Extragranular storage of the neuron blocking agent meta-iodobenzylguanidine (MIBG) in human neuroblastoma cells

The Plasma Membrane Monoamine Transporter is Highly Expressed in Neuroblastoma and Functions as an mIBG Transporter

Neuroblastoma (NB) is a pediatric cancer with low survival rates in high-risk patients. 131I-mIBG has emerged as a promising therapy for high-risk NB and kills tumor cells by radiation. Consequently, 131I-mIBG tumor uptake and retention are major determinants for its therapeutic efficacy. mIBG enters NB cells through the norepinephrine transporter (NET), and accumulates in mitochondria through unknown mechanisms. Here we evaluated the expression of monoamine and organic cation transporters in high-risk NB tumors and explored their relationship with MYCN amplification and patient survival. We found that NB mainly expresses NET, the plasma membrane monoamine transporter (PMAT), and the vesicular membrane monoamine transporter 1/2 (VMAT1/2), and that the expression of these transporters is significantly reduced in MYCN-amplified tumor samples. PMAT expression is the highest and correlates with overall survival in high-risk NB patients without MYCN amplification. Immunostaining showed that PMAT resides intracellularly in NB cells and co-localizes with mitochondria. Using cells expressing PMAT, mIBG was identified as a PMAT substrate. In mitochondria isolated from NB cell lines, mIBG uptake was reduced by ∼50% by a PMAT inhibitor. Together, our data suggest that PMAT is a previously unrecognized transporter highly expressed in NB and could impact intracellular transport and therapeutic response to 131I-mIBG. SIGNIFICANCE STATEMENT: This study identified that plasma membrane monoamine transporter (PMAT) is a novel transporter highly expressed in neuroblastoma and its expression level is associated with overall survival rate in high-risk patients without MYCN amplification. PMAT is expressed intracellularly in neuroblastoma cells, transports meta-iodobenzylguanidine (mIBG) and thus could impact tumor retention and response to 131I-mIBG therapy. These findings have important clinical implications as PMAT could represent a novel molecular marker to help inform disease prognosis and predict response to 131I-mIBG therapy.

68Ga-DOTATATE PET/CT for Neuroblastoma Staging: Utility for Clinical Use

Metaiodobenzylguanidine (MIBG) imaging has been the standard for neuroblastoma staging for many decades. Novel agents such as ¹⁸F-DOPA and ⁶⁸Ga-DOTATATE are being used nowadays in academic centers. During the coronavirus disease 2019 (COVID-19) pandemic, procurement of ¹²³I-MIBG has proved particularly challenging, necessitating the use of ⁶⁸Ga-DOTATATE PET. ⁶⁸Ga-DOTATATE is Food and Drug Administration-approved for imaging of somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumors. Methods: ⁶⁸Ga-DOTATATE PET/CT imaging was performed for staging of 3 pediatric patients with neuroblastoma at our institution. A review of the literature was also completed. Results: ⁶⁸Ga-DOTATATE PET/CT scans were successfully performed on all patients. All patients showed ⁶⁸Ga-DOTATATE-avid disease. PET scans showed an excellent spatial resolution and demonstrated high accuracy in concordance with current European Association of Nuclear Medicine guidelines. Conclusion: We have presented ⁶⁸Ga-DOTATATE PET/CT imaging for staging of neuroblastoma and believe it can reliably be used as an alternative to ¹²³I-MIBG. It has technical, clinical, and practical advantages making it an attractive option. Further multicenter studies are required before it can be recommended for standard clinical use.

The Current Status and Future Potential of Theranostics to Diagnose and Treat Childhood Cancer

In theranostics (i.e., therapy and diagnostics) radiopharmaceuticals are used for both therapeutic and diagnostic purposes by targeting one specific tumor receptor. Biologically relevant compounds, e.g., receptor ligands or drugs, are labeled with radionuclides to form radiopharmaceuticals. The possible applications are multifold: visualization of biological processes or tumor biology in vivo, diagnosis and tumor staging, therapy planning, and treatment of specific tumors. Theranostics research is multidisciplinary and allows for the rapid translation of potential tumor targets from preclinical research to “first-in-man” clinical studies. In the last decade, the use of theranostics has seen an unprecedented value for adult cancer patients. Several radiopharmaceuticals are routinely used in clinical practice (e.g., [⁶⁸Ga/¹⁷⁷Lu]DOTATATE), and dozens are under (pre)clinical development. In contrast to these successes in adult oncology, theranostics have scarcely been developed to diagnose and treat pediatric cancers. To date, [^123/131I]meta-iodobenzylguanidine ([^123/131I]mIBG) is the only available and approved theranostic in pediatric oncology. mIBG targets the norepinephrine transporter, expressed by neuroblastoma tumors. For most pediatric tumors, including neuroblastoma, there is a clear need for novel and improved radiopharmaceuticals for imaging and therapy. The strategy of theranostics for pediatric oncology can be divided in (1) the improvement of existing theranostics, (2) the translation of theranostics developed in adult oncology for pediatric purposes, and (3) the development of novel theranostics for pediatric tumor-specific targets. Here, we describe the recent advances in theranostics development in pediatric oncology and shed a light on how this methodology can affect diagnosis and provide additional treatment options for these patients.

