Silicon-Fluoride Acceptors (SiFA) for 18F-Radiolabeling: From Bench to Bedside

Fluorine-18 (18F) is undoubtedly one of the most frequently applied radionuclides for the development of new radiotracers for positron emission tomography (PET) in the context of clinical cancer, neurological, and metabolic imaging. Until recently, the available radiochemical methodologies to introduce 18F into organic molecules ranging from small- to medium- and large-sized compounds were limited to a few applicable protocols. With the advent of late-stage fluorination of small aromatic, nonactivated compounds and various noncanonical labeling strategies geared toward the labeling of peptides and proteins, the molecular toolbox for PET radiotracer development was substantially extended. Especially, the noncanonical labeling methodologies characterized by the formation of Si-18F, B-18F, and Al-18F bonds give access to kit-like 18F-labeling of complex and side-group unprotected compounds, some of them already in clinical use. This chapter will particularly focus on silicon-fluoride acceptor (SiFA) chemistry and cover the history of its conceptual design and its translation into the clinical practice.

Type I inhibitors of tropomyosin receptor kinase (Trk): a 2020-2022 patent update

INTRODUCTION

Trk inhibitors are significant in the realm of personalized medicine as they target specific genetic alterations, such as NTRK gene fusions, leading to improved treatment outcomes for cancer patients. By tailoring the treatment to the genetic characteristics of the tumor rather than the tumor type, Trk inhibitors offer the potential for more effective and precise therapies, resulting in enhanced response rates and prolonged survival for patients with NTRK fusion-positive cancers.

AREAS COVERED

Patents covering type I inhibitors targeting the Trk family are discussed, building upon our prior review series on Trk inhibitors. Relevant patents were identified through the Web of Science database, Google, and Google Patents.

EXPERT OPINION

The field of Trk inhibitors has evolved significantly, as reflected in the current patent literature, which emphasizes the selective structural refinement of clinical champions. Efforts now concentrate on enhancing efficacy against on-target resistance mechanisms, with modifications made to improve potency, reduce toxicity, and enhance pharmacokinetics. Combination therapies show potential to address off-target resistance mechanisms and improve treatment outcomes. Challenges remain in accurately diagnosing NTRK gene alterations and integrating screening into routine clinical practice. Trk inhibitors have surpassed their conventional role of inhibition and are now seeing new applications in radiopharmaceutical development and as molecular targeting agents.

Toward in vivo proof of binding of 18F-labeled inhibitor [18F]TRACK to peripheral tropomyosin receptor kinases

BACKGROUND

Tropomyosin receptor kinases (TrkA, TrkB, TrkC) are a family of tyrosine kinases primarily expressed in neuronal cells of the brain. Identification of oncogenic alterations in Trk expression as a driver in multiple tumor types has increased interest in their role in human cancers. Recently, first- and second-generation ¹¹C and ¹⁸F-labeled Trk inhibitors, e.g., [¹⁸F]TRACK, have been developed. The goal of the present study was to analyze the direct interaction of [¹⁸F]TRACK with peripheral Trk receptors in vivo to prove its specificity for use as a functional imaging probe.

METHODS

In vitro uptake and competition experiments were carried out using the colorectal cancer cell line KM12. Dynamic PET experiments were performed with [¹⁸F]TRACK, either alone or in the presence of amitriptyline, an activator of Trk, entrectinib, a Trk inhibitor, or unlabeled reference compound TRACK in KM12 tumor-bearing athymic nude mice as well as B6129SF2/J and corresponding B6;129S2-Ntrk2^tm1Bbd/J mice. Western blot and immunohistochemistry experiments were done with KM12 tumors, brown adipose tissue (BAT), and brain tissue samples.

RESULTS

Uptake of [¹⁸F]TRACK was increasing over time reaching 208 ± 72% radioactivity per mg protein (n = 6/2) after 60 min incubation time. Entrectinib and TRACK competitively blocked [¹⁸F]TRACK uptake in vitro (IC₅₀ 30.9 ± 3.6 and 29.4 ± 9.4 nM; both n = 6/2). [¹⁸F]TRACK showed uptake into KM12 tumors (SUV_mean,60 min 0.43 ± 0.03; n = 6). Tumor-to-muscle ratio reached 0.9 (60 min) and 1.2 (120 min). In TrkB expressing BAT, [¹⁸F]TRACK uptake reached SUV_mean,60 min 1.32 ± 0.08 (n = 7). Activation of Trk through amitriptyline resulted in a significant radioactivity increase of 21% in KM12 tumor (SUV_mean,60 min from 0.53 ± 0.01 to 0.43 ± 0.03; n = 6; p < 0.05) and of 21% in BAT (SUV_mean,60 min from 1.32 ± 0.08; n = 5 to 1.59 ± 0.07; n = 6; p < 0.05) respectively. Immunohistochemistry showed TrkB > TrkA expression on BAT fat cells, but TrkA > TrkB in whole brain. WB analysis showed sevenfold higher TrkB expression in BAT versus KM12 tumor tissue.

CONCLUSION

The present data show that radiotracer [¹⁸F]TRACK can target peripheral Trk receptors in human KM12 colon cancer as well as brown adipose tissue as confirmed through in vitro and in vivo blocking experiments. Higher TrkB versus TrkA protein expression was detected in brown adipose tissue of mice confirming a peripheral functional role of brain-derived neurotrophic factor in adipose tissue.

Synthesis and Preclinical Evaluation of [18F]SiFA-PSMA Inhibitors in a Prostate Cancer Model

Positron emission tomography (PET) imaging of prostate-specific membrane antigen (PSMA) with gallium-68 (⁶⁸Ga) and fluorine-18 (¹⁸F) radiotracers has aroused tremendous interest over the past few years. The use of organosilicon-[¹⁸F]fluoride acceptors (SiFA) conjugated to urea-based peptidomimetic PSMA inhibitors provides a "kit-like" multidose synthesis technology. Nine novel ¹⁸F-labeled SiFA-bearing PSMA inhibitors with different linker moieties were synthesized and analyzed for their in vitro binding against [¹²⁵I]I-TAAG-PSMA in LNCaP cells. IC₅₀ values ranged from 58-570 nM. Among all compounds, [¹⁸F]SiFA-Asp₂-PEG₃-PSMA (IC₅₀ = 125 nM) showed the highest tumor uptake in LNCaP tumors (SUV_60min 0.73). A substantial increase in molar activity (A_m) (from 7.5 ± 0.5 to 86 ± 3 GBq/μmol) led to a significant increase in LNCaP tumor uptake (SUV_60min 1.18; Δ 0.45 corresponding to +62%). In vivo blocking with DCFPyL resulted in -32% uptake after 60 min. The SiFA-isotopic exchange chemistry offers a method that is readily adaptable for a "kit-type" labeling procedure and clinical translation.

