Preclinical studies of a PARP targeted, Meitner-Auger emitting, theranostic radiopharmaceutical for metastatic ovarian cancer.

Advanced ovarian cancer currently has few therapeutic options. Poly(ADP-ribose) polymerase (PARP) inhibitors bind to nuclear PARP and trap the protein-inhibitor complex to DNA. This work investigates a theranostic PARP inhibitor for targeted radiopharmaceutical therapy of ovarian cancer in vitro and PET imaging of healthy mice in vivo. METHODS: [77Br]RD1 was synthesized and assessed for pharmacokinetics and cytotoxicity in human and murine ovarian cancer cell lines. [76Br]RD1 biodistribution and organ uptake in healthy mice were quantified through longitudinal PET/CT imaging and ex vivo radioactivity measurements. Organ-level dosimetry following [76/77Br]RD1 administration was calculated using RAPID, an in-house platform for absorbed dose in mice, and OLINDA for equivalent and effective dose in human. RESULTS: The maximum specific binding (Bmax), equilibrium dissociation constant (Kd), and nonspecific binding slope (NS) were calculated for each cell line. These values were used to calculate the cell specific activity uptake for cell viability studies. The half maximal effective concentration (EC50) was measured as 0.17 (95 % CI: 0.13-0.24) nM and 0.46 (0.13-0.24) nM for PARP(+) and PARP(-) expressing cell lines, respectively. The EC50 was 0.27 (0.21-0.36) nM and 0.30 (0.22-0.41) nM for BRCA1(-) and BRCA1(+) expressing cell lines, respectively. When measuring the EC50 as a function of cellular activity uptake and nuclear dose, the EC50 ranges from 0.020 to 0.039 Bq/cell and 3.3-9.2 Gy, respectively. Excretion through the hepatobiliary and renal pathways were observed in mice, with liver uptake of 2.3 ± 0.4 %ID/g after 48 h, contributing to estimated absorbed dose values in mice of 19.3 ± 0.3 mGy/MBq and 290 ± 10 mGy/MBq for [77Br]RD1 and [76Br]RD1, respectively. CONCLUSION: [77Br]RD1 cytotoxicity was dependent on PARP expression and independent of BRCA1 status. The in vitro results suggest that [77Br]RD1 cytotoxicity is driven by the targeted Meitner-Auger electron (MAe) radiotherapeutic effect of the agent. Further studies investigating the theranostic potential, organ dose, and tumor uptake of [76/77Br]RD1 are warranted.

Intermetallic cobalt-gallium targets for production of germanium radioisotopes.

Increasing interest in targeted radionuclide therapy motivates the development of new radionuclides. The unique emission spectrum from 71Ge make it an ideal candidate for probing microdosimetric effects of low energy electrons absent confounding photon dose. This work reports a novel intermetallic target of Co and Ga for accelerator production of no-carrier-added 69/71Ge and a new method to isolate the Ge in high yields and purities.

Cyclotron production and radiochemical separation of 55Co and 58mCo from 54Fe, 58Ni and 57Fe targets.

This work presents the production with a cyclotron of the positron emitter 55Co via the 54Fe(d,n) and 58Ni(p,α) reactions and the Auger electron emitter 58mCo via the 57Fe(d,n) reaction after high current (40µA p and 60µA d) irradiation on electroplated targets. High specific activity radionuclides (up to 55.6 GBq/µmol 55Co and 31.8GBq/µmol 58mCo) with high radionuclidic purity (99.995% 55Co from 54Fe, 98.8% 55Co from 58Ni, and 98.7% 58mCo from 57Fe at end of bombardment, EoB), in high activity concentration (final separated radionuclide in < 0.6mL) and with almost quantitative overall activity separation yield (> 92%) were obtained after processing of the irradiated targets with novel radiochemical separation methods based on HCl dissolution and the resin N,N,N',N'-tetrakis-2-ethylhexyldiglycolamide (DGA, branched). One hour long irradiations using 38-65, 110-214 and 59-78mg of enriched 54Fe (99.93%), 58Ni (99.48%) and 57Fe (95.06%), respectively, electroplated over a 1.0cm2 surface, yielded 582 ± 66MBq 55Co, 372 ± 14MBq 55Co and 810 ± 186MBq 58mCo, respectively, decay corrected to EoB. The separation methods allow for the recovery of the costly enriched target materials, which were reconstituted into metallic targets after novel electroplating methods, with an overall recycling efficiency of 93 ± 4% for iron. The produced radionuclides were used to radiolabel the angiogenesis marker antibody TRC105 conjugated to the chelator NOTA as a demonstration of their quality.

Separation of 44Ti from proton irradiated scandium by using solid-phase extraction chromatography and design of 44Ti/44Sc generator system.

