Characterization of membrane potential-dependent uptake of the novel PET tracer 18F-fluorobenzyl triphenylphosphonium cation

Membrane potential-dependent uptake of 18F-triphenylphosphonium--a new voltage sensor as an imaging agent for detecting burn-induced apoptosis

BACKGROUND

Mitochondrial dysfunction has been closely related to many pathologic processes, such as cellular apoptosis. Alterations in organelle membrane potential are associated with mitochondrial dysfunction. A fluorine-18 labeled phosphonium compound: (18)F-triphenylphosphonium ((18)F-TPP) was prepared to determine its potential use as a mitochondria-targeting radiopharmaceutical to evaluate cellular apoptosis.

METHODS

Studies were conducted in both ex vivo cell lines and in vivo using a burned animal model. Uptake of (18)F-TPP was assessed in PC-3 cells by gamma counting under the following conditions: graded levels of extracellular potassium concentrations, incubation with carbonyl cyanide m-chlorophenylhydrazone and staurosporine. Apoptosis was studied in a burn animal model using terminal deoxynucleotidyl transferase dUTP nick end labeling staining and simultaneous assessment of (18)F-TPP uptake by biodistribution.

RESULTS

We found that stepwise membrane depolarization by potassium (K) resulted in a linear decrease in (18)F-TPP uptake, with a slope of 0.62 ± 0.08 and a correlation coefficient of 0.936 ± 0.11. Gradually increased concentrations of m-chlorophenylhydrazone lead to decreased uptake of (18)F-TPP. Staurosporine significantly decreased the uptake of (18)F-TPP in PC-3 cells from 14.2 ± 3.8% to 5.6 ± 1.3% (P < 0.001). Burn-induced significant apoptosis (sham: 4.4 ± 1.8% versus burn: 24.6 ± 6.7 %; P < 0.005) and a reduced uptake of tracer in the spleens of burn-injured animals as compared with sham burn controls (burn: 1.13 ± 0.24% versus sham: 3.28 ± 0.67%; P < 0.005). Biodistribution studies demonstrated that burn-induced significant reduction in (18)F-TPP uptake in spleen, heart, lung, and liver, which were associated with significantly increased apoptosis.

CONCLUSIONS

(18)F-TPP is a promising new voltage sensor for detecting mitochondrial dysfunction and apoptosis in various tissues.

Clinical translation of molecular imaging agents used in PET studies of cancer

Over recent years, there has been a rapid expansion in our knowledge of the factors that regulate tumor growth; this has resulted in the identification of new therapeutic targets and improvements in the long-term survival of cancer patients. New noninvasive biomarkers of drug targets and pathway modulation in vivo are needed to guide therapy selection and detect drug resistance early so that alternative, more effective treatments can be offered. The translation of new therapeutics into the clinic is disappointingly slow, expensive, and subject to high rates of attrition often occurring at late stages (phase 3) of development. In an attempt to mitigate these delays and failures, there has been resurgence in the development of new molecular imaging probes for studies with positron emission tomography (PET) to characterize tumor biology. In the assessment of therapeutic effects, PET allows imaging of entire tumor burden in a noninvasive repeatable manner. This chapter focuses on the clinical translation of PET imaging agents from bench to bedside. New probes are being used to study a diverse range of processes such as angiogenesis, apoptosis, fatty acid metabolism, and growth factor receptor expression. In the future, validation of these novel imaging probes could allow more innovative therapies to be adapted earlier in the clinic leading to improved patient outcomes.

Biodistribution and radiation dosimetry of 18F-CP-18, a potential apoptosis imaging agent, as determined from PET/CT scans in healthy volunteers

(18)F-CP-18, or (18S,21S,24S,27S,30S)-27-(2-carboxyethyl)-21-(carboxymethyl)-30-((2S,3R,4R,5R,6S)-6-((2-(4-(3-F18-fluoropropyl)-1H-1,2,3-triazol-1-yl)acetamido)methyl)-3,4,5-trihydroxytetrahydro-2H-pyran-2-carboxamido)-24-isopropyl-18-methyl-17,20,23,26,29-pentaoxo-4,7,10,13-tetraoxa-16,19,22,25,28-pentaazadotriacontane-1,32-dioic acid, is being evaluated as a tissue apoptosis marker for PET imaging. The purpose of this study was to determine the biodistribution and estimate the normal-organ radiation-absorbed doses and effective dose from (18)F-CP-18.

