Cytotoxic and antitumor effects of the norepinephrine analogue meta-iodo-benzylguanidine (MIBG)

Novel Meta-iodobenzylguanidine and Etoposide Complex: Physicochemical Characterization and Mathematical Modeling of Anticancer Activity

It is hypothesized that meta-iodobenzylguanidine (MIBG) complexation with etoposide (VP-16) will improve drug solubility and specificity towards BE(2)C neuroblastoma (NB) cells, 90% of which are known to be MIBG avid. After MIBG and VP-16 interaction, the dry complex was analyzed for crystalline structure, surface morphology, solubility, and size distribution by X-ray powder diffraction (P-XRD), scanning electron microscopy (SEM), infrared (FTIR) and UV spectroscopy, and dynamic light scattering. After exposure to the complex, the cell viability and decay rates were assessed by the MTS assay and estimated using exponential decay models (EDM). Multi-factorial ANOVA and an independent t-test were used to assess for cell viability and solubility data, respectively. The resulting (1: 3 w/w) VP-16: MIBG complex had a mean diameter and zeta potential of 458.5 nm and 0.951 mV, respectively. It dramatically increased the drug apparent water solubility (~ 12-folds). This was ascribed to the formation of a VP-16/MIBG nanocrystalline state mainly governed by cation-π interactions, evidenced by FTIR, SEM, and P-XRD data following the complexation. The EDM relating percent cell viability to drug concentration yielded an excellent fit (r2 > 0.95) and enabled to estimate the IC50 values of both native drug and its complex: 6.2 μM and 5.23 μM, respectively (indicating a conservation of drug anticancer activity). The statistical results were consistent with those of the exponential decay models, indicating that MIBG does not inhibit the anticancer activity of VP-16. This study indicates that the VP-16/MIBG complexation improves VP-16 solubility without antagonizing its anticancer activity. Moreover, the efficiency of the EDM for drug IC50 estimation provides alternative mathematical method for such in vitro cytotoxicity studies.

Noradrenergic uptake in the liver on 123 I-mIBG imaging: Influence of heart failure and diabetes

BACKGROUND AND AIM

Imaging noradrenergic uptake in the liver with norepinephrine analog ¹²³ I-meta-iodobenzylguanidine (mIBG) was explored in normal controls and patients with heart failure (HF).

METHODS

A total of 961 HF (343 with diabetes mellitus [DM]) and 94 control subjects underwent anterior planar mIBG images including upper abdomen at 15 min (early) and 3 h 50 min (late) post-injection. Decay-corrected liver activity normalized to injected activity and body surface area (counts/pixel [cpp]/MBq/m² ) was compared in three groups: HF with DM; HF without DM; and controls. Associations with plasma norepinephrine, liver function tests, and level of cardiac innervation were explored.

RESULTS

In controls, liver mIBG activity decreased over time (early: 2.78 vs late: 2.43 cpp/MBq/m² , P < 0.0001); in HF subjects, activity increased during this interval (HF without DM: 2.85 vs 2.93 [P = 0.005]; HF with DM: 2.37 vs 2.43 [P = 0.054]). Early liver activity was lower in HF with DM subjects than in the other groups (P < 0.001); late liver activity was higher in HF without DM than in the other two groups (P < 0.01). Subjects with elevated plasma norepinephrine (> 520 pg/mL) or ≥ 1 abnormal liver function test had lower early and late liver activity. In subjects with preserved cardiac mIBG uptake, HF subjects had higher and control subjects lower liver activity than comparable subjects with decreased cardiac innervation.

CONCLUSIONS

In HF subjects, liver mIBG activity increased over time, reversing the normal washout pattern, suggesting a compensatory change in sympathetic nerve function. DM, abnormal liver function tests, and decreased cardiac innervation were associated with decreased liver mIBG uptake in HF.

Norepinephrine-Transporter-Targeted and DNA-Co-Targeted Theranostic Guanidines

High risk neuroblastoma often recurs, even with aggressive treatments. Clinical evidence suggests that proliferative activities are predictive of poor outcomes. This report describes syntheses, characterization, and biological properties of theranostic guanidines that target norepinephrine transporter and undergo intracellular processing, and subsequently their catabolites are efficiently incorporated into DNA of proliferating neuroblastoma cells. Radioactive guanidines are synthesized from 5-radioiodo-2'-deoxyuridine, a molecular radiotherapy platform with clinically proven minimal toxicities and DNA-targeting properties. The transport of radioactive guanidines into neuroblastoma cells is active as indicated by the competitive suppression of cellular uptake by meta-iodobenzylguanidine. The rate of intracellular processing and DNA uptake is influenced by the agent's catabolic stability and cell population doubling times. The radiotoxicity is directly proportional to DNA uptake and duration of exposure. Biodistribution of 5-[¹²⁵I]iodo-3'-O-(ε-guanidinohexanoyl)-2'-deoxyuridine in a mouse neuroblastoma model shows significant tumor retention of radioactivity. Neuroblastoma xenografts regress in response to the clinically achievable doses of this agent.

