Synthesis, in Vitro Evaluation, and Radiolabeling of Fluorinated Puromycin Analogues: Potential Candidates for PET Imaging of Protein Synthesis

Visualisation of in vivo protein synthesis during mycobacterial infection through [68Ga]Ga-DOTA-puromycin µPET/MRI

Radiolabelled puromycin analogues will allow the quantification of protein synthesis through nuclear medicine-based imaging. A particularly useful application could be the non-invasive longitudinal visualisation of mycobacterial activity through direct quantification of puromycin binding. This study assesses the value of [68Ga]Ga-DOTA-puromycin in the visualisation of mycobacteria through positron emission tomography combined with magnetic resonance imaging (µPET/MRI). The radiopharmaceutical was produced by previously published and validated methods. [68Ga]Ga-DOTA-Puromycin imaging was performed on severe immunodeficient mice infected with Bacille Calmette-Guérin-derived M. Bovis (BCG). Acute and chronic infection stages were examined by µPET/MRI. A follow-up group of animals acted as controls (animals bearing S. aureus-derived infection and sterile inflammation) to assess tracer selectivity. [68Ga]Ga-DOTA-puromycin-µPET/MRI images revealed the acute, widespread infection within the right upper shoulder and armpit. Also, [68Ga]Ga-DOTA-puromycin signal sensitivity measured after a 12-week period was lower than that of [18F]FDG-PET in the same animals. A suitable correlation between normalised uptake values (NUV) and gold standard histopathological analysis confirms accurate tracer accumulation in viable bacteria. The radiopharmaceutical showed infection selectivity over inflammation but accumulated in both M. Bovis and S. Aureus, lacking pathogen specificity. Overall, [68Ga]Ga-DOTA-puromycin exhibits potential as a tool for non-invasive protein synthesis visualization, albeit without pathogen selectivity.

Pilot Evaluation of S-(3-[18F]Fluoropropyl)-D-Homocysteine and O-(2-[18F]Fluoroethyl)-D-Tyrosine as Bacteria-Specific Radiotracers for PET Imaging of Infection

PURPOSE

There is currently no ideal radiotracer for imaging bacterial infections. Radiolabelled D-amino acids are promising candidates because they are actively incorporated into the peptidoglycan of the bacterial cell wall, a structural feature which is absent in human cells. This work describes fluorine-18 labelled analogues of D-tyrosine and D-methionine, O-(2-[18F]fluoroethyl)-D-tyrosine (D-[18F]FET) and S-(3-[18F]fluoropropyl)-D-homocysteine (D-[18F]FPHCys), and their pilot evaluation studies as potential radiotracers for imaging bacterial infection.

PROCEDURES

D-[18F]FET and D-[18F]FPHCys were prepared in classical fluorination-deprotection reactions, and their uptake in Staphylococcus aureus and Pseudomonas aeruginosa was evaluated over 2 h. Heat killed bacteria were used as controls. A clinically-relevant foreign body model of S. aureus infection was established in Balb/c mice, as well as a sterile foreign body to mimic inflammation. The ex vivo biodistribution of D-[18F]FPHCys in the infected and inflamed mice was evaluated after 1 h, by dissection and gamma counting. The uptake was compared to that of [18F]FDG.

RESULTS

In vitro uptake of both D-[18F]FET and D-[18F]FPHCys was specific to live bacteria. Uptake was higher in S. aureus than in P. aeruginosa for both radiotracers, and of the two, higher for D-[18F]FPHCys than D-[18F]FET. Blocking experiments with non-radioactive D-[19F]FPHCys confirmed specificity of uptake. In vivo, D-[18F]FPHCys had greater accumulation in S. aureus infection compared with sterile inflammation, which was statistically significant. As anticipated, [18F]FDG showed no significant difference in uptake between infection and inflammation.

CONCLUSIONS

D-[18F]FPHCys uptake was higher in infected tissues than inflammation, and represents a fluorine-18 labelled D-AA with potential to detect a S. aureus reference strain (Xen29) in vivo. Additional studies are needed to evaluate uptake of this radiotracer in clinical isolates.

