5-Hydroxypyrroloindoline Affords Tryptathionine and 2,2'-bis-Indole Peptide Staples: Application to Melanotan-II.

With peptides increasingly favored as drugs, natural product motifs, namely the tryptathionine staple, found in amatoxins and phallotoxins, and the 2,2'-bis-indole found in staurosporine represent unexplored staples for unnatural peptide macrocycles. We disclose the efficient condensation of a 5-hydroxypyrroloindoline with either a cysteine-thiol or a tryptophan-indole to form a tryptathionine or 2-2'-bis-indole staple. Judicious use of protecting groups provides for chemoselective stapling using α-MSH, which provides a basis for investigating both chemoselectivity and affinity. Both classes of stapled peptides show nanomolar Ki's, with one showing a sub-nanomolar Ki value.

Diphenylphosphinylhydroxylamine (DPPH) Affords Late-Stage S-imination to access free-NH Sulfilimines and Sulfoximines.

Sulfilimines, as potential aza-isosteres of sulfoxides, are valued as building blocks, auxiliaries, ligands, bioconjugation handles, and as precursors to versatile S(VI) scaffolds including sulfoximines and sulfondiimines. Here, we report a thioether imination methodology that exploits O-(diphenylphosphinyl)hydroxyl amine (DPPH). Under mild, metal-free, and biomolecule-compatible conditions, DPPH enables late-stage S-imination on peptides, natural products, and a clinically trialled drug, and shows both excellent chemoselectivity and broad functional group tolerance. This methodological report is extended to an efficient and high-yielding one-pot reaction for accessing free-NH sulfoximines with diverse substrates including ones of potential clinical importance. In the presence of a rhodium catalyst, sulfoxides are S-iminated in higher yields to afford free-NH sulfoximines. S-imination was validated on an oxidatively delicate amatoxin to give sulfilimine and sulfoximine congeners. Interestingly, these new sulfilimine and sulfoximine-amatoxins show cytotoxicity. This method is further extended to create sulfilimine and sulfoximine-Fulvestrant and buthionine analogues.

Toward tryptathionine-stapled one-bead-one-compound (OBOC) libraries: solid phase synthesis of a bioactive octretoate analog.

New somatostatin analogs are highly desirable for diagnosing and treating neuroendocrine tumors (NETs). Here we describe the solid-phase synthesis of a new octreotate (TATE) analog where the disulfide bond is replaced with a tryptathionine (Ttn) staple as part of an effort to prototyping a one-bead-one-compound (OBOC) library of Ttn-stapled peptides. Library design provides the potential for on- and off-bead screening. To validate our method, we labelled Ttn-TATE with a fluorescent dye to demonstrate binding to soluble somatostatin receptor subtype-2 and staining of Ar42J rat prostate cancer cells. By exploring this staple in the context of a ligand of known affinity, this method paves the way for an OBOC library construction of bioactive octreotate analogs and, more broadly speaking, tryptathionine-staped peptide macrocycles.

The "Ins and Outs" of Prostate Specific Membrane Antigen (PSMA) as Specific Target in Prostate Cancer Therapy.

Prostate-specific membrane antigen (PSMA) is expressed in epithelial cells of the prostate gland and is strongly upregulated in prostatic adenocarcinoma, with elevated expression correlating with metastasis, progression, and androgen independence. Because of its specificity, PSMA is a major target of prostate cancer therapy; however, detectable levels of PSMA are also found in other tissues, especially in salivary glands and kidney, generating bystander damage of these tissues. Antibody target therapy has been used with relative success in reducing tumor growth and prostate specific antigen (PSA) levels. However, since antibodies are highly stable in plasma, they have prolonged time in circulation and accumulate in organs with an affinity for antibodies such as bone marrow. For that reason, a second generation of PSMA targeted therapeutic agents has been developed. Small molecules and minibodies have had promising clinical trial results, but concerns about their specificity had arisen with side effects due to accumulation in salivary glands and kidneys. Herein we study the specificity of small molecules and minibodies that are currently being clinically tested. We observed a high affinity of these molecules for PSMA in prostate, kidney and salivary gland, suggesting that their effect is not prostate specific. The search for specific prostate target agents must continue so as to optimally treat patients with prostate cancer, while minimizing deleterious effects in other PSMA expressing tissues.

