Cysteine, a chelating moiety for synthesis of technetium-99m radiopharmaceuticals: II. Attempt to synthesize renal tubular radiopharmaceuticals

NW/. Med. Viol. Vol. 17, No. 8, pp. 757-762, 1990 Inr. J. Radiac. Appl. Instrum. Parr B Printed in Great Britain. All rights reserved

0883-2897/90 $3.00 + 0.00 Copyright Q 1990 Pergamon Press plc

Cysteine, a Chelating Moiety for Synthesis of Technetium-99m Radiopharmaceuticals: II. Attempt to Synthesize Renal Tubular Radiopharmaceuticals SOMA Department

SANYAL

and SOMENATH

BANERJEE*

of Nuclear Medicine, Indian Institute of Chemical Biology, 4 Raja S. C. Mullick Road, Calcutta 700 032. India (Received

4 April

1990)

IV-acyl glycine residue is known to interact with renal tubular transport enzymes and thereby promotes renal tubular secretion. Recently, a similar renal property was also observed with dioxotechnetium aminocarboxy chelates. Therefore, a radiopharmaceutical was designed containing both the above groups with the expectation that an efficient %Tc labelled renal tubular agent could be developed by this process with which evaluation of various renal parameters will be possible. The synthesized compound, though secreted through the renal tubular pathway, did not show the expected efficiency. It was indicated that the renal property of the molecule was entirely due to chelated dioxotechnetium moiety and the expected effect of the acyl glycine residue was not observed in the molecule.

Introduction

known agents for inhibiting renal tubular transport (Beyer, 1954; Mudge and Taggart, 1950). Therefore it is anticipated that if an organic molecule, which possesses the required chemical characteristics for renal tubular transport, is attached to cysteine by S-substitution, it is possible that one can obtain a radiopharmaceutical which can secrete efficiently through the renal tubular pathway. The importance of such an agent for determining ERPF, an important parameter for renal function evaluation is well established (Fritzberg et al., 1981). It is well known that to interact with renal tubular transport enzymes, a substrate must possess anionic characteristics as well as hydrogen bonding capability, the latter occurring in at least two centres placed at definite distances from the anionic site (Despopoulos, 1965). A simple compound possessing the above characteristics is represented by structure I (Fig. l), with carboxylate ion behaving as an anionic site and the possibility of hydrogen bonding at the carboxylate carbonyl and amide carbonyl groupings (Despopoulos, 1965). This structure may be able to bind to the sulfur atom of cysteine through a methylene group, the desired compound being represented as 2 (Fig. 1). After 99mTc chelation, ligand 2 is expected (Chattopadhyay and Banerjee, 1988) to have the chelate structure 6, which satisfies the chemical structural requirement for interacting with the transport enzymes at three sites (a,b,c), with two

Cysteine possesses good potential as a chelating moiety for converting a bioactive molecule to a ligand for %Tc chelation. Attachment of organic molecule to cysteine is possible by electrophilic attack, either at the sulfhydryl or the amino centre. Also the carboxylic acid group could be activated to promote a nucleophilic attack at that site. Because of these different modes of synthetic possibilities, cysteine offers an appreciable versatility in ligand synthesis, and a wide selection of biomolecules is possible for conversion to *“Tc-based radiopharmaceuticals. Also the ligand thus produced as a result of this synthetic reaction still contains two donating groups which are well known (Hagen et al., 1977; Chattopadhyay and Banerjee, 1988; Halpern et al., 1972) to produce stable 99mTcchelates. S-substituted cysteines should offer a special advantage for the synthesis of renal tubular radiopharmaceuticals because of the presence of the r-aminocarboxy donating group in the ligand. We have established (Chattopadhyay and Banerjee, 1988) that technetium chelates of this class of ligand are excreted through the renal tubular secretory pathway, since excretion is depressed either in the presence of probenecid or 2,4_dinitrophenol, which are both well --*Author

for correspondence. 757

758

SOMASANYAL and -;-NH-CH2-C-O

BANEWEE

of structure 1 could be observed in 6. The results are described in the following sections.

