Synthesis of a biologically active triazole-containing analogue of cystatin A through successive peptidomimetic alkyne-azide ligations

Protein click chemistry and its potential for medical applications

A revolution in chemical biology occurred with the introduction of click chemistry. Click chemistry plays an important role in protein chemistry modifications, providing specific, sensitive, rapid, and easy-to-handle methods. Under physiological conditions, click chemistry often overlaps with bioorthogonal chemistry, defined as reactions that occur rapidly and selectively without interfering with biological processes. Click chemistry is used for the posttranslational modification of proteins based on covalent bond formations. With the contribution of click reactions, selective modification of proteins would be developed, representing an alternative to other technologies in preparing new proteins or enzymes for studying specific protein functions in different biological processes. Click-modified proteins have potential in diverse applications such as imaging, labeling, sensing, drug design, and enzyme technology. Due to the promising role of proteins in disease diagnosis and therapy, this review aims to highlight the growing applications of click strategies in protein chemistry over the last two decades, with a special emphasis on medicinal applications.

Triazole-Modified Peptidomimetics: An Opportunity for Drug Discovery and Development

Peptidomimetics play a fundamental role in drug design due to their preferential properties regarding natural peptides. In particular, compounds possessing nitrogen-containing heterocycles have been intensively studied in recent years. The triazolyl moiety incorporation decreases the molecule susceptibility to enzymatic degradation, reduction, hydrolysis, and oxidation. In fact, peptides containing triazole rings are a typical example of peptidomimetics. They have all the advantages over classic peptides. Both efficient synthetic methods and biological activity make these systems an interesting and promising object of research. Peptide triazole derivatives display a diversity of biological properties and can be obtained via numerous synthetic strategies. In this review, we have highlighted the importance of the triazole-modified peptidomimetics in the field of drug design. We present an overview on new achievements in triazolyl-containing peptidomimetics synthesis and their biological activity as inhibitors of enzymes or against cancer, viruses, bacteria, or fungi. The relevance of above-mentioned compounds was confirmed by their comparison with unmodified peptides.

1,2,3-Triazoles as Biomimetics in Peptide Science

Natural peptides are an important class of chemical mediators, essential for most vital processes. What limits the potential of the use of peptides as drugs is their low bioavailability and enzymatic degradation in vivo. To overcome this limitation, the development of new molecules mimicking peptides is of great importance for the development of new biologically active molecules. Therefore, replacing the amide bond in a peptide with a heterocyclic bioisostere, such as the 1,2,3-triazole ring, can be considered an effective solution for the synthesis of biologically relevant peptidomimetics. These 1,2,3-triazoles may have an interesting biological activity, because they behave as rigid link units, which can mimic the electronic properties of amide bonds and show bioisosteric effects. Additionally, triazole can be used as a linker moiety to link peptides to other functional groups.

1,5-Disubstituted 1,2,3-Triazoles as Amide Bond Isosteres Yield Novel Tumor-Targeting Minigastrin Analogs

1,5-Disubstituted 1,2,3-triazoles (1,5-Tz) are considered bioisosteres of cis-amide bonds. However, their use for enhancing the pharmacological properties of peptides or proteins is not yet well established. Aiming to illustrate their utility, we chose the peptide conjugate [Nle15]MG11 (DOTA-dGlu-Ala-Tyr-Gly-Trp-Nle-Asp-Phe-NH2) as a model compound since it is known that the cholecystokinin-2 receptor (CCK2R) is able to accommodate turn conformations. Analogs of [Nle15]MG11 incorporating 1,5-Tz in the backbone were synthesized and radiolabeled with lutetium-177, and their pharmacological properties (cell internalization, receptor binding affinity and specificity, plasma stability, and biodistribution) were evaluated and compared with [Nle15]MG11 as well as their previously reported analogs bearing 1,4-disubstituted 1,2,3-triazoles. Our investigations led to the discovery of novel triazole-modified analogs of [Nle15]MG11 with nanomolar CCK2R-binding affinity and 2-fold increased tumor uptake. This study illustrates that substitution of amides by 1,5-disubstituted 1,2,3-triazoles is an effective strategy to enhance the pharmacological properties of biologically active peptides.