Molecular imaging of norepinephrine transporter-expressing tumors: current status and future prospects

The human norepinephrine transporter (hNET) is a transmembrane protein responsible for reuptake of norepinephrine in presynaptic sympathetic nerve terminals and adrenal chromaffin cells. Neural crest tumors, such as neuroblastoma, paraganglioma and pheochromocytoma often show high hNET expression. Molecular imaging of these tumors can be done using radiolabeled norepinephrine analogs that target hNET. Currently, the most commonly used radiopharmaceutical for hNET imaging is meta-[123I]iodobenzylguanidine ([123I]MIBG) and this has been the case since its development several decades ago. The γ-emitter, iodine-123 only allows for planar scintigraphy and single photon emission computed tomography imaging. These modalities typically have a poorer spatial resolution and lower sensitivity than positron emission tomography (PET). Additional practical disadvantages include the fact that a two-day imaging protocol is required and the need for thyroid blockade. Therefore, several PET alternatives for hNET imaging are actively being explored. This review gives an in-depth overview of the current status and recent developments in clinical trials leading to the next generation of clinical PET ligands for imaging of hNET-expressing tumors.

Subcellular storage and release mode of the novel 18F-labeled sympathetic nerve PET tracer LMI1195

BACKGROUND

¹⁸F-N-[3-bromo-4-(3-fluoro-propoxy)-benzyl]-guanidine (¹⁸F-LMI1195) is a new class of PET tracer designed for sympathetic nervous imaging of the heart. The favorable image quality with high and specific neural uptake has been previously demonstrated in animals and humans, but intracellular behavior is not yet fully understood. The aim of the present study is to verify whether it is taken up in storage vesicles and released in company with vesicle turnover.

RESULTS

Both vesicle-rich (PC12) and vesicle-poor (SK-N-SH) norepinephrine-expressing cell lines were used for in vitro tracer uptake studies. After 2 h of ¹⁸F-LMI1195 preloading into both cell lines, effects of stimulants for storage vesicle turnover (high concentration KCl (100 mM) or reserpine treatment) were measured at 10, 20, and 30 min. ¹³¹I-meta-iodobenzylguanidine (¹³¹I-MIBG) served as a reference. Both high concentration KCl and reserpine enhanced ¹⁸F-LMI1195 washout from PC12 cells, while tracer retention remained stable in the SK-N-SH cells. After 30 min of treatment, ¹⁸F-LMI1195 releasing index (percentage of tracer released from cells) from vesicle-rich PC12 cells achieved significant differences compared to cells without treatment condition. In contrast, such effect could not be observed using vesicle-poor SK-N-SH cell lines. Similar tracer kinetics after KCl or reserpine treatment were also observed using ¹³¹I-MIBG. In case of KCl exposure, Ca²+-free buffer with the calcium chelator, ethylenediaminetetracetic acid (EDTA), could suppress the tracer washout from PC12 cells. This finding is consistent with the tracer release being mediated by Ca²+ influx resulting from membrane depolarization.

CONCLUSIONS

Analogous to ¹³¹I-MIBG, the current in vitro tracer uptake study confirmed that ¹⁸F-LMI1195 is also stored in vesicles in PC12 cells and released along with vesicle turnover. Understanding the basic kinetics of ¹⁸F-LMI1195 at a subcellular level is important for the design of clinical imaging protocols and imaging interpretation.

Impact of medications on mIBG uptake, with specific attention to the heart: Comprehensive review of the literature

BACKGROUND

A critical review of the literature on drug interactions with mIBG uptake was performed to allow formulation of contemporary guidance regarding withholding medications prior to clinical imaging studies.

METHODS

Published information was extracted on the experimental system used, the quantitative characteristics of the measurements, and whether any data directly examining cardiac tissues were included. Level of evidence for each medication category was assessed on a qualitative scale of very low, low, medium, or high. Strength of medication effect for inhibition of mIBG uptake was judged as none, weak, moderate, or strong.

RESULTS

The only medications for which level of evidence was judged high were labetalol and reserpine. Level of evidence was judged medium for tricyclic antidepressants, calcium channel blockers, and antiarrhythmics (specifically amiodarone). Evidence was judged sufficient to recommend withholding labetalol and the tricyclic antidepressants prior to mIBG cardiac imaging. Mechanistic evidence was sufficient to suggest consideration of withdrawal of sympathomimetic amines and serotonin-norepinephrine reuptake inhibitors (SNRIs).

CONCLUSIONS

As there is strong evidence for inhibition of mIBG uptake in only a small number of compounds, clinical decisions regarding withdrawal of concomitant medications should be individualized by considering the potential consequences of a false-positive (artificially low cardiac uptake) imaging result.

Nothing but NET: a review of norepinephrine transporter expression and efficacy of 131I-mIBG therapy

Neuroblastoma is unique amongst common pediatric cancers for its expression of the norepinephrine transporter (NET), enabling tumor-selective imaging and therapy with radioactive analogues of norepinephrine. The majority of neuroblastoma tumors are avid for (123)I-metaiodobenzaguanidine (mIBG) on imaging, yet the therapeutic response to (131) I-mIBG is only 30% in clinical trials, and off-target effects cause short- and long-term morbidity. We review the contemporary understanding of the tumor-selective uptake, retention, and efflux of meta-iodobenzylguanidine (mIBG) and strategies currently in development for improving its efficacy. Combination treatment strategies aimed at enhancing NET are likely necessary to reach the full potential of (131)I-mIBG therapy.

