CROSS REFERENCE TO RELATED APPLICATIONS
This application is a national stage application of PCT/US2010/060085 which was filed on Dec. 13, 2010, which is a non-provisional of U.S. Patent Application No. 61/285,955 which was filed on Dec. 11, 2009.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
This invention was made with government support under cooperative agreements awarded by the National Institutes of Health under grant numbers DK40949 and DK074176. The U.S. government may have certain rights to the invention.
- Top of Page
This invention relates to polypeptides that are resistant to thermal degradation. More particularly, this invention relates to thermally stabilized insulin analogues. Even more particularly, this invention relates to insulin analogues that are chemically and thermally stabilized by the incorporation of the element chlorine into an amino acid of the insulin analogue. Chlorine is classified as a halogen and is distinguished from the ordinary constituents of proteins by its atomic radius, electronegativity, stereoelectronic distribution of partial charges, and transmitted effects on the stereoelectronic properties of neighboring atoms in a molecule.
BACKGROUND OF THE INVENTION
- Top of Page
The engineering of ultra-stable proteins, including therapeutic agents and vaccines, may have broad societal benefits in regions of the developing world where electricity and refrigeration are not consistently available. An example of a therapeutic protein susceptible to thermal degradation is provided by insulin. The challenge posed by its chemical and physical degradation is deepened by the pending epidemic of diabetes mellitus in Africa and Asia. Because chemical degradation rates of insulin analogues correlate inversely with their relative stabilities, the design of ultra-stable formulations may enhance the safety and efficacy of insulin replacement therapy in such challenged regions.
The utility of some halogen substitutions in small organic molecules is known in medicinal chemistry. Fluorinated functional groups are critical to the efficacy of such widely prescribed small molecules as atorvastatin (Liptor™), an inhibitor of cholesterol biosynthesis, and fluoxetine hydrochloride (Prozac™), a selective serotonin reuptake inhibitor used in the treatment of depression and other affective disorders. Although the atomic radius of fluorine is similar to that of hydrogen, its large inductive effects modify the stereo-electronic properties of these drugs, in turn enhancing their biological activities. Similar considerations of physical organic chemistry pertain to the incorporation of larger halogen atoms, such as chlorine. The small molecule montelukast sodium (Singulair™) is a leukotriene inhibitor whose pharmaceutical properties are enhanced by covalent incorporation of a chlorine atom. Additionally, the use of fluorine-substituted amino acids in an insulin analogue is provided in International Patent Application No. PCT/US2009/52477 filed 31 Jul. 2009.
Modulation of the chemical, physical, and biological properties of proteins by the site-specific incorporation of chlorine atoms into modified amino acids are less well characterized in the scientific literature than are the above effects of incorporation of fluorine atoms.
Aromatic side chains may engage in a variety of weakly polar interactions, involving not only neighboring aromatic rings but also other sources of positive- or negative electrostatic potential. Examples include main-chain carbonyl- and amide groups in peptide bonds.
Administration of insulin has long been established as a treatment for diabetes mellitus. Insulin is a small globular protein that plays a central role in metabolism in vertebrates. Insulin contains two chains, an A chain, containing 21 residues and a B chain containing 30 residues. The hormone is stored in the pancreatic β-cell as a Zn2+-stabilized hexamer, but functions as a Zn2+-free monomer in the bloodstream. Insulin is the product of a single-chain precursor, proinsulin, in which a connecting region (35 residues) links the C-terminal residue of B chain (residue B30) to the N-terminal residue of the A chain (FIG. 1A). Although the structure of proinsulin has not been determined, a variety of evidence indicates that it consists of an insulin-like core and disordered connecting peptide (FIG. 1B). Formation of three specific disulfide bridges (A6-A11, A7-B7, and A20-B19; FIGS. 1A and 1B) is thought to be coupled to oxidative folding of proinsulin in the rough endoplasmic reticulum (ER). Proinsulin assembles to form soluble Zn2+-coordinated hexamers shortly after export from ER to the Golgi apparatus. Endoproteolytic digestion and conversion to insulin occurs in immature secretory granules followed by morphological condensation. Crystalline arrays of zinc insulin hexamers within mature storage granules have been visualized by electron microscopy (EM).
