Coordination modes of multidentate ligands in fac-[Re(CO)(3)(polyaminocarboxylate)] analogues of (99m)Tc radiopharmaceuticals. dependence on aqueous solution reaction conditions

NOTA and NODAGA [99mTc]Tc- and [186Re]Re-Tricarbonyl Complexes: Radiochemistry and First Example of a [99mTc]Tc-NODAGA Somatostatin Receptor-Targeting Bioconjugate

With the long-term goal of developing theranostic agents for applications in nuclear medicine, in this work we evaluated the well-known NOTA and NODAGA chelators as bifunctional chelators (BFCs) for the [^99mTc/¹⁸⁶Re]Tc/Re-tricarbonyl core. In particular, we report model complexes of the general formula fac-[M(L)(CO)₃]⁺ (M = Re, ^99mTc, ¹⁸⁶Re) where L denotes NOTA-Pyr (1) or NODAGA-Pyr (2), which are derived from conjugation of NOTA/NODAGA with pyrrolidine (Pyr). Further, as proof-of-principle, we synthesized the peptide bioconjugate NODAGA-sst₂-ANT (3) and explored its complexation with the fac-[Re(CO)₃]⁺ and fac-[^99mTc][Tc(CO)₃]⁺ cores; sst₂-ANT denotes the somatostatin receptor (SSTR) antagonist 4-NO₂-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH₂. Rhenium complexes Re-1 through Re-3 were synthesized and characterized spectroscopically, and receptor binding affinity was demonstrated for Re-3 in SSTR-expressing cells (AR42J, IC₅₀ = 91 nM). Radiolabeled complexes [^99mTc]Tc/[¹⁸⁶Re]Re-1/2 and [^99mTc]Tc-3 were prepared in high radiochemical yield (>90%, determined by radio-HPLC) by reacting [^99mTc]/[¹⁸⁶Re][Tc/Re(OH₂)₃(CO)₃]⁺ with 1-3 and correlated well with the respective Re-1 through Re-3 standards in comparative HPLC studies. All radiotracers remained intact through 24 h (^99mTc-labeled complexes) or 48 h (¹⁸⁶Re-labeled complexes) against 1 mM l-histidine and 1 mM l-cysteine (pH 7.4, 37 °C). Similarly, rat serum stability studies displayed no decomposition and low nonspecific binding of 9-24% through 4 h. Biodistribution of [^99mTc]Tc-3 in healthy CF-1 mice demonstrated a favorable pharmacokinetic profile. Rapid clearance was observed within 1 h post-injection, predominantly via the renal system (82% of the injected dose was excreted in urine by 1 h), with low kidney retention (% ID/g: 11 at 1 h, 5 at 4 h, and 1 at 24 h) and low nonspecific uptake in other organs/tissues. Our findings establish NOTA and NODAGA as outstanding BFCs for the fac-[M(CO)₃]⁺ core in the design and development of organometallic radiopharmaceuticals. Future in vivo studies of [^99mTc]Tc- and [¹⁸⁶Re]Re-tricarbonyl complexes of NODAGA/NOTA-biomolecule conjugates will further probe the potential of these chelates for nuclear medicine applications in diagnostic imaging and targeted radiotherapy, respectively.

fac-99mTc/Re-tricarbonyl complexes with tridentate aminocarboxyphosphonate ligands: suitability of the phosphonate group in chelate ligand design of new imaging agents

Ligands that coordinate via dianionic phosphonate groups have not been widely utilized in radiopharmaceuticals. N-(phosphonomethyl)iminodiacetic acid (1, PMIDA) and N-(phosphonomethyl)glycine (2, PMG) were investigated as new chelators for the ^99mTc/Re-tricarbonyl "core" (fac-M(CO)₃, M = ^99mTc, Re) present in a major class of radiopharmaceuticals. fac-M(CO)₃(PMIDA) and fac-M(CO)₃(PMG) complexes were studied by HPLC and ¹H/¹³C/³¹P NMR methods for M = Re (Re-1 and Re-2) and by HPLC for M = ^99mTc ( ^99m Tc-1 and ^99m Tc-2). Re-1 and ^99m Tc-1 complexes exhibit a similar pH-dependent equilibrium between geometric linkage isomers (M-1a and M-1b). However, only one isomer exists for M-2 under all conditions. Structural characterization by X-ray crystallography reveals the presence of a bond between a phosphonate oxygen and the Re(I) center in fac-Re(CO)₃(PMG) (Re-2). Detailed comparisons of NMR data for Re-2 conclusively demonstrate that the phosphonate group is coordinated in Re-1b (isomer favored at high pH) but not in Re-1a, which has a dangling N-(phosphonomethyl) group. To our knowledge, Re-1b and Re-2 and their ^99mTc analogs are the first well-documented examples of complexes with these metal-tricarbonyl cores having a dianionic phosphonate group directly coordinated in a fac-M(CO)₃-O-P grouping. Pharmacokinetic studies using Sprague-Dawley rats reveal that ^99m Tc-2 is a robust tracer. Hence, phosphonate groups should be considered in designing ^99mTc and ^186/188Re radiopharmaceuticals, including agents with bioactive moieties attached to dangling carboxylate or phosphonate groups.

