Pharmacokinetic analysis and uptake of 18F-FBPA-Fr after ultrasound-induced blood-brain barrier disruption for potential enhancement of boron delivery for neutron capture therapy

Theranostics in Neurooncology: Heading Toward New Horizons

Therapeutic approaches to brain tumors remain a challenge, with considerable limitations regarding delivery of drugs. There has been renewed and increasing interest in translating the popular theranostic approach well known from prostate and neuroendocrine cancer to neurooncology. Although far from perfect, some of these approaches show encouraging preliminary results, such as for meningioma and leptomeningeal spread of certain pediatric brain tumors. In brain metastases and gliomas, clinical results have failed to impress. Perspectives on these theranostic approaches regarding meningiomas, brain metastases, gliomas, and common pediatric brain tumors will be discussed. For each tumor entity, the general context, an overview of the literature, and future perspectives will be provided. Ongoing studies will be discussed in the supplemental materials. As most theranostic agents are unlikely to cross the blood-brain barrier, the delivery of these agents will be dependent on the successful development and clinical implementation of techniques enhancing permeability and retention. Moreover, the international community should strive toward sufficiently large and randomized studies to generate high-level evidence on theranostic approaches with radioligand therapies for central nervous system tumors.

α-Synuclein Radiotracer Development and In Vivo Imaging: Recent Advancements and New Perspectives

α-Synucleinopathies including idiopathic Parkinson's disease, dementia with Lewy bodies and multiple systems atrophy share overlapping symptoms and pathological hallmarks. Selective neurodegeneration and Lewy pathology are the main hallmarks of α-synucleinopathies. Currently, there is no imaging biomarker suitable for a definitive early diagnosis of α-synucleinopathies. Although dopaminergic deficits detected with single-photon emission computed tomography (SPECT) and positron emission tomography (PET) radiotracers can support clinical diagnosis by confirming the presence of dopaminergic neurodegeneration, dopaminergic imaging cannot visualize the preceding disease process, nor distinguish α-synucleinopathies from tauopathies with dopaminergic neurodegeneration, especially at early symptomatic disease stage when clinical presentation is often overlapping. Aggregated α-synuclein (αSyn) could be a suitable imaging biomarker in α-synucleinopathies, because αSyn aggregation and therefore, Lewy pathology is evidently an early driver of α-synucleinopathies pathogenesis. Additionally, several antibodies and small molecule compounds targeting aggregated αSyn are in development for therapy. However, there is no way to directly measure if or how much they lower the levels of aggregated αSyn in the brain. There is clearly a paramount diagnostic and therapeutic unmet medical need. To date, aggregated αSyn and Lewy pathology inclusion bodies cannot be assessed ante-mortem with SPECT or PET imaging because of the suboptimal binding characteristics and/or physicochemical properties of current radiotracers. The aim of this narrative review is to highlight the suitability of aggregated αSyn as an imaging biomarker in α-synucleinopathies, the current limitations with and lessons learned from αSyn radiotracer development, and finally to propose antibody-based ligands for imaging αSyn aggregates as a complementary tool rather than an alternative to small molecule ligands. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

Safety and efficacy of focused ultrasound induced blood-brain barrier opening, an integrative review of animal and human studies

The blood-brain barrier, while fundamental in maintaining homeostasis in the central nervous system, is a bottleneck to achieving efficacy for numerous therapeutics. Improved brain penetration is also desirable for reduced dose, cost, and systemic side effects. Transient disruption of the blood-brain barrier with focused ultrasound (FUS) can facilitate drug delivery noninvasively with precise spatial and temporal specificity. FUS technology is transcranial and effective without further drug modifications, key advantages that will accelerate adoption and translation of existing therapeutic pipelines. In this review, we performed a comprehensive literature search to build a database and provide a synthesis of ultrasound parameters and drug characteristics that influence the safety and efficacy profile of FUS to enhance drug delivery.