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.

Destabilization of MYC/MYCN by the mitochondrial inhibitors, metaiodobenzylguanidine, metformin and phenformin

In the present study, we investigated the anticancer effects of the mitochondrial inhibitors, metaiodobenzylguanidine (MIBG), metformin and phenformin. ¹³¹I-MIBG has been used for scintigraphic detection and the targeted radiotherapy of neuroblastoma (NB), a pediatric malignancy. Non-radiolabeled MIBG has been reported to be cytotoxic to NB cells in vitro and in vivo. However, the mechanisms behind its growth suppressive effects have not yet been fully elucidated. Metformin and phenformin are diabetes medications that are being considered in anticancer therapeutics. We investigated the anticancer mechanisms of action of MIBG and metformin in NB. Our data revealed that both drugs suppressed NB cell growth and that the combination drug treatment was more potent. MIBG reduced MYCN and MYC expression in MYCN-amplified and non-MYCN-amplified NB cells in a dose- and time-dependent manner. Metformin was less effective than MIBG in destabilizing MYC/MYCN. The treatment of NB cells with metformin or MIBG resulted in an increased expression of genes encoding biomarkers for favorable outcome in NB [(ephrin (EFN)B2, EFNB3, EPH receptor B6 (EPHB6), neurotrophic tyrosine kinase, receptor, type 1 (NTRK1), CD44 and Myc-interacting zinc finger protein (MIZ-1)] and tumor suppressor genes [(early growth response 1 (EGR1), EPH receptor A2 (EPHA2), growth arrest and DNA-damage-inducible, beta (GADD45B), neuregulin 1 (NRG1), TP53 apoptosis effector (PERP) and sel-1 suppressor of lin-12-like (C. elegans) (SEL1L)]. Accordingly, metformin and MIBG augmented histone H3 acetylation in these cells. Phenformin also exhibited histone modification and was more effective than metformin in destabilizing MYC/MYCN in NB cells. Our data suggest that the destabilization of MYC/MYCN by MIBG, metformin and phenformin and their effects on histone modification are important mechanisms underlying their anticancer effects.

Radiolabeled metaiodobenzylguanidine for the treatment of neuroblastoma

INTRODUCTION

Neuroblastoma is the most common pediatric extracranial solid cancer. This tumor is characterized by metaiodobenzylguanidine (MIBG) avidity in 90% of cases, prompting the use of radiolabeled MIBG for targeted radiotherapy in these tumors.

METHODS

The available English language literature was reviewed for original research investigating in vitro, in vivo and clinical applications of radiolabeled MIBG for neuroblastoma.

RESULTS

MIBG is actively transported into neuroblastoma cells by the norepinephrine transporter. Preclinical studies demonstrate substantial activity of radiolabeled MIBG in neuroblastoma models, with (131)I-MIBG showing enhanced activity in larger tumors compared to (125)I-MIBG. Clinical studies of (131)I-MIBG in patients with relapsed or refractory neuroblastoma have identified myelosuppression as the main dose-limiting toxicity, necessitating stem cell reinfusion at higher doses. Most studies report a response rate of 30-40% with (131)I-MIBG in this population. More recent studies have focused on the use of (131)I-MIBG in combination with chemotherapy or myeloablative regimens.

CONCLUSIONS

(131)I-MIBG is an active agent for the treatment of patients with neuroblastoma. Future studies will need to define the optimal role of this targeted radiopharmaceutical in the therapy of this disease.

Meta-[123I]iodobenzylguanidine is selectively radiotoxic to neuroblastoma cells at concentrations that spare cells of haematopoietic lineage

BACKGROUND

The Auger electron-emitting agents meta-[125I]iodobenzylguanidine (125I-MIBG) and 123I-MIBG have been proposed as alternatives to 131I-MIBG for the treatment of neuroblastoma, due to the absence of a cross-fire effect which may minimize bone marrow toxicity. However, the differential toxicity of 123I-MIBG towards neuroblastoma cells and cells of haematopoietic lineage has not been studied.

OBJECTIVE

To compare the toxic effects of 123I-MIBG on SK-N-SH and SK-N-BE(2) neuroblastoma cells and on cells of haematopoietic lineage, specifically HL-60 human myeloid leukemia cells and bone marrow stem cells (BMSCs) from human adult donors.

METHODS

The antiproliferative effects of exchange-labelled or no carrier added (n.c.a.) 123I-MIBG, unlabelled MIBG or the trimethylsilylbenzylguanidine (MTBG) precursor used to prepare n.c.a. 123I-MIBG against SK-N-SH or SK-N-BE(2) cells or HL-60 cells were evaluated using a cell proliferation assay. The toxicity of 123I-MIBG towards SK-N-SH cells or BMSCs from healthy adult human donors was studied using a clonogenic assay.