Imidazolium-methylene-trifluoroborate: A novel radioprosthetic group validated with preclinical 18 F-Positron Emission Tomography imaging of Prostate Specific Membrane Antigen in mice.

Organotrifluoroborates have gained acceptance as radioprosthetic groups for radiofluorination. Of these, the zwitterionic prosthetic group "AMBF3 " with a quaternary dimethylammonium ion dominates the trifluoroborate space. Herein, we report on imidazolium-methylene trifluoroborate (ImMBF3 ) as an alternative radioprosthetic group and report on its properties in the context of a PSMA-targeting EUK ligand that was previously been conjugated to AMBF3 . The ImMBF3 is readily synthesized from imidazole and conjugated via CuAAC "click" chemistry to give a structure similar to PSMA-617. 18 F-labeling proceeded in one step per our previous reports and imaged in LNCaP-xenograft bearing mice. The [18 F]-PSMA-617-ImMBF3 tracer proved to be less polar (LogP7.4 = -2.95 ± 0.03) while showing a significantly lower solvolytic rate (t1/2 = 8100 min) and slightly higher molar activity (Am) at 174 ± 38 GBq/µmol. Tumor uptake was measured at 13.7 ± 4.8%ID/g and a tumor:muscle ratio of 74.2 ± 35.0, tumor:blood ratio of 21.4 ± 7.0, tumor:kidney ratio of 0.29 ± 0.14, and tumor:bone ratio of 23.5 ± 9.5. In comparison with previously reported PSMA-targeting EUK-AMBF3 conjugates, we have altered the LogP7.4 value, tuned the solvolytic half-life of the prosthetic, and increased radiochemical conversion while achieving similar tumor uptake, contrast ratios, and molar activities compared with AMBF3 bioconjugates.

Salt Metathesis: Tetrafluoroborate Anion Rapidly Fluoridates Organoboronic Acids to give Organotrifluoroborates.

Tetrafluoroborate (BF4 - ) has long been used as a spectator counter anion. Herein, we report an unprecedented salt metathesis between a variety of BF4 - salts and a series of organoboronic acids yielding the corresponding organotrifluoroborates. We identified conditions for fast and efficient fluoridation (<1 h) with minimal workup. Fundamentally, this work discloses the proclivity of BF4 - to exchange fluoride atoms with organoboronates, highlighting the lability of BF4 - .

Selection of M2+ -Independent RNA-Cleaving DNAzymes with Side-Chains Mimicking Arginine and Lysine.

Sequence-specific cleavage of RNA by nucleic acid catalysts in the absence of a divalent metal cation (M2+ ) has remained an important goal in biomimicry with potential therapeutic applications. Given the lack of functional group diversity in canonical nucleotides, modified nucleotides with amino acid-like side chains were used to enhance self-cleavage rates at a single embedded ribonucleoside site. Previous works relied on three functional groups: an amine, a guanidine and an imidazole ensconced on three different nucleosides. However, to date, few studies have systematically addressed the necessity of all three modifications, as the value of any single modified nucleoside is contextualized at the outset of selection. Herein, we report on the use of only two modified dNTPs, excluding an imidazole, i. e. 5-(3-guanidinoallyl)-2'-dUTP (dUga TP) and 5-aminoallyl-2'-dCTP (dCaa TP), to select in-vitro self-cleaving DNAzymes that cleave in the absence of M2+ in a pH-independent fashion. Cleavage shows biphasic kinetics with rate constants that are significantly higher than in unmodified DNAzymes and compare favorably to certain DNAzymes involving an imidazole.

Design, Synthesis, and Biochemical Evaluation of Alpha-Amanitin Derivatives Containing Analogs of the trans-Hydroxyproline Residue for Potential Use in Antibody-Drug Conjugates.

Alpha-amanitin, an extremely toxic bicyclic octapeptide extracted from the death-cap mushroom, Amanita phalloides, is a highly selective allosteric inhibitor of RNA polymerase II. Following on growing interest in using this toxin as a payload in antibody-drug conjugates, herein we report the synthesis and biochemical evaluation of several new derivatives of this toxin to probe the role of the trans-hydroxyproline (Hyp), which is known to be critical for toxicity. This structure activity relationship (SAR) study represents the first of its kind to use various Hyp-analogs to alter the conformational and H-bonding properties of Hyp in amanitin.

