Preclinical evaluation of gastrin derivatives labelled with 111In: radiolabelling, affinity profile and pharmacokinetics in rats

BACKGROUND

Cholecystokinin receptor subtype 2 (CCK-2) is overexpressed in various tumours like medullary thyroid carcinomas and small cell lung cancer. Radiolabelled peptides that bind with high affinity and specificity to CCK-2 receptors, thus hold great potential for visualizing such tumours.

METHODS

We compared four 111In labelled gastrin analogues, called minigastrins (MG), namely MG11, MG45, MG47 and MG48 linked to metal chelating DOTA in preclinical experiments. The radiolabelled peptides were tested for peptide binding in CCK-2 receptor-bearing cell line AR42J and for their pharmacokinetics in normal rats.

RESULTS

The experiments suggest that all gastrin analogues had similar and relatively rapid internalization into AR42J cells. Binding to CCK-2 receptors in AR42J cells was saturable for all agents but there were some differences in receptor affinity. This biodistribution study in rats showed a rapid decrease in blood radioactivity, predominantly renal clearance and saturable uptake of the radiopharmaceutical and/or its metabolites in the CCK-2 receptor-positive stomach. Higher kidney accumulation of radioactivity was only found for 111In-DOTA-minigastrin 48.

CONCLUSIONS

The data suggest that the 111In-DOTA-minigastrin analogues studied are promising candidates for the scintigraphy of CCK-2 receptor-expressing tumours; 111In-DOTA-MG47 and 111In-DOTA-MG11 are the most promising.

Distribution, elimination, and renal handling of (99m)technetium-Demogastrin 1

Radiolabeled cholecystokinin/gastrin (CCK) receptor-targeting peptides are promising compounds for radiodiagnosis and radiotherapy of certain malignancies. This study evaluated the pharmacokinetic profile of a CCK-2 receptor-specific peptide, Demogastrin 1, labeled with technetium-99m ((99m)Tc-Demogastrin 1), in rats. To investigate the fate of (99m)Tc-Demogastrin 1 in the rat, biodistribution and elimination studies in vivo were performed, and elimination parameters in perfused rat liver and kidney were determined. Biodistribution studies showed that (99m)Tc-Demogastrin 1 was rapidly cleared from the blood and most organs. A significant amount of radioactivity was detected in the CCK-2 receptor-rich organs, such as the stomach. Low radioactivity was found in the CCK-1 receptor-rich organs. Radioactivity in bowels and stomach declined relatively slowly. High and long-term retention of radioactivity in the kidneys was observed. Elimination of (99m)Tc-Demogastrin 1 via the bile was negligible. A high and rapid renal excretion was observed in elimination experiments in vivo. In the perfused kidney, glomerular filtration was found to be the main renal excretion mechanism of (99m)Tc-Demogastrin 1. Demogastrin 1 was distributed preferentially to the organs expressing CCK-2 receptors. The decisive elimination route of (99m)Tc-Demogastrin 1 in rats was urinary excretion. A high and prolonged renal retention may limit potential clinical use of the compound.

Preclinical evaluation of wound treatment with hyaluronan-iodine hydrogel

A mixture of hyaluronan and iodine complex KI3 (Hyiodine®) has been developed to support wound healing. In this study, the effect of hyaluronan, KI3, and their combination on progression of wound healing in excision skin wounds in rats was determined.

METHODS

To evaluate the possible toxic effects of iodine after Hyiodine application for wound healing, iodine systemic absorption from the Hyiodine-treat- ed wounds and its distribution profile were quantitatively described and compared after intravenous iodide administration using 131I as a radiolabel.

RESULTS

Treatment of rats with Hyiodine resulted in an enhancement of wound healing, proved by greater degrees of wound contraction and reduction in mean wound healing time compared to other treated rats on hours 1, 2, 24, and 216. Even if wound therapy with Hyiodine resulted in systemic iodine uptake, the estimated iodine absorbed dose in human therapy is tolerable, and the theoretical risk of its systemic toxic effects is minimal.

CONCLUSION

A hyaluronan-iodine hydrogel has a great potential for effective treatment of wounds. .

