Spontaneous degradation via diketopiperazine formation of peptides containing a tetrahydroisoquinoline-3-carboxylic acid residue in the 2-position of the peptide sequence

In vivo degradation forms, anti-degradation strategies, and clinical applications of therapeutic peptides in non-infectious chronic diseases

Current medicinal treatments for diseases comprise largely of two categories: small molecular (chemical) (e.g., aspirin) and larger molecular (peptides/proteins, e.g., insulin) drugs. Whilst both types of therapeutics can effectively treat different diseases, ranging from well-understood (in view of pathogenesis and treatment) examples (e.g., flu), to less-understood chronic diseases (e.g., diabetes), classical small molecule drugs often possess significant side-effects (a major cause of drug withdrawal from market) due to their low- or non-specific targeting. By contrast, therapeutic peptides, which comprise short sequences from naturally occurring peptides/proteins, commonly demonstrate high target specificity, well-characterized modes-of-action, and low or non-toxicity in vivo. Unfortunately, due to their small size, linear permutation, and lack of tertiary structure, peptidic drugs are easily subject to rapid degradation or loss in vivo through chemical and physical routines, thus resulting in a short half-life and reduced therapeutic efficacy, a major drawback that can reduce therapeutic efficiency. However, recent studies demonstrate that the short half-life of peptidic drugs can be significantly extended by various means, including use of enantiomeric or non-natural amino acids (AAs) (e.g., L-AAs replacement with D-AAs), chemical conjugation [e.g., with polyethylene glycol], and encapsulation (e.g., in exosomes). In this context, we provide an overview of the major in vivo degradation forms of small therapeutic peptides in the plasma and anti-degradation strategies. We also update on the progress of small peptide therapeutics that are either currently in clinical trials or are being successfully used in clinical therapies for patients with non-infectious diseases, such as diabetes, multiple sclerosis, and cancer.

Silk-based stabilization of biomacromolecules

Silk fibroin is a high molecular weight amphiphilic protein that self-assembles into robust biomaterials with remarkable properties including stabilization of biologicals and tunable release kinetics correlated to processing conditions. Cells, antibiotics,monoclonal antibodies and peptides, among other biologics, have been encapsulated in silk using various processing approaches and material formats. The mechanistic basis for the entrapment and stabilization features, along with insights into the modulation of release of the entrained compounds from silks will be reviewed with a focus on stabilization of bioactive molecules.

Stability of protein pharmaceuticals: an update

In 1989, Manning, Patel, and Borchardt wrote a review of protein stability (Manning et al., Pharm. Res. 6:903-918, 1989), which has been widely referenced ever since. At the time, recombinant protein therapy was still in its infancy. This review summarizes the advances that have been made since then regarding protein stabilization and formulation. In addition to a discussion of the current understanding of chemical and physical instability, sections are included on stabilization in aqueous solution and the dried state, the use of chemical modification and mutagenesis to improve stability, and the interrelationship between chemical and physical instability.

Synthesis and pharmacological characterization of a novel, highly potent, peptidomimetic delta-opioid radioantagonist, [3H]Tyr-Tic-(2S,3R)-beta-MePhe-Phe-OH

[(3)H]Tyr-Tic-(2S,3R)-beta-MePhe-Phe-OH (where Tic: 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) with a specific radioactivity of 53.7 Ci/mmol was synthesized and characterized in receptor binding assays at 25 degrees C in rat brain membranes. The specific binding was saturable and displayed high affinity, with a K(D) of 0.16+/-0.005 nM and B(max) of 85.9+/-6.3 fmol/mg protein. NaCl increased its affinity by about 4-fold in membranes of rat brain and Chinese Hamster Ovary Cells stably transfected with the human delta-opioid receptors (hDOR-CHO) showing that the new ligand is an antagonist. The prototypic delta-opioid ligands were much more potent than mu- or kappa-specific ligands in competition assays. The autoradiographic distribution of the binding sites of the new ligand agreed with the known locations of the delta-opioid receptors in rat brain. The unlabeled new ligand was about 7-fold more potent than the parent peptide in competing for the binding sites of [(3)H]Tyr-Tic-(2S,3R)-beta-MePhe-Phe-OH in rat brain membranes. Likewise, the threo-beta-methyl analog was 3.8-fold more potent than the parent compound in antagonizing the effect of DPDPE in the [(35)S]GTPgammaS functional assay in hDOR-CHO membranes. The new, highly potent, conformationally constrained antagonist may be a valuable pharmacological tool in understanding the structural and topographical requirements of peptide ligand binding to the delta-opioid receptors.

