Experimental studies and nuclear model calculations on proton-induced reactions on Se, Se and Se with particular reference to the production of the medically interesting radionuclides Br and Br

Excitation functions of the reactions ^natSe(p,x)^75,76,77,82Br, ⁷⁶Se(p,xn)^75,76Br, ⁷⁶Se(p,x)⁷⁵Se and ⁷⁷Se(p,xn)^76,77Br were measured from their respective thresholds up to 40 MeV, with particular emphasis on data for the production of the medically important radionuclides ⁷⁶Br and ⁷⁷Br. The conventional stacked-foil technique was used. The samples were prepared by a sedimentation process. Irradiations were performed using the compact cyclotron CV 28 and the injector of COSY, both at the Research Centre Jülich. In order to validate the data, nuclear model calculations were performed using the code ALICE-IPPE which is based on the preequilibrium-evaporation model. Good agreement was found between the experimental and theoretical data, except in the high-energy region where the calculated data were somewhat higher. All the measured excitation curves were compared with the data available in the literature. From the experimental data the theoretical yields of all the investigated radionuclides were calculated and plotted as a function of proton energy. The calculated yield of ⁷⁷Br from the ^natSe(p,x)⁷⁷Br process over the energy range E_p=25→15 is 72.7 MBq/μA h and from the ⁷⁷Se(p,n)⁷⁷Br reaction over E_p=15→6 MeV it is 86.2 MBq/μA h. The yield of ⁷⁶Br from the ⁷⁶Se(p,n)⁷⁶Br reaction for E_p=15→8 is 360.1 MBq/μA h and from the ⁷⁷Se(p,2n)⁷⁶Br reaction for E_p=28→18 MeV it is 879.2 MBq/μA h. The radionuclidic impurity levels are discussed.     

The Br(p,x)Se nuclear processes: a convenient route for the production of radioselenium tracers relevant to amino acid labelling

A possible route for the production of no-carrier-added (n.c.a.) ⁷³Se (T_1/2=7.1 h) and ⁷⁵Se (120 d) is introduced. -2-Amino-4-([⁷³Se]methyl-seleno) butanoic acid ( -[⁷³Se]selenomethionine) with an overall radiochemical yield of >40% could be prepared via a 3-step polymer-supported synthesis after successful separation of ⁷³Se from KBr targets. Excitation functions for the ^natBr(p,x) ^72,73,75Se processes were measured from threshold up to 100 MeV utilizing pellets of pressed KBr. Targets were irradiated at the NAC cyclotron with proton beams having primary energies of 40.4, 66.8 and 100.9 MeV. The calculated ⁷³Se yield (EOB) for 1 h irradiation in 1 μA of beam at the optimum proton energy range of 62→42 MeV is 81.4 MBq (2.2 mCi), and the calculated ⁷⁵Se yield (EOB) for the overall range 62 MeV→threshold for the same irradiation conditions is 0.97 MBq (0.026 mCi).     

Preparation of high specific activity (86)Y using a small biomedical cyclotron

⁸⁶Y is an attractive PET radionuclide due to its intermediate half-life. ⁸⁶Y was produced via the ⁸⁶Sr(p,n)⁸⁶Y nuclear reaction. Enriched SrCO₃ or SrO was irradiated with 2-6 μA of beam current for <4 h on a CS-15 cyclotron. It was shown that the SrO target could withstand at least 6 μA of beam current, a significant improvement over a maximum of 2 μA on the SrCO₃ target. Average yields of 4.5 mCi/μA·h were achieved with SrO, which represent 71% of the theoretical yield, compared to 2.3 mCi/μA·h with SrCO₃. The radioisotopic contaminants were ^86mY (220%), ⁸⁷Y (0.27%), ^87mY (0.43%) and ⁸⁸Y (0.024%). ⁸⁶Y was isolated in an electrochemical cell consisting of three Pt electrodes. The solution was electrolyzed at 2000 mA (40 min) using two Pt plate electrodes. A second electrolysis (230 mA for 20 min) was performed using one Pt plate and a Pt wire. On average, 97.1% of the ⁸⁶Y was recollected on the Pt wire after a second electrolysis. The ⁸⁶Y was collected from the Pt wire using 2.8 M HNO₃/EtOH (3:1). After evaporation, ⁸⁶Y was reconstituted in 100 μl of 0.1 M HCl. Target materials were recovered as SrCO₃ and then converted to SrO by thermal decomposition at 1150°C. Specific activity of ⁸⁶Y was determined to be 29±19 mCi/μg via titration of ⁸⁶Y(OAc)₃ with DOTA or DTPA. We have established techniques for the routine, economical production of high purity, high specific activity ⁸⁶Y on a small biomedical cyclotron that are translatable to other institutions.     