1,4-Disubstituted 1,2,3-Triazoles as Amide Bond Surrogates for the Stabilisation of Linear Peptides with Biological Activity

Peptides represent an important class of biologically active molecules with high potential for the development of diagnostic and therapeutic agents due to their structural diversity, favourable pharmacokinetic properties, and synthetic availability. However, the widespread use of peptides and conjugates thereof in clinical applications can be hampered by their low stability in vivo due to rapid degradation by endogenous proteases. A promising approach to circumvent this potential limitation includes the substitution of metabolically labile amide bonds in the peptide backbone by stable isosteric amide bond mimetics. In this review, we focus on the incorporation of 1,4-disubstituted 1,2,3-triazoles as amide bond surrogates in linear peptides with the aim to increase their stability without impacting their biological function(s). We highlight the properties of this heterocycle as a trans-amide bond surrogate and summarise approaches for the synthesis of triazole-containing peptidomimetics via the Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC). The impacts of the incorporation of triazoles in the backbone of diverse peptides on their biological properties such as, e.g., blood serum stability and affinity as well as selectivity towards their respective molecular target(s) are discussed.

Chemo- and regioselective click reactions through nickel-catalyzed azide–alkyne cycloaddition

Nickel-catalyzed [3 + 2] cycloaddition reactions of unsymmetrical alkynes and organic azides afford substituted 1,2,3-triazoles with high levels of chemo- and regioselectivity.

Ligation Technologies for the Synthesis of Cyclic Peptides

Cyclic peptides have been attracting a lot of attention in recent decades, especially in the area of drug discovery, as more and more naturally occurring cyclic peptides with diverse biological activities have been discovered. Chemical synthesis of cyclic peptides is essential when studying their structure-activity relationships. Conventional peptide cyclization methods via direct coupling have inherent limitations, like the susceptibility to epimerization at the C-terminus, poor solubility of fully protected peptide precursors, and low yield caused by oligomerization. In this regard, chemoselective ligation-mediated cyclization methods have emerged as effective strategies for cyclic peptide synthesis. The toolbox for cyclic peptide synthesis has been expanded substantially in the past two decades, allowing more efficient synthesis of cyclic peptides with various scaffolds and modifications. This Review will explore different chemoselective ligation technologies used for cyclic peptide synthesis that generate both native and unnatural peptide linkages. The practical issues and limitations of different methods will be discussed. The advance in cyclic peptide synthesis will benefit the biological and medicinal study of cyclic peptides, an important class of macrocycles with potentials in numerous fields, notably in therapeutics.

Semisynthesis of an Active Enzyme by Quantitative Click Ligation

The incorporation of clickable noncanonical amino acids (ncAAs) has proven to an invaluable tool in chemical biology and protein science research. Nevertheless, the number of examples in which the method is used for preparative purposes is extremely limited. We report the synthesis of an active enzyme by quantitative, Cu(I)-catalyzed ligation of two inactive protein halves, expressed and equipped with an azide and alkyne moiety, respectively, through ncAA incorporation. The reported quantitative conversion is exceptional given the large size of the protein fragments and the fact that no linker or excess of either of the polypeptides was used. The triazole bridge formed between the ncAA side chains was shown to effectively mimic a natural protein loop, providing an enzyme with the same activity as its natural counterpart. We envision that this strategy, termed split-click protein chemistry, can be used for the production of proteins that are difficult to express as full-length entities. It also paves the way for the design of new proteins with tailor-made functionalities.

How are 1,2,3-triazoles accommodated in helical secondary structures?

