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. Author manuscript; available in PMC: 2018 Apr 1.
Published in final edited form as: Nucl Med Biol. 2016 Dec 27;47:48–55. doi: 10.1016/j.nucmedbio.2016.12.008

Monoanionic 99mTc-tricarbonyl-aminopolycarboxylate complexes with uncharged pendant groups: Radiosynthesis and evaluation as potential renal tubular tracers

Malgorzata Lipowska a,*, Jeffrey Klenc a, Nashwa Jarkas a, Luigi G Marzilli b, Andrew T Taylor a
PMCID: PMC5340577  NIHMSID: NIHMS845494  PMID: 28110124

Abstract

Introduction

99mTc(CO)3-nitrilotriacetic acid, 99mTc(CO)3(NTA), is a new renal tubular agent with pharmacokinetic properties comparable to those of 131I-OIH but the clearance of 99mTc(CO)3(NTA) and 131I-OIH are still less than the clearance of PAH, the gold standard for the measurement of effective renal plasma flow. At physiological pH, dianionic 99mTc(CO)3(NTA) has a mononegative inner metal-coordination sphere and a mononegative uncoordinated carboxyl group. To evaluated alternate synthetic approaches, we assessed the importance of an uncoordinated carboxyl group, long considered essential for tubular transport, by evaluating the pharmacokinetics of three analogs with the 99mTc(CO)3(NTA) metal-coordination sphere but with uncharged pendant groups.

Methods

99mTc(CO)3 complexes with N-(2-acetamido)iminodiacetic acid (ADA), N-(2-hydroxyethyl)iminodiacetic acid (HDA) and N-(fluoroethyl)iminodiacetic acid (FEDA) were prepared using a tricarbonyl kit and isolated by HPLC. The pharmacokinetics were evaluated in Sprague-Dawley rats, with 131I-OIH as an internal control; urine was analyzed for metabolites. Plasma protein binding and erythrocyte uptake were determined from the 10 min blood samples. Re(CO)3(FEDA), the analog of 99mTc(CO)3(FEDA), was prepared and characterized.

Results

99mTc(CO)3(ADA), 99mTc(CO)3(HDA) and 99mTc(CO)3(FEDA) were efficiently prepared as a single species with high radiochemical purities (>99%). These new monoanionic 99mTc(CO)3 tracers with uncharged dangling groups all showed rapid blood clearance and high specificity for renal excretion. Activity in the urine, as a percent of 131I-OIH at 10 and 60 min, was 96% and 99% for ADA, 96% and 100% for HDA, and 100% and 99% for FEDA, respectively. Each new tracer was excreted unchanged in the urine. The Re(CO)3(FEDA) structure adds compelling evidence that such 99mTc(CO)3(NTA) analogs have metal-coordination spheres identical to that of 99mTc(CO)3(NTA).

Conclusions

New tracers lacking the negatively charged pendant carboxyl group previously thought to be essential for rapid renal extraction, 99mTc(CO)3(ADA), 99mTc(CO)3(HDA) and 99mTc(CO)3(FEDA), exhibit pharmacokinetics in rats comparable to those of 99mTc(CO)3(NTA) and 131I-OIH. Furthermore, these encouraging results in rats warrant evaluation of this new tracer type in humans.

Keywords: 99mTc-tricarbonyl, Renal radiopharmaceutical, Kidney, Biodistribution

1. Introduction

Non-invasive renal scintigraphy has an important role in the determination of relative and absolute renal function and in the evaluation of patients with suspected obstruction or renovascular hypertension. The accuracy of scan interpretation and camera-based measurements of renal function are optimized by a high renal extraction efficiency, providing a rapid renal clearance and high target-to-background ratio. Because only 20% of renal plasma flow in humans is filtered, the extraction efficiency of glomerular filtration rate (GFR) tracers such as 99mTc-diethylenetriaminepentaacetic acid (99mTc-DTPA) cannot be greater than 20%; consequently, to achieve a high extraction efficiency, it is necessary to design new tracers that are extracted by the renal tubules in a manner similar to that for 131I-o-iodohippurate (131I-OIH), the radioactive standard for the measurement of effective renal plasma flow (ERPF) [1–3]. 99mTc-based renal agents have attracted the most interest owing to the widespread availability of 99mTc and its superior nuclear medicine properties (t1/2 = 6 h, 140 keV γ-radiation) over the beta emission, 8-day half-life and 364 keV photon of 131I. 99mTc-mercaptoacetyltriglycine (99mTc-MAG3) is currently the best available commercial replacement for 131I-OIH, and it is used in approximately 70% of all radionuclide renal scans performed in the United States [4–7]. 99mTc-MAG3 is almost exclusively cleared from plasma by the tubular secretion and by the same organic anion transporter (OAT1) system as 131I-OIH [8, 9], although its clearance in humans is only ~ 60% that of 131I-OIH [4, 5]. A number of other alternatives have been proposed over the decades, including 99mTc-ethylenedicysteine (99mTc-EC) [10, 11] and 99mTc-mercaptoacetamide-ethylenecysteine (99mTc-MAEC) [12]; the 99mTc-EC and 99mTc-MAEC renal clearances exceed that of 99mTc-MAG3 but are still lower than that of 131I-OIH.

99mTc-MAG3, 99mTc-EC and 99mTc-MAEC all have a 99mTcVO core and a dangling arm with a carboxyl group, which is deprotonated at physiological pH, giving the complexes a dianionic overall charge. The negatively charged uncoordinated carboxyl group has been postulated to be a critical structural component essential for recognition of the tracer by the OAT1 tubular transporter receptor localized at the basolateral membrane of the proximal tubules [8, 13, 14]. Attempts to determine other features important for recognition by the OAT1 transporter included the preparation of a number of 99mTc-MAG3 analogs to assess the effect of modification of the backbone structure of the MAG3 ligand to further elucidate the structural requirements for an efficient tubular transport. These analogs include the diamide [15], triamide [16], and oxamide [17] derivatives of MAG3, ones with a methyl substituent on the glycine moieties of MAG3 [18], and ones obtained by replacement of one or more glycyl groups of MAG3 by other amino acids [19, 20]. Animal studies showed that some of these 99mTc-MAG3-analogs were cleared rapidly from the blood and excreted into the urine, closely resembling the biological behavior of 99mTc-MAG3, but none of them exhibited better renal kinetics than 99mTc-MAG3. It should be pointed out that all of these 99mTc-MAG3 analogs contained an uncoordinated carboxyl group, a structural component considered to be essential for rapid renal clearance. In contrast, two 99mTc-MAG3 derivatives, in which the free carboxyl group of MAG3 was replaced with a larger group, a mononegative sulfonate or a dinegative phosphonate group, were evaluated to determine the effect of charge distributed over a larger group and of total charge on tubular transport [21]. These two 99mTc-MAG3 derivatives had a 40–50% lower clearance compared to that of 99mTc-MAG3, suggesting that the difference in charge is not the critical factor in tubular transport but that a larger size of the dangling negative group may decrease renal excretion.