A small heterobifunctional ligand provides stable and water dispersible core-shell CdSe/ZnS quantum dots (QDs)

We describe a simple method to prepare water dispersible core-shell CdSe/ZnS quantum dots (QDs) 1 by capping QDs with a new thiol-containing heterobifunctional dicarboxylic ligand 4 (DHLA-EDADA). This ligand, obtained on a gram scale through a few synthetic steps, provides a compact layer on the QDs, whose hydrodynamic size in H2O is 15 nm ± 3 nm. The colloidal stability is dramatically enhanced with respect to the well-known (±) α-lipoic acid (DHLA). The ligand affinity towards QDs and the water dispersibility of nanocrystals 1 are addressed by the dithiol groups of DHLA, which chelate the zinc of the shell, and by the dicarboxylic groups of the ethylenediamine-N,N-diacetic acid (EDADA) residue, respectively. The effects of pH, buffer solutions, and biological medium on the stability of QDs 1 were assessed by monitoring the photoluminescence (PL) and hydrodynamic size over time. Highly fluorescent QD dispersions, stable over extended periods of time and over broad pH ranges and buffer types, were obtained. Furthermore, we show that the DHLA-EDADA ligand 4 also endows QDs with functional groups suitable for further conjugation and for metal ion detection. As a case study to illustrate the potential of our approach, we report the preparation and characterization of a highly luminescent orange light emitting polymer-QD 1 composite film.

Intermolecular Carbonyl···Carbonyl Interactions in Transition-Metal Complexes

We performed a comprehensive analysis of intermolecular carbonyl-carbonyl interactions in transition-metal complexes. Those interactions can be classified in two main types depending on the organometallic or organic nature of the donor carbonyl: M-CO···CO and R-CO···CO, respectively. By means of a combined structural and computational study we unraveled their geometrical features and strength. Moreover, electronic structure, natural bond orbitals, energy decomposition analysis, and quantum theory of atoms in molecules calculations were performed to try to understand their nature. Remarkably, we discovered that these carbonyl-carbonyl contacts have several features of the n → π* interaction. The charge transfer from an oxygen lone pair to an empty antibonding π orbital of the acceptor carbonyl is also accompanied by an electrostatic O^δ-···C^δ+ interaction. To the best of our knowledge this is the first report of an intermolecular n → π* interaction in metal complexes. These results might be significant, for instance, for the catalytic activation of carbonyl-containing small molecules with metal compounds or in the design of hybrid organic-inorganic materials, metal-organic frameworks, and other extended structures.

Re(CO)3([18F]FEDA), a novel 18F PET renal tracer: Radiosynthesis and preclinical evaluation

INTRODUCTION

Our previous work demonstrated that the ^99mTc renal tracer, ^99mTc(CO)₃(FEDA) (^99mTc-1), has a rapid clearance comparable in rats to that of ¹³¹I-OIH, the radioactive gold standard for the measurement of effective renal plasma flow. The uncharged fluoroethyl pendant group of ^99mTc-1 provides a route to the synthesis of a structurally analogous rhenium-tricarbonyl ¹⁸F renal imaging agent, Re(CO)₃([¹⁸F]FEDA) (¹⁸F-1). Our goal was to develop an efficient one-step method for the preparation of ¹⁸F-1 and to compare its pharmacokinetic properties with those of ¹³¹I-OIH in rats.

METHODS

¹⁸F-1 was prepared by the nucleophilic ¹⁸F-fluorination of its tosyl precursor. The labeled compound was isolated by HPLC and subsequently evaluated in Sprague-Dawley rats using ¹³¹I-OIH as an internal control and by dynamic PET/CT imaging. Plasma protein binding (PPB) and erythrocyte uptake (RCB) were determined and the urine was analyzed for metabolites.

