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Biomedical subjects

G Vaidyanathan

Publications and source records attributed to G Vaidyanathan.

51 records · Page 3Linked to original sources
PubMed1992

Fluorine-18-labeled monoclonal antibody fragments: a potential approach for combining radioimmunoscintigraphy and positron emission tomography.

Monoclonal antibody fragments labeled with 18F could be useful for PET if selective tumor uptake could be achieved within a few half-lives of this nuclide. To evaluate this possibility, the F(ab')2 fragment of Mel-14, an antibody reactive with gliomas and other tumors, was labeled by reaction with N-succinimidyl-4-[18F]fluorobenzoate. The in-vitro binding properties of 18F-labeled Mel-14 F(ab')2 were nearly identical to those observed when this F(ab')2 was labeled by reaction with N-succinimidyl-4-[125I]iodobenzoate (18F, affinity constant = (6.7 +/- 1.1) x 10(8) M-1; 125I, affinity constant = (8.8 +/- 0.6) x 10(8) M-1). The tissue distribution of the two labeled fragments was compared in paired-label studies performed in athymic mice with subcutaneous D-54 MG human glioma xenografts. Uptake of both nuclides in tumor was rapid with levels as high as 18.7% +/- 1.1% injected dose/g for 18F and 19.4% +/- 1.0% injected dose/g for 125I observed by 4 hr after injection. Tumor-to-normal tissue ratios for 18F-labeled Mel-14 F(ab')2 at 4 hr ranged between 0.8:1 for kidneys to 40:1 for brain. These results suggest that it may be feasible to use 18F-labeled antibody fragments for imaging tumors with PET.

Animals↗
PubMedDate not supplied

Radiolabeled guanine derivatives for the in vivo mapping of O(6)-alkylguanine-DNA alkyltransferase: 6-(4-[(18)F]Fluoro-benzyloxy)-9H-purin-2-ylamine and 6-(3-[(131)I]Iodo-benzyloxy)-9H-purin-2-ylamine.

Two radiolabeled analogues of 6-benzyloxy-9H-purin-2-ylamine (O(6)-benzylguanine; BG) potentially useful in the in vivo mapping of O(6)-alkylguanine-DNA alkyltransferase (AGT) were synthesized. Fluorine-18 labeling of the known 6-(4-fluoro-benzyloxy)-9H-purin-2-ylamine (FBG; 6) was accomplished by the condensation of 4-[(18)F]fluorobenzyl alcohol with 2-aminopurin-6-yltrimethylammonium chloride (4) or 2-amino-6-chloropurine in average decay-corrected radiochemical yields of 40 and 25%, respectively. Unlabeled 6-(3-iodo-benzyloxy)-9H-purin-2-ylamine (IBG; 7) was prepared from 4 and 3-iodobenzyl alcohol. Radioiodination of 9, prepared from 7 in two steps, and subsequent deprotection gave [(131)I]7 in about 70% overall radiochemical yield. The IC(50) values for the inactivation of AGT from CHO cells transfected with pCMV-AGT were 15 nM for IBG and 50 nM for FBG. The binding of [(18)F]6 and [(131)I]7 to purified AGT was specific and saturable with both exhibiting similar IC(50) values (5-6 microM).

Animals↗
PubMedDate not supplied

Protein radiohalogenation: observations on the design of N-succinimidyl ester acylation agents.

In previous studies we have demonstrated that antibodies radioiodinated with N-succinimidyl 3-iodobenzoate (SIB) are less susceptible to loss of radioiodine in vivo than antibodies iodinated directly by electrophilic substitution on their tyrosine residues with Iodogen. Since the Bolton-Hunter reagent, N-succinimidyl 3-(4-hydroxy-3-iodophenyl)propionate, is identical with SIB except that it contains a hydroxyl group on the aromatic ring and a two-methylene spacer, a comparison of their coupling chemistry and in vivo behavior was performed to better understand the structural requirements for a useful iodinated acylation agent. Protein concentration and pH had a significant effect on the coupling efficiency of both SIB and the Bolton-Hunter reagent; however, protein-labeling yields with SIB were generally higher by a factor of 2. Paired-label biodistribution studies in mice demonstrated that thyroid uptake (a monitor of dehalogenation) of antibody labeled by the Bolton-Hunter method was twice that of antibody labeled with SIB but only 7% of that observed for antibody labeled with Iodogen. These results suggest that even minor differences in iodination site can profoundly alter the retention of label on a protein in vivo.