On the Viability of Tadalafil-Based 18F-Radiotracers for In Vivo Phosphodiesterase 5 (PDE5) PET Imaging

Phosphodiesterase 5 (PDE5) is a clinically relevant biomarker and therapeutic target for many human pathologies, yet a noninvasive agent for the assessment of PDE5 expression has yet to be realized. Such agents would improve our understanding of the nitric oxide (NO)/cyclic guanosine 3',5'-monophosphate (cGMP)/PDE5 pathway in human pathologies and potentially lead to novel uses of PDE5 inhibitors to manage lung conditions like SARS-CoV-2-mediated pulmonary inflammatory responses. In this study, efforts were made to produce an ¹⁸F-labeled analogue of the PDE5 inhibitor tadalafil to visualize PDE5 expression in vivo with positron emission tomography (PET). However, during the late-stage fluorination step, quantitative epimerization of the tadalafil C12a stereocenter occurred, yielding a less active epi-isomer. In vivo dynamic microPET images in mice revealed that the epimerized radiotracer, [¹⁸F]epi-18, rapidly accumulated in the liver with negligible uptake in tissues of known PDE5 expression.

Recent Advances in the Clinical Translation of Silicon Fluoride Acceptor (SiFA) 18F-Radiopharmaceuticals

The incorporation of silicon fluoride acceptor (SiFA) moieties into a variety of molecules, such as peptides, proteins and biologically relevant small molecules, has improved the generation of ¹⁸F-radiopharmaceuticals for medical imaging. The efficient isotopic exchange radiofluorination process, in combination with the enhanced [¹⁸F]SiFA in vivo stability, make it a suitable strategy for fluorine-18 incorporation. This review will highlight the clinical applicability of [¹⁸F]SiFA-labeled compounds and discuss the significant radiotracers currently in clinical use.

Tropomyosin receptor kinase inhibitors: an updated patent review for 2016-2019

Introduction: Tropomyosin receptor kinases (Trks) control processes in the fields of growth, survival, and differentiation of neuronal processes. They also play a crucial role in neurodegenerative diseases as well as different types of cancer. Interest in developing Trk inhibitors to target NTRK fusion-driven cancers has escalated in the last decade, leading to the FDA approval of the pan-Trk inhibitors entrectinib and larotrectinib. The development of next-generation inhibitors that overcome resistance mutations arising from treatment with these first generation inhibitors has been the focus in recent years.Area covered: In this updated patent review for 2016-2019, patents covering inhibitors targeting the Trk family are discussed as a continuation of the previous reviews, Tropomyosin receptor kinase inhibitors: an updated patent review for 2010-2016 - Parts 1 & 2. The status of Trk inhibitors in clinical trials is also evaluated. For the identification of relevant patents and clinical trials, Web of Science, Google, Google Patents, and patent referencing were used.Expert opinion: The FDA approval of larotrectinib and entrectinib is a prime example of how basket clinical trial design targeting oncogenic drivers, regardless of tumor histology, is a viable approach to drug discovery and embodies the shift toward personalized medicine.

Efficient radiosynthesis and preclinical evaluation of [18 F]FOMPyD as a positron emission tomography tracer candidate for TrkB/C receptor imaging

Herein we report an efficient radiolabeling of a ¹⁸ F-fluorinated derivative of dual inhibitor GW2580, with its subsequent evaluation as a positron emission tomography (PET) tracer candidate for imaging of two neuroreceptor targets implicated in the pathophysiology of neurodegeneration: tropomyosin receptor kinases (TrkB/C) and colony stimulating factor receptor (CSF-1R). [¹⁸ F]FOMPyD was synthesized from a boronic acid pinacolate precursor via copper-mediated ¹⁸ F-fluorination concerted with thermal deprotection of the four Boc groups on a diaminopyrimidine moiety in an 8.7±2.8% radiochemical yield, a radiochemical purity >99%, and an effective molar activity of 187±93 GBq/μmol. [¹⁸ F]FOMPyD showed moderate brain permeability in wild-type rats (SUV_max = 0.75) and a slow washout rate. The brain uptake was partially reduced (ΔAUC_40-90 = 11.6%) by administration of the nonradioactive FOMPyD (up to 30 μg/kg). In autoradiography, [¹⁸ F]FOMPyD exhibits ubiquitous distribution in rat and human brain tissues with relatively high nonspecific binding revealed by self-blocking experiment. The binding was blocked by TrkB/C inhibitors, but not with a CSF-1R inhibitor, suggesting selective binding to the former receptor. Although an unfavorable pharmacokinetic profile will likely preclude application of [¹⁸ F]FOMPyD as a PET tracer for brain imaging, the concomitant one-pot copper-mediated ¹⁸ F-fluorination/Boc-deprotection is a practical technique for the automated radiosynthesis of acid-sensitive PET tracers.

18F-Labeling of Radiotracers Functionalized with a Silicon Fluoride Acceptor (SiFA) for Positron Emission Tomography

The para-substituted di-tert-butylfluorosilylbenzene structural motif known as the silicon-fluoride acceptor (SiFA) is a useful tag in the radiochemist's toolkit for incorporating radioactive [¹⁸F]fluoride into tracers for use in positron emission tomography. In comparison to conventional radiolabeling strategies, isotopic exchange of fluorine-19 from SiFA with [¹⁸F]fluoride is carried out at room temperature and requires minimal reaction participants. The formation of by-products is thus negligible, and purification is greatly simplified. However, while the precursor molecule used for labeling and the final radiolabeled product are isotopically discrete, they are chemically identical and are thus inseparable during purification procedures. The SiFA tag is also susceptible to degradation under the basic conditions arising from the processing and drying of [¹⁸F]fluoride. The '4 drop method', wherein only the first 4 drops of eluted [¹⁸F]fluoride are used from the solid-phase extraction, reduces the amount of base in the reaction, facilitates lower molar amounts of precursor, and reduces degradation.

