Synthesis and preclinical evaluation of a selective MET kinase positron emission tomography tracer.

The tyrosine kinase MET (hepatocyte growth factor receptor) is activated or mutated in a wide range of cancers and is often correlated with a poor prognosis. Precision medicine with positron emission tomography (PET) can potentially aid in the assessment of tumor biochemistry and heterogeneity, which can prompt the selection of the most effective therapeutic regimes. The selective MET inhibitor PF04217903 (1) formed the basis for a bioisosteric replacement, leading to the deoxyfluorinated analog [18 F]2. [18 F]2 could be synthesized with a "hydrous fluoroethylation" protocol in 6.3 ± 2.6% radiochemical yield and a molar activity of >50 GBq/µmol. In vitro autoradiography indicated that [18 F]2 selectively binds to MET in PC3 tumor tissue, and in vivo biodistribution in mice showed predominantly a hepatobiliary excretion along with a low retention of radiotracer in other organs.

Design, synthesis and biological evaluation of 6-substituted quinolines derived from cabozantinib as c-Met inhibitors.

Based on the cabozantinib scaffold, novel c-Met inhibitors were rationalized from the limited knowledge of structure-activity relationships for the quinoline 6-position. Emphasis was given to modifications capable of engaging in additional polar interactions with the c-Met active site. In addition, ortho-fluorinations of the terminal benzene ring were explored. Fifteen new molecules were synthesized and evaluated in a c-Met enzymatic binding assay. A wide range of substituents were tolerated in the quinoline 6-position, while the ortho-fluorinations performed were shown to give considerable reductions in the c-Met binding affinity. The antiproliferative effects of the compounds were evaluated in the NCI60 cancer cell line panel. Most notably, compounds 15b and 18b were able to inhibit cell proliferation more efficiently than cabozantinib in leukemia, CNS, and breast cancer cell lines. The in vitro data agreed well with the in silico docking results, where additional hydrogen bonding was identified in the enzymatic pocket for the para-amino substituted 15b and 18b.

A Survey of Molecular Imaging of Opioid Receptors.

The discovery of endogenous peptide ligands for morphine binding sites occurred in parallel with the identification of three subclasses of opioid receptor (OR), traditionally designated as µ, δ, and κ, along with the more recently defined opioid-receptor-like (ORL1) receptor. Early efforts in opioid receptor radiochemistry focused on the structure of the prototype agonist ligand, morphine, although N-[methyl-11C]morphine, -codeine and -heroin did not show significant binding in vivo. [11C]Diprenorphine ([11C]DPN), an orvinol type, non-selective OR antagonist ligand, was among the first successful PET tracers for molecular brain imaging, but has been largely supplanted in research studies by the µ-preferring agonist [11C]carfentanil ([11C]Caf). These two tracers have the property of being displaceable by endogenous opioid peptides in living brain, thus potentially serving in a competition-binding model. Indeed, many clinical PET studies with [11C]DPN or [11C]Caf affirm the release of endogenous opioids in response to painful stimuli. Numerous other PET studies implicate µ-OR signaling in aspects of human personality and vulnerability to drug dependence, but there have been very few clinical PET studies of µORs in neurological disorders. Tracers based on naltrindole, a non-peptide antagonist of the δ-preferring endogenous opioid enkephalin, have been used in PET studies of δORs, and [11C]GR103545 is validated for studies of κORs. Structures such as [11C]NOP-1A show selective binding at ORL-1 receptors in living brain. However, there is scant documentation of δ-, κ-, or ORL1 receptors in healthy human brain or in neurological and psychiatric disorders; here, clinical PET research must catch up with recent progress in radiopharmaceutical chemistry.

One-step synthesis of [18 F]cabozantinib for use in positron emission tomography imaging of c-Met.

