Positron emission tomography radiopharmaceuticals for imaging brain Beta-amyloid

Alzheimer's disease (AD) is defined histologically by the presence of extracellular β-amyloid (Aβ) plaques and intraneuronal neurofibrillary tangles in the cerebral cortex. The diagnosis of dementia, along with the prediction of who will develop dementia, has been assisted by magnetic resonance imaging and positron emission tomography (PET) by using [(18)F]fluorodeoxyglucose (FDG). These techniques, however, are not specific for AD. Based on the chemistry of histologic staining dyes, several Aβ-specific positron-emitting radiotracers have been developed to image neuropathology of AD. Among these, [(11)C]PiB is the most studied Aβ-binding PET radiopharmaceutical in the world. The histologic and biochemical specificity of PiB binding across different regions of the AD brain was demonstrated by showing a direct correlation between Aβ-containing amyloid plaques and in vivo [(11)C]PiB retention measured by PET imaging. Because (11)C is not ideal for commercialization, several (18)F-labeled tracers have been developed. At this time, [(18)F]3'-F-PiB (Flutemetamol), (18)F-AV-45 (Florbetapir), and (18)F-AV-1 (Florbetaben) are undergoing extensive phase II and III clinical trials. This article provides a brief review of the amyloid biology and chemistry of Aβ-specific (11)C and (18)F-PET radiopharmaceuticals. Clinical trials have clearly documented that PET radiopharmaceuticals capable of assessing Aβ content in vivo in the brains of AD subjects and subjects with mild cognitive impairment will be important as diagnostic agents to detect in vivo amyloid brain pathology. In addition, PET amyloid imaging will also help test the amyloid cascade hypothesis of AD and as an aid to assess the efficacy of antiamyloid therapeutics currently under development in clinical trials.

Maternal age affects brain metabolism in adult children of mothers affected by Alzheimer's disease

Cognitively normal (NL) individuals with a maternal history of late-onset Alzheimer's disease (MH) show reduced brain glucose metabolism on FDG-PET as compared to those with a paternal history (PH) and those with negative family history (NH) of Alzheimer's disease (AD). This FDG-PET study investigates whether metabolic deficits in NL MH are associated with advancing maternal age at birth. Ninety-six NL individuals with FDG-PET were examined, including 36 MH, 24 PH, and 36 NH. Regional-to-whole brain gray matter standardized FDG uptake value ratios were examined for associations with parental age across groups using automated regions-of-interest and statistical parametric mapping. Groups were comparable for clinical and neuropsychological measures. Brain metabolism in AD-vulnerable regions was lower in MH compared to NH and PH, and negatively correlated with maternal age at birth only in MH. There were no associations between paternal age and metabolism in any group. Evidence for a maternally inherited, maternal age-related mechanism provides further insight on risk factors and genetic transmission in late-onset AD.

Antiprostate-specific membrane antigen (PSMA)-based radioimmunotherapy: A combined analysis of radiolabeled-J591 studies

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Background: J591 is a monoclonal antibody which selectively binds the external domain of PSMA with high affinity. Two phase I trials of radiolabeled-J591 have been published; two additional studies have been completed. 90Y is a beta-emitting particle optimal for tumor lesions 28-42 mm in size; 177Lu is best suited for lesions 1-3 mm in diameter [O'Donoghue J Nuc Med 1995].

Methods: With WCMC IRB approval, long-term follow-up of 4 clinical trials and the ongoing studies was analyzed. Prospectively collected data were supplemented with retrospective additions when necessary. Median survival (OS) was calculated by Kaplan- Meier methodology.

Results: Between 10/00 and 7/10, 132 pts with metastatic CRPC received radiolabeled J591 (103 received 177Lu-J591, 29 90Y-J591) with a median follow-up of 68.5 months (mo). Median age 70.3 yrs; all progressed after multiple lines of hormonal therapy, 41.7% received prior chemo, 48.5% received post-J591 chemo. Median Halabi nomogram score for the group was 146 (range 97- 196). OS for the entire group was 16.7 mo [95% CI 13.8, 19.7]. 26 (19.7%) experienced > 30% PSA decline, with OS of 22.4 mo (vs 13.6 mo for those with any PSA increase, p=0.08; p=0.006 at phase II doses). Pts receiving 177Lu-J591 had more 30% PSA declines than 90Y-J591 (21.3% vs 6.9%, p=0.06). 37.9% had measurable disease; those who received 90Y-J591 were more likely to have measurable response than 177Lu-J591 [p=0.04]. All objective tumor responses also had significant PSA declines. Of 15 pts with baseline and follow-up CTC counts (CellSearch methodology), 12 (80%) became or remained favorable at follow-up; 3 became or remained unfavorable.