METHODS

Successive whole-body PET/CT scans were obtained at approximately 7, 45, 90, 130, and 170 min after intravenous injection of (18)F-CP-18 in 7 healthy human volunteers. Blood samples and urine were collected between the PET/CT scans, and the biostability of (18)F-CP-18 was assessed using high-performance liquid chromatography. The PET scans were analyzed to determine the radiotracer uptake in different organs. OLINDA/EXM software was used to calculate human radiation doses based on the biodistribution of the tracer.

RESULTS

(18)F-CP-18 was 54% intact in human blood at 135 min after injection. The tracer cleared rapidly from the blood pool with a half-life of approximately 30 min. Relatively high (18)F-CP-18 uptake was observed in the kidneys and bladder, with diffuse uptake in the liver and heart. The mean standardized uptake values (SUVs) in the bladder, kidneys, heart, and liver at around 50 min after injection were approximately 65, 6, 1.5, and 1.5, respectively. The calculated effective dose was 38 ± 4 μSv/MBq, with the urinary bladder wall having the highest absorbed dose at 536 ± 61 μGy/MBq using a 4.8-h bladder-voiding interval for the male phantom. For a 1-h voiding interval, these doses were reduced to 15 ± 2 μSv/MBq and 142 ± 15 μGy/MBq, respectively. For a typical injected activity of 555 MBq, the effective dose would be 21.1 ± 2.2 mSv for the 4.8-h interval, reduced to 8.3 ± 1.1 mSv for the 1-h interval.

CONCLUSION

(18)F-CP-18 cleared rapidly through the renal system. The urinary bladder wall received the highest radiation dose and was deemed the critical organ. Both the effective dose and the bladder dose can be reduced by frequent voiding. From the radiation dosimetry perspective, the apoptosis imaging agent (18)F-CP-18 is suitable for human use.

Rat imaging and in vivo stability studies using [11C]-dimethyl-diphenyl ammonium, a candidate agent for PET-myocardial perfusion imaging

BACKGROUND

PET myocardial perfusion imaging (MPI) holds several advantages over SPECT for diagnosing coronary artery disease. The short half-lives of prevailing PET-MPI agents hamper wider clinical application of PET in nuclear cardiology; prompting the development of novel PET-MPI agents. We have previously reported on the potential of radiolabeled ammonium salts, and particularly on that of [(11)C]dimethyl-diphenyl-ammonium ([(11)C]DMDPA), for cardiac PET imaging. This study was designed to improve the radiosynthesis and increase the yield of [(11)C]DMDPA, characterize more meticulously the kinetics of radioactivity distribution after its injection via micro-PET/CT studies, and further explore its potential for PET-MPI.

METHODS

The radiosynthetic procedure of [(11)C]DMDPA was improved with respect to the previously reported one. The kinetics of radioactivity distribution following injection of [(11)C]DMDPA were investigated in juvenile and young adult male SD rats using microPET/CT, and compared to those of [(13)N]NH3. Furthermore, the metabolic fate of [(11)C]DMDPA in vivo was examined after its injection into rats.

RESULTS

Following a radiosynthesis time of 25-27 min, 11.9 ± 1.1 GBq of [(11)C]DMDPA was obtained, with a 43.7% ± 4.3% radiochemical yield (n = 7). Time activity curves calculated after administration of [(11)C]DMDPA indicated rapid, high and sustained radioactivity uptake in hearts of both juvenile and young adult rats, having a two-fold higher cardiac radioactivity uptake compared to [(13)N]NH3. Accordingly, at all time points after injection to both juvenile and young adult rats, image quality of the left ventricle was higher with [(11)C]DMDPA compared to [(13)N]NH3. In vivo stability studies of [(11)C]DMDPA indicate that no radioactive metabolites could be detected in plasma, liver and urine samples of rats up to 20 min after injection, suggesting that [(11)C]DMDPA is metabolically stable in vivo.

CONCLUSIONS

This study further illustrates that [(11)C]DMDPA holds, at least in part, essential qualities required from a PET-MPI probe. Owing to the improved radiosynthetic procedure reported herein, [(11)C]DMDPA can be produced in sufficient amounts for clinical use.

