Lu - DOTMP , 153 Sm - DOTMP , 175 Yb - EDTMP and 186 / 188 Re - CTMP : Novel Agents for Bone Pain Palliation and Their Comparison with 153 Sm - EDTMP

Designing ideal radiopharmaceuticals for use as bone pain palliatives require the use of a moderate energy β emitter with a stable carrier molecule. Cyclic polyaminophosphonate ligands are known to form complexes with higher thermodynamic stability and kinetic inertness. The present study therefore envisages the use of a few moderate energy β emitters, viz. Lu (T1/2 = 6.71 d, Eβmax= 497 keV), Sm (T1/2 = 46.27 h, Eβmax= 810 keV), Yb (T1/2 = 4.2 d, Eβmax= 480 keV) and Re (T1/2 = 90 h, Eβmax= 1.07 MeV) as the radioisotopes and cyclic polyazamacrocyclic tetramethyl phosphonates namely, 1,4,7,10tetraazacyclododecane-1,4,7,10-tetramethylene phosphonic acid (DOTMP) and 1,4,8,11tetraazacyclotetradecane-1,4,8,11-tetramethylene phosphonic acid (CTMP), apart from the widely used ethylenediaminetetramethylene phosphonic acid (EDTMP) for the development of potential bone pain palliation agents. All the radionuclides under investigation can be produced with adequate specific activity using moderate flux reactors. The comparatively longer half-lives of Lu, Yb and Re will provide much needed logistic advantages in countries with limited reactor facilities. In the present study, LuDOTMP, Sm-DOTMP, Yb-EDTMP and Re-CTMP complexes were prepared with high radiochemical purities (>98%) under optimized reaction conditions. All the radiolabeled complexes exhibited excellent stability at room temperature. Their potential for bone pain palliation could be seen from the biodistribution studies carried out in Wistar rats, wherein selective skeletal uptake (1.82-5.23% of injected activity per gram in tibia at 3 h post-injection) with rapid blood clearance and minimal uptake in any of the major organs was observed. Scintigraphic studies carried out in rabbits also demonstrated significant accumulation of activity in skeleton and insignificant retention of activity in other vital organs. A comparison of the biological behaviour exhibited by the radiolabeled phosphonates under investigation with that of Sm-EDTMP has also been made in order to find out the efficacy of the developed agents. Lu-DOTMP, Sm-DOTMP, Yb-EDTMP ..... Founder’s Day Special Issue, 2005 23 Introduction ncidences of bone metastases arising in a large number of patients suffering from breast, lung and prostate carcinoma are on an increase [1,2]. Intravenous administration of bone seeking radiopharmaceuticals wherein β /conversion electron is incorporated constitutes the most suitable modality for palliation of severe pain in patients suffering from bone metastases [1,3-5]. P [Eβ(max) = 1.71 MeV, T1/2 = 14.3 d] in the form of sodium orthophosphate [6] was the first radionuclide to be used in bone pain palliation followed by Sr [Eβ(max) = 1.40 MeV, T1/2 = 50.5 d] in the form of strontium chloride [7,8]. The major factor in designing effective radiopharmaceuticals for palliative treatment of bone pain is maximizing radiation dose to the bone lesion and minimizing radiation induced bone marrow suppression [9]. Considerable bone marrow suppression due to the presence of higher energy β particle is the major constraint towards the widespread use of P and Sr [1,4,10]. The lack of imagable γ photons and long half-life (especially in case of Sr) are often sited as drawbacks towards the use of these isotope for bone-pain palliation. Sm with its ideally suited decay characteristics, such as, T1/2 = 46.27 h, Eβmax = 0.81 MeV and 103 keV (28%) γ photon [1,10-12] has emerged as an efficient and popular candidate. Additionally, the ease of production of Sm in large quantities with adequate radionuclidic purity by neutron activation of even natural samarium is an added advantage [13]. However, in the Indian context, due to logistic reasons, Sm with 46.27 h half life needs to be produced in adequately high specific activity for administration of required dose to patients, which in turn necessitates handling of high amount of activity during processing. In this context, Lu could be regarded as an attractive alternative radioisotope for bone pain palliation. Lu decays with a half life of 6.71 d by emission of β particles with Emax of 497 keV (78.6%), 384 keV (9.1%) and 176 keV (12.2%) to stable Hf [14]. It also emits γ photons of 113 keV (6.4%) and 208 keV (11%) [14], which is ideally suited for imaging the in vivo localization. Although the physical half life of Lu is relatively longer (compared to Sm or Re), it is within reasonable limits for therapeutic purpose and will in addition provide logistic advantages for facilitating supply to places far away from the reactors. Lu can be produced in adequate specific activity by irradiation of natural Lu target (Lu, 2.6%) in moderate neutron flux (~10 n/cm/s) owing to the very high reaction cross section (σ = 2100 barns) [14]. In the present work, while various isotopes are being evaluated, we have explored the possibility of the use of Yb also as a radionuclide for evaluation of radiopharmaceuticals for bone pain palliation. Yb has excellent radionuclidic properties suitable for developing various radiotherapeutic agents [15], and decays by emission of β particles with Emax of 480 keV to stable Lu with a half-life of 4.2 days. Yb also emits γ photons of 113 keV (1.9%), 282 keV (3.1%), 396 keV (6.5%) which are suitable for carrying simultaneous scintigraphic studies [14]. Owing to significantly large Yb thermal neutron cross section of 69 b [14], it is possible to produce Yb adequate specific activity for preparing agents for bone pain palliation using medium flux reactors. The lesser decay loss owing to the comparatively longer half-lives during the preparation and transportation of agents prepared using Lu and