More recently, a completely new class of renal tracers based on the 99mTcI(CO)3 core and highly hydrophilic ligands has been developed and evaluated in rats; ligands examined in such 99mTc-tricarbonyl tracers include nitrilotriacetic acid [in 99mTc(CO)3(NTA)] [22], aspartic-N-monoacetic acid [in 99mTc(CO)3(ASMA)] [23], carboxymethylmercaptosuccinic acid [in 99mTc(CO)3(CMSA)] [24], thiodisuccinic acid [in 99mTc(CO)3(TDSA)] [24], lanthionine [in 99mTc(CO)3(LAN)] [25], ethylenediamine-N,N’-diacetic acid [in 99mTc(CO)3(ENDAC)] [26], and carboxymethylthioethyliminodiacetic acid [in 99mTc(CO)3(CMT-IDA)] [27]. The 99mTc(CO)3(NTA), 99mTc(CO)3(ASMA) and 99mTc(CO)3(CMSA) tracers were rapidly extracted by the kidney and eliminated in the urine in rats almost as fast as 131I-OIH [22–24]. These three 99mTc-tricarbonyl tracers have a negatively charged inner coordination sphere and a dianionic overall charge at physiological pH [24, 28, 29]; in addition, they also contain the free carboxylate group found in 99mTc-MAG3, 99mTc-EC and 99mTc-MAEC and the other 99mTcVO renal tracers. Furthermore, 99mTc(CO)3(NTA) has been shown to have pharmacokinetic properties comparable to those of 131I-OIH in normal volunteers [30] and also in patients with severely reduced renal function [31].

To better assess the importance of a negatively charged uncoordinated carboxyl group for OAT1 transporter recognition and tubular secretion, we conducted a series of studies in rats to evaluate the pharmacokinetics of three new monoanionic 99mTc(CO)3(NTA) analogs with uncharged pendant groups but with inner coordination spheres identical to that in 99mTc(CO)3(NTA) (Figure 1).

Figure 1.

Figure 1

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Aminopolycarboxylate ligands investigated in this study and their 99mTc/Re-tricarbonyl complexes, indicating charges of complexes at physiological pH 7.4.

2. Materials and methods

2.1. General

N-(2-acetamido)iminodiacetic acid (ADA, 2) and N-(2-hydroxyethyl)iminodiacetic acid (HDA, 3) (both from Acros Organics), dimethyl iminodiacetate hydrochloride (Alfa Aesar) and 1-bromo-2-fluoroethane (Accela) were used as received. An aqueous stock solution (0.1 M) of [Re(CO)3(H2O)3]OTf was prepared as previously reported [32]. Re(CO)3(ADA) and Re(CO)3(HDA) were synthesized as nonradioactive reference compounds as previously described [29]. All other reagent-grade chemicals and solvents were obtained from commercial suppliers and used without further purification. 1H/19F NMR spectra were recorded on a Varian 400 MHz spectrometer, and chemical shifts are reported in δ units using the residual solvent peak as reference; electrospray mass spectrometry (ESI-MS, negative mode) was performed on a Thermo Finnigan LTQ-FT instrument. 99mTc-pertechnetate (99mTcO4−) in 0.9% saline was received from Triad Isotopes. The “CRS Isolink kit” (Center for Radiopharmaceutical Science, Paul Scherrer Institute, Villigen, Switzerland) was used for the preparation of the [99mTc(CO)3(H2O)3]+ precursor according to the manufacturer’s insert. High performance liquid chromatography (HPLC) separation and quality control of the 99mTc tracers were performed using a Beckman Gold Nouveau system equipped with a model 166 ultraviolet light-visible light detector (monitored at 254 nm), a Model 170 radioisotope detector, and a Beckman C18 RP Ultrasphere octyldecyl silane column (5 µm, 4.6 × 250 mm); data were acquired using the 32 Karat software (Beckman Coulter). The mobile phase consisted of aqueous 0.05 M triethylammonium phosphate buffer pH 2.5 (solvent A) and methanol (solvent B), a flow rate was 1 mL/min, and the gradient methods used were the same as reported previously [24]. Tissue/organ radioactivity was measured with an automated 2480 Wizard 2 gamma counter (Perkin Elmer) that corrects for spillover from 131I into the 99mTc window based on a prior normalization process.

All animal experiments followed the principles of laboratory animal care and were approved by the Institutional Animal Care and Use Committee of Emory University.

Note: all specific 99mTc(CO)3 complexes mentioned in this work have a facial geometry; therefore, the fac- designation is omitted. Also, the M(CO)3(L) nomenclature (M = 99mTc, Re; L = ligand) is used as a general reference to the 99mTc and Re complexes, whereas a specific form defied is designated as [M(CO)3(L)]− (monoanion).

2.2. Chemistry

2.2.1. Dimethyl N-(2-fluoroethyl)iminodiacetate (5)

Dimethyl iminodiacetate hydrochloride (1 g, 5.0 mmol) and 1-bromo-2-fluoroethane (0.63 g, 5.0 mmol) were combined with acetonitrile (5 mL) and diisopropylamine (1.2 mL, 11 mmol) in an oven-dried 10 mL sealed tube. The tube was heated in an oil bath at 90 °C for 3 d. The reaction mixture was concentrated, and the crude yellow solid was purified by flash chromatography (97% CHCl3, 3% MeOH) to yield the product as a slightly yellow oil (0.77 g, 74%), which was used for subsequent reaction without further purification. 1H NMR (CDCl3) δ: 4.58 (doublet of triplets, 2H, JFH = 48 Hz, JHH = 5.6 Hz), 3.71 (s, 6H), 3.64 (s, 4H), 3.10 (doublet of triplets, 2H, JF-H = 28 Hz, JH-H = 5.6 Hz). 19F NMR (400 MHz, CDCl3, TFA reference) δ: −222.41 ppm. HRMS (M+, ESI) Calcd for C8H14O4NFNa: 230.07991, found: 230.07986.

2.2.2. N-(2-fluoroethyl)iminodiacetic acid (FEDA, 4)

Compound 5 (80 mg, 0.4 mmol) was dissolved in 80 µL of methanol before the addition of 2 M NaOH (400 µL). TLC of the reaction mixture (dichloromethane : methanol, 9:1) already showed both the product (Rf = 0) and the starting material (Rf = 0.9) in under a minute. The solution was stirred at room temperature for 1 h to ensure reaction completion and loaded directly onto a C18 reverse phase flash column without concentrating or neutralizing the ligand. The product was eluted with water into 4 mL fractions, and was found in fraction 10, as evidenced by the brown stain observed by spotting on TLC and dipping in a basic permanganate solution. The fraction was concentrated to give the pure product as a colorless oil (46 mg, 63%). 1H NMR ( D2O, pH 14) δ: 4.78 (doublet of triplets, 2H, JFH = 47 Hz, JHH = 4.8 Hz), 3.33 (s, 4H), 3.01 (doublet of triplets, 2H, JF-H = 29 Hz, JH-H = 4.8 Hz). 19F NMR (300 MHz, D2O, TFA reference) δ: −220.8 ppm. HRMS (M−, ESI), Calcd m/z for C6H9O4NF (negative mode): 178.05211, found: 178.05199.

