Production and supply of high specific activity radioisotopes for radiotherapy applications

High specific activity radioisotopes such as Lu-177, Pm-149, Ho-166 and Rh-105 can be produced by indirect methods involving neutron irradiation of isotopically enriched (e.g. Ru-104) targets producing parent radioisotopes that beta decay to form the desired daughter radioisotopes. For example, Lu-177 can be produced by direct (n, gamma) irradiation of Lu-176. However, only about 20 percent of the Lu-176 atoms are converted to Lu-177 and the long-lived impurity Lu-177m (half-life = 160 days) is also produced in small quantities. Direct irradiation of Yb-176 results in the production of Yb-177 (half-life = 1.9 hr) that beta decays to form Lu-177, with the further advantage that this route of production avoids long-lived Lu-177m. Chemical separation of the Lu-177 from the Yb target results in a high specific activity Lu-177 that can then be used for radiotherapy. Separation of Rh-105 from irradiated Ru-104 targets is also being investigated by volatilization of the Ru

Development of an in vitro model for assessing the in vivo stability of lanthanide chelates.

An in vitro model was developed to evaluate the in vivo stability of lanthanide polyaminocarboxylate complexes. The ligand-to-metal ratios for the chelates EDTA, CDTA, DTPA, MA-DTPA (monoamide-DTPA) and DOTA with the lanthanides lanthanum, samarium, and lutetium were optimized to achieve > or = 98% complexation yield for the resultant radiolanthanide complexes. The exchange of the radiolanthanides from their EDTA, CDTA, DTPA, MA-DTPA and DOTA complexes with Ca(2+) was determined by in vitro adsorption and in vitro column studies using hydroxyapatite (HA), an in vitro bone model. In vitro serum stability of these radiolanthanide complexes was used as an additional indicator of in vivo stability, although the mechanism of instability in serum will be different than with bone. The in vitro studies were consistent with the expected findings that the smallest lanthanide (Lu) formed the most stable complexes. In vivo studies were done to validate the in vitro model. Biodistribution studies in normal CF-1 mice showed that in vivo stability of the complex (i.e., the more lanthanide remaining in complex form) could be assessed by a combination of the urinary, bone and liver uptake. For example, biodistribution studies demonstrate that high urinary excretion correlated with complex stability, while high liver plus bone uptake correlated with complex instability. The urinary excretion of the EDTA complexes decreased from (177)Lu to (140)La indicating a loss in stability in the direction of (140)La, consistent with the in vitro studies. The more stable a lanthanide complex is, the lower its exchange with HA in vitro will be, and the lower its combined bone plus liver uptake and higher its urinary excretion will be in vivo. This investigation indicates that the in vivo stability can be determined by a screening method that measures the degree of exchange from the lanthanide chelate with hydroxyapatite (HA) and its serum stability.

198Au-labeled hydroxymethyl phosphines as models for potential therapeutic pharmaceuticals.

The development of novel gold-198 complexes with water-soluble phosphines is reported. A series of cationic and hydrophilic 198Au complexes containing the ligands tris(hydroxymethyl)phosphine (THP, 1) 1,2-bis[bis(hydroxymethyl)phosphino]benzene (HMPB, 2), and 1,2-bis[bis(hydroxymethyl)phosphino]ethane (HMPE, 3) were prepared and evaluated as models for potential gold-199 radiopharmaceuticals. The 198Au complexes were formed in high radiochemical purity by simply mixing H198AuCl4 with the respective ligand. The complexes were shown to exhibit high in vitro stability over wide pH ranges and temperatures. However, only the 198Au(HMPB)2+ complex was found to exhibit good in vivo stability. HPLC analyses indicated that the 198Au complexes with these three phosphine ligands produced singular species with similar retention times as compared to their known macroscopic complexes.

Reactor-produced radionuclides at the University of Missouri Research Reactor.