Preclinical Research Highlighting Contemporary Targeting Mechanisms of Radiolabelled Compounds for PET Based Infection Imaging

It is important to constantly monitor developments in the preclinical imaging arena of infection. Firstly, novel radiopharmaceuticals with the correct characteristics must be identified to funnel into the clinic. Secondly, it must be evaluated if enough innovative research is being done and adequate resources are geared towards the development of radiopharmaceuticals that could feed into the Nuclear Medicine Clinic in the near future. It is proposed that the ideal infection imaging agent will involve PET combined with CT but more ideally MRI. The radiopharmaceuticals currently presented in preclinical literature have a wide selection of vectors and targets. Ionic formulations of PET-radionuclides such 64CuCl2 and 68GaCl2 are evaluated for bacterial infection imaging. Many small molecule based radiopharmaceuticals are being investigated with the most prominent targets being cell wall synthesis, maltodextrin transport (such as [18F]F-maltotriose), siderophores (bacterial and fungal infections), the folate synthesis pathway (such as [18F]F-PABA) and protein synthesis (radiolabelled puromycin). Mycobacterial specific antibiotics, antifungals and antiviral agents are also under investigation as infection imaging agents. Peptide based radiopharmaceuticals are developed for bacterial, fungal and viral infections. The radiopharmaceutical development could even react quickly enough on a pandemic to develop a SARS-CoV-2 imaging agent in a timely fashion ([64Cu]Cu-NOTA-EK1). New immuno-PET agents for the imaging of viruses have recently been published, specifically for HIV persistence but also for SARS-CoV2. A very promising antifungal immuno-PET agent (hJ5F) is also considered. Future technologies could include the application of aptamers and bacteriophages and even going as far as the design of theranostic infection. Another possibility would be the application of nanobodies for immuno-PET applications. Standardization and optimization of the preclinical evaluation of radiopharmaceuticals could enhance clinical translation and reduce time spent in pursuing less than optimal candidates.

Tinker, Tailor, Soldier, Spy: The Diverse Roles That Fluorine Can Play within Amino Acid Side Chains

Side chain-fluorinated amino acids are useful tools in medicinal chemistry and protein science. In this review, we outline some general strategies for incorporating fluorine atom(s) into amino acid side chains and for elaborating such building blocks into more complex fluorinated peptides and proteins. We then describe the diverse benefits that fluorine can offer when located within amino acid side chains, including enabling 19F NMR and 18F PET imaging applications, enhancing pharmacokinetic properties, controlling molecular conformation, and optimizing target-binding.

Automated radiosynthesis and preclinical in vivo evaluation of [18F]Fluoroethylpuromycin as a potential radiotracer for imaging protein synthesis with PET

PURPOSE

From a series of fluorinated analogues of puromycin, we recently identified [¹⁸F]fluoroethylpuromycin (FEPURO) as a potential candidate for imaging the rate of protein synthesis in vivo. Herein, we describe the automation of the radiosynthesis, and evaluation of [¹⁸F]FEPURO in vivo.

PROCEDURES

[¹⁸F]FEPURO was radiosynthesised in an automated module. PET imaging was conducted in Wistar rats under control and blocking conditions using the protein synthesis inhibitor cycloheximide. Biodistribution and metabolite studies at 30, 60 and 120 min were conducted in healthy rats.

RESULTS

Automation of the radiosynthesis resulted in reduction of the synthesis time by half from the manual method. A steady increase in the SUV was observed in the time-activity curves for the whole brain as expected for a protein synthesis marker. However, rapid in vivo metabolism of [¹⁸F]FEPURO within 15 min in plasma as well as the brain (4 % of parent 30 min p.i.) indicated formation of the [¹⁸F]FET radio-metabolite in >90 % thus suggesting that observed increase in the brain uptake was due to the radiometabolite.

CONCLUSIONS

[¹⁸F]FEPURO is not a suitable PET radiotracer for imaging protein synthesis rates in brain in vivo due to its rapid metabolism. Further structural modifications to prevent in vivo metabolism are underway.