Extensive X-ray crystallographic studies have been undertaken of Zn2+-coordinated insulin hexamers, the physiological storage form. Multiple crystal forms have been described in vitro, defining three structural families, designated T6, T3Rf3 and R6. In these hexamers two Zn ions are believed to lie along the central axis of the hexamer, each coordinated by three histidines (HisB10); additional low-affinity Zn-binding sites have been observed in some crystal forms. The T-state protomer resembles the structure of an insulin monomer in solution. The R-state protomer exhibits a change in the secondary structure of the B-chain: the central α-helix extends to B1 (the R state) or to B3 (frayed Rf state).
Insulin functions in the bloodstream as a monomer, and yet it is the monomer that is believed to be most susceptible to fibrillation and most forms of chemical degradation. The structure of an insulin monomer, characterized in solution by NMR, is shown in FIG. 1D. The A-chain consists of an N-terminal α-helix (residues A1-A8), non-canonical turn (A9-A12), second α-helix (A12-A18), and C-terminal extension (A19-A21). The B chain contains an N-terminal arm (B1-B6), O-turn (B7-B10), central α-helix (B9-B19), β-turn (B20-B23), β-strand (B24-B28), and flexible C-terminal residues B29-B30. The two chains pack to form a compact globular domain stabilized by three disulfide bridges (cystines A6-A11, A7-B7, and A20-B19).
Absorption of regular insulin is limited by the kinetic lifetime of the Zn-insulin hexamer, whose disassembly to smaller dimers and monomers is required to enable transit through the endothelial lining of capillaries. The essential idea underlying the design of Humalog® and Novolog® is to accelerate disassembly. This is accomplished by destabilization of the classical dimer-forming surface (the C-terminal anti-parallel β-sheet). Humalog® contains substitutions ProB28→Lys and LysB29→Pro, an inversion that mimics the sequence of IGF-I. Novolog® contains the substitution ProB28→Asp. Although the substitutions impair dimerization, the analogs are competent for assembly of a phenol- or meta-cresol-stabilized zinc hexamer. This assembly protects the analog from fibrillation in the vial, but following subcutaneous injection, the hexamer rapidly dissociates as the phenol (or m-cresol) and zinc ions diffuse away. The instability of these analogs underlies their reduced shelf life on dilution by the patient or health-care provider. It would be useful for an insulin analogue to augment the intrinsic stability of the insulin monomer while retaining the variant dimer-related β-sheet of Humalog®.
Use of zinc insulin hexamers during storage is known and represents a classical strategy to retard physical degradation and chemical degradation of a formulation in the vial or in the reservoir of a pump. Because the zinc insulin hexamer is too large for immediate passage into capillaries, the rate of absorption of insulin after subcutaneous injection is limited by the time required for dissociation of hexamers into smaller dimers and monomer units. Therefore, it would advantageous for an insulin analogue to be both (a) competent to permit hexamer assembly at high protein concentration (as in a vial or pump) and yet (b) sufficiently destabilized at the dimer interface to exhibit accelerated disassembly—hence predicting ultra-rapid absorption from the subcutaneous depot. These structural goals walk a fine line between stability (during storage) and instability (following injection).
Amino-acid substitutions in insulin have been investigated for effects on thermodynamic stability and biological activity. No consistent relationship has been observed between stability and activity. Whereas some substitutions that enhance thermodynamic stability also enhance binding to the insulin receptor, other substitutions that enhance stability impede such binding. The effects of substitution of ThrA8 by several other amino acids has been investigated in wild-type human insulin and in the context of an engineered insulin monomer containing three unrelated substitutions in the B-chain (HisB10→Asp, ProB28→Lys, and LysB29→Pro) have been reported. Examples are also known in the art of substitutions that accelerate or delay the time course of absorption. Such substitutions (such as AspB28 in Novalog® and [LySB28, ProB29] in Humalog®) can be and often are associated with more rapid fibrillation and poorer physical stability. Indeed, in one study a series of ten analogues of human insulin was tested for susceptibility to fibrillation, including AspB28-insulin and AspB10-insulin. All ten were found to be more susceptible to fibrillation at pH 7.4 and 37° C. than is human insulin. The ten substitutions were located at diverse sites in the insulin molecule and are likely to be associated with a wide variation of changes in classical thermodynamic stability. Although a range of effects has been observed, no correlation exists between activity and thermodynamic stability.