First-in-Human Brain Imaging of [18F]TRACK, a PET tracer for Tropomyosin Receptor Kinases

The tropomyosin receptor kinase TrkA/B/C family is responsible for human neuronal growth, survival, and differentiation from early nervous system development stages onward. Downregulation of TrkA/B/C receptors characterizes numerous neurological disorders including Alzheimer's disease (AD). Abnormally expressed Trk receptors or chimeric Trk fusion proteins are also well-characterized oncogenic drivers in a variety of neurogenic and non-neurogenic human neoplasms and are currently the focus of intensive clinical research. Previously, we have described the clinical translation of a highly selective and potent carbon-11-labeled pan-Trk radioligand and the preclinical characterization of the optimized fluorine-18-labeled analogue, [¹⁸F]TRACK, for in vivo Trk positron emission tomography (PET) imaging. We describe herein central nervous system selectivity assessment and first-in-human study of [¹⁸F]TRACK.

Targeting Prostate-Specific Membrane Antigen (PSMA) with F-18-Labeled Compounds: the Influence of Prosthetic Groups on Tumor Uptake and Clearance Profile

PURPOSE

Prostate-specific membrane antigen (PSMA) is an important biomarker expressed in the majority of prostate cancers. The favorable positron emission tomography (PET) imaging profile of the PSMA imaging agent 2-(3-(1-carboxy-5-[(6-[¹⁸F]fluoro-pyridine-3-carbonyl)-amino]-pentyl)-ureido)-pentane-dioic acid [¹⁸F]DCFPyL in preclinical prostate cancer models and in prostate cancer patients stimulated the development and validation of other fluorine-containing PSMA inhibitors to further enhance pharmacokinetics and simplify production methods. Here, we describe the synthesis and radiopharmacological evaluation of various F-18-labeled PSMA inhibitors which were prepared through different prosthetic group chemistry strategies.

PROCEDURES

Prosthetic groups N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB), 4-[¹⁸F]fluorobenzaldehyde, and 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) were used for bioconjugation reactions to PSMA-binding lysine-urea-glutamate scaffold via acylation and oxime formation. All fluorine-containing PSMA inhibitors were tested for their PSMA inhibitory potency in an in vitro competitive binding assay in comparison to an established reference compound [¹²⁵I]TAAG-PSMA. Tumor uptake and clearance profiles of three F-18-labeled PSMA inhibitors ([¹⁸F]4, [¹⁸F]7, and [¹⁸F]8) were studied with dynamic PET imaging using LNCaP tumor-bearing mice.

RESULTS

F-18-labeled PSMA inhibitors were synthesized in 32-69 % radiochemical yields using (1) acylation reaction at the primary amino group of the lysine residue with [¹⁸F]SFB and (2) oxime formation with 4-[¹⁸F]fluorobenzaldehyde and [¹⁸F]FDG using the respective aminooxy-functionalized lysine residue. Compound 7 displayed an IC₅₀ value of 6 nM reflecting very high affinity for PSMA. Compounds 4 and 8 showed IC₅₀ values of 13 and 62 nM, respectively. The IC₅₀ value of reference compound DCFPyL was 13 nM. Dynamic PET imaging revealed the following SUV_60min for radiotracer uptake in PSMA(+) LNCaP tumors: 0.98 ([¹⁸F]DCFPyL), 2.11 ([¹⁸F]7), 0.40 ([¹⁸F]4), and 0.19 ([¹⁸F]8).

CONCLUSION

The observed tumor uptake and clearance profiles demonstrate the importance of the selected prosthetic group on the pharmacokinetic profile of analyzed PSMA-targeting radiotracers. Radiotracer [¹⁸F]7 displayed the highest uptake and retention in LNCaP tumors, which exceeded uptake values of reference compound [¹⁸F]DCFPyL by more than 100 %. Despite the higher kidney and liver uptake and retention of compound [¹⁸F]7, the simple radiosynthesis and the exceptionally high tumor uptake (SUV_60min 2.11) and retention make radiotracer [¹⁸F]7 an interesting alternative to radiotracer [¹⁸F]DCFPyL for PET imaging of PSMA in prostate cancer.

Automated synthesis of PET radiotracers by copper-mediated 18 F-fluorination of organoborons: Importance of the order of addition and competing protodeborylation

In this paper, we describe the use of Cu-mediated [18 F]fluorodeboronation for the automated production of positron emission tomography radiotracers suitable for clinical use. Two recurrent issues with the method, low radiochemical conversion on automation and protoarene byproduct purification issues, have been successfully addressed. The new method was utilized to produce sterile injectable doses of [18 F]-(±)-IPMICF17, a positron emission tomography radiotracer for tropomyosin receptor kinase B/C, using an automated synthesis module. The product was isolated in 1.9 ± 0.1% isolated radiochemical yield, excellent radiochemical purity (>99%), and high specific activity (5294 ± 1227 Ci/mmol). Quality control testing confirmed that doses were suitable for clinical use.

Recent Advances in 18F Radiochemistry: A Focus on B-18F, Si-18F, Al-18F, and C-18F Radiofluorination via Spirocyclic Iodonium Ylides

Straightforward radiosynthesis protocols for ¹⁸F-labeled radiopharmaceuticals are an indispensable but often overlooked prerequisite to successfully perform molecular imaging studies in vivo by PET. In recent years, thanks to the expansion of the ¹⁸F chemical toolbox, structurally diverse and novel clinically relevant radiopharmaceuticals have been synthesized with both high efficiency and ready implementation. This article provides an overview of recent ¹⁸F-labeling methodologies, specifically for B-¹⁸F, Si-¹⁸F, Al-¹⁸F, and iodine (III)-mediated radiofluorination via the spirocyclic iodonium ylide technology.