Structure-Guided Discovery of Potent Antifungals that Prevent Ras Signaling by Inhibiting Protein Farnesyltransferase

Infections by fungal pathogens are difficult to treat due to a paucity of antifungals and emerging resistances. Next-generation antifungals therefore are needed urgently. We have developed compounds that prevent farnesylation of Cryptoccoccus neoformans Ras protein by inhibiting protein farnesyltransferase with 3-4 nanomolar affinities. Farnesylation directs Ras to the cell membrane and is required for infectivity of this lethal pathogenic fungus. Our high-affinity compounds inhibit fungal growth with 3-6 micromolar minimum inhibitory concentrations (MICs), 4- to 8-fold better than Fluconazole, an antifungal commonly used in the clinic. Compounds bound with distinct inhibition mechanisms at two alternative, partially overlapping binding sites, accessed via different inhibitor conformations. We showed that antifungal potency depends critically on the selected inhibition mechanism because this determines the efficacy of an inhibitor at low in vivo levels of enzyme and farnesyl substrate. We elucidated how chemical modifications of the antifungals encode desired inhibitor conformation and concomitant inhibitory mechanism.

P2O5-Promoted Cyclization of Di[aryl(hetaryl)methyl] Malonic Acids as a Pathway to Fused Spiro[4.4]nonane-1,6-Diones

Conventional spiro-linked conjugated materials are attractive for organic optoelectronic applications due to the unique combination of their optical and electronic properties. However, spiro-linked conjugated materials with conjugation extension directed along the main axis of the molecule are still only rare examples among the vast number of spiro-linked conjugated materials. Herein, the synthesis, leading to π-extended spiro-linked conjugated materials─spiro[4.4]nonane-1,6-diones and spiro[5.5]undecane-1,7-diones─has been developed and optimized. The proposed design concept starts from readily available malonic esters and contains several steps: double alkylation of malonic ester with bromomethylaryl(hetaryl)s; conversion of a malonic ester into the corresponding malonic acid; electrophilic spirocyclization of the latter into the annulated spiro[4.4]nonane-1,6-dione or spiro[5.5]undecane-1,7-dione in the presence of phosphorus pentoxide. On the basis of these insights, the developed method yielded spiro-linked conjugated cores fused with benzene, thiophene, and naphthalene, decorated with active halogen atoms. The structures of the synthesized spirocycles were determined by single-crystal X-ray diffraction analysis. Benzene fused spiro[4.4]nonane-1,6-dione decorated with bromine atoms was transformed into V-shape phenylene-thiophene co-oligomer type spirodimers via Stille coupling. The spiro-bis(4-n-dodecylphenyl)-2,2'-bithiophene derivative possessed high photoluminescence properties in both solution and solid state with a photoluminescence quantum yield (PL QY) of 38%.

In Vivo Imaging of the Tumor-Associated Enzyme NCEH1 with a Covalent PET Probe

Herein, we report the development of an ¹⁸ F-labeled, activity-based small-molecule probe targeting the cancer-associated serine hydrolase NCEH1. We undertook a focused medicinal chemistry campaign to simultaneously preserve potent and specific NCEH1 labeling in live cells and animals, while permitting facile ¹⁸ F radionuclide incorporation required for PET imaging. The resulting molecule, [¹⁸ F]JW199, labels active NCEH1 in live cells at nanomolar concentrations and greater than 1000-fold selectivity relative to other serine hydrolases. [¹⁸ F]JW199 displays rapid, NCEH1-dependent accumulation in mouse tissues. Finally, we demonstrate that [¹⁸ F]JW199 labels aggressive cancer tumor cells in vivo, which uncovered localized NCEH1 activity at the leading edge of triple-negative breast cancer tumors, suggesting roles for NCEH1 in tumor aggressiveness and metastasis.

