Identification of a novel class of covalent modifiers of hemoglobin as potential antisickling agents

Targeted modification of furan-2-carboxaldehydes into Michael acceptor analogs yielded long-acting hemoglobin modulators with dual antisickling activities

Sickle cell disease (SCD) is the most common genetic disorder, affecting millions of people worldwide. Aromatic aldehydes, which increase the oxygen affinity of human hemoglobin to prevent polymerization of sickle hemoglobin and inhibit red blood cell (RBC) sickling, have been the subject of keen interest for the development of effective treatment against SCD. However, the aldehyde functional group metabolic instability has severly hampered their development, except for voxelotor, which was approved in 2019 for SCD treatment. To improve the metabolic stability of aromatic aldehydes, we designed and synthesized novel molecules by incorporating Michael acceptor reactive centers into the previously clinically studied aromatic aldehyde, 5-hydroxymethylfurfural (5-HMF). Eight such derivatives, referred to as MMA compounds were synthesized and studied for their functional and biological activities. Unlike 5-HMF, which forms Schiff-base interaction with αVal1 nitrogen of hemoglobin, the MMA compounds covalently interacted with βCys93, as evidenced by reverse-phase HPLC and disulfide exchange reaction, explaining their RBC sickling inhibitory activities, which at 2 mM and 5 mM, range from 0% to 21% and 9% to 64%, respectively. Additionally, the MMA compounds showed a second mechanism of sickling inhibition (12%-41% and 13%-62% at 2 mM and 5 mM, respectively) by directly destabilizing the sickle hemoglobin polymer. In vitro studies demonstrated sustained pharmacologic activities of the compounds compared to 5-HMF. These findings hold promise for advancing SCD therapeutics.

Can Crude Oil Exploration Influence the Phytochemicals and Bioactivity of Medicinal Plants? A Case of Nigerian Vernonia amygdalina and Ocimum gratissimum

The Nigerian Niger-Delta crude oil exploration often results in spills that affect indigenous medicinal plant biodiversity, likely changing the phytochemical profile of surviving species, their bioactivity or toxicity. In crude oil-rich Kokori and crude oil-free Abraka, classic examples of indigenous plants occupying the medicine-food interface include Vernonia amygdalina (VAL) and Ocimum gratissimum leaves (OGL). These plants are frequently utilised during pregnancy and in anaemia. To date, no scientific investigation has been reported on the potential changes to the phytochemical or bioactivity of the study plants. To discuss the similarities and dissimilarities in antisickling bioactivity and phytochemicals in VAL and OGL collected from Kokori (VAL-KK and OGL-KK) and Abraka (VAL-AB and OGL-AB), in silico, in vitro and comparative UPLC-QTOF-MS analysis was performed. Nine unique compounds were identified in OGL-KK, which have never been reported in the literature, while differences in antisickling potentials were observed in VAL-KK, OGL-KK and, VAL-AB, OGL-AB. Our findings show that VAL-AB and OGL-AB are richer and more diverse in phytochemicals and displayed a slightly higher antisickling activity than VAL-KK and OGL-KK. Ligand-based pharmacophore modelling was performed to understand the potential compounds better; this study may provide a basis for explaining the effect of crude oil spills on secondary metabolites and a reference for further research.

Modulating hemoglobin allostery for treatment of sickle cell disease: current progress and intellectual property

INTRODUCTION

Sickle cell disease (SCD) is a debilitating inherited disorder that affects millions worldwide. Four novel SCD therapeutics have been approved, including the hemoglobin (Hb) modulator Voxelotor.

AREAS COVERED

This review provides an overview of discovery efforts toward modulating Hb allosteric behavior as a treatment for SCD, with a focus on aromatic aldehydes that increase Hb oxygen affinity to prevent the primary pathophysiology of hypoxia-induce erythrocyte sickling.

EXPERT OPINION

The quest to develop small molecules, especially aromatic aldehydes, to modulate Hb allosteric properties for SCD began in the 1970s; however, early promise was dogged by concerns that stalled support for research efforts. Persistent efforts eventually culminated in the discovery of the anti-sickling agent 5-HMF in the 2000s, and reinvigorated interest that led to the discovery of vanillin analogs, including Voxelotor, the first FDA approved Hb modulator for the treatment of SCD. With burgeoning interest in the field of Hb modulation, there is a growing landscape of intellectual property, including drug candidates at various stages of preclinical and clinical investigations. Hb modulators could provide not only the best chance for a highly effective oral therapy for SCD, especially in the under-developed world, but also a way to treat a variety of other human conditions.