Cabozantinib is an FDA-approved kinase inhibitor for the treatment of medullary thyroid cancer and advanced renal cell carcinoma, which exerts its therapeutic effect by inhibiting, among others, the tyrosine kinase c-Met. Noninvasive imaging techniques are becoming increasingly important clinically to ensure drug efficacy, staging, monitoring, and patient stratification. PET isotope labelled tyrosine kinase inhibitors have, for the same reason, potential as PET tracers for imaging of various cancers. On the basis of cabozantinib, we synthesized the novel boronic acid pinacol ester 4 as a labelling precursor, where the boronic ester moiety replaces the fluorine native to this kinase inhibitor. By this, we wanted to explore whether recently developed Cu-mediated fluorination methods are adaptable to more complex substrates and thereby provide easy access to [18 F]cabozantinib directly. Hydrolysis was implemented before preparative purification due to challenges with on-column hydrolysis of the precursor 4, and [18 F]cabozantinib was obtained in ≥99% radiochemical purity and in 2.8 ± 0.05% (n = 4) isolated decay corrected yield in a synthesis time of 90 minutes. The molar activity of representative batches was determined to be 17 ± 8 GBq/µmol.

Recent Development of Non-Peptide GnRH Antagonists.

The decapeptide gonadotropin-releasing hormone, also referred to as luteinizing hormone-releasing hormone with the sequence (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) plays an important role in regulating the reproductive system. It stimulates differential release of the gonadotropins FSH and LH from pituitary tissue. To date, treatment of hormone-dependent diseases targeting the GnRH receptor, including peptide GnRH agonist and antagonists are now available on the market. The inherited issues associate with peptide agonists and antagonists have however, led to significant interest in developing orally active, small molecule, non-peptide antagonists. In this review, we will summarize all developed small molecule GnRH antagonists along with the most recent clinical data and therapeutic applications.

Synthesis and in vitro evaluation of small-molecule [18F] labeled gonadotropin-releasing hormone (GnRH) receptor antagonists as potential PET imaging agents for GnRH receptor expression.

Two novel small molecule gonadotropin-releasing hormone (GnRH) receptor antagonists (12 and 13) of the furamide-class were synthesized and evaluated in vitro for their receptor binding affinities for the rat GnRH receptor. Radiolabeling with no carrier added fluorine-18 of the appropriate precursors was investigated in a one-step reaction. LogP (Octanol/PBS pH 7.4) and serum stability of the compounds were investigated. The antagonists showed low nM affinity for the rat GnRH receptor. (18)F-radiolabled compounds were obtained in high radiochemical purity (>95%) and specific activity (>75 GBq/µmol). These findings suggest this class of compounds holds promise as potential probes for PET targeting of GnRH-receptor expression.

A new automated and putatively versatile synthesis of the PSMA-ligand derivative [18F]DCFPyL using the FASTlabTM synthesizer.

BACKGROUND: Noninvasive molecular imaging using peptides and biomolecules labelled with positron emitters has become important for detection of cancer and other diseases with PET (positron emission tomography). The positron emitting radionuclide fluorine-18 is widely available in high yield from cyclotrons and has favorable decay (t1/2 109.7 min) and imaging properties. 18F-Labelling of biomolecules and peptides for use as radiotracers is customarily achieved in a two-step approach, which can be challenging to automate. 6-[18F]Fluoronicotinic acid 2,3,5,6-tetrafluorophenyl ester ([18F]F-Py-TFP) is a versatile 18F-prosthetic group for this purpose, which can be rapidly be produced in an one-step approach on solid support. This work details an automated procedure on the cassette-based GE FASTlab™ platform for the labeling of a peptidomimetic, exemplified by the case of using the Glu-CO-Lys motif to produce [18F]DCFPyL, a ligand targeting the prostate specific membrane antigen (PSMA). RESULTS: From fluorine-18 delivery a fully automated two-step radiosynthesis of [18F]DCFPyL was completed in 56 min with an overall end of synthesis yield as high as 37% using solid phase extraction (SPE) purification on the GE FASTlab™ platform. CONCLUSIONS: Putatively, this radiolabeling methodology is inherently amenable to automation with a diverse set of synthesis modules, and it should generalize for production of a broad spectrum of biomolecule-based radiotracers for use in PET imaging.

