Abstract
1787
Objectives Neuroblastoma (NB) is a pediatric malignancy and most tumor cells express the norepinephrine transporter (NET) enabling uptake of NET ligands. Meta-iodobenzylguanidine (MIBG) is a NET-specific ligand used as a highly specific imaging agent and targeted radiotherapeutic. NB patients frequently require sedation during targeted radiotherapy. Dexmedetomidine has been increasingly utilized to achieve safe and efficacious sedation. There are theoretical concerns that this highly selective alpha-2 adrenergic receptor agonist may interfere with active uptake of MIBG through the NET transporter. Here we analyzed the impact of [125I]MIBG uptake in the presence of dexmedetomidine in human NB derived cellular models.
Methods Carrier-free [125I]MIBG was synthesized using UltraTrace® resin (Molecular Insight Pharmaceuticals, Inc., Tarrytown NY) through radioiododestannylation from a tin precursor bound by solid state polymer. Human NB cell lines (BE2C, SKNSH and IMR5) were used for the study and NET (SLC6A2) protein expression was determined by western blot. We then performed [125I]MIBG internalization studies using [125I]MIBG alone or in combination with either 10 μM desipramine or 50 μM dexmedetomidine. Additionally, dexmedetomidine and desipramine competitive inhibition studies were performed and a nonlinear regression curve was fitted for dose response curves and concentration at 50% maximal inhibition (IC50) was calculated for each of the 3 cell lines. Finally, NET inhibitor dissociation studies were performed in which after pre-incubation with either desipramine or dexmedetomidine, cells were washed and [125I]MIBG added.
Results Uptake of [125I]MIBG into NB cells correlated directly with NET expression. Time course experiments showed that the addition of either the tricyclic antidepressant desipramine, known to actively compete with MIBG for NET-mediated active transport into cells, or dexmedetomidine, both inhibited [125I]MIBG uptake. We next explored the concentrations of desipramine and dexmedetomidine required to inhibit [125I]MIBG uptake in NB cells. There was a three log difference in potency for all 3 NB lines, with an IC50 of 0.001-0.004 μM for desipramine and 1.3-2.4 μM for dexmedetomidine (Table 1). Thus we show dose dependent inhibition of [125I]MIBG uptake by dexmedetomidine, but at several logs lower potency than the known NET inhibitor desipramine. Finally, dexmedetomidine rapidly dissociated from NET upon removal of inhibitor containing solutions, while desipramine inhibition was only partially reversible. We next analyzed published pharmacokinetic (PK) data in pediatric patients to place our in-vitro data into context. The loading dose for dexmedetomidine for children hospitalized in the PICU ranges from 0.5 to 1.0 μg/kg over 10 min and maintenance doses range from 0.2 to 2.0 μg/kg/hr. Based on simulated concentration time profiles to determine target concentrations for adequate sedation, our pooled analysis demonstrated that the extrapolated Cmax with a bolus dose of 0.6 μg/kg was 0.5 µg/L, which is equivalent to 0.0025 µM/L. Similarly, the steady state concentration at an infusion rate of 0.68 μg/kg/hr is 0.3-1.6 µg/L (95% prediction interval) across different pediatric age groups. The molecular weight for dexmedetomidine is 200.28 g/mol, and therefore, these concentrations translate into molar concentrations of 0.0015-0.008 µM/L. In summary, review of pediatric dexmedetomidine PK data shows that the concentrations achieved in the serum are much lower than those required to block MIBG uptake.
Conclusions Here we show that dexmedetomidine competitively inhibits MIBG uptake, but only at concentrations much higher than those that are clinically relevant. Based on these data, the concurrent usage of dexmedetomidine with MIBG can be considered to not negatively impact potential imaging quality or anti-tumor activity when given as a targeted radiotherapeutic. $$graphic_550083F7-FF89-4CD4-9A39-5E8C8C53FE3F$$ $$graphic_9BAB643F-589F-487B-B5E7-8F99EECBE10E$$ $$graphic_B338437E-D12D-4588-99AE-C042BBCA67D0$$ $$graphic_759E058B-A769-485F-9E14-CB35759D5C9B$$