Comparative evaluation of [18F]DiFA and its analogs as novel hypoxia positron emission tomography and [18F]FMISO as the standard

[Chemical Design of Radiohalogenated Agents Using Neopentyl Structure]

Many useful radionuclides exist among the halogen elements. Fluorine-18 (18F) is used for positron emission tomography (PET) diagnosis, iodine-123 and iodine-131 (131I) for single photon emission computed tomography (SPECT) diagnosis, 131I for nuclear medicine therapy, and iodine-125 (125I) for research. Astatine-211 (211At), which can be produced by a cyclotron and is attracting attention as a versatile α-ray emitting radionuclide, also belongs to the halogen family. Therefore, if a labeling agent that can stably hold radio-halogens can be developed, it would be useful for the development of radiotheranostic agents that can be expanded from nuclear medicine diagnosis using PET and SPECT to nuclear medicine therapy using β--rays and even α-rays. Currently, benzoic acid derivatives are widely used as labeling agents for radio-halogens. The compounds labeled with 18F or radioiodine using this structure retain the radionuclide stably in vivo, but when 211At is labeled using this structure, 211At is rapidly released from the structure in vivo. Therefore, it is desirable to develop labeling agents that can stably hold 18F to 211At. Under these circumstances, we have found that a neopentyl structure with diol can stably retain 211At and 125I in vivo. Furthermore, this structure can also stably retain 18F in vivo. In this review, I would like to introduce the characteristics of neopentyl diol as a radio-halogens labeling agent and the development of radiotheranositc agents using neopentyl diol.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

Background

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

Results

This commentary of highlights has resulted in 23 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals and also a contribution in relation to MRI-agents is included.

Conclusion

Trends in (radio)chemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

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.

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 Aβ_1-42 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 Aβ_1-42 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.

Positron Emission Tomography After Ischemic Brain Injury: Current Challenges and Future Developments

Positron emission tomography (PET) is widely used in clinical and animal studies, along with the development of diverse tracers. The biochemical characteristics of PET tracers may help uncover the pathophysiological consequences of cardiac arrest (CA) and ischemic stroke, which include cerebral ischemia and reperfusion, depletion of oxygen and glucose, and neuroinflammation. PubMed was searched for studies of the application of PET for "cardiac arrest," "ischemic stroke," and "targeted temperature management." Available studies were included and classified according to the biochemical properties involved and metabolic processes of PET tracers, and were summarized. The mechanisms of ischemic brain injuries were investigated by PET with various tracers to elucidate the pathological process from the initial decrease of cerebral blood flow (CBF) to the subsequent abnormalities in energy and oxygen metabolism, to the monitoring of inflammation. In general, the trends of cerebral blood flow and oxygen metabolism after ischemic attack are not unidirectional but closely related to the time point of injury and recovery. Glucose metabolism after injury showed significant differences in different brain regions whereas global cerebral metabolic rate of glucose (CMRglc) declined. PET monitoring of neuroinflammation shows comparable efficacy to immunostaining. The technology of PET targeting in brain metabolism and the development of tracers provide new tools to track and evaluate the brain's pathological changes after ischemic brain injury. Despite no existing evidence for an available PET-based prediction method, discoveries of new tracers are expected to provide more possibilities for the whole field.

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.