The science of puromycin: From studies of ribosome function to applications in biotechnology

Puromycin is a naturally occurring aminonucleoside antibiotic that inhibits protein synthesis by ribosome-catalyzed incorporation into the C-terminus of elongating nascent chains, blocking further extension and resulting in premature termination of translation. It is most commonly known as a selection marker for cell lines genetically engineered to express a resistance transgene, but its additional uses as a probe for protein synthesis have proven invaluable across a wide variety of model systems, ranging from purified ribosomes and cell-free translation to intact cultured cells and whole animals. Puromycin is comprised of a nucleoside covalently bound to an amino acid, mimicking the 3′ end of aminoacylated tRNAs that participate in delivery of amino acids to elongating ribosomes. Both moieties can tolerate some chemical substitutions and modifications without significant loss of activity, generating a diverse toolbox of puromycin-based reagents with added functionality, such as biotin for affinity purification or fluorophores for fluorescent microscopy detection. These reagents, as well as anti-puromycin antibodies, have played a pivotal role in advancing our understanding of the regulation and dysregulation of protein synthesis in normal and pathological processes, including immune response and neurological function. This manuscript reviews the current state of puromycin-based research, including structure and mechanism of action, relevant derivatives, use in advanced methodologies and some of the major insights generated using such techniques both in the lab and the clinic.

Preparative Method for Asymmetric Synthesis of (S)-2-Amino-4,4,4-trifluorobutanoic Acid

Enantiomerically pure derivatives of 2-amino-4,4,4-trifluorobutanoic acid are in great demand as bioisostere of leucine moiety in the drug design. Here, we disclose a method specifically developed for large-scale (>150 g) preparation of the target (S)-N-Fmoc-2-amino-4,4,4-trifluorobutanoic acid. The method employs a recyclable chiral auxiliary to form the corresponding Ni(II) complex with glycine Schiff base, which is alkylated with CF3-CH2-I under basic conditions. The resultant alkylated Ni(II) complex is disassembled to reclaim the chiral auxiliary and 2-amino-4,4,4-trifluorobutanoic acid, which is in situ converted to the N-Fmoc derivative. The whole procedure was reproduced several times for consecutive preparation of over 300 g of the target (S)-N-Fmoc-2-amino-4,4,4-trifluorobutanoic acid.

Applications of fluorine-containing amino acids for drug design

Fluorine-containing amino acids are becoming increasingly prominent in new drugs due to two general trends in the modern pharmaceutical industry. Firstly, the growing acceptance of peptides and modified peptides as drugs; and secondly, fluorine editing has become a prevalent protocol in drug-candidate optimization. Accordingly, fluorine-containing amino acids represent one of the more promising and rapidly developing areas of research in organic, bio-organic and medicinal chemistry. The goal of this Review article is to highlight the current state-of-the-art in this area by profiling 42 selected compounds that combine fluorine and amino acid structural elements. The compounds under discussion represent pharmaceutical drugs currently on the market, or in clinical trials as well as examples of drug-candidates that although withdrawn from development had a significant impact on the progress of medicinal chemistry and/or provided a deeper understanding of the nature and mechanism of biological action. For each compound, we present features of biological activity, a brief history of the design principles and the development of the synthetic approach, focusing on the source of tailor-made amino acid structures and fluorination methods. General aspects of the medicinal chemistry of fluorine-containing amino acids and synthetic methodology are briefly discussed.

Practical Method for Preparation of (S)-2-Amino-5,5,5-trifluoropentanoic Acid via Dynamic Kinetic Resolution

This work reports an operationally convenient ∼20 g scale synthesis of (S)-2-amino-5,5,5-trifluoropentanoic acid and its Fmoc-derivative via dynamic kinetic resolution of the corresponding racemate.

p53-Dependent Apoptotic Effect of Puromycin via Binding of Ribosomal Protein L5 and L11 to MDM2 and its Combination Effect with RITA or Doxorubicin

