A Novel PET Probe "[18F]DiFA" Accumulates in Hypoxic Region via Glutathione Conjugation Following Reductive Metabolism

Development of a Neopentyl 211At-Labeled Activated Ester Providing In Vivo Stable 211At-Labeled Antibodies for Targeted Alpha Therapy

In this study we developed a neopentyl 211At-labeled activated ester that incorporates a triazole spacer and applied it to the synthesis of an 211At-labeled cetuximab. The activated ester was synthesized via the nucleophilic 211At-astatination of a neopentyl sulfonate carrying two long alkyl chains that serve as a lipid tag, which was followed by the hydrolysis of an acetal. Additionally, we developed a novel Resin-Assisted Purification and Deprotection (RAPD) protocol involving a solid-phase extraction of the protected 211At-labeled compound from the mixture of the labeling reaction, hydrolysis of the acetal on the resin, and finally an elution of the 211At-labeled activator from the resin. This method allows the synthesis of an 211At-labeled activated ester with high purity through a simplified procedure that circumvents the need for HPLC purification. Using this 211At-labeled activated ester, we efficiently synthesized 211At-labeled cetuximab in 27±1 % radiochemical yield with 95 % radiochemical purity. This 211At-activated ester demonstrated high reactivity, and enabled the completion of the reaction with the antibody within 10 min. In comparative biodistribution studies between 211At-labeled cetuximab and the corresponding 125I-labeled cetuximab in normal mice, both the thyroid and stomach showed radioactivity levels that were less than 1.0 % of the injected dose.

18 F-Fluorination of Nitroimidazolyl-Containing Sultone: A Direct Access to a Highly Hydrophilic Radiotracer for High-Performance Positron Emission Tomography Imaging of Hypoxia

Hypoxia, characterized by nonphysiological levels of oxygen tension, is a key phenomenon common to the majority of malignant tumors with poor prognosis. Many efforts have been made to develop hypoxia imaging for diagnosis, staging, and monitoring of diseases, as well as for evaluating therapies. PET Imaging using 18F-fluoronitroimidazoles (i.e., [18F]FMISO as a lead radiotracer) has demonstrated potential for clinical investigations, but the poor contrast and prolonged acquisition times (>2.5 h) strongly limit its accuracy and routine developments. Here, we report an original [18F]fluoronitroimidazole bearing a sulfo group ([18F]FLUSONIM) that displays highly hydrophilic properties and rapid clearance, providing high-performance hypoxia specific PET imaging. We describe the synthesis and radiosynthesis of [18F]FLUSONIM, its in vivo preclinical evaluation by PET imaging in healthy rats and a rhabdomyosarcoma rat model, as well as its radiometabolization and histological studies. [18F]FLUSONIM was prepared in a single step by high yielding radiofluorination of a sultone precursor, highlighting the advantages of this new radiolabeling approach not yet explored for radiopharmaceutical development. PET imaging experiments were conducted by systematically comparing [18F]FLUSONIM to [18F]FMISO as a reference. The overall results unequivocally demonstrate that the developed radiopharmaceutical meets the criteria of an ideal candidate for hypoxia PET imaging-rapid and efficient radiosynthesis, total stability, exclusive urinary elimination, high specificity for hypoxic regions, unprecedented tumor/background ratios, short acquisition delays (<60 min), and promising potential for further preclinical and clinical applications.

In vivo stable 211At-labeled prostate-specific membrane antigen-targeted tracer using a neopentyl glycol structure

BACKGROUND

Prostate cancer is a common cancer among men worldwide that has a very poor prognosis, especially when it progresses to metastatic castration-resistant prostate cancer (mCRPC). Therefore, novel therapeutic agents for mCRPC are urgently required. Because prostate-specific membrane antigen (PSMA) is overexpressed in mCRPC, targeted alpha therapy (TAT) for PSMA is a promising treatment for mCRPC. Astatine-211 (211At) is a versatile α-emitting radionuclide that can be produced using a cyclotron. Therefore, 211At-labeled PSMA compounds could be useful for TAT; however, 211At-labeled compounds are unstable against deastatination in vivo. In this study, to develop in vivo stable 211At-labeled PSMA derivatives, we designed and synthesized 211At-labeled PSMA derivatives using a neopentyl glycol (NpG) structure that can stably retain 211At in vivo. We also evaluated their biodistribution in normal and tumor-bearing mice.