RESULTS

¹⁸F-1 was efficiently prepared as a single species with high radiochemical purity (>99%) and it displayed high radiochemical stability in vitro and in vivo. PPB was 87% and RCB was 21%. Biodistribution studies confirmed rapid renal extraction and high specificity for renal excretion, comparable to that of ¹³¹I-OIH, with minimal hepatic/gastrointestinal elimination. The activity in the urine, as a percentage of ¹³¹I-OIH, was 92% and 95% at 10 and 60 min, respectively. All other organs (heart, spleen, lungs) showed a negligible tracer uptake (<0.4% ID). Dynamic microPET/CT imaging demonstrated rapid transit of ¹⁸F-1 through the kidneys and into the bladder; there was no demonstrable activity in bone verifying the absence of free [¹⁸F]fluoride.

CONCLUSIONS

¹⁸F-1 exhibited a high specificity for the kidney, rapid renal excretion comparable to that of ¹³¹I-OIH and high in vivo radiochemical stability. Not only is ¹⁸F-1 a promising PET renal tracer, but it provides a route to the development of a pair of analogous ¹⁸F/^99mTc renal imaging agents with almost identical structures and comparable pharmacokinetic properties. These promising in vivo results warrant subsequent evaluation in humans.

Fluoroquinolones as imaging agents for bacterial infection

Diagnosis of deep-seated bacterial infection is difficult, as neither standard anatomical imaging nor radiolabeled, autologous leukocytes distinguish sterile inflammation from infection. Two recent imaging efforts are receiving attention: (1) radioactive derivatives of sorbitol show good specificity with Gram-negative bacterial infections, and (2) success in combining anatomical and functional imaging for cancer diagnosis has rekindled interest in 99mTc-fluoroquinolone-based imaging. With the latter, computed tomography (CT) would be combined with single-photon-emission-computed tomography (SPECT) to detect 99mTc-fluoroquinolone-bacterial interactions. The present minireview provides a framework for advancing fluoroquinolone-based imaging by identifying gaps in our understanding of the process. One issue is the reliance of 99mTc labeling on the reduction of sodium pertechnetate, which can lead to colloid formation and loss of specificity. Specificity problems may be reduced by altering the quinolone structure (for example, switching from ciprofloxacin to sitafloxacin). Another issue is the uncharacterized nature of 99mTc-ciprofloxacin binding to, or sequestration in, bacteria: specific interactions with DNA gyrase, an intracellular fluoroquinolone target, are unlikely. Labeling with 68Ga rather than 99mTc enables detection by positron emission tomography, but with similar biological uncertainties. Replacing the C6-F of the fluoroquinolone with 18F provides an alternative to pertechnetate and gallium that may lead to imaging based on drug interactions with gyrase. Gyrase-based imaging requires knowledge of fluoroquinolone action, which we update. We conclude that quinolone-based probes show promise for the diagnosis of infection, but improvements in specificity and sensitivity are needed. These improvements include the optimization of the quinolone structure; such chemistry efforts can be accelerated by refining microbiological assays.

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

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 is 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 evaluate 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 10min 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 60min, 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.

Structure and Properties of fac-[Re(I)(CO)3(NTA)](2-) (NTA(3-) = Trianion of Nitrilotriacetic Acid) and fac-[Re(I)(CO)3(L)](n-) Analogues Useful for Assessing the Excellent Renal Clearance of the fac-[(99m)Tc(I)(CO)3(NTA)](2-) Diagnostic Renal Agent

We previously identified two new agents based on the [(99m)Tc(V)O](3+) core with renal clearances in human volunteers 30% higher than that of the widely used clinical tracer (99m)Tc-MAG3 (MAG3(5-) = penta-anion of mercaptoacetyltriglycine). However, renal agents with even higher clearances are needed. More recently, we changed our focus from the [(99m)Tc(V)O](3+) core to the discovery of superior tracers based on the fac-[(99m)Tc(I)(CO)3](+) core. Compared to (99m)Tc-MAG3, fac-[(99m)Tc(I)(CO)3(NTA)](2-) (NTA(3-) = trianion of nitrilotriacetic acid) holds great promise by virtue of its efficient renal clearance via tubular secretion and the absence of hepatobiliary elimination, even in patients with severely reduced renal function. We report here NMR, molecular (X-ray) structure, and solution data on fac-[Re(I)(CO)3(NTA)](2-) with a -CH2CO2(-) dangling monoanionic chain and on two fac-[Re(I)(CO)3(L)](-) analogues with either a -CH2CONH2 or a -CH2CH2OH dangling neutral chain. In these three fac-[Re(I)(CO)3(L)](n-) complexes, the fac-[Re(I)(CO)3(N(CH2CO2)2)](-) moiety is structurally similar and has similar electronic properties (as assessed by NMR data). In reported and ongoing studies, the two fac-[(99m)Tc(I)(CO)3(L)](-) analogues with these neutral dangling chains were found to have pharmacokinetic properties very similar to those of fac-[(99m)Tc(I)(CO)3(NTA)](2-). Therefore, we reach the unexpected conclusion that in fac-[(99m)Tc(I)(CO)3(L)](n-) agents, renal clearance is affected much more than anticipated by features of the core plus the chelate rings (the [(99m)Tc(I)(CO)3(N(CH2CO2)2)](-) moiety) than by the presence of a negatively charged dangling carboxylate chain.

