Oligopeptides as potential antiaggregation agents for deoxyhemoglobin S

Rational Drug Design of Peptide-Based Therapies for Sickle Cell Disease

Sickle cell disease (SCD) is a group of inherited disorders affecting red blood cells, which is caused by a single mutation that results in substitution of the amino acid valine for glutamic acid in the sixth position of the β-globin chain of hemoglobin. These mutant hemoglobin molecules, called hemoglobin S, can polymerize upon deoxygenation, causing erythrocytes to adopt a sickled form and to suffer hemolysis and vaso-occlusion. Until recently, only two drug therapies for SCD, which do not even fully address the manifestations of SCD, were approved by the United States (US) Food and Drug Administration. A third treatment was newly approved, while a monoclonal antibody preventing vaso-occlusive crises is also now available. The complex nature of SCD manifestations provides multiple critical points where drug discovery efforts can be and have been directed. These notwithstanding, the need for new therapeutic approaches remains high and one of the recent efforts includes developments aimed at inhibiting the polymerization of hemoglobin S. This review focuses on anti-sickling approaches using peptide-based inhibitors, ranging from individual amino acid dipeptides investigated 30-40 years ago up to more promising 12- and 15-mers under consideration in recent years.

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.

A potent new dipeptide inhibitor of cell sickling and haemoglobin S gelation

A dipeptide L-lysine-L-phenylalanine is shown to inhibit both cell sickling and the gelation of solutions of sickle-cell haemoglobin. The effect on deoxyhaemoglobin solutions and gels was followed by centrifugation; a progressive inhibition of gelation was observed up to 30 mM Lys-Phe. The haemoglobin concentration at the plateau (26 g/dl) suggests that an effect might be seen in vivo if suitable concentrations of Lys-Phe (about 20 mM) can be maintained in the blood stream. Additional studies of lag time before onset of gelation support this. Oxygen dissociation curves of sickle cells showed an effect of Lys-Phe only after incubation for 3 h before measurement, the P50 decreasing from 51 mmHg (6.8 MPa) to 41 mmHg (5.5 MPa) for cells depleted of 2,3-bisphosphoglycerate. The effect of Lys-Phe on intact sickle cells was more rapid. A marked increase in the number of unsickled cells in the presence of Lys-Phe was observed after only 15 min incubation. This result, together with measurements of uptake both into the cell and onto the cell membrane shows that the compound produces a membrane-mediated antisickling effect in addition to the effect on haemoglobin in solution within the cell. The membrane effect is not due to a change in cell volume. The properties of this dipeptide may be of value in the treatment of impending and early sickle crisis.

Peptide inhibitors of sickle hemoglobin aggregation: effect of hydrophobicity

Thirty-three peptides have been synthesized which have the ability to inhibit aggregation of sickle hemoglobin which occurs upon deoxygenation. Evidence is presented which indicates that the hydrophobicity of the side chains is the predominant factor. A linear correlation exists between the capacity of peptides to inhibit gelation and the additivity of the side-chain hydrophobic contributions. Small di- and tri-peptides are more responsive to changes in their nonpolar content than are larger oligopeptides whose antigelling activity is a slowly varying function of hydrophobicity. With few exceptions, charged substituent groups on the side chain do not appear to contribute to the inhibitory process. Thus, if hydrogen bonding or ionic interactions are involved with these residues, they appear to play only a secondary role in the molecular interactions responsible for inhibiting or delaying the deoxy-HbS gelation.

Modifications of solid phase peptide synthesis to obtain homogeneous oligoprolines in high yield

When oligoprolines containing a [3H]proline in the second residue from the carboxyl terminus are synthesized by the standard solid phase method using stepwise addition of prolines there appears, in addition to the oligoprolines, a steady increase of ninhydrin-negative, but radioactive, impurities. These impurities were found to be N-trifluoroacetyl oligoprolines, and a substantial amount of diketopiperazine was present at the diproline stage. The trifluoroacetyl blockage of the N-termini was almost complete by the time 14 residues were added. To eliminate these side reactions, a block of Boc[3H]Pro2 was coupled to prolyl-resin in order to avoid the diprolyl-resin stage, and 25% CF3CO2H in CHCl3 was replaced by 4N HCl in dioxane as the deprotecting reagent. These changes eliminated the formation of diketopiperazine and trifluoroacetyl impurities, but still a steady increase of ninhydrin-negative, but radioactive, impurity was seen, reaching 22% after three couplings. These side reactions were traced to the neutralization step, and in order to avoid them it became necessary to eliminate the neutralization step, and replace it by incremental additions of Et3N during the coupling step. As a result of these modifications, we now obtain homogeneous length oligoprolines upon cleavage from the resin, with no further purification, when analyzed by reversed-phase liquid chromatography.