Optimized indirect (76)Br-bromination of antibodies using N-succinimidyl para-[76Br]bromobenzoate for radioimmuno PET

Monoclonal antibody 38S1 was radiobrominated with the positron emitter ⁷⁶Br (T_1/2= 16.2 h). Indirect labeling was performed using N-succinimidyl para-(tri-methylstannyl)benzoate (SPMB) as the precursor molecule. SPMB was labeled using Chloramine-T yielding N-succinimidyl para-[⁷⁶Br]bromobenzoate, which was then conjugated to the antibody. Optimization of the labeling conditions and further conjugation gave a total yield ( mean±max error) of 49±2%. The immunoreactivity of the antibodies was retained after labeling. Thus, antibodies intended for positron emission tomography can be labeled with ⁷⁶Br, which gives high yields and preserved immunoreactivity when using the SPMB technique described.     

Evaluation of novel cationic 99mTc(I)-tricarbonyl complexes as potential radiotracers for myocardial perfusion imaging

This report describes the evaluation of three cationic ^99mTc(I)–tricarbonyl complexes — [^99mTc(CO)₃(L)]⁺ (L=N-methoxyethyl-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (ME-PNP), N-[15-crown-5)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (15C5-PNP) and N-[18-crown-6)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (18C6-PNP)) — as potential radiotracers for myocardial perfusion imaging. Biodistribution, imaging and metabolism studies were performed using Sprague–Dawley rats. It was found that bisphosphine ligands have a significant impact on the biodistribution characteristics and clearance kinetics of their cationic ^99mTc(I)–tricarbonyl complexes. Among the three radiotracers evaluated in this study, [^99mTc(CO)₃(15C5-PNP)]⁺ has a very high initial heart uptake and is retained in the rat myocardium for >2 h. It also shows rapid clearance from the liver and lungs. The heart/liver ratio of [^99mTc(CO)₃(15C5-PNP)]⁺ is 2.5 times better than that of ^99mTc-sestamibi at 30 min postinjection. [^99mTc(CO)₃(15C5-PNP)]⁺ is almost identical to ^99mTcN-DBODC5 with respect to heart uptake, heart/lung ratio and heart/liver ratio. Results from metabolism studies show that there is no significant metabolism for [^99mTc(CO)₃(15C5-PNP)]⁺ in the urine, but it does show a small metabolite peak (<10%) in the radio high-performance liquid chromatography chromatogram of the feces sample at 120 min postinjection. Results planar imaging studies demonstrate that [^99mTc(CO)₃(15C5-PNP)]⁺ has a much better liver clearance profile than ^99mTc-sestamibi and might give clinically useful images of the heart as early as 30 min postinjection. [^99mTc(CO)₃(15C5-PNP)]⁺ is a very promising candidate for more preclinical evaluations in various animal models.     

High yield direct Br-bromination of monoclonal antibodies using chloramine-T

Monoclonal antibody (MAb) A33 was labeled with the positron emitter ⁷⁶Br ( T_1/2=16.2 h). Direct labeling was done using the conventional chloramine-T method. After optimization of the labeling conditions, a maximum yield (mean ± max error) of 77 ± 2% was obtained at pH 6.8. In vitro binding of ⁷⁶Br-A33 to SW1222 colonic cancer cells showed that the immunoreactivity was retained. Also, the MAbs 38S1 and 3S193 and the peptide hEGF were ⁷⁶Br-labeled, resulting in labeling yields (mean ± max error) of 75 ± 3%, 63 ± 4%, and 73 ± 0.1%, respectively. We conclude that antibodies and peptides can be labeled conveniently with ⁷⁶Br for the purpose of whole-body tumour imaging by positron emission tomography.     

