Metabolite Identification of a Radiotracer by Electrochemistry Coupled to Liquid Chromatography with Mass Spectrometric and Radioactivity Detection

Caspase-3 probes for PET imaging of apoptotic tumor response to anticancer therapy

Apoptosis is a highly regulated process involved in the normal organism development and homeostasis. In the context of anticancer therapy, apoptosis is also studied intensively in an attempt to induce cell death in cancer cells. Caspase activation is a known key event in the apoptotic process. In particular, active caspase-3 and -7 are the common effectors in several apoptotic pathways, therefore effector caspase activation may be a promising biomarker for response evaluation to anticancer therapy. Quantitative imaging of apoptosis in vivo could provide early assessment of therapeutic effectiveness and could also be used in drug development to evaluate the efficacy as well as potential toxicity of novel treatments. Positron Emission Tomography (PET) is a highly sensitive molecular imaging modality that allows non-invasive in vivo imaging of biological processes such as apoptosis by using radiolabeled probes. Here we describe the development and evaluation of fluorine-18-labeled caspase-3 activity-based probes (ABPs) for PET imaging of apoptosis. ABPs were selected by screening of a small library of fluorine-19-labeled DEVD peptides containing different electrophilic warhead groups. An acyloxymethyl ketone was identified with low nanomolar affinity for caspase-3 and was radiolabeled with fluorine-18. The resulting radiotracer, [18F]MICA-302, showed good labeling of active caspase-3 in vitro and favorable pharmacokinetic properties. A μPET imaging experiment in colorectal tumor xenografts demonstrated an increased tumor accumulation of [18F]MICA-302 in drug-treated versus control animals. Therefore, our data suggest this radiotracer may be useful for clinical PET imaging of response to anticancer therapy.

Structure elucidation and quantification of the reduction products of anticancer Pt(iv) prodrugs by electrochemistry/mass spectrometry (EC-MS)

Pt(iv) prodrugs are a class of promising anticancer agents, which are activated by reduction to the active Pt(ii) species. Consequently, the reduction process is a crucial parameter. Herein, a new approach using electrochemistry (EC) coupled to liquid chromatography (LC) and electrospray ionization-mass spectrometry (ESI-MS) or inductively coupled plasma (ICP)-MS was applied. This enabled getting insights into the differences in the reduction and ligand release of platinum(iv) complexes with varying equatorial core structures.

Comparison of metabolic pathways of different α-N-heterocyclic thiosemicarbazones

Clinical failure of novel drugs is often related to their rapid metabolism and excretion. This highlights the importance of elucidation of their pharmacokinetic profile already at the preclinical stage of drug development. Triapine, the most prominent representative of α-N-heterocyclic thiosemicarbazones, was investigated in more than 30 clinical phase I/II trials, but the results against solid tumors were disappointing. Recent investigations from our group suggested that this is, at least partially, based on the fast metabolism and excretion. In order to establish more detailed structure/activity/metabolism relationships, herein a panel of 10 different Triapine derivatives was investigated for their metabolic pathways. From the biological point of view, the panel consists of terminally dimethylated thiosemicarbazones with nanomolar IC₅₀ values, derivatives with micromolar cytotoxicities comparable to Triapine and a completely inactive representative. To study the oxidative metabolism, a purely instrumental approach based on electrochemistry/mass spectrometry was applied and the results were compared to the data obtained from microsomal incubations. Overall, the investigated thiosemicarbazones underwent the phase I metabolic reactions dehydrogenation, hydroxylation, oxidative desulfuration (to semicarbazone and amidrazone) and demethylation. Notably, dehydrogenation resulted in a ring-closure reaction with formation of thiadiazoles. Although strong differences between the metabolic pathways of the different thiosemicarbazones were observed, they could not be directly correlated to their cytotoxicities. Finally, the metabolic pathways for the most cytotoxic compound were elucidated also in tissues collected from drug-treated mice, confirming the data obtained by electrochemical oxidation and microsomes. In addition, the in vivo experiments revealed a very fast metabolism and excretion of the compound.

Graphical abstract

Instrumentation and applications of electrochemistry coupled to mass spectrometry for studying xenobiotic metabolism: A review

The knowledge of metabolic pathways and biotransformation of xenobiotics, artificial substances foreign to the entire biological system, is crucial for elucidation of degradation routes of potentially toxic substances. Nowadays, there are many methods to simulate xenobiotic metabolism in the human body in vitro. In this review, the metabolism of various substances in the human body is described, followed by a summary of methods used for prediction of metabolic pathways and biotransformation. Above all, focus is placed on the coupling of electrochemistry to mass spectrometry, which is still a relatively new technique. This promising tool can mimic both oxidative phase I and conjugative phase II metabolism. Different experimental arrangements, with or without a separation step, and various applications of this technique are illustrated and critically reviewed.

