Volume 95, Issue 6 - Radiolanthanides in Therapy

Endoradiotherapy (targeted radionuclide therapy) is a systemic approach, involving a radiolabeled targeting vector with a well characterized biochemical strategy to selectively deliver a cytotoxic level of radiation to a disease site on a cellular/molecular level. The group of radiolanthanides has been considered both for imaging and therapy over many years. Some radiolanthanides have been and are increasingly applied for therapeutic purposes. However, the clinical use of endoradiotherapeuticals containing radiolanthanides requires a complex and interdisciplinary approach. It involves, among other factors, the choice of the most suitable lanthanide radionuclide (in terms of nuclear decay parameters such as type and energy of the particles emitted, half-life, decay products etc. ); the identification of the most promising production route; the determination of the relevant production parameters such as excitation functions, nuclear reaction yields, radionuclidic purities, specific activities etc. ; the chemical isolation of the radiolanthanide produced from the target material (except the ( n , γ) production route); the synthesis of the labelling precursor, and labelling of the precursor and the chemical purification and isolation of the labelled radiotherapeutical, ready for i.v. injection; and finally the investigation of pharmacological targeting parameters of the labelled radiotherapeutical in vitro and in vivo (animal experiments).

The possibilities of production of the therapy-related radionuclides 140 Nd, 153 Sm and 169 Yb, and of 88 Y, in no-carrier-added form were investigated. For 88 Y production the nuclear processes nat Sr( p,xn ) 88 Y and nat Rb(α, xn ) 88 Y over the energy ranges E p = 14→9 MeV and E α = 18→12 MeV, respectively, can be utilised, although the former reaction is superior, both in terms of yield and radionuclidic purity. For 140 Nd production the reactions 141 Pr( p , 2 n ) 140 Nd and nat Ce( 3 He, xn ) 140 Nd were investigated in detail. The optimum energy ranges are E p = 30→15 MeV and E 3 He = 35→ 20 MeV, respectively. Both processes yield high-purity 140 Nd. The former reaction, however, leads to a product yield about 17 times higher than the latter reaction, and is therefore to be preferred. In the case of 153 Sm, investigations were done on the 153 Eu( n,p ) 153 Sm reaction with 14 MeV d(Be) neutrons as well as on the 150 Nd(α, n ) 153 Sm reaction. The yield of the ( n,p ) reaction is very low, but the (α, n ) reaction over the energy range E α = 25→15 MeV offers a good potential for production of no-carrier-added 153 Sm in quantities sufficient for therapeutic applications. Also for the radionuclide 169 Yb a novel route, viz. 169 Tm( p,n ) 169 Yb reaction, was investigated. A critical comparison of this route with the commonly used ( n , γ) route, however, showed that, due to the much higher cost involved, the ( p,n ) process should be utilised only when there is a real need of the high specific radioactivity product.

140 Nd was produced by irradiations of CeO 2 and Pr 2 O 3 targets leading to nat Ce( 3 He, xn ) 140 Nd and 141 Pr( p ,2 n ) 140 Nd nuclear reactions. The practical yield of 140 Nd at EOB in the former reaction over the energy range of E 3 He = 33.5 → 0 MeV amounted to 3.5 MBq/μA h and in the latter reaction over the energy range of E p = 18.6→16.2 MeV to 15.5 MBq/μA h. These values correspond to about 41% and 60% of the respective theoretical values. Successful separations of the radionuclide were performed by means of cation-exchange chromatography resulting in decontamination factors of ≥10 8 and ≥7×10 5 for the cerium and praseodymium target materials, respectively. With the no-carrier-added 140 Nd obtained, an efficient 140 Nd/ 140 Pr radionuclide generator system was developed and evaluated. The principle of the radiochemical separation is based on physico-chemical transitions (hot-atom effects) of the daughter 140 Pr following the electron decay process of 140 Nd. The parent radionuclide 140 Nd(III) is quantitatively adsorbed on a solid phase matrix in the form of 140 Nd-DOTA-conjugated complexes. The daughter nuclide 140 Pr is generated in an ionic species and is easily separated using low volumes of various aqueous eluents. The elution yield is at least 93%, if an optimized eluent, such as DTPA solution is applied. The system remains stable at least over three half-lives of 140 Nd, with high radiolytic stability and low 140 Nd breakthrough. This radionuclide generator system 140 Nd ( T 1/2 = 3.37 d) provides the short-lived positron-emitting radiolanthanide 140 Pr ( T 1/2 = 3.4 min) for molecular imaging using positron emission tomography (PET).

The novel bis(methylphosphonate) 2,2′-[4,10-bis(phosphonomethyl)-1,4,7,10-tetraazacyclododecane-1,7-diyl] diacetic acid, trans -H 6 DO2A2P, has been synthesized and characterized by multinuclear NMR spectroscopy ( 1 H, 13 C and 31 P). 153 Sm and 166 Ho complexes with trans -H 6 DO2A2P have been prepared in high yield using a 1:2 metal to ligand molar ratio at 70 °C, pH 8−9. These complexes are hydrophilic, negatively charged and stable in vitro under physiological solutions, up to 48 h. They present a low plasmatic protein binding and some in vitro hydroxyapatite adsorption, mainly for 166 Ho- trans -DO2A2P. Both complexes are stable in vivo , have a fast tissue clearance from most organs and a rapid total excretion from whole animal body. Moderate bone uptake was also observed but the accumulated radioactivity rapidly decreases with time.

