Radiation dose estimation using preclinical imaging with 124I-metaiodobenzylguanidine (MIBG) PET

Aurora Kinase A inhibition enhances DNA damage and tumor cell death with 131I-MIBG therapy in high-risk neuroblastoma

BACKGROUND

Neuroblastoma is the most common extra-cranial pediatric solid tumor. 131I-metaiodobenzylguanidine (MIBG) is a targeted radiopharmaceutical highly specific for neuroblastoma tumors, providing potent radiotherapy to widely metastatic disease. Aurora kinase A (AURKA) plays a role in mitosis and stabilization of the MYCN protein in neuroblastoma. We aimed to study the impact of AURKA inhibitors on DNA damage and tumor cell death in combination with 131I-MIBG therapy in a pre-clinical model of high-risk neuroblastoma.

RESULTS

Using an in vivo model of high-risk neuroblastoma, we demonstrated a marked combinatorial effect of 131I-MIBG and alisertib on tumor growth. In MYCN amplified cell lines, the combination of radiation and an AURKA A inhibitor increased DNA damage and apoptosis and decreased MYCN protein levels.

CONCLUSION

The combination of AURKA inhibition with 131I-MIBG treatment is active in resistant neuroblastoma models.

Aurora Kinase A inhibition enhances DNA damage and tumor cell death with 131I-MIBG therapy in high-risk neuroblastoma

Background

Neuroblastoma is the most common extra-cranial pediatric solid tumor. 131I-metaiodobenzylguanidine (MIBG) is a targeted radiopharmaceutical highly specific for neuroblastoma tumors, providing potent radiotherapy to widely metastatic disease. Aurora kinase A (AURKA) plays a role in mitosis and stabilization of the MYCN protein in neuroblastoma. Here we explore whether AURKA inhibition potentiates a response to MIBG therapy.

Results

Using an in vivo model of high-risk neuroblastoma, we demonstrated a marked combinatorial effect of 131I-MIBG and alisertib on tumor growth. In MYCN amplified cell lines, the combination of radiation and an AURKA A inhibitor increased DNA damage and apoptosis and decreased MYCN protein levels.

Conclusion

The combination of AURKA inhibition with 131I-MIBG treatment is active in resistant neuroblastoma models and is a promising clinical approach in high-risk neuroblastoma.

Biodistribution and radiation dosimetry of 124I-mIBG in adult patients with neural crest tumours and extrapolation to paediatric models

AIM

Positron emission tomography (PET) using 124I-mIBG has been established for imaging and pretherapeutic dosimetry. Here, we report the first systematic analysis of the biodistribution and radiation dosimetry of 124I-mIBG in patients with neural crest tumours and project the results to paediatric patient models.

METHODS

Adult patients with neural crest tumours who underwent sequential 124I-mIBG PET were included in this retrospective single-center analysis. PET data were acquired 4, 24, 48, and/or 120 h after administration of a mean of 43 MBq 124I-mIBG. Whole-body counting and blood sampling were performed at 2, 4, 24, 48 and 120 h after administration. Absorbed organ dose and effective dose coefficients were estimated in OLINDA/EXM 2.2 according to the MIRD formalism. Extrapolation to paediatric models was performed based on mass-fraction scaling of the organ-specific residence times. Biodistribution data for adults were also projected to 123I-mIBG and 131I-mIBG.

RESULTS

Twenty-one patients (11 females, 10 males) were evaluated. For adults, the organs exposed to the highest dose per unit administered activity were urinary bladder (1.54 ± 0.40 mGy/MBq), salivary glands (0.77 ± 0.28 mGy/MBq) and liver (0.65 ± 0.22 mGy/MBq). Mean effective dose coefficient for adults was 0.25 ± 0.04 mSv/MBq (male: 0.24 ± 0.03 mSv/MBq, female: 0.26 ± 0.06 mSv/MBq), and increased gradually to 0.29, 0.44, 0.69, 1.21, and 2.94 mSv/MBq for the 15-, 10-, 5-, 1-years-old, and newborn paediatric reference patients. Projected mean effective dose coefficients for 123I-mIBG and 131I-mIBG for adults were 0.014 ± 0.002 mSv/MBq and 0.18 ± 0.04 mSv/MBq, respectively.

CONCLUSION

PET-based derived radiation dosimetry data for 124I-mIBG from this study agreed well with historical projected data from ICRP 53. The effective dose coefficients presented here may aid in guidance for establishing weight-based activity administration protocols.

Noninvasive estimation of human radiation dosimetry of 18F-FDG by whole-body small animal PET imaging in rats

PURPOSE

Small animal PET provides the biodistribution of administrated radiotracer in vivo and have a potential to contribute on dosimetry study. The aim of this study is to investigate the effect of region-of-interest (ROI)-delineation in whole-body rat PET image toward non-invasive estimation of human dosimetry of 18F-FDG.

METHOD

After administration of 18F-FDG (averaged 11.7 MBq), 3.5-h PET and 20-min CT scans were sequentially performed for three rats by Clairvivo PET/CT system. Seven source organs, and the remainder of the body, were studied to extrapolate %ID(t) and estimate time-integrated activity coefficients [kBq-h/MBq] in human. The mean absorbed dose in each target organ and the effective dose were estimated by MIRD method. Effects of ROI-definitions on both extrapolated %ID(t) in human and estimated doses were also investigated by using (i) small ROIs of high uptake region and (ii) whole organ ROIs.

RESULTS

Averaged effective doses of 18F-FDG in human by using high-uptake and whole-organ ROIs were 27.8 ± 6.54 and 19.3 ± 2.72 μSv/MBq, respectively.

CONCLUSION

The use of small animal PET scanner, which allows repeatedly PET scans, have a potential to contribute on the reduction of the number of experimental animals. However, the ways of ROI drawing influences on the estimated effective dose and safe-side ROI definition may be preferred.

Diagnostic performance of [124I]m-iodobenzylguanidine PET/CT in patients with pheochromocytoma

^123/131I-MIBG scintigraphy has shown a high specificity for imaging pheochromocytoma and paraganglioma however with low sensitivity due to low spatial resolution. ¹²⁴I-MIBG PET may overcome this limitation to improve the staging of patients with (suspected) pheochromocytoma. Methods: We analyzed the sensitivity, specificity, positive and negative predictive values (PPV, NPV) of ¹²⁴I-MIBG PET in 43 consecutive patients with suspected (recurrence of) pheochromocytoma using histopathological (n = 25) and clinical validation (n = 18) as standard of truth. Furthermore, we compared ¹²⁴I-MIBG PET versus contrast enhanced CT (CE-CT) per-patient and per-lesion detection rate of ¹²⁴I-MIBG PET in 13 additional patients with known metastatic malignant pheochromocytoma (MMP). Results: ¹²⁴I-MIBG PET/CT was positive in 19/43 (44%) patients with suspected pheochromocytoma. Presence of pheochromocytoma was confirmed in 22/43 (51%). ¹²⁴I-MIBG PET/CT sensitivity, specificity, PPV, NPV were 86%, 100%, 100%, 88%, respectively. ¹²⁴I-MIBG PET was positive in 11/13 (85%) MMP patients. Combined ¹²⁴I-MIBG PET and CE-CT detected 173 lesions, of which 166 (96%) and 118 (68%) were visible on ¹²⁴I-MIBG PET and CE-CT, respectively. Discussion: ¹²⁴I-MIBG PET detects pheochromocytoma with high accuracy at initial staging and high detection rate at re-staging. Superior diagnostic performance aids guidance of surgical and medical management including personalized ¹³¹I-MIBG therapy.