1,4-Disubstituted-1,2,3-triazole (Tz) is widely used in peptides as a trans-amide bond mimic, despite having hazardous effects on the native peptide activity. The impact of amide bond substitution by Tz on peptide secondary structures is scarcely documented. We performed a Tz scan, by systematically replacing peptide bonds following the Aib residues with Tz on two model peptaibols: alamethicin F50/5 and bergofungin D, which adopt stable α- and 310 helices, respectively. We observed that the Tz insertion, whatever its position in the peptide sequences, abolished their antimicrobial activity. The structural consequences of this insertion were further investigated using CD, NMR and X-ray diffraction. Importantly, five crystal structures that were incorporated with Tz were solved, showing various degrees of alteration of the helical structures, from minor structural perturbation of the helix to partial disorder. Together, these results showed that Tz insertions impair helical secondary structures.

Novel protein science enabled by total chemical synthesis

Chemical synthesis is a well-established method for the preparation in the research laboratory of multiple-tens-of-milligram amounts of correctly folded, high purity protein molecules. Chemically synthesized proteins enable a broad spectrum of novel protein science. Racemic mixtures consisting of d-protein and l-protein enantiomers facilitate crystallization and determination of protein structures by X-ray diffraction. d-Proteins enable the systematic development of unnatural mirror image protein molecules that bind with high affinity to natural protein targets. The d-protein form of a therapeutic target can also be used to screen natural product libraries to identify novel small molecule leads for drug development. Proteins with novel polypeptide chain topologies including branched, circular, linear-loop, and interpenetrating polypeptide chains can be constructed by chemical synthesis. Medicinal chemistry can be applied to optimize the properties of therapeutic protein molecules. Chemical synthesis has been used to redesign glycoproteins and for the a priori design and construction of covalently constrained novel protein scaffolds not found in nature. Versatile and precise labeling of protein molecules by chemical synthesis facilitates effective application of advanced physical methods including multidimensional nuclear magnetic resonance and time-resolved FTIR for the elucidation of protein structure-activity relationships. The chemistries used for total synthesis of proteins have been adapted to making artificial molecular devices and protein-inspired nanomolecular constructs. Research to develop mirror image life in the laboratory is in its very earliest stages, based on the total chemical synthesis of d-protein forms of polymerase enzymes.

Synthesis of aryl-thioglycopeptides through chemoselective Pd-mediated conjugation

We describe herein a Pd-catalyzed methodology for the thioglycoconjugation of iodoaryl peptides and aminoacids. This operationally simple process occurs under semi-aqueous conditions and displays wide substrate scope. The strategy has been successfully applied to both the thioglycosylation of unprotected peptides and the generation of thioglyco-aminoacid building blocks, including those suitable for solid phase peptide synthesis. To demonstrate the broad potential of this technique for late stage functionalization, we successfully incorporated challenging unprotected β-S-GlcNAc- and α-S-GalNAc-derivatives into very long unprotected peptides. This study opens the way to new applications in chemical biology, considering the well-recognized advantages of S-glycosides over O-glycosides in terms of resistance towards both enzymatic and chemical degradation.

Design, Synthesis, and Immunological Evaluation of a Multicomponent Construct Based on a Glycotripeptoid Core Comprising B and T Cell Epitopes and a Toll-like Receptor 7 Agonist That Elicits Potent Immune Responses

We present here for the first time the synthesis and immunological evaluation of a fully synthetic three-component anticancer vaccine candidate that consists of a β-glycotripeptoid core mimicking a cluster of Tn at the surface of tumor cells (B epitope), conjugated to the OVA 323-339 peptide (T-cell epitope) and a Toll-like receptor 7 (TLR7) agonist for potent adjuvanticity. The immunological evaluation of this construct and of precursor components demonstrated the synergistic activity of the components within the conjugate to stimulate innate and adaptive immune cells (DCs, T-helper, and B-cells). Surprisingly, immunization of mice with the tricomponent GalNAc-based construct elicited a low level of anti-Tn IgG but elicited a very high level of antibodies that recognize the TLR7 agonist. This finding could represent a potential vaccine therapeutic approach for the treatment of some autoimmune diseases such as lupus.