A revolution in radiotherapy has been developing in recent years, based on more sophisticated targeting methods including radioactive intra-arterial microspheres, chemically-guided bone agents, labeled monoclonal antibodies, and isotopically-tagged polypeptide receptor-binding agents. The isotopes of choice for these applications are reactor-produced beta emitters such as Sm-153, Re-186, Re-188, Ho-166, Lu-177, and Rh-105. The University of Missouri Research Reactor (MURR) has been in the forefront of research into means of preparing, handling, and supplying these high specific activity isotopes in quantities appropriate not only for research, but also for patient trials in the U.S. and around the world. Considerable effort has been expended to develop techniques for irradiation, handling, and shipping isotopes worldwide. The MURR has also served as a highly reliable production source for isotopes, with one of the best operating histories of any isotope production reactor in the world.

Biodistribution of model 105Rh-labeled tetradentate thiamacrocycles in rats.

105Rh(III)Cl2 complexes with a limited series of [14]ane- and [16]ane- thia macrocycles were prepared and their biodistributions in Sprague-Dawley rats studied. These studies demonstrate that modifications in the structure and composition of the 105Rh-thia macrocycle complexes produce significant differences in their uptake and retention in both the liver and kidneys. The results indicate that the cis-Rh(III)Cl2-[14]ane thiamacrocycles exhibit less kidney retention than the corresponding trans-Rh(III)Cl2-[16]ane thiamacrocycles. In addition, the presence of a side chain containing a carboxylate group will produce decreased retention of activity in the kidneys. HPLC analysis of urine from these animals indicates no observable in vivo metabolism or dissociation of these chelates in the blood stream.

An Rh-105 complex of tetrathiacyclohexadecane diol with potential for formulating bifunctional chelates.

1,5,9,13-Tetrathiacyclohexane-3,11-diol (16S4-diol), a sulfur crown ether analog, was studied as a potential chelating agent to complex no-carrier-added (NCA) grade 105Rh(III) in high yield at low ligand concentrations. trans-[RhCl2(16S4-diol)]chi (chi = Cl, PF6) was prepared using nonradioactive RhCl3.3H2O and characterized by UV-Vis, nuclear magnetic resonance (NMR) and X-ray crystallography. It was shown to have a +1 charge with the Rh(III) metal center coordinated to the four S atoms equatorially and two Cl atoms in trans axial positions. The 105Rh-16S4-diol complex prepared with NCA 105Rh(III)-chloride reagent was found to exhibit identical chromatographic properties as trans-[Rh(III)Cl2(16S4-diol)]+ (including silica and C-18 thin-layer chromatography [TLC] and electrophoresis). The preparation of 105Rh-16S4-diol complex formation optimized for conditions of pH, temperature, time, % ethanol and quantity of 16S4-diol resulted in yields > 90%. Very low quantities of 16S4-diol (3 nmol) complex NCA 105Rh(III) under relatively mild reaction conditions (heating at 64 degrees C for 90 min) in the presence of ethanol (10%), yielded the high specific activity 105Rh-16S4-diol complex as a single cationic species. The 105Rh-16S4-diol complex was shown to be stable for > or = 4 days in physiological buffers at room temperature and in human serum at 37 degrees C.

Transition metal chemistry of main group hydrazides, Part 14: Evaluation of new Tc-99m chelates of thiol functionalized phosphorus hydrazides.

Ligands containing a combination of amine or amide nitrogens and thiol functionalities have been found to form stable chelates with Tc-99m, presumably in oxidation state +5. Two new thio-phosphorus monohydrazides [(MeO)2P(S)NMeNHCH2C6H4SH], SL1 and [(MeO)2P(S)NMeNHC(O)C6H4SH], SL2 were synthesized and their complexation properties with Re(V) and Tc-99m have been studied. Neutral-lipophilic Tc-99m chelates with both SL1 and SL2 were formed in high yields (95-97%) as a single species ascertained by electrophoresis and reversed-phase HPLC. Biodistribution studies show good in vivo stability and primary clearance of both 99mTc chelates is via the hepatobiliary pathway. Re(V) complexes with SL1 and SL2 were also synthesized using the ReOCl3(PPh3)2 precursor to obtain the product ReOCl(L)(PPh3), where L = SL1 or SL2. H+ was lost from the N-atom and the thiol group in these Re chelates. Even though the Tc-99m chelates of SL1 and SL2 formed at tracer levels are not identical to the Re-chelates (different synthons were used), the Re data suggests complexation of Tc-99m by these hydrazido-thiol ligands will be similar to N,S ligand systems previously used. The good in vitro and in vivo stability and high yields of the Tc-99m complexes of SL1 and SL2 indicate the potential hydrazido-thiols hold for use as a basis in formulating new Tc-99m radiopharmaceuticals, particularly when thiol moieties are used in conjunction with multi-functional phosphorous hydrazide compounds.