‘0 0

0

!SOMENATH

Materials and Methods

P S

N”2

Lip-

k-l -

CO2t-i

2,R=CH2X0.NH.CH2.COR’,

R’=OH

3, R = CH2,CONH. CH2. COR’, 4, R= CH2.CONH.CH2.COR’, 5, R= C2H5

R’ = OCH3 R’=NH2

S.CH2.CONH.CH2.C02H a f

Melting points were determined in open capillary tubes and are uncorrected. Infrared spectra were recorded with a Perkin-Elmer 177 spectrometer in a potassium bromide disc. PMR spectra were recorded in a Jeol NMR spectrometer model FX 100 in D20. 99mTcwas obtained by 2-butanone extraction from a 5 N NaOH solution of 99Mo. TLC experiments were performed with precoated TLC cellulose F254 plates (E. Merck, F.R.G.) and two solvent systems: (A) ethanol-water (7:3) and (B) butanol-acetic acid-pyridine-water (5 : 1: 3: 4) were used for each ligand. Various y -countings were performed in a well type y-counter from the Electronic Corporation of India, model LV 4755. Syntheses of ligands Methyl N-(3’thia-S-amino-5’-carboxy) pentanoyl glycinate (3). To a solution of cysteine (1.83 g,

\

S.Cti2.CONHCH2

.C02H C

Proposed Structure

of 6

Fig. 1. Chemical structures of ligands and proposed structure for one representative *Tc chelate.

structure 1 residues (a,c) and one grouping (b). The latter moiety resembles structure 1 (Chattopadhyay and Banerjee, 1988) since (i) both of them are anionic, (ii) negatively charged dioxotechnetium moiety resembles carboxylate ion, and (iii) chelated carboxyls are capable of hydrogen bonding, like the amide carbonyl. It was observed by us (Chattopadhyay and Banerjee, 1988) that technetium complexes possessing the above characteristics undergo tubular secretion because of the above similarity in chemical structure. Therefore, it is expected that molecule 6 should undergo efficient tubular secretion, because the above three centres a, b and c are connected by a flexible carbon
15 mmol) in saturated bicarbonate solution (50 mL), N-chloroacetyl glycine methyl ester (2.5 g, 17 mmol) was added with vigorous stirring. After complete addition, the stirring was continued for another 2 h after which the pH was adjusted to 3. The clear aqueous solution was evaporated and the dry white residue was extracted with boiling absolute ethanol. On cooling in an ice bath, the combined alcoholic extract furnished white crystals (1.58 g, 35%) which were recrystallized from ethanol several times, to furnish the analytical sample; m.p. 176178°C. TLC RfA) 0.75; R:s, 0.52; i.r. vmsx1640 (-CO, NH,), 1750cm-’ (COOMe). NMR 6, 3.16 (m, 2H, S-CH,-C) 3.40 (S, 2H, OC-CH,-S) 3.72, (S, 3H, -CO*OCH,) 4.04 (S, 2H, HN-CH,-C02). Anal. Calcd for CBH14N205S: C, 38.40; H, 5.60; N, 11.20. Found: C, 38.18; H, 5.29; N, 10.95. N-(3’-thia-S-amino-S-carboxy)

pentanoyl glycine

(2). The preceding methyl ester 3 (400 mg, 1.6 mmol) was hydrolyzed by heating with 5 N NaOH (4 mL) at 95°C for 15 min. The solution was then cooled and acidified to pH 3. The aqueous solution was evaporated, the residue dissolved in a small volume of water and an equal volume of ethanol was added when the desired acid crystallized slowly. The material recrystallized from aqueous ethanol to furnish the pure material (2.85 g, 75%); m.p. 174178°C. TLC $A, 0.53; Rfa, 0.20; i.r. v,,,,, 1580 (COO-), 1640 (CO ‘NH,). NMR, 6 3.24 (m, 2H, S-CH,-CH), 3.44 (S, 2H, OC-CH,-S), 4.04 (S, 2H, NH-CH,-CO,H) Anal. Calcd for C,H,,O,N,S: C, 35.59; H, 5.08; N, 11.86. Found: C, 35.25; H, 5.21; N, 12.04. N-(3’-thia-5’.amino-S-carboxy) pentanoyl glycinamide (4). A solution of the methyl ester 3 (0.75 g,

3 mmol) in methanol (30 mL) was saturated with dry ammonia gas at 0°C. The stoppered solution was allowed to stand at room temperature for 48 h after

Cysteine in the synthesis of *“Tc radiopharmaceuticals

which it was evaporated. The deposited crystalline material was filtered and recrystallized from ethanol to furnish the pure material (0.4g, 56%); m.p. 1788180°C. TLC RI*), 0.52; Rf,, 0.23; i.r. v,, 1580 (COO-), 1660 (-CO.NH-), 1665 (CONHI). NMR 6 3.20 (m, 2H, S-CHr-CH), 3.48 (S, 2H, OC-CHr-S), 3.96 (S, 2H, N-CHCONH) Anal. Calcd for CjH,,O,NjS: C, 35.74; H, 5.53; N, 17.87; Found: C, 36.08; H, 5.21; N, 17.54. RadiolabeUing