Yb, confers a definite advantage. Re, a medium energy β emitter (Eβmax = 1.07 MeV, Eγ = 155 KeV, [15%]) [14], could be envisaged as an isotope of choice for treatment of skeletal metastases. With the existing facilities at our end it is possible to produce Re with moderately high specific activity (~7 TBq/g ) without the use of enriched Re target. Multidentate aminomethylenephosphonic acids form well-characterized stable complexes with different β emitting radionuclides and have already proven to be very effective for palliation of bone pain [10,16-21]. Localization of those radiolabeled polyphosphonates in bone is attributed to the affinity of phosphonate group for calcium present in actively growing bones [2124]. Ethylenediaminetetramethylene phosphonic acid (EDTMP) is one of the most widely used ligands which forms stable complexes with various radionuclides all of which have shown I Lu-DOTMP, Sm-DOTMP, Yb-EDTMP ..... Founder’s Day Special Issue, 2005 24 high bone affinity and other favorable pharmacological characteristics in biodistribution studies [1,10,12,16,18-21,24]. Sm-EDTMP (Quadramet) is now considered to be the most promising radiopharmaceutical for pain palliation due to skeletal metastases. This agent shows excellent pharmacokinetics in both animals and humans, such as preferential localization in bone cancer lesion and rapid excretion of the residual activity via the kidneys [9,25]. Since LuEDTMP is well documented [18,19,24], we have explored the possibility of complexation of Yb with EDTMP. The choice of cyclic polyamino phosphonic acid for the development of potential agents for bone pain palliation is based on the more pronounced thermodynamic stability and kinetic inertness of their lanthanide complexes compared to that of their acyclic analogues [26,27]. Thermodynamic stability of the metalloradiopharmaceutical is a very important aspect as the dissociation of the radiometal from the chelate in blood circulation is a possible eventuality in presence of a variety of competing chelators and metal ions in plasma [9,27]. This may result in the accumulation of radioactivity in non-target organs. Similarly, kinetic inertness also plays a significant role for the in-vivo stability of a metal chelate. While fast dissociation kinetics are characteristics of lanthanide metal complexes of acyclic chelators, an accumulated body of literature has shown that corresponding complexes containing macrocyclic chelators are much more kinetically inert [27]. In this direction we have explored the possibility of labeling macrocyclic αaminomethylphosphonates viz. DOTMP for labeling with trivalent lanthanides such as Sm and Lu. However, it has been observed that DOTMP does not complex Re as it does Sm and Lu. This observation could be attributed to the fact that matching of cavity size of the macrocyclic ligand with the ionic radius of the metal ion is essential for complexation [28] as has been demonstrated earlier on the suitability of TETA (1,4,8,11tetraazacyclotetradecane tetracetic acid) for complexation with Cu(II) ion (ionic radius = 0.72 Ao) and not with Y(III) (ionic radius = 0.93Ao). Therefore, we have synthesised another cyclic tetraphosphonate, 1,4,8,11-tetraazacyclo1,4,8,11-tetraaminomethylenephosphonate (CTMP) which is a 14 membered analogue of DOTMP for complexation with Re. Biodistribution studies of a number of lanthanide phosphonate complexes revealed that poor invivo stability leads to accumulation of uncomplexed activity in liver due to the formation of colloidal hydroxides in the physiological pH. It is noteworthy that preparation of Sm-EDTMP for routine clinical applications requires a high ligand-to-metal ratio of ~(250-300):1. The large ligand excess is employed to prevent uptake of Sm in liver. The presence of excess EDTMP in blood prevents the dissociated Sm(III) in the plasma from forming colloidal hydroxide [9,29]. It could be presumed that macrocyclic polyamino phosphonic acids would form highly stable and kinetically inert complex with Sm at a considerably lower ligand-to-metal ratio and demonstrate ideal pharmacological characteristics as an agent for bone pain palliation. Materials and Methods Natural lutetium oxide, samarium oxide and ytterbium oxide (spectroscopic grade >99.99% pure) were obtained from American Potash Inc., USA. Natural rhenium metal (spectroscopic grade >99.995% pure) was obtained from Johnson Matthey Company, UK. Ethylene diamine, 1,4,7,10-tetraazacyclododecane (cyclen), 1,4,8,11-tetraazacyclotetradecane (cyclam), orthophosphorus acid, formaldehyde and stannous chloride dihydrate were obtained from Aldrich Chemical Company, USA. All other chemicals were of AR grade and supplied by reputed chemical manufacturers. Whatman 3 MM chromatography paper was used for paper chromatography and paper electrophoresis studies. All the radionuclides were produced by neutron irradiation at the Dhruva research reactor at our Institute. All radioactivity measurements were made using NaI(Tl) scintillation counter. The radionuclidic purity of the isotopes after chemical processing was ascertained by high-resolution γ ray Lu-DOTMP, Sm-DOTMP, Yb-EDTMP ..... Founder’s Day Special Issue, 2005 25 spectrometry using a HPGe detector coupled to a 4 K Multi Channel Analyzer (MCA) system. Energy vs. efficiency calibration of the HPGe detector was carried out using standard Eu source obtained from Amersham Inc., USA. A pre-calibrated well type ion chamber was used to measure the activity of the radioisotopes produced on irradiation. FT-IR spectra of the synthesized ligands were recorded in a JASCO FT/IR-420 spectrometer and proton spectra were recorded in a 300 MHz Varian VXR 300S NMR spectrometer using D2O as solvent. Scintigraphic images were obtained using a single head digital SPECT gamma camera (MPS GE, USA).