A new 99mTc-complex with a germanium-hydrazide (GeTH) ligand.

A new tetrahydrazido-germanium (GeTH) ligand was synthesized, characterized and complexed with 99mTc. The negatively charged 99mTc-chelate was shown to form in high yields at neutral pH in the absence of other reducing agents and exhibits high in vitro and in vivo stability.

Synthesis, characterization and biodistribution studies of a neutral-lipophilic Tc-99m N3S2 chelate.

Diaminedithiols (DADT) are known to form neutral-lipophilic complexes with 99mTc in aqueous solutions, where they are readily formed in high yields and demonstrate excellent stability. A new triaminedithiol (TADT) ligand was synthesized, characterized and shown to form a neutral-lipophilic 99mTc-chelate. The biodistribution of this 99mTc chelate in rats showed that its uptake in brain or heart following i.v. injection of the 99mTc chelate was low, but activity taken up was retained over a long period of time. The in vivo and in vitro properties of this chelate indicate the possibility that chemical modification of this TADT ligand may produce ligand systems that form 99mTc chelates with suitable diagnostic properties.

Calculation of radiation dose at a bone-to-marrow interface using Monte Carlo modeling techniques (EGS4)

Radiation dose rate profiles at a bone-to-marrow interface were calculated by simulating a uniform radiation source at the center of the endosteal layer in a long bone. Isotopes (153Sm, 186Re, and 166Ho) were assumed to assimilate as surface agents and the dose profiles were calculated on a microscopic scale using the Electron-Gamma Shower (EGS4) computer program. We validated our computational model against published dose factors (delta) for uniform volume distributed sources replicating them to an accuracy of better than 95%. The calculated dose distributions illustrate the relative contribution of atomic electrons, beta, and photon fractions. The backscatter contribution to marrow dose increased from 3% to 4% at the source to 6% to 8% at a marrow depth of 100 microns. Backscattered dose fraction was not significantly different among the three isotopes. The dose contribution from the three isotopes was remarkably similar at ranges between 25 and 125 microns.

Therapeutic radionuclides: production and decay property considerations.

The development of effective therapeutic radiopharmaceuticals requires careful consideration in the selection of the radionuclide. The in vivo targeting and clearance properties of the carrier molecule must be balanced with the decay properties of the attached radionuclide. Radionuclides for therapeutic applications fall into three general categories: beta-particle emitters, alpha-particle emitters, and Auger and Coster-Kronig-electron emitters following electron capture. Alpha particles and Auger electrons deposit their energy over short distances with a high LET that limits the ability of cells to repair damage to DNA. Despite their high levels of cytotoxicity, the relatively short range of alpha particles requires binding of the carrier molecule to most cancer cells within a tumor in order to be effective. Because of the extremely short range of Auger electrons, the radionuclide must be carried directly into the nucleus to elicit high radiotoxicity, making it necessary to deliver the radionuclide to every cell within a tumor cell population. These characteristics impose rigid restrictions on the nature of the carrier molecules for these types of particle emitters but successful targeting of these types of radionuclides could result in high therapeutic ratios. Most beta-emitting radionuclides are produced in nuclear rectors via neutron capture reactions; however, a few are produced in charged-particle accelerators. For radionuclides produced by direct neutron activation, the quantities and specific activities that can be produced are determined in large part by the cross-section of the target isotope and the flux of the reactor. Many applications (e.g., therapeutic bone agents, radiolabeled microspheres, radiocolloids) do not require high-specific activities and can therefore utilize the wide range of radionuclides that can be produced in sufficient quantity by direct neutron activation. Other applications (e.g., MAb labeling) require high-specific activity radionuclides in order to deliver a sufficient number of radionuclide atoms to the target site without saturating the target or compromising the integrity of the carrier molecule. Most radionuclides, produced at NCA levels in reactors, are produced via indirect reactions. High-specific activity beta emitters can also be obtained from radionuclide generator systems where the longer-lived parent radionuclide may be obtained from direct neutron activation, as a fission product, or from charged-particle accelerators. It is essential that the half-life of a radionuclide used in RNT be compatible with the rates of localization in target tissues and clearance of the carrier molecule from normal tissues. This consideration is especially important for the various MAbs and their fragments that are currently under investigation as carrier molecules to RIT.(ABSTRACT TRUNCATED AT 400 WORDS)