Models for B12-conjugated radiopharmaceuticals. Cobaloxime binding to new fac-[Re(CO)3(Me2bipyridine)(amidine)]BF4 complexes having an exposed pyridyl nitrogen

New mononuclear amidine complexes, fac-[Re(CO)3(Me2bipy)(HNC(CH3)(pyppz))]BF4 [(4,4'-Me2bipy (1), 5,5'-Me2bipy (2), and 6,6'-Me2bipy (3)] (bipy = 2,2'-bipyridine), were synthesized by treating the parent fac-[Re(I)(CO)3(Me2bipy)(CH3CN)]BF4 complex with the C2-symmetrical amine 1-(4-pyridyl)piperazine (pyppzH). The axial amidine ligand has an exposed, highly basic pyridyl nitrogen. The reaction of complexes 1-3 with a B12 model, (py)Co(DH)2Cl (DH = monoanion of dimethylglyoxime), in CH2Cl2 yielded the respective dinuclear complexes, namely, fac-[Re(CO)3(Me2bipy)(μ-(HNC(CH3)(pyppz)))Co(DH)2Cl]BF4 [(4,4'-Me2bipy (4), 5,5'-Me2bipy (5), and 6,6'-Me2bipy (6)]. (1)H NMR spectroscopic analysis of all compounds and single-crystal X-ray crystallographic data for 2, 3, 5, and 6 established that the amidine had only the E configuration in both the solid and solution states and that the pyridyl group is bound to Co in 4-6. Comparison of the NMR spectra of 1-3 with spectra of 4-6 reveals an unusually large "wrong-way" upfield shift for the pyridyl H2/6 signal for 4-6. The wrong-way H2/6 shift of (4-Xpy)Co(DH)2Cl (4-Xpy = 4-substituted pyridine) complexes increased with increasing basicity of the 4-Xpy derivative, a finding attributed to the influence of the magnetic anisotropy of the cobalt center on the shifts of the (1)H NMR signals of the pyridyl protons closest to Co. Our method of employing a coordinate bond for conjugating the fac-[Re(I)(CO)3] core to a vitamin B12 model could be extended to natural B12 derivatives. Because B12 compounds are known to accumulate in cancer cells, such an approach is a very attractive method for the development of (99m)Tc and (186/188)Re radiopharmaceuticals for targeted tumor imaging and therapy.

Complexes possessing rare "tertiary" sulfonamide nitrogen-to-metal bonds of normal length: fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes with hydrophilic sulfonamide ligands