Insulin is a small globular protein that is highly amenable to chemical synthesis and semi-synthesis, which facilitates the incorporation of nonstandard side chains. Insulin contains three phenylalanine residues (positions B1, B24, and B25) and a structurally similar tyrosine at position B26. Conserved among vertebrate insulins and insulin-like growth factors, the aromatic ring of PheB24 packs against (but not within) the hydrophobic core to stabilize the super-secondary structure of the B-chain. PheB24 lies at the classical receptor-binding surface and has been proposed to direct a change in conformation on receptor binding. PheB25 projects from the surface of the insulin monomer whereas TyrB26 packs near aliphatic side chains (IleA2, ValA3, and ValB12) at one edge of the core. The B24-related conformational change is proposed to enable PheB25 and TyrB26 to contact distinct domains of the insulin receptor.
The present theory of protein fibrillation posits that the mechanism of fibrillation proceeds via a partially folded intermediate state, which in turn aggregates to form an amyloidogenic nucleus. In this theory, it is possible that amino-acid substitutions that stabilize the native state may or may not stabilize the partially folded intermediate state and may or may not increase (or decrease) the free-energy barrier between the native state and the intermediate state. Therefore, the current theory indicates that the tendency of a given amino-acid substitution in the insulin molecule to increase or decrease the risk of fibrillation is highly unpredictable.
Fibrillation, which is a serious concern in the manufacture, storage and use of insulin and insulin analogues for diabetes treatment, is enhanced with higher temperature, lower pH, agitation, or the presence of urea, guanidine, ethanol co-solvent, or hydrophobic surfaces. Current US drug regulations demand that insulin be discarded if fibrillation occurs at a level of one percent or more. Because fibrillation is enhanced at higher temperatures, diabetic individuals optimally must keep insulin refrigerated prior to use. Fibrillation of insulin or an insulin analogue can be a particular concern for diabetic patients utilizing an external insulin pump, in which small amounts of insulin or insulin analogue are injected into the patient's body at regular intervals. In such a usage, the insulin or insulin analogue is not kept refrigerated within the pump apparatus and fibrillation of insulin can result in blockage of the catheter used to inject insulin or insulin analogue into the body, potentially resulting in unpredictable blood glucose level fluctuations or even dangerous hyperglycemia. At least one recent report has indicated that lispro insulin (an analogue in which residues B28 and B29 are interchanged relative to their positions in wild-type human insulin; trade name Humalog®) may be particularly susceptible to fibrillation and resulting obstruction of insulin pump catheters.
Insulin fibrillation is an even greater concern in implantable insulin pumps, where the insulin would be contained within the implant for 1-3 months at high concentration and at physiological temperature (i.e., 37° C.), rather than at ambient temperature as with an external pump. Additionally, the agitation caused by normal movement would also tend to accelerate fibrillation of insulin. In spite of the increased potential for insulin fibrillation, implantable insulin pumps are still the subject of research efforts, due to the potential advantages of such systems. These advantages include intraperitoneal delivery of insulin to the portal circulatory system, which mimics normal physiological delivery of insulin more closely than subcutaneous injection, which provides insulin to the patient via the systemic circulatory system. Intraperitoneal delivery provides more rapid and consistent absorption of insulin compared to subcutaneous injection, which can provide variable absorption and degradation from one injection site to another. Administration of insulin via an implantable pump also potentially provides increased patient convenience. Whereas efforts to prevent fibrillation, such as by addition of a surfactant to the reservoir, have provided some improvement, these improvements have heretofore been considered insufficient to allow reliable usage of an implanted insulin pump in diabetic patients outside of strictly monitored clinical trials.
As noted above, the developing world faces a challenge regarding the safe storage, delivery, and use of drugs and vaccines. This challenge complicates the use of temperature-sensitive insulin formulations in regions of Africa and Asia lacking consistent access to electricity and refrigeration, a challenge likely to be deepened by the pending epidemic of diabetes in the developing world. Insulin exhibits an increase in degradation rate of 10-fold or more for each 10° C. increment in temperature above 25° C., and guidelines call for storage at temperatures <30° C. and preferably with refrigeration. At higher temperatures insulin undergoes both chemical degradation (changes in covalent structure such as formation of iso-aspartic acid, rearrangement of disulfide bridges, and formation of covalent polymers) and physical degradation (non-native aggregation and fibrillation).