RESULTS

We designed and synthesized 211At-labeled PSMA derivatives containing two glutamic acid (Glu) linkers between the NpG structure and asymmetric urea (NpG-L-PSMA ((L-Glu)2 linker used) and NpG-D-PSMA ((D-Glu)2 linker used)). First, we evaluated the characteristics of 125I-labeled NpG derivatives because 125I was readily available. [125I]I-NpG-L-PSMA and [125I]I-NpG-D-PSMA showed low accumulation in the stomach and thyroid, indicating their high in vivo stability against deiodination. [125I]I-NpG-L-PSMA was excreted in urine as hydrophilic radiometabolites in addition to the intact form. Meanwhile, [125I]I-NpG-D-PSMA was excreted in urine in an intact form. In both cases, no radioactivity was observed in the free iodine fraction. [125I]I-NpG-D-PSMA showed higher tumor accumulation than [125I]I-NpG-L-PSMA. We then developed 211At-labeled PSMA using the NpG-D-PSMA structure. [211At]At-NpG-D-PSMA showed low accumulation in the stomach and thyroid in normal mice, indicating its high stability against deastatination in vivo. Moreover, [211At]At-NpG-D-PSMA showed high accumulation in tumor similar to that of [125I]I-NpG-D-PSMA.

CONCLUSIONS

[211At]At-NpG-D-PSMA showed high in vivo stability against deastatination and high tumor accumulation. [211At]At-NpG-D-PSMA should be considered as a potential new TAT for mCRPC.

Affinity of PET-MRI Tracers for Hypoxic Cells in Breast Cancer: A Systematic Review

Tumour hypoxia is a known microenvironmental culprit for treatment resistance, tumour recurrence and promotion of metastatic spread. Despite the long-known existence of this factor within the tumour milieu, hypoxia is still one of the greatest challenges in cancer management. The transition from invasive and less reliable detection methods to more accurate and non-invasive ways to identify and quantify hypoxia was a long process that eventually led to the promising results showed by functional imaging techniques. Hybrid imaging, such as PET-CT, has the great advantage of combining the structural or anatomical image (offered by CT) with the functional or metabolic one (offered by PET). However, in the context of hypoxia, it is only the PET image taken after appropriate radiotracer administration that would supply hypoxia-specific information. To overcome this limitation, the development of the latest hybrid imaging systems, such as PET-MRI, enables a synergistic approach towards hypoxia imaging, with both methods having the potential to provide functional information on the tumour microenvironment. This study is designed as a systematic review of the literature on the newest developments of PET-MRI for the imaging of hypoxic cells in breast cancer. The analysis includes the affinity of various PET-MRI tracers for hypoxia in this patient group as well as the correlations between PET-specific and MRI-specific parameters, to offer a broader view on the potential for the widespread clinical implementation of this hybrid imaging technique.

Feasibility study of multimodal imaging for redox status and glucose metabolism in tumor

Understanding the tumor redox status is important for efficient cancer treatment. Here, we noninvasively detected changes in the redox environment of tumors before and after cancer treatment in the same individuals using a novel compact and portable electron paramagnetic resonance imaging (EPRI) device and compared the results with glycolytic information obtained through autoradiography using 2-deoxy-2-[18F]fluoro-d-glucose ([18F]FDG). Human colon cancer HCT116 xenografts were used in the mice. We used 3-carbamoyl-PROXYL (3CP) as a paramagnetic and redox status probe for the EPRI of tumors. The first EPRI was followed by the intraperitoneal administration of buthionine sulfoximine (BSO), an inhibitor of glutathione synthesis, or X-ray irradiation of the tumor. A second EPRI was performed on the following day. Autoradiography was performed after the second EPRI. After imaging, the tumor sections were evaluated by histological analysis and the amount of reducing substances in the tumor was measured. BSO treatment and X-ray irradiation significantly decreased the rate of 3CP reduction in tumors. Redox maps of tumors obtained from EPRI can be compared with tissue sections of approximately the same cross section. BSO treatment reduced glutathione levels in tumors, whereas X-ray irradiation did not alter the levels of any of the reducing substances. Comparison of the redox map with the autoradiography of [18F]FDG revealed that regions with high reducing power in the tumor were active in glucose metabolism; however, this correlation disappeared after X-ray irradiation. These results suggest that the novel compact and portable EPRI device is suitable for multimodal imaging, which can be used to study tumor redox status and therapeutic efficacy in cancer, and for combined analysis with other imaging modalities.

Neopentyl glycol-based radiohalogen-labeled amino acid derivatives for cancer radiotheranostics

BACKGROUND

L-type amino acid transporter 1 (LAT1) is overexpressed in various cancers; therefore, radiohalogen-labeled amino acid derivatives targeting LAT1 have emerged as promising candidates for cancer radiotheranostics. However, 211At-labeled amino acid derivatives exhibit instability against deastatination in vivo, making it challenging to use 211At for radiotherapy. In this study, radiohalogen-labeled amino acid derivatives with high dehalogenation stability were developed.

