Stability of 47Sc-complexes with acyclic polyamino-polycarboxylate ligands
The aim of this study was to evaluate acyclic ligands which can be applied for labeling proteins such as monoclonal antibodies and their fragments with scandium radionuclides. Recently, scandium isotopes (47Sc, 44Sc) are more available and their properties are convenient for radiotherapy or PET imaging. They can be used together as “matched pair” in theranostic approach. Because proteins denaturize at temperature above 42 °C, ligands which efficiently form complexes at room temperature, are necessary for labelling such biomolecules. For complexation of scandium radionuclides open chain ligands DTPA, HBED, BAPTA, EGTA, TTHA and deferoxamine have been chosen. We found that the ligands studied (except HBED) form strong complexes within 10 min and that the radiolabelling yield varies between 96 and 99 %. The complexes were stable in isotonic NaCl, but stability of 46Sc-TTHA, 46Sc-BAPTA and 46Sc-HBED in PBS buffer was low, due to formation by Sc3+stronger complexes with phosphates than with the studied ligands. From the radiolabelling studies with n.c.a. 47Sc we can conclude that the most stable complexes are formed by the 8-dentate DTPA and EGTA ligands.
Introduction<!><!>Introduction<!>Materials<!>Radionuclides<!>Syntheses of radiolabelled complexes<!>Stability of complexes<!><!>Labelling of the ligands with n.c.a. 47Sc<!>Conclusion
<p>Radionuclides emitting low and medium energy beta-particles and having several days half-life are attractive candidates for radioimmunotherapy. The most promising radioisotope in this category is 177Lu, which has favourable decay characteristics, as e.g. half-life of 6.71 days.</p><p>This radionuclide can be produced in nuclear reactors by n,γ reaction and is commercially available at high levels of specific activity and chemical purity.</p><p>Having large cross-section of 2090 b [1], 177Lu can be directly produced with a relatively high specific activity by neutron activation of 176Lu (2.6 % of natural abundance). Nevertheless, some amount of stable 176Lu cannot be avoided, which may cause some problems concerning receptor saturation with biomolecules labelled with stable isotope, especially when the number of receptors is limited. For this purpose an alternative production route via neutron capture, starting with enriched 176Yb targets, has been demonstrated [2–4]. In this method, the isotopically enriched 176Yb target undergoes the (n,γ) reaction to produce 177Yb, which subsequently decays by β−-emission (T
1/2 = 1.9 h) to 177Lu. However, this indirect production route requires difficult radiochemical separation of 177Lu from the irradiated Yb2O3 target. Taking into account that ytterbium and lutetium are neighbouring trivalent lanthanides and that the Yb/Lu mass ratio in the irradiated target can be as high as several thousand, separation is a very difficult task [2, 5, 6].</p><p>The usefulness of other radionuclides, which may enhance the therapeutic effect of radiopharmaceuticals, should be also investigated. As a good alternative to application of carrier-free 177Lu Lehenberger et al. [7] proposed application of reactor produced 161Tb. The properties of this radionuclide are similar to those of 177Lu, but separation from the target is considerably easier.</p><!><p>Production parameters of n.c.a. 177Lu, 161Tb and 47Sc</p><!><p>The production method of highly active 47Sc in a nuclear reactor was described by Mausner and coworkers [8, 12]. Enriched 47TiO2 target was irradiated with high energy neutrons (En > 1 MeV) to produce 47Sc in the 47Ti(n,p)47Sc reaction. Various methods of 47Sc separation from metallic Ti and TiO2 targets, based on TBP extraction or cation and anion exchange processes, have been reported [12, 13]. Recently, to avoid the slow dissolution of the target in hot concentrated H2SO4, a new, simple and fast method based on irradiation of the Li247TiF6 or 47TiO2 target, dissolution in HF solution and ion exchange isolation of 47Sc has been investigated [11].