Amino-acid substitutions have been described in insulin that stabilize the protein but augment its binding to the insulin receptor (IR) and its cross-binding to the homologous receptor for insulin-like growth factors (IGFR) in such a way as to confer a risk of carcinogenesis. An example known in the art is provided by the substitution of HisB10 by aspartic acid. Although AspB10-insulin exhibits favorable pharmaceutical properties with respect to stability and pharmacokinetics, its enhanced receptor-binding properties were associated with tumorigenesis in Sprague-Dawley rats. Although there are many potential substitutions in the A- or B chains that can be introduced into AspB10-insulin or related analogues to reduce its binding to IR and IGFR to levels similar to that of human insulin, such substitutions generally impair the stability of insulin (or insulin analogues) and increase its susceptibility to chemical and physical degradation. It would be desirable to discover a method of modification of insulin and of insulin analogues that enabled “tuning” of receptor-binding affinities while at the same time enhancing stability and resistance to fibrillation. Such applications would require a set of stabilizing modifications that reduce binding to IR and IGFR to varying extent so as to offset the potential carcinogenicity of analogues that are super-active in their receptor-binding properties.
Therefore, there is a need for alternative insulin analogues, including those that are stable during storage but are simultaneously fast-acting.
- Top of Page
OF THE INVENTION
It is, therefore, an aspect of the present invention to provide an insulin analogue that provides altered properties, such as improved stability, by chlorine substitution in an amino acid, where the analogue also maintains at least a portion of biological activity of the corresponding non-chlorinated insulin or insulin analogue.
In addition or in the alternative, it is an aspect of the present invention to provide an insulin analogue that is a fast acting insulin but also has improved stability over previous fast-acting insulin analogues.
In general, the present invention provides an insulin analogue comprising a B-chain polypeptide which incorporates a chlorinated amino acid. In one embodiment, the chlorinated amino acid is phenylalanine at position B24. In one particular embodiment, the chlorinated phenylalanine is para-monochloro-phenylalanine. In addition or in the alternative, the insulin analogue may be a mammalian insulin analogue, such as an analogue of human insulin. In one particular set of embodiments, the B-chain polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NOS: 4-8 and polypeptides having three or fewer additional amino acid substitutions thereof.
Also provided is a nucleic acid encoding an insulin analogue comprising a B-chain polypeptide that incorporates a chlorinated phenylalanine at position B24 such as para-monochloro-phenylalanine. In one example, the chlorinated phenylalanine is encoded by a stop codon, such as the nucleic acid sequence TAG. An expression vector may comprise such a nucleic acid and a host cell may contain such an expression vector.
The invention also provides a method of treating a patient. The method comprises administering a physiologically effective amount of an insulin analogue or a physiologically acceptable salt thereof to the patient, wherein the insulin analogue or a physiologically acceptable salt thereof contains a chlorinated amino acid. In one embodiment, an insulin B-chain incorporates a chlorinated phenylalanine at position B24. In one particular embodiment, the chlorinated phenylalanine is para-monochloro-phenylalanine. In addition or in the alternative, the insulin analogue may a mammalian insulin analogue, such as an analogue of human insulin. Furthermore, the B-chain polypeptide may comprise an amino acid sequence selected from the group consisting of SEQ ID NOS: 4-8 and polypeptides having three or fewer additional amino-acid substitutions thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
- Top of Page
FIG. 1A is a schematic representation of the sequence of human proinsulin including the A- and B-chains and the connecting region shown with flanking dibasic cleavage sites (filled circles) and C-peptide (open circles).
FIG. 1B is a structural model of proinsulin, consisting of an insulin-like moiety and a disordered connecting peptide (dashed line).
FIG. 1C is a schematic representation of the sequence of human insulin indicating the position of residue B24 in the B-chain.
FIG. 1D is a ribbon model of an insulin monomer showing aromatic residue of PheB24 in relation to the three disulfide bridges. The adjoining side chains of LeuB15 (in black) and PheB24 (in gray) are shown. The A- and B-chain chains are shown, and the disulfide bridges as balls and sticks.
FIG. 1E is a space-filling model of insulin showing the PheB24 side chain within a pocket at the edge of the hydrophobic core.
FIG. 2A is a representation of ortho-monochlorinated-phenylalanine (2Cl -Phe).
FIG. 2B is a representation of meta-monochlorinated-phenylalanine (3Cl-Phe).
FIG. 2C is a representation of para-monochlorinated-phenylalanine (4Cl-Phe).
FIG. 3A is a graph showing the results of in vitro receptor-binding assays using isolated insulin receptor (isoform B): human insulin (triangles), KP-insulin (squares), and 4Cl-PheB24-KP-insulin (inverted triangles).
FIG. 3B is a graph showing the results of in vitro receptor-binding assays employing IGF-1R: human insulin (triangles), KP-insulin (squares), 4Cl-PheB24-KP-insulin (inverted triangles), and native IGF-I (circles).