Synthesis of novel cyclopeptides containing heterocyclic skeletons

Cyclopeptides can be considered as naturally biologically active compounds. Over the last several decades, many attempts have been made to synthesize complex naturally occurring cyclopeptides, and great progress has been achieved to advance the field of total synthesis. Moreover, cyclopeptides containing heterocyclic skeletons have been recently developed into powerful reactions and approaches. This review aims to highlight recent advances in the synthesis of cyclopeptides containing heterocyclic skeletons such as triazole, oxazole, thiazole, and tetrazole.

Foldamer Tertiary Structure through Sequence-Guided Protein Backbone Alteration

The prospect of recreating the complex structural hierarchy of protein folding in synthetic oligomers with backbones that are artificial in covalent structure ("foldamers") has long fascinated chemists. Foldamers offer complex functions from biostable scaffolds and have found widespread applications in fields from biomedical to materials science. Most precedent has focused on isolated secondary structures or their assemblies. In considering the goal of complex protein-like tertiary folding patterns, a key barrier became apparent. How does one design a backbone with covalent connectivity and a sequence of side-chain functional groups that will support defined intramolecular packing of multiple artificial secondary structures? Two developments were key to overcoming this challenge. First was the recognition of the power of blending α-amino acid residues with monomers differing in backbone connectivity to create "heterogeneous-backbone" foldamers. Second was the finding that replacing some of the natural α-residues in a biological sequence with artificial-backbone variants can result in a mimic that retains both the fold and function of the native sequence and, in some cases, gains advantageous characteristics. Taken together, these precedents lead to a view of a protein as chemical entity having two orthogonal sequences: a sequence of side-chain functional groups and a separate sequence of backbone units displaying those functional groups. In this Account, we describe our lab's work over the last ∼10 years to leverage the above concept of protein sequence duality in order to develop design principles for constructing heterogeneous-backbone foldamers that adopt complex protein-like tertiary folds. Fundamental to the approach is the utilization of a variety of artificial building blocks (e.g., d-α-residues, Cα-Me-α-residues, N-Me-α-residues, β-residues, γ-residues, δ-residues, polymer segments) in concert, replacing a fraction of α-residues in a given prototype sequence. We provide an overview of the state-of-the-art in terms of design principles for choosing substitutions based on consideration of local secondary structure and retention of key side-chain functional groups. We survey high-resolution structures of backbone-modified proteins to illustrate how diverse artificial moieties are accommodated in tertiary fold contexts. We detail efforts to elucidate how backbone alteration impacts folding thermodynamics and describe how such data informs the development of improved design rules. Collectively, a growing body of results by our lab and others spanning multiple protein systems suggests there is a great deal of plasticity with respect to the backbone chemical structures upon which sequence-encoded tertiary folds can manifest. Moreover, these efforts suggest sequence-guided backbone alteration as a broadly applicable strategy for generating foldamers with complex tertiary folding patterns. We conclude by offering some perspective regarding the near future of this field, in terms of unanswered questions, technological needs, and opportunities for new areas of inquiry.

Towards new strategies to manage livestock reproduction using kisspeptin analogs

The discovery of the hypothalamic neuropeptide kisspeptin and its receptor (KISS1R) have dramatically improved our knowledge about the central mechanisms controlling reproduction. Kisspeptin neurons could be considered the hub where internal and external information controlling reproduction converge. The information is here elaborated and the command dispatched to GnRH neurons, the final output of the brain system controlling reproduction. Several studies have shown that in mammals administration of kisspeptin could finely modulate many aspects of reproduction from puberty to ovulation. For example in ewes kisspeptin infusion triggered ovulation during the non-breeding season and in prepubertal rat repeated injections advanced puberty onset. However, especially in livestock, the suboptimal pharmacological properties of endogenous kisspeptin, notably it short half-life and consequently its poor pharmacodynamics, fetters its use to experimental setting. To overcome this issue synthetic KISS1R agonists, mainly based on kisspeptin backbone, were created. Their more favorable pharmacological profile, longer half-life and duration of action, allowed to perform promising initial experiments for controlling ovulation and puberty. Additional experiments and further refinement of analogs would still be necessary to exploit fully the potential of targeting the kisspeptin system. Nevertheless, it is already clear that this new strategy may represent a breakthrough in the field of reproduction control.