Tertiary sulfonamide nitrogen-to-metal bonds of normal length are very rare. We recently discovered such a bond in one class of fac-[Re(CO)3(N(SO2R)(CH2Z)2)](n) complexes (Z = 2-pyridyl) with N(SO2R)dpa ligands derived from di-(2-picolyl)amine (N(H)dpa). fac-[M(CO)3(N(SO2R)(CH2Z)2)](n) agents (M = (186/188)Re, (99m)Tc) could find use as radiopharmaceutical bioconjugates when R is a targeting moiety. However, the planar, electron-withdrawing 2-pyridyl groups of N(SO2R)dpa destabilize the ligand to base and create relatively rigid chelate rings, raising the possibility that the rare M-N(sulfonamide) bond is an artifact of a restricted geometry. Also, the hydrophobic 2-pyridyl groups could cause undesirable accumulation in the liver, limiting future use in radiopharmaceuticals. Our goal is to identify a robust, hydrophilic, and flexible N(CH2Z)2 chelate framework. New C2-symmetric ligands, N(SO2R)(CH2Z)2 with (Z = CH2NH2; R = Me, dmb, or tol), were prepared by treating N(H)dien(Boc)2, a protected diethylenetriamine (N(H)dien) derivative, with methanesulfonyl chloride (MeSO2Cl), 3,5-dimethylbenzenesulfonyl chloride (dmbSO2Cl), and 4-methylbenzenesulfonyl chloride (tolSO2Cl). Treatment of fac-[Re(CO)3(H2O)3](+) with these ligands, designated as N(SO2R)dien, afforded new fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes. Comparing the fac-[Re(CO)3(N(SO2Me)dien)]PF6 and fac-[Re(CO)3(N(SO2Me)dpa)]PF6 complexes, we find that the Re(I)-N(sulfonamide) bonds are normal in length and statistically identical and that the methyl (13)C NMR signal has an unusually upfield shift compared to that in the free ligand. We attribute this unusual upfield shift to the fact that the sulfonamide N undergoes an sp(2)-to-sp(3) rehybridization upon coordination to Re(I) in both complexes. Thus, the sulfonamide N of N(SO2R)dien ligands is a good donor, even though the chelate rings are conformationally flexible. Addition of the strongly basic and potentially monodentate ligand, 4-dimethylaminopyridine, did not affect the fac-[Re(CO)3(N(SO2tol)dien)]PF6 complex, even after several weeks. This complex is also stable to heat in aqueous solution. These results indicate that N(SO2R)dien ligands form fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes sufficiently robust to be utilized for radiopharmaceutical development.

Synthesis and characterization of fac-Re(CO)3-aspartic-N-monoacetic acid, a structural analogue of a potential new renal tracer, fac-99mTc(CO)3(ASMA)

The reaction of an aminopolycarboxylate ligand, aspartic-N-monoacetic acid (ASMA), with [Re(CO)3(H2O)3]+ was examined. The tridentate coordination of ASMA to this ReI tricarbonyl precursor yielded fac-Re(CO)3(ASMA) as a mixture of diastereomers. The chemistry is analogous to that of the TcI tricarbonyl complex, which yields fac-99mTc(CO)3(ASMA) under similar conditions. The formation, structure, and isomerization of fac-Re(CO)3(ASMA) products were characterized by HPLC, 1H NMR spectroscopy, and X-ray crystallography. The two major fac-Re(CO)3(ASMA) diastereomeric products each have a linear ONO coordination mode with two adjacent five-membered chelate rings, but they differ in the endo or exo orientation of the uncoordinated acetate group, in agreement with expectations based on previous studies. Conditions have been identified for the expedient isomerization of fac-Re(CO)3(ASMA) to a mixture consisting primarily of one major product. Because different isomeric species typically have different pharmacokinetic characteristics, these conditions may provide for the practical isolation of a single 99mTc(CO)3(ASMA) species, thus allowing the isolation of the isomer that has optimal imaging and pharmacokinetic characteristics. This information will aid in the design of future 99mTc radiopharmaceuticals.

New monodentate amidine superbasic ligands with a single configuration in fac-[Re(CO)3(5,5'- or 6,6'-Me2bipyridine)(amidine)]BF4 complexes

Treatment of two precursors, fac-[Re(CO)(3)(L)(CH(3)CN)]BF(4) [L = 5,5'-dimethyl-2,2'-bipyridine (5,5'-Me(2)bipy) (1) and 6,6'-dimethyl-2,2'-bipyridine (6,6'-Me(2)bipy) (2)], with five C(2)-symmetrical saturated heterocyclic amines yielded 10 new amidine complexes, fac-[Re(CO)(3)(L)(HNC(CH(3))N(CH(2)CH(2))(2)Y)]BF(4) [Y = CH(2), (CH(2))(2), (CH(2))(3), NH, or O]. All 10 complexes possess the novel feature of having only one isomer (amidine E configuration), as established by crystallographic and (1)H NMR spectroscopic methods. We are confident that NMR signals of the other possible isomer (amidine Z configuration) would have been detected, if it were present. Isomers are readily detected in closely related amidine complexes because the double-bond character of the amidine C-N3 bond (N3 is bound to Re) leads to slow E to Z isomer interchange. The new fac-[Re(CO)(3)(L)(HNC(CH(3))N(CH(2)CH(2))(2)Y)]BF(4) complexes have C-N3 bonds with essentially identical double-bond character. However, the reason that the Z isomer is so unstable as to be undetectable in the new complexes is undoubtedly because of unfavorable clashes between the equatorial ligands and the bulky N(CH(2)CH(2))(2)Y ring moiety of the axial amidine ligand. The amidine formation reactions in acetonitrile (25 °C) proceeded more easily with 2 than with 1, indicating that the distortion in 6,6'-Me(2)bipy resulting from the proximity of the methyl substituents to the inner coordination sphere enhanced the reactivity of the coordinated CH(3)CN. Reaction times for 1 and 2 exhibited a similar dependence on the basicity and ring size of the heterocyclic amine reactants. Moreover, when the product of the reaction of 1 with piperidine, fac-[Re(CO)(3)(5,5'-Me(2)bipy)(HNC(CH(3))N(CH(2)CH(2))(2)CH(2))]BF(4), was challenged in acetonitrile-d(3) or CDCl(3) with a 5-fold excess of the strong 4-dimethylaminopyridine ligand, there was no evidence for replacement of the amidine ligand after two months, thus establishing that the piperidinylamidine ligand is a robust ligand. This chemistry offers promise as a suitable means for preparing isomerically pure conjugated fac-[(99m)Tc(CO)(3)L](n±) imaging agents, including conjugates with known bioactive heterocyclic amines.