Scaling Amatoxin Synthesis with an Improved Route to (2S,3R,4R)-Dihydroxyisoleucine Exemplified by a Toxic, Clickable α-Amanitin Analogue.

Here we report a scalable synthesis of the key amino acid residue, (2S,3R,4R)-4,5-dihydroxyisoleucine (DHIle) in α-amanitin, that in turn enables the scalable synthesis of an equipotent analogue, Asn(N-ethylazide)-S,6'-dideoxy-α-amanitin, suitable for CuAAC conjugation to empower studies on therapeutic antibody-drug conjugates.

Chiral Benzothiazole Monofluoroborate Featuring Chiroptical and Oxygen-Sensitizing Properties: Synthesis and Photophysical Studies.

Advances in personalized medicine are prompting the development of multimodal agents, that is, molecules that combine properties promoting various diagnostic and therapeutic applications. General approaches exploit chemical conjugation of therapeutic agents with contrast agents or the design of multimodal nanoplatforms. Herein, we report the design of a single molecule that exhibits potential for different diagnostic modes as well as the ability to sensitize oxygen, thus offering potential for photodynamic therapy. Exceptionally, this work involves the synthesis and chiral resolution of an enantiomeric pair of chiral monofluoroborates that contain a stereogenic boron atom. Combining experimental and theoretical chiroptical studies allowed the unambiguous determination of their absolute configuration. Photophysical investigations established the ability of this compound to sensitize oxygen even in the absence of heavy atoms within its structure. The synthesis of a chiral benzothiazole monofluoroborate paves a way to multimodal diagnostic tools (fluorescence and nuclear imaging) while also featuring potential therapeutic applications owing to its ability to activate oxygen to its singlet state for use in photodynamic therapy.

Toward 18 F-Labeled Theranostics: A Single Agent that Can Be Labeled with 18 F, 64 Cu, or 177 Lu.

We report a single-molecule radiotracer that can be labeled independently with 18 F-fluoride or radiometals (64 Cu, 177 Lu) in a single step. A prostate-specific membrane antigen (PSMA)-targeting ligand, armed with both an organotrifluoroborate and a metal-chelator (DOTA), was designed to optionally afford 18 F-, 64 Cu- or 177 Lu-labeled products that were injected into mice bearing prostate cancer (LNCaP) xenografts. PET/CT images and ex vivo biodistribution data show high, specific tumor uptake irrespective of which radionuclide is used, thereby demonstrating a new approach to combining, in a single molecule, 18 F-labeling capabilities for PET imaging with radiometalation for potential imaging and therapeutic applications.

Rapid, High-Yielding Solid-Phase Synthesis of Cathepsin-B Cleavable Linkers for Targeted Cancer Therapeutics.

Antibody-drug conjugates (ADCs) constitute an emerging class of anticancer agents that deliver potent payloads selectively to tumors while avoiding systemic toxicity associated with conventional chemotherapeutics. Critical to ADC development is a serum-stable linker designed to decompose inside targeted cells thereby releasing the toxic payload. A protease-cleavable linker comprising a valine-citrulline (Val-Cit) motif has been successfully incorporated into three FDA-approved ADCs and is found in numerous preclinical candidates. Herein, we present a high-yielding and facile synthetic strategy for a Val-Cit linker that avoids extensive protecting group manipulation and laborious chromatography associated with previous syntheses and provides yields that are up to 10-fold higher than by standard methods. This method is easily scalable and takes advantage of cost-effective coupling reagents and high loading 2-chlorotrityl chloride (2-CTC) resin. Modularity allows for introduction of various conjugation handles in final stages of the synthesis. Facile access to such analogues serves to expand the repertoire of available enzymatically cleavable linkers for ADC generation. This methodology empowers a robust and facile library generation and future exploration into linker analogues containing unnatural amino acids as a selectivity tuning tool.

A Metal-Free DOTA-Conjugated 18F-Labeled Radiotracer: [18F]DOTA-AMBF3-LLP2A for Imaging VLA-4 Over-Expression in Murine Melanoma with Improved Tumor Uptake and Greatly Enhanced Renal Clearance.