FIG. 3C is a graph comparing the results of in vitro receptor-binding assays using isolated insulin receptor (isoform B): human insulin (solid line), KP-insulin (dashed line), 4Cl-PheB24-KP-insulin (triangles) 4F-PheB24-KP-insulin (squares).
FIG. 3D is a graph comparing the results of in vitro receptor-binding assays using isolated insulin receptor (isoform B): human insulin (solid line), KP-insulin (dashed line), 4Cl-PheB26-KP-insulin (triangles) 4F-PheB26-KP-insulin (squares).
FIG. 4 is a graph showing the hypoglycemic action of subcutaneous of 4Cl -PheB24-KP-insulin in STZ induced diabetic Lewis rats over time (inverted triangles) relative to diluent alone (circles), human insulin (crosses), and KP-insulin (squares).
FIGS. 5A-C are graphs showing averaged traces of insulin cobalt solutions showing characteristic spectral profiles from 400-750 nm before and after addition of 2 mM EDTA. Samples were dissolved in 50 mM Tris (pH 7.4), 50 mM phenol, and 0.2 mM CoCl2. NaSCN was then added to a final concentration of 1 mM. Solid lines show data pre-EDTA extraction. Dashed lines show data post-EDTA extraction. Panel A: wild type insulin; Panel B: KP-insulin; Panel C; 4Cl-PheB24-KP-insulin.
FIG. 5D is a graph showing the kinetics of hexamer dissociation after addition of 2 mM EDTA as monitored at 574 nm (25° C. and pH 7.4). Data were normalized to time zero for each sample: wild type (solid line), KP-insulin (dashed line), and 4Cl -PheB24-KP-insulin (dotted line).
FIG. 6 is a graph showing a plot of the mean filtered glucose infusion rate versus time after insulin dose for KP-insulin (Lispro insulin) and 4Cl-PheB24-KP-insulin (4-Cl-Lispro insulin) at a dosage of 0.2 Units per kilogram of bodyweight.
FIG. 7 is a bar graph summarizing 20 pharmacodynamic studies in pigs demonstrating significant improvement in ½ T-max late in 4Cl-PheB24-KP-insulin over KP-insulin at five different dosing levels.
FIG. 8 is a bar graph summarizing 14 pharmacodynamic studies in pigs suggesting improvement in ½ T-max early in 4Cl-PheB24-KP-insulin over KP-insulin at three different dosing levels.
FIG. 9 is a summary of ten, matched pharmacodynamics studies comparing the relative potencies 4Cl-PheB24-KP-insulin with that of KP insulin as measured by area under the curve (AUC) in which the slightly reduced average potency for 4-Cl-KP was found not to be statistically significant (p=0.22).
DESCRIPTION OF EMBODIMENTS
The present invention is directed an insulin analogue that provides greater stability by chlorine substitution in an amino acid, where the analogue then maintains at least a portion of biological activity of the corresponding non-chlorinated insulin or insulin analogue. Particularly, the present invention provides insulin analogues that provide greater stability by substitution of chlorine in an amino acid, where the analogue then maintains at least a portion of biological activity of the corresponding non-chlorinated insulin or insulin analogue. In one particular embodiment, the present invention provides insulin analogues that contain a para-monochloro-phenylalanine (4Cl -PheB24) residue as a substitution for wild type phenylalanine at position B24.
The present invention is not limited, however, to human insulin and its analogues. It is also envisioned that these substitutions may also be made in animal insulins such as porcine, bovine, equine, and canine insulins, by way of non-limiting examples.
Furthermore, in view of the similarity between human and animal insulins, and use in the past of animal insulins in human diabetic patients, it is also envisioned that other minor modifications in the sequence of insulin may be introduced, especially those substitutions considered “conservative.” For example, additional substitutions of amino acids may be made within groups of amino acids with similar side chains, without departing from the present invention. These include the neutral hydrophobic amino acids: Alanine (Ala or A), Valine (Val or V), Leucine (Leu or L), Isoleucine (Ile or I), Proline (Pro or P), Tryptophan (Trp or W), Phenylalanine (Phe or F) and Methionine (Met or M). Likewise, the neutral polar amino acids may be substituted for each other within their group of Glycine (Gly or G), Serine (Ser or S), Threonine (Thr or T), Tyrosine (Tyr or Y), Cysteine (Cys or C), Glutamine (Glu or Q), and Asparagine (Asn or N). Basic amino acids are considered to include Lysine (Lys or K), Arginine (Arg or R) and Histidine (His or H). Acidic amino acids are Aspartic acid (Asp or D) and Glutamic acid (Glu or E). Unless noted otherwise or wherever obvious from the context, the amino acids noted herein should be considered to be L-amino acids. In one example, the insulin analogue of the present invention contains three or fewer conservative substitutions other than the 4Cl-PheB24 substitution of the present invention. In another example, the insulin analogue of the present invention contains one or fewer conservative substitutions other than the 4Cl-PheB24 substitution of the present invention.