Acetylation↗
PubMedDate not supplied

Radioiodination of antibodies via N-succinimidyl 2,4-dimethoxy-3-(trialkylstannyl)benzoates.

We have previously shown that use of N-succinimidyl 3-iodobenzoate (SIB) for radioiodination of monoclonal antibodies (MAbs) decreases the loss of radioiodine in vivo compared to MAbs labeled by using conventional methods. Herein, the synthesis of N-succinimidyl 2,4-dimethoxy-3-(trialkylstannyl)benzoates (alkyl = Me, Bu) are described as is their use as precursors for the radiosynthesis of N-succinimidyl 2,4-dimethoxy-3-iodobenzoate (SDMIB). A MAb F(ab')2 fragment labeled with SDMIB retained its ability to bind specifically to tumor homogenates. Paired-label tissue distribution studies indicate that the thyroid uptake (an indicator of deiodination) of hydrolyzed SDMIB was about 20 times that of hydrolyzed SIB. In contrast, thyroid uptake for SDMIB, when conjugated to a MAb, was only 1.4-2.8 times that for SIB and was considerably lower than levels reported in the literature for MAbs labeled by using direct, electrophilic iodination methods. Although MAbs labeled with SDMIB are significantly more inert to dehalogenation than those labeled by conventional methods, compared to the original SIB reagent, addition of two methoxy groups decreased retention of label in vivo.

Animals↗
PubMedDate not supplied

A polar substituent-containing acylation agent for the radioiodination of internalizing monoclonal antibodies: N-succinimidyl 4-guanidinomethyl-3-[131I]iodobenzoate ([131I]SGMIB).

The objective of this study was to develop an acylation agent for the radioiodination of monoclonal antibodies that would maximize retention of the label in tumor cells following receptor- or antigen-mediated internalization. The strategy taken was to add a polar substituent to the labeled aromatic ring to impede transport of labeled catabolites across lysosomal and cell membranes after antibody degradation. Preparation of unlabeled N-succinimidyl 4-guanidinomethyl-3-iodobenzoate (SGMIB) was achieved in six steps from 3-iodo-4-methylbenzoic acid. Preparation of 4-guanidinomethyl-3-[131I]iodobenzoic acid from the silicon precursor, 4-(N1,N2-bis-tert-butyloxycarbonyl)guanidinomethyl-3-trimethylsilylbenzoic acid proceeded in less than 5% radiochemical yield. A more successful approach was to prepare [131I]SGMIB directly from the tin precursor, N-succinimidyl 4-(N1,N2-bis-tert-butyloxycarbonyl)guanidinomethyl-3-trimethylstannylbenzoate, which was achieved in 60-65% radiochemical yield. A rapidly internalizing anti-epidermal growth factor receptor variant III antibody L8A4 was labeled using [131I]SGMIB in 65% conjugation efficiency and with preservation of immunoreactivity. Paired-label in vitro internalization assays demonstrated that the amount of radioactivity retained in cells after internalization for L8A4 labeled with [131I]SGMIB was 3-4-fold higher than that for L8A4 labeled with 125I using either Iodogen or [125I]SIPC. Catabolite assays documented that the increased retention of radioiodine in tumor cells for antibody labeled using [131I]SGMIB was due to positively charged, low molecular weight species. These results suggest that [131I]SGMIB warrants further evaluation as a reagent for labeling internalizing antibodies.

Acylation↗
PubMedDate not supplied

1-(m-[211At]astatobenzyl)guanidine: synthesis via astato demetalation and preliminary in vitro and in vivo evaluation.

No-carrier-added 1-(m-[211At]astatobenzyl)guanidine ([211At]MABG) was synthesized by astato demetalation using two different routes. The overall yield for the two-step approach from 3-(tri-n-butylstannyl)benzylamine was 13%. N-Chlorosuccinimide-mediated astato desilylation of 1-[3-(trimethylsilyl)benzyl]guanidine in acetic acid gave poor yields. In trifluoroacetic acid, the reaction worked well. The radiochemical yield was independent of reaction time and the amount of precursor used; however, the temperature of the reaction had a marked effect. Yields of 85% were obtained in 5 min at 70 degrees C using 0.5 mumol of the precursor. The percentage specific binding in vitro of [211At]MABG was nearly constant over a 2-log activity range and was comparable to that of no-carrier-added [131]MIBG. The accumulation of [211At]MABG in the heart and adrenals of normal mice was similar to that observed for no-carrier-added [131]MIBG.