New 2',6'-dimethyl-L-tyrosine (Dmt) opioid peptidomimetics based on the Aba-Gly scaffold. Development of unique mu-opioid receptor ligands

The Aba-Gly scaffold, incorporated into Dmt-Tic ligands (H-Dmt-Tic-Gly-NH-CH2-Ph, H-Dmt-Tic-Gly-NH-Ph, H-Dmt-Tic-NH-CH2-Bid), exhibited mixed micro/delta or delta opioid receptor activities with micro agonism. Substitution of Tic by Aba-Gly coupled to -NH-CH2-Ph (1), -NH-Ph (2), or -Bid (Bid=1H-benzimidazole-2-yl) (3) shifted affinity (Ki(micro)=0.46, 1.48, and 19.9 nM, respectively), selectivity, and bioactivity to micro-opioid receptors. These compounds represent templates for a new class of lead opioid agonists that are easily synthesized and suitable for therapeutic pain relief.

Dmt and opioid peptides: a potent alliance

The introduction of the Dmt (2',6'-dimethyl-L-tyrosine)-Tic pharmacophore into the design of opioid ligands produced an extraordinary family of potent delta-opioid receptor antagonists and heralded a new phase in opioid research. First reviewed extensively in 1998, the incorporation of Dmt into a diverse group of opioid molecules stimulated the opioid field leading to the development of unique analogues with remarkable properties. This overview will document the crucial role played by this residue in the proliferation of opioid peptides with high receptor affinity (K(i) equal to or less than 1 nM) and potent bioactivity. The discussion will include the metamorphosis between delta-opioid receptor antagonists to delta-agonists based solely on subtle structural changes at the C-terminal region of the Dmt-Tic pharmacophore as well as their behavior in vivo. Dmt may be considered promiscuous due to the acquisition of potent mu-agonism by dermorphin and endomorphin derivatives as well as by a unique class of opioidmimetics containing two Dmt residues separated by alkyl or pyrazinone linkers. Structural studies on the Dmt-Tic compounds were enhanced tremendously by x-ray diffraction data for three potent and biologically diverse Dmt-Tic opioidmimetics that led to the development of pharmacophores for both delta-opioid receptor agonists and antagonists. Molecular modeling studies of other unique Dmt opioid analogues illuminated structural differences between delta- and mu-receptor ligand interactions. The future of these compounds as therapeutic applications for various medical syndromes including the control of cancer-associated pain is only a matter of time and perseverance.

Diketopiperazines in peptide and combinatorial chemistry

Diketopiperazines (DKPs), the smallest cyclic peptides, represent an important class of biologically active natural products and their research has been fundamental to many aspects of peptide chemistry. The advent of combinatorial chemistry has revived interest in DKPs for two reasons: firstly, they are simple heterocyclic scaffolds in which diversity can be introduced and stereochemically controlled at up to four positions; secondly, they can be prepared from readily available alpha-amino acids using very robust chemistry. Here synthetic methods, conformation, as well as applications of DKPs are summarized and discussed critically.

The TIPP opioid peptide family: development of delta antagonists, delta agonists, and mixed mu agonist/delta antagonists

The discovery of the prototype delta opioid antagonists TIPP (H-Tyr-Tic-Phe-Phe-OH) and TIP (H-Tyr-Tic-Phe-OH) in 1992 was followed by extensive structure-activity relationship studies, leading to the development of analogues that are of interest as pharmacological tools or as potential therapeutic agents. Stable TIPP-derived delta opioid antagonists with subnanomolar delta receptor binding affinity and extraordinary delta receptor selectivity include TIPP[Psi] (H-Tyr-TicPsi[CH(2)NH]Phe-Phe-OH] and TICP[Psi] (H-Tyr-TicPsi[CH(2)NH]Cha-Phe-OH); Cha: cyclohexylalanine), which are widely used in opioid research. Theoretical conformational analyses in conjunction with the pharmacological characterization of conformationally constrained TIPP analogues led to a definitive model of the receptor-bound conformation of H-Tyr-Tic-(Phe-Phe)-OH-related delta opioid antagonists, which is characterized by all-trans peptide bonds. Further structure-activity studies revealed that the delta antagonist vs delta agonist behavior of TIP(P)-derived compounds depended on very subtle structural differences in diverse locations of the molecule and suggested a delta receptor model involving a number of different inactive receptor conformations. A further outcome of these studies was the identification of a new class of potent and very selective dipeptide delta agonists of the general formula H-Tyr-Tic-NH-X (X = arylalkyl), which are of interest for drug development because of their low molecular weight and lipophilic character. Most interestingly, TIPP analogues containing a C-terminal carboxamide group displayed a mixed mu agonist/delta antagonist profile, and thus were expected to be analgesics with a low propensity to produce tolerance and physical dependence. This turned out to be the case with the TIPP-derived mu agonist/delta antagonist DIPP-NH(2)[Psi] (H-Dmt-TicPsi[CH(2)NH]Phe-Phe-NH(2)); Dmt: 2',6'- dimethyltyrosine).

