The significance of cobalt-55 positron emission tomography in ischemic stroke

Benefit of Later-Time-Point PET Imaging of HER3 Expression Using Optimized Radiocobalt-Labeled Affibody Molecules

HER3-binding affibody molecules are a promising format for visualization of HER3 expression. Cobalt-55, a positron-emitting isotope, with a half-life of 17.5 h, allows for next-day imaging. We investigated the influence of the charge of the radiocobalt–chelator complex on the biodistribution of anti-HER3 affibody molecule (HE)₃-Z_HER3 and compared the best radiocobalt-labeled variant with a recently optimized gallium-labeled variant. Affibody conjugates (HE)₃-Z_HER3-X (X = NOTA, NODAGA, DOTA, DOTAGA) were labeled with [⁵⁷Co]Co (surrogate for ⁵⁵Co). Affinity measurements, binding specificity and cellular processing were studied in two HER3-expressing cancer cell lines. Biodistribution was studied 3 and 24 h post-injection (pi) in mice with HER3-expressing BxPC-3 xenografts and compared to [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA. Micro-single-photon emission tomography/computed tomography (microSPECT/CT) and micro-positron emission tomography/computed tomography (microPET/CT) imaging was performed 3 and 24 h pi. Stably labeled conjugates bound to HER3 with subnanomolar affinity. [⁵⁷Co]Co-(HE)₃-Z_HER3-DOTA had the best tumor retention and a significantly lower concentration in blood than other conjugates, leading to superior tumor-to-blood and tumor-to-liver ratios 24 h pi. Compared to [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA 3 h pi, [⁵⁷Co]Co-(HE)₃-Z_HER3-DOTA provided superior imaging contrast in liver 24 h pi. Concluding, the composition and charge of the [⁵⁷Co]Co–chelator complex influenced the uptake in tumors and normal tissue. [⁵⁷Co]Co-(HE)₃-Z_HER3-DOTA provided the best imaging properties among the cobalt-labeled conjugates. Delayed imaging of HER3 expression with [⁵⁷Co]Co-(HE)₃-Z_HER3-DOTA improved imaging contrast compared to early-time-point imaging with [⁶⁸Ga]Ga-(HE)₃-Z_HER3-NODAGA.

Cyclotron Production of High-Specific Activity 55Co and In Vivo Evaluation of the Stability of 55Co Metal-Chelate-Peptide Complexes

This work describes the production of high-specific activity 55Co and the evaluation of the stability of 55Co-metal-chelate-peptide complexes in vivo. 55Co was produced via the 58Ni(p,α)55Co reaction and purified using anion exchange chromatography with an average recovery of 92% and an average specific activity of 1.96 GBq/μmol. 55Co-DO3A and 55Co-NO2A peptide complexes were radiolabeled at 3.7 MBq/μg and injected into HCT-116 tumor xenografted mice. Positron emission tomography (PET) and biodistribution studies were performed at 24 and 48 hours postinjection and compared to those of 55CoCl2. Both 55Co-metal-chelate complexes demonstrated good in vivo stability by reducing the radiotracers' uptake in the liver by sixfold at 24 hours with ~ 1% ID/g and at 48 hours with ~ 0.5% ID/g and reducing uptake in the heart by fourfold at 24 hours with ~ 0.7% ID/g and sevenfold at 48 hours with ~ 0.35% ID/g. These results support the use of 55Co as a promising new radiotracer for PET imaging of cancer and other diseases.

Magnetic Resonance Imaging after Seizures in Patients with an Ischemic Stroke

BACKGROUND

Seizures related to ischemic strokes are harmful. Their pathogenesis is not very well understood. The present study investigates whether diffusion-weighted imaging (DWI) can detect if those seizures are due to recurrent infarction or responsible for secondary ischemic changes.

PATIENTS AND METHODS

DWI was obtained within 8 days in 60 patients with seizures (7 early and 53 late onset) related to an ischemic stroke.

RESULTS

In 30 patients, positive DWI with decreased apparent diffusion coefficient was found. In 11 patients with late-onset seizures, the DWI showed a positive rim, surrounding the old infarct, while in 12 patients a large positive zone corresponding to a new infarct was observed. All 7 patients with early-onset seizures had positive DWI corresponding to the establishing infarct. A large positive zone on DWI was mainly observed in cardioembolic stroke.

CONCLUSIONS

The positive rim around the old infarct can be considered as cytotoxic edema. Recurrent acute infarction, mainly of cardioembolic origin, seems to be a significant cause of so-called late-onset seizures, making the subdivision into early- and late-onset seizures arbitrary.