RESULTS

123I-MIBG was strongly growth inhibitory to SK-N-SH or SK-N-BE(2) cells at concentrations (IC50 185-370 mBq.ml(-1); IC90 740 mBq.ml(-1)) that were sparing to HL-60 cells. Treatment of SK-N-SH cells with 74 mBq of 123I-MIBG decreased colony formation by >90%, whereas colonies from all three populations of stem cells were formed at amounts up to 370 mBq. It was discovered that the MTBG precursor was non-specifically toxic towards both SK-N-SH cells and HL-60 cells, suggesting the need to purify n.c.a. 123I-MIBG for clinical use.

CONCLUSION

Our results suggest that 123I-MIBG is a promising novel radiotherapeutic agent for neuroblastoma. For the first time, we report that the MTBG precursor used to prepare n.c.a. 123I-MIBG was toxic towards neuroblastoma cells as well as to HL-60 cells, representing cells of the haematopoietic lineage, suggesting the need for purification.

Effects of tumour acidification with glucose+MIBG on the spontaneous metastatic potential of two murine cell lines

In addition to hypoxia, acidic extracellular pH (pH_e) is recognised as one of the microenvironmental characteristics of solid tumours. A number of studies have examined ways to increase tumour acidity in order to improve tumour-specific targeting of certain drugs and the effectiveness of hyperthermia. However, previous data have shown that exposure of murine tumour cells to acid conditions in culture can enhance their metastatic potential when injected subsequently into mice, raising the concern that deliberate tumour acidification might increase the probability of metastasis. In this study, we examined the effects of in vivo tumour acidification and hypoxia on the spontaneous metastatic potential of the murine KHT-C fibrosarcoma and B16F1 melanoma cell lines. A tumour-specific increase in extracellular acidity, demonstrated by measurements with pH electrodes, was achieved by daily intraperitoneal injections of meta-iodo-benzylguanidine (MIBG) and/or glucose. This method of tumour acidification during tumour growth did not significantly enhance the spontaneous metastatic potential of the two murine cell lines.

Effect of Mild Hyperglycemia ± Meta-Iodo-Benzylguanidine on the Radiation Response of R3230 Ac Tumors

The effects of glucose or meta-iodo-benzylguanidine (MIBG) on oxygen utilization (QO2) of several tumor cell lines were studied using a Clark-type electrode chamber. For in vivo studies, rats bearing R3230 Ac rat mammary adenocarcinomas were utilized. To evaluate changes in tumor oxygenation induced by glucose or MIBG, intratumoral pO2 and skeletal muscle pO2 were measured using Eppendorf Histography. To find the effect of mild hyperglycemia (i.p., 1 g/kg) +/- MIBG (i.p., 20 mg/kg) on the radiation response, a growth delay assay was used. Glucose alone produced a approximately 20% inhibition of QO2 in several tumor cells we tested except Q7 tumor cells. MIBG inhibited QO2 in R3230 Ac tumors. The median tumor pO2 for glucose + MIBG was increased from 5.3 mm Hg to 13.8 mm Hg. We hypothesized that combined treatment with glucose + MIBG significantly enhanced radiation-induced tumoricidal effects on R3230 Ac tumors, mainly due to reduction in QO2 and increase in tumor pO2.

Endogenous mono-ADP-ribosylation mediates smooth muscle cell proliferation and migration via protein kinase N-dependent induction of c-fos expression

ADP-ribosylation has been coupled to intracellular events associated with smooth muscle cell vasoreactivity, cytoskeletal integrity and free radical damage. Additionally, there is evidence that ADP-ribosylation is required for smooth muscle cell proliferation. Our investigation employed selective inhibitors to establish that mono-ADP-ribosylation and not poly(ADP-ribosyl)ation was necessary for the stimulation of DNA synthesis by mitogens. Mitogen treatment increased concomitantly the activity of both soluble and particulate mono-ADP-ribosyltransferase, as well as the number of modified proteins. Inclusion of meta-iodobenzylguanidine (MIBG), a selective decoy substrate of arginine-dependent mono-ADP-ribosylation, prevented the modification of these proteins. MIBG also blocked the stimulation of DNA and RNA synthesis, prevented smooth muscle cell migration and suppressed the induction of c-fos and c-myc gene expression. An examination of relevant signal transduction pathways showed that MIBG did not interfere with MAP kinase and phosphatidylinositol 3-kinase stimulation; however, it did inhibit phosphorylation of the Rho effector, PRK1/2. This novel observation suggests that mono-ADP-ribosylation participates in a Rho- dependent signalling pathway that is required for immediate early gene expression.