2.2.3. Na[Re(CO)3(FEDA)] (Re-4)

An aqueous solution of 4 (35 mg, 0.19 mmol) was combined with a stirred aqueous solution of 0.1 M [Re(CO)3(H2O)3]OTf (1.9 mL, 0.19 mmol), and aqueous NaOH was added as needed to maintain the pH at 6. The reaction mixture was heated at 70 °C for 10 min, and HPLC analysis revealed the major product peak with a retention time of 16.4 min. (An identical reaction mixture stirred at room temperature proceeded to completion within 90 min.) The reaction mixture was concentrated to 1 mL and purified over Sephadex G-15 gel. UV-active fractions were analyzed by HPLC, and those without impurities were combined and concentrated to yield the product as a white solid (45 mg, 50%). 1H NMR (400 MHz, D2O, pH 7) δ: 4.75 (doublet of triplets, 2H, JFH = 48 Hz, JHH = 4.8 Hz), 3.94 (d, 2H, J = 16.4 Hz), 3.79 (doublet of triplets, 2H, JF-H = 28 Hz, JH-H = 4.8 Hz), 3.73 (d, 2H, J = 16.4 Hz). 19F NMR (CDCl3, TFA reference) δ: −217.61 ppm. HRMS (M−, ESI) Calcd m/z for C9H8O7NF187Re: 447.98478, found: 447.98438.

2.2.4. X-ray crystallography

Crystallographic data were collected at 100(2) K by using a Bruker D8 diffractometer with APEX2 detector (Mo Kα radiation, λ = 0.71073 Å). X-ray quality crystals of Na[Re(CO)3(FEDA)] (Re-4) were obtained by slow evaporation of a water/ethanol solution of highly purified complex at room temperature. A suitable crystal of Re-4 was selected and mounted on a loop with paratone oil. Using Olex2 [33], the structure was solved with the Superflip [34–36] structure solution program using Charge Flipping and refined with the XL [37] refinement package using Least Squares minimization.

2.3. 99mTc radiolabeling

ADA, HDA and FEDA ligands were labeled in a similar manner to form 99mTc-tricarbonyl complexes as previously described for the NTA ligand [22]. Briefly, a freshly prepared solution of the [99mTc(CO)3(H2O)3]+ precursor (0.5 mL, pH ~ 7–8) was added to a sealed vial containing 0.2 mL of an aqueous ligand solution (1 mg/1 mL, pH ~ 6–7) and the labeling mixture was heated at 70 °C for 30 min. After cooling to room temperature, the 99mTc(CO)3(L) (L = ADA, HDA, FEDA) tracer was purified by HPLC as described above, and the collected aqueous solution of radiotracer was diluted in a physiological phosphate buffer pH 7.4 to obtain a final concentration of 3.7 MBq/mL. Radiochemical purity and identity of each 99mTc(CO)3(L) tracer were determined and confirmed by analytical HPLC by co-injection with the analogous Re(CO)3(L) reference compound and comparing the γ-trace of the 99mTc complex with the UV trace of the corresponding Re complex.

2.4. Biodistribution studies in rats

The biodistribution of 99mTc(CO)3(ADA) (99mTc-2), 99mTc(CO)3(HDA) (99mTc-3) and 99mTc(CO)3(FEDA) (99mTc-4) was evaluated in Sprague-Dawley rats with body weights ranging from 190–250 g by using identical, dual isotope protocol. Rats were anesthetized with ketamine-xylazine (2 mg/kg of body weight) injected intramuscularly, with additional supplemental anesthetic as needed. The bladder was catheterized by use of heat-flared PE-50 tubing (Becton, Dickinson and Co.) for urine collection. Groups of five animals were injected via tail vein with 0.2 mL of a solution containing 99mTc(CO)3(L) (100 µCi/mL) and 131I-OIH (25 µCi/mL) in phosphate-buffered saline (PBS) pH 7.4. One additional aliquot of the 99mTc and 131I tracer solution (0.2 mL) for each time point was diluted to 100 mL, and three 1-mL portions of the resulting solution were used as standards.

Animals were sacrificed at 10 min and 60 min after injection. Tissues of interest, along with blood and urine, were collected and placed in counting vials. Each sample and the standards were counted for radioactivity by using an automated gamma-counter using the 99mTc/131I dual-label program. The percentage of the dose in each tissue or organ was calculated by dividing the counts in each tissue or organ by the total injected counts. The percentage injected dose in whole blood was estimated by assuming a blood volume of 6.5% of total body weight.

One rat in each of the 60 min studies of 99mTc(CO)3(ADA), 99mTc(CO)3(HDA), and 99mTc(CO)3(FEDA) produced very little urine, and two rats in the 10 min study of 99mTc(CO)3(ADA) and one rat in the 10 min study of 99mTc(CO)3(HDA) did not produce urine at all; these six rats probably became hypotensive during anesthesia. All six were eliminated from the combined data analysis even though the urine collected from the rats in the 60 min studies had a high 99mTc/131I ratio ranging from 97–101%.

2.5. Metabolism studies

Two additional rats received an intravenous tail vein bolus injection of ~18 MBq of each 99mTc complex. The bladder was catheterized as described for the biodistribution study, and urine was collected for 15 min, filtered with a 0.2 µm Millex-LG filter to remove foreign particles and analyzed by reversed phase HPLC to determine whether the complex was metabolized or excreted unchanged in the urine.

2.6. Plasma protein binding and erythrocyte uptake

Carotid artery blood samples were obtained at 10 min after intravenous injection of each 99mTc tracer (~100 µCi) and the collected blood samples were immediately centrifuged for 15 min. One milliliter of plasma from the carotid artery sample was used to determine plasma protein binding (PPB) by ultrafiltration (Centrifree micropartition system; Amican Inc.): PPB = [1 – (ultrafiltrate concentration/plasma concentration)] × 100. The blood samples were also placed in capillary tubes and centrifuged to determine the hematocrit. The percent erythrocytes uptake was calculated from the activity counted in the whole blood (counts/g) and packed cells (counts/g) as [(counts/g in erythrocytes × hematocrit)/counts/g in whole blood]. No correction was made for plasma trapped in the red blood cells sample. PPB and erythrocyte uptake were calculated in duplicate and the mean values reported.

