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

Metallic radionuclide-labeled tetrameric 2,6-diisopropylphenyl azides for cancer treatment

This study proposes a new method for radionuclide therapy that involves the use of oligomeric 2,6-diisopropylphenyl azides and a chelator to form stable complexes with metallic radionuclides. The technique works by taking advantage of the endogenous acrolein produced by cancer cells. The azides react with the acrolein to give a diazo derivative that immediately attaches to the nearest organelle, effectively anchoring the radionuclide within the tumor. Preliminary in vivo experiments were conducted on a human lung carcinoma xenograft model, demonstrating the feasibility of this approach for cancer treatment.

Timing and chemotherapy association for 131-I-MIBG treatment in high-risk neuroblastoma

Prognosis of high-risk neuroblastoma is dismal, despite intensive induction chemotherapy, surgery, high-dose chemotherapy, radiotherapy, and maintenance. Patients who do not achieve a complete metastatic response, with clearance of bone marrow and skeletal NB infiltration, after induction have a significantly lowersurvival rate. Thus, it's necessary to further intensifytreatment during this phase. 131-I-metaiodobenzylguanidine (131-I-MIBG) is a radioactive compound highly effective against neuroblastoma, with32% response rate in relapsed/resistant cases, and only hematological toxicity. 131-I-MIBG wasutilized at different doses in single or multiple administrations, before autologous transplant or combinedwith high-dose chemotherapy. Subsequently, it was added to consolidationin patients with advanced NB after induction, but an independent contribution against neuroblastoma and for myelotoxicity is difficult to determine. Despiteresults of a 2008 paper demonstratedefficacy and mild hematological toxicity of 131-I-MIBG at diagnosis, no center had included it with intensive chemotherapy in first-line treatment protocols. In our institution, at diagnosis, 131-I-MIBG was included in a 5-chemotherapy drug combination and administered on day-10, at doses up to 18.3 mCi/kg. Almost 87% of objective responses were observed 50 days from start with acceptable hematological toxicity. In this paper, we review the literature data regarding 131-I-MIBG treatment for neuroblastoma, and report on doses and combinations used, tumor responses and toxicity. 131-I-MIBG is very effective against neuroblastoma, in particular if given to patients at diagnosis and in combination with chemotherapy, and it should be included in all induction regimens to improve early responses rates and consequently long-term survival.

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.

Delivery systems exploiting natural cell transport processes of macromolecules for intracellular targeting of Auger electron emitters

The presence of Auger electrons (AE) among the decay products of a number of radionuclides makes these radionuclides an attractive means for treating cancer because these short-range electrons can cause significant damage in the immediate vicinity of the decomposition site. Moreover, the extreme locality of the effect provides a potential for selective eradication of cancer cells with minimal damage to adjacent normal cells provided that the delivery of the AE emitter to the most vulnerable parts of the cell can be achieved. Few cellular compartments have been regarded as the desired target site for AE emitters, with the cell nucleus generally recognized as the preferred site for AE decay due to the extreme sensitivity of nuclear DNA to direct damage by radiation of high linear energy transfer. Thus, the advantages of AE emitters for cancer therapy are most likely to be realized by their selective delivery into the nucleus of the malignant cells. To achieve this goal, delivery systems must combine a challenging complex of properties that not only provide cancer cell preferential recognition but also cell entry followed by transport into the cell nucleus. A promising strategy for achieving this is the recruitment of natural cell transport processes of macromolecules, involved in each of the aforementioned steps. To date, a number of constructs exploiting intracellular transport systems have been proposed for AE emitter delivery to the nucleus of a targeted cell. An example of such a multifunctional vehicle that provides smart step-by-step delivery is the so-called modular nanotransporter, which accomplishes selective recognition, binding, internalization, and endosomal escape followed by nuclear import of the delivered radionuclide. The current review will focus on delivery systems utilizing various intracellular transport pathways and their combinations in order to provide efficient targeting of AE to the cancer cell nucleus.

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.

131I-MIBG targeting of neuroblastoma cells is acutely enhanced by KCl stimulation through the calcium/calmodulin-dependent kinase pathway

The efficacy of (131)I-metaiodobenzylguanidine (MIBG) therapy relies on norepinephrine transporter (NET) function. The ionic make-up of the extracellular fluid critically controls neuronal cell activity and can also affect substrate transport. In this study, we explored the effect of treatment with elevated KCl concentration on MIBG uptake in SK-N-SH neuroblastoma cells. KCl stimulation caused a rapid increase of (131)I-MIBG uptake in a manner that was calcium-dependent and accompanied by activation of calcium/calmodulin-dependent protein kinase (CaMK)II. The effect was completely abolished by KN93, an inhibitor of CaMKI, II, and IV. STO609, a selective inhibitor of CaMK kinase required for activation of CaMKI and IV, but not CaMKII, only modestly attenuated the response. The KCl effect was also completely abrogated by ML7, a selective inhibitor of myosin light chain kinase (MLCK). This restricted form of CaMK activates myosin, which is required for vesicle trafficking. Saturation kinetic analysis revealed KCl stimulation to increase maximal transport velocity without affecting substrate affinity. In conclusion, KCl stimulation rapidly upregulates NET function through the CaMK pathway via activation of CaMKII and MLCK. These findings allow a better understanding of how NET function is acutely modulated by the ionic environment, which in turn may ultimately help improve the efficacy of (131)I-MIBG therapy.

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.

3-[211At]astato-4-fluorobenzylguanidine: a potential therapeutic agent with prolonged retention by neuroblastoma cells

An analogue of meta-iodobenzylguanidine (MIBG) in which an aromatic hydrogen was replaced with fluorine has been found to possess many properties similar to those of the parent compound. Moreover, 4-fluoro-3-iodobenzylguanidine (FIBG) was retained in vitro by human neuroblastoma cells to a much greater extent than MIBG itself. Since alpha-emitters such as 211At could be valuable for the treatment of micrometastatic disease, an FIBG analogue in which the iodine atom is replaced by 211At would be of interest. In this study, we have evaluated the in vitro and in vivo properties of 3-[211At]astato-4-fluorobenzylguanidine ([211At]AFBG). The specific binding of [211At]AFBG to SK-N-SH human neuroblastoma cells remained fairly constant over 2- to 3-log activity range and was similar to that of [131I]MIBG. The uptake of [211At]AFBG by this cell line was reduced by desipramine, ouabain, 4 degrees C incubation, noradrenaline, unlabelled MIBG and FIBG, suggesting that its uptake is specifically mediated through an active uptake-1 mechanism. Over the 16 h period studied, the amount of [211At]AFBG retained was similar to that of [131I]FIBG, whereas the per cent of retained meta-[211At]astatobenzylguanidine ([211At]MABG) was considerably less than that of [131I]FIBG (53% vs 75%; P < 0.05). The IC50 values for the inhibition of uptake of [131I]MIBG, [211At]MABG, [125I]FIBG and [211At]AFBG by unlabelled MIBG were 209, 300, 407 and 661 nM respectively, suggesting that the affinities of these tracers for the noradrenaline transporter in SK-N-SH cells increase in that order. Compared with [211At]MABG, higher uptake of [211At]AFBG was seen in vivo in normal mouse target tissues such as heart and, to a certain extent, in adrenals. That the uptake of [211At]AFBG in these tissues was related to the uptake-1 mechanism was demonstrated by its reduction when mice were pretreated with desipramine. However, the stability of [211At]AFBG towards in vivo dehalogenation was less than that of [211At]MABG, as evidenced by the higher uptake of 211At in thyroid, spleen, lungs and stomach.