New frontiers in the design and synthesis of imaging probes for PET oncology: current challenges and future directions

Despite being developed over 30 years ago, 2-deoxy-2-[(18)F]fluoro-D-glucose remains the most frequently used radiotracer in PET oncology. In the last decade, interest in new and more specific radiotracers for imaging biological processes of oncologic interest has increased exponentially. This review summarizes the strategies underlying the development of those probes together with their validation and status of clinical translation; a brief summary of new radiochemistry strategies applicable to PET imaging is also included. The article finishes with a consideration of the challenges imaging scientists must overcome to bring about increased adoption of PET as a diagnostic or pharmacologic tool.

Mono- and dicationic Re(I)/(99m)Tc(I) tricarbonyl complexes for the targeting of energized mitochondria

The enhanced negative mitochondrial membrane potential of tumor cells can increase the cell accumulation of triphenylphosphonium (TPP) derivatives, which prompted us to investigate TPP-containing Re(I)/(99m)Tc organometallic compounds as probes for in vivo targeting of energized mitochondria. Novel compounds (Re1-Re4/Tc1-Tc4) were obtained with bifunctional chelators of the pyrazole-diamine (N,N,N-donors) and pyrazole-aminocarboxylic (N,N,O-donors) type, functionalized with TPP pharmacophores that have been introduced at the central amine of the chelators using different spacers. In this way, dicationic (Re1-Re2, Tc1-Tc2) and monocationic (Re3-Re4, Tc3-Tc4) complexes with variable lipophilicity were synthesized. The (99m)Tc complexes (Tc1-Tc4) are highly stable under physiological conditions and their chemical identification was done by HPLC comparison with the Re congeners (Re1-Re4), which were fully characterized by common analytical techniques (electrospray ionization mass spectrometry (ESI-MS), IR, multinuclear NMR). The in vitro biological evaluation of Tc1-Tc4 was performed in a panel of human tumor cell lines (PC-3, MCF-7 and H69), including cell lines overexpressing P-glycoprotein (MCF-7/MDR1 and H69/Lx4), and in isolated mitochondria. All the tested complexes showed a low to moderate cellular and mitochondrial uptake and did not undergo significant P-glycoprotein (Pgp)-mediated efflux processes. In particular, the dication Tc2 and the monocation Tc4 presented the highest cellular and mitochondrial uptake. Their cellular uptake was shown to depend on the mitochondrial (Δψm) and plasma membrane (Δψp) potentials. Altogether, the biological properties of these compounds suggest that they might be relevant for the design of radioactive metalloprobes for in vivo targeting of mitochondria.

Structure--activity relationship and preclinical evaluation of carbon-11-labeled ammonium salts as PET--myocardial perfusion imaging agents

BACKGROUND

Due to the limited availability of suitable positron emission tomography (PET) tracers, the majority of myocardial perfusion imaging (MPI) scans is performed using SPECT rather than PET.

AIM

The aim of this study is to design and synthesize carbon-11-labeled ammonium salt derivatives and explore their structure–activity relationship (SAR) and their potential as PET–MPI agents.

METHODS AND RESULTS

Three carbon-11-labeled ammonium salts were developed. SAR of the labeled compounds were explored vis-à-vis the effects of charge density and lipophilicity on the distribution kinetics in mice. These studies pointed at [11C]4 as the lead compound. Comparative microPET/CT scans in healthy rats, using both [11C]4 and [13 N]–NH3, substantiated the potential of [11C]4 ([11C]-DMDPA). A proof of concept for the potential of radiolabeled ammonium salts as MPI agents has been demonstrated in a newly developed swine model of permanent partial coronary artery occlusion.

CONCLUSIONS

SAR studies of 11C-labeled ammonium salts suggest that both lipophilicity and charge density affect the performance of these compounds as MPI probes. In a swine model, the labeled lead successfully visualized the defect regions in the myocardium. The data presented call for the development of fluorine-18 analogues, to increase clinical impact.

Mechanistic evaluation of a novel small molecule targeting mitochondria in pancreatic cancer cells

Background

Pancreatic cancer is one of the deadliest cancers with a 5-year survival rate of 6%. Therapeutic options are very limited and there is an unmet medical need for safe and efficacious treatments. Cancer cell metabolism and mitochondria provide unexplored targets for this disease. We recently identified a novel class of triphenylphosphonium salts, TP compounds, with broad- spectrum anticancer properties. We examined the ability of our prototypical compound TP421– chosen for its fluorescent properties – to inhibit the growth of pancreatic cancer cells and further investigated the molecular mechanisms by which it exerts its anticancer effects.

