Lory Hansen

Synthesis of new N2S2 ligands and Re(V)O(N2S2) analogues of 99mTc renal imaging agents. Characterization by NMR spectroscopy, molecular mechanics calculations, and X-ray crystallography

Abstract Two new N2S2 ligands [S-trityl- l -cysteine-acetamide-ethanethiol ethyl ester (Tr- l -CAATH2-Et, 1) and S-trityl- d -penicillamine-acetamide-(2-methyl-2-propanethiol) ethyl ester (Tr- d -TMCAATH2-Et, 2)] were prepared. These ligands, with the electron-withdrawing carboxyl group separated by only two bonds from the NH amine group, belong to monoamide-monoamine-dithiol (MAMA) class of chelates that typically form M(V)O(N2S2) (M=Tc, Re) complexes with the ligand secondary amine pKa of ∼6–7 and both syn and anti isomers. Re(V)O(N2S2) complexes were prepared from 1 and 2 with the dangling ester either syn or anti to the ReO intact or hydrolyzed; the latter, ReO( l -CAATH2) and ReO( d -TMCAATH2) have a carboxyl group needed for renal imaging when M=Tc. The gross structure of syn-ReO( l -CAATH-Et) (3) is typical of Re(V)O(N2S2) complexes. However, within the cys moiety of 3, an intramolecular H-bond between the ester carbonyl oxygen O(4) and the NH (N(2)⋯O(4) distance=2.80 A) causes the cys chelate ring to adopt an unusual conformation with the αC (C(5)) atom 0.42 A below the plane defined by the cys S, Re, and N(2) atoms with respect to the ReO group. From molecular mechanics (MM) analysis of ReO( l -CAATH2) models, the lowest-energy syn-ReO( l -CAATH2) structure has a corresponding region similar to that of the X-ray structure for 3, an N(2)H endo with respect to ReO group, a N(2)H⋯O(4) hydrogen bond (2.69 A N(2)⋯O(4) distance) contributing ∼5 kcal mol−1 to its stability and an αC atom deviating 0.24 A from the cys S, Re, and N(2) plane. Computed structures with an exo N(2)H are ∼6 kcal mol−1 less stable than the lowest-energy endo analogue. Below pH ∼7, the 1H NMR spectrum of syn-ReO( d -TMCAATH2) exhibited changes in two pH regions, consistent with sequential protonation of the amine and of the carboxylic group. These experimental and MM results reinforce previous hypotheses about the factors favoring one syn form in one ionization state of M(V)O(N2S2) compounds at physiological pH.

Mixed-ligand rhenium(V) oxo monomers with triphenylphosphine oxide and lopsided and symmetrical heterocyclic ligands. Putative intermediates in rhenium(V) oxo synthetic chemistry☆

Normally, ReOCl3 (Me2S) (OPPh3) (1) serves as a useful source of ReO3+ or ReOClx(3−x)+ for the synthesis of Re complexes by ligand exchange. Complexes of the type ReOCl3(OPPh3)(L) (L = 1,5,6-trimethylbenzimidazole (Me3Bzm) (2); 3,5-lutidine (3,5-lut) (3); pyridine (py) (4)) were prepared from 1 and one equivalent of L. Formation of these mixed-ligand complexes is unsual because the normally labile OPPh3 in 1 was retained. Addition of non-coordinating triethylamine (NEt3) gave [HNEt3][ReOCl4(OPPh3] (5). The anion in 5 has been populated to be an intermediate in some synthetic schemes. ReOCl3(OPPh3)(Me3Bzm) (2) was characterized by X-ray crystallography. Crystallographic data are: C28H27Cl3N2O2PRe, P21/c, a = 18.503(3), b = 9.780(2), c = 15.735(4) A, β = 97.56(2)°, Dcalc = 1.76 g cm−3, Z = 4, R = 0.041, Rw = 0.066 for 3193 independent reflections. In 2, the pseudo-octahedral Re has the OPPh3 ligand trans to the oxo ligand, and one of the OPPh3 phenyl rings lies nearly over the five-membered ring of Me3Bzm. Reaction of ReOCl3(OPPh3)(Me3Bzm) (2) with one equivalent of 3,5-lut or py resulted in the precipitation of mixed-ligand species best formulated as Re2O3Cl4(Me3Bzm)2(3,5-lut)2 (6) and Re2O3Cl4(Me3Bzm)2(py)2 (7), respectively.