153 Sm and 166 Ho complexes with two series of tetraazamacrocyclic ligands containing methylcarboxylate and/or methylphosphonate pendant arms were synthesized and their charge, lipophilicity, protein binding and in vitro and in vivo behaviour evaluated. The first series has the same backbone, a 14-membered tetraazamacrocycle containing a pyridine unit with different pendant arms, namely methylcarboxylates (ac 3 py14) or methylphosphonates (MeP 2 py14 and P 3 py14). The second series comprises 12- to 14-membered tetraazamacrocycles having methylcarboxylates and/or methylphosphonates as pendant arms ( trans -DO2A2P, TRITA, TRITP, TETA and TETP). The 153 Sm/ 166 Ho complexes with the 14-membered tetraazamacrocycles containing the pyridine unit are neutral, hydrophilic, have a significant plasmatic protein binding, are unstable in vivo and present a slow rate of radioactivity excretion and high hepatic retention. 153 Sm/ 166 Ho complexes with the 12- to 14-membered tetraazamacrocycles are quantitatively prepared, except those with TETP. These complexes are hydrophilic, have an overall negative charge and present a medium to low plasmatic protein binding. The 153 Sm/ 166 Ho- trans -DO2A2P, 153 Sm/ 166 Ho-TRITA and 166 Ho-TRITP complexes are stable in vitro and in vivo , presenting a rapid clearance from main organs and a high rate of whole body radioactivity excretion. Biological profile of 153 Sm/ 166 Ho-TRITA complexes makes them promising candidates for therapy when conjugated to a biomolecule, while 166 Ho-TRITP is potentially useful for bone targeting due to its considerable uptake by bone.

In this paper we report and discuss the biodistribution studies with Wistar rats of a series of 153 Sm(III)-glycoconjugates, based on DO3A and DO2A( cis ) scaffolds (DO3A=1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane; DO2A( cis ) = 1,4-bis(carboxymethyl)-1,4,7,10-tetraazacyclododecane). The effects of changing the sugar type (galactose, lactose and glucose), valency (mono and divalent) and topology on the targeting ability of the liver asialoglycoprotein receptor (ASGPR) are evaluated. Divalent glycoconjugates with different topologies were generated by a pendant glycodendrimeric (generation 1) architecture on a DO3A scaffold and by a linear DO2A( cis )-bis derivative. The results show that the galactose conjugates are more target efficient than the lactose analogues, while the glucose conjugates have no liver targeting ability. Divalent galactose conjugates are more efficiently targeted to the liver than the monovalent ones, while the dendrimeric topology of DO3A-Gal 2 has higher targeting efficiency than that of the DO2A( cis )-Gal 2 .

Vascular Endothelial Growth Factor (VEGF) is one of the molecules which regulate angiogenesis, a phenomenon observed in many diseases, including cancer. VG76e, an anti-VEGF monoclonal antibody, was labeled with 177 Lu via p-SCN-Bz-DOTA and CHX-A″-DTPA chelating systems, in order to investigate its possible therapeutic use. Labeling was performed by a 30 min incubation of 177 LuCl 3 and each immunoconjugate, at 37 °C. Radiochemical analysis showed the formation of a single radioactive species, at a yield higher than 98%, for both immunoconjugates. Kits have been formulated for both VG76e-DOTA and VG76e-DTPA. Stability studies, in the presence of a competitor excess, showed that both radiolabeled species remained sufficiently stable (95%) for at least 48 h. Biodistribution results in normal mice were similar for both radioimmunoconjugates, with no significant bone uptake. Gamma camera images of tumor-bearing mice showed satisfactory visualization of the tumor 24 h p.i., while a higher uptake was observed at 48 h p.i. Our findings indicate that both the bifunctional chelating agents p-SCN-Bz-DOTA and CHX-A″-DTPA can be used for the labeling of VG76e with 177 Lu, with high labeling yield and stability. Their in vivo behaviour in normal and tumor-bearing mice looks promising and they can be successfully used for tumor imaging studies.

Radiometallated analogues of the regulatory peptide somatostatin are of interest in the in vivo localization and targeted radiotherapy of somatostatin receptor-overexpressing tumors. An important aspect of their use in vivo is a fast and efficient labeling (complexation) protocol for radiometals along with a high specific activity. We describe in this manuscript synthetic methods for the coupling of two chelators (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid=DOTA) to the bioactive peptide [Tyr 3 , Thr 8 ]-octreotide (TATE) in order to increase the specific activity (radioactivity in Bq per mole peptide). The full chelator-linker-peptide conjugate was assembled on solid support using standard Fmoc chemistry. Two DOTA-chelators were linked to the peptide using lysine or N,N′-bis(3-aminopropyl)-glycine (Apg); in addition, pentasarcosine (Sar 5 ) was used as a spacer between the chelators and the peptide to probe its influence on biology and pharmacology. Complexation rates with In 3+ and Y 3+ salts and the corresponding radiometals were high, the bis-DOTA-derivatives showed higher complexation rates and gave higher specific activity than DOTA-TATE. Pharmacological and biological data of the complexed molecules did not show significant differences if compared to the parent peptide [ 111/nat In-DOTA]-TATE except for [( 111/nat In-DOTA) 2 -Apg]-TATE which showed a lower binding affinity and rate of internalization into tumor cells. The biodistribution of [( 111/nat In-DOTA)-Lys( 111/nat In-DOTA)]-TATE in the rat tumor model (AR4-2J) showed a high and specific (as shown by a blocking experiment) tracer uptake in somatostatin receptor-positive tissue but a lower tumor uptake compared to [ 111/nat In-DOTA]-TATE.