Clinical and clinicopathologic response of canine bone tumor patients to treatment with samarium-153-EDTMP.

Forty dogs with spontaneous skeletal neoplasia were treated with 153Sm-EDTMP (ethylenediaminetetramethylene phosphonic acid). Both primary and metastatic lesions were treated. Two treatment regimes, a single (37 MBq (1.0 mCi)/kg dose or two 37 MBq (1.0 mCi)/kg doses separated by 1 wk) were tested. Response to treatment was varied. Large lesions with minimal tumor bone formation responded poorly, while primary lesions with substantial ossification usually exhibited a transient response. Small lesions with minimal lysis, metastatic lesions, and axial skeleton lesions generally responded well. The major adverse side effects of treatment were platelet and white blood cell count depression below baseline values for up to 4 wk (p less than 0.05). Minor depression of packed cell volume and transient elevation of serum alkaline phosphatase were also noted (p less than 0.05). No significant differences (p greater than 0.05) between the two treatment groups, either in treatment effect or undesirable side effects, were detected.

Clinical and clinicopathologic effects of samarium-153-EDTMP administered intravenously to normal beagle dogs.

A study was undertaken to determine the degree of acute bone marrow and vital organs injury sustained when dogs were administered doses of 153Sm-EDTMP calculated to irradiate an acute bone lesion arising from cancer metastasis to a dose considered palliative or even therapeutic (20-160 Gy). The study revealed significant (p less than 0.05) temporary depression of the bone marrow in all doses in the therapeutic (greater than 40 Gy) range. Palliative (20 Gy) doses caused significant leukocyte depression but insignificant (p greater than 0.05) depression of platelet and packed cell volumes when compared to control animals. A mild transient rise in the levels of serum alkaline phosphatase occurred immediately following radioisotope administration. All hematologic parameters had returned to normal by six weeks after the last injection of radioisotope. The study indicates potential for this compound as a safe, therapeutic radiopharmaceutical for treatment of cancer bone metastasis.

Human pharmacokinetics of samarium-153 EDTMP in metastatic cancer.

Samarium-153 ethylenediaminetetramethylene phosphonic acid ([153Sm]EDTMP) has been proposed to palliate pain resulting from osteoblastic metastatic bone cancer. Encouraging results in dogs with primary malignant bone cancer provided the catalysis for human biodistribution studies in five patients with metastatic skeletal carcinoma. The objective was to assess the preferential localization of [153Sm]EDTMP in bony lesions and compare it to the 99mTc-labeled phosphonates. Blood clearance of [153Sm]EDTMP was rapid with minimal accumulation in nonosseous tissues. Both radiopharmaceuticals showed identical lesion uptake in 23 paired lesions (p greater than 0.05). This indicates that the two radiopharmaceuticals concentrate in metastatic skeletal lesions by the same mechanism and since [153Sm]EDTMP emits beta radiation it may be therapeutically useful in ameliorating metastatic bony cancer pain.

153Sm-EDTMP and 186Re-HEDP as bone therapeutic radiopharmaceuticals.