Different radioactivity uptake between somatostatin analogues labelled with ¹¹¹In and ⁹⁰/⁸⁸Y in rat kidney

Somatostatin receptor targeting is a valuable method to treat somatostatin receptor-positive tumours. In peptide receptor radionuclide therapy, it is essential to determine the highest activity that can be safely administered to the patient. As (90)Y emits no gamma rays, absorbed doses for (90)Y are usually estimated using the same peptide labelled with (111)In. The aim of the study was to determine if replacement of (90/88)Y by (111)In affects the biodistribution profile of five selected somatostatin analogues in preclinical experiments.

MATERIALS AND METHODS

Radiolabelled peptides were administered intravenously to male Wistar rats.

RESULTS

The peptides under study labelled either with (111)In or with (88/90)Y showed similar distribution profiles in all tissues excepting the kidney. The kidney radioactivity uptake was significantly lower for (88/90)Y-labeled peptide in comparison with the one of (111)In.

CONCLUSION

We conclude that a radiation-absorbed dose after (90)Y-labelled somatostatin analogues appears to be lower than that predicted by the (111)In-labelled peptide.

Receptor affinity and preclinical biodistribution of radiolabeled somatostatin analogs

In this study, we investigated the relationship between affinity to somatostatin receptor subtype2 (SSTR(2)) and uptake of radioactivity from a group of radiolabeled somatostatin analogues in somatostatin receptor-rich tissues of rats. Organs with a high density of somatostin receptors (namely the adrenals and pancreas bearing mainly SSTR(2); this receptor subtype is also the most abundant in somatostatin receptor-positive tumors) were chosen as markers of specific binding of the peptides in vivo. Accumulations of radioactivity in these organs 24 h and 48 h after intravenous administration of six (111)In-labeled octreotide and octreotate derivatives with predominant affinity to SSTR(2) were correlated with affinity to SSTR(2) determined in vitro (IC(50) values). For correlation between adrenal uptake of radioactivity and IC(50), the best fit for exponential dependence was found; for that of pancreas, however, linear dependence was the most suitable. In cases where the values for the peptide with affinity to SSTR subtypes 2, 3 and 5, namely (111)In-DOTA-Nal(3)-octreotide were included in the group of studied agents, substantially less correlations were obtained. Our results showed that uptake of radioactivity in tissues with a high density of somatostatin receptors correlates with somatostatin receptor affinity of receptor-specific peptides; however, other factors (the affinity to particular receptor subtypes, the overall pharmacokinetic profile in the body etc.) may contribute to this observed relationship.

Preclinical evaluation of (177)lu-nimotuzumab: a potential tool for radioimmunotherapy of epidermal growth factor receptor-overexpressing tumors

BACKGROUND

The humanized monoclonal antibody Nimotuzumab (h-R3) has demonstrated an exceptional and better clinical profile than other monoclonal antibodies for immunotherapy of epidermal growth factor receptor-overexpressing tumors. This work deals with the preparation and radiolabeling optimization of (177)Lu-Nimotuzumab and their preclinical evaluation.

METHODS

Nimotuzumab was conjugated with S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (p-SCN-Bn-DOTA), testing different molar ratios. The immunoconjugates were characterized. The radiolabeling with (177)Lu was optimized. Radioimmunoconjugates stability was tested in 2-[bis[2-[bis(carboxymethyl)amino]ethyl]amino]acetic acid (DTPA) excess and human serum. In vitro studies were performed in tumor model cell lines. Receptor-specific binding was tested by competitive inhibition. (177)Lu-Nimotuzumab in vivo studies were conducted in healthy and xenograft animals.

RESULTS

Nimotuzumab conjugates were obtained with high purity. Radiolabeling yield and specific activities ranged from 63.6% to 94.5% and from 748 to 1142 MBq/mg, respectively. The stability in DTPA excess and human serum was 95.9% and 93.2% after 10 days, respectively. The radioimmunoconjugate showed specific receptor binding in tumor cell lines. Biodistribution in healthy animals showed the typical behavior of the immunoconjugates based on monoclonal antibodies. The study in xenografts mice demonstrated uptake of (177)Lu-Nimotuzumab in the tumor and reticuloendothelial organs.

CONCLUSIONS

(177)Lu-Nimotuzumab was obtained with high purity and specific activities under optimal conditions without significant loss in immunoreactivity and might be a potential radioimmunoconjugate for radioimmunotherapy of tumors with epidermal growth factor receptor overexpression.