Structural Characterization of the Rhenium(V) Oxo Complex of Mercaptoacetyltriglyclne in its Dianionic Form

Dianionic [MO(MAG3)]2−(MAG3 = penta-anionic form of mercaptoacetyltriglycine, M = 186Re, 99mTc) complexes have important applications in nuclear medicine. In vivo the complexes have a deprotonated carboxyl group that is important to their biodistribution. The solid-state structures of 99Tc and Re complexes with mercaptoacetyltriglycine reported previously are monoanions with protonated carboxyl groups. In the present work, we report the preparation and X-ray crystal structure of Na2[ReO(MAG3)]·5H2O (1), which contains the physiologically relevant dianion. The dianion is a distorted square pyramid with the nitrogen and sulphur donor atoms forming the base and the oxo ligand at the apex. The terminal carboxyl group is deprotonated, uncoordinated and has a syn orientation with respect to the oxo ligand. The syn conformation of the dianion in 1 differs in conformation from the anti-monoanion in [Bu4N][ReO(MAG3H)] but is similar to the syn-monoanion in [Ph4P][99TcO(MAG3 H)].

Effect on advanced age on p-aminohippurate-induced inhibition of renal tubular secretion in male fischer 344 rats

The effect of aging on glomerular filtration, effective renal plasma flow and on the responsiveness of the renal tubular anion secretory system to inhibition by 4-aminobenzoylglycine (p-aminohippurate, PAH) was examined in young (5-month) and old (22-month) Fischer 344 male rats. Plasma clearance, protein binding and renal extraction of [131I]o-iodohippurate, [125I]iothalamate and HPLC-purified [99mTc]mercaptoacetyltriglycine (MAG3), were used as in vivo probes of renal function. The effect of advanced age, without concomitant PAH, on the disposition of these markers was initially determined in ketamine anesthetized, temperature-maintained male rats, ages 5, 14 and 22 months by means of constant infusion clearance studies. Aging per se decreased (P less than 0.05) the kidney-weight normalized or body weight-normalized GFR and effective renal plasma flow rates. GFR values averaged 1.67, 1.43 and 1.32 ml/min per g kidney for the 5-, 14- and 22-month-old rats, respectively. Kidney- or body weight-normalized clearances of MAG3 and o-iodohippurate showed similar (25-27%) decreases, whereas the absolute values (ml/min) for GFR, o-iodohippurate and MAG3 clearance rates were not altered by aging. The effective filtration fraction, extraction ratio and plasma protein binding were also unchanged by advanced age. Overall, the age-related decreases in renal function were minimal in Fischer-344 rats, compared to other species. Differences in data normalization, species and gender account, in part, for discrepancies observed when comparing results in different studies on the effects of advanced age on renal function. Subsequently, we examined the effect of aging on the renal responsiveness to inhibition of tubular anion secretion using constant rate PAH infusion studies, adjusted for age-related changes in renal function. Aging did not alter PAH-induced inhibition of iodohippurate secretion. Inhibition of MAG3 elimination was more pronounced in the old rats compared to the young controls.

Synthesis of the Sulphonate and Phosphonate Derivatives ofMercaptoacetyltriglycine. X-Ray Crystal Structure of Na2[ReO(Mercaptoacetylglycylglycylaminomethanesulphonate)]·3H2O

Mercaptoacetyltriglycine forms complexes with 186/188Re and 99mTc radionuclides that are useful in nuclear medicine because they are substrates of the renal anion transport system. However, the renal clearance of [MO(MAG3)]2-(MAG3 = penta-anionic form of mercaptoacetyltriglycine, M = Re, Tc) complexes are less than ideal. Organic sulphonates are also transported by the renal anion transport system and phosphonates are similar to sulphonates in size and shape. In an effort to develop new ligands that form Re and Tc complexes and have improved renal clearances compared to [MO(MAG3)]2- complexes, the sulphonate and phosphonate derivatives of mercaptoacetyltriglycine were synthesized. The dianion [ReO(MAG2-AMS)]2- (MAG2-AMS = penta-anionic form of mercaptoacetylglycylglycylaminomethanesulphonic acid) was prepared for characterization by exchange reaction of ReOCl3(Me2S)(OPPh3) and isolated as the disodium salt. The structure of Na2[ReO(MAG2-AMS)]·3H2O (6) was determined by X-ray diffraction. The coordination geometry is pseudo square pyramidal, with the nitrogen and sulfur donor atoms forming a square base and the oxo ligand at the apex. The deprotonated sulphonate group has a syn conformation with respect to the oxo ligand. The renal clearances of [99mTcO(MAG2-AMS)]2- and [99mTcO(MAG2-AMP)]3- were similar in rats and suggest that the difference in total charge between the SO3- and PO32- groups is not important to renal clearance. However, their renal clearances were 40-50% less than that of [99mTcO(MAG3)]2- suggesting that the size and shape of the large tetrahedral SO3- and PO32- groups of [99mTcO(MAG2-AMS)]2- and [99mTcO(MAG2-AMP)]3- inhibit recognition by the renal transport system compared to the small planar CO2- group of [99mTcO(MAG3)]2-.