Meta-iodobenzylguanidine induces growth inhibition and apoptosis of neuroendocrine gastrointestinal tumor cells

Neuroendocrine gastrointestinal tumors take up, decarboxylate and store large amounts of monoamines. Radioactive-labeled monoamines like the norepinephrine analogue meta-iodobenzylguanidine (MIBG) have been used for the imaging of neuroendocrine tumors for many years. MIBG is selectively taken up via norepinephrine transporters (NETs) localized in the plasma membrane of neuroendocrine gastrointestinal tumor cells and thereby offers the possibility for specific and innovative therapeutic approaches. We investigated the antiproliferative, cytotoxic, cell cycle-arresting and apoptosis-inducing effects of MIBG in the neuroendocrine gastrointestinal tumor cell line STC-1 and for control in the nonneuroendocrine colorectal cancer cell line HT-29. RT-PCR revealed the expression of NET in STC-1 but not in HT-29 cells. MIBG dose-dependently induced cytotoxicity and growth inhibition of STC-1 cells. It potently induced apoptosis in STC-1 cells as assessed by changes in the mitochondrial membrane potential, activation of caspase-3 and DNA fragmentation. Moreover, MIBG altered the expression of several genes involved in proliferation, apoptosis and stress responses as shown by cDNA arrays. In contrast, neither cytotoxicity, nor growth inhibition nor induction of apoptosis were detected in response to MIBG in the NET-deficient colorectal cancer cell line HT-29. Our data show that MIBG induces growth inhibition and apoptosis in neuroendocrine gastrointestinal tumor cells. MIBG did not arrest the cell cycle in either cell line. Thus, monoamine transporters in the plasma membrane of neuroendocrine gastrointestinal tumor cells are promising targets for innovative and specific treatment strategies of these tumors.

Guanidino-containing drugs in cancer chemotherapy: biochemical and clinical pharmacology

The pharmacology and clinical application of three guanidino-containing compounds are reviewed in this commentary with special focus on a new member of this group of drugs, CHS 828 [N-(6-(4-chlorophenoxy)hexyl)-N'-cyano-N"-4-pyridylguanidine]. m-Iodobenzylguanidine (MIBG) and methylglyoxal bis(guanylhydrazone) (MGBG) have been extensively studied, preclinically as well as clinically, and have established use as anticancer agents. MIBG has structural similarities to the neurotransmitter, norepinephrine, and MGBG is a structural analog of the natural polyamine spermidine. CHS 828 is a pyridyl cyanoguanidine newly recognized as having cytotoxic effects when screening antihypertensive compounds. Apart from having the guanidino groups in common, there are many differences between these drugs in both structure and their mechanisms of action. However, they all inhibit mitochondrial function, a seemingly unique feature among chemotherapeutic drugs. In vitro in various cell lines and primary cultures of patient tumor cells and in vivo in various tumor models, CHS 828 has cytotoxic properties unlike any of the standard cytotoxic drugs with which it has been compared. Among these are non-cross-resistance to standard drugs and pronounced activity in tumor models acknowledged to be highly drug-resistant. Similar to MIBG, CHS 828 induces an early increase in extracellular acidification, due to stimulation of the glycolytic flux. Furthermore, ATP levels decrease, and the syntheses of DNA and protein are shut off after approximately 30 hr of exposure, indicating active cell death. CHS 828 is now in early clinical trials, the results of which are eagerly awaited.

Early stimulation of acidification rate by novel cytotoxic pyridyl cyanoguanidines in human tumor cells: comparison with m-iodobenzylguanidine

CHS 828, a newly recognized pyridyl cyanoguanidine, has shown promising antitumor activity both in vitro and in vivo and is presently in early phase I clinical trial in collaboration with EORTC. In this study, the effects of CHS 828 and a series of analogues on extracellular acidification and cytotoxicity were compared with those of m-iodobenzylguanidine (MIBG) in human tumor cells. The extracellular acidification rate was measured using the Cytosensor microphysiometer, and determination of cytotoxicity and proliferation was [(14)C] performed by the fluorometric microculture cytotoxicity assay (FMCA) and measurement of [(14)C]thymidine and leucine uptake. CHS 828 significantly increased the acidification rate during the first 15-24 hr in a concentration-dependent manner. This effect was abolished by removal of glucose from the medium, substituted with 10 mM of pyruvate, indicating stimulated glycolysis as the source of the increased acidification rate. However, CHS 828 induced cytotoxicity at concentrations well below those that affected the rate of acidification; when a series of closely related pyridylguanidine analogues were tested and compared, no apparent relationship between cytotoxicity and acidification could be discerned. Furthermore, comparable increases in the acidification rate were evident in one subline with high-grade resistance to the cytotoxic actions of CHS 828. The results indicate that CHS 828 may share the inhibitory actions of MIBG on mitochondrial respiration with a subsequent increase in glycolysis and acidification rate. However, this mechanism of action appears neither necessary nor sufficient to fully explain the cytotoxic actions of CHS 828 in human tumor cells, actions which remain to be mechanistically clarified.