Nuclear Medicine Imaging in Neuroblastoma: Current Status and New Developments

Neuroblastoma is the most common extracranial solid malignancy in children. At diagnosis, approximately 50% of patients present with metastatic disease. These patients are at high risk for refractory or recurrent disease, which conveys a very poor prognosis. During the past decades, nuclear medicine has been essential for the staging and response assessment of neuroblastoma. Currently, the standard nuclear imaging technique is meta-[¹²³I]iodobenzylguanidine ([¹²³I]mIBG) whole-body scintigraphy, usually combined with single-photon emission computed tomography with computed tomography (SPECT-CT). Nevertheless, 10% of neuroblastomas are mIBG non-avid and [¹²³I]mIBG imaging has relatively low spatial resolution, resulting in limited sensitivity for smaller lesions. More accurate methods to assess full disease extent are needed in order to optimize treatment strategies. Advances in nuclear medicine have led to the introduction of radiotracers compatible for positron emission tomography (PET) imaging in neuroblastoma, such as [¹²⁴I]mIBG, [¹⁸F]mFBG, [¹⁸F]FDG, [⁶⁸Ga]Ga-DOTA peptides, [¹⁸F]F-DOPA, and [¹¹C]mHED. PET has multiple advantages over SPECT, including a superior resolution and whole-body tomographic range. This article reviews the use, characteristics, diagnostic accuracy, advantages, and limitations of current and new tracers for nuclear medicine imaging in neuroblastoma.

Human dosimetry of free 211At and meta-[211At]astatobenzylguanidine (211At-MABG) estimated using preclinical biodistribution from normal mice

BACKGROUND

211At is one of the ideal nuclides for targeted radionuclide therapies (TRTs). Meta-[211At]astatobenzylguanidine (211At-MABG) has been proposed for the treatment of pheochromocytoma. To effectively use these radiopharmaceuticals, dosimetry must be performed. It is important to determine the absorbed doses of free 211At and 211At-MABG to determine the organs that may be at risk when using TRTs. The aim of this study was to estimate human dosimetry from preclinical biodistribution of free 211At and 211At-MABG in various organs in normal mice.

METHODS

Male C57BL/6 N mice were administered 0.13 MBq of free 211At or 0.20 MBq of 211At-MABG by tail-vein injection. The mice were sacrificed at 5 min, and at 1, 3, 6, and 24 h after the injection (n = 5 for each group). The percentage of injected activity per mass in organs and blood (%IA/g) was determined. The human absorbed doses of free 211At and 211At-MABG were calculated using the Organ Level INternal Dose Assessment/EXponential Modeling (OLINDA/EXM) version 2.0 and IDAC-Dose 2.1.

RESULTS

High uptake of free 211At was observed in the lungs, spleen, salivary glands, stomach, and thyroid. The absorbed doses of free 211At in the thyroid and several tissues were higher than those of 211At-MABG. The absorbed doses of 211At-MABG in the adrenal glands, heart wall, and liver were higher than those of free 211At.

CONCLUSIONS

The absorbed doses of 211At-MABG in organs expressing the norepinephrine transporter were higher than those of free 211At. In addition, the biodistribution of free 211At was different from that of 211At-MABG. The absorbed dose of free 211At may help predict the organs potentially at risk during TRTs using 211At-MABG due to deastatination.

Roles of PET/Computed Tomography in the Evaluation of Neuroblastoma

Neuroblastoma is one of the most common pediatric malignant tumors. Functional imaging plays an important role in the diagnosis, staging, and therapy response monitoring of neuroblastoma. Although metaiodobenzylguanidine scan with single-photon emission computed tomography/computed tomography remains the mainstay in functional imaging of the neuroblastomas, PET/CT has begun to show increased utility in this clinical setting.

124I-MIBG PET/CT to Monitor Metastatic Disease in Children with Relapsed Neuroblastoma

The metaiodobenzylguanidine (MIBG) scan is one of the most sensitive noninvasive lesion detection modalities for neuroblastoma. Unlike ¹²³I-MIBG, ¹²⁴I-MIBG allows high-resolution PET. We evaluated ¹²⁴I-MIBG PET/CT for its diagnostic performance as directly compared with paired ¹²³I-MIBG scans. Methods: Before ¹³¹I-MIBG therapy, standard ¹²³I-MIBG imaging (5.2 MBq/kg) was performed on 7 patients, including whole-body (anterior-posterior) planar imaging, focused-field-of-view SPECT/CT, and whole-body ¹²⁴I-MIBG PET/CT (1.05 MBq/kg). After therapy, 2 of 7 patients also completed ¹²⁴I-MIBG PET/CT as well as paired ¹²³I-MIBG planar imaging and SPECT/CT. One patient underwent ¹²⁴I-MIBG PET/CT only after therapy. We evaluated all 8 patients who showed at least 1 ¹²³I-MIBG-positive lesion with a total of 10 scans. In 8 pairs, ¹²³I-MIBG and ¹²⁴I-MIBG were performed within 1 mo of each other. The locations of identified lesions, the number of total lesions, and the curie scores were recorded for the ¹²³I-MIBG and ¹²⁴I-MIBG scans. Finally, for 5 patients who completed at least 3 PET/CT scans after administration of ¹²⁴I-MIBG, we estimated the effective dose of ¹²⁴I-MIBG. Results: ¹²³I-MIBG whole-body planar scans, focused-field-of-view SPECT/CT scans, and whole-body ¹²⁴I-MIBG PET scans found 25, 32, and 87 total lesions, respectively. There was a statistically significant difference in lesion detection for ¹²⁴I-MIBG PET/CT versus ¹²³I-MIBG planar imaging (P < 0.0001) and ¹²³I-MIBG SPECT/CT (P < 0.0001). The curie scores were also higher for ¹²⁴I-MIBG PET/CT than for ¹²³I-MIBG planar imaging and SPECT/CT in 6 of 10 patients. ¹²⁴I-MIBG PET/CT demonstrated better detection of lesions throughout the body, including the chest, spine, head and neck, and extremities. The effective dose estimated for patient-specific ¹²⁴I-MIBG was approximately 10 times that of ¹²³I-MIBG; however, given that we administered a very low activity of ¹²⁴I-MIBG (1.05 MBq/kg), the effective dose was only approximately twice that of ¹²³I-MIBG despite the large difference in half-lives (100 vs. 13.2 h). Conclusion: The first-in-humans use of low-dose ¹²⁴I-MIBG PET for monitoring disease burden demonstrated tumor detection capability superior to that of ¹²³I-MIBG planar imaging and SPECT/CT.