I-labelling of an internally Se-labelled monoclonal antibody—Biodistribution in tumour-bearing nude mice

A monoclonal antibody, C215, was first internally labelled with ⁷⁵Se-methionine and then labelled with ¹²⁵I. The biodistribution of the dual-labelled [¹²⁵I][⁷⁵Se]C215 was studied in tumour-bearing nude mice killed 3 days after injection. The biodistribution of the dual-labelled [¹²⁵I][⁷⁵Se]C215 was compared with the biodistribution of single-labelled [¹³¹I]C215 and [⁷⁵Se]C215. Iodine-labelled antibodies seem to be damaged during iodination, affecting the disappearance rate and tumour uptake. There were no signs of dehalogenation of circulating antibodies or antibodies taken up in the tumour.     

Experimental studies on the proton-induced activation reactions of molybdenum in the energy range 22–67 MeV

The production cross-sections of ^99,93mMo, ^96,95,95m,94Tc, ^96,95,92m,90Nb, ^89,88,86Zr and ^88,87,86Y radionuclides for proton-induced reactions on molybdenum were measured with molybdenum targets of natural isotopic composition using a stacked-foil activation technique in the energy range 22–67 MeV. The thick target integral yields were also deduced for each reaction using the measured cross-sections from the respective threshold up to 67 MeV. The results have given new data for all of the investigated radionuclides. The results of the present experiment showed excellent agreement with the earlier reported data in the lower energy region.     

Determination of specific radioactivity for (76)Br-labeled compounds measuring the ratio between (76)Br and (79)Br using packed capillary liquid chromatography mass spectrometry

Packed capillary liquid chromatography with electrospray mass spectrometry was used for direct determination of the specific radioactivity by calculation of isotope ratios between the ⁷⁶Br- and ⁷⁹Br-labeled analogues of N-((3-aminomethyl)benzyl)-4-bromobenzamide. Using 20 μL injections on packed capillary columns, sufficient mass sensitivity was attained for the determination on an injected amount of radioactivity corresponding to approximately 2 MBq (0.3 pmol of the ⁷⁶Br isotopic analogue).     

Molecular recognition and stability of Tc-UBI 29–41 based on experimental and semiempirical results

^99mTc-UBI 29–41 is an antimicrobial peptide fragment that directly radiolabeled with ^99mTc shows high in vitro and in vivo stability, rapid background clearance, minimal accumulation in non-target tissues and rapid detection of infection sites. Molecular mechanics (MM) calculation has been an essential tool in explaining experimental results associated with molecular recognition and stability. This work is an attempt to explain the ^99mTc-UBI 29–41 specificity for bacteria and to understand from a structural point of view, the experimental results indicative of a molecular recognition and stability not well favored for two other cationic peptides (^99mTc-Tat-1-Scr and ^99mTc-Tat-2-Scr ) used as control. Structures of ^99mTc-UBI, ^99mTc-Tat-1-Scr, ^99mTc-Tat-2-Scr and of the corresponding free cationic peptides were built and the optimized structures, in the best stable configurations, were calculated by a MM procedure. In order to correlate the calculated and experimental results, in vitro stability tests with cysteine challenge and stability to dilution in human serum and in saline solution, were performed for the three labeled cationic peptides. The three complexes can be represented by the general formula [Tc(V)(O)(H₂O)₂(Lys_n=1,2-Arg_n=0,1-peptide)]^10+,11+. The potential energies were 104.5, 95.6 and 90.8 kcal/mol for ^99mTc-Tat-1-Scr, ^99mTc-Tat-2-Scr and ^99mTc-UBI 29–41, respectively. Experimental and calculated results were in good agreement. It is thus possible to predict and explain that in similar solution media ^99mTc-Tat-2-Scr would be more stable than ^99mTc-Tat-1-Scr and why ^99mTc-UBI shows the highest stability. In conclusion, the in vitro specific binding to bacteria and the accumulation at infection sites in humans of ^99mTc-labeled UBI could be the result of its high thermodynamic stability, selectivity and stereospecificity.     