Conclusions: Radiolabled J591 is tolerable and efficacious. As predicted based upon their physical properties, 177Lu-J591 appears more effective for lower volume disease, with objective responses in larger volume disease only with 90Y-J591. Current trials utilizing 177Lu-J591 focus on predictive biomarkers, dose fractionation to improve tolerance and efficacy, combination with chemotherapy, and “salvage radioimmunotherapy” to delay the onset of metastases in men with progressive biochemical (micrometastatic) disease best suitable for 177Lu-J591.

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Imaging inflammation in a patient with epilepsy due to focal cortical dysplasia

BACKGROUND AND PURPOSE

Evidence from animal models and examination of human epilepsy surgery specimens indicates that inflammation plays an important role in epilepsy. Positron emission tomography (PET) using [C11]PK11195, a marker of activated microglia, provides a means to visualize neuroinflammation in vivo in humans. We hypothesize that in patients with active epilepsy, [C11]PK11195 PET (PK-PET) may be able to identify areas of focally increased inflammation corresponding to the seizure onset zone.

METHODS

A young woman with intractable epilepsy underwent PK-PET as part of an approved research study. PK-PET results were compared with results from other clinical studies.

RESULTS

PK-PET revealed an area of focally increased radiotracer uptake in the right frontal lobe corresponding to this patient's seizure focus as identified by ictal and interictal 18F-fluorodeoxyglucose (FDG)-PET and EEG. Routine brain magnetic resonance imaging (MRI) was initially considered normal, though high-resolution studies showed possible subtle dysplasia of the right frontal lobe. The patient underwent a right frontal lobe resection, and pathological evaluation showed focal cortical dysplasia with activated microglia.

CONCLUSIONS

PK-PET can identify neuroinflammation associated with subtle focal cortical dysplasia, and may therefore have a clinical role in guiding epilepsy surgery for patients with difficult-to-localize seizure foci.

Safety and maximum tolerated dose of superselective intraarterial cerebral infusion of bevacizumab after osmotic blood-brain barrier disruption for recurrent malignant glioma. Clinical article

OBJECT

The authors assessed the safety and maximum tolerated dose of superselective intraarterial cerebral infusion (SIACI) of bevacizumab after osmotic disruption of the blood-brain barrier (BBB) with mannitol in patients with recurrent malignant glioma.

METHODS

A total of 30 patients with recurrent malignant glioma were included in the current study.

RESULTS

The authors report no dose-limiting toxicity from a single dose of SIACI of bevacizumab up to 15 mg/kg after osmotic BBB disruption with mannitol. Two groups of patients were studied; those without prior bevacizumab exposure (naïve patients; Group I) and those who had received previous intravenous bevacizumab (exposed patients; Group II). Radiographic changes demonstrated on MR imaging were assessed at 1 month postprocedure. In Group I patients, MR imaging at 1 month showed a median reduction in the area of tumor enhancement of 34.7%, a median reduction in the volume of tumor enhancement of 46.9%, a median MR perfusion (MRP) reduction of 32.14%, and a T2-weighted/FLAIR signal decrease in 9 (47.4%) of 19 patients. In Group II patients, MR imaging at 1 month showed a median reduction in the area of tumor enhancement of 15.2%, a median volume reduction of 8.3%, a median MRP reduction of 25.5%, and a T2-weighted FLAIR decrease in 0 (0%) of 11 patients.

CONCLUSIONS

The authors conclude that SIACI of mannitol followed by bevacizumab (up to 15 mg/kg) for recurrent malignant glioma is safe and well tolerated. Magnetic resonance imaging shows that SIACI treatment with bevacizumab can lead to reduction in tumor area, volume, perfusion, and T2-weighted/FLAIR signal.

Anti-prostate-specific membrane antigen-based radioimmunotherapy for prostate cancer

Despite recent advances, advanced prostate cancer is suboptimally responsive to current chemotherapeutic agents. Radiolabeled monoclonal antibody therapy that targets prostate-specific membrane antigen (PSMA) shows promise and is an area of active investigation. J591 is a deimmunized IgG monoclonal antibody developed to target the extracellular domain of PSMA. Preclinical and early phase clinical studies using radiolabeled J591 have demonstrated efficacy in targeting tumor cells and decreasing levels of prostate-specific antigen. Radiolabeled J591 is well-tolerated, nonimmunogenic, and can be administered in multiple doses. The dose-limiting toxicity is reversible myelosuppression with little nonhematologic toxicity. Future studies will include approaches to optimize patient selection and incorporate novel strategies to improve the success of anti-PSMA radioimmunotherapy.