RESULTS

We designed and synthesized new radiohalogen-labeled amino acid derivatives ([211At]At-NpGT, [125I]I-NpGT, and [18F]F-NpGT) in which L-tyrosine was introduced into the neopentyl glycol (NpG) structure. The radiolabeled amino acid derivatives were recognized as substrates of LAT1 in the in vitro studies using C6 glioma cells. In a biodistribution study using C6 glioma-bearing mice, these agents exhibited high stability against in vivo dehalogenation and similar biodistributions. The similarity of [211At]At-NpGT and [18F]F-NpGT indicated that these pairs of radiolabeled compounds would be helpful in radiotheranostics. Moreover, [211At]At-NpGT exhibited a dose-dependent inhibitory effect on the growth of C6 glioma-bearing mice.

CONCLUSIONS

[211At]At-NpGT exhibited a dose-dependent inhibitory effect on the tumor growth of glioma-bearing mice, and its biodistribution was similar to that of other radiohalogen-labeled amino acid derivatives. These findings suggest that radiotheranostics using [18F]F-NpGT and [123/131I]I-NpGT for diagnostic applications and [211At]At-NpGT and [131I]I-NpGT for therapeutic applications are promising.

1-(N,N-Dialkylcarbamoyl)-1,1-difluoromethanesulfonyl ester as a stable and effective precursor for a neopentyl labeling group with astatine-211

Radiohalogens with a short half-life are useful radioisotopes for radiotheranostics. Astatine-211 is an α-emitting radiohalogen and is expected to be applicable to targeted α therapy. A neopentyl labeling group is an effective hydrophilic labeling unit for various radiohalogens, which includes 211At. In this study, a 1-(N,N-dialkylcarbamoyl)-1,1-difluoromethanesulfonyl (CDf) ester was developed as a stable precursor for labeling with 211At, 77Br and 125I through a neopentyl labeling group. The CDf ester remained stable in an acetonitrile solution at room temperature and enabled the successful syntheses of 211At-labeled compounds in a highly radiochemical conversion in the presence of K2CO3. 77Br- and 125I-labeled compounds can be prepared from the CDf ester without a base. The utility of the CDf ester was demonstrated in the synthesis of a benzylguanidine with a neopentyl 211At-labeling group. The developed method afforded a 32% radiochemical yield of 211At-labeled benzylguanidine. However, a partial deastatination was observed under acidic conditions during the removal of an N-Boc protecting group. Deprotecting these groups under milder acidic conditions may improve the radiochemical yield. In conclusion, the CDf ester facilitates the syntheses of 211At, 125I and 77Br-labeled compounds that use a neopentyl labeling group for radiotheranostic applications. Further optimization of protecting groups and reaction conditions should enhance the total radiochemical yield of the 211At-labeled compounds.

Timing of hypoxia PET/CT imaging after 18F-fluoromisonidazole injection in non-small cell lung cancer patients

Positron emission tomography (PET)/computed tomography (CT) using the radiotracer 18F-Fluoromisonidazole (FMISO) has been widely employed to image tumour hypoxia and is of interest to help develop novel hypoxia modifiers and guide radiation treatment planning. Yet, the optimal post-injection (p.i.) timing of hypoxic imaging remains questionable. Therefore, we investigated the correlation between hypoxia-related quantitative values in FMISO-PET acquired at 2 and 4 h p.i. in patients with non-small cell lung cancer (NSCLC). Patients with resectable NSCLC participated in the ATOM clinical trial (NCT02628080) which investigated the hypoxia modifying effects of atovaquone. Two-hour and four-hour FMISO PET/CT images acquired at baseline and pre-surgery visits (n = 58) were compared. Cohort 1 (n = 14) received atovaquone treatment, while cohort 2 (n = 15) did not. Spearman's rank correlation coefficients (ρ) assessed the relationship between hypoxia-related metrics, including standardised uptake value (SUV), tumour-to-blood ratio (TBR), and tumour hypoxic volume (HV) defined by voxels with TBR ≥ 1.4. As the primary imaging-related trial endpoint used to evaluate the action of atovaquone on tumour hypoxia in patients with NSCLC was change in tumour HV from baseline, this was also assessed in patients (n = 20) with sufficient baseline 2- and 4-h scan HV to reliably measure change (predefined as ≥ 1.5 mL). Tumours were divided into four subregions or distance categories: edge, outer, inner, and centre, using MATLAB. In tumours overall, strong correlation (P < 0.001) was observed for SUVmax ρ = 0.87, SUVmean ρ = 0.91, TBRmax ρ = 0.83 and TBRmean ρ = 0.81 between 2- and 4-h scans. Tumour HV was moderately correlated (P < 0.001) with ρ = 0.69 between 2- and 4-h scans. Yet, in tumour subregions, the correlation of HV decreased from the centre ρ = 0.71 to the edge ρ = 0.45 (P < 0.001). SUV, TBR, and HV values were consistently higher on 4-h scans than on 2-h scans, indicating better tracer-to-background contrast. For instance, for TBRmax, the mean, median, and interquartile range were 1.9, 1.7, and 1.6-2.0 2-h p.i., and 2.6, 2.4, and 2.0-3.0 4-h p.i., respectively. Our results support that FMISO-PET scans should be performed at 4 h p.i. to evaluate tumour hypoxia in NSCLC.Trial registration: ClinicalTrials.gov, NCT02628080. Registered 11/12/2015, https://clinicaltrials.gov/ct2/show/NCT02628080 .