Biodistribution of 99mTc-MAA on SPECT/CT performed for 90Y radioembolization therapy planning: a pictorial review

Purpose

To evaluate the frequency of 99mTc-MAA uptake in extrahepatic organs during 90Y radioembolization therapy planning.

Methods

This retrospective case series of 70 subjects who underwent 99mTc-MAA hepatic artery perfusion studies between January 2014 and July 2016 for 90Y radioembolization therapy planning at our institution involved direct image review for all subjects, with endpoints recorded: lung shunt fraction, extrahepatic radiotracer uptake, time from MAA injection to imaging.

Results

Combined planar and SPECT/CT imaging findings in the 70 subjects demonstrated lung shunt fraction measurements of less than 10% in 53 (76%) subjects and greater than 10% in 17 (24%) subjects. All patients demonstrated renal cortical uptake, 23 (33%) demonstrated salivary gland uptake, 23 (33%) demonstrated thyroid uptake, and 32 (46%) demonstrated gastric mucosal uptake, with significant overlap between these groups. The range of elapsed times between MAA injection and initial imaging was 41-138 min, with a mean of 92 min. There was no correlation between time to imaging and the presence of extrahepatic radiotracer uptake at any site.

Conclusions

During hepatic artery perfusion scanning for 90Y radioembolization therapy planning, extrahepatic uptake is common, particularly in the kidney, salivary gland, thyroid and gastric mucosa, and is hypothesized to result from breakdown of 99mTc-MAA over time. Given the breakdown to smaller aggregates and ultimately pertechnetate, this should not be a contraindication to actual Y-90 microsphere therapy. Although we found no correlation between time to imaging and extrahepatic uptake, most of our injection to imaging times were relatively short.

Laser-preparation of geometrically optimised samples for X-ray nano-CT

A robust and versatile sample preparation technique for the fabrication of cylindrical pillars for imaging by X‐ray nano‐computed tomography (nano‐CT) is presented. The procedure employs simple, cost‐effective laser micro‐machining coupled with focused‐ion beam (FIB) milling, when required, to yield mechanically robust samples at the micrometre length‐scale to match the field‐of‐view (FOV) for nano‐CT imaging. A variety of energy and geological materials are exhibited as case studies, demonstrating the procedure can be applied to a variety of materials to provide geometrically optimised samples whose size and shape are tailored to the attenuation coefficients of the constituent phases. The procedure can be implemented for the bespoke preparation of pillars for both lab‐ and synchrotron‐based X‐ray nano‐CT investigations of a wide range of samples.

Tropomyosin receptor kinase inhibitors: an updated patent review for 2010-2016 - Part II

INTRODUCTION

TrkA/B/C receptor activation supports growth, survival, and differentiation of discrete neuronal populations during development, adult life, and ageing but also plays numerous roles in human disease onset and progression. Trk-specific inhibitors have therapeutic applications in cancer and pain and thus constitute a growing area of interest in oncology and neurology. There has been substantial growth in the number of structural classes of Trk inhibitors and the number of industrial entrants to the Trk inhibitor field over the past six years. Areas covered: In Part II of this two-part review, the discussion of recent patent literature covering Trk family inhibitors is continued from Part I and clinical research with Trk inhibitors is considered. Expert opinion: Trk has been molecularly targeted for over a decade resulting in the progressive evolution of structurally diversified Trk inhibitors arising from scaffold hopping and HTS efforts. Correspondingly, there have been a growing number of clinical investigations utilizing Trk inhibitors in recent years, with a particular focus on the treatment of NTRK-fusion positive cancers and chronic pain. The observed potential of Trk inhibitors to cause adverse CNS side effects however suggests the need for a more rigorous consideration of BBB permeation capabilities during drug development.

Tropomyosin receptor kinase inhibitors: an updated patent review for 2010-2016 - Part I

INTRODUCTION

Tropomyosin receptor kinases (TrkA/B/C) play crucial roles in the development and maintenance of the nervous system, and aberrant expression of Trk has been implicated in neurological disorders as well as neural and non-neural neoplasms. Patent activity encompassing Trk inhibitors has grown substantially over the last 6 years, recognized by a rise in the number of pharmaceutical entrants to the field and the escalation of novel inhibitor chemotypes. Area covered: In Part I of this two part review, a biological and structural overview of Trk is provided in the context of Trk as a therapeutic target for cancer and pain, followed by the report of recent patent literature claiming small molecule inhibitors of Trk family kinases or which describe inhibitors developed for other kinase targets but include noteworthy Trk inhibition/application. The discussion of the patent literature continues in Part II of this review, which includes an in-depth view of the current clinical applications of Trk inhibitors. Expert opinion: Substantial synthetic efforts in Trk inhibitor development has propagated numerous and diverse inhibitor chemotypes, including TrkA-specific inhibitors. While many novel Trk inhibitors remain the original progeny of Trk-specific development programs, kinase inhibitors initially developed for other kinases have also been successfully repositioned for Trk.

Antiproton stopping power data for radiation therapy simulations

Stopping powers of H, He, H₂, and H₂O targets for antiprotons have been calculated using a convergent close-coupling method. For He and H₂ targets electron-electron correlations are fully accounted for using a multiconfiguration approximation. Two-electron processes are included using an independent-event model. The water molecule is described using a neon-like structure model with a pseudo-spherical potential. Results are tabulated for the purpose of Monte Carlo simulations to model antiproton transport through matter for radiation therapy.