3. Results and discussion

3.1. Chemistry

Figure 1 shows the general structures of ligands 1–4 and their 99mTc/Re tricarbonyl complexes (99mTc-1 to 99mTc-4 and Re-1 to Re-4, respectively). The Re complexes were prepared as nonradioactive reference compounds and for chemical characterization of their analogous 99mTc tracers since Re and 99mTc complexes have essentially identical coordination parameters and physical properties. We have previously reported extensive solution and solid-state studies of Re(CO)3(NTA) (Re-1) and its analogs with the ADA and HDA ligands (Re-2 and Re-3, respectively) [29]; here we present the synthesis and characterization of a new Re(CO)3 complex with the FEDA ligand, Re-4.

The FEDA ligand (4) was prepared in good yield (68%) by the N-alkylation reaction of dimethyl iminodiacetate with 1-bromo-2-fluoroethane followed by the basic hydrolysis of the methyl ester groups in 5 (Scheme 1). Standard characterization techniques were used to confirm the structure of the isolated material as the desired compound 4 (see the Supporting Information). Reaction of 4 with the [Re(CO)3(H2O)3]+ precursor at 70 °C in the mildly acidic (pH ~ 6) aqueous conditions produced the Re(CO)3(FEDA) complex (Re-4) in a moderate yield (50%, Scheme 1).

Scheme 1.

Scheme 1

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Synthesis of the FEDA ligand (4) and its Re(CO)3(FEDA) complex (Re-4).

The molecular structure of the [Re(CO)3(FEDA)]− anion (Figure 2), confirming the coordination environment of Re-4, was established through single-crystal X-ray analysis. The complex crystallized in the triclinic space group P-1, with two chemically equivalent Na[Re(CO)3(FEDA)] molecules bridged by Na+ ions. As expected, the structure of Re-4 revealed the tridentate facial coordination of the FEDA ligand to the metal core, with the ONO donor set forming two five-membered chelate rings, and leaving the uncharged fluoroethyl group dangling. This distorted octahedral complex has a negatively charged inner metal-coordination sphere, [Re(CO)3(N(CH2COO)2)]−, identical to that present in the previously reported Re-tricarbonyl complexes with NTA, ADA and HDA (Re-1, Re-2, and Re-3, respectively, Figure 1) [29]. All bond lengths and angles fall within expected values and are comparable to those reported for other octahedral complexes containing the [Re(CO)3]+ core and the same donor atoms [29, 38–40]. Crystallographic details, bond lengths and angles, are given in the Supporting Information (Tables S1–S2). As anticipated, the 1H and 19F NMR analyses of Re-4 are in full agreement with the solid X-ray structural analysis. 1H NMR spectrum of Re-4 in D2O at pH 7 exhibits two strongly coupled doublets for the carboxylate-coordinated acetate moiety (J= 16.4 Hz, AB spin system) and a typical coupling (JH-F) between fluorine and hydrogen atoms of the dangling fluoroethyl group can also be observed (see Section 2.2.3; Supporting Information). The 1H NMR signals of Re-4 showed no change for weeks, demonstrating the high stability of the complex in aqueous solution and further confirming that the formation of the iminodiacetate tridentate chelate with two five-membered rings leads to a kinetically inert ONO coordination sphere.

Figure 2.

Figure 2

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ORTEP view of the anion in Na[Re(CO)3(FEDA)]•2H2O (Re-4•2H2O) with 50% probability for thermal ellipsoids. Selected bond lengths [Å] and angles [°]: Re(1)-N(1) 2.246(3); Re(1)-O(4) 2.151(2); Re(1)-O(6) 2.145(2); Re(1)-C(1) 1.917(4); Re(1)-C(2) 1.909(3); Re(1)-C(3) 1.920(3); C(1)-Re(1)-N(1) 97.03(12); C(2)-Re(1)-N(1) 96.87(12); C(3)-Re(1)-N(1) 172.18(12); C(1)-Re(1)-O(6) 171.27(12); C(2)-Re(1)-O(4) 173.63(12); O(4)-Re(1)-N(1) 78.22(9); O(6)-Re(1)-N(1) 77.19(9); O(6)-Re(1)-O(4) 77.06(9).

3.2. Radiochemistry

The 99mTc(CO)3(ADA) (99mTc-2), 99mTc(CO)3(HDA) (99mTc-3), and 99mTc(CO)3(FEDA) (99mTc-4) radiotracers were synthesized in high yield (>90%) and in a similar manner as was previously reported for 99mTc(CO)3(NTA) (99mTc-1) (Scheme 2) [22]. Formation of each 99mTc tracer was analyzed by reversed phase HPLC utilizing a radiometric (γ) detector. Each radiotracer was purified by gradient HPLC and obtained as a single product with radiochemical purity > 99%. The chemical identity of each 99mTc radiotracer was verified by comparing its HPLC retention time (γ detector) to that of a co-inj ected and well-characterized Re analog (determined by using a UV detector; Table 1). Both 99mTc and Re complexes showed similar retention times when co-injected, confirming the formation of the same product at the n.c.a (99mTc) and macroscopic levels (Re).

Scheme 2.

Scheme 2

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Radiosynthesis of 99mTc(CO)3(ADA) (99mTc-2), 99mTc(CO)3(HDA) (99mTc-3) and 99mTc(CO)3(FEDA) (99mTc-4) in the reaction of the aminopolycarboxylate ligands 2–4 with the [99mTc(CO)3(H2O)3]+ precursor.

Table 1.

HPLC retention times (min)a of the 99mTc(CO)3(L) radiotracers and the Re(CO)3(L) analogs with aminopolycarboxylate ligands 1–4.

Ligand (L) 99mTc-L Re-L
NTA (1) 14.7 14.4
ADA (2) 12.5 12.2
HDA (3) 15.4 15.1
FEDA (4) 16.7 16.4
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a

The 0.3 min time delay between the γ-signal of the 99mTc tracer and the UV peak (at 254 nm) of the corresponding Re complex arises from the serial (in-line) arrangement of the radiometric and UV detectors.

The in vitro stability of 99mTc-2, 99mTc-3, and 99mTc-4 in aqueous solution at physiological pH was assessed at various time intervals by HPLC for up to 24 h, and no measurable decomposition or reoxidation to 99mTcO4- was observed, which was in close agreement with the findings for their highly stable Re analogs [29].