Synthesis and evaluation of a [18F]BODIPY-labeled caspase-inhibitor

BODIPYs (boron dipyrromethenes) are fluorescent dyes which show high stability and quantum yields. They feature the possibility of selective ¹⁸F-fluorination at the boron-core. Attached to a bioactive molecule and labeled with [¹⁸F]fluorine, the resulting compounds are promising tracers for multimodal imaging in vivo and can be used for PET and fluorescence imaging. A BODIPY containing a phenyl and a hydroxy substituent on boron was synthesized and characterized. Fluorinated and hydroxy substituted dyes were coupled to an isatin-based caspase inhibitor via cycloaddition and the resulting compounds were evaluated in vitro in caspase inhibition assays. The metabolic stability and the formed metabolites were investigated by incubation with mouse liver microsomes and LC-MS analysis. Subsequently the fluorophores were labeled with [¹⁸F]fluorine and an in vivo biodistribution study using dynamic PET was performed.

Isatin sulfonamides: potent caspases-3 and -7 inhibitors, and promising PET and SPECT radiotracers for apoptosis imaging

Caspases-3 and -7 play an essential role in apoptosis. Isatin sulfonamides have been identified as potent inhibitors of these executing caspases. Besides pharmacological application, these compounds can also serve as recognition units to target caspases using positron emission tomography (PET) and single-photon emission computed tomography (SPECT) when labeled with a positron or a gamma emitter. Fluorinated, alkylated, arylated isatin derivatives, in addition to derivatives modified with heterocycles, have been prepared in order to improve their binding potency, selectivity and metabolic stability. Structural optimization has led to stable, highly active inhibitors, which after labeling have been applied in PET studies in tumor mouse models and for first preclinical and clinical investigations with healthy human volunteers. The results support further development of such radiotracers for clinical apoptosis imaging.

Characterization of [(99m)Tc]Duramycin as a SPECT Imaging Agent for Early Assessment of Tumor Apoptosis

PURPOSE

We investigated the usefulness of [(99m)Tc]duramycin for monitoring early response to cancer therapy in mice, with an eye towards clinical translation.

PROCEDURES

[(99m)Tc]Duramycin was injected in healthy CD1-/- mice to estimate human [(99m)Tc]duramycin radiation dose. [(99m)Tc]Duramycin single-photon emission computed tomography (SPECT) imaging of apoptosis was evaluated in a mouse model of colorectal cancer treated with irinotecan and validated ex vivo using autoradiography, cleaved caspase-3, and TdT-mediated dUTP nick-end labeling (TUNEL) histology of the tumors.

RESULTS

The mean effective dose was estimated to be 3.74 × 10(-3) ± 3.43 × 10(-4) mSv/MBq for non-purified and 3.19 × 10(-3) ± 2.16 × 10(-4) mSv/MBq for purified [(99m)Tc]duramycin. [(99m)Tc]Duramycin uptake in vivo following therapy increased significantly in apoptotic irinotecan-treated tumors (p = 0.008). Radioactivity in the tumors positively correlated with cleaved caspase-3 (r = 0.85, p < 0.001) and TUNEL (r = 0.92, p < 0.001) staining.

CONCLUSION

[(99m)Tc]Duramycin can be used to detect early chemotherapy-induced tumor cell death, and thus, may be a prospective candidate for clinical SPECT imaging of tumor response to therapy.

Novel fluorine-18 labeled 5-(1-pyrrolidinylsulfonyl)-7-azaisatin derivatives as potential PET tracers for in vivo imaging of activated caspases in apoptosis

The programmed type I cell death, defined as apoptosis, is induced by complex regulated signaling pathways that trigger the intracellular activation of executioner caspases-3, -6 and -7. Once activated, these enzymes initiate cellular death through cleavage of proteins which are responsible for DNA repair, signaling and cell maintenance. Several radiofluorinated inhibitors of caspases-3 and -7, comprising a moderate lipophilic 5-(1-pyrrolidinylsulfonyl)isatin lead structure, are currently being investigated for imaging apoptosis in vivo by us and others. The purpose of this study was to increase the intrinsic hydrophilicity of the aforementioned lead structure to alter the pharmacokinetic behavior of the resulting caspase-3 and -7 targeted radiotracer. Therefore, fluorinated and non-fluorinated derivatives of 5-(1-pyrrolidinylsulfonyl)-7-azaisatin were synthesized and tested for their inhibitory properties against recombinant caspases-3 and -7. Fluorine-18 has been introduced by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) of an alkyne precursor with 2-[(18)F]fluoroethylazide. Using dynamic micro-PET biodistribution studies in vivo the kinetic behavior of one promising PET-compatible 5-pyrrolidinylsulfonyl 7-azaisatin derivative has been compared to a previously described isatin based radiotracer.