3-Iodobenzylguanidine↗
PubMedDate not supplied

Radioiodination of proteins using N-succinimidyl 4-hydroxy-3-iodobenzoate.

N-Succinimidyl 4-hydroxy-3-[131I]iodobenzoate ([131I]SHIB) was synthesized from 4-hydroxybenzoic acid in two steps. The overall radiochemical yield was 40-56%. A monoclonal antibody (mAb) was labeled in 10-15% yield by reaction with [131I]SHIB. The specific binding of [131I]SHIB mAb to tumor homogenates in vivo was 78 +/- 3%, compared to 84 +/- 3% for the same mAb labeled using N-succinimidyl 3-[125I]iodobenzoate ([125I]SIB). Paired-label studies in normal mice demonstrated similar tissue distributions of 131I and 125I except in thyroid. In thyroid, uptake of the two isotopes was similar on day 1; however, 131I levels increased gradually to 2-3 times those of 125I by day 6. Our results indicate that loss of label in vivo from mAbs labeled using SHIB is somewhat higher than seen with SIB but significantly lower than that observed when direct iodination methods are used.

Animals↗
PubMedDate not supplied

Improved synthesis of N-succinimidyl 4-[18F]fluorobenzoate and its application to the labeling of a monoclonal antibody fragment.

Our previously reported method for the 18F labeling of antibodies using N-succinimidyl 4-[18F]fluorobenzoate (SFB) involved a rather long synthesis time. Here we present an improved method for the synthesis of SFB which reduces the synthesis time by about 45 min. A reaction time of 5-8 min (versus 25 min for the original procedure) was sufficient in the fluorination step to form 4-[18F]fluorobenzaldehyde in high yield. In the original method, 30-35 min was necessary to convert 4-[18F]fluorobenzoic acid to SFB using dicyclohexylcarbodiimide and N-hydroxysuccinimide. When N,N'-disuccinimidyl carbonate was used, facile conversion of 4-fluorobenzoic acid to SFB was seen at a micromolar level. At a tracer level, no product was formed at room temperature; however, complete consumption of starting material was observed. Heating at 150 degrees C resulted in the formation of SFB in more than 80% yield in 1-3 min. HPLC purification of SFB was necessary since use of crude SFB, or SFB purified using a silica solid-phase cartridge column, resulted in lower protein coupling yields. Furthermore, use of crude SFB resulted in cross-linking and lower immunoreactivity of antibody. Largely as a result of the considerable reduction in total labeling time, these modifications have increased the amount of 18F-labeled antibody available per 100 mCi of [18F]fluoride by 30-35%.

Animals↗
PubMedDate not supplied

Synthesis of ring- and side-chain-substituted m-iodobenzylguanidine analogues.

With the goal of developing MIBG analogues with improved targeting properties especially for oncologic applications, several radioiodinated ring- and side-chain-substituted MIBG analogues were synthesized. Except for 3-[(131)I]iodo-4-nitrobenzylguanidine and N-hydroxy-3-[(131)I]iodobenzylguanidine, the radioiodinated analogues were prepared at no-carrier-added levels from their respective tin precursors. The radiochemical yields generally were in the range of 70-90% except for 3-amino-5-[(131)I]iodobenzylguanidine for which a radiochemical yield of about 40% was obtained. While the silicon precursor N(1),N(2)-bis(tert-butyloxycarbonyl)-N(1)-(4-nitro-3-trimethylsilylbenzyl)guanidine did not yield 3-[(131)I]iodo-4-nitrobenzylguanidine, its deprotected derivative, N(1)-(4-nitro-3-trimethylsilylbenzyl)guanidine was radioiodinated in a modest yield of 20% providing 3-[(131)I]iodo-4-nitrobenzylguanidine. Exchange radioiodination of 3-iodo-4-nitrobenzylguanidine gave 3-[(131)I]iodo-4-nitrobenzylguanidine in 80% radiochemical yield. No-carrier-added [(131)I]NHIBG was prepared from its silicon precursor N(1)-hydroxy-N(3)-(3-trimethylsilylbenzyl)guanidine in 85% radiochemical yield.

3-Iodobenzylguanidine↗
PubMedDate not supplied

Biological evaluation of ring- and side-chain-substituted m-iodobenzylguanidine analogues.