Inhibition of ADP-ribosylation suppresses aberrant accumulation of lipidated apolipoprotein B in the endoplasmic reticulum

ApoB-crescent, an endoplasmic reticulum (ER)-lipid droplet amalgamation structure, is a useful marker to indicate aberrant lipidated apolipoprotein B accumulation in the hepatocyte ER. Blockade of the ER-to-Golgi transport by either vesicle transport inhibitors or dominant-negative Arf1 caused a significant increase in ApoB-crescents. However, a low concentration of Brefeldin A induced the same result without affecting protein secretion, suggesting ADP-ribosylation as an additional mechanism. ADP-ribosylation inhibitors not only suppressed the increase of ApoB-crescents, but also rapidly dissolved existing ApoB-crescents. These results implicate the involvement of ADP-ribosylation in the ApoB-crescent formation and maintenance process at the ER.

Pediatrics: diagnosis of neuroblastoma

Neuroblastoma is the most common pediatric extracranial soft-tissue tumor, accounting for approximately 8% of childhood malignancies. Its prognosis is widely variable, ranging from spontaneous regression to fatal disease despite multimodality therapy. Multiple imaging and clinical tests are needed to accurately assess patient risk with risk groups based on disease stage, patient age, and biological tumor factors. Approximately 60% of patients with neuroblastoma have metastatic disease, most commonly involving bone marrow or cortical bone. Metaiodobenzylguanidine (mIBG) scintigraphy plays an important role in the assessment of neuroblastoma, allowing whole-body disease assessment. mIBG is used to define extent of disease at diagnosis, assess disease response during therapy, and detect residual and recurrent disease during follow-up. mIBG is highly sensitive and specific for neuroblastoma, concentrating in >90% of tumors. mIBG was initially labeled with (131)I, but (123)I-mIBG yields higher quality images at a lower patient radiation dose. (123)I-mIBG (AdreView; GE Healthcare, Arlington Heights, IL) was approved for clinical use in children by the Food and Drug Administration in 2008 and is now commercially available throughout the United States. The use of single-photon emission computed tomography and single-photon emission computed tomography/computed tomography in (123)I-mIBG imaging has improved certainty of lesion detection and localization. Fluorodeoxyglucose positron-emission tomography has recently been compared with mIBG and found to be most useful in neuroblastomas which fail to or weakly accumulate mIBG.

Reserpine-induced reduction in norepinephrine transporter function requires catecholamine storage vesicles

Treatment of rats with reserpine, an inhibitor of the vesicular monoamine transporter (VMAT), depletes norepinephrine (NE) and regulates NE transporter (NET) expression. The present study examined the molecular mechanisms involved in regulation of the NET by reserpine using cultured cells. Exposure of rat PC12 cells to reserpine for a period as short as 5min decreased [(3)H]NE uptake capacity, an effect characterized by a robust decrease in the V(max) of the transport of [(3)H]NE. As expected, reserpine did not displace the binding of [(3)H]nisoxetine from the NET in membrane homogenates. The potency of reserpine for reducing [(3)H]NE uptake was dramatically lower in SK-N-SH cells that have reduced storage capacity for catecholamines. Reserpine had no effect on [(3)H]NE uptake in HEK-293 cells transfected with the rat NET (293-hNET), cells that lack catecholamine storage vesicles. NET regulation by reserpine was independent of trafficking of the NET from the cell surface. Pre-exposure of cells to inhibitors of several intracellular signaling cascades known to regulate the NET, including Ca(2+)/Ca(2+)-calmodulin dependent kinase and protein kinases A, C and G, did not affect the ability of reserpine to reduce [(3)H]NE uptake. Treatment of PC12 cells with the catecholamine depleting agent, alpha-methyl-p-tyrosine, increased [(3)H]NE uptake and eliminated the inhibitory effects of reserpine on [(3)H]NE uptake. Reserpine non-competitively inhibits NET activity through a Ca(2+)-independent process that requires catecholamine storage vesicles, revealing a novel pharmacological method to modify NET function. Further characterization of the molecular nature of reserpine's action could lead to the development of alternative therapeutic strategies for treating disorders known to be benefitted by treatment with traditional competitive NET inhibitors.