Bioavailability and toxicity after oral administration of m-iodobenzylguanidine (MIBG)

meta-iodobenzylguanidine (MIBG) radiolabelled with iodine-131 is used for diagnosis and treatment of neuroadrenergic neoplasms such as phaeochromocytoma and neuroblastoma. In addition, non-radiolabelled MIBG, administered i.v., is used in several clinical studies. These include palliation of the carcinoid syndrome, in which MIBG proved to be effective in 60% of the patients. Oral MIBG administration might be convenient to maintain palliation and possibly improve the percentage of responders. We have, therefore, investigated the feasibility of oral administration of MIBG in an animal model. Orally administered MIBG demonstrated a bioavailability of 59%, with a maximal tolerated dose of 60 mg kg(-1). The first and only toxicity encountered was a decrease in renal function, measured by a reduced clearance of [51Cr]EDTA and accompanied by histological tubular damage. Repeated MIBG administration of 40 mg kg(-1) for 5 sequential days or of 20 mg kg(-1) for two courses of 5 sequential days with a 2-day interval did not affect renal clearance and was not accompanied by histological abnormalities in kidney, stomach, intestines, liver, heart, lungs, thymus, salivary glands and testes. Because of a sufficient bioavailability in absence of gastrointestinal toxicity, MIBG is considered suitable for further clinical investigation of repeated oral administration in patients.

MIBG inhibits respiration: potential for radio- and hyperthermic sensitization

INTRODUCTION

Meta-iodobenzylguanidine (MIBG) in its 131I-labeled form is clinically used as a tumor-targeted radiopharmaceutical in the diagnosis and treatment of adrenergic tumors. This well established drug may have additional clinical applications as a radiosensitizer or hyperthermic agent, ie., MIBG reportedly inhibits mitochondrial respiration in vitro. The mechanism for MIBG inhibition of cellular oxygen consumption is uncertain. Moreover, MIBG reportedly stimulates glycolysis both in vitro and in vivo. Our studies show the effect of MIBG on 9L glioma oxygen consumption and redox status with tumors cells in vitro and in vivo.

MATERIALS AND METHODS

The effects on electron transfer were determined by following oxygen consumption with a Clark oxygen electrode. Fluorescence measurements were used to determine effects of MIBG on intracellular electron acceptors, NADPH and flavoproteins, in vitro and in vivo. 31P-NMR was used to determine alterations in tumor cell pH in vivo.

RESULTS

Our results show the inhibition of oxygen utilization with MIBG for cell suspensions in vitro. The same results were demonstrated for tumor cell suspensions rapidly isolated from tumors grown in rats. Moreover, NAD(P)H and flavoprotein (Fp) fluorescence changes were observed to rapidly occur following MIBG addition in vitro. Changes in intracellular pH measured with 31P-NMR, in vivo, precede the changes in fluorescence of NAD(P)H and Fp obtained with frozen sections of tumor.

CONCLUSIONS

We conclude that 31P-NMR measurements and fluorescence changes, following MIBG injection, can be used as criterion for selecting the proper time to treat tumors with ionizing radiation or hyperthermia.

Iodopyridine-for-iodobenzene substitution for use with low molecular weight radiopharmaceuticals: application to m-iodobenzylguanidine

Substituting a pyridine ring for a benzene ring in the acylation agent N-succinimidyl 3-iodobenzoate has resulted in a useful approach for the radiohalogenation of monoclonal antibodies, peptides, and labeled biotin conjugates. It was hypothesized that such a substitution in m-iodobenzylguanidine (MIBG), a radiotracer used in the detection and treatment of neuroendocrine tumors, might result in an analogue with more rapid normal tissue clearance, thereby facilitating its use for tumor therapy. For the preparation of this analogue, 3-guanidinomethyl-5-iodopyridine (GMIP; 9b), the silicon precursor 4 was synthesized starting from 5-bromonicotinic acid. Attempts to convert 4 to 9b under various conditions were not successful. Radioiodinated 9b could be prepared by the iododestannylation of the tin precursor 8 in 65-70% radiochemical yield. A number of in vitro, in vivo, and ex vivo studies showed that pyridine-for-benzene substitution in MIBG yielded a compound that no longer was taken up by the uptake-1 pathway.

Alteration in cellular calcium and mitochondrial functions in the rat liver during cold preservation

BACKGROUND

Preservation injury is multifactorial and its mechanism is still incompletely defined. Calcium may play an important role in preservation injury.

METHODS

The effects of hypothermia on cytosolic free calcium concentration ([Ca2+]I) and total cellular calcium content in isolated rat hepatocytes were investigated by using fura-2 fluorescence and atomic absorption spectroscopy. Fura-2 loaded cells were placed into a prechilled (7 degrees C) cuvette equipped with a stirrer or preserved in the University of Wisconsin (UW) solution for up to 48 hr. In some experiments, cells were pretreated with inhibitors of Ca2+ release from mitochondria (m-iodobenzylguanidine [MIBG]) and from endoplasmic reticulum (ryanodine [RYA]) for 20 min at 37 degrees C. Mitochondrial functions after preservation were evaluated by measuring ATP and respiratory rates.

RESULTS

Cooling to 7 degrees C caused a rapid increase in [Ca2+]I that was substantially blocked by MIBG and RYA pretreatment. The elevated calcium gradually leaked out of the cells into the Ca2+-free medium. In long-term storage of the cells in the UW solution, there was a marked decrease in both cytosolic free calcium and total cellular calcium. Pretreatment of the livers with MIBG before cold preservation in the UW solution resulted in a stimulation of ATP regeneration in tissue slices. MIBG pretreatment also improved mitochondrial respiratory functions after cold preservation.