As used in this specification and the claims, various amino acids in insulin or an insulin analogue may be noted by the amino acid residue in question, followed by the position of the amino acid, optionally in superscript. The position of the amino acid in question includes the A or B chain of insulin where the substitution is located. Thus, PheB24 denotes a phenylalanine at the twenty fourth amino acid of the B chain of insulin, while PheB25 denotes a phenylalanine at the twenty fifth amino acid of the B chain of insulin. A chlorinated amino acid may be indicated with the prefix “Cl-.” Therefore, chlorinated phenylalanine may be abbreviated “Cl-Phe.” In the case of phenylalanine, the position of the chlorine substituents or substituents on the phenyl side chain may be further indicated by the number of the carbon to which the chlorine is attached. Therefore, ortho-monochloro-phenylalanine (shown in FIG. 2B) is abbreviated “2Cl-Phe,” meta-monochloro-phenylalanine (shown in FIG. 2C) is abbreviated “3Cl-Phe” and para-monochloro-phenylalanine (shown in FIG. 2D) is abbreviated 4Cl-Phe.
The phenylalanine at B24 is an invariant amino acid in functional insulin and contains an aromatic side chain. The biological importance of PheB24 in insulin is indicated by a clinical mutation (SerB24) causing human diabetes mellitus. As illustrated in FIGS. 1D and 1E, and while not wishing to be bound by theory, PheB24 is believed to pack at the edge of a hydrophobic core at the classical receptor binding surface. The models are based on a crystallographic protomer (2-Zn molecule 1; Protein Databank identifier 4INS). Lying within the C-terminal β-strand of the B-chain (residues B24-B28), PheB24 adjoins the central α-helix (residues B9-B19). One face and edge of the aromatic ring sit within a shallow pocket defined by LeuB15 and CysB19; the other face and edge are exposed to solvent (FIG. 1E). This pocket is in part surrounded by main-chain carbonyl and amide groups and so creates a complex and asymmetric electrostatic environment.
It is envisioned that the substitutions of the present invention may be made in any of a number of existing insulin analogues. For example, the chlorinated PheB24 substitution provided herein may be made in insulin analogues such as Lispro (KP) insulin, insulin Aspart, other modified insulins or insulin analogues, or within various pharmaceutical formulations, such as regular insulin, NPH insulin, lente insulin or ultralente insulin, in addition to human insulin. Aspart insulin contains an AspB28 substitution and is sold as Novalog whereas Lispro insulin contains LysB28 and ProB29 substitutions and is known as and sold under the name Humalog®. These analogues are described in U.S. Pat. Nos. 5,149,777 and 5,474,978. Both of these analogues are known as fast-acting insulins.
While not wishing to be bound by theory, the chloro substitution at the para position of an aromatic ring is believed to be buried within the dimer interface (4Cl -PheB24) and is also believed to accelerate hexamer disassembly by creating an unfavorable alignment of chloro-aromatic electrostatic dipole moments, pairwise juxtaposed at each dimer interface within the hexamer. This is believed to permit the insulin analogue to be formulated in the presence of Zn2+ ions and still retain the ability to be a fast-acting (meal time) insulin analogue.
A chlorinated-Phe substitution, including one at B24, may also be introduced into analogues of human insulin that, while not previously clinically used, are still useful experimentally, such as DKP insulin, described more fully below, or miniproinsulin, a proinsulin analogue containing a dipeptide (Ala-Lys) linker between the A chain and B chain portions of insulin in place of the normal 35 amino acid connecting region between the C-terminal residue of the B chain and the N-terminal residue of the A chain (See FIG. 1B). Incorporation of chlorinated-Phe at position B24 in DKP-insulin (or other insulin analogues that contain AspB10 or that exhibit receptor-binding affinities higher than that of human insulin) can reduce their receptor-binding affinities to be similar to or below that of human insulin and so potentially enable clinical use. In this manner the cross-binding of insulin analogues to the mitogenic IGFR may also be reduced.
The amino-acid sequence of human proinsulin is provided, for comparative purposes, as SEQ ID NO: 1.
SEQ ID NO: 1