Prepubertal gonadotropin-releasing hormone analog leads to exaggerated behavioral and emotional sex differences in sheep.

In mammals, sex specialization is reflected by differences in brain anatomy and function. Measurable differences are documented in reproductive behavior, cognition, and emotion. We hypothesized that gonadotropin-releasing hormone (GnRH) plays a crucial role in controlling the extent of the brain's sex specificity and that changes in GnRH action during critical periods of brain development, such as puberty, will result in altered sex-specific behavioral and physiological patterns. We blocked puberty in half of the 48 same-sex Scottish mule Texel cross sheep twins with GnRH analog (GnRHa) goserelin acetate every 3 weeks, beginning just before puberty. To determine the effects of GnRHa treatment on sex-specific behavior and emotion regulation in different social contexts, we employed the food acquisition task (FAT) and measurement of heart rate variability (HRV). ANOVA revealed significant sex and sex×treatment interaction effects, suggesting that treated males were more likely to leave their companions to acquire food than untreated, while the opposite effect was observed in females. Concordant results were seen in HRV; treated males displayed higher HRV than untreated, while the reverse pattern was found in females, as shown by significant sex and sex×treatment interaction effects. We conclude that long-term prepubertal GnRHa treatment significantly affected sex-specific brain development, which impacted emotion and behavior regulation in sheep. These results suggest that GnRH is a modulator of cognitive function in the developing brain and that the sexes are differentially affected by GnRH modulation.

Preclinical evaluation of [18F]cabozantinib as a PET imaging agent in a prostate cancer mouse model.

INTRODUCTION: Cabozantinib is a tyrosine kinase inhibitor (TKI) approved for the treatment of medullary thyroid cancer, renal cell carcinoma and hepatocellular carcinoma, and is currently in clinical trials for the treatment of prostate cancer and others. It exerts its therapeutic effect mainly through inhibition of the tyrosine kinases MET (hepatocyte growth factor receptor) and VEGFR2 (vascular endothelial growth factor receptor 2), in addition to several other kinases involved in cancer. PET imaging with TKIs such as [18F]cabozantinib could potentially aid in cancer diagnosis and guide treatment. This study aims to evaluate the utility of [18F]cabozantinib as a PET imaging probe in PC3 tumor xenografted mice. METHODS: [18F]cabozantinib was evaluated in non-tumor and tumor bearing (PC3 xenografted) male mice by ex vivo biodistribution studies and in vivo µPET imaging. Pretreatment studies were performed in the tumor bearing mice with the MET inhibitor PF04217903. Mouse plasma was analyzed with HPLC to quantify radiometabolites. To further evaluate the binding specificity of [18F]cabozantinib, in vitro autoradiography studies on heart and PC3 tumor sections were performed in the presence of authentic cabozantinib or specific MET and VEGFR2 inhibitors. RESULTS: Tissue distribution studies in non-tumor bearing mice revealed slow blood clearance, absence of brain uptake and a high myocardial uptake. In the tumor bearing mice, tumor uptake was low (0.58 ± 0.20% ID/g at 30 min post tracer injection), which was confirmed by µPET imaging. No differences in tissue distribution and kinetics were observed in both biodistributions and µPET studies after pretreatment with the MET inhibitor PF04217903. At 30 min post tracer injection, 60 ± 3% of the recovered radioactivity in plasma in non-tumor bearing mice was present as intact tracer. [18F]cabozantinib binding in vitro to heart and tumor tissues was partly blocked in the presence of selective MET and VEGFR2 inhibitors (up to 40% block). The fraction of non-specific binding was relatively high for both tissues (66% for heart and 39% for tumor). CONCLUSION: [18F]cabozantinib exhibits non-favorable properties as a PET imaging probe, demonstrated by slow excretion kinetics along with low tumor uptake and high non-specific binding in tumor and heart tissue. The results reflect cabozantinibs multi-kinase activity, making PET imaging of tumor specific kinase expression with [18F]cabozantinib challenging.