Neopentyl Glycol as a Scaffold to Provide Radiohalogenated Theranostic Pairs of High In Vivo Stability

The high in vivo stability of 2,2-dihydroxymethyl-3-[¹⁸F]fluoropropyl-2-nitroimidazole ([¹⁸F]DiFA) prompted us to evaluate neopentyl as a scaffold to prepare a radiotheranostic system with radioiodine and astatine. Three DiFA analogues with one, two, or without a hydroxyl group were synthesized. While all ¹²⁵I-labeled compounds remained stable against nucleophilic substitution, only a ¹²⁵I-labeled neopentyl glycol was stable against cytochrome P450 (CYP)-mediated metabolism and showed high stability against in vivo deiodination. ²¹¹At-labeled neopentyl glycol also remained stable against both nucleophilic substitution and CYP-mediated metabolism. ²¹¹At-labeled neopentyl glycol showed the biodistribution profiles similar to those of its radioiodinated counterpart in contrast to the ¹²⁵I/²¹¹At-labeled benzoate pair. The urine analyses confirmed that ²¹¹At-labeled neopentyl glycol was excreted in the urine as a glucuronide conjugate with the absence of free [²¹¹At]At^-. These findings indicate that neopentyl glycol would constitute a promising scaffold to prepare a radiotheranostic system with radioiodine and ²¹¹At.

Enzymatic synthesis of fluorinated compounds

Fluorinated compounds are widely used in the fields of molecular imaging, pharmaceuticals, and materials. Fluorinated natural products in nature are rare, and the introduction of fluorine atoms into organic compound molecules can give these compounds new functions and make them have better performance. Therefore, the synthesis of fluorides has attracted more and more attention from biologists and chemists. Even so, achieving selective fluorination is still a huge challenge under mild conditions. In this review, the research progress of enzymatic synthesis of fluorinated compounds is summarized since 2015, including cytochrome P450 enzymes, aldolases, fluoroacetyl coenzyme A thioesterases, lipases, transaminases, reductive aminases, purine nucleoside phosphorylases, polyketide synthases, fluoroacetate dehalogenases, tyrosine phenol-lyases, glycosidases, fluorinases, and multienzyme system. Of all enzyme-catalyzed synthesis methods, the direct formation of the C-F bond by fluorinase is the most effective and promising method. The structure and catalytic mechanism of fluorinase are introduced to understand fluorobiochemistry. Furthermore, the distribution, applications, and future development trends of fluorinated compounds are also outlined. Hopefully, this review will help researchers to understand the significance of enzymatic methods for the synthesis of fluorinated compounds and find or create excellent fluoride synthase in future research.Key points• Fluorinated compounds are distributed in plants and microorganisms, and are used in imaging, medicine, materials science.• Enzyme catalysis is essential for the synthesis of fluorinated compounds.• The loop structure of fluorinase is the key to forming the C-F bond.

Eribulin improves tumor oxygenation demonstrated by 18F-DiFA hypoxia imaging, leading to radio-sensitization in human cancer xenograft models

PURPOSE

Eribulin, an inhibitor of microtubule dynamics, is known to show antitumor effects through its remodeling activity in the tumor vasculature. However, the extent to which the improvement of tumor hypoxia by eribulin affects radio-sensitivity remains unclear. We utilized 1-(2,2-dihydroxymethyl-3-¹⁸F-fluoropropyl)-2-nitroimidazole (¹⁸F-DiFA), a new PET probe for hypoxia, to investigate the effects of eribulin on tumor hypoxia and evaluate the radio-sensitivity during eribulin treatment.

