cl-. We detected intrinsic fluorescence from bound bezafibrate that introduced uncertainty in the comparison with other effectors. The results support the global allostery model by showing that conformational changes propagate from the effector binding site to the interdimeric interfaces in both quaternary states.

Austrian Moderate Altitude Study (AMAS 2000): Erythropoietic Activity and Hb–O2 Affinity During a 3-Week Hiking Holiday at Moderate Altitude in Persons with Metabolic Syndrome

Moderate altitude hypoxia (1500 to 2500 m) is known to stimulate erythropoiesis and to improve oxygen transport to tissue by a reduction of Hb-O(2) affinity. Whether this adaptation also occurs in tourists with metabolic syndrome has not yet been investigated sufficiently. Thus, we performed a prospective field study to measure erythropoietic parameters and oxygen transport properties in 24 male volunteers with metabolic syndrome during a 3- week holiday program at 1700 m consisting of four guided, individually adapted hiking tours per week. The following examinations were performed: baseline investigations at 500 m (T1); examinations at moderate altitude on day 1 (T2), day 4 (T3), day 9 (T4), and day 19 (T5); and postaltitude tests (T6) 7 to 10 days after return. On day 1 and day 19, a walk on a standardized hiking test route with oxygen saturation (SpO(2)) measure points was performed. Hemoglobin, packed cell volume, and red cell count showed changes over time, with higher values at T5 as compared to baseline. Reticulocyte count and erythropoietin (EPO) were increased at T2 and increased further until T5. EPO declined toward prealtitude values. P50-value (blood PO(2) at 50% hemoglobin oxygen saturation at actual pH) increased during the altitude sojourn (maximum increase at T5 by +0.40 kPa). At T5 all volunteers had a higher SpO(2) before, during, and at the end of the test route compared to T1. During adaptation to moderate altitude, persons with metabolic syndrome exhibit an increase in EPO and a rightward shift of the oxygen dissociation curve that is similar to healthy subjects.

Liganded hemoglobin structural perturbations by the allosteric effector L35

Effector binding to liganded hemoglobin (Hb) provides a new understanding of structural determinants of Hb function. L35, a bezafibrate-related compound, is one of the more potent synthetic regulators of Hb oxygen (O(2)) affinity. In the presence of inositol hexaphosphate and bezafibrate (or derivatives), liganded Hb at low pH (pH approximately 6.5) exhibits extremely low O(2) affinity and very low cooperativity. In this study, the nature of L35 binding to COHbA at pH 6.35, an altered R-state, is presented. Solution-active site-specific spectroscopic probings by front-face fluorescence and circular dichroism reveal that L35 induces a global heterogeneous conformation in COHbA at pH 6.35 that includes: a T-like structural feature at the alpha1beta2 interface; an R-like structural feature within the heme environment; and an intermediate-like state at the central cavity. These long-range structural perturbations appear to stem from L35 binding to two classes of binding sites: the central cavity (primarily at the alphaalpha cleft) and the surface. These results indicate that L35 induces an allosteric transition species, characterized by domain-specific tertiary and quaternary-like conformation within a global R-quaternary structure.

R-state hemoglobin bound to heterotropic effectors: models of the DPG, IHP and RSR13 binding sites

We performed a docking study followed by a 500-ps molecular dynamics simulation of R-state human adult hemoglobin (HbA) complexed to different heterotropic effectors [2,3-diphosphoglycerate (DPG), inositol hexaphosphate (IHP), and 2-[4-[(3,5-dichlorophenylcarbamoyl)-]methyl]-phenoxy]-2-methylpropionic acid (RSR13)) to propose a molecular basis for recently reported interactions of effectors with oxygenated hemoglobin. The simulations were carried out with counterions and explicit solvation. As reported for T-state HbA, the effector binding sites are also located in the central cavity of the R-state and differ depending on effector anionic character. DPG and IHP bind between the alpha-subunits and the RSR13 site spans the alpha1-, alpha2- and beta2-subunits. The generated models provide the first report of the molecular details of R-state HbA bound to heterotropic effectors.

Clinical potential of nonhemoglobin oxygen therapeutics in cardiac and general surgery

Significant efforts have been made over the past 70 years to find a solution that could substitute for blood. Over the years, the focus has shifted to developing a solution capable of delivering oxygen to the tissues. Fluorocarbons (FC) are highly inert solutions with a high solubility for all gases, making them a prime candidate to become such an oxygen delivery agent. Although clinical research efforts into the use of these agents as substitutes for blood transfusions continue at present, the rapid disappearance of emulsified FCs from the vascular space and accumulation in the liver and spleen may well limit their usefulness as transfusion substitutes. Because of their ability to dissolve significant quantities of oxygen and carbon dioxide, these agents may be more attractive as oxygen delivery agents during periods of local or global organ ischemia, including preservation of organs for transplantation. FCs have also been tested in animal models of cardiopulmonary bypass, and may be efficacious in adsorbing the gases present in air emboli. Recently a second class of oxygen therapeutics (allosteric modifiers) has been developed, and these agents enhance oxygen delivery by shifting the oxygen dissociation curve to the right, thus increasing tissue PO(2). Allosteric modifiers have been shown to effectively shift the p50 of hemoglobin 10mm Hg at clinically relevant dosages, and have been shown (in animal models) to reduce cerebral infarct size following carotid ligation and to improve myocardial performance following myocardial ischemia. Despite significant research efforts, however, none of the solutions under development are currently approved for clinical use by the Food and Drug Administration, with the exception of myocardial contrast imaging agents.