A number of ring- and side-chain-substituted m-iodobenzylguanidine analogues were evaluated for their lipophilicity, in vitro stability, uptake by SK-N-SH human neuroblastoma cells in vitro, and biodistribution in normal mice. As expected, the lipophilicity of m-iodobenzylguanidine increased when a halogen was introduced onto the ring and decreased with the addition of polar hydroxyl, amino, and nitro substitutents. Most of the derivatives showed reasonable stability up to 24 h in PBS at 37 degrees C. While N(1)-hydroxy-N(3)-3-[(131)I]iodobenzylguanidine and 3,4-dihydroxy-5-[(131)I]iodobenzylguanidine generated a more nonpolar product in addition to the free iodide, 3-[(131)I]iodo-4-nitrobenzylguanidine decomposed to a product more polar than the parent compound. The specific uptake of 4-chloro-3-[(131)I]iodobenzylguanidine, 3-[(131)I]iodo-4-nitrobenzylguanidine, and N(1)-hydroxy-N(3)-3-[(131)I]iodobenzylguanidine by SK-N-SH human neuroblastoma cells in vitro, relative to that of m-[(125)I]iodobenzylguanidine, was 117 +/- 10%, 50 +/- 4%, and 12 +/- 2%, respectively. The specific uptake of the known m-iodobenzylguanidine analogues 4-hydroxy-3-[(131)I]iodobenzylguanidine and 4-amino-3-[(131)I]iodobenzylguanidine was 80 +/- 4% and 66 +/- 4%, respectively. None of the other m-iodobenzylguanidine derivatives showed any significant specific uptake by SK-N-SH cells. Heart uptake of 4-chloro-3-[(131)I]iodobenzylguanidine in normal mice was higher than that of m-[(125)I]iodobenzylguanidine at later time points (11 +/- 1% ID/g versus 3 +/- 1% ID/g at 24 h; p < 0.05) while uptake of 3-[(131)I]iodo-4-nitrobenzylguanidine and of N(1)-hydroxy-N(3)-3-[(131)I]iodobenzylguanidine in the heart was lower than that of m-iodobenzylguanidine at all time points. In accordance with the in vitro results, none of the other novel m-iodobenzylguanidine derivatives showed any significant myocardial or adrenal uptake in vivo.

3-Iodobenzylguanidine↗
PubMedDate not supplied

Synthesis and evaluation of glycosylated octreotate analogues labeled with radioiodine and 211At via a tin precursor.

Carbohydration of N-terminus and substitution of a threonine for the threoninol residue at the C-terminus of Tyr3-octreotide (TOC) has resulted in improved pharmacokinetics and tumor targeting of its radioiodinated derivatives. Yet, these peptides are very susceptible to in vivo deiodination due to the similarity of monoiodotyrosine (MIT) to thyroid hormone. The goal of this work was to develop octreotate analogues containing both a sugar moiety and a nontyrosine prosthetic group on which a radioiodine or 211At can be introduced. Solid-phase synthesis and subsequent modifications delivered an iodo standard of the target peptide N(alpha)-(1-deoxy-D-fructosyl)-N(epsilon)-(3-iodobenzoyl)-Lys0-octreotate (GIBLO) and the corresponding tin precursor N(alpha)-(1-deoxy-D-fructosyl)-N(epsilon)-[(3-tri-n-butylstannyl)benzoyl]-Lys0-octreotate (GTBLO). GIBLO displaced [125I]TOC from somatostatin receptor subtype 2 (SSTR2)-positive AR42J rat pancreatic tumor cell membranes with an IC50 of 0.46 +/- 0.05 nM suggesting that GIBLO retained affinity to SSTR2. GTBLO was radiohalogenated to [131I]GIBLO and N(alpha)-(1-deoxy-D-fructosyl)-N(epsilon)-(3-[211At]astatobenzoyl)-Lys0-octreotate ([211At]GABLO) in 21.2 +/- 4.9% and 46.8 +/- 9.5% radiochemical yields, respectively. From a paired-label internalization assay using D341 Med medulloblastoma cells, the maximum specific internalized radioactivity from [131I]GIBLO was 1.78 +/- 0.8% of input dose compared to 9.67 +/- 0.43% for N(alpha)-(1-deoxy-D-fructosyl)-[125I]iodo-Tyr3-octreotate ([125I]I-Gluc-TOCA). Over a 4 h period, the extent of internalization of [131I]GIBLO and [211At]GABLO was similar in this cell line. In D341 Med murine subcutaneous xenografts, the uptake of [125I]I-Gluc-TOCA at 0.5, 1 and 4 h was 21.5 +/- 4.0% ID/g, 18.8 +/- 7.7% ID/g, and 0.9 +/- 0.4% ID/g, respectively. In comparison, these values for [131I]GIBLO were 6.9 +/- 1.2% ID/g, 4.7 +/- 1.4% ID/g, and 0.8 +/- 0.5% ID/g. Both in vitro and in vivo catabolism studies did not suggest the severance of the lys0 along with its appendages from the peptide. Taken together, although GIBLO maintained affinity to SSTR2, its tumor uptake both in vitro and in vivo was substantially lower than that of I-Gluc-TOCA suggesting other factors such as net charge and overall geometry of the peptide may be important.