METHODS

Mice bearing human breast cancer MDA-MB-231 cells or human lung cancer NCI-H1975 cells were administered a single dose of eribulin. After administration, mice were injected with ¹⁸F-DiFA and pimonidazole, and tumor hypoxia regions were analyzed. For the group that received combined treatment with radiation, ¹⁸F-DiFA PET/CT imaging was performed before tumors were locally X-irradiated. Tumor size was measured every other day after irradiation.

RESULTS

Eribulin significantly reduced ¹⁸F-DiFA accumulation levels in a dose-dependent manner. Furthermore, the reduction in ¹⁸F-DiFA accumulation levels by eribulin was most significant 7 days after treatment. These results were also supported by reduction of the pimonidazole-positive hypoxic region. The combined treatment showed significant retardation of tumor growth in comparison with the control, radiation-alone, and drug-alone groups. Importantly, tumor growth after irradiation was inversely correlated with ¹⁸F-DiFA accumulation.

CONCLUSION

These results demonstrated that ¹⁸F-DiFA PET/CT clearly detected eribulin-induced tumor oxygenation and that eribulin efficiently enhanced the antitumor activity of radiation by improving tumor oxygenation.

Repurposing 18F-FMISO as a PET tracer for translational imaging of nitroreductase-based gene directed enzyme prodrug therapy

Nitroreductases (NTR) are a family of bacterial enzymes used in gene directed enzyme prodrug therapy (GDEPT) that selectively activate prodrugs containing aromatic nitro groups to exert cytotoxic effects following gene transduction in tumours. The clinical development of NTR-based GDEPT has, in part, been hampered by the lack of translational imaging modalities to assess gene transduction and drug cytotoxicity, non-invasively. This study presents translational preclinical PET imaging to validate and report NTR activity using the clinically approved radiotracer, ¹⁸F-FMISO, as substrate for the NTR enzyme.

Methods: The efficacy with which ¹⁸F-FMISO could be used to report NfsB NTR activity in vivo was investigated using the MDA-MB-231 mammary carcinoma xenograft model. For validation, subcutaneous xenografts of cells constitutively expressing NTR were imaged using ¹⁸F-FMISO PET/CT and fluorescence imaging with CytoCy5S, a validated fluorescent NTR substrate. Further, examination of the non-invasive functionality of ¹⁸F-FMISO PET/CT in reporting NfsB NTR activity in vivo was assessed in metastatic orthotopic NfsB NTR expressing xenografts and metastasis confirmed by bioluminescence imaging. ¹⁸F-FMISO biodistribution was acquired ex vivo by an automatic gamma counter measuring radiotracer retention to confirm in vivo results. To assess the functional imaging of NTR-based GDEPT with ¹⁸F-FMISO, PET/CT was performed to assess both gene transduction and cytotoxicity effects of prodrug therapy (CB1954) in subcutaneous models.

Results:¹⁸F-FMISO retention was detected in NTR⁺ subcutaneous xenografts, displaying significantly higher PET contrast than NTR^- xenografts (p < 0.0001). Substantial ¹⁸F-FMISO retention was evident in metastases of orthotopic xenografts (p < 0.05). Accordingly, higher ¹⁸F-FMISO biodistribution was prevalent ex vivo in NTR⁺ xenografts. ¹⁸F-FMISO NfsB NTR PET/CT imaging proved useful for monitoring in vivo NTR transduction and the cytotoxic effect of prodrug therapy.

Conclusions:¹⁸F-FMISO NfsB NTR PET/CT imaging offered significant contrast between NTR⁺ and NTR^- tumours and effective resolution of metastatic progression. Furthermore, ¹⁸F-FMISO NfsB NTR PET/CT imaging proved efficient in monitoring the two steps of GDEPT, in vivo NfsB NTR transduction and response to CB1954 prodrug therapy. These results support the repurposing of ¹⁸F-FMISO as a readily implementable PET imaging probe to be employed as companion diagnostic test for NTR-based GDEPT systems.

Effects of 18F-fluorinated neopentyl glycol side-chain on the biological characteristics of stilbene amyloid-β PET ligands

INTRODUCTION

The 2,2-dihydroxymethyl-1-[18F]fluoropropane group, also called 18F-labelled neopentyl glycol side-chain, is a novel 18F-labelling group for positron emission tomography (PET) imaging agents. The aim of using this group is to develop simple purification with solid-phase extraction without high-performance liquid chromatography. However, the effects of the neopentyl 18F-labelling group on the characteristics of brain imaging agents are unknown. Here, we added this side-chain to compounds with an aminostilbene structure to evaluate their effects on the biological properties of aminostilbene as an amyloid-β (Aβ) radioligand.