An Investigation of Structure-Activity Relationships of Azolylacryloyl Derivatives Yielded Potent and Long-Acting Hemoglobin Modulators for Reversing Erythrocyte Sickling

Aromatic aldehydes that bind to sickle hemoglobin (HbS) to increase the protein oxygen affinity and/or directly inhibit HbS polymer formation to prevent the pathological hypoxia-induced HbS polymerization and the subsequent erythrocyte sickling have for several years been studied for the treatment of sickle cell disease (SCD). With the exception of Voxelotor, which was recently approved by the U.S. Food and Drug Administration (FDA) to treat the disease, several other promising antisickling aromatic aldehydes have not fared well in the clinic because of metabolic instability of the aldehyde moiety, which is critical for the pharmacologic activity of these compounds. Over the years, our group has rationally developed analogs of aromatic aldehydes that incorporate a stable Michael addition reactive center that we hypothesized would form covalent interactions with Hb to increase the protein affinity for oxygen and prevent erythrocyte sickling. Although, these compounds have proven to be metabolically stable, unfortunately they showed weak to no antisickling activity. In this study, through additional targeted modifications of our lead Michael addition compounds, we have discovered other novel antisickling agents. These compounds, designated MMA, bind to the α-globin and/or β-globin to increase Hb affinity for oxygen and concomitantly inhibit erythrocyte sickling with significantly enhanced and sustained pharmacologic activities in vitro.

Hemoglobin: Structure, Function and Allostery

This chapter reviews how allosteric (heterotrophic) effectors and natural mutations impact hemoglobin (Hb) primary physiological function of oxygen binding and transport. First, an introduction about the structure of Hb is provided, including the ensemble of tense and relaxed Hb states and the dynamic equilibrium of Hb multistate. This is followed by a brief review of Hb variants with altered Hb structure and oxygen binding properties. Finally, a review of different endogenous and exogenous allosteric effectors of Hb is presented with particular emphasis on the atomic interactions of synthetic ligands with altered allosteric function of Hb that could potentially be harnessed for the treatment of diseases.

100 years of sickle cell disease research: etiology, pathophysiology and rational drug design (part 1)

Background

Sickle cell disease (SCD) is a chronic hemolytic disease caused by an altered hemoglobin molecule (HbS) and was first termed as a molecular disease. Glutamic acid in the normal hemoglobin molecule (HbA), was replaced by valine in HbS at the sixth position of both β-chains. This alteration was proved to be due to a single point mutation GTG instead of GAG in the genetic code. Since the discovery of sickle cell disease in 1910, great efforts have been done to study this disease on a molecular level. These efforts aimed to identify the disease etiology, pathophysiology, and finally to discover efficient treatment. Despite the tremendous work of many research groups all over the world, the only approved drug up to this moment, for the treatment of SCD is the hydroxyurea.

Main text

In this review, the antisickling pharmaco-therapeutics will be classified into two major groups: hemoglobin site directed modifiers and ex-hemoglobin effectors. The first class will be discussed in details, here in, focusing on the most important figures in the way of the rational drug design for SCD treatment aiming to help scientists solve the mystery of this problem and to get clear vision toward possible required therapy for SCD.

Conclusion

Despite the large number of the antisickling candidates that have been reached clinical studies yet, none of them has been introduced to the market. This may be due to the fact that hemoglobin is a large molecule with different target sites, which requires highly potent therapeutic agent. With this potency, these drugs should be safe, with acceptable oral pharmacokinetic and pharmacodynamic properties. Such ideal drug candidate needs more efforts to be developed.

Structural modification of azolylacryloyl derivatives yields a novel class of covalent modifiers of hemoglobin as potential antisickling agents