Biodistribution of two octreotate analogs radiolabeled with indium and yttrium in rats

BACKGROUND

In this study, two octreotate derivatives N-[4-carboxy-4-[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]butanoyl]-Tyr(3)-octreotate (DOTAGA-tate) and N-[[4,10-bis(carboxymethyl)-7-(1(1,3-dicarboxypropyl))-1,4,7,10-tetraaza-cyclododec-1-yl]acetyl]-Tyr(3)-octreotate (DOTA-t-GA-tate) were radio-labeled with (111)In or (88)Y and their biodistribution profiles together with their elimination characteristics in rats were compared.

MATERIALS AND METHODS

Radiolabeling of the peptides with high radiochemical purity was carried out in an acetate buffer with gentisic acid as radioprotective compound. Biodistribution profiles of the radiolabeled peptides were determined in intact male Wistar rats after an intravenous dose of 1 microg/kg. For elimination pathways analysis, studies in intact rats in metabolic cages and perfused rat kidney and liver were carried out.

RESULTS

Fast radioactivity clearance from rat tissues (excepting somatostatin receptor-rich organs and the kidney) was determined for all agents under study. Profound radioactivity uptake in organs with a high density of somatostatin receptors (namely the adrenals and pancreas as biomarkers of somatostatin receptor-positive tissue) was slightly higher for radiolabeled DOTAGA-tate when compared with DOTA-t-GA-tate. Significantly higher accumulation in kidney and somewhat lower urinary elimination of (111)In-labeled peptides in comparison with that of (88)Y-agents were determined. Perfused rat kidney experiments confirmed that glomerular filtration was the main elimination mechanism for the compounds under study; their bile clearances in the perfused rat liver were negligible.

CONCLUSION

(111)In((88)Y)-DOTAGA-tates exhibited higher distribution into somatostatin receptor-rich organs when compared with the corresponding radiolabeled DOTA-t-GA-tates. Higher uptake of (111)In-labeled peptides in the kidney is attributed to its different coordination properties.

Preparation and the kinetic stability of hyaluronan radiolabeled with 111In, 125I and 14C

Three different procedures for the labeling of hyaluronan (HA) with (111)In, (125)I and (14)C radionuclides were compared, and the kinetic stability of radiolabeled HA under different conditions (saline, artificial gastric juice and plasma) was established. Modification of HA structure with bifunctional chelating agents (DTPA) or with the prosthetic group (tyramine or tyrosine) was essential prior (111)In and (125)I labeling. These chemical labeling techniques were fast, simple and inexpensive, and labeled agents with a high specific activity were obtained. The only disadvantage of these methods was the occurrence of unknown functional groups in the HA molecule requiring further characterization of the compound. Conversely, HA labeling with (14)C by biotechnological synthesis was found to be rather expensive and time-consuming process. Although, the final product (14)C-HA was identical to natural HA its low specific activity presents certain limitation for its application in biological experiments. Stability studies showed that (14)C-HA and (125)I-Tm-HA were stable in all studied mediums. In the case of (125)I-Trs-HA, stability slightly decreased in rat plasma and in artificial gastric juice with increasing time. The least stable was (111)In-DTPA-HA, which degraded completely after 48h in artificial gastric juice. Kinetic stability studies may provide primary information concerning the properties of radiolabeled HA in vitro, which is essential for the use and explanation of its behavior in biological experiments.

Dendrimers: Analytical characterization and applications

This review focuses on analytical techniques used for separation and characterization of dendrimers and their derivatives. These macromolecules have been attractive material for a development of new drug carriers and imaging agents. They are also interesting for many biological and industrial applications. The review mentions a few of them.

Comparison of 111In-DOTA-NOC and 111I-DOTA-TATE distribution in the target and dose-limiting tissues: conflicting results in vitro and in vivo

BACKGROUND

In this study, some important biological characteristics of two radiolabelled somatostatin analogues 111In-DOTA-1-Nal3-octreotide (DOTA-NOC) and 111In-DOTA-Tyr3-octreotate (DOTA-TATE) were compared.