Two new radiopharmaceuticals, 186Re-1-hydroxyethylidenediphosphonate (186Re-HEDP) and 153Sm-ethylenediaminetetramethylenephosphonate (153Sm-EDTMP), have been proposed as palliative treatments for metastatic bone cancer. Biolocalization properties of these chelates in animals as well as the physical decay and production properties of the respective radionuclides are consistent with those of a therapeutic agent. Subtherapeutic doses of both agents have been administered to human cancer patients to determine their biokinetics and skeletal localization. The 186Re-HEDP studies were conducted at the University of Cincinnati while the 153Sm-EDTMP studies were conducted at the University of Missouri-Columbia. The pharmacokinetics of these agents in humans were consistent with those found in animals. Imaging studies show that the retained activity localizes primarily in the skeleton with high selective uptake in skeletal lesions; there is no visualization of other organs or soft tissue. This paper will review the development and preparation procedures for these radiopharmaceuticals and briefly summarize the animal and patient data.

The chemistry of rhenium and technetium as related to the use of isotopes of these elements in therapeutic and diagnostic nuclear medicine.

The beta emitting isotopes 186Re and 188Re are logical choices on which to base therapeutic radiopharmaceuticals that might be expected to be analogous to diagnostic radiopharmaceuticals based on 99mTc. However, the chemistry of rhenium is sufficiently different from that of technetium so that the development of Re radiopharmaceuticals often cannot be predicated on the known chemistry and biological behavior of 99mTc radiopharmaceuticals. The relevant chemical differences involve the greater stability of the higher oxidation states of Re (and thus the greater tendency of reduced Re radiopharmaceuticals to undergo re-oxidation to perrhenate), and the greater substitution inertness of reduced Re complexes. These differences are illustrated in the preparation and use of 186Re (Sn)-HEDP and 99mTc(Sn)-HEDP diphosphonate radiopharmaceuticals designed, respectively, for palliative therapy and diagnosis of metastatic cancer to bone, and in the preparation and biodistribution of tr[186Re(DMPE)2Cl2]+ and [186Re(DMPE)3]+, analogs to the potential myocardial perfusion imaging agents tr-[99mTc(DMPE)2Cl2]+ and [99mTc(DMPE)3]+. [HEDP = (1-hydroxyethylidene)diphosphonate; DMPE = 1,2-bis(dimethylphosphino)ethane].

186Re-HEDP: a potential therapeutic bone agent.

A lyophilized kit preparation of HEDP for labelling with 186Re has been developed as a potential bone agent useful for both diagnosis and therapy. High pressure liquid chromatographic analysis showed that the 186Re-HEDP is a mixture of several components. Preliminary in vivo studies in several animal species showed that both the 186Re-HEDP mixture and the HPLC-purified main component are bone-seeking agents with slow blood and soft-tissue clearance rates. HPLC purification slightly improved the quality of the image. In mice and rats with trauma-induced osteogenetic activity in one leg, the bone uptake of the traumatized leg was 1.7-1.9 times higher than of the normal leg. The lesion-to-normal bone ratio was 4-5.4.

Myocardial scintigraphy with 99mTc-tris-DMPE in man.

Cardiac scintigraphy was performed in six patients with a documented previous myocardial infarction, in one patient with mitral regurgitation, and in four healthy volunteers following administration of 99mTc-tris-DMPE. An intense early blood pool phase permitted gated blood pool scintigraphy and left ventricular ejection fraction calculation. A myocardial phase 12-14 h later permitted myocardial perfusion imaging. The rest myocardial perfusion image quality with 99mTc-tris-DMPE appeared to be superior to the resting image quality obtained with 99mTc-dichloro-DMPE but was inferior to the resting image quality obtained with 201Tl.

Biodistribution of lipophilic 99mTc complexes of cyclam derivatives.

Several n-alkyl-cyclam derivatives were synthesized which form stable single component cationic chelates with 99mTc. These results suggest that the cyclam moiety in these derivatives complexes 99mTc in the same manner as the underivatized macrocycle. Biodistribution studies in mice show that all of these chelates are cleared from circulation by both the kidneys and liver. The ratio and rates of clearance by these organ systems is related to lipid solubility. None of the lypophilic-cationic-99mTc agents show any significant myocardial uptake. Also, these chelates show no significant ability to penetrate the blood-brain-barrier.