DOTA is commonly used for radiometal chelation in molecular imaging. Yet in the absence of a radiometal, DOTA is hypothesized to promote renal clearance of 18F-labeled peptide tracers. In light of an increasing interest in the use of F18 for PET, here the effect of DOTA is evaluated for the first time with an 18F-labeled tracer and is found to significantly improve the quality of images acquired through positron emission tomography (PET). We chose to image the peptide LLP2A that recognizes the transmembrane protein very-late antigen 4 (VLA-4) that is overexpressed by many cancers. Since it is known that [18F]RBF3-PEG2-LLP2A derivatives gave low tumor uptake values and significant GI tract accumulation, this ligand thus represents an ideal means of testing the additive effects of a DOTA group on clearance while permitting a facile, user-friendly, one-step 18F-labeling. A newly designed RBF3-LLP2A bioconjugate with an appended DOTA moiety increased tumor uptake nearly 3-fold and reduced GI accumulation by more than 10-fold. The DOTA-AMBF3-PEG2-LLP2A was radiolabeled by isotope exchange and was purified by semiprep HPLC and C18 cartridge elution. Male C57BL/6J mice bearing B16-F10 melanoma tumors that overexpress the VLA-4 target were used to evaluate [18F]DOTA-AMBF3-PEG2-LLP2A using a combination of static and dynamic PET scans, biodistribution studies, and blocking controls at 1 h post injection (p.i.). The precursor peptide was synthesized and 18F-labeled to provide formulations with mean (±SD) radiochemical purities of 95.9 ± 1.8%, in radiochemical yields of 4.8 ± 2.9% having molar activities of 131.7 ± 50.3 GBq/µmol. In vivo static PET images of [18F]DOTA-AMBF3-PEG2-LLP2A provided clear tumor visualization, and biodistribution studies showed that tumor uptake was 9.46 ± 2.19% injected dose per gram of tissue (%ID/g) with high tumor/muscle and tumor/blood contrast ratios of ∼8 and ∼10, respectively. Blocking confirmed the specificity of [18F]DOTA-AMBF3-PEG2-LLP2A to VLA-4 in the tumor and the bone marrow. Dynamic PET showed clearance of [18F]DOTA-AMBF3-PEG2-LLP2A mainly via the renal pathway, wherein accumulation in the intestines was reduced 10-fold compared to our previously investigated LLP2A's, while spleen uptake was at levels similar to previously reported LLP2A-chelator radiotracers. [18F]DOTA-AMBF3-PEG2-LLP2A represents a promising VLA-4 radiotracer and provides key evidence as to how a DOTA appendage can significantly reduce GI uptake in favor of urinary excretion. Implications for the development of dual-isotope theranostics that exploit the use fluorine-18 for imaging and DOTA to chelate therapeutic metal cations for therapy are discussed.

Fluorescent Isoindole Crosslink (FlICk) Chemistry: A Rapid, User-friendly Stapling Reaction.

The stabilization of peptide secondary structure via stapling is a ubiquitous goal for creating new probes, imaging agents, and drugs. Inspired by indole-derived crosslinks found in natural peptide toxins, we employed ortho-phthalaldehydes to create isoindole staples, thus transforming inactive linear and monocyclic precursors into bioactive monocyclic and bicyclic products. Mild, metal-free conditions give an array of macrocyclic α-melanocyte-stimulating hormone (α-MSH) derivatives, of which several isoindole-stapled α-MSH analogues (Ki ≈1 nm) are found to be as potent as α-MSH. Analogously, late-stage intra-annular isoindole stapling furnished a bicyclic peptide mimic of α-amanitin that is cytotoxic to CHO cells (IC50 =70 µm). Given its user-friendliness, we have termed this approach FlICk (fluorescent isoindole crosslink) chemistry.

Synthesis of the Death-Cap Mushroom Toxin α-Amanitin.

α-Amanitin is an extremely toxic bicyclic octapeptide isolated from the death-cap mushroom, Amanita phalloides. As a potent inhibitor of RNA polymerase II, α-amanitin is toxic to eukaryotic cells. Recent interest in α-amanitin arises from its promise as a payload for antibody-drug conjugates. For over 60 years, A. phalloides has been the only source of α-amanitin. Here we report a synthesis of α-amanitin, which surmounts the key challenges for installing the 6-hydroxy-tryptathionine sulfoxide bridge, enantioselective synthesis of (2 S,3 R,4 R)-4,5-dihydroxy-isoleucine, and diastereoselective sulfoxidation.