A radioiodinated MIBG-octreotate conjugate exhibiting enhanced uptake and retention in SSTR2-expressing tumor cells

Several neuroendocrine tumors are known to express both the somatostatin receptor subtype 2 (SSTR2) and the norepinephrine transporter (NET), and radiopharmaceuticals directed toward both these targets such as MIBG and octreotide derivatives are routinely used in the clinic. To investigate the possibility of targeting both NET and SSTR2 conjointly, a conjugate of radioiodinated MIBG and octreotate was synthesized. Attempts to synthesize the radioiodinated target compound (MIBG-octreotate; [ (131)I] 12a) from a tin precursor were futile; however, it could be accomplished from a bromo precursor by exchange radioiodination in 3-36% ( n = 10) radiochemical yields. The total uptake of [ (131)I] 12a in SK-N-SH human neuroblastoma cells transfected to express SSTR2 (SK-N-SHsst2) was similar to that for [ (125)I]MIBG at all time points (34.9 +/- 2.4% vs 43.8 +/- 1.2% at 4 h; p < 0.05), while it was substantially lower (5.4 +/- 0.3% vs 35.9 +/- 1.2%) in the SH-SY5Y cell line, a subclone of SK-N-SH line that is known to express SSTR2. The NET blocker desipramine reduced the uptake of [ (131)I] 12a only to a small extent, further suggesting a limited role of NET in its binding and accumulation. Uptake of [ (131)I] 12a in SK-N-SHsst2 cells was 8-10-fold higher ( p < 0.05) than that of [ (125)I]I-Gluc-TOCA, an octreotide analogue, at all time points over a 4 h period and was reduced to about 20% by 10 muM octreotide demonstrating that the uptake of [ (131)I] 12a in this cell line is predominantly mediated by SSTR2. The intracellularly trapped radioactivity in SK-N-SHsst2 cells was substantially higher for [ (131)I] 12a compared to that for [ (125)I]OIBG-octreotate, an isomeric congener of 12a. Because MIBG has more specific NET-mediated uptake than OIBG, this suggests at least a partial role for NET-mediated uptake of [ (131)I] 12a in this cell line. While further refinement in the structure of the conjugate-probably interposition of a flexible and/or cleavable linker between the MIBG and octreotate moieties-may be necessary to make it a substrate/ligand for both NET and SSTR2, this conjugate is demonstrated to be much superior than I-Gluc-TOCA with respect to the uptake in SSTR2-expressing cells.

Neuroblastoma and other neuroendocrine tumors

Neuroblastoma is the most common extracranial solid tumor of childhood. It commonly presents in children younger than 2 years of age, with 90% being younger than 5 years of age. There is marked variability in clinical behavior ranging from spontaneous regression or differentiation into benign tumors to rapid and progressive fatal disease. Approximately 50% of patients will have metastases at presentation. The management is dependent on age, stage of disease, and biological and biochemical markers. Nuclear medicine plays an important role in the initial staging, as a prognostic indicator, for assessment of response to treatment, and also in therapy. The most common nuclear medicine diagnostic studies are (99m)Tc-disphosphonate bone scintigraphy and (123)I-MIBG (metaiodobenzylguanidine) scintigraphy. Bone scintigraphy has been the main investigational modality to diagnose skeletal metastases. Whole body imaging with (123)I-MIBG has become the preferred diagnostic test because this agent accumulates in neuroblastoma in 90% to 95% of cases and will accumulate in the primary tumor and metastases particularly in bone, bone marrow, lymph nodes, and soft tissues. MIBG can be used to assess therapy response and is a significant prognostic indicator. Other diagnostic techniques include positron emission tomography (PET)/computed tomography, mainly using (18)F-fluorodeoxyglucose. Other more experimental PET agents, as well as radiolabeled antibodies and octreotide, also are being investigated. Therapy has mainly focused on palliation and has been used alone or in combination with chemotherapy in high-risk refractory or relapsed patients. Major attention is being placed on stratification of patients to try and reduce the side effects associated with intensive megatherapy in the low to intermediate risk patients. Neuroendocrine tumors (NETs) are rare in childhood, but nuclear medicine techniques, mainly using MIBG and somatostatin receptor agents, have a role in diagnosis, staging, and a limited role in therapy. Newer radiopharmaceuticals, including PET agents, are being evaluated for the assessment of NET. Nuclear medicine techniques play a major role in the management of neuroblastoma and NET.

Long-term radioiodine retention and regression of liver cancer after sodium iodide symporter gene transfer in wistar rats

Radioiodine therapy of nonthyroid cancers after sodium iodide symporter (NIS) gene delivery has been proposed as a potential application of gene therapy. However, it seems to be precluded by the rapid efflux of taken up iodine from most transduced xenografted tumors. We present an in vivo kinetic study of NIS-related hepatic iodine uptake in an aggressive model of hepatocarcinoma induced by diethylnitrosamine in immunocompetent Wistar rats. We followed the whole-body iodine distribution by repeated imaging of live animals. We constructed a rat NIS (rNIS) adenoviral vector, Ad-CMV-rNIS, using the cytomegalovirus (CMV) as a promoter. Injected in the portal vein in 5 healthy and 25 hepatocarcinoma-bearing rats and liver tumors in 9 hepatocarcinoma-bearing rats, Ad-CMV-rNIS drove expression of a functional NIS protein by hepatocytes and allowed marked (from 20 to 30% of the injected dose) and sustained (>11 days) iodine uptake. This contrasts with the massive iodine efflux found in vitro in human hepatic tumor cell lines. In vivo specific inhibition of NIS by sodium perchlorate led to a rapid iodine efflux from the liver, indicating that the sustained uptake was not attributable to an active retention mechanism but to permanent recycling of the effluent radioiodine via the high hepatic blood flow. Radioiodine therapy after Ad-CMV-rNIS administration achieved a strong inhibition of tumor growth, the complete regression of small nodules, and prolonged survival of hepatocarcinoma-bearing rats. This demonstrates for the first time the efficacy of NIS-based radiotherapy in a relevant preclinical model of nonthyroid human carcinogenesis.