To a solution of either of the ligands (2-S) (10 mg) in freshly boiled nitrogen purged water (0.3 mL) ““TcO, was added (0.2mL, 1.5 mCi, 55.5 MBq) followed by addition of stannous chloride solution (50 pL) under nitrogen; the latter reagent was prepared by dissolving stannous chloride dihydrate (IO mg) in 6 N HCl (100 pL) and diluting to 25 mL. The final pH of the solution was 4.01 and the solution was allowed to stand at room temperature for 30 min before further use. Determination

qf radiochemical purity

The radiochemical purity of %“‘Tc chelates was evaluated by two TLC experiments for each complex using silica gel as stationary phase, and either acetone or 0.9% saline solution as developer. In a separate experiment the R, values of possible contaminants like TcO; and reduced hydrolysed technetium (Sn-Tc) were also recorded. From these results the percentage of purity of individual chelate was determined using standard procedure (Neirinckx et al., 1987). Sephadex gel chromatography

A Sephadex gel chromatographic system consisting of a gel column (G25M, 1.I x 28 mm) was utilized for determination of radiochemical purity, purification and separation of constituent radiolabelled material. After application of the sample (100 p L), the column was eluted with nitrogen purged water and fractions of 1 mL were collected and analysed either in a dose callibrator or y-counter depending on the amount of radioactivity present. In a separate experiment the elution volumes of Sn-Tc and 99”Tc0; were also determined. HPLC analysis

The retention times of 99mTc chelates were measured using a reverse-phase HPLC system with a p-Bondapak C,8 column (3.9 x 30cm). After injecting the complex (5-10 PL), the column was eluted isocratically with methanol-water (9: 1) at the rate of I .OmL/min under 1.1 x 10’ lb/in* pressure. From the recorder the number of components and their retention times were determined. Biodistribution studies

Male rats (15&200 g) anaesthetized with urethane (25%. 0.6mL/lOOg body wt) were hydrated

solution

759

via the femoral vein with saline solution (0.9%, 6 mL) for 1 h, 1 mL of the solution being injected every tenth min. After hydration was complete the animals were allowed to stabilize for 1 h and then the desired radiopharmaceutical (50 PCi, 1.85 MBq) was injected. In inhibition experiments probenecid (50mg/kg) was injected as 3% aqueous solution 10 min before the radiopharmaceutical injection. The animals were sacrificed by injecting air intravenously and the desired organs were taken out. The blood and urine were collected by puncture of heart and urinary bladder, respectively. Intestinal radioactivity was determined by homogenizing the tissue in water, and counting the aliquots. Other organs were washed once with saline and blotted dry to remove the residual blood. The results were expressed as percent dose/organ by counting the samples in a y-counter against the suitably diluted aliquots of the injected solution as standard. Results Condensation of cysteine with IV-chloroacetyl glycine methyl ester produced the corresponding Ssubstituted cysteine, 3, in moderate yield. Hydrolysis and amonolysis of 3 produced the corresponding acid 2 and the amide 4 respectively, in good yields. These three previously unreported compounds had the expected analytical and spectroscopic data. S-ethyl cysteine 5 was prepared by reducing cysteine with sodium and liquid ammonia followed by in situ alkylation with ethyl iodide according to the known procedure (Armstrong and Lewis, 1951). Radiolabelling of these ligands with %“‘Tc was performed using stannous chloride as the reducing agent. The reaction mixture was analyzed by TLC using two solvent systems for each ligand and it was observed that the desired chelates were produced in 65-70% yield and the only contaminant present was reduced hydrolysed technetium (Sn-Tc). To remove Sn-Tc from the desired chelate, Sephadex gel chromatography was used, since we observed (Chattopadhyay and Banerjee, 1988) that Sn-Tc does not elute out of this column. However, this chromatography also showed that there were two components present in each chelate preparation, one eluting at 13-15 mL and the other at 22-26 mL (Table I). Under the same experimental conditions pertechnetate eluted at 17-l 8 mL, with no detectable amount of pertechnetate being present in the preparation (Fig. 2). The purified chelate was again analyzed by TLC, and Sn-Tc was found to be absent in the purified complex. The presence of two compounds in each chelate was confirmed by HPLC experiment where two peaks with different retention times were obtained. To correlate the Sephadex gel chromatographic data and HPLC results, the components separated by Sephadex gel chromatography were individually subjected to HPLC analysis. Earlier it was observed