CONCLUSIONS

Thus, the loss of mitochondrial function after liver preservation in the UW solution may be related to the effects of hypothermia on calcium metabolism. Approaches to help maintain calcium homeostasis during storage may improve organ preservation.

The effect of the neuroblastoma-seeking agent meta-iodobenzylguanidine (MIBG) on NADH-driven superoxide formation and NADH-driven lipid peroxidation in beef heart submitochondrial particles

In this paper we report the effects of the neuroblastoma-seeking agent meta-iodobenzylguanidine (MIBG) on NADH-driven superoxide formation and NADH-driven lipid peroxidation in beef heart submitochondrial particles. MIBG is a structural analogue of noradrenaline and is capable of inhibiting complex I and complex III of the respiratory chain. The results of our studies show that MIBG enhanced both NADH-driven superoxide formation and NADH-driven lipid peroxidation at concentrations that are likely to exist inside mitochondria of the target cells of neuroblastoma patients treated with [131I]MIBG. The effect of MIBG is comparable to that of rotenone (an inhibitor of complex I) rather than that of antimycin (an inhibitor of complex III). These results suggest that the formation of superoxide and lipid peroxidation contributes to the cytotoxicity of [131I]MIBG.

Comparison of somatostatin analogue and metaiodobenzylguanidine scintigraphy for the detection of carcinoid tumours

The purpose of this prospective study was to compare the ability of radiolabelled somatostatin analogue (RSA) and metaiodobenzylguanidine (MIBG) scintigraphy to display carcinoid tumours. Forty patients were studied after radiological assessment based on clinical symptomatology. These patients had radiologically demonstrated tumours (n=28), resected tumours discovered to be of the carcinoid type (n=5) or clinically and biologically suspected carcinoid tumours (n=7). They underwent indium-111 DTPA-pentetreotide or iodine-123-Tyr-3-octreotide and 131I-MIBG scintigraphy. The results were compared with those of complementary surgical or morphological examinations and analysed according to the site of the tumour and the symptomatology. In the case of 31 patients with a total of 55 tumoral sites, the sensitivity of the initial radiological assessment, of RSA and of MIBG was 96%, 86% and 64%, respectively, for the detection of at least one tumour per patient, but 51%, 85% and 51%, respectively, for the total number of sites. No site was detected solely by MIBG. The concordance between RSA and MIBG was better when all sites were considered (kappa index+0.44) than for only extrahepatic abdominal tumoral sites (kappa index+0.095). Abdominal, thoracic or bone marrow tumours were more easily detected with RSA than with MIBG. Hepatic invasion (21 cases) was more easily detected by radiology (sensitivity 100%) than by RSA and MIBG, both of which displayed a sensitivity of 80%, but with differences in uptake intensity. Tumour detection using MIBG was more significantly linked with flush (P<0.01) than with diarrhoea (P>0.10). In the assessment of carcinoid tumours, RSA scintigraphy should be carried out initially (just after hepatic ultrasonography) and supplemented by MIBG, as comparison of the studies serves to guide therapeutic options and might be valuable for prognosis.

Meta-iodobenzylguanidine inhibits complex I and III of the respiratory chain in the human cell line Molt-4

In this paper we report the effects of meta-iodobenzylguanidine (MIBG), a structural analogue of norepinephrine, on cell proliferation and several parameters related to mitochondrial respiration in Molt-4 cells. In micromolar concentrations, MIBG completely inhibited the proliferation of Molt-4 cells. In intact Molt-4 cells, a progressive increase in the lactate to pyruvate ratio was observed after incubation of these cells with glucose and increasing concentrations of MIBG. In Molt-4 cells permeabilized with digitonin, MIBG inhibited mitochondrial ATP synthesis when malate was used as a substrate. Succinate-driven synthesis of ATP was also inhibited by MIBG, although higher concentrations were required. These results indicate that apart from inhibition of complex I, MIBG inhibits at least one other complex of the respiratory chain. Measurement of the activities of the individual enzyme complexes in the presence of MIBG revealed that complex III is the other enzyme complex susceptible to inhibition by MIBG. Although maximal inhibition of ATP synthesis was observed at a concentration of 10 microM, maximal inhibition of cell proliferation was observed at a concentration of 50 microM of MIBG. This suggests that MIBG also influences other cellular processes apart from mitochondrial oxidative phosphorylation, resulting in additional inhibition of cell proliferation.

Meta-iodobenzylguanidine (MIBG) inhibits malate and succinate driven mitochondrial ATP synthesis in the human neuroblastoma cell line SK-N-BE(2c)

In this paper, we report on our studies of the effects of MIBG, a structural analogue of norepinephrine, on SK-N-BE(2c) cells. In micromolar concentrations, MIBG caused almost complete inhibition of the proliferation of SK-N-BE(2c) cells. In intact SK-N-BE(2c) cells, addition of MIBG led to a decrease of the ATP to ADP ratio. A progressive increase of the lactate to pyruvate ratio (due to increased lactate production) was observed after incubation of the cells with glucose and increasing concentrations of MIBG. In cells treated with digitonin, MIBG inhibited malate driven ATP synthesis. Comparable inhibition of ATP synthesis with succinate as a substrate required higher concentrations of MIBG. These results indicate that, apart from inhibition of complex I, MIBG was capable of inhibiting at least one other complex of the respiratory chain. Although maximal inhibition of ATP synthesis was observed at a concentration of 10 microM, optimal inhibition of cell proliferation occurred at a MIBG concentration > 25 microM. This suggests that MIBG also influences other cellular processes apart from mitochondrial ATP synthesis, resulting in additional inhibition of cell proliferation.