Preclinical evaluation of 99mTc(CO)3-aspartic-N-monoacetic acid, a renal radiotracer with pharmacokinetic properties comparable to 131I-o-iodohippurate

In an ongoing effort to develop a renal tracer with pharmacokinetic properties comparable to p-aminohippurate and superior to those of both (99m)Tc-mercaptoacetyltriglycine and (131)I-o-iodohippurate ((131)I-OIH), we evaluated a new renal tricarbonyl radiotracer based on the aspartic-N-monoacetic acid (ASMA) ligand, (99m)Tc(CO)(3)(ASMA). The ASMA ligand features 2 carboxyl groups and an amine function for the coordination of the {(99m)Tc(CO)(3)}(+) core as well as a dangling carboxylate to facilitate rapid renal clearance.

METHODS

rac-ASMA and l-ASMA were labeled with a (99m)Tc-tricarbonyl precursor, and radiochemical purity of the labeled products was determined by high-performance liquid chromatography. Using (131)I-OIH as an internal control, we evaluated biodistribution in normal rats with (99m)Tc(CO)(3)(ASMA) isomers and in rats with renal pedicle ligation with (99m)Tc(CO)(3)(rac-ASMA). Clearance studies were conducted in 4 additional rats. In vitro radiotracer stability was determined in phosphate-buffered saline, pH 7.4, and in challenge studies with cysteine and histidine. (99m)Tc(CO)(3)(ASMA) metabolites in urine were analyzed by high-performance liquid chromatography.

RESULTS

Both (99m)Tc(CO)(3)(ASMA) preparations had greater than 99% radiochemical purity and were stable in phosphate-buffered saline, pH 7.4, for 24 h. Challenge studies on both revealed no significant displacement of the ligand. In normal rats, the percentage injected dose in urine at 10 and 60 min for both preparations averaged, respectively, 103% and 106% that of (131)I-OIH. The renal clearances of (99m)Tc(CO)(3)(rac-ASMA) and (131)I-OIH were comparable (P = 0.48). The tracer was excreted unchanged in the urine, proving its in vivo stability. In pedicle-ligated rats, (99m)Tc(CO)(3)(rac-ASMA) had less excretion into the bowel (P < 0.05) than did (131)I-OIH and was better retained in the blood (P < 0.05).

CONCLUSION

Both (99m)Tc(CO)(3)(ASMA) complexes have pharmacokinetic properties in rats comparable to or superior to those of (131)I-OIH, and human studies are warranted for their further evaluation.

Design and synthesis of a bombesin peptide-conjugated tripodal phosphino dithioether ligand topology for the stabilization of the fac-[M(CO)3]+ core (M=(99 m)Tc or Re)

A new tumor-seeking tridentate topology consisting of a phosphino dithioether ((HOCH(2))(2)PCH(2)CH(2)S(CH(2))(n)CH(2)SR; PS(2)) ligand framework for the production of kinetically inert and in vivo stable facial [(99m)Tc(CO)(3)(PS(2))](+) or [Re(CO)(3)(PS(2))](+) is described. The X-ray crystal structure of fac-Re(CO)(3)(PS(2))PF(6) is reported. The bioconjugation strategies for incorporating bombesin (BBN) peptides on to the PS(2) tripodal framework and, thereby, de novo designing of GRP receptor-seeking Tc(PS(2)-BBN)(CO)(3) are developed.