Subtleties of structure-agonist versus antagonist relationships of opioid peptides and peptidomimetics

The development of novel delta opioid antagonists and delta opioid agonists structurally derived from the prototype delta antagonist TIPP (H-Tyr-Tic-Phe-Phe-OH), is reviewed. Both delta antagonists and delta agonists with extraordinary potency and unprecedented delta receptor selectivity were discovered. Some of them are already widely used as pharmacological tools and are also of interest as potential therapeutic agents for use in analgesia. The results of the performed structure-activity studies revealed that the delta antagonist versus delta agonist behavior of this class of compounds depended on very subtle structural differences in diverse locations of the molecule. These observations can be best explained with a receptor model involving a number of different inactive and active receptor conformations.

Synthesis and binding characteristics of [3H] H-Tyr-Ticpsi[CH2-NH] Cha-Phe-OH, a highly specific and stable delta-opioid antagonist

Substitution of the Phe3 aromatic ring in H-Tyr-Ticpsi[CH2-NH]Phe-Phe-OH with cyclohexylalanine (Cha) has been reported to result in a compound, H-Tyr-Ticpsi[CH2-NH]Cha-Phe-OH (TICP[psi]), showing substantially increased delta-opioid antagonist potency and high delta selectivity. TICP[psi] was radiolabeled by catalytic tritiation of its precursor Tyr(3',5'-I2)1TICP[psi]. Binding characteristics of the new tritiated pseudopeptide were determined using the radioligand binding assay in rat brain membranes. On the basis of the results of saturation binding studies performed at 25 degrees C, an equilibrium dissociation constant (Kd) of 0.35 nM and a receptor density (Bmax) of 112 fmol/mg protein were calculated. This new tritiated ligand exhibits high affinity for delta-opioid receptors, whereas its binding to mu and kappa receptors is weak. A study of [H3]TICP[psi] binding displacement by various receptor-selective opioids showed the following rank order of potency: delta > kappa = mu. These receptor binding characteristics of the ligand, together with its high specific radioactivity (41.3 Ci/mmol) and stability, makes it a useful tool for labeling delta-opioid receptors, both in vitro and in vivo.

Slide-binding characterization and autoradiographic localization of delta opioid receptors in rat and mouse brains with the tetrapeptide antagonist [3H]TIPP

Slide-binding and autoradiographic studies were performed on cryostat sections from brains of adult Sprague-Dawley rats and BALB C mice to describe the binding characteristics of the tetrapeptide [3H]TIPP, an antagonist with high specificity and affinity for the delta opioid receptors. Steady-state binding of [3H]TIPP to cryostat sections of brain paste was reached in 120-180 min of incubation. Specific [3H]TIPP binding resulted in maximal numbers of binding sites (Bmax) of 15.59 and 23.91 fmol/mg protein, and dissociation constants (Kd) of 0.46 and 0.85 nM for rat and mouse brain paste sections, respectively. TIPP displayed the highest affinity for delta opioid receptors in inhibiting specific [3H]TIPP binding, with IC50 values of 0.82 nM and 0.14 nM in rat and mouse brain sections, respectively. While DPDPE was also effective in displacing the specific binding of [3H]TIPP (IC50 = 3.18 +/- 0.53 nM and 0.63 +/- 0.42 nM in rat and mouse brain paste sections, respectively), other subclass-selective or nonopioid ligands were much less effective, or ineffective. Autoradiographic localization of [3H]TIPP binding revealed the characteristic distribution of delta opioid receptors in both species. In consequence of its antagonistic nature, and of its unnatural amino acid residue, which makes this ligand more resistant to biodegradation, [3H]TIPP is a superior ligand for evaluation of the binding characteristics and autoradiogaphic distribution of the delta opioid receptors.