MATERIALS AND METHODS

Rats were used for in vivo biodistribution experiments and in vitro cell models (OK and AR42J cell lines) were used for simulating the internalization in the kidney and in subtype 2 somatostatin receptor (SSTR2)-positive tissues, respectively.

RESULTS

Significantly higher radioactivity concentrations in rat organs with high density of somatostatin receptors after 111In-DOTA-NOC administration in comparison with 111In-DOTA-TATE were observed. The predominant urine excretion was associated with accumulation of the radioactivity in the kidney, where higher retention of 111In-DOTA-TATE compared to 111In-DOTA-NOC was detected. In the OK cell line the opposite results were found. No significant differences in the in vitro internalization and externalization of radioactivity to AR42J cell line were found for either peptide suggesting their same affinity for SSTR2.

CONCLUSION

Preclinical experiments indicated that 111In-DOTA-NOC is a very promising peptide for somatostatin receptor-positive tumour visualization. The conflict between the in vitro and in vivo kidney handling showed that the transfer of results from in vitro to in vivo conditions and their interpretation should be performed very carefully because both types of experiments can be affected by different factors, making their simple comparison difficult.

Radiolabelling of glucose-Tyr3-octreotate with 125I and analysis of its metabolism in rats: comparison with radiolabelled DOTA-Tyr3-octreotate

BACKGROUND

Somatostatin analogues labelled with radiometals or radiohalogens are useful for the imaging and treatment of somatostatin receptor-containing tumours. In this study, the procedures for the radioiodination of glucose-Tyr3-octreotate (gluc-Tyr3-tate) and radiolabelling of DOTA-Tyr3-octreotate (DOTA-Tyr3-tate) with 111In, 177Lu and 125I were compared and their metabolism in rats was analyzed. The usefulness of high performance liquid chromatography (HPLC) analysis and instant thin-layer chromatography on silica gel (ITLC-SG) for both radiochemical purity determination and analysis of metabolism in urine was investigated.

MATERIALS AND METHODS

For labelling with radiometals, the formation of a complex with the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) functionality of the peptide was employed. Radioiodination was performed by the chloramime-T method. The radiochemical purity of radiolabelled peptides and the analyses of rat urine were determined by HPLC and/or ITLC-SG methods. Male Wistar rats were used in the elimination studies.

RESULTS

DOTA-Tyr3-tate was simply radiolabelled with radiometals with high yield and high radiochemical purity. Stopping of the reaction was a critical step for radioiodination, therefore labelling of gluc-Tyr3-tate and DOTA-Tyr3-tate with 125I was not so simple and the reaction product had to be purified by preparative HPLC analysis. Whereas 111In-DOTA-Tyr3-tate and 177Lu-DOTA-Tyr3-tate were eliminated in rat urine in a practically unchanged form, a significant proportion of metabolites was observed with radioiodinated peptides, particularly at longer time intervals.

CONCLUSION

Labelling of DOTA-Tyr3-tate with radiometals is simple and the radiochemical purity of prepared compounds is very high, while iodination of the peptides demands purification of the product by preparative HPLC. The analysis of rat urine showed that excretion of radioiodinated peptides included a significant proportion of metabolites.

Biodistribution and elimination characteristics of two 111In-labeled CCK-2/gastrin receptor-specific peptides in rats

BACKGROUND

Due to their high CCK-2/gastrin receptor selectivity, high affinity, and rapid background clearance, radiolabeled minigastrins (MG) are emerging as promising new tools in the diagnosis and therapy of CCK-2/gastrin receptor-positive tumors. In this study, the pharmacokinetic profile, particularly the excretion mode, of two 111In-labeled minigastrins was compared in rats. The first tracer, 111In-MG-0 is based on (D)Glu1-MG, while the second, 111In-MG-11, is its des-(Glu)5-derivative, expected to be less retained in renal tissue.

MATERIALS AND METHODS

The fate of 111In-MG-0 and 111In-MG-11 in the body of rats was investigated during biodistribution and bioelimination experiments, while the respective elimination parameters were determined in perfused rat liver and kidney models.