From Unorthodox to Established: The Current Status of (18)F-Trifluoroborate- and (18)F-SiFA-Based Radiopharmaceuticals in PET Nuclear Imaging

Unorthodox (18)F-labeling strategies not employing the formation of a carbon-(18)F bond are seldom found in radiochemistry. Historically, the formation of a boron- or silicon-(18)F bond has been introduced very early on into the repertoire of labeling chemistries, but is without translation into any clinical radiotracer besides inorganic B[(18)F]F4(-) for brain tumor diagnosis. For many decades these labeling methodologies were forgotten and have just recently been revived by a handful of researchers thinking outside the box. When breaking with established paradigms such as the inability to obtain labeled compounds of high specific activity via isotopic exchange or performing radiofluorination in aqueous media, the research community often reacts skeptically. In 2005 and 2006, two novel labeling methodologies were introduced into radiochemistry for positron emission tomography (PET) tracer development: RBF3(-) labeling reported by Perrin et al. and the SiFA methodology by Schirrmacher, Jurkschat, and Waengler et al. which is based on isotopic exchange (IE). Both labeling methodologies have been complemented by other noncanonical strategies to introduce (18)F into biomolecules of diagnostic importance, thus profoundly enriching the landscape of (18)F radiolabeling. B- and Si-based labeling strategies finally revealed that IE is a viable alternative to established and traditional radiochemistry with the advantage of simplifying both the labeling effort as well as the necessary purification of the radiotracer. Hence IE will be the focus of this contribution over other noncanonical labeling methods. Peptides for tumor imaging especially lend themselves favorably toward one-step labeling via IE, but small molecules have been described as well, taking advantage of these new approaches, and have been used successfully for brain imaging. This Review gives an account of both radiochemistries centered on boron and silicon, describing the very beginnings of their basic research, the path that led to optimization of their chemistries, and the first encouraging preclinical results paving the way to their clinical use. This side by side approach will give the reader the opportunity to follow the development of a new basic discovery into a clinically applicable radiotracer including all the hurdles that have had to be overcome.

Development of subnanomolar radiofluorinated (2-pyrrolidin-1-yl)imidazo[1,2-b]pyridazine pan-Trk inhibitors as candidate PET imaging probes

Dysregulation of tropomyosin receptor kinases (TrkA/B/C) expression and signalling is recognized as a hallmark of numerous neurodegenerative diseases including Parkinson's, Huntington's and Alzheimer's disease.

Synthesis of high-mannose 1-thio glycans and their conjugation to protein

The oligosaccharides Man4 and Man5, substructures of the high-mannose glycans of HIV glycoprotein gp120, were synthesized with a terminal 1-thiomannopyranose residue. The anomeric thiol can be readily converted to an azidomethyl aglycone through reaction with dichloromethane and displacement with sodium azide. The resulting oligomannans were then conjugated to ubiquitin utilizing thiol alkylation or azide/alkyne reactive tethers of minimal length. By combining high efficiency conjugation reactions and a short tether, we sought to establish conjugation conditions that would permit high density clustering of oligomannans in conjugate vaccines that could produce antibodies able to bind gp120 and potentially neutralize virus. LC-UV-MS was used to separate, identify and quantify the ubiquitin glycoconjugates with differing degrees of oligomannan incorporation. Binding of the HIV protective monoclonal antibody 2G12 and concanavalin A to microtitre plates coated with glycoconjugates was measured by ELISA.

Self-adjuvanting glycopeptide conjugate vaccine against disseminated candidiasis

Our research on pathogenesis of disseminated candidiasis led to the discovery that antibodies specific for Candida albicans cell surface β-1, 2–mannotriose [β-(Man)₃] protect mice. A 14 mer peptide Fba, which derived from the N-terminal portion of the C. albicans cytosolic/cell surface protein fructose-bisphosphate aldolase, was used as the glycan carrier and resulted in a novel synthetic glycopeptide vaccine β-(Man)₃-Fba. By a dendritic cell-based immunization approach, this conjugate induced protective antibody responses against both the glycan and peptide parts of the vaccine. In this report, we modified the β-(Man)₃-Fba conjugate by coupling it to tetanus toxoid (TT) in order to improve immunogenicity and allow for use of an adjuvant suitable for human use. By new immunization procedures entirely compatible with human use, the modified β-(Man)₃-Fba-TT was administered either alone or as a mixture made with alum or monophosphoryl lipid A (MPL) adjuvants and given to mice by a subcutaneous (s.c.) route. Mice vaccinated with or, surprisingly, without adjuvant responded well by making robust antibody responses. The immunized groups showed a high degree of protection against a lethal challenge with C. albicans as evidenced by increased survival times and reduced kidney fungal burden as compared to control groups that received only adjuvant or DPBS buffer prior to challenge. To confirm that induced antibodies were protective, sera from mice immunized against the β-(Man)₃-Fba-TT conjugate transferred protection against disseminated candidiasis to naïve mice, whereas C. albicans-absorbed immune sera did not. Similar antibody responses and protection induced by the β-(Man)₃-Fba-TT vaccine was observed in inbred BALB/c and outbred Swiss Webster mice. We conclude that addition of TT to the glycopeptide conjugate results in a self-adjuvanting vaccine that promotes robust antibody responses without the need for additional adjuvant, which is novel and represents a major step forward in vaccine design against disseminated candidiasis.

Endoscopic full-thickness resection of gastric tumors using a novel grasp-and-snare technique: feasibility in ex vivo and in vivo porcine models

BACKGROUND AND STUDY AIMS

Endoscopic full-thickness resection (EFTR) is a less-invasive method of en bloc removal of gastrointestinal tract tumors. The aim of this study was to evaluate the feasibility of a grasp-and-snare EFTR technique using a novel tissue-lifting device that provides more secure tissue anchoring and manipulation.

METHODS

EFTR of normal gastric tissue and model stomach tumors was performed using a double-channel therapeutic endoscope with a prototype tissue-lifting device through one channel and a prototype hexagonal snare through the other. The lifting device was advanced through the open snare and anchored to the gastric wall immediately adjacent the model tumor. The tissue-lifting device was then partially retracted into the endoscope, causing the target tissue, including tumor, to evert into the gastric lumen. The open snare was then placed distal to the tumor around uninvolved gastric tissue. Resection was performed with a blended electrosurgical current through the snare. In the live pigs, EFTR was followed by laparotomy to asses for complications.

RESULTS

24 EFTRs were performed -- 14 in explanted stomachs and 10 in live pigs. In total, 23/24 resections resulted in full-thickness gastric defects. Resection specimens measured up to 5.0 cm when stretched and pinned on a histology stage. Gross margins were negative in 17/20 model tumor resections. Two resections were complicated by gastric mural bleeding. There was no evidence of adjacent organ injury.

CONCLUSIONS

EFTR of gastric tumors using the grasp-and-snare technique is feasible in pigs. This technique is advantageous in that eversion of the gastric wall avoids injury to external organs, continuous luminal insufflation is not required, and the involved techniques are familiar to endoscopists. Additional research is necessary to further evaluate safety and reliable closure.