METHODS

Biodistributions of four novel 18F-labelled stilbene compounds with different lengths of polyethylene glycol (PEG) linkers, called [18F]Cpd-0, -1, -2, and -4, (PEG = 0, 1, 2, and 4), and [18F]AV-1 in normal mice were evaluated. Metabolite analysis of [18F]Cpd-0 and -1 was performed with mouse plasma and brain. A competitive binding assay of [18F]AV-1 binding to Aβ1-42 fibrils was performed to determine the binding properties of the compounds.

RESULTS

[18F]Cpd-0, -1, and -2 demonstrated moderate initial brain uptake in mice (3.1-4.2% injected dose/g at 2 min post-injection) followed by fast clearance, and in vivo defluorination of these compounds was negligible. [18F]Cpd-4 exhibited low brain uptake and high bone uptake. Compared with [18F]Cpd-1, the percentage of [18F]Cpd-0 in mouse brain was high at 10 min post-injection. A competitive binding assay revealed partial interference effects by the neopentyl glycol side-chain on binding of stilbene compounds to Aβ1-42 fibrils.

CONCLUSIONS

Aminostilbene compounds with two or fewer PEG linkers containing an 18F-labelled neopentyl glycol side-chain demonstrated preferable pharmacokinetic properties as a brain imaging radioligand in normal mice. These side-chains can be used as an alternative labelling group for imaging agents targeting the brain.

Synthesis and evaluation of gallium-68-labeled nitroimidazole-based imaging probes for PET diagnosis of tumor hypoxia

OBJECTIVE

In this study, we designed and synthesized four novel 68Ga-radiolabeled compounds ([68Ga]DN-3, [68Ga]DN-4, [68Ga]NN-3, and [68Ga]NN-4) composed of a nitroimidazole and two types of bifunctional chelates (DOTA or NOTA) via several alkyl linkers of different length. Then, we evaluated their properties as hypoxia imaging probes for positron emission tomography (PET) compared with conventional compounds ([68Ga]DN-2 and [68Ga]NN-2).

METHODS

The precursors of 68Ga-radiolabeled compounds were synthesized through a two-step reaction, and then reacted with 68GaCl3 to be 68Ga-radiolabeled compounds. FaDu cells were treated with 68Ga-radiolabeled compounds and then incubated under normoxic (21% O2) or hypoxic (1% O2) conditions. The radioactivity of these cells was measured 2 h after incubation. The biodistribution and PET/CT imaging of 68Ga-radiolabeled compounds in FaDu-bearing Balb/c nude mice were evaluated 2 h after intravenous injection.

RESULTS

The 68Ga-radiolabeled compounds were synthesized with radiochemical purities over 95%. In the in vitro study, the levels of 68Ga-radiolabeled compounds were significantly higher in hypoxic cells than in normoxic cells. In hypoxic cells, the compounds we designed in this study demonstrated higher accumulation than the conventional compounds. In the in vivo biodistribution study, [68Ga]DN-3 exhibited the highest accumulation in tumor. In the in vivo PET/CT imaging study, the tumor tissues of the FaDu-xenografted mice were visualized at 2 h after intravenous administration of 68Ga-radiolabeled compounds.

CONCLUSIONS

Our study suggested that the length of the linkers connecting nitroimidazole to a bifunctional chelate affect PET imaging of hypoxic tumors with 68Ga-radiolabeled compounds.

Increased [18F]FMISO accumulation under hypoxia by multidrug-resistant protein 1 inhibitors

Background

[¹⁸F]Fluoromisonidazole ([¹⁸F]FMISO) is a PET imaging probe widely used for the detection of hypoxia. We previously reported that [¹⁸F]FMISO is metabolized to the glutathione conjugate of the reduced form in hypoxic cells. In addition, we found that the [¹⁸F]FMISO uptake level varied depending on the cellular glutathione conjugation and excretion ability such as enzyme activity of glutathione-S-transferase and expression levels of multidrug resistance-associated protein 1 (MRP1, an efflux transporter), in addition to the cellular hypoxic state. In this study, we evaluated whether MRP1 activity affected [¹⁸F]FMISO PET imaging.