New renal function imaging agents. I. Synthesis and biological characterization of a [Tc]diaminomercapto(thio)ether (DAMTE)

The present study describes the synthesis of a [^99mTc]diaminomercapto(thio)ether (DAMTE-derivative) as a first compound of a new class of ^99mTc-complexes which is tubular excreted.

10-Benzoyl-8-keto-7-aza-2-amino-4,10-dithia-decanoic acid (CO₂-DAMTE 3) was synthesized by the reaction of succinimidyl-S-benzoyl-thioglycolate and (S)-2-aminoethyl- -cysteine. The respective technetium complex, ^99mTc---CO₂---DAMTE was obtained in radiochemical yields of about 70% using stannous chloride as reducing agent. Hydrolysis of the protecting group was performed either prior to the complexation of pertechnetate (“cold kit”) or during the labelling reaction (“hot kit”). Organ distribution was determined in Wistar rats. Within 24 h 40% of the activity were excreted into the feces and 43% into the urine, whereas 10% were retained in the kidneys. In contrast, a first human study showed a very fast renal elimination of ^99mTc---CO₂---DAMTE, a low liver uptake (< 10%) and no retention in the kidneys. The renal clearance of approx. 240 mL/min/1.73 m² in addition to the protein binding of > 95% suggests an effective tubular excretion of the compound.     

A re-evaluation of k0 and related nuclear data for the 555.8 keV gamma-line emitted by the Rh-Rh mother–daughter pair for use in NAA

A re-evaluation is made of the k₀-factor and related nuclear data for the 555.8 keV gamma-ray of the ^104mRh-¹⁰⁴Rh mother–daughter pair that are important in neutron activation analysis (NAA). This study considers that the relevant level is also fed by the 4.34 min ^104mRh mother (with an absolute gamma-ray emission probability γ₂=0.13%) and not only, as assumed in former work, by the 42.3 s ¹⁰⁴Rh daughter isotope (with γ₃=2.0%). In view of this, generalised equations were developed for both the experimental determination and the analytical use of the k₀-factor and of the associated parameters k₀(m)/k₀(g), Q₀(m) and Q₀(g) [(m):^104mRh; (g): ¹⁰⁴Rh], requiring the introduction of the γ₂ and γ₃ data and also of the ^104mRh→¹⁰⁴Rh fractional decay factor F₂(=0.9987). The experimental determinations were based on irradiations performed in the BR1 reactor in Mol and the WWR-M reactor in Budapest. Furthermore, considering the special formation of the 555.8 keV gamma-ray, the procedure for true-coincidence correction was revised as well. All this led to the compilation and recommendation of a new set of ‘k₀-NAA’ data.     

(E)-[125I]-5-AOIBV: a SPECT radioligand for the vesicular acetylcholine transporter

The premise that, over the course of Alzheimer's disease (AD), changes in the levels of the vesicular acetylcholine transporter (VAChT) occur in parallel with changes to other cholinergic marker proteins provides the basis for the applicability of benzovesamicol derivatives as radioligands for AD studies by single photon emission computed tomography or positron emission tomography. We report the synthesis of enantiopure benzovesamicol derivatives: (R,R) or (S,S)-(E)-2-hydroxy-5-(3-iodoprop-2-en-1-oxy)-3-(4-phenylpiperidino)tetralin [(R,R)-AOIBV: K_d=0.45 nM or (S,S)-5-AOIBV: K_d=4.3 nM] and their corresponding tributyltin precursors for radioiodination. (R,R or S,S)-5-AOIBV was labeled with iodine-125 from their corresponding n-tributyltin precursors. Both compounds were obtained with radiochemical and optical purity greater than 97% and in radiochemical yields ranging 34–36%. To determine if these compounds could provide an advantage when compared to [¹²⁵I]-iodo benzovesamicol (IBVM), IBVM was also labeled and used as the reference compound in all ex vivo experiments. Ex vivo biodistribution experiments in rats revealed that [¹²⁵I]-(R,R)-5-AOIBV displayed the most suitable pharmacological profile as the radioactivity distribution corresponded well with the known VAChT brain density. Moreover, pre-injection of vesamicol prevented the uptake of [¹²⁵I]-(R,R)-5-AOIBV in striatum, cortex and hippocampus, demonstrating selectivity for the VAChT. However, even if time activity curves of [¹²⁵I]-(R,R)-5-AOIBV confirmed that this compound could be used to visualize the VAChT in vivo, at each point of the kinetic study, [¹²⁵I]-(R,R)-5-AOIBV showed a lower specific binding compared to [¹²⁵I]-IBVM. These results made [¹²⁵I]-( R,R)-5-AOIBV inferior to [¹²⁵I]-IBVM for the VAChT exploration in vivo.     