Quantitative Imaging of Alpha-Emitting Therapeutic Radiopharmaceuticals

Targeted alpha therapy (TAT) is an active area of drug development as a highly specific and highly potent therapeutic modality that can be applied to many types of late-stage cancers. In order to properly evaluate its safety and efficacy, understanding biokinetics of alpha-emitting radiopharmaceuticals is essential. Quantitative imaging of alpha-emitting radiopharmaceuticals is often possible via imaging of gammas and positrons produced during complex decay chains of these radionuclides. Analysis of the complex decay chains for alpha-emitting radionuclides (Tb-149, At-211, Bi-212 (decayed from Pb-212), Bi-213, Ra-223, Ac-225, and Th-227) with relevance to imageable signals is attempted in this mini-review article. Gamma camera imaging, single-photon emission computed tomography, positron emission tomography, bremsstrahlung radiation imaging, Cerenkov luminescence imaging, and Compton cameras are briefly discussed as modalities for imaging alpha-emitting radiopharmaceuticals.

Preclinical Pharmacokinetics and Dosimetry Studies of 124I/131I-CLR1404 for Treatment of Pediatric Solid Tumors in Murine Xenograft Models

Cancer is the second leading cause of death for children between the ages of 5 and 14 y. For children diagnosed with metastatic or recurrent solid tumors, for which the utility of external-beam radiotherapy is limited, the prognosis is particularly poor. The availability of tumor-targeting radiopharmaceuticals for molecular radiotherapy (MRT) has demonstrated improved outcomes in these patient populations, but options are nonexistent or limited for most pediatric solid tumors. 18-(p-iodophenyl)octadecylphosphocholine (CLR1404) is a novel antitumor alkyl phospholipid ether analog that broadly targets cancer cells. In this study, we evaluated the in vivo pharmacokinetics of ¹²⁴I-CLR1404 (CLR 124) and estimated theranostic dosimetry for ¹³¹I-CLR1404 (CLR 131) MRT in murine xenograft models of the pediatric solid tumors neuroblastoma, rhabdomyosarcoma, and Ewing sarcoma. Methods: Tumor-bearing mice were imaged with small-animal PET/CT to evaluate the whole-body distribution of CLR 124 and, correcting for differences in radioactive decay, predict that of CLR 131. Image volumes representing CLR 131 provided input for Geant4 Monte Carlo simulations to calculate subject-specific tumor dosimetry for CLR 131 MRT. Pharmacokinetics for CLR 131 were extrapolated to adult and pediatric humans to estimate normal-tissue dosimetry. In neuroblastoma, a direct comparison of CLR 124 with ¹²⁴I-metaiodobenzylguanidine (¹²⁴I-MIBG) in an MIBG-avid model was performed. Results: In vivo pharmacokinetics of CLR 124 showed selective uptake and prolonged retention across all pediatric solid tumor models investigated. Subject-specific tumor dosimetry for CLR 131 MRT presents a correlative relationship with tumor-growth delay after CLR 131 MRT. Peak uptake of CLR 124 was, on average, 22% higher than that of ¹²⁴I-MIBG in an MIBG-avid neuroblastoma model. Conclusion: CLR1404 is a suitable theranostic scaffold for dosimetry and therapy with potentially broad applicability in pediatric oncology. Given the ongoing clinical trials for CLR 131 in adults, these data support the development of pediatric clinical trials and provide detailed dosimetry that may lead to improved MRT treatment planning.

Guidelines on nuclear medicine imaging in neuroblastoma

Nuclear medicine has a central role in the diagnosis, staging, response assessment and long-term follow-up of neuroblastoma, the most common solid extracranial tumour in children. These EANM guidelines include updated information on ¹²³I-mIBG, the most common study in nuclear medicine for the evaluation of neuroblastoma, and on PET/CT imaging with ¹⁸F-FDG, ¹⁸F-DOPA and ⁶⁸Ga-DOTA peptides. These PET/CT studies are increasingly employed in clinical practice. Indications, advantages and limitations are presented along with recommendations on study protocols, interpretation of findings and reporting results.

Improving PET Quantification of Small Animal [68Ga]DOTA-Labeled PET/CT Studies by Using a CT-Based Positron Range Correction

PURPOSE

Image quality of positron emission tomography (PET) tracers that emits high-energy positrons, such as Ga-68, Rb-82, or I-124, is significantly affected by positron range (PR) effects. PR effects are especially important in small animal PET studies, since they can limit spatial resolution and quantitative accuracy of the images. Since generators accessibility has made Ga-68 tracers wide available, the aim of this study is to show how the quantitative results of [⁶⁸Ga]DOTA-labeled PET/X-ray computed tomography (CT) imaging of neuroendocrine tumors in mice can be improved using positron range correction (PRC).

PROCEDURES

Eighteen scans in 12 mice were evaluated, with three different models of tumors: PC12, AR42J, and meningiomas. In addition, three different [⁶⁸Ga]DOTA-labeled radiotracers were used to evaluate the PRC with different tracer distributions: [⁶⁸Ga]DOTANOC, [⁶⁸Ga]DOTATOC, and [⁶⁸Ga]DOTATATE. Two PRC methods were evaluated: a tissue-dependent (TD-PRC) and a tissue-dependent spatially-variant correction (TDSV-PRC). Taking a region in the liver as reference, the tissue-to-liver ratio values for tumor tissue (TLR_tumor), lung (TLR_lung), and necrotic areas within the tumors (TLR_necrotic) and their respective relative variations (ΔTLR) were evaluated.

RESULTS

All TLR values in the PRC images were significantly different (p < 0.05) than the ones from non-PRC images. The relative differences of the tumor TLR values, respect to the case with no PRC, were ΔTLR_tumor 87 ± 41 % (TD-PRC) and 85 ± 46 % (TDSV-PRC). TLR_lung decreased when applying PRC, being this effect more remarkable for the TDSV-PRC method, with relative differences respect to no PRC: ΔTLR_lung = - 45 ± 24 (TD-PRC), - 55 ± 18 (TDSV-PRC). TLR_necrotic values also decreased when using PRC, with more noticeable differences for TD-PRC: ΔTLR_necrotic = - 52 ± 6 (TD-PRC), - 48 ± 8 (TDSV-PRC).

CONCLUSION

The PRC methods proposed provide a significant quantitative improvement in [⁶⁸Ga]DOTA-labeled PET/CT imaging of mice with neuroendocrine tumors, hence demonstrating that these techniques could also ameliorate the deleterious effect of the positron range in clinical PET imaging.

Norepinephrine Transporter as a Target for Imaging and Therapy

The norepinephrine transporter (NET) is essential for norepinephrine uptake at the synaptic terminals and adrenal chromaffin cells. In neuroendocrine tumors, NET can be targeted for imaging as well as therapy. One of the most widely used theranostic agents targeting NET is metaiodobenzylguanidine (MIBG), a guanethidine analog of norepinephrine. ¹²³I/¹³¹I-MIBG theranostics have been applied in the clinical evaluation and management of neuroendocrine tumors, especially in neuroblastoma, paraganglioma, and pheochromocytoma. ¹²³I-MIBG imaging is a mainstay in the evaluation of neuroblastoma, and ¹³¹I-MIBG has been used for the treatment of relapsed high-risk neuroblastoma for several years, however, the outcome remains suboptimal. ¹³¹I-MIBG has essentially been only palliative in paraganglioma/pheochromocytoma patients. Various techniques of improving therapeutic outcomes, such as dosimetric estimations, high-dose therapies, multiple fractionated administration and combination therapy with radiation sensitizers, chemotherapy, and other radionuclide therapies, are being evaluated. PET tracers targeting NET appear promising and may be more convenient options for the imaging and assessment after treatment. Here, we present an overview of NET as a target for theranostics; review its current role in some neuroendocrine tumors, such as neuroblastoma, paraganglioma/pheochromocytoma, and carcinoids; and discuss approaches to improving targeting and theranostic outcomes.