Computed tomographic changes of the brain and clinical outcome of patients with seizures and epilepsy after an ischaemic hemispheric stroke

It is not well established whether seizures and epilepsy after an ischaemic stroke increase the disability of patients. Seventy-two patients with delayed seizures after a hemispheric infarct (37 with a single seizure and 35 with epilepsy) were included in the study. The modified Rankin scale was used to compare disability of the patients at 1 month after stroke and at 2 weeks after single or the last seizure, in case of epilepsy. The size of the X-ray hypoattenuation zone was compared on computed tomographic (CT) scans, performed in the weeks after the stroke and 1 week after single or repeated seizures. Lesion size was determined by superimposing the CT slices on digital cerebral vascular maps, on which the contours of the infarct area were delineated. The extent of the infarcts was expressed as the percentage fraction of the total surface area of the cerebral hemisphere. Groups with a single seizure and with epilepsy were mutually compared. Infarcts predominated in the parieto-temporal cortical regions. In the overall group the median Rankin score worsened significantly after seizures. The average size of the X-ray hypoattenuation zone was also significantly increased on the CT scans after the seizures, compared with those after stroke, without clear evidence of recent infarction. Mutual comparison of patients with a single seizure episode and of those with epilepsy showed only a trend of more severe disability and of increase in lesion size in the post-stroke epilepsy group. Delayed seizures and epilepsy after ischaemic stroke are accompanied by an increase in lesion size on CT and by worsening of the disability of the patients. This study does not allow to determine whether this is due to stroke recurrence or due to additional damage as a result of the seizures themselves.

Vascular cognitive impairment in patients with late-onset seizures after an ischemic stroke

BACKGROUND

Cognitive impairment and seizures are both common conditions in patients with cerebrovascular disease.

PURPOSE

The present study investigates whether the occurrence of late-onset seizures, following an ischemic stroke, contributes to vascular cognitive impairment.

PATIENTS AND METHODS

The mean Mini-Mental State Examination (MMSE) and the median modified Rankin (mR) scores were compared between 125 patients who developed late-onset seizures (66 with a single seizure and 59 with repeated seizures or epilepsy) following an ischemic stroke and 125 patients who did not during, at least, a 2-year follow-up.

RESULTS

There were no differences in age, gender, etiology and degree of neurological impairment on admission for their stroke between the groups with and without seizures. Although the mean MMSE score was similar between both groups the median mR score was significantly higher in the seizure patients. Comparing the patients with a single seizure to the non-seizure ones showed the same results. On the other hand, comparison of the patients with epilepsy to the non-seizure group revealed, in addition to the higher median mR score, a significantly lower mean MMSE score in the former group.

CONCLUSION

Repeated seizures following an ischemic stroke promote vascular cognitive impairment.

Cobalt-55 positron emission tomography in symptomatic atherosclerotic carotid artery disease: borderzone versus territorial infarcts

BACKGROUND

Borderzone as well as territorial infarcts can occur in severe atherosclerotic carotid artery disease. It remains controversial whether the borderzone distribution of infarcts is due to hypoperfusion or due to artery-to-artery embolism.

PURPOSE

The present study investigates whether cobalt-55 (55Co) positron emission tomography (PET) shows a different pattern of ischaemia according to the topography of the infarct in severe atherosclerotic carotid artery disease.

PATIENTS AND METHODS

Five patients with a cortical borderzone and seven with a territorial infarct, due to symptomatic carotid artery disease, were investigated with 55Co PET 3-6 months after stroke. Average 55Co counts in the infarct area, the adjacent cortical zone, the deep white matter and, contralateral cerebral cortex and white matter, were compared to the values in the cerebellum used as reference.

RESULTS

No differences in 55Co ratio were observed in the different regions of interest (ROIs) between patients with cortical borderzone and those with territorial infarcts. The 55Co uptake was similar or lower than the reference value for all ROIs in all individual patients.

CONCLUSION

In patients with borderzone as well as with territorial infarcts no evidence was found for subclinical ischaemic injury in or around the infarcts. These data do support the conclusion that cortical borderzone infarcts may not be due to ongoing chronic haemodynamic impairment, but by no means is this conclusive evidence.

Cobalt-55 positron emission tomography in vascular dementia: significance of white matter changes

BACKGROUND

Vascular dementia (VaD) is still used as a covering term to indicate the relationship between cerebrovascular disease and the progressive cognitive disorder. The contribution of white matter changes (WMCs), seen with computed tomography (CT) and magnetic resonance imaging (MRI) of the brain, to dementia had not been fully elucidated. Cobalt-55 (55Co) positron emission tomography (PET) allows us to distinguish between recent and old infarcts.