Animals↗
PubMedDate not supplied

No-carrier-added (4-fluoro-3-[131I]iodobenzyl)guanidine and (3-[211At]astato-4-fluorobenzyl)guanidine.

With 3-bromo-4-fluorotoluene as starting material, [4-fluoro-3-(trimethylsilyl)benzyl]guanidine was prepared in five steps in 1.5% overall yield. Radioiodination of this silicon precursor using N-chlorosuccinimide in trifluoroacetic acid at room temperature for 5 min gave (4-fluoro-3-[131I]-iodobenzyl)guanidine ([131I]FIBG) in 50-60% radiochemical yield. A byproduct which had a retention time in two HPLC systems similar to that of (m-iodobenzyl)guanidine (MIBG) was formed in about 30% yield. [131I]FIBG was stable up to 3 h under these conditions of iodination, indicating that the byproduct is not generated as a result of [131I]FIBG degradation. Using hydrogen peroxide as the oxidant in aqueous medium and a reaction time of 30 min at 50 degrees C, yields of [131I]FIBG could be increased to 75-80%, with less than 7% of the byproduct formed under these conditions. Astatination of the silicon precursor using N-chlorosuccinimide in trifluoroacetic acid at 70 degrees C gave 65-70% radiochemical yield of (3-[211At]astato-4-fluorobenzyl)guanidine ([211At]AFBG) in 10-15 min; about 17% of the byproduct formation was seen. Astatination of the silicon precursor under aqueous conditions using hydrogen peroxide was not successful.

Amino Acids↗
PubMedDate not supplied

Method for radioiodination of proteins using N-succinimidyl 3-hydroxy-4-iodobenzoate.

A conjugation method has been developed for the radioiodination of proteins which should be adaptable to kit formulation. m-Hydroxybenzoic acid was converted to 3-hydroxy-4-[131I]iodobenzoic acid in 65% radiochemical yield using Chloramine-T as the oxidant. This intermediate was then converted to N-succinimidyl 3-hydroxy-4-[131I]iodobenzoate ([131I]mSHIB) in 75% yield by reaction with N-hydroxysuccinimide and dicyclohexylcarbodiimide in a reaction time of only 10 min. Monoclonal antibody (mAb) 81C6 was labeled in 40-60% yield by reaction with [131I]mSHIB. Performing purifications of radioiodinated compounds using cartridges instead of HPLC did not alter conjugation efficiency, mAb immunoreactivity, or tissue distribution. Thyroid uptake of labeled mAb was low but up to 2.4 times higher than that seen when the mAb was labeled with N-succinimidyl 3-[125I]-iodobenzoate. These results suggest that [131I]mSHIB may be a useful reagent for the radioiodination of proteins, particularly in contexts when less complicated purification methods would be advantageous.

Animals↗
PubMedDate not supplied

Iodopyridine-for-iodobenzene substitution for use with low molecular weight radiopharmaceuticals: application to m-iodobenzylguanidine.

Substituting a pyridine ring for a benzene ring in the acylation agent N-succinimidyl 3-iodobenzoate has resulted in a useful approach for the radiohalogenation of monoclonal antibodies, peptides, and labeled biotin conjugates. It was hypothesized that such a substitution in m-iodobenzylguanidine (MIBG), a radiotracer used in the detection and treatment of neuroendocrine tumors, might result in an analogue with more rapid normal tissue clearance, thereby facilitating its use for tumor therapy. For the preparation of this analogue, 3-guanidinomethyl-5-iodopyridine (GMIP; 9b), the silicon precursor 4 was synthesized starting from 5-bromonicotinic acid. Attempts to convert 4 to 9b under various conditions were not successful. Radioiodinated 9b could be prepared by the iododestannylation of the tin precursor 8 in 65-70% radiochemical yield. A number of in vitro, in vivo, and ex vivo studies showed that pyridine-for-benzene substitution in MIBG yielded a compound that no longer was taken up by the uptake-1 pathway.

3-Iodobenzylguanidine↗