Activation of diketopiperazine formation by alkylammonium carboxylate salts and aprotic dipolar protophobic solvents

Diketopiperazine formation from the N-terminal residues of a peptide chain is accelerated by aprotic dipolar protophobic solvents and catalyzed in organic solvents by alkylammonium carboxylate salts. The t1/2 for the first-order reaction of H-Ala-Pro-NH2 x TFA falls from 20 d in methanol to 3.6 min in acetonitrile containing 0.02 mol dm(-3) triethylammonium acetate; for H-Ala-Ala-NH2 x TFA in the same reaction media t1/2 falls from an unmeasurably long time to 1.3 d.

Influence of sample pH on the conformational backbone dynamics of a pseudotripeptide (H-Tyr-Tic psi [CH2-NH]Phe-OH) incorporating a reduced peptide bond: an NMR investigation

In the present paper we investigate the influence of sample pH on the conformational and dynamical properties of the pseudotripeptide H-Tyr-Tic psi [CH2-NH]Phe-OH (TIP[psi]; Tic: 1,2,3,4,-tetrahydroisoquinoline-3-carboxylic acid) using various one- and two-dimensional nmr techniques in conjunction with molecular modeling. Studies were conducted at three different pH levels corresponding to the zwitterionic peptide containing a formal positive charge (pH 3.1), the deprotonated molecule (pH 9.1), and a situation at neutral pH (pH 7.2) involving both protonated and deprotonated states of the reduced peptide bond. Analysis of the one-dimensional1H-nmr spectra reveals that in solution TIP[psi] is in slow dynamic exchange between conformations containing cis and trans configurations of the Tyr-Tic bond. An nmr pH dependence study of the cis:trans ratio indicated that the exchange process was governed by the protonation state of the reduced bone amine. From the nmr data, reduced peptide bond pK alpha values of 6.5 and 7.5 were determined for the cis and trans conformers, respectively. It was concluded that conformations containing a trans Tyr-Tic bond are stabilized at low pH by an intramolecular hydrogen bond between the Tyr carbonyl and the reduced peptide bond protonated amine. This observation was corroborated by molecular mechanics investigations that revealed low energy trans structures compatible with nmr structural data, and furthermore, were consistently characterized by the existence of a strong N+ H ... O = C interaction closing a seven-membered cycle. The dynamics of cis-trans isomerization about the Tyr-Tic peptide bond were probed by nmr exchange experiments. The selective presaturation of exchanging resonances carried out at several temperatures between 50 and 70 degrees C allowed the determination of isomerization rate constants as well as thermodynamic activation parameters. delta G not equal to values were in close agreement with the cis-->trans energy barrier found in X-Pro peptide fragments (approximately 83 kJ/mol). A large entropic barrier determined for the trans-->cis conversion of TIP[psi] (5.7 JK-1 mol-1 at pH 3.1;6.5 J K-1 mol-1 at pH 9.1) is discussed in terms of decreased solvent molecular ordering around the conformers possessing a trans Tyr-Tic bond. Evidence that the neutral form of the reduced peptide bond gains rigidity upon protonation was obtained from relaxation measurements in the rotating frame. T1 rho measurements of several protons in the vicinity of the reduced peptide bond were made as a function of spin-lock field.(ABSTRACT TRUNCATED AT 400 WORDS)

Solution and solid-state structure of the diketopiperazine of tyrosyl-tetrahydroisoquinoline-3-carboxylic acid

delta-Selective antagonism of [L-Tic2]-peptides, including the simple dipeptide Tyr-L-Tic-NH2, is linked to the Tyr-Tic-"recognition site". In order to gain further information on the conformational preferences of the Tyr-Tic-moiety we have undertaken a structural study of a cyclic analog, the diketopiperazine of Tyr-Tic. A conformational study of cyclo[-Tyr-Tic-], that is almost devoid of opioid activity, can also be useful to discriminate between the role of the two aromatic rings and of the basic nitrogen in determining antagonism. The structure of cyclo[-Tyr-Tic-] has been solved in a DMSO/water solution at 278 K by NMR spectroscopy and in the solid state by X-ray diffraction methods. The two informations are almost identical, with an arrangement of the aromatic rings rather different from that of the putative bioactive conformation of the parent linear dipeptide. This difference points to the importance of conformational effects and is in agreement with the hypothesis that the positive center may be not essential for antagonism.