Enhancing Boron Uptake in Brain Glioma by a Boron-Polymer/Microbubble Complex with Focused Ultrasound

Boron neutron capture therapy (BNCT) is a promising radiotherapy for treating glioblastoma multiforme (GBM). However, the penetration of drugs (e.g., sodium borocaptate and BSH) for BNCT into brain tumors is limited by cerebral vesicular protective structures, the blood-brain barrier, and the blood-brain tumor barrier (BTB). Although BSH has been reported to be selectively taken up by tumors, it is rapidly excreted from the body and cannot achieve a high tumor-to-normal brain ratio (T/N ratio) and tumor-to-blood ratio (T/B ratio). Despite the development of large-molecular weight boron compounds, such as polymers and nanoparticles, to enhance the permeation and retention effect, their effects remain insufficient for clinical use. To improve the efficiency of boron delivery to the tumor site, we propose combinations of self-assembled boron-containing polyanion [polyethylene glycol- b-poly(( closo-dodecaboranyl)thiomethylstyrene) (PEG- b-PMBSH)] nanoparticles (295 ± 2.3 nm in aqueous media) coupled with cationic microbubble (B-MB)-assisted focused ultrasound (FUS) treatment. Upon FUS sonication (frequency = 1 MHz, pressure = 0.3-0.7 MPa, duty cycle = 0.5%, sonication = 1 min), B-MBs can simultaneously achieve safe BTB opening and boron drug delivery into tumor tissue. Compared with the MBs of the PEG- b-PMBSH mixture group (B + MBs), B-MBs showed 3- and 2.3-fold improvements in the T/N (4.4 ± 1.4 vs 1.3 ± 0.1) and T/B ratios (1.4 ± 0.6 vs 0.1 ± 0.1), respectively, after 4 min of FUS sonication. The spatial distribution of PEG- b-PMBSH was also improved by the complex of PEG- b-PMBSH with MBs. The findings presented herein, in combination with the expanding clinical application of FUS, may improve BNCT and treatment of GBM.

4-Borono-2-18F-fluoro-L-phenylalanine PET for boron neutron capture therapy-oriented diagnosis: overview of a quarter century of research

4-¹⁰B-Borono-2-¹⁸F-fluoro-L-phenylalanine (¹⁸F-FBPA) was developed for monitoring the pharmacokinetics of 4-¹⁰B-borono-L-phenylalanine (¹⁰B-BPA) used in boron neutron capture therapy (BNCT) with positron emission tomography (PET). The tumor-imaging potential of ¹⁸F-FBPA was demonstrated in various animal models. Accumulation of ¹⁸F-FBPA was higher in melanomas than in non-melanoma tumors in animal models and cell cultures. ¹⁸F-FBPA was incorporated into tumors mediated mainly by L-type amino acid transporters in in vitro and in vivo models. Tumoral distribution of ¹⁸F-FBPA was primarily related to the activity of DNA synthesis. ¹⁸F-FBPA is metabolically stable but is incorporated into melanogenesis non-enzymatically. These in vitro and in vivo characteristics of ¹⁸F-FBPA corresponded well to those of ¹⁰B-BPA. Nuclear magnetic resonance and other studies using non-radioactive ¹⁹F-^10/11B-FBPA also contributed to characterization. The validity and reliability of ^18/19F-FBPA as an in vivo probe of ¹⁰B-BPA were confirmed by comparison of the pharmacokinetics of ¹⁸F-FBPA and ¹⁰B-BPA and direct measurement of both ¹⁸F and ¹⁰B in tumors with various doses of both probes administered by different routes and methods. Clinically, based on the kinetic parameters of dynamic ¹⁸F-FBPA PET, the estimated ¹⁰B-concentrations in tumors with continuous ¹⁰B-BPA infusion were similar to those measured directly in surgical specimens. The significance of ¹⁸F-FBPA PET was verified for the estimation of ¹⁰B-concentration and planning of BNCT. Later ¹⁸F-FBPA PET has been involved in ¹⁰B-BPA BNCT of patients with intractable tumors such as malignant brain tumors, head and neck tumors, and melanoma. Usually a static PET scan is used for screening patients for BNCT, prediction of the distribution and accumulation of ¹⁰B-BPA, and evaluation of treatment after BNCT. In some clinical trials, a tumor-to-normal tissue ratio of ¹⁸F-FBPA > 2.5 was an inclusion criterion for BNCT. Apart from BNCT, ¹⁸F-FBPA was demonstrated to be a useful PET probe for tumor diagnosis in nuclear medicine: better tumor-to-normal brain contrast compared with ¹¹C-methionine, differentiation of recurrent and radiation necrosis after radiotherapy, and melanoma-preferential uptake. Further progress in ¹⁸F-FBPA studies is expected for more elaborate evaluation of ¹⁰B-concentrations in tumors and normal tissues for successful ¹⁰B-BPA BNCT and for radiosynthesis of ¹⁸F-FBPA to enable higher ¹⁸F-activity amounts and higher molar activities.