A quantitative assay for reductive metabolism of a pesticide in fish using electrochemistry coupled with liquid chromatography tandem mass spectrometry

This is the first study to use electrochemistry to generate a nitro reduction metabolite as a standard for a liquid chromatography-mass spectrometry-based quantitative assay. This approach is further used to quantify 3-trifluoromethyl-4-nitrophenol (TFM) reductive metabolism. TFM is a widely used pesticide for the population control of sea lamprey (Petromyzon marinus), an invasive species of the Laurentian Great Lakes. Three animal models, sea lamprey, lake sturgeon (Acipenser fulvescens), and rainbow trout (Oncorhynchus mykiss), were selected to evaluate TFM reductive metabolism because they have been known to show differential susceptibilities to TFM toxicity. Amino-TFM (aTFM; 3-trifluoromethyl-4-aminophenol) was the only reductive metabolite identified through liquid chromatography-high-resolution mass spectrometry screening of liver extracts incubated with TFM and was targeted for electrochemical synthesis. After synthesis and purification, aTFM was used to develop a quantitative assay of the reductive metabolism of TFM through liquid chromatography and tandem mass spectrometry. The concentrations of aTFM were measured from TFM-treated cellular fractions, including cytosolic, nuclear, membrane, and mitochondrial protein extracts. Sea lamprey extracts produced the highest concentrations (500 ng/mL) of aTFM. In addition, sea lamprey and sturgeon cytosolic extracts showed concentrations of aTFM substantially higher than those of rainbow trout. However, other fractions of lake sturgeon extracts tend to show aTFM concentrations similar to those of rainbow trout but not with sea lamprey. These data suggest that the level of reductive metabolism of TFM may be associated with the sensitivities of the animals to this particular pesticide.

Unexpected benzimidazole ring formation from a quinoneimide species in the presence of ammonium acetate as supporting electrolyte used in the coupling of electrochemistry with mass spectrometry

RATIONALE

Electrochemistry (EC) coupled to mass spectrometry (MS) has been used to study different phase-I reactions. Despite of the versatility of EC/MS, the effect of the nature of the supporting electrolyte on the formation of oxidation products has seldom been discussed during EC/MS experiments. Here, we present a comparison of two different supporting electrolytes and their effect on the identification of unstable intermediate oxidation species is discussed.

METHODS

The oxidation of acebutolol was performed with a coulometric cell in the presence of two supporting electrolytes namely ammonium acetate and lithium acetate. Ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UPLC/QTOFMS) using a binary gradient (water/acetonitrile) with positive electrospray ionization was used to identify the oxidation products in the presence and absence of glutathione. Chemical structure elucidations of the oxidation products were performed by high-resolution mass spectrometry (HRMS) and were also supported by nuclear magnetic resonance (NMR) measurements.

RESULTS

From the electrochemical study and HRMS measurements, we demonstrate that the quinoneimide species resulting from the oxidative hydrolyses of acebutolol gives a benzimidazole ring product in the presence of ammonium acetate. Through the example of the oxidation of acebutolol, a correlation between the supporting electrolyte nature and oxidation product formation was established. The obtained results were supported by quantum mechanical calculations.

CONCLUSIONS

We present here evidence of the side reactions induced by the presence of ammonia as supporting electrolyte during EC/MS measurements. Acebutolol was used as a model to postulate an uncommon and unexpected side reaction leading to benzimidazole ring formation. The findings may help to understand the identification of the intermediate species in the oxidative degradation process.

Mass spectrometric methods for monitoring redox processes in electrochemical cells

Electrochemistry (EC) is a mature scientific discipline aimed to study the movement of electrons in an oxidation-reduction reaction. EC covers techniques that use a measurement of potential, charge, or current to determine the concentration or the chemical reactivity of analytes. The electrical signal is directly converted into chemical information. For in-depth characterization of complex electrochemical reactions involving the formation of diverse intermediates, products and byproducts, EC is usually combined with other analytical techniques, and particularly the hyphenation of EC with mass spectrometry (MS) has found broad applicability. The analysis of gases and volatile intermediates and products formed at electrode surfaces is enabled by differential electrochemical mass spectrometry (DEMS). In DEMS an electrochemical cell is sampled with a membrane interface for electron ionization (EI)-MS. The chemical space amenable to EC/MS (i.e., bioorganic molecules including proteins, peptides, nucleic acids, and drugs) was significantly increased by employing electrospray ionization (ESI)-MS. In the simplest setup, the EC of the ESI process is used to analytical advantage. A limitation of this approach is, however, its inability to precisely control the electrochemical potential at the emitter electrode. Thus, particularly for studying mechanistic aspects of electrochemical processes, the hyphenation of discrete electrochemical cells with ESI-MS was found to be more appropriate. The analytical power of EC/ESI-MS can further be increased by integrating liquid chromatography (LC) as an additional dimension of separation. Chromatographic separation was found to be particularly useful to reduce the complexity of the sample submitted either to the EC cell or to ESI-MS. Thus, both EC/LC/ESI-MS and LC/EC/ESI-MS are common.

