Ex vivo imaging and analysis of ROS generation correlated with microglial activation in rat model with acute neuroinflammation induced by intrastriatal injection of LPS

Neuroinflammation and oxidative stress are hallmarks of neurodegenerative diseases. Microglia, the major important regulators of neuroinflammation, are activated in response to excessive generation of reactive oxygen species (ROS) from damaged cells and resulting in elevated and sustained damages. However, the relationship between microglia and ROS-regulatory system in the early stages of neuroinflammation prior to the appearance of neuronal damages have not been elucidated in detail. In this study, we analyzed the time-dependent changes in ROS generation during acute neuroinflammation in rats that were given an intrastriatal injection of lipopolysaccharide (LPS). We evaluated the effects of minocycline, an anti-inflammatory antibiotic, and N,N'-dimethylthiourea (DMTU), a radical scavenger, to understand the correlation between activated microglia and ROS generation. Ex vivo fluorescence imaging using dihydroethidium (DHE) clearly demonstrated an increased ROS level in the infused side of striatum in the rats treated with LPS. The level of ROS was changed in time-dependent manner, and the highest level of ROS was observed on day 3 after the infusion of LPS. Immunohistochemical studies revealed that time-dependent changes in ROS generation were well correlated to the presence of activated microglia. The inhibition of microglial activation by minocycline remarkably reduced ROS levels in the LPS-injected striatum, which indicated that the increased ROS generation caused by LPS was induced by activated microglia. DMTU decreased ROS generation and resulted in remarkable inhibitory effect on microglial activation. This study demonstrated that ROS generation during acute neuroinflammation induced by LPS was considerably associated with microglial activation, in an intact rat brain. The results provides a basis for understanding the interaction of ROS-regulatory system and activated microglia during neuroinflammation underlying neurodegenerative diseases.

Influence of Linker Molecules in Hexavalent RGD Peptides on Their Multivalent Interactions with Integrin αvβ3

Multivalent RGD peptides have been used as an excellent targeting vector to integrin α_vβ₃-positive tumors. However, little attention has been paid to the influence of linker molecules in multivalent RGD peptides on their dissociation kinetics from tumor cells. In this study, we evaluated the dissociation kinetics of ^99mTc-labeled hexavalent RGD peptides which have (CH₂-CH₂-O)_n (n = 4, [^99mTc][Tc(L1)₆]⁺ and n = 12, [^99mTc][Tc(L2)₆]⁺) or (_DPro-Gly)_n (n = 1, [^99mTc][Tc(L3)₆]⁺; n = 6, [^99mTc][Tc(L4)₆]⁺; and n = 9, [^99mTc][Tc(L5)₆]⁺) as a linker molecule. The results showed that [^99mTc][Tc(L4)₆]⁺ and [^99mTc][Tc(L5)₆]⁺ displayed slower dissociation kinetics and [^99mTc][Tc(L4)₆]⁺ showed exceptionally high in vitro cellular uptake (203.1 ± 16.7% dose/mg protein) and the highest tumor to blood ratio (138.1 ± 26.3 at 4 h p.i.) in tumor bearing nude mice. These findings indicate that the use of appropriate length of (_DPro-Gly)_n would maximize the binding of multivalent RGD peptides to clustered integrin α_vβ₃.

Aryl isocyanide derivative for one-pot synthesis of purification-free 99mTc-labeled hexavalent targeting probe

INTRODUCTION

^99mTc-labeled hexavalent probes can be readily synthesized by the coordination of six equivalent isocyanide ligands towards Tc^I, and alkyl isocyanide ligands have been extensively used for preparing such probes. However, high ligand concentration (>1 mM) is generally required due to their insufficient coordination ability to Tc^I.

METHODS AND RESULTS

In this study, we revealed that aryl isocyanide ligands, which have greater π-accepting ability compared with alkyl ones, provided ^99mTc-labeled hexavalent probes in high radiochemical yields (>95%) even at low ligand concentration (50 μM). We applied this finding to the synthesis of a ^99mTc-labeled hexavalent RGD probe, targeting integrin α_vβ₃. This ^99mTc-labeled probe was prepared in a 5 min reaction at ligand concentration of 50 μM, and exhibited high tumor localization in vivo without post-labeling purification.