Evaluation of D-isomers of 4-borono-2-18F-fluoro-phenylalanine and O-11C-methyl-tyrosine as brain tumor imaging agents: a comparative PET study with their L-isomers in rat brain glioma

Background

The potential of the D-isomerization of 4-borono-2-¹⁸F-fluoro-phenylalanine (¹⁸F-FBPA) to improve its target tumor to non-target normal brain tissue ratio (TBR) was evaluated in rat brain glioma and compared with those of L- and D-¹¹C-methyl-tyrosine (¹¹C-CMT).

The L- or D-isomer of ¹⁸F-FBPA was injected into rats through the tail vein, and their whole body kinetics and distributions were assessed using the tissue dissection method up to 90 min after the injection. The kinetics of L- and D-¹⁸F-FBPA or L- and D-¹¹C-CMT in the C-6 glioma-inoculated rat brain were measured for 90 or 60 min, respectively, using high-resolution animal PET, and their TBRs were assessed.

Results

Tissue dissection analyses showed that D-¹⁸F-FBPA uptake was significantly lower than that of L-¹⁸F-FBPA in the brain and abdominal organs, except for the kidney and bladder, reflecting the faster elimination rate of D-¹⁸F-FBPA than L-¹⁸F-FBPA from the blood to the urinary tract. PET imaging using ¹⁸F-FBPA revealed that although the brain uptake of D-¹⁸F-FBPA was significantly lower than that of L-¹⁸F-FBPA, the TBR of the D-isomer improved to 6.93 from 1.45 for the L-isomer. Similar results were obtained with PET imaging using ¹¹C-CMT with a smaller improvement in TBR to 1.75 for D-¹¹C-CMT from 1.33 for L-¹¹C-CMT.

Conclusions

The present results indicate that D-¹⁸F-FBPA is a better brain tumor imaging agent with higher TBR than its original L-isomer and previously reported tyrosine-based PET imaging agents. This improved TBR of D-¹⁸F-FBPA without any pre-treatments, such as tentative blood-brain barrier disruption using hyperosmotic agents or sonication, suggests that the D-isomerization of BPA results in the more selective accumulation of ¹⁰B in tumor cells that is more effective and less toxic than conventional L-BPA.

Reliable radiosynthesis of 4-[10B]borono-2-[18F]fluoro-L-phenylalanine with quality assurance for boron neutron capture therapy-oriented diagnosis

Objective

The aim of this study was to establish a reliable and routine method for the preparation of 4-[¹⁰B]borono-2-[¹⁸F]fluoro-l-phenylalanine (l-[¹⁸F]FBPA) for boron neutron capture therapy-oriented diagnosis using positron emission tomography.

Methods

To produce l-[¹⁸F]FBPA by electrophilic fluorination of 4-[¹⁰B]borono-l-phenylalanine (l-BPA) with [¹⁸F]acetylhypofluorite ([¹⁸F]AcOF) via [¹⁸F]F₂ derived from the ²⁰Ne(d,α)¹⁸F nuclear reaction, several preparation parameters and characteristics of l-[¹⁸F]FBPA were investigated, including: pre-irradiation for [¹⁸F]F₂ production, the carrier F₂ content in the Ne target, l-BPA-to-F₂ ratios, separation with high-performance liquid chromatography (HPLC) using 10 different eluents, enantiomeric purity, and residual trifluoroacetic acid used as the reaction solvent by gas chromatography-mass spectrometry.

Results

The activity yields and molar activities of l-[¹⁸F]FBPA (n = 38) were 1200 ± 160 MBq and 46–113 GBq/mmol, respectively, after deuteron-irradiation for 2 h. Two 5 min pre-irradiations prior to [¹⁸F]F₂ production for ¹⁸F-labeling were preferable. For l-[¹⁸F]FBPA synthesis, 0.15–0.2% of carrier F₂ in Ne and l-BPA-to-F₂ ratios > 2 were preferable. HPLC separations with five of the 10 eluents provided injectable l-[¹⁸F]FBPA without any further formulation processing, which resulted in a synthesis time of 32 min. Among the five eluents, 1 mM phosphate-buffered saline was the eluent of choice. The l-[¹⁸F]FBPA injection was sterile and pyrogen-free, and contained very small amounts of D-enantiomer (< 0.1% of l-[¹⁸F]FBPA), l-BPA (< 1% of l-FBPA), and trifluoroacetic acid (< 0.5 ppm).