Catabolism of 4-fluoro-3-iodobenzylguanidine and meta-iodobenzylguanidine by SK-N-SH neuroblastoma cells

BACKGROUND

A fluorine substituted derivative of meta-iodobenzylguanidine (MIBG), 4-fluoro-3-iodobenzylguanidine (FIBG), is retained in SK-N-SH human neuroblastoma cells in vitro to a higher degree than the MIBG.

METHOD

To investigate whether the higher retention of FIBG is due to differences in the catabolic degradation of the two tracers, in vitro paired-label studies were performed using SK-N-SH cells.

RESULTS

No detectable amount of benzyl amines, benzoic acids or hippuran derivatives, potential catabolites of these tracers, were seen in either case. Even after 48 h, the cell culture supernatants contained exclusively intact I-MIBG and I-FIBG. In contrast, in some cases, HPLC analysis of cell lysates indicated the presence of a very polar compound(s) as the predominant species with smaller quantities of intact tracers. The per cent total radioactivity in the lysate at each time point that was associated with intact I-FIBG was (average [range]) 25.4% [20.3-30.5], 22.5% [19.3-25.6], and 18.8% [14.3-23.3], at 0 h, 24 h and 48 h, respectively. The corresponding values for I-MIBG were 24.3% [21.0-27.5], 19.1% [11.7-26.5] and 17.4% [14.6-20.1]. No significant amount of activity was associated with high molecular weight species for either halobenzylguanidine, indicating that protein binding was not a major factor.

Meta-iodobenzylguanidine derivatives containing a second guanidine moiety

Radioiodinated meta-iodobenzylguanidine (MIBG) is used in the diagnosis and therapy of various neuroendocrine tumors. To investigate whether an additional guanidine function in the structure of MIBG will yield analogues that may potentially enhance tumor-to-target ratios, two derivatives-one with a guanidine moiety and another with a guanidinomethyl group at the 4-position of MIBG-were prepared. In the absence of any uptake-1 inhibiting conditions, the uptake of 4-guanidinomethyl-3-[(131)I]iodobenzylguanidine ([(131)I]GMIBG) by SK-N-SH cells in vitro was 1.7+/-0.1% of input counts, compared to a value of 40.3+/-1.4% for [(125)I[MIBG suggesting that guanidinomethyl group at the 4-position negated the biological properties of MIBG. On the other hand, 4-guanidino-3-[(131)I]iodobenzylguanidine ([(131)I]GIBG) had an uptake (5.6+/-0.3%) that was 12-13% that of [(125)I]MIBG (46.1+/-2.7%), and the ratio of uptake by control over DMI-treated (nonspecific) cultures was higher for [(131)I]GIBG (20.9+/-0.3) than [(125)I]MIBG itself (15.0+/-2.7). The exocytosis of [(131)I]GIBG and [(125)I]MIBG from SK-N-SH cells was similar. The uptake of [(131)I]GIBG in the mouse target tissues, heart and adrenals, as well as in a number of other tissues was about half that of [(125)I]MIBG. These results suggest that substitution of guanidine functions, especially a guanidinomethyl group, in MIBG structure may not be advantageous.

Local delivery of (131)I-MIBG to treat peritoneal neuroblastoma

Internal radiotherapy involving systemic administration of iodine-131 metaiodobenzylguanidine ((131)I-MIBG) in neural crest tumours such as neuroblastoma has shown considerable success. Although peritoneal seeding of neuroblastoma occurs less often than metastases to organs such as the liver, no effective treatments exist in this clinical setting. Previous reports have demonstrated the effectiveness of peritoneal application of chemotherapeutic drugs or radiolabelled monoclonal antibodies in several kinds of carcinomas. Local delivery of (131)I-MIBG should produce more favourable dosimetry in comparison with its systemic administration in the treatment of peritoneal neuroblastoma. In the current investigation, a peritoneal model of neuroblastoma was established in Balb/c nu/nu mice by i.p. injection of SK-N-SH neuroblastoma cells. Two weeks after cell inoculation, comparative biodistribution studies were performed following i.v. or i.p. administration of (131)I-MIBG. Mice were treated with 55.5 MBq of (131)I-MIBG administered either i.v. or i.p. at 2 weeks. Intraperitoneal injection of (131)I-MIBG produced significantly higher tumour accumulation than did i.v. injection ( P<0.01). Therapeutic ratios of i.p. injection were 4- to 14-fold higher than those of i.v. injection. Radiotherapy with i.v. administered (131)I-MIBG failed to improve the survival of mice; mean survival of untreated mice and mice treated with i.v. administration of (131)I-MIBG was 59.3+/-3.9 days and 60.6+/-2.8 days, respectively. On the other hand, radiotherapy delivered via i.p. administration of (131)I-MIBG prolonged survival of mice to 94.7+/-17.5 days ( P<0.02 vs untreated controls and mice treated with i.v. (131)I-MIBG therapy). Radiation doses absorbed by tumours at 55.5 MBq of (131)I-MIBG were estimated to be 4,140 cGy with i.p. injection and 450 cGy with i.v. injection. These results indicate the benefits of locoregional delivery of (131)I-MIBG in the treatment of peritoneal neuroblastoma.