Scandium-44g (half-life 3.97h [1]) shows promise for positron emission tomography (PET) imaging of longer biological processes than that of the current gold standard, 18F, due to its favorable decay parameters. One source of 44gSc is the long-lived parent nuclide 44Ti (half-life 60.0 a). A 44Ti/44gSc generator would have the ability to provide radionuclidically pure 44gSc on a daily basis. The production of 44Ti via the 45Sc(p,2n) reaction requires high proton beam currents and long irradiation times. Recovery and purification of no-carrier added (nca) 44Ti from scandium metal targets involves complex separation chemistry. In this study, separation systems based on solid phase extraction chromatography were investigated, including branched diglycolamide (BDGA) resin and hydroxamate based ZR resin. Results indicate that ZR resin in HCl media represents an effective 44Ti/44gSc separation system.

Large scale accelerator production of 225Ac: Effective cross sections for 78-192MeV protons incident on 232Th targets.

Actinium-225 and 213Bi have been used successfully in targeted alpha therapy (TAT) in preclinical and clinical research. This paper is a continuation of research activities aiming to expand the availability of 225Ac. The high-energy proton spallation reaction on natural thorium metal targets has been utilized to produce millicurie quantities of 225Ac. The results of sixteen irradiation experiments of thorium metal at beam energies between 78 and 192MeV are summarized in this work. Irradiations have been conducted at Brookhaven National Laboratory (BNL) and Los Alamos National Laboratory (LANL), while target dissolution and processing was carried out at Oak Ridge National Laboratory (ORNL). Excitation functions for actinium and thorium isotopes, as well as for some of the fission products, are presented. The cross sections for production of 225Ac range from 3.6 to 16.7mb in the incident proton energy range of 78-192MeV. Based on these data, production of curie quantities of 225Ac is possible by irradiating a 5.0gcm-2 232Th target for 10 days in either BNL or LANL proton irradiation facilities.

Application of ion exchange and extraction chromatography to the separation of actinium from proton-irradiated thorium metal for analytical purposes.

Actinium-225 (t1/2=9.92d) is an α-emitting radionuclide with nuclear properties well-suited for use in targeted alpha therapy (TAT), a powerful treatment method for malignant tumors. Actinium-225 can also be utilized as a generator for (213)Bi (t1/2 45.6 min), which is another valuable candidate for TAT. Actinium-225 can be produced via proton irradiation of thorium metal; however, long-lived (227)Ac (t1/2=21.8a, 99% ß(-), 1% α) is co-produced during this process and will impact the quality of the final product. Thus, accurate assays are needed to determine the (225)Ac/(227)Ac ratio, which is dependent on beam energy, irradiation time and target design. Accurate actinium assays, in turn, require efficient separation of actinium isotopes from both the Th matrix and highly radioactive activation by-products, especially radiolanthanides formed from proton-induced fission. In this study, we introduce a novel, selective chromatographic technique for the recovery and purification of actinium isotopes from irradiated Th matrices. A two-step sequence of cation exchange and extraction chromatography was implemented. Radiolanthanides were quantitatively removed from Ac, and no non-Ac radionuclidic impurities were detected in the final Ac fraction. An (225)Ac spike added prior to separation was recovered at ≥ 98%, and Ac decontamination from Th was found to be ≥ 10(6). The purified actinium fraction allowed for highly accurate (227)Ac determination at analytical scales, i.e., at (227)Ac activities of 1-100 kBq (27 nCi to 2.7 µCi).

Cyclotron produced 44gSc from natural calcium.

(44g)Sc was produced by 16MeV proton irradiation of unenriched calcium metal with radionuclidic purity greater than 95%. The thick target yield at saturation for (44g)Sc was 213 MBq/µA, dwarfing the yields of contaminants (43)Sc,(44 m)Sc, (47)Sc and (48)Sc for practical bombardment times of 1-2h. Scandium was isolated from the dissolved calcium target by filtration, and reconstituted in small volumes of dilute HCl. Reactions with the chelate 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) indicated a reactivity of 54 ± 14 Gbq/µmol at end-of-bombardment.

Very high specific activity 66/68Ga from zinc targets for PET.

This work describes the production of very high specific activity (66/68)Ga from (nat)Zn(p,n) and (66)Zn(p,n) using proton irradiations between 7 and 16 MeV, with emphasis on (66)Ga for use with common bifunctional chelates. Principal radiometallic impurities are (65)Zn from (p,x) and (67)Ga from (p,n). Separation of radiogallium from target material is accomplished with cation exchange chromatography in hydrochloric acid solution. Efficient recycling of Zn target material is possible using electrodeposition of Zn from its chloride form, but these measures are not necessary to achieve high specific activity or near-quantitative radiolabeling yields from natural targets. Inductively coupled plasma mass spectroscopy (ICP-MS) measures less than 2 ppb non-radioactive gallium in the final product, and the reactivity of (66)Ga with common bifunctional chelates, decay corrected to the end of irradiation, is 740 GBq/µmol (20 Ci/µmol) using natural zinc as a target material. Recycling enriched (66)Zn targets increased the reactivity of (66)Ga with common bifunctional chelates.

Cross sections of the 36Ar(d,α)34mCl, 40Ar(d,α)38Cl, and 40Ar(d,p)41Ar nuclear reactions below 8.4 MeV.