Conclusions

l-[¹⁸F]FBPA injection was reliably prepared by the electrophilic fluorination of l-BPA with [¹⁸F]AcOF followed by HPLC separation with 1 mM phosphate-buffered saline.

Pharmacokinetics of doxorubicin in glioblastoma multiforme following ultrasound-Induced blood-brain barrier disruption as determined by microdialysis

The goal of this study was to investigate the in vivo extracellular kinetics of doxorubicin (Dox) in glioblastoma multiforme (GBM)-bearing mice following focused ultrasound (FUS)-induced blood-brain barrier (BBB) disruption using microdialysis. An intracranial brain tumor model in NOD-scid mice using human brain GBM 8401 cells was used in this study. Prior to each sonication, simultaneous intravenous administration of Dox and microbubbles, and the Dox concentration in the brains was quantified by high performance liquid chromatography (HPLC). Drug administration with sonication elevated the tumor-to-normal brain Dox ratio of the target tumors by about 2.35-fold compared with the control tumors. The mean peak concentration of Dox in the sonicated GBM dialysate was 10 times greater than without sonication, and the area under the concentration-time curve was 3.3 times greater. This study demonstrates that intracerebral microdialysis is an effective means of evaluating real-time target BBB transport profiles and offers the possibility of investigating the pharmacokinetics of drug delivery in the sonicated brain.

Focused ultrasound enhanced molecular imaging and gene therapy for multifusion reporter gene in glioma-bearing rat model

The ability to monitor the responses of and inhibit the growth of brain tumors during gene therapy has been severely limited due to the blood-brain barrier (BBB). A previous study has demonstrated the feasibility of noninvasive in vivo imaging with ¹²³I-2′-fluoro-2′-deoxy-5-iodo-1-β-D-arabinofuranosyluracil (¹²³I-FIAU) for monitoring herpes simplex virus type 1 thymidine kinase (HSV1-tk) cancer gene expression in an experimental animal model. Here, we tested the enhancement of SPECT with ¹²³I-FIAU and ganciclovir (GCV) treatment in brain tumors after BBB disruption induced by focused ultrasound (FUS) in the presence of microbubbles. We established an orthotopic F98 glioma-bearing rat model with trifusion reporter genes. The results of this study showed that the rat model of HSV1-tk-expressing glioma cells could be successfully detected by SPECT imaging after FUS-induced BBB disruption on day 10 after implantation. Compared to the control group, animals receiving the GCV with or without sonication exhibited a significant antitumor activity (P < 0.05) of glioma cells on day 16 after implantation. Moreover, combining sonication with GCV significantly inhibited tumor growth compared with GCV alone. This study demonstrated that FUS may be used to deliver a wide variety of theranostic agents to the brain for molecular imaging and gene therapy in brain diseases.

Controllable permeability of blood-brain barrier and reduced brain injury through low-intensity pulsed ultrasound stimulation

It has been shown that the blood-brain barrier (BBB) can be locally disrupted by focused ultrasound (FUS) in the presence of microbubbles (MB) while sustaining little damage to the brain tissue. Thus, the safety issue associated with FUS-induced BBB disruption (BBBD) needs to be investigated for future clinical applications. This study demonstrated the neuroprotective effects induced by low-intensity pulsed ultrasound (LIPUS) against brain injury in the sonicated brain. Rats subjected to a BBB disruption injury received LIPUS exposure for 5 min after FUS/MB application. Measurements of BBB permeability, brain water content, and histological analysis were then carried out to evaluate the effects of LIPUS. The permeability and time window of FUS-induced BBBD can be effectively modulated with LIPUS. LIPUS also significantly reduced brain edema, neuronal death, and apoptosis in the sonicated brain. Our results show that brain injury in the FUS-induced BBBD model could be ameliorated by LIPUS and that LIPUS may be proposed as a novel treatment modality for controllable release of drugs into the brain.