SoMASANYALand

760

SOMENATHBANERJEE

Table I. Labelling yield and chromatographic data for %Tc RmTc complex of 2 3 4 5 TcO, Sn-Tc

Thin layer chromatography G,

Sephadex gel chromatography Peak A

%

I I I I

8 0 0 I 0

12-13 14-15 12-13 12-14

Fig. 2. Sephadex gel chromatographic analysis of *Tc-2 showing the position of the two components (A) and (B). Absence of activity at 17 mL indicates that no pertechnetate was present in the chelating mixture.

that components which eluted at 12-15 mL in Sephadex gel chromatography had retention times of 2.39-2.82 min in HPLC, whereas the fraction that eluted at 22-25 mL was retained in the HPLC column for 3.754.17 min (Table 1). The biodistribution data of the two components prepared from each of the four hgands (2-5) and separated by Sephadex gel chromatography at the 15 and 30 min time points, are depicted in Tables 2 and 3. It appears from these data that there is no significant excretion of these chelates by the hepatobiliary pathway, and they preferred the renal route as the major excretory channel. The significant difference in biodistribution that was observed between components (A) and (B) is that component (B), which was derived from each of the four ligands, exhibited a Table

2. Biodistribution radiouharmaceuticals

I5min ‘J’I-OIH %Tc-DTPA Tc-2A Tc-2B Tc-3A Tc-3B Tc-QA Tc-4B TC-M Tc-SB

HPLC Peak A

Peak B

Yield of chelation

2.46 2.69 2.82 2.39

3.7s 4.17 3.92 4.03

81 78 71 72

24-2s 22-23 2&22 22-24

2.3 -

17-18 Did not elute

is ib

240

Peak B

chelates of ligands 2-5

considerable amount of kidney retention compared to component (A). WmTc-2, however, did not show any increase in renal excretion compared to the other chelates. “mT~-24 behaved essentially similar to WmTc-5 regarding their biodistribution-only a moderate increase of renal excretion was observed for the former chelates, probably because of the presence of hydrophilic groups present in the molecule. To determine the exact renal handling mechanism of these two chelates, the biodistribution of the two components for each of the WmTc chelates derived from ligands 2 and 5 were determined after i.v. administration of probenecid. It was observed in all cases that there was definite depression of renal excretion in probenecid treated animals compared to control animals for component (B) obtained only from @“‘Tc-2 and 99mT~-5. On the other hand, component (A) from *Tc-2 and 99mTc-5 exhibited undepressed renal excretion in probenecid treated animals (Table 4). This suggests two different renal excretory pathways for the two components.

Discussion After chelation in the presence of stannous chloride, hgands 2-5 were found to contain appreciable amounts of reduced hydrolyzed technetium (Sn-Tc), and were purified by the Sephadex gel chromatographic method (Chattopadhyay and Banerjee, 1988). During chromatography it was observed that the chelate separated into two components (A and B) which had appreciable differences in elution volume. Essentially similar results were obtained by HPLC analysis of the chelate mixture, where the presence of the two components having different retention times, could

be demonstrated.

Therefore,

of *Tc chelates of ligands 2-S along with two standard renal at I5 min wst iniection ~exoressed in oercent dose oer ornan)

Blood

Liver

7.12+0.50 8.59 + 2.09 12.28 + 2.10 II.12 +0.26 12.71 + 2.86 9.51 f 0.82 13.25 f 2.67 13.00 + 3.69 9.87 f 0.77 12.85 f 1.29

3.29 f 0.38 4.36 f 0.51 4.76 * 0.72 4.95f0.18 3.55 f 0.29 2.43 & 0.50 2.81 f 0.39 5.40 f 0.91 5.60 * 0.62 6.03 k 0.43

Results are mean for 4-6 rats&SD.