Effect of an arginine-specific ADP-ribosyltransferase inhibitor on differentiation of embryonic chick skeletal muscle cells in culture

Primary cultures of embryonic chick skeletal myogenic cells were used as an experimental model to examine the possible role of mono(ADP-ribosyl)ation reactions in myogenic differentiation. Initial studies demonstrated arginine-specific mono(ADP-ribosyl)transferase activity in the myogenic cell cultures. We then examined the effect of a novel inhibitor of cellular arginine-specific mono(ADP-ribosyl)transferases, meta-iodobenzylguanidine (MIBG), on differentiation of cultured embryonic chick skeletal myoblasts. MIBG reversibly inhibited both proliferation and differentiation of embryonic chick myoblasts grown in culture. Micromolar (15-60 microM) concentrations of MIBG blocked myoblast fusion, the differentiation-specific increase in creatine phosphokinase activity, and both DNA and protein accumulation in myogenic cell cultures. Meta-iodobenzylamine, an analog of MIBG missing the guanidine group, had no effect. Low concentrations of methylglyoxal bis-guanylhydrazone, a substrate for cholera toxin with a higher Km than MIBG, also had no effect, but higher concentrations reversibly inhibited fusion. These findings suggest a possible role for mono(ADP-ribosyl)ation reactions in myogenesis. In addition, the total arginine-specific mono(ADP-ribosyl)transferase activity increased with differentiation in the myogenic cell cultures, and this increase was also blocked by MIBG treatment. Because high levels of activity were found in the membrane fraction derived from later, myotube cultures, the membrane fraction from 96-h cultures was incubated with [32P]NAD+ and subjected to electrophoresis and autoradiography. Three proteins, migrating at 21, 20, and 17 kDa, that were ADP-ribosylated in the absence, but not the presence, of MIBG were identified. These proteins may be endogenous substrates for this enzyme.

Mitochondrial effects of the guanidino group-containing cytostatic drugs, m-iodobenzylguanidine and methylglyoxal bis (guanylhydrazone)

The involvement of mitochondrial damage in the antiproliferative effects of m-iodobenzylguanidine [MIBG] and methylglyoxal bis (guanylhydrazone) [methylGAG] was studied in human neuroblastoma SK-N-SH, mouse neuroblastoma N1E115 and mouse lymphosarcoma S49 cells. Proliferation of SK-N-SH cells was insensitive to MIBG (100 microM gave 15% inhibition), but sensitive to methylGAG (IC50 = 50 microM). MIBG and methylGAG were approximately equitoxic to N1E115 cells (IC50 of 92 and 87 microM, respectively). S49 cells were most sensitive to both MIBG (IC50 = 11 microM) and methylGAG (IC50 = 5 microM). In isolated sonicated mitochondria, MIBG inhibited respiration a complex I of the respiratory chain (EC50 = 0.5 mM), whereas methylGAG was much less effective (EC50 greater than 15 mM). In intact cells, MIBG at 31 microM impaired mitochondrial respiration and stimulated the glycolytic flux. In contrast, equimolar concentrations of methylGAG had no effect on oxygen consumption, ATP content, glucose consumption and lactate production. MethylGAG significantly increased putrescine levels in N1E115 and S49 cells within 12 hr via inhibition of S-adenosylmethionine decarboxylase. No such effects were seen in SK-N-SH cells for up to 48 hr. Equimolar concentrations of MIBG had no effect on the putrescine levels in the various cell lines, suggesting that MIBG did not inhibit S-adenosylmethionine decarboxylase. It is concluded that the antiproliferative mechanisms of the guanidino compounds are essentially different. MIBG inhibited mitochondrial respiration at complex I with concomitant stimulation of the glycolytic flux but was essentially without effect on polyamine levels. On the other hand, cytotoxicity of methylGAG was not associated with mitochondrial dysfunction.

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.

Impaired mitochondrial respiration and stimulated glycolysis by m-iodobenzylguanidine (MIBG)

m-Iodobenzylguanidine (MIBG) is a functional analogue of the neurotransmitter norepinephrine. Radio-iodinated 131I-MIBG is used clinically as a tumor-targeted radiopharmaceutical agent in the diagnosis and treatment of adrenergic tumors. Native MIBG has previously been demonstrated to be cytotoxic in cultured cells and to produce anti-tumor responses in animals when non-toxic schedules are used. In this study the effect of MIBG was investigated on isolated rat liver mitochondria and on various tumor cell lines (human neuroblastoma SK-N-SH, mouse neuroblastoma N1E115 and mouse lymphosarcoma S49). Results revealed that MIBG inhibits respiration of isolated liver mitochondria at complex I of the respiratory chain, without affecting F1 ATP-ase. In cell lines, impairment of the mitochondrial respiration was evident from reduced oxygen consumption and decreased intracellular ATP levels. In response to this effect, the glycolytic flux was stimulated as shown by increased glucose consumption and lactic acid production. Cytotoxicity of MIBG was proportional to drug-induced alterations in glucose metabolism.