We have measured the cross section for production of the medically interesting isotope (34m)Cl, along with (38)Cl and (41)Ar, using deuteron bombardments of (36)Ar and (40)Ar below 8.4 MeV. ALICE/ASH analytical codes were employed to determine the shape of nuclear excitation functions, and experiments were performed using the University of Wisconsin tandem electrostatic accelerator to irradiate thin targets of argon gas.

Specific α4ß2 nicotinic acetylcholine receptor binding of [F-18]nifene in the rhesus monkey.

OBJECTIVE: [F-18]Nifene is a PET radioligand developed to image α4ß2* nicotinic acetylcholine receptors (nAChR) in the brain. This work assesses the in vivo binding and imaging characteristics of [F-18]nifene in rhesus monkeys for the development of PET experiments examining nAChR binding. METHODS: Dynamic PET imaging experiments with [F-18]nifene were acquired in four anesthetized Macaca mulatta (rhesus) monkeys using a microPET P4 scanner. Data acquisition was initiated with a bolus injection of 109 ± 17 MBq [F-18]nifene and the time course of the radioligand in the brain was measured for up to 120 min. For two experiments, a displacement dose of (-)nicotine (0.03 mg kg(-1) , i.v.) was given 45-60 min post injection and followed 30 min later with a second [F-18]nifene injection to measure radioligand nondisplaceable uptake. Time activity curves were extracted in the regions of the antereoventral thalamus (AVT), lateral geniculate nucleus region (LGN), frontal cortex, and the cerebellum (CB). RESULTS: The highest levels of [F-18]nifene uptake were observed in the AVT and LGN. Target-to-CB ratios reached maximum values of 3.3 ± 0.4 in the AVT and 3.2 ± 0.3 in the LGN 30-45 min postinjection. Significant binding of [F-18]nifene was observed in the subiculum, insula cortex, temporal cortex, cingulate gyrus, frontal cortex, striatum, and midbrain areas. The (-)nicotine displaced bound [F-18]nifene to near background levels within 15 min postdrug injection. No discernable displacement was observed in the CB, suggesting its potential as a reference region. Logan graphical estimates using the CB as a reference region yielded binding potentials of 1.6 ± 0.2 in the AVT and 1.3 ± 0.1 in the LGN. The postnicotine injection displayed uniform nondisplaceable uptake of [F-18]nifene throughout gray and white brain matter. CONCLUSIONS: [F-18]Nifene exhibits rapid equilibration and a moderately high target to background binding profile in the α4ß2* nAChR rich regions of the brain, thus providing favorable imaging characteristics as a PET radiotracer for nAChR assay.

In vivo kinetics of [F-18]MEFWAY: a comparison with [C-11]WAY100635 and [F-18]MPPF in the nonhuman primate.

UNLABELLED: [F-18]Mefway was developed to provide an F-18 labeled positron emission tomography (PET) neuroligand with high affinity for the serotonin 5-HT(1A) receptor to improve the in vivo assessment of the 5-HT(1A) system. The goal of this work was to compare the in vivo kinetics of [F-18]mefway, [F-18]MPPF, and [C-11]WAY100635 in the rhesus monkey. METHODS: Each of four monkeys were given bolus injections of [F-18]mefway, [C-11]WAY100635, and [F-18]MPPF and scans were acquired with a microPET P4 scanner. Arterial blood was sampled to assay parent compound throughout the time course of the PET experiment. Time activity curves were extracted in the high 5-HT(1A) binding areas of the anterior cingulate cortex (ACG), mesial temporal cortex, raphe nuclei, and insula cortex. Time activity curves were also extracted in the cerebellum, which was used as a reference region. The in vivo kinetics of the radiotracers were compared based on the nondisplaceable distribution volume (V(ND) ) and binding potential (BP(ND) ). RESULTS: At 30 min, the fraction of radioactivity in the plasma due to parent compound was 19%, 28%, and 29% and cleared from the arterial plasma at rates of 0.0031, 0.0078, and 0.0069 (min⁻¹) ([F-18]mefway, [F-18]MPPF, [C-11]WAY100635). The BP(ND) in the brain regions were mesial temporal cortex: 7.4 ± 0.6, 3.1 ± 0.4, 7.0 ± 1.2, ACG: 7.2 ± 1.2, 2.1 ± 0.2, 7.9 ± 1.2; raphe nuclei: 3.7 ± 0.6, 1.3 ± 0.3, 3.3 ± 0.7; and insula cortex: 4.2 ± 0.6, 1.2 ± 0.1, 4.7 ± 1.0 for [F-18]mefway, [F-18]MPPF, and [C-11]WAY100635 respectively. CONCLUSIONS: In the rhesus monkey, [F-18]mefway has similar in vivo kinetics to [C-11]WAY100635 and yields greater than 2-fold higher BP(ND) than [F-18]MPPF. These properties make [F-18]mefway a promising radiotracer for 5-HT(1A) assay, providing higher counting statistics and a greater dynamic range in BP(ND).