3.3. Animal studies

The biodistribution studies of 99mTc(CO)3(ADA) (99mTc-2), 99mTc(CO)3(HDA) (99mTc-3), and 99mTc(CO)3(FEDA) (99mTc-4) in normal rats were performed by simultaneous intravenous administration of the 99mTc radiotracer along with 131I-OIH, followed by analysis of radioactivity in various organs at 10 and 60 min post injection. 131I-OIH served as an internal control because it is the radioactive standard for measurement of ERPF. The results are summarized in Table 2 and are expressed as a percent of injected dose (%ID) per organ. For comparison, we have included our previously published biodistribution data for 99mTc(CO)3(NTA) (99mTc-1) [22]. All three new tridentate coordinated 99mTc(CO)3 complexes (99mTc-2 to 99mTc-4) are efficiently excreted from all organs and tissues through the urinary pathway. The blood clearance was rapid for all three 99mTc tracers, with percent injected dose remaining in the blood ranging from 3.2 ± 0.1% to 5.1 ± 0.9% at 10 min and 0.3 ± 0.2% to 0.6 ± 0.3% at 60 min; these values are comparable to the percent dose in the blood of 99mTc(CO)3(NTA) at the same time points, 4.2 ± 0.9% and 0.4 ± 0.2%, respectively. 99mTc-2, 99mTc-3 and 99mTc-4 were all rapidly eliminated in the urine and demonstrated high specificity renal excretion (Figure 3). The activity in the urine as a percentage of 131I-OIH at 10 and 60 min post injection was 96 ± 2% and 99 ± 1% for 99mTc-2, 96 ± 3% and 100 ± 1% for 99mTc-3, and 100 ± 4% and 99 ± 2% for 99mTc-4, respectively, vs. 108 ± 9% and 101 ± 5%, respectively, for 99mTc(CO)3(NTA) (99mTc-1) [22]. The percent of the injected dose present in the liver at 10 min post-injection was similar, ~ 6%, for each of the three new 99mTc tracers and ranged from 0.8 ± 0.1% to 2.1 ± 0.5% at 60 min, values only minimally higher than the liver activity of 99mTc(CO)3(NTA) at the same time points. Less than 1% of the total activity of either complex was present in the spleen, heart and lungs at either 10 or 60 min post injection.

Table 2.

Percent injected dose of 99mTc(CO)3(NTA) (99mTc-1) [22], 99mTc(CO)3(ADA) (99mTc-2), 99mTc(CO)3(HDA) (99mTc-3), 99mTc(CO)3(FEDA) (99mTc-4), and 131I-OIH in blood, urine and selected organs at 10 and 60 minutes in normal ratsa.

Blood Liver Bowelb Spleen Heart Lung Kidney Urine % Urine
99mTc/131I
10 min
99mTc-2 3.2 ± 0.1 5.9 ± 0.6 1.3 ± 0.2 0.0 ± 0.0 0.1 ± 0.0 0.4 ± 0.1 7.0 ± 0.3 55.1 ± 5.7 96 ± 2
131I-OIH 4.6 ± 0.2 3.0 ± 0.5 1.4 ± 0.4 0.1 ± 0.0 0.1 ± 0.1 0.4 ± 0.1 4.2 ± 0.5 57.7 ± 7.0
99mTc-3 5.0 ± 0.8 6.2 ± 0.8 3.3 ± 0.4 0.1 ± 0.0 0.1 ± 0.0 0.4 ± 0.1 6.5 ± 0.2 38.4 ± 6.5 96 ± 3
131I-OIH 6.2 ± 0.7 3.1 ± 0.4 2.2 ± 0.4 0.1 ± 0.0 0.2 ± 0.1 0.4 ± 0.1 4.8 ± 0.5 40.1 ± 6.6
99mTc-4 5.1 ± 0.9 5.9 ± 0.8 2.9 ± 0.5 0.1 ± 0.0 0.1 ± 0.0 0.4 ± 0.2 6.6 ± 2.1 54.1 ± 7.2 100 ± 4
131I-OIH 5.8 ± 1.0 3.0 ± 0.9 1.8 ± 0.2 0.1 ± 0.0 0.2 ± 0.0 0.4 ± 0.1 4.8 ± 0.8 54.2 ± 9.2
99mTc-1 4.2 ± 0.9 2.7 ± 1.2 0.8 ± 0.3 0.0 ± 0.0 0.1 ± 0.1 0.4 ± 0.2 6.1 ± 0.7 57.1 ± 8.4 108 ± 9
131I-OIH 5.6 ± 1.0 4.0 ± 0.7 2.5 ± 0.5 0.1 ± 0.1 0.2 ± 0.1 0.5 ± 0.2 5.6 ± 0.6 53.2 ± 7.2
60 min
99mTc-2 0.6 ± 0.3 2.1 ± 0.5 3.7 ± 0.2 0.0 ± 0.0 0.0 ± 0.0 0.1 ± 0.1 0.9 ± 0.2 81.8 ± 2.5 99 ± 1
131I-OIH 0.9 ± 0.4 0.8 ± 0.2 1.2 ± 0.2 0.0 ± 0.0 0.0 ± 0.0 0.1 ± 0.1 0.5 ± 0.1 82.6 ± 2.3
99mTc-3 0.5 ± 0.2 1.0 ± 0.3 3.2 ± 0.7 0.0 ± 0.0 0.0 ± 0.0 0.1 ± 0.0 0.6 ± 0.2 81.3 ± 2.2 100 ± 1
131I-OIH 0.8 ± 0.1 0.7 ± 0.1 1.1 ± 0.2 0.0 ± 0.0 0.1 ± 0.0 0.1 ± 0.1 0.5 ± 0.2 81.4± 2.0
99mTc-4 0.3 ± 0.2 0.8 ± 0.1 2.9 ± 1.4 0.1 ± 0.2 0.0 ± 0.0 0.1 ± 0.1 0.7 ± 0.2 79.2 ± 6.4 99 ± 2
131I-OIH 0.5 ± 0.4 0.5 ± 0.1 1.2 ± 0.5 0.1 ± 0.1 0.1 ± 0.0 0.1 ± 0.1 0.5 ± 0.2 80.24 ± 5.3
99mTc-1 0.4 ± 0.2 0.3 ± 0.2 0.4 ± 0.1 0.0 ± 0.0 0.0 ± 0.0 0.1 ± 0.1 0.4 ± 0.2 93.0 ± 4.0 101 ± 5
131I-OIH 0.5 ± 0.0 0.6 ± 0.3 1.7 ± 0.7 0.0 ± 0.0 0.0 ± 0.0 0.1 ± 0.1 0.5 ± 0.0 91.1 ± 3.7
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Data are presented as mean ± SD.

a

NTA (10 and 60 min n = 5); ADA (10 min n = 3, 60 min n = 4); HDA (10 and 60 min n = 4), FEDA (10 min n = 5, 60 min n = 4).

b

Bowel includes intestines and stomach.

Figure 3.

Figure 3

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Bar graphs comparing the activity in urine, as a percentage of 131I-OIH (99mTc/131I), of 99mTc(CO)3(NTA) (99mTc-1) [22], 99mTc(CO)3(ADA) (99mTc-2), 99mTc(CO)3(HDA) (99mTc-3) and 99mTc(CO)3(FEDA) (99mTc-4) in rats at 10 min and 60 min post-injection.