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.

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

Human SK-N-SH neuroblastoma cells accumulate and store the adrenal imaging agent metaiodobenzylguanidine (MIBG) with minor involvement of specialized cytoplasmic storage granules (Smets LA et al., Active uptake and extravesicular storage of meta-iodo-benzylguanidine in human neuroblastoma SK-N-SH cells. Cancer Res 49: 2941-2944, 1989). In the present study the mechanism of extravesicular MIBG retention was investigated and compared with granular storage of MIBG and norepinephrine (NE) in PC-12 pheochromocytoma cells. SK-N-SH cells concentrated both MIBG and NE by neuron-specific Uptake-1 but long-term retention was only observed with MIBG. Retention of accumulated NE was, however, promoted by inhibition of intracellular catecholamine degradation with pyrogallol. Drug release by controlled cell permeabilization and by KCl-induced exocytosis indicated that MIBG was mainly stored as freely diffusible, cytoplasmic molecules. SK-N-SH cells were depleted from stored MIBG by the Uptake-1 inhibitor imipramine but poorly so by the granule-depleting drug reserpine. Conversely, PC-12 cells were depleted by reserpine but insensitive to imipramine. The data suggest that extravesicular retention of MIBG in SK-N-SH cells is not based on intracellular sequestration but is solely due to efficient re-uptake of accumulated drug after leaking from the cells. The accumulation of MIBG in SK-N-SH cells, reflecting "pure" Uptake-1, appears to be a powerful system for exploring various cellular and molecular aspects of catecholamine uptake.

Meta-iodobenzylguanidine (MIBG), a novel high-affinity substrate for cholera toxin that interferes with cellular mono(ADP-ribosylation)

Meta-iodobenzylguanidine (MIBG) is a guanidine analogue of the neurotransmitter norepinephrine. Radioiodinated [131I]MIBG is clinically used as a tumor-targeted radiopharmaceutical in the diagnosis and treatment of adrenergic tumors. Moreover, non-radiolabelled MIBG exerts several cell-biological effects, tentatively ascribed to interference with cellular mono(ADP-ribosyl) transferases (Smets, L.A., Bout, B. and Wisse, J. (1988) Cancer Chemother. Pharmacol. 21, 9-13; Smets, L.A., Metwally, E.A.G., Knol, E. and Martens, M. (1988) Leukemia Res. 12, 737-743). In the present study it was investigated whether MIBG could serve as an acceptor for the ribosyl transferase activity of cholera toxin and of erythrocyte membranes. MIBG appeared a substrate for the cholera toxin-catalyzed transfer of the ADP-ribose moiety of NAD to arginine-like residues with the highest affinity for this enzyme reported as yet (Km = 6.5 microM). MIBG was also ADP-ribosylated by the mono(ADP-ribosyl)transferase(s) of turkey erythrocyte membranes. Moreover, the drug appeared a potent affector of the ADP-ribose linkage to membrane proteins by these enzymes. Interference by MIBG was stronger than by related guanyltyramine, the monoamine precursors of MIBG, meta-iodobenzylamine had no effect at all. In contrast, the drug failed to affect endogenous, O-linked poly(ADP-ribose) polymerase, induced in nuclei of S49-leukemia cells by deoxyribonuclease. Since MIBG is the first described drug that specifically interferes with the cellular N-linked mono(ADP-ribosyl) transferase reactions, it may be an important tool to elucidate the physiological role of this posttranscriptional protein modification.

Potentiation of glucocorticoid-induced lysis in refractory and resistant leukemia cells by inhibitors of ADP-ribosylation

Meta-iodo-benzylguanidine (MIBG; 3 x 10(-5) M), a novel inhibitor of mono(ADP-ribosylation)-and the general ribosylation inhibitor nicotinamide (NA; 5-20 mM) both stimulated the glucocorticoid-mediated lysis of sensitive L1210 leukemia cells and even induced susceptibility in various human and murine lines refractory or resistant to dexamethasone (DEX). Potentiation and induction of DEX-sensitivity by ADP-ribosylation inhibitors was accompanied by an increase in saturable 3H-DEX binding sites and by a 2-3 fold increase in the affinity of intracellular receptors for hormone binding. Moreover, the ribosylation inhibitors converted the glucocorticoid antagonist RU-486 into a potent agonist for cytolysis of L1210 cells. We conclude that the cytolytic action of glucocorticoid hormones in leukemic cells is negatively controlled by (mono)ADP-ribosylation of receptor proteins.