The intracellular polymerization and the concomitant sickling processes, central to the pathology of sickle cell disease, can be mitigated by increasing the oxygen affinity of sickle hemoglobin (HbS). Attempts to develop azolylacryloyl derivatives to covalently interact with βCys93 and destabilize the low-O2-affinity T-state (deoxygenated) HbS to the polymer resistant high-O2-affinity R-state (liganded) HbS were only partially successful. This was likely due to the azolylacryloyls carboxylate moiety directing the compounds to also bind in the central water cavity of deoxygenated Hb and stabilizing the T-state. We now report a second generation of KAUS compounds (KAUS-28, KAUS-33, KAUS-38, and KAUS-39) without the carboxylate moiety designed to bind exclusively to βCys93. As expected, the compounds showed reactivity with both free amino acid l-Cys and the Hb βCys93. At 2 mM concentrations, the compounds demonstrated increased Hb affinity for oxygen (6% to 15%) in vitro, while the previously reported imidazolylacryloyl carboxylate derivative, KAUS-15 only showed 4.5% increase. The increased O2 affinity effects were sustained through the experimental period of 12 h for KAUS-28, KAUS-33, and KAUS-38, suggesting conserved pharmacokinetic profiles. When incubated at 2 mM with red blood cells from patients with homozygous SS, the compounds inhibited erythrocyte sickling by 5% to 9%, respectively in correlation with the increase Hb-O2 affinity. These values compare to 2% for KAUS-15. When tested with healthy mice, KAUS-38 showed very low toxicity.

Antisickling Drugs Targeting βCys93 Reduce Iron Oxidation and Oxidative Changes in Sickle Cell Hemoglobin

Sickle cell disease is a genetic blood disorder caused by a single point mutation in the β globin gene where glutamic acid is replaced by valine at the sixth position of the β chain of hemoglobin (Hb). At low oxygen tension, the polymerization of deoxyHbS into fibers occurs in red blood cells (RBCs) leading to an impaired blood vessel transit. Sickle cell hemoglobin (HbS), when oxidized with hydrogen peroxide (H₂O₂), stays longer in a highly oxidizing ferryl (Fe⁴⁺) form causing irreversible oxidation of βCys93 to a destabilizing cysteic acid. We have previously reported that an antisickling drug can be designed to bind specifically to βCys93 and effectively protect against its irreversible oxidation by H₂O₂. Here, we report oxygen dissociation, oxidation, and polymerization kinetic reactions for four antisickling drugs (under different preclinical/clinical developmental stages) that either site-specifically target βCys93 or other sites on the HbS molecule. Molecules that specifically bind to or modify βCys93, such as 4,4′-di(1,2,3-triazolyl) disulfide (TD-3) and hydroxyurea (HU) were contrasted with molecules that target other sites on Hb including 5-hydroxymethyl-2-furfural (5-HMF) and L-glutamine. All reagents induced a left shift in the oxygen dissociation curve (ODC) except L-glutamine. In the presence of H₂O₂ (2.5:1, H₂O₂:heme), both TD-3 and HU reduced the ferryl heme by 22 and 37%, respectively, which corresponded to a 3- to 2-fold reduction in the levels of βCys93 oxidation as verified by mass spectrometry. Increases in the delay times prior to polymerization of HbS under hypoxia were in the following order: TD-3 > HU > 5-HMF = L-glutamine. Designing antisickling agents that can specifically target βCys93 may provide a dual antioxidant and antisickling therapeutic benefits in treating this disease.

New developments in anti-sickling agents: can drugs directly prevent the polymerization of sickle haemoglobin in vivo?

The hallmark of sickle cell disease is the polymerization of sickle haemoglobin due to a point mutation in the β-globin gene (HBB). Under low oxygen saturation, sickle haemoglobin assumes the tense (T-state) deoxygenated conformation that can form polymers, leading to rigid erythrocytes with impaired blood vessel transit, compounded or initiated by adhesion of erythrocytes to endothelium, neutrophils and platelets. This process results in vessel occlusion and ischaemia, with consequent acute pain, chronic organ damage, morbidity and mortality. Pharmacological agents that stabilize the higher oxygen affinity relaxed state (R-state) and/or destabilize the lower oxygen affinity T-state of haemoglobin have the potential to delay the sickling of circulating red cells by slowing polymerization kinetics. Relevant classes of agents include aromatic aldehydes, thiol derivatives, isothiocyanates and acyl salicylates derivatives. The aromatic aldehyde, 5-hydroxymethylfurfural (5-HMF) increases oxygen affinity of sickle haemoglobin and reduces hypoxia-induced sickling in vitro and protects sickle cell mice from effects of hypoxia. It has completed pre-clinical testing and has entered clinical trials as treatment for sickle cell disease. A related molecule, GBT440, has shown R-state stabilization and increased oxygen affinity in preclinical testing. Allosteric modifiers of haemoglobin as direct anti-sickling agents target the fundamental pathophysiological mechanism of sickle cell disease.

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.