Methods

FaDu human pharyngeal squamous cell carcinoma cells were pretreated with MRP1 inhibitors (cyclosporine A, lapatinib, or MK-571) for 1 h, incubated with [¹⁸F]FMISO for 4 h under hypoxia, and their radioactivity was then measured. FaDu tumor-bearing mice were intravenously injected with [¹⁸F]FMISO, and PET/CT images were acquired at 4 h post-injection (1st PET scan). Two days later, the same mice were pretreated with MRP1 inhibitors (cyclosporine A, lapatinib, or MK-571) for 1 h, and PET/CT images were acquired (2nd PET scan).

Results

FaDu cells pretreated with MRP1 inhibitors exhibited significantly higher radioactivity than those without inhibitor treatment (cyclosporine A: 6.91 ± 0.27, lapatinib: 10.03 ± 0.47, MK-571: 10.15 ± 0.44%dose/mg protein, p < 0.01). In the in vivo PET study, the SUV_mean ratio in tumors [calculated as after treatment (2nd PET scan)/before treatment of MRP1 inhibitors (1st PET scan)] of the mice treated with MRP1 inhibitors was significantly higher than those of control mice (cyclosporine A: 2.6 ± 0.7, lapatinib: 2.2 ± 0.7, MK-571: 2.2 ± 0.7, control: 1.2 ± 0.2, p < 0.05).

Conclusion

In this study, we revealed that MRP1 inhibitors increase [¹⁸F]FMISO accumulation in hypoxic cells. This suggests that [¹⁸F]FMISO-PET imaging is affected by MRP1 inhibitors independent of the hypoxic state.

Rational design of fluorescent probes for targeted in vivo nitroreductase visualization

Three fluorescent probes were made by conjugation of para-, ortho-, and meta-nitrobenzene to the BODIPY core via a thiolether bond. It revealed that the linkage and nitro substituent position significantly influence the capability of NTR detection.

The Roles of Hypoxia Imaging Using 18F-Fluoromisonidazole Positron Emission Tomography in Glioma Treatment

Glioma is the most common malignant brain tumor. Hypoxia is closely related to the malignancy of gliomas, and positron emission tomography (PET) can noninvasively visualize the degree and the expansion of hypoxia. Currently, ¹⁸F-fluoromisonidazole (FMISO) is the most common radiotracer for hypoxia imaging. The clinical usefulness of FMISO PET has been established; it can distinguish glioblastomas from lower-grade gliomas and can predict the microenvironment of a tumor, including necrosis, vascularization, and permeability. FMISO PET provides prognostic information, including survival and treatment response information. Because hypoxia decreases a tumor’s sensitivity to radiation therapy, dose escalation to an FMISO-positive volume is an attractive strategy. Although this idea is not new, an insufficient amount of evidence has been obtained regarding this concept. New tracers for hypoxia imaging such as ¹⁸F-DiFA are being tested. In the future, hypoxia imaging will play an important role in glioma management.

Biodistribution and radiation dosimetry of the novel hypoxia PET probe [18F]DiFA and comparison with [18F]FMISO

Background

To facilitate hypoxia imaging in a clinical setting, we developed 1-(2,2-dihydroxymethyl-3-[¹⁸F]-fluoropropyl)-2-nitroimidazole ([¹⁸F]DiFA) as a new tracer that targets tumor hypoxia with its lower lipophilicity and efficient radiosynthesis. Here, we evaluated the radiation dosage, biodistribution, human safety, tolerability, and early elimination after the injection of [¹⁸F]DiFA in healthy subjects, and we performed a preliminary clinical study of patients with malignant tumors in a comparison with [¹⁸F]fluoromisonidazole ([¹⁸F]FMISO).

Results

The single administration of [¹⁸F]DiFA in 8 healthy male adults caused neither adverse events nor abnormal clinical findings. Dynamic and sequential whole-body scans showed that [¹⁸F]DiFA was rapidly cleared from all of the organs via the hepatobiliary and urinary systems. The whole-body mean effective dose of [¹⁸F]DiFA estimated by using the medical internal radiation dose (MIRD) schema with organ level internal dose assessment/exponential modeling (OLINDA/EXM) computer software 1.1 was 14.4 ± 0.7 μSv/MBq. Among the organs, the urinary bladder received the largest absorbed dose (94.7 ± 13.6 μSv/MBq). The mean absorbed doses of the other organs were equal to or less than those from other hypoxia tracers. The excretion of radioactivity via the urinary system was very rapid, reaching 86.4 ± 7.1% of the administered dose. For the preliminary clinical study, seven patients were subjected to [¹⁸F]FMISO and [¹⁸F]DiFA positron emission tomography (PET) at 48-h intervals to compare the two tracers’ diagnostic ability for tumor hypoxia. The results of the tumor hypoxia evaluation by [¹⁸F]DiFA PET at 1 h and 2 h were not significantly different from those obtained with [¹⁸F]FMISO PET at 4 h ([¹⁸F]DiFA at 1 h, p = 0.32; [¹⁸F]DiFA at 2 h, p = 0.08). Moreover, [¹⁸F]DiFA PET at both 1 h (k = 0.68) and 2 h (k = 1.00) showed better inter-observer reproducibility than [¹⁸F]FMISO PET at 4 h (k = 0.59).