Nuclear Medicine in Pediatric and Adolescent Tumors

Nuclear medicine has an important role in the management of many cancers in pediatric age group with multiple imaging modalities and radiopharmaceuticals targeting various biological uptake mechanisms. 18-Flourodeoxyglucose is the radiotracer of choice especially in patients with sarcoma and lymphoma. (18)FDG-PET, for sarcoma and lymphomas, is proved to be superior to conventional imaging in staging and therapy response. Although studies are limited in pediatric population, (18)FDG-PET/CT has found its way through international guidelines. Limitations and strengths of PET imaging must be noticed before adapting PET imaging in clinical protocols. Established new response criteria using multiple parameters derived from (18)FDG-PET would increase the accuracy and repeatability of response evaluation. Current data suggest that I-123 metaiodobenzylguanidine (MIBG) remains the tracer of choice in the evaluation of neuroblastoma (NB) because of its high sensitivity, specificity, diagnostic accuracy, and prognostic value. It is valuable in determining the response to therapy, surveillance for disease recurrence, and in selecting patients for I-131 therapy. SPECT/CT improves the diagnostic accuracy and the interpretation confidence of MIBG scans. (18)FDG-PET/CT is an important complementary to MIBG imaging despite its lack of specificity to NB. It is valuable in cases of negative or inconclusive MIBG scans and when MIBG findings underestimate the disease status as determined from clinical and radiological findings. F-18 DOPA is promising tracer that reflects catecholamine metabolism and is both sensitive and specific. F-18 DOPA scintigraphy provides the advantages of PET/CT imaging with early and short imaging times, high spatial resolution, inherent morphologic correlation with CT, and quantitation. Regulatory and production issues currently limit the tracer's availability. PET/CT with Ga-68 DOTA appears to be useful in NB imaging and may have a unique role in selecting patients for peptide receptor radionuclide therapy with somatostatin analogues. C-11 hydroxyephedrine PET/CT is a specific PET tracer for NB, but the C-11 label that requires an on-site cyclotron production and the high physiologic uptake in the liver and kidneys limit its use. I-124 MIBG is useful for I-131 MIBG pretherapeutic dosimetry planning. Its use for diagnostic imaging as well as the use of F-18 labeled MIBG analogues is currently experimental. PET/MR imaging is emerging and is likely to become an important tool in the evaluation. It provides metabolic and superior morphological data in one imaging session, expediting the diagnosis and lowering the radiation exposure. Radioactive iodines not only detect residual tissue and metastatic disease but also are used in the treatment of differentiated thyroid cancer. However, these are not well documented in pediatric age group like adult patients. Use of radioactivity in pediatric population is very important and strictly controlled because of the possibility of secondary malignities; therefore, management of oncological cases requires detailed literature knowledge. This article aims to review the literature on the use of radionuclide imaging and therapy in pediatric population with thyroid cancer, sarcomas, lymphoma, and NB.

A phantom study: Should 124 I-mIBG PET/CT replace 123 I-mIBG SPECT/CT?

PURPOSE

The isotope ¹²³ I is commonly labeled with meta-iodobenzylguanidine (mIBG) for imaging of neuroendocrine tumors, such as pheochromocytomas and neuroblastomas. ¹²³ I-mIBG SPECT/CT imaging is performed for staging, follow-up and selection of patients for treatment with ¹³¹ I mIBG. As an alternative to ¹²³ I, ¹²⁴ I-mIBG PET/CT may be used, potentially taking advantage of the superior PET image quality. The purpose of this study was to investigate whether ¹²⁴ I PET/CT improves image quality as compared with ¹²³ I SPECT/CT for equal patient effective radiation dose (in mSv).

METHODS

Phantom measurements were performed using the NEMA-2007 image quality phantom. SPECT and PET reconstruction settings were used with and without time-of-flight (TOF) and point-spread-function (PSF) modeling. As a measure of image quality, the contrast-to-noise ratio (CNR) was calculated. The ratio of the ¹²³ I to ¹²⁴ I activity concentration was determined at which the contrast-to-noise ratio was equal for both modalities. This metric was defined as the contrast equivalent activity ratio (CEAR).

RESULTS

CEARs of 47.7, 25.6, 23.1, 14.6, 10.0, and 9.1 were obtained for a TOF and PSF modeled ¹²⁴ I reconstruction method and an attenuation and scatter-corrected ¹²³ I reconstruction method for sphere sizes of 10 to 37 mm, respectively. As the effective radiation dose of ¹²⁴ I-mIBG is higher than of ¹²³ I-mIBG (in mSv/MBq), an equal effective dose corresponds to a CEAR of 5 to 10. Therefore, CEARs higher than 5 to 10 indicate that ¹²⁴ I PET/CT outperforms ¹²³ I SPECT/CT in the sense of image quality for equal patient effective radiation dose.

CONCLUSION

The CEAR is much larger than a factor of 5 to 10 (needed for equal patient effective radiation dose) for most of the reconstruction methods and sphere sizes. Therefore, ¹²⁴ I-mIBG PET/CT is expected to improve image quality and/or may be used to reduce effective patient dose as compared with ¹²³ I-mIBG SPECT/CT.

I-131 Metaiodobenzylguanidine Therapy of Pheochromocytoma and Paraganglioma

Pheochromocytomas and paragangliomas are rare tumors arising from chromaffin cells. Available therapeutic modalities consist of chemotherapy, tyrosine kinase inhibitors, and I-131 metaiodobenzylguanidine (MIBG). I-131 MIBG is taken up via specific receptors and localizes into many but not all pheochromocytomas and paragangliomas. Because these tumors are rare, most therapy studies are retrospective presentations of clinical experience. Numerous retrospective studies and a few prospective studies have shown favorable responses in this disease, including symptomatic, biochemical, and objective responses. In this report, we review the experience of using I-131 MIBG therapy for targeting pheochromocytoma and paragangliomas.

Neuroblastoma: MIBG Imaging and New Tracers

Neuroblastoma is an embryonic tumor of the peripheral sympathetic nervous system, and is metastatic or otherwise high risk for relapse in nearly 50% of cases, with a long-term survival of <40%. Therefore, exact staging with radiological and nuclear medicine imaging methods is crucial for finding the adequate therapeutic choice. The tumor cells express the norepinephrine transporter, which makes metaiodobenzylguanidine (MIBG), an analogue of norepinephrine, an ideal tumor-specific agent for imaging. On the contrary, MIBG imaging has several disadvantages such as limited spatial resolution, limited sensitivity in small lesions, need for two or even more acquisition sessions, and a delay between the start of the examination and result. Most of these limitations can be overcome with positron emission tomography (PET) using different radiotracers. Furthermore, for operative or biopsy planning, a combination with morphological imaging methods is indispensable. This article would discuss the therapeutic strategy for primary and follow-up diagnosis in neuroblastoma using MIBG scintigraphy and different new PET tracers as well as multimodality imaging.