Methodology/Principal Findings

TP421 exhibited sub-micromolar IC₅₀ values in all the pancreatic cancer cell lines tested using MTT and colony formation assays. TP421 localized predominantly to mitochondria and induced G₀/G₁ arrest, ROS accumulation, and activation of several stress-regulated kinases. Caspase and PARP-1 cleavage were observed indicating an apoptotic response while LC3B-II and p62 were accumulated indicating inhibition of autophagy. Furthermore, TP421 induced de-phosphorylation of key signaling molecules involved in FAK mediated adhesion that correlated with inhibition of cell migration.

Conclusions/Significance

TP421 is a representative compound of a new promising class of mitochondrial-targeted agents useful for pancreatic cancer treatment. Because of their unique mechanism of action and efficacy further development is warranted.

Biomarkers and molecular probes for cell death imaging and targeted therapeutics

Cell death is a critically important biological process. Disruption of homeostasis, either by excessive or deficient cell death, is a hallmark of many pathological conditions. Recent research advances have greatly increased our molecular understanding of cell death and its role in a range of diseases and therapeutic treatments. Central to these ongoing research and clinical efforts is the need for imaging technologies that can locate and identify cell death in a wide array of in vitro and in vivo biomedical samples with varied spatiotemporal requirements. This review article summarizes community efforts over the past five years to identify useful biomarkers for dead and dying cells, and to develop molecular probes that target these biomarkers for optical, radionuclear, or magnetic resonance imaging. Apoptosis biomarkers are classified as either intracellular (caspase enzymes, mitochondrial membrane potential, cytosolic proteins) or extracellular (plasma membrane phospholipids, membrane potential, surface exposed histones). Necrosis, autophagy, and senescence biomarkers are described, as well as unexplored cell death biomarkers. The article discusses possible chemotherapeutic and theranostic strategies, and concludes with a summary of current challenges and expected eventual rewards of clinical cell death imaging.

Imaging inflammation: molecular strategies to visualize key components of the inflammatory cascade, from initiation to resolution

Dysregulation of inflammation is central to the pathogenesis of innumerable human diseases. Understanding and tracking the critical events in inflammation are crucial for disease monitoring and pharmacological drug discovery and development. Recent progress in molecular imaging has provided novel insights into spatial associations, molecular events and temporal sequelae in the inflammatory process. While remaining a burgeoning field in pre-clinical research, increasing application in man affords researchers the opportunity to study disease pathogenesis in humans in situ thereby revolutionizing conventional understanding of pathophysiology and potential therapeutic targets. This review provides a description of commonly used molecular imaging modalities, including optical, radionuclide and magnetic resonance imaging, and details key advances and translational opportunities in imaging inflammation from initiation to resolution.

Imaging apoptosis with positron emission tomography: 'bench to bedside' development of the caspase-3/7 specific radiotracer [(18)F]ICMT-11

The capacity to evade apoptosis has been defined as one of the hallmarks of cancer and, thus, effective anti-cancer therapy often induces apoptosis. A biomarker for imaging apoptosis could assist in monitoring the efficacy of a wide range of current and future therapeutics. Despite the potential, there are limited clinical examples of the use of positron emission tomography for imaging of apoptosis. [(18)F]ICMT-11 is a novel reagent designed to non-invasively image caspase-3 activation and, hence, drug-induced apoptosis. Radiochemistry development of [(18)F]ICMT-11 has been undertaken to improve specific radioactivity, reduce content of stable impurities, reduce synthesis time and enable automation for manufacture of multi-patient dose. Due to the promising mechanistic and safety profile of [(18)F]ICMT-11, the radiotracer is transitioning to clinical development and has been selected as a candidate radiotracer by the QuIC-ConCePT consortium for further evaluation in preclinical models and humans. A successful outcome will allow use of the radiotracer as qualified method for evaluating the pharmaceutical industry's next generation therapeutics.

Radiolabeled isatin binding to caspase-3 activation induced by anti-Fas antibody

INTRODUCTION

Noninvasive imaging methods that can distinguish apoptosis from necrosis may be useful in furthering our understanding of diseases characterized by apoptotic dysregulation as well as aiding drug development targeting apoptotic pathways. We evaluated the ability of radiolabeled isatins to quantify caspase-3 activity induced by the activation of the extrinsic apoptotic pathway by the anti-Fas antibody in mice.