RESULTS

During biodistribution both compounds were rapidly cleared from the blood and most non-target organs whereas activity levels in the bowel and stomach declined slowly. The overall contribution of hepatobiliary excretion of 111In-MG-0 and 111In-MG-11 was relatively small. In the perfused rat liver their elimination into the bile was negligible. In contrast, renal excretion was the major excretion pathway for both analogs, mainly via glomerular filtration. However, kidney levels were substantially higher and retention was more prolonged in the case of 111In-MG-0 as compared to 111In-MG-11.

CONCLUSION

The presence of the (Glu)5-chain in 111Ln-MG-0 appears to be implicated in the prolonged radioactivity retention in the kidney of rats.

99mTc demotate 1: biodistribution and elimination characteristics in rats

BACKGROUND AND METHODS

In this study, we investigated the biodistribution and elimination characteristics of a new radiolabelled somatostatin analogue, 99mTc demotate 1, in rats by in-vivo biodistribution and elimination experiments, perfused rat liver and kidney experiments and micro-autoradiography of renal tissue.

RESULTS

Rapid clearance from blood and most organs was found. High and long-term uptake in organs with high density of somatostatin receptors (the adrenals and pancreas) and in stomach and intestine was reduced in non-radiolabelled octreotide pretreated animals. The predominant urine excretion was associated with an accumulation of 99mTc demotate 1 in the kidney, mainly in the renal cortex. This uptake was not affected by non-radiolabelled octreotide pretreatment.

CONCLUSION

99mTc demotate 1 is a prospective radiopharmaceutical for use in human medicine in somatostatin receptor-positive tumour imaging and its potential should be confirmed in further experiments and clinical trials.

Pharmacokinetics of radiolabelled quinlukast in rats

Pharmacokinetics together with in vivo metabolism and elimination of quinlukast, a potential anti-asthmatic and anti-inflammatory drug, were designed in rats. For this purpose, bile duct cannulated rats and an in situ perfused rat liver preparation were employed. 3H-radiolabelled compound was administered i.v. or loaded to the perfusion medium, respectively. Quinlukast represented the main form of radioactivity determined in plasma; in comparison with the parent drug metabolites were present in lower levels in the systemic circulation. The pharmacokinetic parameters related to the whole animal were calculated from quinlukast rat plasma concentration-time course. The distribution of quinlukast in the body was relatively fast (distribution half-life was approx. 6 min), the elimination half-life exceeded 2h. Binding of quinlukast to rat plasma proteins was very high (approx. 99.7%) and this binding influenced distribution volumes of quinlukast. Both the volume of the central compartment and the volume at a steady state were approx. 115 and 430 ml, respectively. The experiments showed that the biliary clearance was the major route of elimination of this compound from the systemic circulation of rats. In agreement with the determined elimination half-life approx. 42% of the radioactivity was found in the bile, with <0.5% appearing in the urine. The majority of the eliminated radioactivity in the bile was in the form of polar metabolites; only a small part of the parent compound was determined. Two hours after intravenous administration, polar metabolites - but no parent drug - were detected in the urine.

Analysis of accumulation of 99mTc-octreotide and 99mTc-EDDA/HYNIC-Tyr3-octreotide in the rat kidneys

The aim of this study was to compare renal handling and distribution of (99m)Tc-octreotide and (99m)Tc-EDDA/HYNIC-Tyr(3)-octreotide (HYNIC-TOC) in rats. In kidney perfusion experiments, the renal clearance value of (99m)Tc-octreotide was three times lower than that of (99m)Tc-EDDA/HYNIC-TOC. The predominant renal excretion of (99m)Tc-EDDA/HYNIC-TOC was associated with a high and long-term renal accumulation up to 48 hrs. Microautoradiographic results indicated that (99m)Tc-EDDA/HYNIC-TOC was retained mainly in the renal medulla within the cells of the collecting ducts and in the surrounding tissue. Lower positivity was found in the proximal and distal tubular cells. We conclude that the mechanism of renal accumulation of somatostatin analogues renal accumulation is complex and that proximal tubular reabsorption is probably not the main mechanism for uptake of (99m)Tc-EDDA/HYNIC-TOC in the kidneys. The presence of the somatostatin receptors, differences in the tonicity level within kidneys and other possible mechanisms could participate in their renal accumulation.