Radiosynthesis of no-carrier-added meta-[124I]iodobenzylguanidine for PET imaging of metastatic neuroblastoma

Meta-iodobenzylguanidine (mIBG) has been radiolabelled at the no-carrier-added level with [¹²⁴I] for a proof of concept study to assess the diagnostic accuracy of [¹²⁴I]mIBG PET/CT in detecting metastatic deposits in patients diagnosed with metastatic neuroblastoma. Radiolabelling of mIBG was achieved via the iododesilylation reaction between [¹²⁴I]sodium iodide and meta-trimethylsilylbenzylguanidine. [¹²⁴I]mIBG was produced in 62–70 % radioiodide incorporation yield from [¹²⁴I]sodium iodide. The average amount of formulated [¹²⁴I]mIBG was 359 MBq (range 344–389 MBq) with an average specific radioactivity of 4.1 TBq μmol⁻¹ (range 1.8–5.9 TBq μmol⁻¹) at end of synthesis.

An overview of (124)I production at a medical cyclotron by ALICE/ASH, EMPIRE-3.2.2 and TALYS-1.6 codes

Excitation functions of proton, deuteron and alpha induced nuclear reactions on enriched tellurium and antimony isotopes and also natural antimony were calculated by ALICE/ASH, EMPIRE-3.2.2 and TALYS-1.6 nuclear codes. Therefrom, the production yield of (124)I nuclide and its formed impurities were calculated by using the evaluation results. The calculated cross sections were compared with available experimental values in literatures. According to results, (124)Te(p,n)(124)I reaction is the method of choice due to formation of higher amount of (124)I nuclide and low levels of (125)I as an major concern in (124)I production.

Patient-specific dosimetry using pretherapy [¹²⁴I]m-iodobenzylguanidine ([¹²⁴I]mIBG) dynamic PET/CT imaging before [¹³¹I]mIBG targeted radionuclide therapy for neuroblastoma

PURPOSE

Iodine-131-m-iodobenzylguanidine ([(131)I]mIBG)-targeted radionuclide therapy (TRT) is a standard treatment for recurrent or refractory neuroblastoma with response rates of 30-40 %. The aim of this study is to demonstrate patient-specific dosimetry using quantitative [(124)I]mIBG positron emission tomography/X-ray computed tomography (PET/CT) imaging with a GEometry ANd Tracking 4 (Geant4)-based Monte Carlo method for better treatment planning.

PROCEDURES

A Monte Carlo dosimetry method was developed using the Geant4 toolkit with voxelized anatomical geometry and source distribution as input. The presegmented hybrid computational human phantoms developed by the University of Florida and the National Cancer Institute (UF/NCI) were used as a surrogate to characterize the anatomy of a given patient. S values for I-131 were estimated by the phantoms coupled with Geant4 and compared with those estimated by OLINDA|EXM and MCNPX for the newborn model. To obtain patient-specific biodistribution of [(131)I]mIBG, a 10-year-old girl with relapsed neuroblastoma was imaged with [(124)I]mIBG PET/CT at four time points prior to the planned [(131)I]mIBG TRT. The organ- and tumor-absorbed doses of the clinical case were estimated with the Geant4 method using the modified UF/NCI 10-year-old phantom with tumors and the patient-specific residence time.

RESULTS

For the newborn model, the Geant4 S values were consistent with the MCNPX S values. The S value ratio of the Geant4 method to OLINDA|EXM ranged from 0.08 to 6.5 of all major organs. The [(131)I]mIBG residence time quantified from the pretherapy [(124)I]mIBG PET/CT imaging of the 10-year-old patient was mostly comparable to those previously reported. Organ-absorbed dose for the salivary glands was 98.0 Gy, heart wall 36.5 Gy, and liver 34.3 Gy, while tumor-absorbed dose ranged from 143.9 to 1,641.3 Gy in different sites.

CONCLUSIONS

Patient-specific dosimetry for [(131)I]mIBG TRT was accomplished using pretherapy [(124)I]mIBG PET/CT imaging and a Geant4-based Monte Carlo dosimetry method. The Geant4 method with quantitative pretherapy imaging can provide dose estimates to normal organs and tumors with more realistic simulation geometry, and thus may improve treatment planning for [(131)I]mIBG TRT.

Preclinical evaluation of investigational radiopharmaceutical RISAD-P intended for use as a diagnostic and molecular radiotherapy agent for prostate cancer

BACKGROUND

The androgen receptor (AR) plays a dominant role in the pathogenesis of prostate cancer. 5-Radioiodo-3'-O-(17β-succinyl-5α-androstan-3-one)-2'-deoxyuridin-5'-yl phosphate (RISAD-P) is an AR-targeting reagent developed for noninvasive assessment of AR and proliferative status of the AR-expressing tumors, and for molecular radiotherapy with Auger electron-emitting radionuclides. In this study, the preclinical toxicity and targeting potential of RISAD-P was evaluated.

METHODS

Effects of nonradioactive ISAD-P and RISAD-P labeled with (123) I, (124) I, and (125) I were evaluated in male mice. Expanded-acute single dose toxicity studies, hematologic toxicity, liver and kidney function, pharmacokinetics, biodistribution, and imaging studies were conducted. Imaging and pilot therapy studies were conducted in transgenic mice.

RESULTS

RISAD-P is not toxic at doses projected for clinical use. Its tissue distribution compares favorably with the distribution reported for (18) F-dihydrotestosterone derivatives. RISAD-P has excellent prostate cancer targeting properties. One hour after (125) IRISAD-P administration, nearly 10% of the injected dose is associated with prostate tumor. The tumor clearance is biphasic and plateaus between 24 and 48 hr post-injection. The estimated radiation doses calculated for 1 g tumor using the MIRD convention are well within the therapeutic range with values of 170, 250, 1,240 Gy × MBq(-1)  × g(-1) for (125) I-, (123) I-, and (124) I-labeled RISAD-P, respectively. The transient uptake of radioactivity is observed in the genitourinary tract and stomach. Without the potassium iodide blockade, thyroid uptake is also observed.

CONCLUSIONS

Biodistribution, toxicity, and radiation dosimetry studies suggest that RISAD-P holds characteristics of a promising candidate for imaging of AR expression and tumor proliferation, as well as molecular radiotherapy for metastatic or locally, regionally advanced prostate cancer.

