Multi-modality megatherapy with [131I]meta-iodobenzylguanidine, high dose melphalan and total body irradiation with bone marrow rescue: feasibility study of a new strategy for advanced neuroblastoma

The effect of cisplatin pretreatment on the accumulation of MIBG by neuroblastoma cells in vitro

[131I]meta-iodobenzylguanidine ([131I]MIBG) provides a means of selectively delivering radiation to neuroblastoma cells and is a promising addition to the range of agents used to treat neuroblastoma. As MIBG is now being incorporated into multimodal approaches to therapy, important questions arise about the appropriate scheduling and sequencing of the various agents employed. As the ability of neuroblastoma cells to actively accumulate MIBG is crucial to the success of this therapy, the effect of chemotherapeutic agents on this uptake capacity needs to be investigated. We report here our initial findings on the effect of cisplatin pretreatment on the neuroblastoma cell line SK-N-BE (2c). After treating these cells with therapeutically relevant concentrations of cisplatin (2 microM and 20 microM), a stimulation in uptake of [131I]MIBG was observed. Reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that this effect was due to increased expression of the noradrenaline transporter. These results suggest that appropriate scheduling of cisplatin and [131I]MIBG may lead to an increase in tumour uptake of this radiopharmaceutical with consequent increases in radiation dose to the tumour.

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.

The current status of targeted radiotherapy in clinical practice

Biologically targeted radiotherapy in clinical practice requires a molecule which has a relative specificity for tumour tissue--the missile--coupled to a radionuclide with appropriate physical characteristics--the warhead. When administered to a patient this combination should result in selective irradiation of the target tumour cells with relative sparing of normal tissues. Simple ions and small molecules which follow physiological pathways as either the natural substrates or analogues form the best examples of biological targeting. Clinically valuable results are seen with, for instance, iodine uptake by normal and malignant thyroid cells, incorporation of the calci-mimetic element strontium in areas of increased bone metabolism and accumulation of the catecholamine analogue meta-iodobenzylguanidine in neuroblastoma. The use of monoclonal antibodies as targeting vehicles has not proved to be a panacea, yet some patients with lymphoma, hepatoma and ovarian carcinoma have obtained benefit. Current clinical studies in targeted radiotherapy focus on the integration of radionuclide treatment with conventional treatments, and the optimization of such combined approaches. The development of modifications to offset the limitations inherent in the use of crude antibodies also offers an opportunity for improved clinical outcomes.

The radiation dose to the urinary bladder in radio-iodine therapy

A new MIRD dynamic model has been used to provide estimates of the dose to the urinary bladder resulting from the administration of the therapeutic agents 131I as iodide (for thyroid carcinoma) and 131I meta-iodobenzylguanidine (MIBG) (for neuroendocrine tumours). Because the latter agent is used for therapeutic purposes in children, dose estimates were obtained for subjects aged 1 year and upwards. Those parameters likely to influence the bladder dose were also investigated, making use of the inherent flexibility of the model. For an administration of 1 GBq of either 131I as iodide or 131I MIBG to an adult subject, the radiation dose to the inner surface of the bladder was estimated to be approximately 1100 mGy, which is nearly twice the value estimated using a constant-volume bladder model. The new model produced dose estimates for children (within the range 1000-2750 mGy GBq-1 of 131I MIBG) which were approximately 50% greater than those derived using a constant-volume bladder model. The urine flow rate was found to have the greatest effect on the bladder dose, a flow of twice the normal rate resulting in a reduction in the bladder dose by a factor of two. On the other hand, a reduction in the urine flow rate to half the normal value was estimated to increase the radiation dose by a factor of two. This was true for subjects of all ages. With normal voiding, the average dose to the bladder wall from 131I beta-radiation was estimated to be 5-13% of the surface beta dose for subjects of different ages, the values being greater in children. This has to be compared with a photon contribution to the average bladder wall dose amounting to 10-20% of the combined surface dose both from beta-particles and from photons.

Enhanced tumour uptake and in vitro radiotoxicity of no-carrier-added [131I]meta-iodobenzylguanidine: implications for the targeted radiotherapy of neuroblastoma

In vitro and in vivo neuroblastoma models were used to determine whether improvements in tumour targeting in vivo and therapeutic efficacy in vitro could result from the use of no-carrier-added (n.c.a.) [131I]MIBG. Results were compared with use of the conventional therapy MIBG preparation (ex. [131I]MIBG) of lower specific activity which is produced by iodide exchange reaction. The efficacy of n.c.a. [131I]MIBG was compared with that of [131I]MIBG over a range of specific activities by the assessment of neuroblastoma spheroid growth delay. Whereas n.c.a. [131I]MIBG at a radioactivity concentration of 2 MBq/ml prevented the regrowth of 84% of spheroids, toxicity was significantly reduced by the addition of non-radiolabelled MIBG to the incubation medium. The time-dependent biodistribution of n.c.a. [131I]MIBG in nude mice bearing human neuroblastoma xenografts was compared with that of the conventional therapy radiopharmaceutical. The n.c.a. agent gave improved tumour uptake but also significantly greater accumulation in normal tissues known to accumulate MIBG such as heart, adrenal and skin. However, uptake and retention in the blood was unaltered. For all tissues examined, the 3-day calculations were undertaken to predict organ to tumour dose ratios which would result in human neuroblastoma patients with each of the [131I]MIBG preparations. These results suggest that significant therapeutic gain may be achieved by the use of n.c.a. [131I]MIBG as a treatment agent in neuroblastoma. neuroblastoma.