Among ribosomal proteins essential for protein synthesis, the functions of ribosomal protein L5 (RPL5) and RPL11 still remain unclear to date. Here, the roles of RPL5 and RPL11 were investigated in association with p53/p21 signaling in the antitumor effect of puromycin mainly in HCT116 and H1299 cancer cells. Cell proliferation assays using 3-[4,5-dimethylthiazole-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assays and colony formation assays, cell cycle analysis, Reverse transcription polymerase chain reaction (RT-PCR) and Western blotting were performed in cancer cells. Puromycin exerted cytotoxic and anti-proliferative effects in p53 wild-type HCT116 more than in p53 null H1299 cells. Consistently, puromycin increased sub-G1, cleaved Poly (ADP-ribose) polymerase (PARP), activated p53, p21, and Mouse double minute 2 homolog (MDM2), and attenuated expression of c-Myc in HCT116 cells. Notably, puromycin upregulated the expression of RPL5 and RPL11 to directly bind to MDM2 in HCT116 cells. Conversely, deletion of RPL5 and RPL11 blocked the activation of p53, p21, and MDM2 in HCT116 cells. Also, puromycin enhanced the antitumor effect with reactivating p53 and inducing tumor apoptosis (RITA) or doxorubicin in HCT116 cells. These findings suggest that puromycin induces p53-dependent apoptosis via upregulation of RPL5 or RPL11 for binding with MDM2, and so can be used more effectively in p53 wild-type cancers by combination with RITA or doxorubicin.

Technetium-99m-labeled lapachol as an imaging probe for breast tumor identification

OBJECTIVE

Breast cancer is a health problem worldwide with high incidence and mortality rates. It is well known that the development of more sensitive and specific diagnostic methods is of great importance since an early diagnosis is essential to successfully treat tumors. Lapachol is a natural compound, belonging to the naphthoquinone group that has been widely used in traditional medicine to treat various illnesses, including cancer. The aim of this study was to evaluate technetium-99m (^99mTc) labeled lapachol as an imaging probe for breast cancer identification.

METHODS

To achieve this purpose, lapachol was labeled with ^99mTc, radiochemical purity and in vitro stability were determined. Blood clearance, in healthy mice, and biodistribution, in 4T1 tumor-bearing mice, were also evaluated.

RESULTS

Lapachol was successfully labeled with ^99mTc, with high values of radiochemical yield (95.9±3.4%). In vitro stability showed that the radiolabeled complex remained stable for up to 24h, with values above 90% for both saline and plasma (95.6±3.6% and 96.4±1.7%, respectively). The radiolabeled complex decays in a biphasic manner, with a half-life of distribution and elimination equal to 3.3 and 50.0min, respectively. Biodistribution and scintigraphic images showed high uptake in organs of excretion (kidneys, liver, and intestine). It could be also noted that tumor uptake was higher than the muscle at all time points. Tumor-to-muscle ratio reaches ∼4.5 at 24h after administration.

CONCLUSION

These findings suggest that ^99mTc-lapachol can be a potential diagnostic agent for breast tumors.

Puromycins B-E, Naturally Occurring Amino-Nucleosides Produced by the Himalayan Isolate Streptomyces sp. PU-14G

The isolation and structure elucidation of four new naturally occurring amino-nucleoside [puromycins B-E (1-4)] metabolites from a Himalayan isolate ( Streptomyces sp. PU-14-G, isolated from the Bara Gali region of northern Pakistan) is reported. Consistent with prior reports, comparative antimicrobial assays revealed the need for the free 2″-amine for anti-Gram-positive bacteria and antimycobacterial activity. Similarly, comparative cancer cell line cytotoxicity assays highlighted the importance of the puromycin-free 2″-amine and the impact of 3'-nucleoside substitution. These studies extend the repertoire of known naturally occurring puromycins and their corresponding SAR. Notably, 1 represents the first reported naturally occurring bacterial puromycin-related metabolite with a 3'- N-amino acid substitution that differs from the 3'- N-tyrosinyl of classical puromycin-type natural products. This discovery suggests the biosynthesis of 1 in Streptomyces sp. PU-14G may invoke a uniquely permissive amino-nucleoside synthetase and/or multiple synthetases and sets the stage for further studies to elucidate, and potentially exploit, new biocatalysts for puromycin chemoenzymatic diversification.