Case report of PET/CT imaging of a patient with neuroblastoma using 18F-FPBG

The use of an (18)F-labeled radiopharmaceutical for positron emission tomography (PET) imaging of sympathetic neuronal activity has a tremendous clinical impact because it circumvents the substantial drawbacks of the prototypical imaging agent, (123)I-metaiodobenzylguanidine (MIBG). We describe a 14-year-old male patient who had neuroblastoma and underwent (18)F-fluoropropylbenzylguanidine (FPBG) PET/computed tomography (CT). We compared the findings of (18)F-FPBG PET/CT with those of (123)I-MIBG γ scintigraphy. (18)F-FPBG PET/CT readily demonstrated a focal lesion in the left tibia, but the lesion was not seen using (123)I-MIBG scintigraphy. Surgical resection proved the presence of a neuroblastoma lesion in the left tibia. To the best of our knowledge, our study was the first to use an (18)F-labeled radiopharmaceutical for evaluating sympathetic neuronal activity in neuroblastoma. We suggest that (18)F-FPBG is a promising PET agent for imaging sympathetic neuronal activity in neuroblastoma.

124 Iodine: a longer-life positron emitter isotope-new opportunities in molecular imaging

(124)Iodine ((124)I) with its 4.2 d half-life is particularly attractive for in vivo detection and quantification of longer-term biological and physiological processes; the long half-life of (124)I is especially suited for prolonged time in vivo studies of high molecular weight compounds uptake. Numerous small molecules and larger compounds like proteins and antibodies have been successfully labeled with (124)I. Advances in radionuclide production allow the effective availability of sufficient quantities of (124)I on small biomedical cyclotrons for molecular imaging purposes. Radioiodination chemistry with (124)I relies on well-established radioiodine labeling methods, which consists mainly in nucleophilic and electrophilic substitution reactions. The physical characteristics of (124)I permit taking advantages of the higher PET image quality. The availability of new molecules that may be targeted with (124)I represents one of the more interesting reasons for the attention in nuclear medicine. We aim to discuss all iodine radioisotopes application focusing on (124)I, which seems to be the most promising for its half-life, radiation emissions, and stability, allowing several applications in oncological and nononcological fields.

Production of iodine-124 and its applications in nuclear medicine

Until recently, iodine-124 was not considered to be an attractive isotope for medical applications owing to its complex radioactive decay scheme, which includes several high-energy gamma rays. However, its unique chemical properties, and convenient half-life of 4.2 days indicated it would be only a matter of time for its frequent application to become a reality. The development of new medical imaging techniques, especially improvements in the technology of positron emission tomography (PET), such as the development of new detectors and signal processing electronics, has opened up new prospects for its application. With the increasing use of PET in medical oncology, pharmacokinetics, and drug metabolism, (124)I-labeled radiopharmaceuticals are now becoming one of the most useful tools for PET imaging, and owing to the convenient half-life of I-124, they can be used in PET scanners far away from the radionuclide production site. Thus far, the limited availability of this radionuclide has been an impediment to its wider application in clinical use. For example, sodium [(124)I]-iodide is potentially useful for diagnosis and dosimetry in thyroid disease and [(124)I]-M-iodobenzylguanidine ([(124)I]-MIBG) has enormous potential for use in cardiovascular imaging, diagnosis, and dosimetry of malignant diseases such as neuroblastoma, paraganglioma, pheochromocytoma, and carcinoids. However, despite that potential, both are still not widely used. This is a typical scenario of a rising new star among the new PET tracers.

Imaging the norepinephrine transporter in neuroblastoma: a comparison of [18F]-MFBG and 123I-MIBG

PURPOSE

The norepinephrine transporter (NET) is a critical regulator of catecholamine uptake in normal physiology and is expressed in neuroendocrine tumors like neuroblastoma. Although the norepinephrine analog, meta-iodobenzylguanidine (MIBG), is an established substrate for NET, (123)I/(131)I-MIBG has several clinical limitations for diagnostic imaging. In the current studies, we evaluated meta-[(18)F]-fluorobenzylguanidine ([(18)F]-MFBG) and compared it with (123)I-MIBG for imaging NET-expressing neuroblastomas.

EXPERIMENTAL DESIGN

NET expression levels in neuroblastoma cell lines were determined by Western blot and (123)I-MIBG uptake assays. Five neuroblastoma cell lines and two xenografts (SK-N-BE(2)C and LAN1) expressing different levels of NET were used for comparative in vitro and in vivo uptake studies.

RESULTS

The uptake of [(18)F]-MFBG in cells was specific and proportional to the expression level of NET. Although [(18)F]-MFBG had a 3-fold lower affinity for NET and an approximately 2-fold lower cell uptake in vitro compared with that of (123)I-MIBG, the in vivo imaging and tissue radioactivity concentration measurements demonstrated higher [(18)F]-MFBG xenograft uptake and tumor-to-normal organ ratios at 1 and 4 hours after injection. A comparison of 4 hours [(18)F]-MFBG PET (positron emission tomography) imaging with 24 hours (123)I-MIBG SPECT (single-photon emission computed tomography) imaging showed an approximately 3-fold higher tumor uptake of [(18)F]-MFBG, but slightly lower tumor-to-background ratios in mice.

CONCLUSIONS

[(18)F]-MFBG is a promising radiopharmaceutical for specifically imaging NET-expressing neuroblastomas, with fast pharmacokinetics and whole-body clearance. [(18)F]-MFBG PET imaging shows higher sensitivity, better detection of small lesions with low NET expression, allows same day scintigraphy with a shorter image acquisition time, and has the potential for lower patient radiation exposure compared with (131)I/(123)I-MIBG.

Synthesis and evaluation of 18F-labeled benzylguanidine analogs for targeting the human norepinephrine transporter

PURPOSE

Both (131)I- and (123)I-labeled meta-iodobenzylguanidine (MIBG) have been widely used in the clinic for targeted imaging of the norepinephrine transporter (NET). The human NET (hNET) gene has been imaged successfully with (124)I-MIBG positron emission tomography (PET) at time points of >24 h post-injection (p.i.). (18)F-labeled MIBG analogs may be ideal to image hNET expression at time points of <8 h p.i. We developed improved methods for the synthesis of known MIBG analogs, [(18)F]MFBG and [(18)F]PFBG and evaluated them in hNET reporter gene-transduced C6 rat glioma cells and xenografts.

METHODS

[(18)F]MFBG and [(18)F]PFBG were synthesized manually using a three-step synthetic scheme. Wild-type and hNET reporter gene-transduced C6 rat glioma cells and xenografts were used to comparatively evaluate the (18)F-labeled analogs with [(123)I]/[(124)I]MIBG.

RESULTS

The fluorination efficacy on benzonitrile was predominantly determined by the position of the trimethylammonium group. The para-isomer afforded higher yields (75 ± 7%) than meta-isomer (21 ± 5%). The reaction of [(18)F]fluorobenzylamine with 1H-pyrazole-1-carboximidamide was more efficient than with 2-methyl-2-thiopseudourea. The overall radiochemical yields (decay-corrected) were 11 ± 2% (n = 12) for [(18)F]MFBG and 41 ± 12% (n = 5) for [(18)F]PFBG, respectively. The specific uptakes of [(18)F]MFBG and [(18)F]PFBG were similar in C6-hNET cells, but 4-fold less than that of [(123)I]/[(124)I]MIBG. However, in vivo [(18)F]MFBG accumulation in C6-hNET tumors was 1.6-fold higher than that of [(18)F]PFBG at 1 h p.i., whereas their uptakes were similar at 4 h. Despite [(18)F]MFBG having a 2.8-fold lower affinity to hNET and approximately 4-fold lower cell uptake in vitro compared to [(123)I]/[(124)I]MIBG, PET imaging demonstrated that [(18)F]MFBG was able to visualize C6-hNET xenografts better than [(124)I]MIBG. Biodistribution studies showed [(18)F]MFBG and (123)I-MIBG had a similar tumor accumulation, which was lower than that of no-carrier-added [(124)I]MIBG, but [(18)F]MFBG showed a significantly more rapid body clearance and lower uptake in most non-targeting organs.