Positron emission tomography of high-grade gliomas

High-grade gliomas [HGG (WHO grades III-IV)] are almost invariably fatal. Imaging of HGG is important for orientating diagnosis, prognosis and treatment planning and is crucial for development of novel, more effective therapies. Given the potentially unlimited number of usable tracing molecules and the elevated number of available radionuclides, PET allows gathering multiple informations on HGG including data on tissue metabolism and drug pharmacokinetics. PET studies on the diagnosis, prognosis and treatment of HGG carried out by most frequently used tracers and radionuclides ((11)C and (18)F) and published in 2014 have been reviewed. These studies demonstrate that a thorough choice of tracers may confer elevated diagnostic and prognostic power to PET imaging of HGG. They also suggest that a combination of PET and MRI may give the most complete and reliable imaging information on HGG and that research on hybrid PET/MRI may be paying back in terms of improved diagnosis, prognosis and treatment planning of these deadly tumours.

Protective effects of low-intensity pulsed ultrasound on aluminum-induced cerebral damage in Alzheimer's disease rat model

The protein expressions of neurotrophic factors can be enhanced by low-intensity pulsed ultrasound (LIPUS) stimulation in the brain. The purpose of this study was to demonstrate the protective effect of LIPUS stimulation against aluminum-induced cerebral damage in Alzheimer's disease rat model. LIPUS was administered 7 days before each aluminum chloride (AlCl₃) administration, and concomitantly given with AlCl₃ daily for a period of 6 weeks. Neurotrophic factors in hippocampus were measured by western blot analysis. Behavioral changes in the Morris water maze and elevated plus maze were examined in rats after administration of AlCl₃. Various biochemical analyses were performed to evaluate the extent of brain damages. LIPUS is capable of prompting levels of brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), and vascular endothelial growth factor (VEGF) in rat brain. AlCl₃ administration resulted in a significant increase in the aluminum concentration, acetylcholinesterase activity and beta-amyloid (Aβ) deposition in AlCl₃ treated rats. LIPUS stimulation significantly attenuated aluminum concentration, acetylcholinesterase activity, Aβ deposition and karyopyknosis in AlCl₃ treated rats. Furthermore, LIPUS significantly improved memory retention in AlCl₃-induced memory impairment. These experimental results indicate that LIPUS has neuroprotective effects against AlCl₃-induced cerebral damages and cognitive dysfunction.

FBPA PET in boron neutron capture therapy for cancer: prediction of (10)B concentration in the tumor and normal tissue in a rat xenograft model

Background

Boron neutron capture therapy (BNCT) is a molecular radiation treatment based on the ¹⁰B (n, α) ⁷Li nuclear reaction in cancer cells, in which delivery of ¹⁰B by 4-borono-phenylalanine conjugated with fructose (BPA-fr) to the cancer cells is of critical importance. The PET tracer 4-borono-2-¹⁸ F-fluoro-phenylalanine (FBPA) has been used to predict the accumulation of BPA-fr before BNCT. However, because of the difference in chemical structure between BPA-fr and FBPA and the difference in the dose administered between BPA-fr (therapeutic dose) and FBPA (tracer dose), the predictive value of FBPA PET for BPA-fr accumulation in the tumor and normal tissues is not yet clearly proven. We conducted this study to validate FBPA PET as a useful test to predict the accumulation of BPA-fr in the tumor and normal tissues before BNCT.

Methods

RGC-6 rat glioma cells (1.9 × 10⁷) were implanted subcutaneously in seven male F344 rats. On day 20 after the tumor implantation, dynamic PET scan was performed on four rats after injection of FBPA for 1 h. Whole-body PET/CT was performed 1 h after intravenous injection of the FBPA solution (30.5 ± 0.7 MBq, 1.69 ± 1.21 mg/kg). PET accumulation of FBPA in the tumor tissue and various normal tissues was estimated as a percentage of the injected dose per gram (%ID/g). One hour after the PET/CT scan, BPA-fructose (167.32 ± 18.65 mg/kg) was injected intravenously, and the rats were dissected 1 h after the BPA-fr injection. The absolute concentration of ¹⁰B in the autopsied tissues and blood was measured by inductively coupled plasma optical emission spectrometry (ICP-OES).

Results

The highest absolute concentration of ¹⁰B determined by ICP-OES was found in the kidney (4.34 ± 0.84 %ID/g), followed by the pancreas (2.73 ± 0.63 %ID/g), and the tumor (1.44 ± 0.44 %ID/g). A significant positive correlation was found between the accumulation levels of BPA-fr and FBPA (r = 0.91, p < 0.05).

Conclusions

FBPA PET can reliably predict accumulation of BPA-fr in the tumor as well as normal tissues.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-014-0070-2) contains supplementary material, which is available to authorized users.