Oxotechnetium 99mTcO[SN(R)S][S] complexes as potential 5-HT1A receptor imaging agents

A series of ^99mTcO[SN(R)S][S] complexes carrying the 1-(2-methoxyphenyl)piperazine moiety on the tridentate ligand [SN(R)S] was synthesized. For structural characterization and for in vitro binding assays the analogous oxorhenium complexes were prepared. As demonstrated by appropriate competition binding tests in rat hippocampal preparations, all oxorhenium analogues showed affinity for the 5-HT_1A receptor binding sites with IC₅₀ values at the nanomolar range (IC₅₀= 5.8–103 nM). All ^99mTcO[SN(R)S]/[S] complexes showed significant brain uptake in rats at 2 min p.i. (0.24–1.31% ID). However, a clear correlation between distribution of radioactivity in the brain and distribution of 5-HT_1A receptors could not be established.     

In vivo evaluation of copper-64-labeled monooxo-tetraazamacrocyclic ligands

Copper-64 (T_1/2=12.7 h; β⁺: 0.653 MeV, 17.4%; β⁻: 0.578 MeV, 39%) has applications in positron emission tomography (PET) imaging and radiotherapy, and is conveniently produced on a biomedical cyclotron. Tetraazamacrocyclic ligands are the most widely used bifunctional chelators (BFCs) for attaching copper radionuclides to antibodies and peptides due to their relatively high kinetic stability. In this paper, we evaluated three monooxo-tetraazamacrocyclic ligands with different ring sizes and oxo group positions. H1 [1,4,7,10-tetraazacyclotridecan–11-one], H2 [1,4,8,11-tetraazacyclotetradecan-5-one] and H3 [1,4,7,10-tetraazacyclotridecan-2-one] were radiolabeled with ⁶⁴Cu in high radiochemical yields under mild conditions. The three ⁶⁴Cu-labeled complexes are all +1 charged, as determined by their electrophoretic mobility. While they demonstrated >95% stability in rat serum out to 24 h, both biodistribution and microPET imaging studies revealed high uptake and long retention of the compounds in major clearance organs (e.g., blood, liver and kidney), which suggests that ⁶⁴Cu dissociated from the complexes in vivo. Of the three complexes, ⁶⁴Cu-2⁺, which has a cyclam backbone (1,4,8,11-tetraazacyclotetradecane), exhibited the lowest nontarget organ accumulation. The data from these studies may invalidate the candidacy of the monooxo-tetraazamacrocyclics as BFCs for copper radiopharmaceuticals. However, the data presented here suggest that neutral or negatively charged Cu(II) complexes of tetraazamacrocyclic ligands with a cyclam backbone (tetradecane) are optimal for copper radiopharmaceutical applications.     

An improved (99m)Tc-aprotinin kit formulation: quality control analysis of radiotracer stability and cold kit shelf life

^99mTc-aprotinin scintigraphy has been demonstrated to be a useful noninvasive imaging technique for amyloid deposits located in extraabdominal regions of patients. The aim of this study was to develop an improved aprotinin cold kit formulation, to validate the kit for long-term stability, as well as to assess the radiotracer stability by novel quality control methods. The aprotinin cold kit formulation of Trasylol, pyrophosphate (PYP)-chelated stannous reductant and an alkaline buffer, was dispensed into nitrogen-filled vials and aliquots frozen at −20°C. After 0, 1, 2, 3 and 6 months of storage, three samples were reconstituted with 750–850 MBq of ^99mTc-pertechnetate, followed by quality control analyses by paper chromatography methods at 25, 85 and 265 min postreconstitution (pr). Cation-exchange cartridge quality control methods were also investigated. The cold kits proved to be stable to long-term storage for up to 6 months, and the radiotracer was stable for at least 4 h pr. ^99mTc-aprotinin was formed at greater than 95% efficiency at all time points tested with ^99mTcO₂ present as the major impurity (1–4%) and ^99mTc-pertechnetate and ^99mTc-PYP present in trace amounts. An alternative, rapid, safe and reliable method was found in Oasis MCX–BSA-treated cartridges using saline as the eluting solution to assay for ^99mTc-aprotinin.     