METHODS

The behavior of three different radiolabeled isatins ([(18)F]WC-II-89, [(18)F]WC-IV-3 and [(11)C]WC-98) was characterized in mice with and without anti-Fas antibody treatment by microPET imaging and biodistribution studies. The activity of [(18)F]WC-II-89 was also compared with [(99m)Tc]mebrofenin. The effect of pan-caspase inhibition with quinolyl-valyl-O-methylaspartyl-[2,6-difluorophenoxy]-methyl ketone (Q-VD-OPh) on [(18)F]WC-II-89 uptake was studied. Caspase-3 activity was confirmed by a fluorometric enzyme assay.

RESULTS

All three tracers behaved similarly in microPET and biodistribution studies. Increased retention of all tracers was observed in the livers of treated animals and several other organs, all of which demonstrated increased caspase-3 enzyme activity; however, impaired hepatobiliary excretion made attribution of these findings to caspase-3 activity difficult. The isatin [(18)F]WC-II-89 was retained at statistically significantly higher levels in the organs after anti-Fas antibody treatment while [(99m)Tc]mebrofenin activity cleared, suggesting specific binding to activated caspase-3, but the magnitude of increased binding was still relatively low. Caspase inhibition with Q-VD-OPh partially blocked [(18)F]WC-II-89 retention but completely blocked caspase-3 enzyme activity in the liver.

CONCLUSIONS

The radiolabeled isatins appear to bind specifically to caspase-3 in vivo, but their sensitivity is limited. Further optimization is required for these tracers to be useful for clinical applications.

18F-fluorobenzyl triphenyl phosphonium: a noninvasive sensor of brown adipose tissue thermogenesis

Recent studies have proposed activation of brown adipose tissue (BAT) thermogenesis as a new strategy to combat obesity. Currently, there is no effective noninvasive imaging agent to directly detect unstimulated BAT and quantify the core mechanism of mitochondrial thermogenesis. We investigated an approach to detect BAT depots and monitor thermogenesis using the mitochondria-targeting voltage sensor radiolabeled fluorobenzyltriphenyl phosphonium (FBnTP).

METHODS

(18)F-FBnTP, (14)C-FBnTP, (18)F-FDG, and (99m)Tc-sestamibi uptake in BAT at room temperature (n = 8) and cold-treated (n = 8) Lewis rats was assayed. The effect of the cold condition on (18)F-FBnTP retention in BAT was assessed in 8 treated and 16 control rats. The effect of the noradrenergic inhibitor propranolol on (14)C-FBnTP response to cold stimulation was investigated in an additional 8 treated and 8 control mice.

RESULTS

At room temperature, (18)F-FBnTP accumulated in BAT to an extent similar to that in the heart, second only to the kidney and twice as much as (99m)Tc-sestamibi. Prior exposure to cold (4°C) for 4 h resulted in an 82% decrease of (14)C-FBnTP uptake and an 813% increase of (18)F-FDG uptake in BAT. (99m)Tc-sestamibi uptake was not affected by cold. Administration of (18)F-FBnTP at room temperature 60 min before 120 and 240 min of exposure to cold resulted in marked washout of the tracer from BAT. Propranolol significantly diminished the effect of cold on (14)C-FBnTP and (18)F-FDG uptake into BAT.

CONCLUSION

The intense uptake of (18)F-FBnTP into BAT at room temperature and the response to cold stimulation suggest the unique potential advantage of (18)F-FBnTP not only in detecting unstimulated BAT at high contrast but also in quantifying the mitochondrial thermogenic activity. (18)F-FBnTP PET may serve as a useful technique to assess BAT volume and function.

PET/CT in cancer research: from preclinical to clinical applications

The identification of genetic and biochemical mechanisms underlying tumor growth and progression along with the unraveling of human genoma provided a plethora of new targets for cancer detection, treatment and monitoring. Simultaneously, the extraordinary development of a number of imaging technologies, including hybrid systems, allowed the visualization of biochemical, molecular and physiological aberrations linked to underlying mutations in a given tumor. In vivo evaluation of complex biological processes such as proliferation, apoptosis, angiogenesis, metastasis, gene expression, receptor-ligand interactions, transport of substrates and metabolism of nutrients in human cancers is feasible using PET/CT and radiolabeled molecular probes. Some of these compounds are in preclinical phases of evaluation whereas others have been already applied in clinical settings. Here we provide prominent examples on how some biological processes and target expression can be visualized by PET/CT in animal tumor models and cancer patients for the noninvasive detection of well-known markers of tumor aggressiveness, invasiveness and resistance to treatment and for the evaluation of tumor response to therapy.