Radiosynthesis and biodistribution of a prosthetic group (¹8F-FENMA) conjugated to cyclic RGD peptides.

We have recently reported a new N-methylaminooxy-based prosthetic group for the site-selective introduction of ¹8F-fluorine under mild acidic aqueous conditions into model peptides functionalized with a Michael acceptor moiety. To further investigate the utility of this methodology, the radiosynthesis of two cyclic RGD peptides was carried out, and in vivo biodistribution and microPET studies were performed in tumor-bearing mice. A cyclic RGD peptide was functionalized with the Michael acceptors trans-ß-nitrostyrene carboxylic acid and 3-vinylsulfonylpropionic acid. Radiolabeling was then performed with the prosthetic group O-(2-(2-[¹8F]fluoroethoxy)ethyl)-N-methylhydroxylamine (¹8F-FENMA) yielding the ¹8F-conjugates in moderate yields (8.5-12%). Biodistribution, blocking, and microPET imaging studies were performed in a mouse xenograft model. The vinylsulfonyl-modified conjugate demonstrated good in vitro plasma stability. Biodistribution and microPET studies revealed excellent tumor uptake with low background in key organs and renal elimination as the predominant route of excretion. Blocking studies with coinjected nonlabeled RGD peptide confirmed the in vivo specificity for the integrin α(v)ß3. On the other hand, ¹8F-FENMA-nitrostyrene-RGD, although stable at conjugation pH 5, was found to rapidly degrade at physiological pH through loss of the ¹8F-prosthetic group.

Correction: Towards dual inhibitors of the MET kinase and WNT signaling pathway; design, synthesis and biological evaluation.

[This corrects the article DOI: 10.1039/C9RA08954C.].

Towards dual inhibitors of the MET kinase and WNT signaling pathway; design, synthesis and biological evaluation.

Both the kinase MET and the WNT signaling pathway are attractive targets in cancer therapy, and synergistic effects have previously been observed in animal models upon simultaneous inhibition. A strategy towards a designed multiple ligand of MET and WNT signaling is pursued based on the two hetero biaryl systems present in both the MET inhibitor tepotinib and WNT signaling inhibitor TC-E 5001. Initial screening was conducted to find the most suitable ring systems for further optimization, whereas a second screen explored modifications towards pyridazinones and triazolo pyridazines. Up to 54% reduction of WNT signaling activity at 10 µM concentration was achieved, however, only low affinities towards MET were observed. Overall, the thiophene substituted pyridazinone 40 was the best dual MET and WNT signaling inhibitor, with a 17% and 19% reduction of activity, respectively. Although further optimizations are needed to achieve more potent dual inhibitors, the strategy presented herein can be valuable towards the development of a dual inhibitor of MET and WNT signaling.

Improved Drug Delivery to Brain Metastases by Peptide-Mediated Permeabilization of the Blood-Brain Barrier.

Patients with melanoma have a high risk of developing brain metastasis, which is associated with a dismal prognosis. During early stages of metastasis development, the blood-brain barrier (BBB) is likely intact, which inhibits sufficient drug delivery into the metastatic lesions. We investigated the ability of the peptide, K16ApoE, to permeabilize the BBB for improved treatment with targeted therapies preclinically. Dynamic contrast enhanced MRI (DCE-MRI) was carried out on NOD/SCID mice to study the therapeutic window of peptide-mediated BBB permeabilization. Further, both in vivo and in vitro assays were used to determine K16ApoE toxicity and to obtain mechanistic insight into its action on the BBB. The therapeutic impact of K16ApoE on metastases was evaluated combined with the mitogen-activated protein kinase pathway inhibitor dabrafenib, targeting BRAF mutated melanoma cells, which is otherwise known not to cross the intact BBB. Our results from the DCE-MRI experiments showed effective K16ApoE-mediated BBB permeabilization lasting for up to 1 hour. Mechanistic studies showed a dose-dependent effect of K16ApoE caused by induction of endocytosis. At concentrations above IC50, the peptide additionally showed nonspecific disturbances on plasma membranes. Combined treatment with K16ApoE and dabrafenib reduced the brain metastatic burden in mice and increased animal survival, and PET/CT showed that the peptide also facilitated the delivery of compounds with molecular weights as large as 150 kDa into the brain. To conclude, we demonstrate a transient permeabilization of the BBB, caused by K16ApoE, that facilitates enhanced drug delivery into the brain. This improves the efficacy of drugs that otherwise do not cross the intact BBB.