CONCLUSION

[(18)F]MFBG and [(18)F]PFBG were synthesized in reasonable radiochemical yields under milder conditions. [(18)F]MFBG is a better PET ligand to image hNET expression in vivo at 1-4 h p.i. than both [(18)F]PFBG and [(123)I]/[(124)I]MIBG.

Other PET tracers for neuroendocrine tumors

In this article the applicability of (124)I-MIBG and (11)C-5-HTP PET for the detection of abdominal gastro-enteropancreatic neuroendocrine tumors is discussed. (124)I-MIBG is a positron-emitting variant of (123)I-MIBG and therefore suited for PET imaging. Due to the better intrinsic characteristics of PET, (124)I-MIBG PET has a higher spatial resolution than (123)I-MIBG and may therefore lead to better lesion detection in patients with neuroblastoma and pheochromocytoma. Research is ongoing to develop (18)F-labeled analogues and to assess its place in staging patients with gastro-enteropancreatic neuroendocrine tumors when compared with (18)F-FDOPA and (68)Ga-somatostatin PET analogues.

Tumor dosimetry using [124I]m-iodobenzylguanidine microPET/CT for [131I]m-iodobenzylguanidine treatment of neuroblastoma in a murine xenograft model

PURPOSE

[(124)I]m-iodobenzylguanidine ((124)I-mIBG) provides a quantitative tool for pretherapy tumor imaging and dosimetry when performed before [(131)I]m-iodobenzylguanidine ((131)I-mIBG) targeted radionuclide therapy of neuroblastoma. (124)I (T (1/2) = 4.2 days) has a comparable half-life to that of (131)I (T (1/2) = 8.02 days) and can be imaged by positron emission tomography (PET) for accurate quantification of the radiotracer distribution. We estimated expected radiation dose in tumors from (131)I-mIBG therapy using (124)I-mIBG microPET/CT imaging data in a murine xenograft model of neuroblastoma transduced to express high levels of the human norepinephrine transporter (hNET).

PROCEDURES

In order to enhance mIBG uptake for in vivo imaging and therapy, NB 1691-luciferase (NB1691) human neuroblastoma cells were engineered to express high levels of hNET protein by lentiviral transduction (NB1691-hNET). Both NB1691 and NB1691-hNET cells were implanted subcutaneously and into renal capsules in athymic mice. (124)I-mIBG (4.2-6.5 MBq) was administered intravenously for microPET/CT imaging at 5 time points over 95 h (0.5, 3-5, 24, 48, and 93-95 h median time points). In vivo biodistribution data in normal organs, tumors, and whole-body were collected from reconstructed PET images corrected for photon attenuation using the CT-based attenuation map. Organ and tumor dosimetry were determined for (124)I-mIBG. Dose estimates for (131)I-mIBG were made, assuming the same in vivo biodistribution as (124)I-mIBG.

RESULTS

All NB1691-hNET tumors had significant uptake and retention of (124)I-mIBG, whereas unmodified NB1691 tumors did not demonstrate quantifiable mIBG uptake in vivo, despite in vitro uptake. (124)I-mIBG with microPET/CT provided an accurate three-dimensional tool for estimating the radiation dose that would be delivered with (131)I-mIBG therapy. For example, in our model system, we estimated that the administration of (131)I-mIBG in the range of 52.8-206 MBq would deliver 20 Gy to tumors.

CONCLUSIONS

The overexpression of hNET was found to be critical for (124)I-mIBG uptake and retention in vivo. The quantitative (124)I-mIBG PET/CT is a promising new tool to predict tumor radiation doses with (131)I-mIBG therapy of neuroblastoma. This methodology may be applied to tumor dosimetry of (131)I-mIBG therapy in human subjects using (124)I-mIBG pretherapy PET/CT data.

Pharmacodynamics, tissue distribution, toxicity studies and antitumor efficacy of the vascular targeting fusion toxin VEGF121/rGel

As a part of an ongoing assessment of its mechanism of action, we evaluated the in vivo pharmacokinetics, tissue distribution, toxicity and antitumor efficacy of VEGF(121)/rGel, a novel fusion protein. Pharmacokinetic studies showed that VEGF(121)/rGel cleared from the circulation in a biphasic manner with calculated half-lives of 0.3 and 6h for the alpha and beta phases, respectively. Pharmacokinetic evaluation of (64)Cu-DOTA-VEGF(121)/rGel showed relatively high blood retention 30 min after injection (26.6 ± 1.73% ID/g), dropping to 11.8 ± 2.83% and 0.82 ± 0.11% ID/g at 60 and 240 min post injection, respectively. Tissue uptake studies showed that kidneys, liver and tumor had the highest drug concentrations 48 h after administration. The maximum tolerated dose (MTD), based on a QOD×5 i.v. administration schedule, was found to be 18 mg/kg with an LD(50) of 25mg/kg. Treatment of BALB/c mice with VEGF(121)/rGel at doses up to the MTD caused no alterations in hematologic parameters. However, aspartate aminotransferase (AST) and alanine aminotransferase (ALT) parameters increased in a dose-related manner. The no-observable-adverse-effect-level (NOAEL) was determined to be 20% of the MTD (3.6 mg/kg). VEGF(121)/rGel treatment of mice bearing orthotopically-placed MDA-MB-231 breast tumors caused increased vascular permeability of tumor tissue by 53% compared to saline-treated controls. Immunohistochemical analysis showed significant tumor hypoxia and necrosis as a consequence of vascular damage. In summary, VEGF(121)/rGel appears to be an effective therapeutic agent causing focused damage to tumor vasculature with minimal toxic effects to normal organs. This agent appears to be an excellent candidate for further clinical development.

Dose escalation study of no-carrier-added 131I-metaiodobenzylguanidine for relapsed or refractory neuroblastoma: new approaches to neuroblastoma therapy consortium trial

(131)I-metaiodobenzylguanidine (MIBG) is specifically taken up in neuroblastoma, with a response rate of 20%-37% in relapsed disease. Nonradioactive carrier MIBG molecules inhibit uptake of (131)I-MIBG, theoretically resulting in less tumor radiation and increased risk of cardiovascular toxicity. Our aim was to establish the maximum tolerated dose of no-carrier-added (NCA) (131)I-MIBG, with secondary aims of assessing tumor and organ dosimetry and overall response.