Preparation and Crystal Structure of a Rhenium Analogue of the Cationic Renal Agent, Tc-99m Diaminocyclohexane

We report here a chemical study on a Re analogue of one of the few cationic Tc-99m tracers previously investigated as an agent for effective renal plasma flow (ERPF) measurement. Cationic Tc-99m tracers have the potential for overcoming problems associated with common anionic Tc-99m tracers in patients who have developed a uremic state. The Tc-99m-DACH tracer, prepared from 1,2-diaminocyclohexane (1,2-DACH), is the only cationic renal agent tested in humans and has seven possible isomers. The complex isolated from the reaction of the racemic mixture, (±)-trans-1,2-DACH, and ReIO2(PPh3)2 after conversion to the BPh4- salt was found by X-ray crystallography to be the meso isomer, trans-[ReO2 (trans-R,R-1,2-DACH)(trans-S,S-l,2-DACH)][BPh4]·MeOH·2H2O (1). The structural parameters for 1 are normal. The complex is highly symmetrical, suggesting that the analogous meso Tc-99m-DACH agent is also symmetrical. Studies of other Tc-99m-DACH agents that were made from cis-1,2-DACH or individual trans-1,2-DACH enantiomers show that the biodistribution is not very dependent on the starting 1,2-DACH ligand stereochemistry; these agents must be less symmetrical than the meso Tc-99m-DACH agent analogue of 1. Thus, the overall charge and lipophilicity (similar for all Tc-99m-DACH isomers) exert a greater influence on biodistribution than the specific structural features of the different Tc-99m-DACH isomers.

Biodistribution and renal excretion of isomers of the cationic tracer, 99mTc diaminocyclohexane (DACH): Biodistribution of cationic renal tracers

The buildup of organic anions in the plasma in the uremic state can competitively inhibit the tubular extraction of para-aminohippurate or (131)I ortho-iodohippurate (OIH) and lead to spuriously low measurements of effective renal plasma flow (ERPF). This problem can be circumvented by the use of cationic tracers. The cationic renal tracer, (99m)Tc labeled diaminocyclohexane ((99m)Tc DACH), has a clearance of 80% of OIH in mice but its clearance in humans is relatively low, only 30% of OIH. The (99m)Tc DACH isomer(s) used in prior studies, however, was not clearly defined and may have consisted of a single isomer or a combination of isomers. Since the anionic isomers of some (99m)Tc renal tracers have been shown to have widely different clearances, the biodistribution and urine excretion of the (99m)Tc cis-, trans-S,S, trans-R,R and +/-trans-DACH isomers were compared in Sprague-Dawley rats at 10 minutes and 60 minutes postinjection to determine if one of the (99m)Tc DACH isomers may be a significantly better renal tracer than the others. The red cell binding of (99m)Tc +/- trans-DACH was also determined. All of the isomers showed a high degree of specificity for the kidney with minimal secretion into the gastrointestinal tract. Urine excretion of the 4 tracers, however, was only 38-48% that of OIH at 10 minutes and 66-84% that of OIH at 60 minutes. Red cell binding was 6.9%. Cationic renal tracers have the potential to provide a more accurate measurement of ERPF than anionic tracers. Based on the animal data, however, it is unlikely that any of the (99m)Tc DACH isomers will have a substantially higher clearance in humans than the form of (99m)Tc DACH originally tested. Development of alternative cationic renal tracers is warranted.