A novel prosthetic group for site-selective labeling of peptides for positron emission tomography.

Efficient methodologies for the radiolabeling of peptides with [(18)F]fluoride are a prerequisite to enabling commercialization of peptide-containing radiotracers for positron emission tomography (PET) imaging. It was the purpose of this study to investigate a novel chemoselective ligation reaction comprising conjugation of an [(18)F]-N-methylaminooxy-containing prosthetic group to a functionalized peptide. Twelve derivatives of general formula R1-CO-NH-Lys-Gly-Phe-Gly-Lys-OH were synthesized where R1 was selected from a short list of moieties anticipated to be reactive toward the N-methylaminooxy group. Conjugation reactions were initially carried out with nonradioactive precursors to assess, in a qualitative manner, their general suitability for PET chemistry with only the most promising pairings progressing to full radiochemical assessment. Best results were obtained for the ligation of O-[2-(2-[(18)F]fluoroethoxy)ethyl]-N-methyl-N-hydroxylamine 18 to the maleimidopropionyl-Lys-Gly-Phe-Gly-Lys-OH precursor 10 in acetate buffer (pH 5) after 1 h at 70 degrees C. The non-decay-corrected isolated yield was calculated to be 8.5%. The most encouraging result was observed with the combination 18 and 4-(2-nitrovinyl)benzoyl-Lys-Gly-Phe-Gly-Lys-OH, 9, where the conjugation reaction proceeded rapidly to completion at 30 degrees C after only 5 min. The corresponding non-decay-corrected radiochemical yield for the isolated (18)F-labeled product 27 was 12%. The preliminary results from this study demonstrate the considerable potential of this novel strategy for the radiolabeling of peptides.

Crystal structure of 6,7-dimeth-oxy-1-(4-nitro-phen-yl)quinolin-4(1H)-one: a mol-ecular scaffold for potential tubulin polymerization inhibitors.

The protein tubulin is central for maintaining normal cellular processes, and mol-ecules inter-fering with the tubulin dynamics have potential in the treatment of cancerous diseases. The title compound, C17H14N2O5, was prepared as a lead compound in a project dedicated to the development of therapeutic agents binding to the colchicine binding site on tubulin, thereby inter-fering with the cell division in cancer cells. It holds many of the main structural characteristics for colchicine binding and has the potential for further modification and functionalization. In the title mol-ecule, the benzene ring is inclined to the quinoline ring by 76.10 (8)°. In the crystal, mol-ecules are linked by two pairs of C-H⋯O hydrogen bonds, forming tubular-like arrangements, propagating along the direction of the diagonals of the ab plane, and enclosing R22(26) and R22(16) ring motifs.

Didanosine ester prodrugs: synthesis, albumin binding properties and pharmacokinetic studies in rats.

Three half-ester derivatives 10-12 of 5'-O-2',3'-dideoxydidanosine (DDI, 1) have been synthesized. The compounds exhibited excellent correlation between partition coefficients LogP and relative in vitro bovine serum albumin binding. Using high-performance liquid chromatography-mass spectrometry (LC-MS), DDI (1) was quantitatively determined in rat plasma after intravenous injection of the azelaic acid monoester derivative (11) of DDI. The pharmacokinetic data obtained for DDI were consistent with literature. The pharmacokinetic profile of 11 showed no significant difference in AUC(0-360) or curve shape compared to the parent drug DDI (1). The data indicate that the prodrug was converted to DDI within minutes after administration. High relative protein binding in vitro holds a promise for validity of the concept using more stable linker bonds.