METHODS

Eligible patients were 1-30 y old with resistant neuroblastoma, (131)I-MIBG uptake, and cryopreserved hematopoietic stem cells. A diagnostic dose of NCA (131)I-MIBG was followed by 3 dosimetry scans to assess radiation dose to critical organs and soft-tissue tumors. The treatment dose of NCA (131)I-MIBG (specific activity, 165 MBq/μg) was adjusted as necessary on the basis of critical organ tolerance limits. Autologous hematopoietic stem cells were infused 14 d after therapy to abrogate prolonged myelosuppression. Response and toxicity were evaluated on day 60. The NCA (131)I-MIBG was escalated from 444 to 777 MBq/kg (12-21 mCi/kg) using a 3 + 3 design. Dose-limiting toxicity (DLT) was failure to reconstitute neutrophils to greater than 500/μL within 28 d or platelets to greater than 20,000/μL within 56 d, or grade 3 or 4 nonhematologic toxicity by Common Terminology Criteria for Adverse Events (version 3.0) except for predefined exclusions.

RESULTS

Three patients each were evaluable at 444, 555, and 666 MBq/kg without DLT. The dose of 777 MBq/kg dose was not feasible because of organ dosimetry limits; however, 3 assigned patients were evaluable for a received dose of 666 MBq/kg, providing a total of 6 patients evaluable for toxicity at 666 MBq/kg without DLT. Mean whole-body radiation was 0.23 mGy/MBq, and mean organ doses were 0.92, 0.82, and 1.2 mGy/MBq of MIBG for the liver, lung, and kidney, respectively. Eight patients had 13 soft-tissue lesions with tumor-absorbed doses of 26-378 Gy. Four of 15 patients had a complete (n = 1) or partial (n = 3) response, 1 had a mixed response, 4 had stable disease, and 6 had progressive disease.

CONCLUSION

NCA (131)I-MIBG with autologous peripheral blood stem cell transplantation is feasible at 666 MBq/kg without significant nonhematologic toxicity and with promising activity.

Promising therapeutic targets in neuroblastoma

Neuroblastoma, the most common extracranial solid tumor in children, is derived from neural crest cells. Nearly half of patients present with metastatic disease and have a 5-year event-free survival of <50%. New approaches with targeted therapy may improve efficacy without increased toxicity. In this review we evaluate 3 promising targeted therapies: (i) (131)I-metaiodobenzylguanidine (MIBG), a radiopharmaceutical that is taken up by human norepinephrine transporter (hNET), which is expressed in 90% of neuroblastomas; (ii) immunotherapy with monoclonal antibodies targeting the GD2 ganglioside, which is expressed on 98% of neuroblastoma cells; and (iii) inhibitors of anaplastic lymphoma kinase (ALK), a tyrosine kinase that is mutated or amplified in ~10% of neuroblastomas and expressed on the surface of most neuroblastoma cells. Early-phase trials have confirmed the activity of (131)I-MIBG in relapsed neuroblastoma, with response rates of ~30%, but the technical aspects of administering large amounts of radioactivity in young children and limited access to this agent have hindered its incorporation into treatment of newly diagnosed patients. Anti-GD2 antibodies have also shown activity in relapsed disease, and a recent phase III randomized trial showed a significant improvement in event-free survival for patients receiving chimeric anti-GD2 (ch14.18) combined with cytokines and isotretinoin after myeloablative consolidation therapy. A recently approved small-molecule inhibitor of ALK has shown promising preclinical activity for neuroblastoma and is currently in phase I and II trials. This is the first agent directed to a specific mutation in neuroblastoma, and marks a new step toward personalized therapy for neuroblastoma. Further clinical development of targeted treatments offers new hope for children with neuroblastoma.

Impact of expression system on the function of the C6.5 diabody PET radiotracer

The ability of engineered antibodies to rapidly and selectively target tumors that express their target antigen makes them well suited for use as radioimaging tracers. The combination of molecular size and bivalent nature makes diabody molecules a particularly promising structure for use as radiotracers for diagnostic imaging. Previous data have demonstrated that the anti-HER2 C6.5 diabody (C6.5db) is an effective radiotracer in preclinical models of HER2-positive cancer. The aim of this study was to evaluate the impact on radiotracer performance, associated with expressing the C6.5db in the Pichia pastoris (P-C6.5db) system as compared to Escherichia coli (E. C6.5db). Glycosylation of P-C6.5db led to faster blood clearance and lower overall tumor uptake than seen with E. coli-produced C6.5db. However, P-C6.5db achieved high tumor/background ratios that are critical for effective imaging. Dosimetry measurements determined in this study for both (124)I-P-C6.5db and (124)I-E-C6.5db suggest that they are equivalent to other radiotracers currently being administered to patients.

Pheochromocytoma and paraganglioma: current functional and future molecular imaging

Paragangliomas are neural crest-derived tumors, arising either from chromaffin sympathetic tissue (in adrenal, abdominal, intra-pelvic, or thoracic paraganglia) or from parasympathetic tissue (in head and neck paraganglia). They have a specific cellular metabolism, with the ability to synthesize, store, and secrete catecholamines (although most head and neck paragangliomas do not secrete any catecholamines). This disease is rare and also very heterogeneous, with various presentations (e.g., in regards to localization, multifocality, potential to metastasize, biochemical phenotype, and genetic background). With growing knowledge, notably about the pathophysiology and genetic background, guidelines are evolving rapidly. In this context, functional imaging is a challenge for the management of paragangliomas. Nuclear imaging has been used for exploring paragangliomas for the last three decades, with MIBG historically as the first-line exam. Tracers used in paragangliomas can be grouped in three different categories. Agents that specifically target catecholamine synthesis, storage, and secretion pathways include: 123 and 131I-metaiodobenzylguanidine (123/131I-MIBG), 18F-fluorodopamine (18F-FDA), and 18F-fluorodihydroxyphenylalanine (18F-FDOPA). Agents that bind somatostatin receptors include 111In-pentetreotide and 68Ga-labeled somatostatin analog peptides (68Ga-DOTA-TOC, 68Ga-DOTA-NOC, 68Ga-DOTA-TATE). The non-specific agent most commonly used in paragangliomas is 18F-fluorodeoxyglucose (18F-FDG). This review will first describe conventional scintigraphic exams that are used for imaging paragangliomas. In the second part we will emphasize the interest in new PET approaches (specific and non-specific), considering the growing knowledge about genetic background and pathophysiology, with the aim of understanding how tumors behave, and optimally adjusting imaging technique for each tumor type.

Matched pairs dosimetry: 124I/131I metaiodobenzylguanidine and 124I/131I and 86Y/90Y antibodies

The technological advances in imaging and production of radiopharmaceuticals are driving an innovative way of evaluating the targets for antineoplastic therapies. Besides the use of imaging to better delineate the volume of external beam radiation therapy in oncology, modern imaging techniques are able to identify targets for highly specific medical therapies, using chemotherapeutic drugs and antiangiogenesis molecules. Moreover, radionuclide imaging is able to select targets for radionuclide therapy and to give the way to in vivo dose calculation to target tissues and to critical organs. This contribution reports the main studies published on matched pairs dosimetry with (124)I/(131)I- and (86)Y/(90)Y-labelled radiopharmaceuticals, with an emphasis on metaiodobenzylguanidine (MIBG) and monoclonal antibodies.