</p><p>As shown in Table 1, the advantage of 47Sc production contrary to that of 177Lu and 161Tb, is relatively easy isolation of the radionuclide from the target, but the disadvantage is smaller cross-section of the nuclear reaction.</p><p>To date, only few reports concerning labelling studies with Sc radionuclides were published [14–17] Authors of these papers concluded, that for labelling peptides, such as octreotide, the macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), is the most suitable. The stability of the DOTA complexes results from their kinetical inertness, therefore efficient labelling of DOTA-conjugates requires elevated (90 °C) temperature. Due to its rather long half-life, 47Sc can be also considered for preparation of alternative therapeutic agents with longer pharmacokinetics, such as 47Sc-labelled proteins, as e.g. monoclonal antibodies, its fragments and nanobodies. Unfortunately, because proteins denaturize at temperature above 42 °C, ligands other than DOTA, which form complexes at room temperature, are necessary for labelling such biomolecules. The acyclic chelators are not as kinetically stable as the macrocyclic chelators (DOTA, NOTA, TETA etc.), but formation of their complexes at room temperature is much faster. In this paper we report formation and stability studies on 47Sc complexes with various acyclic polydentate ligands, which exhibit faster than DOTA kinetics of complex formation.</p><!><p>The analytical grade reagents were used for radiochemical investigations. Water was obtained from a Millipore water purification system. Ammonium acetate and hydrofluoric acid (40 %) were purchased from Sigma-Aldrich. All resins (Chelex® 100, Dowex® 1X8, Dowex® 50WX4) were purchased in 100–200 mesh size from Dow Chemical.</p><p>The analytical grade TiO2 was obtained from Sigma-Aldrich. Enriched 47TiO2 was supplied by Isoflex, San Francisco, USA. The isotopic composition was: 46Ti-4.0, 47Ti-65.8, 48Ti-26.8, 49Ti-1.8, 50Ti-1.6 %.</p><p>We have chosen the following acyclic ligands for the studies: 8-dentate diethylenetriaminepentaacetic acid (DTPA), 6-dentate N,N-bis(2-hydroxybenzyl)ethylenediamine-N,N-diacetic acid (HBED), 6-dentate 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA), 8-dentate ethylene glycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA), 10-dentate triethylenetetraamine-N,N′,N′′,N′′′-hexaacetic acid (TTHA) and deferoxamine (DFO). For comparison, cyclic 8-dentate ligand, the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) has been chosen.</p><!><p>For reasons of availability we used in preliminary experiments instead of 47Sc, the longer lived 46Sc radionuclide. The former is produced by fast neutron irradiation of the 47Ti target, while 46Sc can be produced in a simple way by direct thermal neutron irradiation of natural scandium. The 46Sc radionuclide was obtained by neutron irradiation of the Sc2O3 target at a neutron flux of 1.5 × 1014 n cm−2 s−1 for 200 h in the Maria research reactor in Świerk (Poland). The specific activity of the radionuclide was 3.8 GBq/mg. The target was then dissolved in 0.1 M HCl.</p><p>47Sc was produced by fast neutrons in the 47Ti(n,p)47Sc reaction on enriched (66 %) 47Ti target. The 2 mg samples of 47TiO2 target were irradiated in the fast neutron channel of nuclear reactor in Świerk (Poland) for 143 h at fast neutron flux (>1 MeV) of 3 × 1013 n cm−2 s−1, and thermal neutron flux of 2.5 1014 n cm−2 s−1. The irradiated 47TiO2 was then dissolved in 1 ml of concentrated hydrofluoric acid (2–4 h with gentle heating at 80 °C) and the solution was diluted to 20 ml with 1 M HF. The solution was next passed through anion exchange bed (Dowex® 1X8). 