Utility of current risk stratification tests for predicting major arrhythmic events after myocardial infarction

OBJECTIVES

We surveyed the literature to estimate prediction values for five common tests for risk of major arrhythmic events (MAEs) after myocardial infarction. We then determined feasibility of a staged risk stratification using combinations of noninvasive tests, reserving an electrophysiologic study (EPS) as the final test.

BACKGROUND

Improved approaches are needed for identifying those patients at highest risk for subsequent MAE and candidates for implantable cardioverter-defibrillators.

METHODS

We located 44 reports for which values of MAE incidence and predictive accuracy could be inferred: signal-averaged electrocardiography; heart rate variability; severe ventricular arrhythmia on ambulatory electrocardiography; left ventricular ejection fraction; and EPS. A meta-analysis of reports used receiver-operating characteristic curves to estimate mean values for sensitivity and specificity for each test and 95% confidence limits. We then simulated a clinical situation in which risk was estimated by combining tests in three stages.

RESULTS

Test sensitivities ranged from 42.8% to 62.4%; specificities from 77.4% to 85.8%. A three-stage stratification yielded a low-risk group (80.0% with a two-year MAE risk of 2.9%), a high-risk group (11.8% with a 41.4% risk) and an unstratified group (8.2% with an 8.9% risk equivalent to a two-year incidence of 7.9%).

CONCLUSIONS

Sensitivities and specificities for the five tests were relatively similar. No one test was satisfactory alone for predicting risk. Combinations of tests in stages allowed us to stratify 91.8% of patients as either high-risk or low-risk. These data suggest that a large prospective study to develop a robust prediction model is feasible and desirable.

Dysrhythmia hazard after hospitalization for myocardial infarction: two ECG prognostic methods compared

We retrieved reports of heart rate variability and signal-averaged electrocardiograms (SAECG) used to predict risk of a dysrhythmic event. From each report the number of cases with and without events was extracted to establish accurate values for true positive rate (tpr = sensitivity) and false positive rate (fpr = 1 minus specificity). For all the heart rate variability reports, these values were collected and tpr values were plotted versus fpr. The (fpr,tpr) data were summarized by a meta ROC graph using the method of Moses and Shapiro. A composite weighted mean value and 95% confidence interval were also derived. A summary meta-ROC curve for the SAECG reports was similarly obtained., Meta-ROC analysis of multiple reports better summarizes the performances of different prognostic methods and allows the effect of combining tests for a larger population to be simulated.

Neurally mediated cardiac syncope: autonomic modulation after normal saline infusion

OBJECTIVES

This study assessed the heart variability response to orthostatic stress during tilt table testing before and after normal saline administration.

BACKGROUND

The efficacy of sodium chloride and mineralocortoid in the treatment of neurally mediated cardiac syncope is attributed to intravascular volume expansion; however, their modulation of autonomic nervous system activity has not been evaluated.

METHODS

Heart rate variability analysis was performed on 12 adolescents with a history of syncope or presyncope (mean age 15.2+/-0.7 years) during tilt table testing. Subjects were upright 80 degrees for 30 min or until syncope. After normal saline administration, the patient was returned upright for 30 min. Heart rate variability analysis data were analyzed by an autoregression model (Burg method).

RESULTS

All subjects reproducibly developed syncope during control tilt table testing; median time to syncope was 9.4+/-2.1 min. After normal saline infusion, none of the subjects developed syncope after 30 min upright. In the control tilt, there was an initial increase followed by a progressive decrease in low frequency power until syncope. Repeat tilt after normal saline administration demonstrates that low frequency power increased but the magnitude of initial change was blunted when compared with control. In addition, low frequency power increased during normal saline tilt sequence compared with the control tilt, during which it decreased.

CONCLUSIONS

Normal saline blunted low frequency power stimulation and prevented paradoxical low frequency power (sympathetic) withdrawal. Increasing intravascular volume with normal saline alters autonomic responses that may trigger neurally mediated syncope reflexes.

NHLBI workshop on the utilization of ECG databases: preservation and use of existing ECG databases and development of future resources

Baseline examinations and periodic reexaminations in longitudinal population studies, together with ongoing surveillance for morbidity and mortality, provide unique opportunities for seeking ways to enhance the value of the electrocardiogram (ECG) recorded with digital technology as an inexpensive and noninvasive tool for prognosis and diagnosis. Clinicians, epidemiologists, and engineers from industry, government, and academic medical centers gathered at a workshop sponsored by the National Heart, Lung, and Blood Institute (NHLBI) on June 11-12, 1997, to discuss the research potential of ECG databases, their preservation and accession, and standards for recording and storage. Databases considered were those acquired in ongoing and future NHLBI-funded studies and in clinical settings in which the ECG continues to provide valuable information for evaluation and treatment. The accessibility of existing databases, the quality of their data, and the availability of ancillary demographic and clinical information were major themes. Also discussed were appropriate statistical methodologies to be used with these data for developing and testing ECG algorithms. The workshop participants affirmed the value of these databases and urged the establishment of an ECG advisory and review group to (1) resolve technical and proprietary issues for the utilization of currently existing databases; (2) develop standards for recording, storage, and utilization of ECGs in future NHLBI-supported studies; (3) oversee the creation of a national ECG database resource, consisting of an archive of ECG databases from past and ongoing NHLBI-supported studies, and a registry of ECG databases that would eventually include digital ECGs from populations currently underrepresented in the demographic spectrum of the NHLBI databases.

Panoramic display of the orderly sequenced 12-lead ECG

The standard 12-lead electrocardiogram (ECG) has been developed over many years. The ECG has had a long and successful history of providing diagnostic information in clinical medicine. Cardiac arrhythmias have been elucidated by deductive reasoning from continuous ECG recordings with confirmation from electrophysiologic studies. Recently, there has been renewed interest in the morphology of the QRS complex, ST-segment, and T wave, which raises the important question of considering whether the usual method of display provides maximal diagnostic capabilities. The conventional display provides a logical visualization of precordial lead recordings representing the horizontal plane, but does not provide a logical visualization of the limb lead recordings representing the frontal plane. Many clinical problems require the consideration of serial ECGs necessitating the comparison of separate pages. An alternate format presenting serial recordings on a single page would be advantageous. Some automated ECG analysis systems already include the capability for multiple display formats, but these have not yet been widely used in clinical practice. This point of view paper introduces a new display format for the standard 12-lead ECG that includes: (1) a presentation of an orderly sequence of leads to facilitate scanning through different points in space and (2) a presentation of recordings of 12-lead sequences to facilitate scanning through different points in time. This display format could either replace or supplement the conventional ECG format.