Antisickling activity of amino acid benzyl esters

The sickling of homozygous sickel cells upon deoxygenation is inhibited in the presence of 3 mM L-phenylalanine benzyl ester (Phe-OBzl) or benzyl esters of other aromatic or hydrophobic amino acids. Phe-OBzl was found to permeate into erythrocytes rapidly, and the deoxygenated cells maintained considerable flexibility as measured by their ability to pass through 3-micron pores. The osmotic fragility of the cells was unchanged and the oxygen dissociation curve was shifted slightly from a 50% saturation values of 28.5 mm Hg to 31.0 mm Hg. At lower concentrations of Phe-OBzl some antisickling activity was seen. The Phe-OBzl antisickling activity may involve both binding to deoxyhemoglobin S and modification of the erythrocyte membrane. This class of compounds has considerable potential as therapeutic agents for the treatment of sickle cell disease.

Do amino acids reverse the sickling of erythrocytes containing hemoglobin S?

Homoserine, asparagine and glutamine do not restore the deformability of deoxygenated sickle cells in spite of noticeable morphological changes. These amino acids also do not raise the minimum gelling concentration of deoxyhemoglobin S. The use of these compounds as anti-sickling agents is therefore doubtful.

Stereospecific inhibitors of the gelation of sickle hemoglobin

During the last decade there have been major advances in understanding the structure of the gel of deoxyhemoglobin S and the mechanism of its formation. These advances have allowed the development of a new strategy for the inhibition of gelation, i.e., stereospecific competitive inhibitors of the polymerization process. For this purpose we have used equilibrium solubility measurements to study the effects of amino acids and peptides on deoxyhemoglobin S solubility. We have found that aromatic amino acids and short peptides containing these amino acids significantly increase solubility; longer peptides, however, decrease solubility by an excluded-volume related effect. Recently we have used computer graphics projections of the surface of the hemoglobin molecules in the crystal form to design peptide inhibitors. In addition, we have developed 13C nuclear magnetic resonance spectroscopic methods to quantitate the gel of deoxyhemoglobin S in hemoglobin solutions an in cells. These spectroscopic methods allow us to study the mechanism of intracellular gelation and to test the effects of inhibitors on sickle cells.

Inhibition of sickle hemoglobin gelation by amino acids and related compounds

The effects of amino acids, several aromatic compounds, and peptides on the gelation and solubility of deoxyhemoglobin S have been studied. The aromatic amino acids (tryptophan, phenylalanine, and possibly tyrosine) significantly inhibited the rate of gel formation and increased solubility. The dipeptide L-Thr-L-Phe, the tripeptide L-Lys-L-Phe-L-Phe, and various phenylalanine analogues (hydrocinnamic acid, phenethylamine, benzamine, and amphetamine) also inhibited gelation. However, aromaticity is not a sufficient condition for inhibiting gelation as shown by the fact that several aromatic compounds (acetylsalicylic acid, salicyclic acid, aniline, and phenol) enhanced gelation. Surprisingly, several oligopeptides (betaS1--12, betaS4--8, betaS3--13, and betaS4--10) also enhanced gelation. All of these additives follow the supersaturation relationship that the delay time for gelation is proportional to the ratio of the total hemoglobin concentration to the solubility of deoxyhemoglobin S to the nth power (n approximately 35). A possible mechanism for the action of these inhibitors is considered in terms of a specific site of interaction on the hemoglobin molecule. Although none of these compounds may prove to be efficacious in treatment of sickle cell anemia, they should yield information about the structure and process of formation of the deoxyhemoglobin S gel.