A chiral cobalt(ii) complex catalyzed asymmetric formal [3+2] cycloaddition for the synthesis of 1,2,4-triazolines

A chiral N,N′-dioxide/Co(BF₄)₂·6H₂O complex catalysed highly efficient asymmetric [3+2] cycloaddition reaction of 5-alkoxyoxazoles with azodicarboxylate compounds.

Crystal-to-Crystal Synthesis of Triazole-Linked Pseudo-proteins via Topochemical Azide-Alkyne Cycloaddition Reaction

Isosteric replacement of amide bond(s) of peptides with surrogate groups is an important strategy for the synthesis of peptidomimetics (pseudo-peptides). Triazole is a well-recognized bio-isostere for peptide bonds, and peptides with one or more triazole units are of great interest for different applications. We have used a catalyst-free and solvent-free method, viz., topochemical azide-alkyne cycloaddition (TAAC) reaction, to synthesize pseudo-proteins with repeating sequences. A designed β-sheet-forming l-Ala-l-Val dipeptide containing azide and alkyne at its termini (N₃-Ala-Val-NHCH₂C≡CH, 1) was synthesized. Single-crystal XRD analysis of the dipeptide 1 showed parallel β-sheet arrangement along the b-direction and head-to-tail arrangement of such β-sheets along the c-direction. This head-to-tail arrangement along the c-direction places the complementary reacting motifs, viz., azide and alkyne, of adjacent molecules in proximity. The crystals of dipeptide 1, upon heating at 85 °C, underwent crystal-to-crystal polymerization, giving 1,4-triazole-linked pseudo-proteins. This TAAC polymerization was investigated by various time-dependent techniques, such as NMR, IR, DSC, and PXRD. The crystal-to-crystal nature of this transformation was revealed from polarizing microscopy and PXRD experiments, and the regiospecificity of triazole formation was evidenced from various NMR techniques. The MALDI-TOF spectrum showed the presence of pseudo-proteins >7 kDa.

Toward the Optimization of Bombesin-Based Radiotracers for Tumor Targeting

The peptide bombesin (BBN) is a peptide with high affinity for the gastrin-releasing peptide receptor (GRPr), a receptor that is overexpressed by, for example, breast and prostate cancers. Thus, GRPr agonists can be used as cancer-targeting vectors to shuttle diagnostic and therapeutic agents into tumor cells. With the aim of optimizing the tumor targeting properties of a radiolabeled [Nle(14)]BBN(7-14) moiety, novel BBN(7-14)- and BBN(6-14)-based radioconjugates were synthesized, labeled with Lu-177, and fully evaluated in vitro and in vivo. The effect of residue and backbone modification on several parameters such as the internalization of the radiolabeled peptides into PC3 and AR42J tumor cells, their affinity toward the human GRPr, metabolic stability in blood plasma, and biodistribution in mice bearing GRPr-expressing PC3 xenografts was studied. As a result of our investigations, a novel radiolabeled GRPr agonist with a high tumor uptake and a high tumor-to-kidney ratio was identified.

Radiolabeled analogs of neurotensin (8–13) containing multiple 1,2,3-triazoles as stable amide bond mimics in the backbone

Substitution of multiple amide bonds by metabolically stable 1,2,3-triazoles yields novel tumour-targeting neurotensin-based peptidomimetics with interesting biological properties.

Synthesis and anticancer evaluation of complex unsaturated isosteviol-derived triazole conjugates

BACKGROUND

For the last two decades, diterpenoid isosteviol and its derivatives have gained significant attention for novel chemical transformation in the drug discovery field.

RESULTS

An efficient way towards the synthesis of structurally diverse isosteviol derivatives was described here employing unsaturated functionalities as attractive templates for further transformation such as epoxidation. These structurally diverse compounds exhibited promising cytotoxic activities on different types of cancer cell lines, leading to drug discovery derived from natural products for the treatment of cancer.

CONCLUSION

In this work, novel isosteviol derivatives with Michael acceptors were synthesized to expand the diversity and complexity of a class of isosteviol-derived triazole conjugates to facilitate the development of potential antitumor agents.