Specialist outreach clinics in general practice

OBJECTIVES

To establish the extent and nature of specialist outreach clinics in primary care and to describe specialists' and general practitioners' views on outreach clinics.

DESIGN

Telephone interviews with hospital managers. Postal questionnaire surveys of specialists and general practitioners.

SETTING

50 hospitals in England and Wales.

SUBJECTS

50 hospital managers, all of whom responded. 96 specialists and 88 general practitioners involved in outreach clinics in general practice, of whom 69 (72%) and 46 (52%) respectively completed questionnaires. 122 additional general practitioner fundholders, of whom 72 (59%) completed questionnaires.

MAIN OUTCOME MEASURES

Number of specialist outreach clinics; organisation and referral mechanism; waiting times; perceived benefits and problems.

RESULTS

28 of the hospitals had a total of 96 outreach clinics, and 32 fundholders identified a further 61 clinics. These clinics covered psychiatry (43), medical specialties (38), and surgical specialties (76). Patients were seen by the consultant in 96% (107) of clinics and general practitioners attended at only six clinics. 61 outreach clinics had shorter waiting times for first outpatient appointment than hospital clinics. The most commonly reported benefits for patients were ease of access and shorter waiting times.

CONCLUSIONS

Specialist outreach clinics cover a wide range of specialties and are popular, especially in fundholding practices. These clinics do not seem to have increased the interaction between general practitioners and specialists.

Improved detection of echocardiographic left ventricular hypertrophy using a new electrocardiographic algorithm

OBJECTIVES

The purpose of this study was to use the Framingham data base to devise and test an improvement in an electrocardiographic (ECG) voltage criterion for detecting left ventricular hypertrophy that is gender specific and adjusts for age and obesity.

BACKGROUND

Electrocardiographic detection of left ventricular hypertrophy has been receiving increasing attention. The "Cornell" ECG voltage, defined as the sum of voltages for the R wave of lead aVL and S wave of lead V3, has been shown to correlate strongly with echocardiographically estimated left ventricular mass. Because the magnitude of this voltage varies with both age and obesity, we have proposed a simple formula for its adjustment for these two variables.

METHODS

Using linear regression, the adjustment formula was estimated from data on 1,468 men and 1,883 women from the Framingham Heart Study cohort who were free of myocardial infarction and who had both an ECG and an echocardiogram recorded during the same clinic examination. A modified receiver operating characteristic curve method was used to compare sensitivities at the same specificity levels. The adjustment formula was estimated from one randomly chosen half of the study cohort and applied to the other half for evaluation.

RESULTS

Significant improvement in sensitivity for the detection of left ventricular hypertrophy was realized at all levels of specificity. At a specificity level of 98%, the adjustment increased the sensitivity of the Cornell voltage from 10% to 17% in men and from 12% to 22% in women. For severe hypertrophy, defined as a left ventricular mass > 3 SD above the gender-specific mean, the sensitivity increased from 23% to 38% for men and from 22% to 55% for women at a specificity level of 95%.

CONCLUSIONS

This approach can substantially enhance the utility of the ECG for the detection of left ventricular hypertrophy. If these results are validated in other population groups, this approach may prove valuable in the screening of hypertensive populations and for the monitoring of patients undergoing treatment for hypertension.

A kinetic model of CD4+ lymphocytes with the human immunodeficiency virus (HIV)

This report describes a kinetic model of in vitro cytopathology involving interactions of human immunodeficiency virus (HIV) with CD4+ helper T lymphocytes. The model uses nonlinearly coupled, ordinary differential equations to simulate the dynamics of infected and uninfected cells and free virions. It is assumed that resting cells are more readily infected than activated cells, but once infected, only activated cells produce more virus. Resting cells can be activated by some appropriate stimulus (e.g. phytohemagglutinin, soluble antigen). The model predicts that the initial inoculum of virus is taken up by resting cells and without stimulation the system comes to a steady state of two populations, namely infected and uninfected cells. Stimulation of this system produces two additional populations, namely infected and uninfected activated cells which, along with the previous populations, exhibit cyclic behavior of growth, viral expression/release, and death. Additional stimuli enhance or diminish the cyclic behavior depending upon their occurrence in time. These simulations suggest a similar dynamics in human HIV infection and may explain a major factor responsible for the widely varying depletion rate of (CD4+) helper T cells in AIDS patients.

A filter to suppress ECG baseline wander and preserve ST-segment accuracy in a real-time environment

Accurate monitoring of the ST-segment displacements in real-time environments can be distorted by the nonlinear phase response of a baseline filter such as the single-pole, high-pass (0.5 Hz) filter that is standard in the industry today. The authors have previously constructed a four-pole null phase (1.0 Hz) filter that is nearly ideal in suppressing baseline wander while preserving ST-segment accuracy; however, this foreward/backward filter requires capture of a large ECG segment before filtering, thereby producing a delay that is unacceptable in a real-time environment. As a practical compromise, a two-pole, phase-compensated (1.0 Hz) filter was constructed while introducing a small time delay (160 ms). It performs much better than the "standard filter" and almost as well as the "ideal" filter in several tests, namely (1) suppression of baseline wander in a series of ECGs, (2) suppression of artificial baseline, (3) response to a triangular impulse wave (American Heart Association test), and (4) J-point displacement in several ECGs.