Apoptosis imaging: beyond annexin V

Induction of apoptosis is the primary mechanism through which most chemotherapies cause tumor cell death. Early assessment of tumor response is required to manage patients in terms of quality of life versus intensive chemotherapy. Although imaging with radiolabeled annexin V has been intensively investigated, it is still not sufficiently mature for clinical application. This article will summarize various alternative imaging techniques for visualization of phosphatidylserine externalization, activity of caspases, and mitochondrial membrane potential. Such imaging studies will promote the identification of novel molecular targets and the development of highly specific apoptosis-detecting imaging probes with potential clinical applications. It is highly possible that quantitative imaging of apoptosis will greatly improve clinical decision making in apoptosis-related diseases.

Imaging the life and death of tumors in living subjects: Preclinical PET imaging of proliferation and apoptosis

Cancer is characterized by deregulation of cell proliferation and altered cell death apoptosis, which constitutes, in almost all instances, the minimal common platform upon which all neoplastic evolution occurs. The most implicit and clinically attractive anticancer strategies, therefore, consist of eliminating tumor cells by preventing their expansion and ultimately inducing cell death apoptosis. In this context, the non-invasive molecular assessment of tumor cell proliferation and apoptosis status using PET imaging constitutes a major strategy in preclinical studies to assess the efficacy of new anticancer therapeutics using small animal PET imaging, and in clinical settings for the monitoring of treatment responses in patients. For this purpose, a variety of PET tracers targeting specific molecular entities allowing the non-invasive measurement of biological processes, including cell proliferation and apoptosis, are under development for use in preclinical studies and clinical trials to non-invasively image in vivo the lifeline of tumors.

Small-molecule biomarkers for clinical PET imaging of apoptosis

Apoptosis is a fundamental biologic process. Molecular imaging of apoptosis in vivo may have important implications for clinical practice, assisting in early detection of disease, monitoring of disease course, assessment of treatment efficacy, or development of new therapies. Although a PET probe for clinical imaging of apoptosis would be highly desirable, this is yet an unachieved goal, mainly because of the required challenging integration of various features, including sensitive and selective detection of the apoptotic cells, clinical aspects such as favorable biodistribution and safety profiles, and compatibility with the radiochemistry and imaging routines of clinical PET centers. Several approaches are being developed to address this challenge, all based on novel small-molecule structures targeting various steps of the apoptotic cascade. This novel concept of small-molecule PET probes for apoptosis is the focus of this review.

Cardiac positron emission tomography

Positron emission tomography (PET) is a powerful, quantitative imaging modality that has been used for decades to noninvasively investigate cardiovascular biology and physiology. Due to limited availability, methodologic complexity, and high costs, it has long been seen as a research tool and as a reference method for validation of other diagnostic approaches. This perception, fortunately, has changed significantly within recent years. Increasing diversity of therapeutic options for coronary artery disease, and increasing specificity of novel therapies for certain biologic pathways, has resulted in a clinical need for more accurate and specific diagnostic techniques. At the same time, the number of PET centers continues to grow, stimulated by PET's success in oncology. Methodologic advances as well as improved radiotracer availability have further contributed to more widespread use. Evidence for diagnostic and prognostic usefulness of myocardial perfusion and viability assessment by PET is increasing. Some studies suggest overall cost-effectiveness of the technique despite higher costs of a single study, because unnecessary follow-up procedures can be avoided. The advent of hybrid PET-computed tomography (CT), which enables integration of PET-derived biologic information with multislice CT-derived morphologic information, and the key role of PET in the development and translation of novel molecular-targeted imaging compounds, have further contributed to more widespread acceptance. Today, PET promises to play a leading diagnostic role on the pathway toward a future of high-powered, comprehensive, personalized, cardiovascular medicine. This review summarizes the state-of-the-art in current imaging methodology and clinical application, and outlines novel developments and future directions.