Expanding the Balz-Schiemann Reaction: Organotrifluoroborates Serve as Competent Sources of Fluoride Ion for Fluoro-Dediazoniation.

The Balz-Schiemann reaction endures as a method for the preparation of (hetero)aryl fluorides yet is eschewed due to the need for harsh conditions or high temperatures along with the need to isolate potentially explosive diazonium salts. In a departure from these conditions, we show that various organotrifluoroborates (RBF3 - s) may serve as fluoride ion sources for solution-phase fluoro-dediazoniation in organic solvents under mild conditions. This methodology was successfully extended to a one-pot process obviating aryl diazonium salt isolation. Sterically hindered (hetero)anilines are fluorinated under unprecedentedly mild conditions in good-to-excellent yields. Taken together, this work expands the repertoire of RBF3 - s to act as fluorine ion sources in an update to the classic Balz-Schiemann reaction.

Melanoma Imaging Using 18F-Labeled α-Melanocyte-Stimulating Hormone Derivatives with Positron Emission Tomography.

Melanocortin 1 receptor (MC1R) is specifically expressed in the majority of melanomas, a leading cause of death related to skin cancers. Accurate staging and early detection is crucial in managing melanoma. Based on the α-melanocyte-stimulating hormone (αMSH) sequence, MC1R-targeted peptides have been studied for melanoma imaging, predominately for use with single-photon emission computed tomography, with few attempts made for positron emission tomography (PET). 18F is a commonly used PET isotope due to readily available cyclotron production, pure positron emission, and a favorable half-life (109.8 min). In this study, we aim to design and evaluate αMSH derivatives that enable radiolabeling with 18F for PET imaging of melanoma. We synthesized three imaging probes based on the structure of Nle4-cyclo[Asp5-His-d-Phe7-Arg-Trp-Lys10]-NH2 (Nle-CycMSHhex), with a Pip linker (CCZ01064), an Acp linker (CCZ01070), or an Aoc linker (CCZ01071). 18F labeling was enabled by an ammoniomethyl-trifluoroborate (AmBF3) moiety. In vitro competition binding assays showed subnanomolar inhibition constant ( Ki) values for all three peptides. The 18F radiolabeling was performed via a one-step 18F-19F isotope exchange reaction that resulted in high radiochemical purity (>95%) and good molar activity (specific activity) ranging from 40.7 to 66.6 MBq/nmol. All three 18F-labeled peptides produced excellent tumor visualization with PET imaging in C57BL/6J mice bearing B16-F10 tumors. The tumor uptake was 7.80 ± 1.77, 5.27 ± 2.38, and 5.46 ± 2.64% injected dose per gram of tissue (%ID/g) for [18F]CCZ01064, [18F]CCZ01070, and [18F]CCZ01071 at 1 h post-injection (p.i.), respectively. Minimal background activity was observed except for kidneys at 4.99 ± 0.20, 4.42 ± 0.54, and 13.55 ± 2.84%ID/g, respectively. The best candidate [18F]CCZ01064 was further evaluated at 2 h p.i., which showed increased tumor uptake at 11.96 ± 2.31%ID/g and further reduced normal tissue uptake. Moreover, a blocking study was performed for CCZ01064 at 1 h p.i., where tumor uptake was significantly reduced to 1.97 ± 0.60%ID/g, suggesting the tumor uptake was receptor mediated. In conclusion, [18F]CCZ01064 showed high tumor uptake, low normal tissue uptake, and fast clearance and is therefore a suitable and promising candidate for PET imaging of melanoma.

[(18)F]-Organotrifluoroborates as Radioprosthetic Groups for PET Imaging: From Design Principles to Preclinical Applications.