Radioisotope diagnosis and therapy of malignant pheochromocytoma

The availability of radiopharmaceuticals to depict primary malignant pheochromocytoma and its metastases has markedly changed the approach to these unusual cancers. Whole body screening afforded by scintigraphy allows remote tumor involvement to be identified and provides staging information necessary to guide subsequent therapy. The avid accumulation by malignant pheochromocytoma of some radiopharmaceuticals used for scanning has shown promise in therapeutic trials. In this paper, we discuss radiopharmaceuticals presently employed in malignant pheochromocytoma for both diagnostic and therapeutic uses and potential future compounds that may find their way into clinical practice in the approach to these and other related neoplasms.

Synthesis and characterization of fluorescent ligands for the norepinephrine transporter: potential neuroblastoma imaging agents

Radiolabeled m-iodobenzylguanidine (MIBG) is a tumor-seeking radioactive drug used in the diagnosis and treatment of pheochromocytomas and neuroblastomas. It is transported into the tumor cells by the neuronal norepinephrine (NE) transporter (NET) which is expressed in almost all neuroblastoma cells. Here, we describe the synthesis and some pharmacological properties of a series of fluorescent compounds structurally related to the NET substrate, MIBG, or to the NET inhibitors, (-)-(2R,3S)-cocaine and nisoxetine. Three of 10 synthesized fluorescent compounds, 1-(1-naphthylmethyl)guanidinium sulfate (1), 1-[2-(dibenz[b, f]azepin-5-yl)ethyl]guanidinium sulfate (2), and (2R, 3S)-2beta-ethoxycarbonyl-3beta-tropanyl 5-(dimethylamino)naphthalene-1-sulfonate (6), exhibited high affinity (IC(50) about 50 nM) for the NET. The nisoxetine derivatives 8 (rac-N-[(3-methylamino-1-phenyl)propyl]-5-(dimethylamino)-1-naphthale nesulfonamide) and 9 (rac-4-[(3-methylamino-1-phenyl)propyl]amino-7-nitro-2,1, 3-benzoxadiazole) and especially the guanidine derivative 4 (1-[4-(4-phenyl-1,3-butadienyl)benzyl]guanidinium sulfate) which are characterized by intermediate affinity for the NET (IC(50) 370-850 nM) caused significant and nisoxetine-sensitive cell fluorescence. At least the guanidine derivative 4 might represent a potentially useful agent for imaging of neuroblastoma cells.

Menadione inhibits MIBG uptake in two neuroendocrine cell lines

In this paper we report on our studies of the effect of menadione on the uptake of MIBG in the neuroendocrine cell lines PC12 and SK-N-SH. Menadione inhibits the uptake of MIBG in both cell lines in a dose-dependent manner. Inhibition of MIBG uptake is most pronounced in the PC12 cell line. Comparison of the inhibitory action of menadione on the uptake and retention of MIBG with that of imipramine and reserpine suggests that menadione inhibits uptake 1 mediated uptake as well as granular storage.

In vitro evaluation of N-(fluoro)isopropyl norephedrine as potential cardiac imaging agents for PET

N-Isopropylnorephedrine (INE) and N-fluoroisopropylnorephedrine (FINE) were found to have a poor affinity for either beta-adrenoceptors and the norepinephrine carrier protein. The low affinity of both compounds for Uptake-1 is probably due to the introduction of a bulky substituent on the nitrogen atom. It is concluded that INE and FINE cannot be used for cardiac imaging with PET.

Release mechanisms of [125I]meta-iodobenzylguanidine in neuroblastoma cells: evidence of a carrier-mediated efflux

[131I]metaiodobenzylguanidine ([131I]MIBG) is selectively taken up and stored by tumours derived from the neural crest, and is used for diagnosis and treatment of neuroblastoma (NB). The antitumoral effect of [131I]MIBG is closely related to the intracellular level of the radiopharmaceutical compound, which is dependent on uptake and storage/release mechanisms. While MIBG uptake is well characterised, storage and release mechanisms are still controversial. In order to better characterise [125I]MIBG release mechanisms, we studied the basal and stimulated efflux of [125I]MIBG in the human NB cell line, SH-SY5Y, preloaded with 0.1 microM [125I]MIBG for 1 h. We found that [125I]MIBG basal efflux is highly temperature-dependent, that [125I]MIBG release, induced by cell depolarisation with high potassium, is mainly calcium-independent, and induced by exchange with cold MIBG or noradrenaline, inversion of the sodium gradient across the cell membrane by veratridine by substitution of sodium chloride with equimolar concentration of lithium chloride. The exposure of NB cells to imipramine, an Uptake-1 inhibitor, also produces a net stimulatory effect on [125I]MIBG release. However, when used in association with other releasing stimuli, such as higher levels of intracellular sodium or external agonists, imipramine abolishes the consequent increase of [125I]MIBG release. Our findings suggest that stimulated [125I]MIBG release is mediated by a carrier, most probably the uptake carrier working in a reverse mode, while a minimal fraction of [125I]MIBG is released by an exocytotic mechanism.