Online monitoring oxidative products and metabolites of nicotine by free radicals generation with Fenton reaction in tandem mass spectrometry

In general, over 70% absorbed nicotine is metabolized to cotinine and trans-3′-hydroxycotinine by cytochrome oxidase P450, and nicotine is also a major addictive and the psychoactive component in cigarettes. As a xenobiotic metabolism, hydrophobic compounds are usually converted into more hydrophilic products through enzyme systems such as cytochrome oxidase P450, sulfotransferases, and UDP-glucuronosyltransferases to deliver drug metabolites out of the cell during the drug metabolic process. In this study, an electrodeless electrochemical oxidation (EEO) reaction via Fenton reaction by producing free radical to react with nicotine to immediately monitor the oxidative products and metabolic derivatives of nicotine by tandem mass spectrometer (MS) is done. Fenton reaction generates free radicals via ferrous ion (Fe²⁺) and hydrogen peroxide (H₂O₂) to oxidize DNA and to degrade proteins in cells. In the EEO method, the oxidative products of nicotine including cotinine, cotinine-N-oxide, trans-3′-hydroxycotinine, nornicotine, norcotinine, 4-oxo-4-(3-pyridyl)-butanoic acid, 4-hydroxy-4-(3-pyridyl)-butanoic acid, and nicotine-N′-oxide were detected by tandem mass spectrometer to simulate the changes of nicotine and its derivatives in a time-dependent manner.

Synthesis, 18F-radiolabeling, and in vivo biodistribution studies of N-fluorohydroxybutyl isatin sulfonamides using positron emission tomography

The effector caspases-3 and -7 play a central role in programmed type I cell death (apoptosis). Molecular imaging using positron emission tomography (PET) by tracking the activity of executing caspases might allow the detection of the early onset as well as therapy monitoring of various diseases induced by dysregulated apoptosis. Herein, four new fluorinated diastereo- and enantiopure isatin sulfonamide-based potent and selective caspase-3 and -7 inhibitors were prepared by cyclic sulfate ring-opening with fluoride. All fluorohydrins exhibited excellent in vitro affinities (up to IC50 = 11.8 and 0.951 nM for caspase-3 and -7, respectively), which makes them appropriate PET radiotracer candidates. Therefore, N-(4-[(18)F]fluoro-3(R)-hydroxybutyl)- and N-(3(S)-[(18)F]fluoro-4-hydroxybutyl)-5-[1-(2(S)-(methoxymethyl)pyrrolidinyl)sulfonyl]isatin were synthesized in 140 min with 24% and 10% overall radiochemical yields and specific activities of 10-127 GBq/μmol using [(18)F]fluoride in the presence of Kryptofix and subsequent acidic hydrolysis. In vivo biodistribution studies in wild-type mice using PET/computed tomography imaging proved fast clearance of the tracer after tail vein injection.

Synthesis of new fluorinated, 2-substituted 5-pyrrolidinylsulfonyl isatin derivatives as caspase-3 and caspase-7 inhibitors: nonradioactive counterparts of putative PET-compatible apoptosis imaging agents

Downstream caspases-3 and -7 are essential to execute the programmed type I cell death (apoptosis). In order to better understand their role, specific inhibitors of these enzymes are required, which after radiolabeling can be applied to non-invasively visualize and monitor apoptotic pathways in vivo using Positron Emission Tomography (PET). Therefore, 2-methoxyethyl-, 2-methoxypropyl-, 2-ethoxymethyl-, 2-(2-fluoroethoxymethyl)-, and 2-(2,2,2-trifluoroethoxymethyl)pyrrolidinyl analogues of (S)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin (2) were prepared and their in vitro binding affinities towards caspases-1, -3, -6 and -7 were evaluated and compared to that of the lead structure 2. While the inhibition potencies against caspases-1 and -6 were in the micromolar range, all synthesized compounds exhibited excellent and selective inhibition of caspases-3 and -7 in the nanomolar range up to IC50=4.79 nM and 7.47 nM, respectively. These highly potent 2-substituted analogues of 2 might be developed as anti-apoptosis agents and some selected fluorinated inhibitors might be useful as potential PET radiotracers for apoptosis imaging after (18)F-labeling.