Antimicrobial peptide shows enhanced activity and reduced toxicity upon grafting to chitosan polymers

Here we report that grafting of a short antimicrobial peptide, anoplin, to chitosan polymers is a strategy for abolishing the hemolytic propensity, and at the same time increasing the activity of the parent peptide. Anoplin-chitosan conjugates were synthesized by CuAAC reaction of multiple peptides through 2-azidoacetyl groups on chitosan.

Combining triazole ligation and enzymatic glycosylation on solid phase simplifies the synthesis of very long glycoprotein analogues

The solid-phase chemical assembly of a protein through iterative chemoselective ligation of unprotected peptide segments can be followed with chemical and/or enzymatic transformations of the resulting immobilized protein, the latter steps thus benefitting from the advantages provided by the solid support. We demonstrate here the usefulness of this strategy for the chemo-enzymatic synthesis of glycoprotein analogues. A linker was specifically designed for application to the synthesis of O-glycoproteins: this new linker is readily cleaved under mild aqueous conditions compatible with very sensitive glycosidic bonds, but is remarkably stable under a wide range of chemical and biochemical conditions. It was utilized for solid-supported N-to-C peptidomimetic triazole ligation followed by enzymatic glycosylation, ultimately leading to a very large MUC1-derived glycoprotein containing 160 amino acid residues, 24 α-GalNAc moieties linked to Ser and Thr, and 3 triazoles as peptide bond mimetics.

Comparison of backbone modification in protein β-sheets by α→γ residue replacement and α-residue methylation

The mimicry of protein tertiary structure by oligomers with unnatural backbones is a significant contemporary research challenge. Among common elements of secondary structure found in natural proteins, sheets have proven the most difficult to address. Here, we report the systematic comparison of different strategies for peptide backbone modification in β-sheets with the goal of identifying the best method for replacing a multi-stranded sheet in a protein tertiary fold. The most effective sheet modifications examined led to native-like tertiary folding behavior with a thermodynamic folded stability comparable to the prototype protein on which the modified backbones are based.

Rational design of triazololipopeptides analogs of kisspeptin inducing a long-lasting increase of gonadotropins

New potent and selective KISS1R agonists were designed using a combination of rational chemical modifications of the endogenous neuropeptide kisspeptin 10 (KP10). Improved resistance to degradation and presumably reduced renal clearance were obtained by introducing a 1,4-disubstituted 1,2,3-triazole as a proteolysis-resistant amide mimic and a serum albumin-binding motif, respectively. These triazololipopeptides are highly potent full agonists of KISS1R and are >100 selective over the closely related NPFF1R. When injected in ewes with a quiescent reproductive system, the best compound of our series induced a much prolonged increase of luteinizing hormone release compared to KP10 and increased follicle-stimulating hormone plasma concentration. Hence, this KISS1R agonist is a new valuable pharmacological tool to explore the potential of KP system in reproduction control. Furthermore, it represents the first step to develop drugs treating reproductive system disorders due to a reduced activity of the hypothalamo-pituitary-gonadal axis such as delayed puberty, hypothalamic amenorrhea, and hypogonadotropic hypogonadism.

Protein backbone engineering as a strategy to advance foldamers toward the frontier of protein-like tertiary structure

A variety of non-biological structural motifs have been incorporated into the backbone of natural protein sequences. In parallel work, diverse unnatural oligomers of de novo design (termed "foldamers") have been developed that fold in defined ways. In this Perspective article, we survey foundational studies on protein backbone engineering, with a focus on alterations made in the context of complex tertiary folds. We go on to summarize recent work illustrating the potential promise of these methods to provide a general framework for the construction of foldamer mimics of protein tertiary structures.