Modification of meta-iodobenzylguanidine uptake in neuroblastoma cells by elevated temperature

Successful imaging or treatment of neuroblastoma with 131I-meta-iodobenzylguanidine (131I-mIBG) depends on the selectivity of active (type 1) uptake of mIBG in neuroblastoma cells relative to passive (type 2) uptake present in most normal tissues. This study investigates the effects of moderately elevated temperature (39-41 degrees C) on the cellular uptake of 131I-mIBG in two neuroblastoma cell lines [SK-N-BE(2c) and IMR-32] and in a non-neuronal (ovarian carcinoma) cell line (A2780). In SK-N-BE(2c), a cell line with high active uptake capacity, the specific (type 1) uptake was reduced by 75% (P < 0.001) at 39 degrees C. Both IMR-32 and A2780 have a low capacity for accumulation of mIBG by active uptake. These cell lines demonstrated a statistically significant increase in accumulation at 39 degrees C, mainly as a result of increased non-specific transport. At 41 degrees C uptake of 131I-mIBG was reduced in all cell lines. Thus, the active component of mIBG uptake is more vulnerable to increased temperature than the passive component. It seems probable that moderately increased temperature will have an unfavourable effect on the therapeutic differential for targeted radiotherapy of neuroblastoma using radiolabelled mIBG.

Meta-[211At]astatobenzylguanidine: further evaluation of a potential therapeutic agent

Meta-[211At]astatobenzylguanidine ([211At]MABG) is an astatinated analogue of meta-iodobenzylguanidine (MIBG) that could be of value for therapeutic applications. The initial goal of this study was to determine whether [211At]MABG is taken up, like MIBG, by a specific uptake-I mechanism. Norepinephrine and desipramine (DMI) decreased [211At]MABG uptake in SK-N-SH human neuroblastoma cells. This uptake was found to be energy-dependent: In mice, pre-treatment with DMI reduced uptake of [211At]MABG at 1 hr post-injection in the adrenal and in the heart. Tetrabenazine at a dose of 40 mg/kg reduced uptake of [211At]MABG in the mouse heart in vivo (69% of control) whereas up to 100 microM of tetrabenazine did not affect the in vitro uptake of [211At]MABG in SK-N-SH cells. In SK-N-SH cells, 53% and 38%, respectively, of the initial uptake of [211At]MABG was retained at 4 hr and 6 hr. For no-carrier-added (n.c.a.) [131I]MIBG these values were similar, 60% and 48%. The ability of SK-N-SH cells to incorporate [3H]thymidine was reduced to less than 50% of control values when treated with as little as 3.2 nCi of [211At]MABG. In contrast, no significant reduction in the thymidine uptake was observed, even with 80 nCi of n.c.a. MIBG.

Uptake of the neuron-blocking agent meta-iodobenzylguanidine and serotonin by human platelets and neuro-adrenergic tumour cells

The adrenomedulla-imaging agent meta-iodobenzylguanidine (MIBG) is concentrated by various tumours of neuroectodermal origin. Radio-iodinated [131I]MIBG is therefore increasingly used for diagnosis and therapy of these disorders. To study the cause of thrombocytopenia associated with [131I]MIBG therapy, we investigated the uptake of MIBG in human platelets in comparison with that of serotonin. Specific imipramine-sensitive uptake of [131I]MIBG was much slower than of [3H]serotonin, but after prolonged incubation high and serotonin-equivalent uptake levels were observed. Accumulation of MIBG saturated at 10- to 100-fold higher concentration than serotonin, and the affinity for uptake and intracellular storage in platelets was much higher for serotonin than for MIBG. Conversely, serotonin was not detectably concentrated by neuroadrenergic Uptake-I in SK-N-SH neuroblastoma and PC12 pheochromocytoma cells. Fluvoxamine inhibited the uptake of norepinephrine and MIBG in PC12 cells, similarly to that of serotonin in platelets. However, the drug was 100-fold more effective in inhibiting platelet transport of MIBG than of serotonin. The results indicate that MIBG uptake in platelets is not mediated by a neuro-adrenergic Uptake-I, but probably proceeds via the serotonin transport system. MIBG concentration by platelets was at least as efficient as in neuro-adrenergic tumour cells and has therefore (radio)biological potential for injuring these cells or precursor megakaryocytes. Platelet uptake of MIBG could be selectively blocked by fluvoxamine in concentrations which minimally affected its accumulation in neuro-adrenergic target cells.