Detection and quantification of the evolution dynamics of apoptosis using the PET voltage sensor 18F-fluorobenzyl triphenyl phosphonium

Apoptosis is a key mechanism in numerous pathologies. However, there are no effective noninvasive means available for an early detection and quantitative assessment of evolution dynamics of the apoptotic process. Here, we have characterized the ability of the novel PET voltage sensor (18)F-fluorobenzyl triphenyl phosphonium ((18)F-FBnTP) to quantify the time-dependent apoptotic action of the taxanes paclitaxel and docetaxel in vitro and in vivo.

METHODS

The duration-dependent treatment effect of paclitaxel on (18)F-FBnTP uptake was assayed in human MDA-MB-231 breast carcinoma cells. The expression of the proapoptotic Bax and antiapoptotic Bcl-2 mitochondrial proteins, release of the apoptogen cytochrome c, and activation of executioner caspase-3 were determined by Western blotting. The fraction of viable cells was determined using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. The effect of docetaxel on (18)F-FBnTP and (18)F-FDG uptake in orthotopic prostate tumors in mice was compared.

RESULTS

(18)F-FBnTP cellular uptake in viable cells declined linearly with the increasing duration of paclitaxel treatment, from 3 to 24 h, and plateaued at 48 h. The extent of decrease of (18)F-FBnTP correlated strongly with the Bax-to-Bcl-2 ratio (R(2) = 0.83) and release of cytochrome c (R(2) = 0.92), but preceded in time the caspase-3 cleavage. The P-glycoprotein blocker verapamil did not interfere with (18)F-FBnTP cellular uptake. (18)F-FBnTP prostate tumor contrast was greater than (18)F-FDG prostate tumor contrast. Docetaxel caused a marked decrease (52.4%) of (18)F-FBnTP tumor uptake, within 48 h, whereas (18)F-FDG was much less affected (12%).

CONCLUSION

The voltage sensor (18)F-FBnTP is a viable means for quantification of paclitaxel pharmacodynamics. (18)F-FBnTP permits the detection of paclitaxel apoptotic action in vivo earlier than does (18)F-FDG. (18)F-FBnTP may afford a novel approach for early detection and quantitative assessment of the cumulative-effect kinetics of proapoptotic drugs and conditions using PET.

Arylphosphonium salts interact with DNA to modulate cytotoxicity

Arylphosphonium salts (APS) are compounds that have both lipophilic and cationic character, allowing them facile transport through plasma membranes or cell walls to accumulate in the cytoplasm or mitochondria of cells. APS molecules preferentially accumulate in tumor cells and are therefore under investigation as tumor imaging agents and mitochondrial targeting molecules. We have generated a systematic set of APS to study their ability to associate with DNA. The chemical structure of the APS determines the extent of its interaction with DNA and therefore its ability to aggregate the DNA. Also, APS compounds blocked DNA amplification in vitro at concentrations below the aggregation threshold, corroborating the structure/interaction relationship. Furthermore, the extent of APS:DNA interaction strongly correlates with bacterial toxicity, implying that APS molecules may deter cellular metabolic DNA pathways. Finally, DNA repair deficient and DNA bypass polymerase deficient bacterial strains were screened for sensitivity to APS. Interestingly, no single pathway for the repair or tolerance of these compounds was solely responsible for APS mediated toxicity. Taken together, these findings suggest that APS compounds may be capable of targeting and regulating unchecked cell growth and therefore show potential applications as a chemotherapeutic agent.

Imaging of apoptosis

Cells can die by several pathways, such as accidental death, apoptosis, autophagy, pyroptosis, and oncosis. These are important in normal physiology and many disease states, such as cancer and cardiovascular disease. Specific biochemical changes occur in cells undergoing apoptosis that provide potential targets for molecular imaging agents. Several of these molecular steps have been evaluated to date, including phosphatidylserine exposure at the extracellular face of the plasma membrane, detected by proteins such as annexin V; caspase activation in the intracellular compartment, detected by labeled enzyme substrates or inhibitors; and mitochondrial membrane potential collapse, detected by reduced levels of phosphonium cations that normally accumulate in healthy mitochondria. Phase I clinical trials have been performed with 1 of these agents, annexin V. Future work will likely include development of new agents that detect targets not exploited by current agents, translational research on the significance of imaging the different forms of cell death, and further improvements in the techniques for labeling existing agents to improve sensitivity and reduce nonspecific background.