Clinical potential of blood substitutes or oxygen therapeutics during cardiac surgery

Several complications and unforeseen adverse side effects have colluded to keep commercially available blood substitutes or oxygen therapeutic agents tantalizingly "just out of reach." Because the three classes of agents under development have different oxygen-delivery mechanisms and side-effect profiles, each can be expected to have its own unique clinical applications, particularly in the cardiac surgery population. The fact that South Africa recently approved one HBOC for use as a transfusion alternative in patients with chronic anemia indicates that initial clinical use is near for a number of these agents. It is to be hoped that they will be used for several applications rather than just for a "transfusion alternative." Despite some frustrating limitations, all of these agents are antigen and pathogen free, have an acceptable side-effect profile, and have a long shelf life. Increasing volunteer-blood-donor shortages, coupled with increasing blood-transfusion needs, and expanding human immunodeficiency virus and hepatitis B and C epidemics, continue to fuel the demand for further development of these products. Transfusion alternatives will eventually become commercially available--the question is "when," not "if." Equally important, these agents' potential for serving as effective oxygen-delivery agents to ischemic tissues heralds an entirely new field of clinical investigation.

Allosteric modification of oxygen delivery by hemoglobin

Hemoglobin affinity for oxygen is altered by pH, temperature, and high altitude, making oxygen more readily available to the tissues. RSR13 (Allos Therapeutics, Denver, CO), an analog of the drugs clofibrate and bezofibrate, causes a dose-dependent, rightward shift of the oxygen dissociation curve in animals and humans. We tested the safety, pharmacodynamic, and pharmacokinetics of RSR13, an allosteric modifier of hemoglobin, in patients having general surgery in a prospective, randomized, double-blinded, placebo-controlled, dose-escalation clinical trial. After the induction of general anesthesia with endotracheal intubation, 26 patients who consented were randomly assigned to receive an infusion of RSR13 or placebo (2:1) in an ascending dose scheme. Doses studied were 10, 20, 30, 40, 50, 60, 75, and 100 mg/kg infused for 30--60 minutes. Samples were taken for determination of RSR13 concentration in plasma, red blood cells, and urine, as well as for determination of the p50 in blood by using three-point tonometry at frequent intervals after the infusion of the study drug. The RSR13 administration resulted in a dose-dependent rightward shift of the oxygen dissociation curve, with the target p50 shift of 10 mm Hg achieved at the 75- and 100-mg/kg doses. No differences were seen between RSR13 and placebo groups in laboratory or hemodynamic findings, with the exception of a transient, limited increase in serum creatinine in 3 patients who received RSR13. These increases peaked at 48 h (2.2, 3.5, and 4.5 mg/dL respectively), were not associated with oliguria, did not require treatment, and did not prolong hospitalization in any patient. The reasons for the unexplained increases in serum creatinine were not evident, but potentially included surgery itself (nephrectomy), patient condition, or the concomitant administration of renally cleared medications or drugs that affect renal blood flow.

IMPLICATIONS

We studied the safety and tolerance of an investigational drug, RSR13 (Allos Therapeutics, Denver, CO), in general surgery patients. This drug, which increases the amount of oxygen available to the body, was well tolerated by the 17 patients who received it. There were clinically relevant increases in serum creatinine in 3 patients, indicating a decrease in renal function, but these increases were short-lived and resolved without treatment.

Modulating the oxygen affinity of human fetal haemoglobin with synthetic allosteric modulators

Improving the delivery of oxygen to the tissues by decreasing the oxygen affinity of haemoglobin has been a major aim of several laboratories over recent years because this may reduce the consequences of anaemia and/or improve tissue oxygenation in cases of decreased blood perfusion. Within the same context, lowering the oxygen affinity may prove valuable in the application of native or recombinant haemoglobin solutions as a blood substitute. The shift of the oxygen equilibrium curve to the right is obtained by various modulators. Among them, the bezafibrate derivatives are considered as a most interesting group. These principles are of the utmost importance in thalassaemia and other haemoglobinopathies where the beneficial effects of the compensatory synthesis of fetal haemoglobin are diminished by the increased oxygen affinity of this pigment. In this paper we present the results of a study initiated to determine whether a potent oxygen affinity modifier, RSR-4, could satisfactorily decrease the oxygen affinity of fetal haemoglobin, thus improving tissue oxygenation. The experiments were carried out on whole blood and on purified haemoglobin solutions and showed that the effector markedly decreased the oxygen affinity of HbF (from 18.7 to 3.73 mmHg in whole blood). At the same time the cooperativity index (n50) and the oxygen saturation levels remained within normal limits under the conditions of the main experiment. These observations have important implications for the potential application of oxygen affinity modifiers in vivo.

Probing the hemoglobin central cavity by direct quantification of effector binding using fluorescence lifetime methods

Time-resolved fluorescence methods have been used to show that 8-hydroxy-1,3,6-pyrenetrisulfonate (HPT), a fluorescent analog of 2,3-diphosphoglycerate, binds to the central cavity of carboxyhemoglobin A (HbACO) at pH 6.35. A direct quantitative approach, based on the distinctive free and bound HPT fluorescent lifetimes of 5.6 ns and approximately 27 ps, respectively, was developed to measure the binding affinity of this probe. HPT binds to a single site and is displaced by inositol hexaphosphate at a 1:1 mol ratio, indicating that binding occurs at the 2,3-diphosphoglycerate site in the central cavity. Furthermore, the results imply that low pH HbACO exists as an altered R state and not an equilibrium mixture of R and T states. The probe was also used to monitor competitive effector binding and to compare the affinity of the binding site in several cross-bridged HbA derivatives.