In addition to evaluating the structure/clearance relationship, we also measured the plasma protein binding (PPB) and erythrocyte uptake since these factors modulate the tubular extraction of renal tracers. For example, the high protein binding of 99mTc-MAG3 relative to 131I-OIH results in 99mTc-MAG3 being less available to the OAT1 transporter during the short period of time that the blood is in contact with the basolateral membrane of the renal tubules as it transits the kidney. In consequence, 131I-OIH is extracted more rapidly by the kidney than 99mTc-MAG3 [41]. Other important modulators of renal transport include the affinity of tracers for plasma proteins and the affinity for the OAT1 transporter. Tracers that are loosely bound to plasma proteins are more available to the OAT1 transporter than tracers with a high plasma protein binding affinity. The relative low binding affinity of 131I-OIH and 99mTc-MAG3 for plasma proteins is illustrated by their high clearance rates and by the fact that the plasma protein binding of both tracers can be reduced 40–50% simply by an infusion of mannitol [42]. The major factor affecting tubular extraction is affinity for the OAT1 transporter. Constant infusion of competitive inhibitors of the OAT1 transporter depress the clearance and extraction efficiency of 99mTc-MAG3 to a greater extent than 131I-OIH and help explain the more rapid clearance of 131I-OIH compared to 99mTc-MAG3 [42, 43]. In our study, PPB was 21% for 99mTc(CO)3(ADA), 40% for 99mTc(CO)3(HDA) and 61% for 99mTc(CO)3(FEDA) (Figure 4) compared to 67% for 99mTc(CO)3(NTA) [22]. The erythrocyte uptake was 12% for 99mTc(CO)3(ADA), 26% for 99mTc(CO)3(HDA) and 20% for 99mTc(CO)3(FEDA) compared to 7% for 99mTc(CO)3(NTA) [22]. In spite of differences in protein binding and erythrocyte uptake, the percent dose in the urine at 10 and 60 minutes for all four tracers was comparable to that for 131I-OIH. This similarity suggests that, for these tracers, a low binding affinity for plasma proteins and a relatively high binding affinity for the OAT1 transporter are the dominant factors affecting tubular transport.

Figure 4.

Figure 4

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Bar graphs comparing plasma protein binding (PPB) and red blood cell uptake (RBC) of 99mTc(CO)3(ADA) (99mTc-2), 99mTc(CO)3(HDA) (99mTc-3) and 99mTc(CO)3(FEDA) (99mTc-4) in rats, with that of 99mTc(CO)3(NTA) (99mTc-1) included for reference [22].

The urine from rats that received the bolus injection of 18.5 MBq of each of the new 99mTc(CO)3 tracers was analyzed by radio-HPLC to determine the in vivo metabolic stability of the complexes. Figure 5 shows typical HPLC traces of analyzed urine and the corresponding injected complex. The corresponding reference HPLC traces demonstrate that all three complexes (99mTc-2 to 99mTc-4) were excreted unchanged in urine.

Figure 5.

Figure 5

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HPLC radiochromatograms (γ-detection) of purified 99mTc(CO)3(ADA) (99mTc-2), 99mTc(CO)3(HDA) (99mTc-3) and 99mTc(CO)3(FEDA) (99mTc-4) before injection (column A) and of rat urine injected with those 99mTc radiotracers 15 min post-injection (column B). The corresponding reference HPLC traces showed that all three complexes were excreted unchanged in urine.

Our biodistribution results indicate that all three 99mTc(CO)3(ADA), 99mTc(CO)3(HDA) and 99mTc(CO)3(FEDA) radiotracers exhibit a rapid renal extraction rate, practically identical to that of 99mTc(CO)3(NTA) despite a monoanionic charge and the absence of a negatively charged dangling carboxyl group. Decades of studies of 99mTcVO renal agents indicated an uncoordinated negative carboxyl group (often in a dangling chain) appeared to be the key factor favoring a high rate of renal extraction [14]. In contrast, our present results lead to the conclusion that the primary factor responsible for the favorable pharmacokinetic properties of 99mTc-2, 99mTc-3 and 99mTc-4 resides chiefly within the [99mTc(CO)3(N(CH2CO2)2)]− inner coordination sphere and much less so in the dangling group. Accordingly, the dangling group may accommodate a wider variety of chemical groups to provide additional functionality to the renal tracers, with the expectation that their pharmacokinetic properties will largely be maintained as long as the [99mTc(CO)3(N(CH2CO2)2)]− moiety is preserved. Nevertheless, the electron-donating properties of the dangling group also still must be taken into consideration when designing a renal tracer because 99mTc(CO)3(NTA) had a significantly higher rate of renal excretion in rats compared to the 99mTc(CO)3(NDAP) tracer [NDAP = nitrilodiacetic-propionic acid] which differs from 99mTc(CO)3(NTA) only in having a more electron-donating propionate pendant group instead of the acetate pendant group in 99mTc(CO)3(NTA) [44].

Finally, the uncharged fluoroethyl pendant group of 99mTc(CO)3(FEDA) provides a potentially important route to the synthesis of a structurally analogous rhenium-tricarbonyl 18F renal imaging agent, Re(CO)3(18F-FEDA). Successful synthesis of Re(CO)3(18F-FEDA) would make available a pair of 18F/99mTc renal imaging agents that have almost identical structures and are quite likely to have identical pharmacokinetic properties. In addition, the development of an 18F renal tubular tracer assumes increasing importance if there are further shortages of molybdenum-99/technetium-99 generators [45, 46].

5. Conclusions

A new series of 99mTc(CO)3(NTA) analogs with uncharged pendant groups has been developed to assess the importance of the negatively charged uncoordinated group in the renal tracers for tubular secretion. Three new complexes, 99mTc(CO)3(ADA) (99mTc-2), 99mTc(CO)3(HDA) (99mTc-3), and 99mTc(CO)3(FEDA) (99mTc-4), having acetamido, hydroxyethyl and fluoroethyl dangling groups, respectively, were successfully radiolabeled via reaction with the [99mTc(CO)3(H2O)3]+ precursor. Despite lacking a charged pendant group, all three monoanionic tracers demonstrated rapid renal extraction and high specificity for renal excretion comparable to that of the more hydrophilic 99mTc(CO)3(NTA), which contains the negatively charged carboxylate pendant group and which is dianionic at physiological pH. All four tracers (99mTc-1 to 99mTc-4), however, share the negatively charged inner metal-coordination [99mTc(CO)3(N(CH2CO2)2)]− sphere, a finding which suggests that the inner-sphere ligand properties dominate over the dangling chain properties and which provides new perspectives into the role of the inner-sphere chelate ring atoms and its dangling chain. The high specificity for renal extraction and the rapid clearance in rats warrants evaluation in humans; these results have formed the basis for two FDA-approved exploratory IND applications to evaluate the pharmacokinetic properties for 99mTc-2 and 99mTc-4 in normal volunteers.

Supplementary Material

Acknowledgments

This research was supported by the National Institutes of Health/National Institute of Diabetes and Digestive and Kidney Diseases grant R37 DK38842. The authors thank Dr. John Bacsa at Emory for discussions regarding crystal preparation and X-ray structure determination studies and Dr. Patricia A. Marzilli for her invaluable comments during the preparation of the manuscript. We also thank Eugene Malveaux for his excellent technical assistance with all animal studies.