Suppression of baseline wander in the ECG using a bilinearly transformed, null-phase filter

The purpose of this study was to design and test a bilinearly transformed, null-phase (BLT/NP) filter for removing baseline wander and to compare it with the cubic spline for performance. For this purpose, the ECG data were filtered to remove high-frequency noise and low-frequency baseline wander to form a set of "clean" ECGs. Artificial low-frequency noise mimicking typical baseline wander was constructed from sine and cosine waves at 0.20 and 0.45 Hz and with amplitudes of 400 and 300 microV, respectively, and added to the "clean" ECGs to form the "test" ECGs. The BLT/NP filter and the cubic spline method each were applied to a "test" ECG to form a "restored" ECG. The measure of performance was the root mean square difference (RMSD) between the "restored" ECG and the initial "clean" ECG. RMSD values showed that on the average the BLT/NP filter performed as well as the cubic spline method and has the advantage that accurate determination of the QRS onset is not required.

Progress on the CSE diagnostic study. Application of McNemar's test revisited

The authors describe an extension of McNemar's test that can be used to compare diagnostic performance when multiple statements are obtained from computer analysis or visual interpretation of the ECG. If two or more diagnostic statements are made, by definition only one can be correct for the cases in the CSE pilot database, which were selected for single, clinically well documented, abnormalities. If one statement stood out from others as being made with the highest degree of certainty, that was accepted as the single interpretation, right or wrong. However, when two or more statements were made with the same degree of certainty and only one statement was correct, then in the previous application of McNemar's test that interpretation was given credit for being correct. In the extension of the method presented in this article, such an interpretation is given partial credit for the one correct statement and partial discredit for any incorrect statement, thereby reporting the results more properly in the sensitivity and specificity statistics for the different diagnostic categories.

Nonparametric comparison of entire ROC curves for computerized ECG left ventricular hypertrophy algorithms using data from the Framingham Heart Study

A computer program may be capable of several different statements for left ventricular hypertrophy (eg, possible LVH, probable LVH, consistent with LVH), but such statements resulting from discretized levels of sensitivity/specificity would represent only isolated points on a receiver-operating characteristic (ROC) curve, which is a plot of all levels of sensitivity versus specificity. Even if two algorithms use the same discrete scales, their performances may not readily be compared. The authors present a comparison methodology for ROC curves using ROC area as a nonparametric measure of the ability of the algorithm to separate the two populations; the ROC area ranges from 0.5 (no ability) to 1.0 (perfect separation) and is unbiased if the normal versus abnormal populations have no common values for the measurement. The methodology compares the performance of ECG algorithms on the same population of cases by testing for significant differences of ROC areas and incorporating correlation of the algorithms in a nonparametric way. To illustrate this methodology, they use ECG and echocardiographic data from the Framingham Heart Study.

Studies of Trypanosoma cruzi clones in inbred mice. III. Histopathological and electrocardiographical responses to chronic infection

Histopathological and electrocardiographical (ECG) changes occur in the heart of C3H/HeN and C57BL/6 mice infected for 1 year with Trypanosoma cruzi clones Sylvio-X10/4 (X10/4), Miranda/78 (M/78), or Miranda/80 (M/80). Heart parasitism and a variable degree of inflammation occurred following infection with clones X10/4 or M/78 but not with M/80. Clone X10/4 caused more extensive myocardial inflammation and fibrosis than clone M/78. Myocardial fibrosis was more extensive in C3H than in C57 mice infected with clone X10/4. The normal ECG pattern of C3H mice is distinctly different from C57 mice. The PR intervals of mice infected with clone X10/4 greater than M/78 greater than M/80 approximately equal to controls. ECG abnormalities occurred more frequently in mice infected with clone X10/4 than in controls or mice infected with either M/78 or M/80 regardless of strain or sex and were generally more severe in C57 than in C3H infected with X10/4. First degree atrioventricular block occurred more frequently in C3H mice infected with clone X10/4 or M/78 and C57 mice infected with X10/4 than in all other groups. Complete atrioventricular dissociation occurred frequently in C57 mice infected with X10/4 and rarely in other mice. These results demonstrate that the myocardial response of mice to T. cruzi infection, both histological and electrophysiological, is modulated by both the mouse strain and the parasite isolate used.

The emerging prominence of computer electrocardiography in large population-based surveys

The year 1987 marks the centennial of electrocardiography. As Beamish reminds us, it was the first and foremost technical aid to the cardiologist. During the history of cardiovascular epidemiology, which dates back roughly forty years, electrocardiography has played a central role as a relatively specific marker for coronary and hypertensive heart disease. The lack of sensitivity of electrocardiography and the advent of new technologies such as ultrasonography for assessment of such heart disease in cardiology and cardiovascular epidemiology has raised questions as to whether electrocardiography might be replaced in the next century. Recent developments in computer electrocardiography have rendered suggestions of the decreased importance of electrocardiography premature. Use of computer analysis of electrocardiograms in epidemiologic studies, which dates back less than three decades, has renewed interest in and expanded the role of electrocardiography in cardiology in general and cardiovascular epidemiology in particular. The computer analysis of electrocardiograms has allowed the processing and analysis of large numbers of electrocardiograms in a rapid, efficient, systematic and consistent manner. This has led to new, more sensitive criteria for detection of left ventricular hypertrophy as a discrete variable. In addition, the numerous waveform amplitude and duration measurements by computer have allowed the development of mathematical models to express the likelihood of coronary heart disease (myocardial infarction) and degree of left ventricular hypertrophy (estimates of left ventricular mass) on continuous scales. These developments have contributed substantially to the potential of electrocardiography (and particularly computer electrocardiography) in future studies and clinical applications.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrocardiographic changes with advancing age. A cross-sectional study of the association of age with QRS axis, duration and voltage

In order to examine the electrocardiographic (ECG) changes which occur with advancing age we defined an apparently healthy reference population derived from prospectively followed subjects of the Framingham Heart Study. Healthy subjects were clinically free of hypertension, coronary artery disease, congestive heart failure and valve disease and were not taking antihypertensive or other cardiac medications. ECG tracings were analyzed by the IBM Bonner (V2) program. Mean values and correlations with age for PR duration, QRS duration and axis, S wave voltage V1 and R wave voltage V5 are presented. With advancing age in men there is a narrowing of QRS, a leftward QRS axis shift, and a loss of S V1 and R V5 amplitude. In women only a leftward QRS axis shift is associated with advancing age. These changes should be considered in defining normal age- and sex-specific reference values. These findings underscore theoretical limitations of commonly-used criteria for the ECG diagnosis of conditions such as left ventricular hypertrophy.