47Sc was quantitatively eluted using the 0.04 M HNO3 + 0.06 M HF solution. For additional purification the eluent was evaporated to dryness and the residue was dissolved in 0.1 M HNO3. Afterwards 47Sc was adsorbed on cation exchange column Dowex® 50WX4 and finally eluted in two 1 ml fractions using 0.25 M ammonium acetate.</p><!><p>The experimental conditions for labelling, such as metal-to-ligand molar ratio and time of reaction were optimized to achieve high complexation yield. The 46Sc complexes were synthesized by mixing 164 nmol of carrier added 46Sc in chloride form with aqueous solutions of chelators in various molar ligand-to-metal ratios (1/1, 2/1, 5/1, 10/1). In the case of n.c.a. 47Sc 1 MBq was used and the ligand amount varied from 2 to 15 nmol. The complexes were prepared at room temperature at pH = 6.0 (20 mM acetate buffer). The radiolabelling yield was determined by thin layer chromatography (TLC) using silica gel plates (Polygram, Macherey–Nagel). The NH3/H2O (1/25) mixture was used as the mobile phase. The distribution of activity on paper strips was measured by cutting the paper into 1-cm pieces and counting in the NaI (Tl) well counter. The 46Sc-complexes moved with the solvent front (R
f = 1), while free 46Sc remained at the starting point.</p><!><p>Stability of the 46,47Sc acyclic complexes in isotonic NaCl solution and PBS buffer was assessed by adding 20 μl of radiocomplex solution to 500 μl of 0.9 % NaCl and 0.01 M PBS buffer. The solutions were incubated at 37 °C and kinetics of the Sc complex dissociation was measured by taking aliquots of the NaCl and PBS solution at different time points and measuring the liberated 46Sc by ITLC analysis.</p><!><p>Labelling yield of the DOTA ligand with 46Sc at room and elevated temperature</p><p>Structures of the ligands</p><p>Labelling yield of DTPA and HBED ligands with 46Sc at room temperature</p><p>Labelling of the ligands with carrier added 46Sc</p><p>Stability of 46Sc-BABTA, 46Sc-TTHA, 46Sc-EGTA, 46Sc-DTPA and 46Sc-HBED complexes in 0.9 % NaCl and 0.1 M PBS buffer</p><p>Labelling of the open chain ligands with n.c.a. 47Sc</p><p>Labelling of the macrocyclic ligand with n.c.a. 47Sc</p><!><p>From comparison of the Table 3 with the Table 4 one can conclude that both ligands, DTPA and EGTA, form complexes with n.c.a. 47Sc, in lower amount of ligand than those formed by DOTA. Particularly in the case of EGTA, only 2 nmol of the ligand is sufficient to achieve labelling higher than 97 %. The reason is that EGTA and DTPA as a 8-dentate ligand, forms strong complexes by binding via four carboxylic oxygen atoms, two ether oxygen atoms and two nitrogen atoms (EGTA) [18] or five carboxylic and tree nitrogen (DTPA). As was reported by Thakur et al. [19], contrary to our results Am3+, Cm3+ and Eu3+ form stronger complexes with DTPA than EGTA. Formation of stronger complexes by Sc3+ with EGTA than DTPA is probably related with stronger interaction of smaller and harder Sc3+ with ether than with carboxylic oxygen atoms.</p><!><p>In this paper formation and in vitro stability of a series of polyamino-polycarboxylate ligands labelled with 46Sc and 47Sc has been studied. We found that acyclic ligands (except HBED) form complexes much faster than the cyclic DOTA. The radiolabelling yield of acyclic ligands was very high and for the Sc:L molar ratio of 1:1 varied after 10 min from 96 to 99 %. The obtained complexes were stable in isotonic NaCl solution. However, stability of 46Sc-TTHA, 46Sc-BABTA and 46Sc-HBED in PBS buffer was low, due to formation by Sc3+stronger complexes with phosphates than with TTHA, BABTA and HBED. We have shown that when using n.c.a. 47Sc only 2 nmol of the EGTA is sufficient to obtain labelling yield greater than 97 %. Therefore, Sc-EGTA is a promising moiety for coupling 47Sc to proteins. However, biological studies on animal models are needed for evaluation the in vivo stability of radiometal-labelled chelates.</p>