HPLC-radiometric determination of quinlukast in biological fluids

An analytical method for the analysis of a novel antiasthmatic drug quinlukast and its metabolites in the plasma, bile and urine was developed. For the analysis, the solid phase extraction method and the C(8) RP-HPLC with radiometric detection of the drug were used. This method enables a quantitative determination of the agent and all of its metabolites (even of those with an unknown structure) in a biological system. The procedure is, therefore, suitable both for the pharmacokinetic analysis of quinlukast and the determination of its elimination pathways.

Comparison of 99mTc-mercaptoacetyltriglycine and [131I]-o-iodohippurate elimination in perfused rat kidney

Renal elimination of two renal radiodiagnostic agents, (99m)Tc-mercaptoacetyltriglycine ((99m)Tc-MAG3) and [(131)I]-o-iodohippurate (OIH) has been studied using the method of the perfused rat kidney. The experiments showed significant differences between renal handling of (99m)Tc-MAG3 and OIH in the perfused rat kidney. While the renal clearance for (99m)Tc-MAG3 was relatively stable, the renal clearance values of OIH rapidly decreased after the OIH administration in a bolus dose. The infusion administration of OIH resulted in stable clearance values during the perfusion. The OIH/(99m)Tc-MAG3 renal clearance ratio was 2.47. Both compounds were bound to proteins in the perfusate to a considerable extent. An analysis of renal handling showed that contribution of tubular secretion to the total excretion was 95% for OIH, and 97% for (99m)Tc-MAG3.

Octreotide and octreotate derivatives radiolabeled with yttrium: pharmacokinetics in rats

Distribution profiles and elimination pathways in rats of two new octreotate derivatives radiolabeled with yttrium, namely Y-DOTAGA-tate and Y-DOTA-t-GA-tate, were compared with those of Y-DOTA-octreotide and Y-DOTA-Tyr(3)-octreotide. All synthetic somatostatin analogues under study were rapidly cleared from the blood and most organs of rats. The main elimination pathway for all peptides under study was urine excretion. High and long-term uptakes of radioactivity in the kidneys and also in organs with high density of somatostatin receptors (the adrenals and pancreas) were found. Radioactivity concentrations in these somatostatin receptor-rich organs were substantially higher for octreotate derivatives in comparison with octreotide analogues; the highest values for Y-DOTAGA-tate were determined. The octreotate derivatives under study appear to be specific ligands for treatment of somatostatin receptor-positive tumors if some mechanism to decrease their kidney retention is provided.

Lanreotide labeled with 99mTc: preparation, preclinical testing and comparison with (111)In-DTPA-octreotide

Somatostatin receptors are known to be present at a high density in a large number of tumors while (111)In-DTPA-octreotide has been routinely used in oncology for imaging somatostatin receptor-positive tumors and metastases. Lanreotide is another somatostatin receptor-specific peptide, shown to be effective in controlling the growth of some human tumors. The aim of this study was to label lanreotide with 99mTc by a direct labeling method and to evaluate the distribution and elimination characteristics of the labeled agent in rats. (111)In-octreotide was used as the reference radiopharmaceutical. For both radiolabeled-peptides the activity in blood and most organs decreased relatively rapidly with time. On the other hand, 99mTc-lanreotide was excreted mainly by the gastrointestinal tract to feces while (111)In-DTPA-octreotide was eliminated mostly into urine. The rat liver perfusion experiments showed that bile clearance of 99mTc-lanreotide was about three-order times higher than for (111)In-DTPA-octreotide. Analysis of the elimination mechanisms of 99mTc-lanreotide and (111)In-DTPA-octreotide in the perfused rat kidney confirmed that both peptides were eliminated mostly by glomerular filtration. Different protein binding of the agents ((111)In-DTPA-octreotide was only weakly bound, whereas 99mTc-lanreotide was strongly bound to proteins) resulted in substantially lower renal clearance of 99mTc-lanreotide when compared with (111)In-DTPA-octreotide. The results indicated that 99mTc-lanreotide could be of value for the scintigraphic imaging of specific tumors.