Footnotes

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References

  • 1.Tubis M, Posnick E, Nordyke A. Preparation and use of 131I labeled sodium iodohippurate in kidney function. Proc Soc Exp Biol Med. 1960;103:497–498. doi: 10.3181/00379727-103-25571. [DOI] [PubMed] [Google Scholar]
  • 2.Burbank MK, Tauxe WN, Maher FT, Hunt JC. Evaluation of radioiodinated hippuran for the estimation of renal plasma flow. Mayo Clin Proc. 1961;36:372–386. [PubMed] [Google Scholar]
  • 3.Stadalnik RC, Vogel JM, Jansholt AL, Krohn KA, Matolo NM, Lagunassolar MC, et al. Renal clearance and extraction parameters of ortho-iodohippurate (I-123) compared with OIH(I-131) and PAH. J Nucl Med. 1980;21:168–170. [PubMed] [Google Scholar]
  • 4.Bubeck B, Brandau W, Weber E, Kalble T, Parekh N, Georgi P. Pharmacokinetics of technetium-99m–MAG3 in humans. J Nucl Med. 1990;31:1285–1293. [PubMed] [Google Scholar]
  • 5.Eshima D, Taylor A. Technetium-99m (99mTc) mercaptoacetyltriglycine: update on the new 99mTc renal tubular function agent. Semin Nucl Med. 1992;22:61–73. doi: 10.1016/s0001-2998(05)80082-0. [DOI] [PubMed] [Google Scholar]
  • 6.Taylor AT. Radionuclides in nephrology, part 1: radiopharmaceuticals, quality control and quantitative indices. J Nucl Med. 2015;55:608–615. doi: 10.2967/jnumed.113.133447. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 7.Esteves FP, Taylor A, Manatunga A, Folks RD, Krishnan M, Garcia EV. 99mTc-MAG3 renography: normal values for MAG3 clearance and curve parameters, excretory parameters, and residual urine volume. AJR. 2006;187:W610–W617. doi: 10.2214/AJR.05.1550. [DOI] [PubMed] [Google Scholar]
  • 8.Shikano N, Kanai Y, Kawai K, Ishikawa N, Endou H. Transport of 99mTc-MAG3 via rat renal organic anion transporter 1. J Nucl Med. 2004;45:80–85. [PubMed] [Google Scholar]
  • 9.Trejtner F, Laznicek M. Analysis of renal handling of radiopharmaceuticals. Q J Nucl Med. 2002;46:181–194. [PubMed] [Google Scholar]
  • 10.Taylor A, Hansen L, Eshima D, Malveaux E, Folks R, Shattuck L, et al. Comparison of technetium-99m–LL-EC isomers in rats and humans. J Nucl Med. 1997;38:821–826. [PubMed] [Google Scholar]
  • 11.Van Nerom CG, Bormans GM, De Roo MJ, Verbruggen AM. First experience in healthy volunteers with 99mTc-L,L-ethylenedicysteine, a new renal imaging agent. Eur J Nucl Med. 1993;20:738–746. doi: 10.1007/BF00180902. [DOI] [PubMed] [Google Scholar]
  • 12.Taylor AT, Lipowska M, Hansen L, Malveaux E, Marzilli LG. 99mTc-MAEC complexes: new renal radiopharmaceuticals combining characteristics of 99mTc-MAG3 and 99mTc-EC. J Nucl Med. 2004;45:885–891. [PubMed] [Google Scholar]
  • 13.Despopoulos A. A definition of substrate specificity in renal transport of organic anions. J Theor Biol. 1965;8:163–192. doi: 10.1016/0022-5193(65)90101-3. [DOI] [PubMed] [Google Scholar]
  • 14.Moran JK. Technetium-99m–EC and other potential new agents in renal nuclear medicine. Semin Nucl Med. 1999;29:91–101. doi: 10.1016/s0001-2998(99)80002-6. [DOI] [PubMed] [Google Scholar]
  • 15.Mang’era K, Vanbilloen HP, Cleynhens BJ, de Groot T, Bormans G, Verbruggen AM, et al. Synthesis and evaluation of the 99mTc-complexes of L-cysteine acetyldiglycine (a hybrid of MAG3 and L,L-EC) and of L-β-homocysteine acetyldiglycine. Nucl Med Biol. 2000;27:781–789. doi: 10.1016/s0969-8051(00)00162-1. [DOI] [PubMed] [Google Scholar]
  • 16.Okarvi SM, Adriaens P, Verbruggen AM. Synthesis and biological characterisctics of technetium-99m triamide derivatives of mercaptoacetyltriglycine (MAG3) J Label Compd Radiopharm. 2003;46:73–84. [Google Scholar]
  • 17.Okarvi SM, Torfs P, Adriaens P, Verbruggen AM. Synthesis, radiolabeling and biological evaluation of terminal oxamide derivatives of mercaptoacetyltriglycine. J Label Compd Radiopharm. 2002;45:407–421. [Google Scholar]
  • 18.Vanbilloen HP, De Roo MJ, Verbruggen AM. Complexes of technetium-99m with tetrapeptides containing one alanyl and three glycyl moieties. Eur J Nucl Med. 1996;23:40–48. doi: 10.1007/BF01736988. [DOI] [PubMed] [Google Scholar]
  • 19.Eshima D, Taylor A, Fritzberg AR, Kasina S, Hansen L, Sorenson JF. Animal evaluation of technetium-99m triamide mercaptide complexes as potential renal imaging agents. J Nucl Med. 1987;28:1180–1186. [PubMed] [Google Scholar]
  • 20.Bormans GM, Cleynhen BJ, De Roo MJ, verbruggen AM. Evaluation of the renal excretion characterisctics of technetium-99m mercaptoacetylglycyl-D-alanylglycine in healthy volunteers. Eur J Nucl Med. 1992;19:271–277. doi: 10.1007/BF00175141. [DOI] [PubMed] [Google Scholar]
  • 21.Hansen L, Marzilli LG, Eshima D, Malveaux E, Folks R, Taylor A. Evaluation of technetium-99m–triamide-mercaptide complexes designed to identify properties favoring renal tubular transport. J Nucl Med. 1994;35:1198–1205. [PubMed] [Google Scholar]
  • 22.Lipowska M, Marzilli LG, Taylor AT. 99mTc(CO)3-nitrilotriacetic acid: a new renal radiopharmaceutical showing pharmacokinetic properties in rats comparable to those of 131I-OIH. J Nucl Med. 2009;50:454–460. doi: 10.2967/jnumed.108.058768. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 23.Lipowska M, Klenc J, Marzilli LG, Taylor AT. Preclinical evaluation of 99mTc(CO)3-aspartic-N-monoacetic acid, a renal radiotracer with pharmacokinetic properties comparable to 131I-o-iodohippurate. J Nucl Med. 2012;53:1277–1283. doi: 10.2967/jnumed.111.102236. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.He H, Lipowska M, Christoforou AM, Marzilli LG, Taylor AT. Initial evaluation of new 99mTc(CO)3 renal imaging agents having carboxyl-rich thioether ligands and chemical characterization of Re(CO)3 analogues. Nucl Med Biol. 2007;34:709–716. doi: 10.1016/j.nucmedbio.2007.06.007. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 25.Lipowska M, He H, Malveaux E, Xu X, Marzilli LG, Taylor A. First evaluation of a 99mTc-tricarbonyl complex, 99mTc(CO)3(LAN), as a new renal radiopharmaceutical in humans. J Nucl Med. 2006;47:1032–1040. [PMC free article] [PubMed] [Google Scholar]