Pharmacokinetics and intracellular distribution of the tumor-targeted radiopharmaceutical m-iodo-benzylguanidine in SK-N-SH neuroblastoma and PC-12 pheochromocytoma cells

Radiodinated meta-iodobenzylguandine (MIBG) is increasingly used for the diagnosis and targeted radiotherapy of neuro-adrenergic tumors. We have investigated various conditions for specific tumor loading and prolonged retention of this radiopharmaceutical in poorly differentiated SK-N-SH neuroblastoma and highly differentiated PC-12 pheochromocytoma cells. At a constant value of drug concentration x incubation time, short incubations were superior to protracted incubations for maximal cell loading. This effect was most pronounced in the SH-N-SH neuroblastoma cells. In highly differentiated pheochromocytoma cells, the levels of MIBG storage remained high and unchanged during incubations up to 46 hr in label-free medium, while primitive SK-N-SH cells lost 40-50% of accumulated drug by diffusion. In PC-12 cells, susceptibility of stored MIBG to exocytotic release induced by acetylcholine or K+ was similar to that of natural norepinephrine (NE) and prevented by the Ca(++)-channel blockers verapamil and nifedipine. Conversely, granule-poor SK-N-SH cells were insensitive to exocytotic release of MIBG. Uptake and retention capacities were minimally impaired by an externally delivered radiation dose of 5 Gy to mimic the radiobiological effect of 131I-MIBG in tumors. In pre-irradiated cultures, drug uptake was even stimulated, probably due to enrichment in non-proliferating cells. An autoradiographic comparison of the (sub)cellular distributions of 3H-norepinephrine and 125I-MIBG showed that routine conditions of cell fixation and sample processing do not yield reliable results regarding localization of MIBG.

Meta-iodobenzylguanidine (mIBG) uptake and storage in the human neuroblastoma cell line SK-N-BE(2C)

Whilst many human neuroblastoma cell lines have been studied to see if they are capable of taking up mIBG, few appear to have this ability. This contrasts markedly to the situation in vivo, where uptake has been demonstrated in the majority of tumours investigated. Here we report on the human neuroblastoma cell line SK-N-BE(2C) and demonstrate that mIBG uptake can occur in this cell line through 2 mechanisms. At low concentrations of mIBG (approximately 10(-8) M) an active transport process predominates, whereas at non-physiological levels (10(-4) M) uptake occurs through passive diffusion. The active transport process is ATP-, Na(+)- and temperature-dependent. Uptake is blocked by 10(-6) M desipramine, an inhibitor of the uptake-I mechanism involved in amine transport. In contrast, desipramine has no effect on the passive diffusion of mIBG into cells. The active transport mechanism for mIBG uptake appears rather promiscuous for biogenic amines, as dopamine, tyramine and nor-adrenaline were highly efficient at blocking mIBG entry to the cell. Serotonin and histamine were capable of interfering with mIBG uptake only at much higher concentrations. Electron microscopy of SK-N-BE(2C) cells revealed a paucity of neurosecretory granules. Biochemical investigations demonstrated the majority of mIBG to be present in the cytoplasm of cells. The availability of a human neuroblastoma cell line that grows well, both as xenograft and in culture, should further our understanding of the cytotoxic effects of mIBG and thus enhance its clinical usefulness.

Intracellular inhibition of mono(ADP-ribosylation) by meta-iodobenzylguanidine: specificity, intracellular concentration and effects on glucocorticoid-mediated cell lysis

meta-Iodobenzylguanidine (MIBG) is a high-affinity substrate for mono(ADP-ribosyl)transferase of cholera toxin and turkey erythrocyte membranes (Loesberg, C., Van Rooij, H. and Smets, L.A.(1990) Biochim. Biophys. Acta 1037, 92-99). In the present study the drug was investigated as a potential inhibitor of intracellular ribosyltransferases by competition with endogenous acceptors. To this end, MIBG was compared with the conventional ADP-ribosylation inhibitors nicotinamide and 3-aminobenzamide in cell-free ribosylation systems and in intact L1210 leukemia cells. Poly(ADP-ribose)polymerase (poly-ADPRP) was assayed by the DNAse-I-induced incorporation of [14C]NAD in nuclei of permeabilized L1210 cells. Mono(ADP-ribosyl)transferase (mono-ADPRT) was assayed as NAD linkage to [125I]iodoguanyltyramine catalysed by turkey erythrocyte membranes or activated cholera toxin. Poly-ADPRP was inhibited by nicotinamide (IC50 = 0.03 mM) and by 3-aminobenzamide (IC50 less than or equal to 0.03 mM) but was insensitive to MIBG. Conversely, mono-ADPRT was inhibited by MIBG (IC50 = approx. 0.1 mM) but not by 3-aminobenzamide and only weakly so by nicotinamide in high concentration (10 mM). In L1210 cells, intracellular levels of nicotinamide equilibrated at 60-70% of the extracellular drug concentrations assayed at 1 and 10 mM. In contrast, MIBG was concentrated 15-fold by nonspecific uptake. The preferential interference of the drugs with endogenous mono- or poly-ADP ribosylations, predicted from inhibitory capacity in vitro and intracellular concentrations, was confirmed by their effect on dexamethasone-induced lysis of L1210 cell lines. Inhibition of endogenous mono-ADPRT with 0.03 mM MIBG or 10 mM nicotinamide induced sensitivity to glucocorticoids in refractory L1210-wt cells. In contrast, inhibition of poly-ADPRP by 3-aminobenzamide or nicotinamide (1 mM each) did not confer susceptibility to refractory cells but enhanced the lytic process in the sensitive subline L1210-H7 or in L1210-wt cells sensitized by MIBG. These results indicate that MIBG is the first substrate for guanidino-specific mono-ADPRT which accumulates in intact mammalian cells and effectively competes with intracellular acceptors for endogenous enzymes.