Acid catalysis in the formation of dioxopiperazines from peptides containing tetrahydroisoquinoline-3-carboxylic acid at position 2

The kinetics of the spontaneous formation of 2,5-dioxopiperazines from peptides containing the Tic (1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) residue in the 2-position of the sequence has been studied in DMSO and water solution. The reaction is first order in Tic-peptide and subject to general-acid catalysis. Moreover, only the fraction of peptide having the amino terminal group in the deprotonated state reacts with appreciable rate. In pure organic solvent, and in aqueous solution with low buffer concentration, the degradation reaction of Tic-peptides is very low; at 20 degrees C for the peptide H-Tyr-Tic-Phe-Phe-NH2, in DMSO and in neutral water in the absence of buffer, the half-lives (t1/2) are 3 x 10(4) and 1.2 x 10(4) h, respectively. The addition of carboxylic acids or buffers to the reaction solutions markedly increases the reaction rate; in 0.01 m HAc in DMSO and in 0.1 M phosphate buffer in water, pH 7.1, t1/2 values for the tetrapeptide are 61 and 121 h, respectively.

Synthesis and binding characteristics of tritiated TIPP[psi], a highly specific and stable delta opioid antagonist

The pseudopeptide H-Tyr-Tic psi [CH2-NH]Phe-Phe-OH (TIPP[psi]) is a delta opioid antagonist with high delta receptor affinity and unprecedented delta selectivity. TIPP[psi] was radiolabelled by catalytic tritiation of its precursor [Tyr(3',5'-I2)1]TIPP[psi]. The resulting radioligand, [3H]TIPP[psi], had a specific activity of 1.77 TBq/mmol (47.9 Ci/mmol) and showed high stability against enzymatic degradation. [3H]TIPP[psi] binding to rat brain membranes was saturable and Scatchard analysis indicated a single binding site with a Kd of 0.98 nM and a Bmax of 105.4 fmol/mg. A study of [3H]TIPP[psi] binding displacement by various receptor-selective opioids showed the expected rank order of potency (delta >> mu > kappa). [3H]TIPP[psi] represents an excellent new radioligand for delta receptor labelling studies in vitro and in vivo.

Comparative analysis of various proposed models of the receptor-bound conformation of H-Tyr-Tic-Phe-OH related delta-opioid antagonists

A molecular mechanics study (grid search and energy minimization) was performed with six delta opioid peptide antagonists containing a tetrahydroisoquinoline-3-carboxylic acid (Tic) residue in the 2-position of the peptide sequence. Compounds examined were the highly potent and selective TIP(P) peptides H-Tyr-Tic-Phe-OH (TIP), H-Tyr-Tic psi[CH2-NH]Phe-OH (TIP[psi]), H-Tyr-Tic-Phe-Phe-OH (TIPP), and H-Tyr-Tic psi[CH2-NH]Phe-Phe-OH (TIPP[psi]), and the weakly active analogues H-Tyr-Tic-NH2 and H-Tyr-Tic-Ala-OH. Low energy conformers of the peptides were examined for their compatibility with three proposed models of the delta receptor-bound conformation. Model 1, based on spatial overlap of the Tyr1 and Phe3 aromatic rings and N-terminal amino group of the peptides with the corresponding aromatic rings and nitrogen atom of the nonpeptide delta-antagonist naltrindole, was ruled out because of the demonstrated importance of the Tic2 aromatic ring for delta antagonism and because of the somewhat elevated energies of the conformers consistent with this model. Models of the receptor-bound conformation based on superimposition of the Tyr1 and Tic2 aromatic rings and N-terminal amino group of the peptides with the corresponding moieties in naltrindole included an all-trans peptide bond conformer [model 2, proposed by B.C. Wilkes and P.W. Schiller (1994) Biopolymers, Vol. 34, pp. 1213-1219] and a conformer with a cis peptide bond between the Tyr1 and Tic2 residues (model 3, originally proposed by P.A. Temussi et al. [(1994) Biochemical and Biophysical Research Communications, Vol. 198, pp. 933-939]. For all six peptides low energy conformers consistent with both model 2 and model 3 were identified; however, peptide conformers corresponding to model 2 showed better coplanarity of the Tyr1 aromatic ring and the phenol ring in naltrindole than peptide conformers corresponding to model 3. Both models remain plausible candidate structures for the receptor-bound conformation of delta antagonists of the TIP(P) class. TIP(P) analogues containing additional conformational constraints need to be developed in order to arrive at a unique model.