Design and synthesis of isosteviol triazole conjugates for cancer therapy

One of the keys for successfully developing drugs against the broad spectrum of cancer cell types is structural diversity. In the current study, we focused on a family of isosteviol derivatives as potential novel antitumor agents. Isosteviol is a tetracyclic diterpenoid obtained by acid hydrolysis of steviol glycoside extracts isolated from abundant Stevia rebaudiana plants. In this work, we have designed and synthesized a panel of isosteviol triazole conjugates using "click" chemistry methodology. Evaluation of these compounds against a series of cancer cell lines derived from primary and metastatic tumors demonstrated that these conjugates exhibit cytotoxic activities with IC50 in the low μM range. In addition, their anti-proliferative activities are cancer cell type specific. Taken together, our studies underscore the importance of structural diversity in achieving cancer cell type specific drug development.

Constraining cyclic peptides to mimic protein structure motifs

Many proteins exert their biological activities through small exposed surface regions called epitopes that are folded peptides of well-defined three-dimensional structures. Short synthetic peptide sequences corresponding to these bioactive protein surfaces do not form thermodynamically stable protein-like structures in water. However, short peptides can be induced to fold into protein-like bioactive conformations (strands, helices, turns) by cyclization, in conjunction with the use of other molecular constraints, that helps to fine-tune three-dimensional structure. Such constrained cyclic peptides can have protein-like biological activities and potencies, enabling their uses as biological probes and leads to therapeutics, diagnostics and vaccines. This Review highlights examples of cyclic peptides that mimic three-dimensional structures of strand, turn or helical segments of peptides and proteins, and identifies some additional restraints incorporated into natural product cyclic peptides and synthetic macrocyclic peptidomimetics that refine peptide structure and confer biological properties.

A bombesin-shepherdin radioconjugate designed for combined extra- and intracellular targeting

Radiolabeled peptides which target tumor-specific membrane structures of cancer cells represent a promising class of targeted radiopharmaceuticals for the diagnosis and therapy of cancer. A potential drawback of a number of reported radiopeptides is the rapid washout of a substantial fraction of the initially delivered radioactivity from cancer cells and tumors. This renders the initial targeting effort in part futile and results in a lower imaging quality and efficacy of the radiotracer than achievable. We are investigating the combination of internalizing radiopeptides with molecular entities specific for an intracellular target. By enabling intracellular interactions of the radioconjugate, we aim at reducing/decelerating the externalization of radioactivity from cancer cells. Using the “click-to-chelate” approach, the ^99mTc-tricarbonyl core as a reporter probe for single-photon emission computed tomography (SPECT) was combined with the binding sequence of bombesin for extracellular targeting of the gastrin-releasing peptide receptor (GRP-r) and peptidic inhibitors of the cytosolic heat shock 90 protein (Hsp90) for intracellular targeting. Receptor-specific uptake of the multifunctional radioconjugate could be confirmed, however, the cellular washout of radioactivity was not improved. We assume that either endosomal trapping or lysosomal degradation of the radioconjugate is accountable for these observations.

Sequential "click" - "photo-click" cross-linker for catalyst-free ligation of azide-tagged substrates

Heterobifunctional linker allows for selective catalyst-free ligation of two different azide-tagged substrates via strained-promoted azide-alkyne cycloaddition (SPAAC). The linker contains an azadibenzocyclooctyne (ADIBO) moiety on one end and a cyclopropenone-masked dibenzocyclooctyne (photo-DIBO) group on the other. The first azide-derivatized substrate reacts only at the ADIBO end of the linker as the photo-DIBO moiety is azide-inert. After the completion of the first SPAAC step, photo-DIBO is activated by brief exposure to 350 nm light from a fluorescent UV lamp. The unmasked DIBO group then reacts with the second azide-tagged substrate. Both click reactions are fast (k = 0.4 and 0.07 M(-1) s(-1), respectively) and produce quantitative yield of ligation in organic solvents or aqueous solutions. The utility of the new cross-linker has been demonstrated by conjugation of azide functionalized bovine serum albumin (azido-BSA) with azido-fluorescein and by the immobilization of the latter protein on azide-derivatized silica beads. The BSA-bead linker was designed to incorporate hydrolytically labile fragment, which permits release of protein under the action of dilute acid. UV activation of the second click reaction permits spatiotemporal control of the ligation process.