Distribution and elimination characteristics of 111In-DTPA-D-phe1-octreotide and 111In-DTPA-L-phe1-octreotide in rats

The present study compares distribution and elimination characteristics of 111In-DTPA-D-Phe1-octreotide and 111In-DTPA-L-Phe1-octreotide in rats and evaluated the effect of the replacement of the terminal L-phenylalanine by D-phenylalanine on pharmacokinetic profiles of the radiolabelled peptides. Both agents exhibited rapid radioactivity clearance from the blood and most organs and tissues with no systematic and significant differences in activity accumulation. The long-term retention and high radioactivity concentrations for both compounds under study were found in the kidneys and organs with a high density of somatostatin receptors, such as the pancreas and adrenals. The residence times in these organs were longer for 111In-DTPA-D-Phe1-octreotide in comparison with 111In-DTPA-L-Phe1-octreotide. The major elimination pathway for both radiolabelled peptides was relatively rapid excretion into the urine. Analysis of the renal handling by an employment of the perfused rat kidney showed that both peptides were eliminated mainly by the mechanism of glomerular filtration. Rat liver perfusion experiments confirmed a very low value of bile clearance of radioactivity for both agents under study.

Pharmacokinetics and renal handling of 99mTc-labeled peptides

99mTc-labeled peptides, particularly those of a lipophilic nature, are often excreted through the hepatobiliary system, and the subsequent accumulation in the intestine may obscure receptor-mediated uptake in tumor sites in the pelvis. We have therefore explored the route and rate of excretion of a small series of Tc-labeled peptides to shed some light on the mechanisms that influence the clearance of these agents.

METHODS

Pharmacokinetic parameters, biodistribution, routes of elimination of 99mTc-complexes of 3 model tetrapeptides--namely, acetyl-N-Gly-Gly-Cys-Gly (AGGCG), acetyl-N-Ser-Ser-Cys-Gly (ASSCG), and acetyl-N-Gly-Gly-Cys-Lys (AGGCL)--were determined in rats in vivo. Renal handling of the complexes was studied in the perfused rat kidney.

RESULTS

After intravenous injection, a relatively fast disappearance of the complexes from blood was found. Although the parameters of distribution in all 3 chelates were very similar, the elimination rate of 99mTc-AGGCG was higher than those of 99mTc-ASSCG and 99mTc-AGGCL. The Tc complexes under study were distributed mainly to the excretory organs (kidneys and liver), and no specific accumulation in other organs or tissues was found. Most of the radioactivity after intravenous administration of the chelates was rapidly eliminated through the urine, but a significant amount was also excreted through the feces, in the following order among the 3 chelates: 99mTc-AGGCL < 99mTc-ASSCG < 99mTc-AGGCG. Different proportions of glomerular filtration and secretion in renal tubules of the complexes were found in the perfused rat kidney. Elimination by glomerular filtration was dominant only in the case of 99mTc-AGGCL, whereas the rate of filtration of 99mTc-AGGCG was very low because of its high protein binding. Various rates of secretion into renal tubules were shown for all 3 agents. This renal excretion pathway was decisive in 99mTc-AGGCG and lowest in 99mTc-AGGCL. 99mTc-ASSCG was eliminated by both mechanisms at similar rates.

CONCLUSION

These studies show that increasing the hydrophilic nature or reducing the negative charge of the peptides will reduce their hepatobiliary excretion, whereas the incorporation of suitable peptide sequences permits them to exploit efficient routes of renal excretion, such as tubular secretion, thereby optimizing the pattern of biodistribution of these radiopharmaceuticals.

Renal handling of iodobenzoates in rats

Renal elimination pathways of three positional isomers of iodobenzoic acid (2-iodobenzoate, 3-iodobenzoate and 4-iodobenzoate radiolabelled with 125I) were compared using the perfused rat kidney in-situ. All agents were eliminated both in a parent form (involving all renal elimination mechanisms i.e. glomerular filtration, tubular secretion, and tubular reabsorption) and also metabolized to a large extent in the kidney. After 3-iodobenzoate and 4-iodobenzoate administration, the major fractions of radioactivity found in urine were in the form of their metabolites, whereas 2-iodobenzoate was eliminated into urine mostly as the parent compound. Proportions of the individual metabolites in the urine of the perfused rat kidney were similar to those in intact rats for all agents. The results suggest that the kidney is the major organ for both the excretion and metabolism of iodobenzoates in rats. The principal renal metabolic reaction for all compounds under study was conjugation with glycine to produce the corresponding hippuric acid derivatives.