PURPOSE

The present study investigates whether 55Co PET can detect the lesions responsible for the progression of the cognitive disorder in VaD patients.

PATIENTS AND METHODS

20 consecutive patients with a previous history of repeated strokes occurring more than 6 months before and with multiple cortical infarcts, lacunes and WMCs on CT and 5 age-matched controls were investigated with 55Co PET. The stroke patients were divided in two groups: 8 with and 12 without VaD. Average 55Co counts in cerebral cortex, deep gray nuclei and white matter were compared to the value in the cerebellum used as reference.

RESULTS

In the control group, the 55Co uptake was identical in the cerebral cortex and in the cerebellum, but lower in the deep gray nuclei and the cerebral white matter. When comparing the stroke groups with the control, the 55Co uptake was similar for the cerebral cortex and deep gray nuclei, but significantly increased in the cerebral white matter. The 55Co uptake was also more increased in the stroke group with VaD compared to the non-demented group.

CONCLUSION

55Co PET shows that the WMCs are due to the ongoing damage of probably ischaemic origin which is more prominent in stroke patients with progressive cognitive decline.

Cobalt-55 positron emission tomography in late-onset epileptic seizures after thrombo-embolic middle cerebral artery infarction

The pathogenesis of late-onset epileptic seizures after thrombo-embolic cerebral infarction is poorly understood. Our previous positron emission tomographic (PET) studies with 15O have demonstrated that post-apoplectic epilepsy is associated with more severe brain ischemia, but we were unable to determine if this was the cause or the consequence of the seizures. Using cobalt-55 (55Co) as PET tracer we can now distinguish recurrent, recent infarction in patients with a previous old infarct in the same vascular territory. In seven out of twelve patients with post-apoplectic seizures an increased uptake of 55Co was observed in the border area and in two of them also within the old infarct core. In the control group, composed of eight seizure-free patients with also an old infarct involving the cortical territory of the middle cerebral artery, no increase in 55Co uptake was observed on PET examination. The present study indicates that in a significant number of patients late-onset epilepsy is the clinical expression of recurrent strokes, occurring in the same vascular territory.

Ischemic penumbra: evidence from functional imaging in man

The ischemic penumbra is defined as tissue with flow within the thresholds for maintenance of function and of morphologic integrity. Penumbra tissue has the potential for recovery and therefore is the target for interventional therapy in acute ischemic stroke. The identification of the penumbra necessitates measuring flow reduced less than the functional threshold and differentiating between morphologic integrity and damage. This can be achieved by multitracer positron emission tomography (PET) and perfusion-weighted (PW) and diffusion-weighted magnetic resonance imaging (DW-MRI) in experimental models, in which the recovery of critically perfused tissue or its conversion to infarction was documented in repeat studies. Neuroimaging modalities applied in patients with acute ischemic stroke--multitracer PET, PW- and DW-MRI, single photon emission computed tomography (SPECT), perfusion, and Xe-enhanced computed tomography (CT)-- often cannot reliably identify penumbra tissue: multitracer studies for the assessment of flow and irreversible metabolic damage usually cannot be performed in the clinical setting; CT and MRI do not reliably detect irreversible damage in the first hours after stroke, and even DW-MRI may be misleading in some cases: determinations of perfusion alone yield a poor estimate of the state of the tissue as long as the time course of changes is not known in individual cases. Therefore, the range of flow values in ischemic tissue found later, either within or outside the infarct, was rather broad. New tracers--for example, receptor ligands or hypoxia markers--might improve the identification of penumbra tissue in the future. Despite these methodologic limitations, the validity of the concept of the penumbra was proven in several therapeutic studies in which thrombolytic treatment reversed critical ischemia and decreased the volume of final infarcts. Such neuroimaging findings might serve as surrogate targets in the selection of other therapeutic strategies for large clinical trials.

Cobalt-55 positron emission tomography in recurrent ischaemic stroke

The present study investigates if Cobalt-55 (55Co) positron emission tomography (PET) allows us to distinguish and detect recent, recurrent strokes in patients who had already suffered a previous infarct in the same vascular territory. Fourteen patients with recurrent strokes underwent a 55Co PET scan of the brain. Recently infarcted areas, less than 2 months old, had a high 55Co uptake ratio, whereas infarcts of 6 months to 1 year had an uptake ratio comparable to normal brain tissue. In infarcts older than 2 years the 55Co uptake ratio was decreased compared to the control values. The evolution in 55Co uptake ratios with time can be explained by the dynamics of the inflammatory response within the infarct core. 55Co PET allows to demonstrate stroke recurrence and suggests that single photon emission tomography, using 57Co as the tracer, could be a more easy alternative to be used in routine neurological practice.