An in vitro approach to evaluate and develop potential Sn-117m based bone-seeking radiopharmaceuticals

It has become standard practice in the development of radiopharmaceuticals to evaluate/assess the efficacy of prospective therapeutic or diagnostic agents by animal models, which generally calls for subjecting a substantial number of animals to intensive test and retest measurements for obtaining representative and conclusive results/data. This work communicates the advantage of combining various analytical modalities with mathematical and computational modeling as a multifaceted tool, for pre-vivo screening of prospective radiopharmaceuticals intended for the treatment of metastases in bone. Ultimately, the methods applied here could help curtail the number of futile tests, and in so doing reduce the number of animals used in the typical trial-and error approach of developing radiopharmaceuticals, in particular metal-chelate type drugs. The benefits being that of an ethical nature as well as cost minimization - in terms of time, facilities and professional consultation hours (vets, radiographers, etc) required. The Sn-117m radionuclide was identified as an ideal radiopharmaceutical component for the treatment of bone pain, owing to its favourable radiation properties – with a half life of 13.6 days and decay emission of conversion (Auger) electrons. The short emission range of the Auger electrons lends itself to minimizing radioxicity/radiation dose to the sensitive bone marrow. In order to best exploit the radiation characteristics of Sn-117m for therapy various methods were considered for the preparation of the isotope with high specific activity. The quintessence of the production techniques attempted employed the Szilard-Chalmers effect, whereby the product atoms or ions are separable from the original target matrix due to chemical and/or structural differences incurred during nuclear bombardment, or as a result of recoil of the atoms from the target lattice due to extreme activation/excitation energy acquired. The nuclear reactions considered were that of neutron capture, Sn-116(n,gamma)Sn-117m, and photonuclear, Sn(IV)-118(gamma,n)Sn(II)-117m. Chemical separation and isolation methods were unique for each reaction, namely recoil capture of Sn-117m in the (n,gamma) reaction followed by chemical extraction – yielding a specific activity of 2.53 MBq/mmol (0.07 % yield); and anion exchange chromatography for (gamma,n) reaction, which produced Sn-117m with a yield of 60% and specific activity of 2.94 GBq/mA/h/mg (349.01 GBq/mA/h/mmol). Two tin-bisphosphonate complexes were studied in this thesis, namely Sn(II)-APDDMP and Sn(IV)-PEI-MP, where the bisphosphonate ligands are N,N-dimethylenephosphonate-1-hydroxy-4-aminopropylidenediphosphonate and N,N’,N’-trimethylenephosphonate-polyethyleneimine, respectively. Using glass electrode potentiometry, the complexes of the former were studied for divalent tin (Sn(II)) and compared against those of the most prominent physiological metal ions, namely Ca(II), Mg(II), Zn(II), which revealed that Ca(II) formed more stable with APDDMP and was therefore prone to displacing the Sn(II). As a result of the dissociation the Sn(II) could then be taken up by amino acids in the plasma thus negating the tumour targeting. The biodistribution of [Sn-117m]Sn(II)-APDDMP was tested in a rodent model, which showed fairly rapid renal clearance of the [Sn-117m]Sn(II). With the aid of blood plasma modeling software, ECCLES, various postulates were tested to explain the observed biodistribution. This confirmed that the complexes did in fact dissociate as a result of Ca(II) competition, resulting in the formation of Sn(II)-complexes with histidine and cysteine, which were then excreted via the kidneys. In an attempt to improve the tumour selectivity and uptake, the water soluble phosphonate polymer PEI-MP was studied, exploiting a phenomenon known as the Enhanced Permeation and Retention effect (EPR), whereby macromolecules, e.g. polymers, selectively accumulate within tumours due to irregularities in the vasculature and poor lymphatic clearance. PEI-MP was complexed with Sn(IV), and in similar fashion as had been performed for Sn(II)-APDDMP, the susceptibility was tested by ECCLES blood plasma modeling. The simulation suggested that the complexes would dissociate within blood plasma, resulting the formation of Sn(IV)-glutamine complexes and Ca-PEI-MP. The stability of Sn(IV)-PEI-MP was lower than that of its Sn(II) counterpart. Therefore, biodistribution studies were averted, given the validity of previous modeling predictions – i.e. Sn(II)-APDDMP and Sn(II)-PEI-MP. When working with tin, and especially Sn(II), an argument ensues as to valence stability of the metal ion(s), that when preparing or injecting a complex of tin with a particular oxidation state, is it certain to remain in that form and not be oxidized (Sn(II)) or reduced (Sn(IV)) within the environment of blood plasma – given the inherent reducing nature of Sn(II) and the oxygen abundance of blood. A study of the tin-phosphonate complexes by P-31 NMR was successful in addressing this question. The coordination of the ligands to Sn(II) and Sn(IV) were distinguishable by the chemical shift of the phosphorous signal. In so doing any interconversion of oxidation states could be easily monitored by changes in intensity of the respective P-31 peaks. The complexes were put through their paces by observing their P-31 spectra in time, and applying oxidative pressure in the form of hydrogen peroxide, and inversely, using glutathione (GSH) to reverse the oxidation. The extreme conditions required to achieve complete conversion was considerably beyond that which can be expected in blood plasma, therefore it was concluded that the tin within the complexes would essentially remain unchanged for the duration of treatment. Concurrently with the blood plasma modeling assessment, the prospective “bone seeking” agents Sn(IV)-PEI-MP and Sn(II)-PEI-MP were tested for their adsorption characteristics with hydroxyapatite, which served as an in vitro model for bone mineral. The adsorption of the complexes as well as the free ligand was adequately described by Langmuir adsorption isotherms – deriving information about the adsorption affinity and the maximum adsorption capacity of each for hydroxyapatite. The ideal combination for optimum complex adsorption was determined, taking into account the polymer, with size fractions of 10-30kDA or 30-50kDa, and the particular valence form of tin. The Sn(II)-PEI-MP complex, with a polymer size of 10-30kDa, was best. This complemented the blood plasma modeling findings, and the adsorption speciation substantiated speciation results obtained by the potentiometry, with M2L complexes forming for Sn(II)-PEI-MP and ML species for Sn(IV)-PEI-MP. Furthermore, the presence of tin favourably enhanced the adsorption of the complex(es), which was evident by: (i) the superior adsorption data/figures of the tin complexes over that of the free ligand; and (ii) dinuclear complexes (M2L) of Sn(II)-PEI-MP. In general this thesis provides a holistic approach to investigating some of the fundamental issues in the development of radiopharmaceuticals – intended particularly for the treatment of bone metastases. The combination of the methods used in this study made it possible to better understand and predict the behaviour of novel drug concepts without the conventional and elaborate animal models – save for the rodent biodistribution study of [Sn-117m]Sn(II)-APDDMP, which, in retrospect, validated the adequacy of the thermodynamic blood plasma model. The study leaves room for adaptation and inclusion of additional techniques – depending on the expected activity of, or hypotheses surrounding the drug being considered, which could help resolve many of the underlying factors influencing efficacy. These could include: cell assays; and dissociation kinetic measurements by free-ion selective radiotracer extraction (FISRE), for example