  • 26.Lipowska M, Hansen L, Marzilli LG, Taylor A. The new renal imaging agent 99mTc(CO)3(ENDAC) and the chemistry of the Re(CO)3(ENDAC) analog. J Nucl Med. 2001;42:259p. [Google Scholar]
  • 27.Bhadwal M, Satpati D, Singhal S, Sarma HD, Venkatesh M, Banerjee S. Preparation of 99mTc(CO)3-carboxymethylthioethyl iminodiacetic acid and evaluation as a potential renal imaging agent. Curr Radiopharm. 2012;5:65–70. doi: 10.2174/1874471011205010065. [DOI] [PubMed] [Google Scholar]
  • 28.Klenc J, Lipowska M, Taylor AT, Marzilli LG. Synthesis and characterization of fac-Re(CO)3-aspartic-N-monoacetic acid: structural analogue of a potential renal tracer, fac-99mTc(CO)3(ASMA) Eur J Inorg Chem. 2012:4334–4341. doi: 10.1002/ejic.201200599. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 29.Klenc J, Lipowska M, Abhayawardhana PL, Taylor AT, Marzilli LG. Structure and properties of fac-[ReI(CO)3(NTA)]2- (NTA3- = trianion of nitrilotriacetic acid) and fac-[ReI(CO)3(L)]n- analogues useful for assessing the excellent renal clearance of the fac-[99mTcI(CO)3(NTA)]2- diagnostic renal agent. Inorg Chem. 2015;54:6281–6290. doi: 10.1021/acs.inorgchem.5b00584. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.Taylor AT, Lipowska M, Marzilli LG. 99mTc(CO)3(NTA): a 99mTc renal tracer with pharmacokinetic properties comparable to those of 131I-OIH in healthy volunteers. J Nucl Med. 2010;51:391–396. doi: 10.2967/jnumed.109.070813. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 31.Taylor AT, Lipowska M, Cai H. 99mTc(CO)3(NTA) and 131I-OIH: comparable plasma clearances in patients with chronic kidney disease. J Nucl Med. 2013;54:578–584. doi: 10.2967/jnumed.112.108357. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 32.He H, Lipowska M, Xu X, Taylor AT, Carlone M, Marzilli LG. Re(CO)3 complexes synthesized via an improved preparation of aqueous fac-[Re(CO)3(H2O)3]+ as an aid in assessing 99mTc imaging agents. Structural characterization and solution behavior of complexes with thioether-bearing amino acids as tridentate ligands Inorg. Chem. 2005;44:5437–5446. doi: 10.1021/ic0501869. [DOI] [PubMed] [Google Scholar]
  • 33.Dolomanov OV, Bourhis LJ, Gildea RJ, Howard JAK, Puschmann H. OLEX2: A complete structure solution, refinement and analysis program. J Appl Cryst. 2009;42:339–341. [Google Scholar]
  • 34.Palatinus L, Chapuis G. SUPERFLIP - a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions. J Appl Cryst. 2007;40:786–790. [Google Scholar]
  • 35.Palatinus L, van der Lee A. Symmetry determination following structure solution in P1. J Appl Cryst. 2008;41:975–984. [Google Scholar]
  • 36.Palatinus L, Prathapa SJ, van Smaalen S. EDMA: a computer program for topological analysis of discrete electron densities. J Appl Cryst. 2012;45:575–580. [Google Scholar]
  • 37.Sheldrick GM. A short history of SHELX . Acta Crystallogr. 2008;A64:112–122. doi: 10.1107/S0108767307043930. [DOI] [PubMed] [Google Scholar]
  • 38.Lipowska M, He H, Xu X, Taylor AT, Marzilli PA, Marzilli LG. Coordination modes of multidentate ligands in fac-[Re(CO)3(polyaminocarboxylate)] analogues of 99mTc radiopharmaceuticals Dependence on aqueous solution reaction conditions. Inorg Chem. 2010;49:3141–3151. doi: 10.1021/ic9017568. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 39.Wang W, Hor ATS, Yan YK. Synthesis and X-ray structures of rhenium(I) carbonyl aminoalkoxide and aminocarboxylate complexes. Inorg Chim Acta. 2006;359:3754–3762. [Google Scholar]
  • 40.Hernandez-Valdes D, Rodriguez-Riera Z, Diaz-Garcia A, Benoist E, Jauregui-Haza U. Influence of the chelator structures in the stability of Re and Tc tricarbonyl complexes with iminodiacetic acid tridentate ligands: a computational study. J Mol Model. 2016;22:179. doi: 10.1007/s00894-016-3043-8. [DOI] [PubMed] [Google Scholar]
  • 41.Eshima D, Eshima L, Hansen L, Lipowska M, Marzilli LG, Taylor A. Effect of protein binding on renal extraction of 131I-OIH and 99mTc-labeled tubular agents. J Nucl Med. 2000;41:2077–2082. [PubMed] [Google Scholar]
  • 42.Taylor A, Eshima D. Effects of altered physiologic states on clearance and distribution of technetium-99m MAG3, iodine-131 OIH, and iodine-125 iothalamate. J Nucl Med. 1988;29:616–622. [PubMed] [Google Scholar]
  • 43.Fritzberg AR, Kasina S, Eshima D, Johnson DL. Synthesis and biological evaluation of technetium-99m MAG3 as a hippuran replacement. J Nucl Med. 1986;27:111–116. [PubMed] [Google Scholar]
  • 44.Lipowska M, Taylor AT, Marzilli LG. Derivatives of 99mTc(CO)3(nitrilotriacetic acid), evaluated as potential renal tubular tracers: properties and in vivo distribution. In: Mazzi U, Eckelman WC, Volkert WA, editors. Techentium and Other Radiometals in Chemistry and Medicine. Padova, Italy: CDEditoriali; 2010. pp. 281–284. [Google Scholar]
  • 45.SNMMI Newsline. NAS Report Warns of U.S. Radioisotope Shortages. J Nucl Med. 2016:19N. [PubMed] [Google Scholar]
  • 46.National Academies of Sciences, Engineering, and Medicine. Molybdenum-99 for Medical Imaging. Washington, DC: The National Academies Press; 2016. [PubMed] [Google Scholar]

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