PET imaging of leptin biodistribution and metabolism in rodents and primates

We have determined the systemic biodistribution of the hormone leptin by PET imaging. PET imaging using (18)F- and (68)Ga-labeled leptin revealed that, in mouse, the hormone was rapidly taken up by megalin (gp330/LRP2), a multiligand endocytic receptor localized in renal tubules. In addition, in rhesus monkeys, 15% of labeled leptin localized to red bone marrow, which was consistent with hormone uptake in rodent tissues. These data confirm a megalin-dependent mechanism for renal uptake in vivo. The significant binding to immune cells and blood cell precursors in bone marrow is also consistent with prior evidence showing that leptin modulates immune function. These experiments set the stage for similar studies in humans to assess the extent to which alterations of leptin's biodistribution might contribute to obesity; they also provide a general chemical strategy for (18)F labeling of proteins for PET imaging of other polypeptide hormones.

Bone marrow recovery and subsequent chemotherapy following radiolabeled anti-prostate-specific membrane antigen (PSMA) monoclonal antibody J591 in patients (pts) with metastatic castration-resistant prostate cancer (metCRPC)

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Background: Reversible myelosuppression is the dose-limiting toxicity of radioimmunotherapy (RIT). Cases of marrow damage, including myelodysplasia and acute leukemia have been reported with the RIT most used to date (that targeting CD20 in Non- Hodgkin's lymphoma), though no statistically significant association exists. In addition, post-RIT chemotherapy (chemo) may theoretically be limited. We have treated metCRPC pts with anti-PSMA RIT since 2000 and analyzed post-therapy outcomes. Methods: Follow-up across 4 prospective phase I and II trials utilizing 177Lu-J591 and 90Y-J591 was performed. Quality of hematologic recovery from post-RIT myelosuppression was documented. Administration of pre- and post-RIT chemo was analyzed. Specific searches for subsequent myelodysplasia and/or leukemia were performed. Results: Median age of the 109 treated pts is 70 (range 47–88). Entry criteria for all trials included progressive metCRPC, PS < 2, normal baseline neutrophil and platelet counts, and hemoglobin > 10 g/dL. 80 received 177Lu-J591 at cumulative doses ranging from 10–120 mCi/m2 and 29 received 90Y-J591 at cumulative doses of 5–40 mCi/m2. 43% received at least 1 line of pre-RIT chemo, 53% received at least 1 line of post-RIT chemo, and 20% have never received chemo to date. All pts with adequate performance status received chemo except per pt choice. Excluding re-treated pts, 98% and 87% of assessable pts had full recovery of neutrophils and platelets respectively. Of the remaining, all but 4 recovered to Gr 1 neutropenia and/or thrombocytopenia. The most common reason for lack of complete hematologic recovery was CRPC progression in bone marrow (16 pts with persistent or recurrent myelosuppression underwent bone marrow biopsy). No cases of post-RIT myelodysplasia and/or leukemia were discovered. Conclusions: Anti-PSMA radioimmunotherapy is well-tolerated with efficacy previously reported. While follow-up is ongoing, reports of irreversible bone marrow damage appear unfounded and all patients with intact performance status willing to undergo post-RIT chemotherapy are able to receive it.

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Site-specific (18)F-labeling of the protein hormone leptin using a general two-step ligation procedure

The protein hormone leptin acts to regulate body fat and energy expenditure. Resistance to this hormone is implicated in human obesity and its pathophysiological consequences. In order to gain insight into the mechanism of leptin resistance, an (18)F-labeled derivative was developed to study the biodistribution of the hormone using positron emission tomography (PET). A two-step, site specific ligation approach was developed for this purpose, in which an aminooxy-reactive group was incorporated at the C-terminus of leptin using expressed protein ligation (EPL), which was subsequently derivatized with [ (18)F]fluorobenzaldehyde using an aniline-accelerated radiochemical oximation reaction. The modified hormone was shown to be biologically active in vitro and in vivo, and it was applied to PET imaging in ob/ ob mice. These protocols will allow for the routine production of site-specifically (18)F radiolabeled leptin, as well as other proteins, for use in PET imaging in systems from mouse to man.