Conclusion

[¹⁸F]DiFA is well tolerated, and its radiation dose is comparable to those of other hypoxia tracers. [¹⁸F]DiFA is very rapidly cleared via the urinary system. [¹⁸F]DiFA PET generated comparable images to [¹⁸F]FMISO PET in hypoxia imaging with shorter waiting time, demonstrating the promising potential of [¹⁸F]DiFA PET for hypoxia imaging and for a multicenter trial.

Electronic supplementary material

The online version of this article (10.1186/s13550-019-0525-6) contains supplementary material, which is available to authorized users.

Hypoxia Imaging and Adaptive Radiotherapy: A State-of-the-Art Approach in the Management of Glioma

Severe hypoxia [oxygen partial pressure (pO₂) below 5–10 mmHg] is more frequent in glioblastoma multiforme (GBM) compared to lower-grade gliomas. Seminal studies in the 1950s demonstrated that hypoxia was associated with increased resistance to low–linear energy transfer (LET) ionizing radiation. In experimental conditions, the total radiation dose has to be multiplied by a factor of 3 to achieve the same cell lethality in anoxic situations. The presence of hypoxia in human tumors is assumed to contribute to treatment failures after radiotherapy (RT) in cancer patients. Therefore, a logical way to overcome hypoxia-induced radioresistance would be to deliver substantially higher doses of RT in hypoxic volumes delineated on pre-treatment imaging as biological target volumes (BTVs). Such an approach faces various fundamental, technical, and clinical challenges. The present review addresses several technical points related to the delineation of hypoxic zones, which include: spatial accuracy, quantitative vs. relative threshold, variations of hypoxia levels during RT, and availability of hypoxia tracers. The feasibility of hypoxia imaging as an assessment tool for early tumor response to RT and for predicting long-term outcomes is discussed. Hypoxia imaging for RT dose painting is likewise examined. As for the radiation oncologist's point of view, hypoxia maps should be converted into dose-distribution objectives for RT planning. Taking into account the physics and the radiobiology of various irradiation beams, preliminary in silico studies are required to investigate the feasibility of dose escalation in terms of normal tissue tolerance before clinical trials are undertaken.

[Accumulation Mechanism of 2-Nitroimidazole-based Hypoxia Imaging Probes Revealed by Imaging Mass Spectrometry]

Hypoxia in tumor tissues plays a pivotal role in tumor progression and angiogenesis, and is associated with cancer therapeutic resistance. For the diagnosis of hypoxia, non-invasive imaging techniques, especially positron emission tomography (PET) with 2-nitroimidazole-based probes, are used, since 2-nitroimidazole-based probes are considered to undergo reductive metabolism on their 2-nitroimidazole moiety and become trapped in hypoxic cells. However, the detailed mechanism of their accumulation remains unclear because of the difficulty in estimating the metabolites by radioisotopic analysis. Imaging mass spectrometry (IMS) can distinguish the distribution patterns of the drug and its metabolites. To clarify the accumulation mechanism of 2-nitroimidazole-based probes in hypoxic cells, we evaluated [¹⁸F]fluoromisonidazole (FMISO), a 2-nitroimidazole-based PET probe, in combination with radioisotopic analysis and IMS. We found that the glutathione conjugate of reduced FMISO (amino-FMISO-GS) was the main FMISO metabolite, and was specifically distributed in the hypoxic regions of tumors. The same phenomenon was observed when we examined another 2-nitroimidazole-based probe, pimonidazole. The in vitro cellular uptake study revealed that FMISO accumulation in hypoxic cells depends on the cell type. In those cells exhibiting higher FMISO uptake, the reactive glutathione level and enzyme (glutathione S-transferase; GST) activity catalyzing the glutathione conjugation reaction was significantly higher, whereas the expression level of the efflux transporter (multidrug resistance-associated protein 1; MRP1) was significantly lower. Our study suggests that 2-nitroimidazole-based probes accumulate in hypoxic cells via glutathione conjugation following reductive metabolism, which depends not only on the glutathione conjugation capacity of the cells but also on hypoxic conditions.