Positron emission tomography (PET) is revolutionizing our ability to visualize in vivo targets for target validation and personalized medicine. Of several classes of imaging agents, peptides afford high affinity and high specificity to distinguish pathologically distinct cell types by the presence of specific molecular targets. Of various available PET isotopes, [(18)F]-fluoride ion is preferred because of its excellent nuclear properties and on-demand production in hospitals at Curie levels. However, the short half-life of (18)F and its lack of reactivity in water continue to challenge peptide labeling. Hence, peptides are often conjugated to a metal chelator for late-stage, one-step labeling. Yet radiometals, while effective, are neither as desirable nor as available as [(18)F]-fluoride ion. Despite considerable past success in identifying semifeasible radiosyntheses, significant challenges continue to confound tracer development. These interrelated challenges relate to (1) isotope/prosthetic choice; (2) bioconjugation for high affinity; (3) high radiochemical yields, (4) specific activities of >1 Ci/µmol to meet FDA microdose requirements; and (5) rapid clearance and in vivo stability. These enduring challenges have been extensively highlighted, while a single-step, operationally simple, and generally applicable means of labeling a peptide with [(18)F]-fluoride ion in good yield and high specific activity has eluded radiochemists and nuclear medicine practitioners for decades. Radiosynthetic ease is of primordial importance since multistep labeling reactions challenge clinical tracer production. In the past decade, as we sought to meet this challenge, appreciation of reactions with aqueous fluoride led us to consider organotrifluoroborate (RBF3(-)) synthesis as a means of rapid aqueous peptide labeling. We have applied principles of mechanistic chemistry, knowledge of chemical reactivity, and synthetic chemistry to design stable RBF3(-)s. Over the past 10 years, we have developed several new [(18)F]-RBF3(-) radioprosthetic groups, all of which guarantee radiosynthetic ease while in most cases providing high tumor:nontumor (T:NT) ratios and moderate-to-high tumor uptake. Although others have developed methods for labeling of peptides with [(18)F]-silylfluorides or [(18)F]-Al-NOTA chelates, this Account focuses on the synthesis of [(18)F]-organotrifluoroborates. In this Account, I detail mechanistic, kinetic, thermodynamic, synthetic, and radiosynthetic approaches that enabled the translation of fundamental principles regarding the chemistry of RBF3(-)s into a tantalizingly close realization of a clinical application of an [(18)F]-organotrifluoroborate-peptide conjugate for imaging of neuroendocrine tumors and the generalization of this method for labeling of several other peptides.

Efficient oxidation of N-protected tryptophan and tryptophanyl-dipeptides by in situ generated dimethyldioxirane provides hexahydropyrroloindoline-containing synthons suitable for peptide synthesis and subsequent tryptathionylation.

A series of hydroxypyrroloindoline (Hpi) containing dipeptides along with the corresponding monomeric Hpi-α-amino acid (Hpi-2-carboxylate), were prepared by reacting a series of N α-protected-tryptophans in aqueous or biphasic [water/cyclopentyl methyl ether (CPME)] solutions containing Oxone® (potassium peroxymonosulfate) and acetone. This procedure avoids the tedious distillation of unstable dimethyldioxirane (DMDO), which is commonly used to oxidize indoles. Monomers N α-Boc-Hpi-OH and N α-Fmoc-Hpi-OH were readily incorporated by solid-phase peptide synthesis (SPPS) into a peptide containing a cysteine; in trifluoroacetic acid (TFA), the Hpi underwent intramolecular dehydrative condensation with the cysteine thiol to afford the anticipated tryptathionine crosslink. This eco- and user-friendly oxidative methodology greatly simplifies the synthesis of Hpi derivatives while enabling the synthesis of tryptathionine crosslinks characteristic of phalloidin and amanitin, two potent peptide toxins of present interest.

Whole cell-SELEX of aptamers with a tyrosine-like side chain against live bacteria.

In an effort to expand the binding and recognition capabilities of aptamers, a nucleoside triphosphate modified with a phenol that mimics the side chain of tyrosine was used in the selection of DNA aptamers against live bacteria. Of multiple modified aptamers that were isolated against Escherichia coli DH5α cells, one aptamer displays high selectivity and affinity for the target cells and is greatly enriched for phenol-modified dU nucleotides (dUy, 47.5%). When the same sequences are synthesized with TTP, no binding is observed. Taken together, these findings highlight the value of using modified nucleotide triphosphates in aptamer selections and portends success in SELEX against an array of whole cells as targets.