Intestine

I .96

f 0.67 4.30 * 0.85 4. I7 -+ 0.85 2.88 f 0.25 5.14 * 0.85 4.79 f 1.31 3.15 f 0.37 4.33 f 0.61 4.01 f 0.60 3.57f0.17

Kidney

Urine

2.74 + 0.31 2.69 + 0.46 6.16_+0.29 8.1 I -+ 0.66 6.05 It 0.30 Il.32 f 0.66 3.19*0.18 8.73 f 1.89 3.46 i_ 0.25 6.08 f 0.28

69.89 f 3.55 30.14 f 2.72 32.27k 1.16 28.18k2.12 35.30 f: 1.68 39.24 f 2.82 40.76 + 5.37 33.19 f 2.26 27.19 + 1.99 23.64 jl I .05

it appears

either

Cysteine in

761

the synthesis of %Tc radiopharmaceuticals

Table 3. Biodistribution of 99"rcchelates of the ligands2-5 along with two standardrenal radiopharmaceuticals at 30min post-injection (expressed in percentdose per organ) 30min 'WOIH %Tc-DTPA Tc-2A Tc-2B Tc-3A Tc-3B Tc4A Tc4B Tc-5A Tc-5B

Blood

Liver

5.90* 1.09 5.73f 2.12 12.03k4.71 8.44+ 0.29 10.55f 3.26 9.34f 3.37 8.79rt1.85 9.36+ 1.38 8.27f 0.58 10.16&-1.65

1.95kO.52 3.44kO.30 5.79* 1.55 4.95kO.33 2.54kO.18 4.27f 0.38 1.91f0.18 5.52+ 1.16 4.38+ 0.94 4.55f 0.78

Intestine 4.28kO.59 2.47+ 0.92 8.50+ 2.03 4.16kO.27 4.01k 0.98 5.28+ 0.18 6.12f 2.27 6.63kO.55 3.84+0.67 3.49+ 0.46

Kidney

Urine

1.62+ 0.60 2.08IO.80 7.69+0.36 10.89kO.19 4.12f 0.27 10.44I2.18 3.55*0.41 8.54kO.64 3.42& 0.20 5.96F 0.32

85.16+ 6.03 30.51k 3.76 45.22+ 3.35 34.99k4.72 50.30* 1.53 44.28+ 0.64 50.84+ 2.16 40.81+ 2.06 35.77+ 2.66 32.03f 0.39

Resultsare mean for46 ratsf SD

of the above chromatographic methods is suitable for resolution of the above chelate mixture. The two components exhibited a considerable difference in their renal properties, as apparent from their biodistribution results. The biodistribution in the presence of probenecid indicated that renal excretion of both of the first components (2A and 5A) were undepressed compared to control animals, whereas definite depression of the same was observed while using the second components (2B and 5B). This indicates that the first components were excreted by efficient glomerular filtration since their renal excretions were comparable to that of *Tc-DTPA. On the other hand, both the B components exhibited much less renal excretion in comparison to that of ‘311-OIH, indicating that the affinity of these “9mTc chelates towards the renal transport enzymes were not as pronounced as observed for the former radiopharmaceutical. The presence of two components in renal radioexhibiting different chromatopharmaceuticals, graphic and renal properties, has been observed for

99mTc-DADS compounds (Fritzberg et al., 1982). The presence of two components was demonstrated for 99mTc-Co2-DADS by HPLC and the fact that they are geometrical isomers was proved by mass spectrometric methods (Costello et al., 1983). Ligands 2-5 are expected to chelate by amino and carboxy grouping and the fact that these types of chelate can exhibit geometrical isomerism is exemplified by the isolation of cis and tram isomers of octahedral Cu (II) and square planer Pt (II) complexes of a-amino acids (Melnik, 1982; Volshtein and Ankova, 1963). We have postulated (Chattopadhyay and Banerjee, 1988) that %Tc complexes of a-amino acids are best represented by an octahedral dioxotoechnetium bis structure, and in analogy to the above Cu(II) and Pt(II) complexes can exhibit geometrical isomerism. Therefore, the fact that components (A) and (B) may bear a c&tram relationship to each other cannot be altogether ruled out. It appears from the above experiments that the expected characteristic effect of structure residue I toward the renal tubular transport enzymes, as

Table 4. Effectof nrobenecidon biodistribution of %Tc Blood 5 min 2-A 2-B S-A 5-B

32.04i:2.88 (20.83k 2.12) 30.09* 1.28 (21.72f 2.94) 21.29+ 1.53 (20.40f 1.92) 18.93_+1.24 (15.1550.65)

lO!?llli 2-A 17.17+ 0.82 (14.84f 1.12) 2-B 19.63+ 2.26 (15.21+ 2.55) 5-A II.53i 1.12) (10.08+ 1.11) 5-B 18.99f 2.29 (14.19kO.50)

Liver 10.90+ (14.36k 12.16* (9.31f 5.72F (7.99+ 8.89+ (9.63+

Intestine

2-B 5-A S-B

15.93+ (12.28+ 16.56+ (11.12i II.67+ (9.87f 13.78k (12.85f

0.48 2.10) 2.20 0.26) 0.59 0.77) I.31 I.ZY)