A CD(4)/T(4) receptor peptide ligand labeled with technetium-99m: synthesis and biological activity

The octapeptide D-Ala-Ser-Thr-Thr-Thr-Asn-Tyr-Thr-NH₂ ([D-Ala¹]TNH₂), an analog of peptide T (H-Ala-Ser-Thr-Thr-Thr-Asn-Tyr-Thr-OH) associated with CD₄/T₄ receptors involved in human immunodeficiency virus infection, was combined with the chelating polyazamacrocycle 1,4,8,11-tetraazacyclotetradecane (cyclam) to afford the bifunctional ligand cyc-[D-Ala¹]TNH₂. This was then reacted with [^99mTcO₄]⁻ and Sn²⁺ to yield the monocationic complex [^99mTc(O)₂(cyc-[D-Ala¹]TNH₂)]⁺. Biological activity of both the cyclam-peptide conjugate and the resulting Tc-99m complex were evaluated by measuring their chemotactic indexes. Results showed that N-cyclam acylation and subsequent labeling with Tc-99m of [D-Ala¹]TNH₂ were tolerated, and both cyc-[D-Ala¹]TNH₂ and [^99mTc(O)₂(cyc-[D-Ala¹]TNH₂)]⁺ retained the high chemotactic capacity of the original octapeptide. Biodistribution of the Tc-99m complex was carried out in rats. Fast blood clearance and no accumulation in organs of interest were observed.     

Biodistribution of 225Ra citrate in mice: retention of daughter radioisotopes in bone

Alpha-particle-emitting radionuclides have potential for therapy of localized disease due to their high linear energy transformation and short pathlengths. Radiometals that home naturally to bone can be exploited for this purpose, and ²²³Ra (t_1/2=11.4 days) recently has been studied for therapy of bone tumors in mice and rats. Actinium-225 (t_1/2=10 days) is also an attractive radioisotope for endoradiotherapy. In a single decay of a ²²⁵Ac nucleus and its subsequent decay daughters, over 27 MeV (90% of total energy) is released by sequential emission of four particles, ranging in energy from 5.7 to 8.4 MeV. Although Ac³⁺ does not home naturally to bone, its parent radioisotope ²²⁵Ra (β⁻, t_1/2=15 days) can be used as an in vivo source for ²²⁵Ac. Thus, injection of ²²⁵Ra takes advantage of the bone-homing properties of radium coupled with the significant amount of energy released from the ²²⁵Ac decay chain. Our data confirm that a large fraction of radium citrate injected intravenously into mice localizes rapidly in bone. Injected doses per gram (ID/g) for ²²⁵Ra range from 25% in skull to about 10% in sternum. Once deposited, the ²²⁵Ra remains in the bone with a biological half life of >40 days. Furthermore, >95% of the daughter radioisotope, ²²⁵Ac, is retained in the bone. However, a significant fraction of one of the daughter radioisotopes, ²¹³Bi, is found in kidney. The biodistribution data indicate that ²²⁵Ra injection should be a powerful agent for killing cells associated with bone; however, the toxicity of this radioisotope which is similar to that of other emitters limits the dose that can be tolerated.     

Alternative quality control for technetium-99m IDA complexes

A quality control procedure for ^99mTc-IDA complexes based on the use of C₁₈ Sep-pak^TM cartridges is developed and the validation of the procedure presented. C₁₈ Sep-pak^TM cartridges are pretreated by washing with 95% ethanol followed by 10⁻³ N hydrochloric acid. A small amount of the ^99mTc-IDA complex is applied, washed with 10⁻³ N hydrochloric acid and eluted with 95% ethanol. The radiochemical purity values obtained for ^99mTc-mebrofenin and ^99mTc-disofenin using this Sep-pak^TM procedure are comparable to those obtained using the standard two strip (ITLC-SG/100% methanol, ITLC-SA/20% saline) procedure.