Kidney

Urine

0.46 0.62) 1.53 1.07) 0.29 0.52) 0.23 1.02)

19.54kO.91 (14.33+ 1.76) 7.05+ 1.07 (13.30+ 1.24) 15.665 0.71 (12.36+ 0.45) 13.50i-O.82 (IS.35+ 2.34)

2.35kO.19 (2.40kO.34) 5.56k 1.29 (2.87+ 0.64) 4.45kO.54 (3.65kO.76) 3.37+0.11 (3.99+ 0.23)

4.78+ 0.04 (5.20f 0.85) 4.26+ 0.40 (8.90kO.51) 4.29f 0.58 (3.55kO.41) 4.04+ 0.24 (6.61F0.37)

24.04+ 0.77 (23.26+ 1.77) 14.39+ 0.79 (18.28+ 1.27) 24.35Il.97 (24.23k 2.15) 15.0+ 2.53 (19.20i0.52)

2.66F 0.30 (4.17kO.85) 3.93kO.81 (2.88f 0.25) 3.82f 0.58 (4.01* 0.60) 4.85kO.47 (3.57*0.17)

4.68+ 0.89 (6.16i 0.29) 3.39f 0.37 (8.11+0.66) 3.97* 0.09 (3.46f 0.25) 4.68+ I.12 (6.08kO.28)

34.80k 0.92 (32.27+ 1.16) 15.74+ 0.43 (28.1852.12) 33.17k 2.85 (27.19f 1.99) 19.85+ 0.89 _. _. .^~ (23&I+_ I.U5)

1.69 1.14) 2.07 0.49) 1.25 0.93) 0.09 0.50)

2.52kO.18 (2.45+O.IS) 2.88+ 0.76 (2.71+ 0.14) 3.04i 0.46 (3.91kO.41) 3.50_+0.48 (4.32+ 0.28)

5.42+ 0.30 (5.06: 0.87) 8.25f 1.07 (7.67+ 0.85) 4.40+ 0.60 (7.86;0.25) 5.38+_0.62 (6.55k 0.79) 4.68f 0.89 (4.76+ 0.72) 6.21+ 0.62 (5.95-+1.92) 7.71*0.45 (5.6050.62) 4.57kO.83 (6.03kO.43)

l5min

2-A

chelates of ligands2-S in rats

6.29f (5.39+ 7.91* (II.56+ 4.37f (5.26f 5.32+ (6.56+

Resultsare mean and SD for4 rats. Controlvaluesare givenin parentheses.

762

SOMA SANYAL and SOMENAM BANERJEE

postulated in the literature (Despopoulos, 1965) was not observed for 99mTc-2B, and the above moiety 1 essentially behaved as a biologically inert unit just like the ethyl group ( 99mTc-5B).It is also well known (Despopoulos, 1965) that any modification of the carboxy group in structure 1 results in a complete loss of renal tubular transport ability, therefore, a decrease in renal excretion is usually observed by this group modification. It is apparent that this effect is also not observed, since the renal excretion of 99mTc-2 was not altered compared to ““Tc-3 or *Tc-4 in the expected way. The above rationale for synthesis of technetium tagged radiopharmaceuticals has previously been utilized (Chervu et al., 1984), which led to the synthesis of wmTc-PAHIDA. The authors claimed that this radiopharmaceutical was excreted by the renal tubular secretory pathway, though the renal clearance was only half that of “‘I-OIH. Recent experiments (Fang et al., 1988), however, proved that the compound was not cleared by renal tubules since its renal excretion was not depressed by probenecid. Therefore, it may be concluded that although several technetium essential radiopharmaceuticals possessing excellent renal tubular secretory properties have been synthesized (Fritzberg et al., 1986) the attempts to synthesize a technetium tagged radiopharmaceutical utilizing the substrate of general structure 1 has been disappointing. With our present knowledge about the technetium based radiopharmaceutical this discrepancy

cannot

be adequately

explained.

Acknowledgement-The authors are thankful to Indian Council of Medical Research for providing a Fellowship to one of them (SS).

References Armstrong, M. D.; Lewis, J. D. Thioether derivatives of cysteine and homocysteine. J. Org. Chem. 16: 749-753; 1951. Beyer, K. H. Factors basic to the development of useful inhibitors of renal transport mechanism. Arch. Ind. Pharmacodyn. 98: 97-117; 1954.

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