CONCLUSION

The present findings indicate that aryl isocyanide ligands would be a useful precursor to a variety of ^99mTc-labeled hexavalent targeting probes for molecular imaging of saturable systems.

ADVANCES IN KNOWLEDGE

Aryl isocyanide is a better precursor than alkyl isocyanide for preparing ^99mTc-labeled hexavalent targeting probe.

IMPLICATION FOR PATIENT CARE

This work provides a straightforward method to prepare molecular imaging agents of high target uptake, which would facilitate nuclear medicine imaging in clinical settings.

The synthesis of a 99mTc-labeled tetravalent targeting probe upon isonitrile coordination to 99mTcI for enhanced target uptake in saturable systems

The difference in 2-proton's acidity between L_β and L_G led to dramatically different results of their reactions with [^99mTc][Tc(CO)₃(OH₂)₃]⁺.

Coordination-Mediated Synthesis of Purification-Free Bivalent 99mTc-Labeled Probes for in Vivo Imaging of Saturable System

In the synthesis of technetium-99m (^99mTc) labeled target-specific ligands, the presence of a large excess of unlabeled ligands over ^99mTc in the injectate hinders target accumulation of ^99mTc-labeled ligands by competing for target molecules. To circumvent the problem, we recently developed a concept of the metal coordination-mediated multivalency, and proved the concept with a ^99mTc-labeled trivalent compound [^99mTc(CO)₃(CN-RGD)₃]⁺. In this study, D-penicillamine (Pen) was selected as a chelating molecule and a cyclic RGDfK peptide was conjugated to Pen via a hexanoic linkage (Pen-Ahx-c(RGDfK)). ^99mTc complexation reaction, and the stability, integrin α_vβ₃ binding affinity, and biodistribution of the ^99mTc-labeled probe were investigated to evaluate the applicability of the concept to bivalent probes. ^99mTc-[Pen-Ahx-c(RGDfK)]₂ was obtained over 95% radiochemical yields under low Pen-Ahx-c(RGDfK) concentration (50 μM). ^99mTc-[Pen-Ahx-c(RGDfK)]₂ showed approximately 10-times higher integrin α_vβ₃ binding affinity than the monovalent compounds, Pen-Ahx-c(RGDfK) and c(RGDyV). In biodistribution studies, the tumor accumulation of ^99mTc-[Pen-Ahx-c(RGDfK)]₂ was decreased to 77% and 43% of HPLC-purified (Pen-Ahx-c(RGDfK)-free) ^99mTc-[Pen-Ahx-c(RGDfK)]₂ by the presence of 5 nmol of unlabeled Pen-Ahx-c(RGDfK) and Re-[Pen-Ahx-c(RGDfK)]₂, respectively. ^99mTc-[Pen-Ahx-c(RGDfK)]₂ provided tumor image without removing unlabeled ligand, while a ^99mTc-labeled monovalent probe prepared from a monovalent ligand could not. These findings indicate the availability of the design concept to prepare ^99mTc-labeled bivalent probes with a variety of ^99mTc core and other metallic radionuclides of clinical relevance.

111In-DTPA-d-Phe-1-Asp0-d-Phe1-octreotide exhibits higher tumor accumulation and lower renal radioactivity than 111In-DTPA-d-Phe1-octreotide

INTRODUCTION

¹¹¹In-DTPA-d-Phe¹-octreotide scintigraphy is an important method of detecting neuroendocrine tumors. We previously reported that a new derivative of ¹¹¹In-DTPA-d-Phe¹-octreotide, ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide, accomplished the reduction of prolonged renal accumulation of radioactivity. The aim of this study was to evaluate the tumor accumulation of ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide in vitro and in vivo by comparing it with ¹¹¹In-DTPA-d-Phe¹-octreotide.

METHODS

The tumor accumulation of this octreotide derivative was determined by measuring its uptake using cultured AR42J cells in vitro and biodistribution studies in vivo. The distribution of the radiotracer and the extent of somatostatin receptor-specific uptake in the tumor were estimated by a counting method using AR42J tumor-bearing mice. The radioactive metabolite species in the tumor and kidney were identified by HPLC analyses at 3 and 24h post-injection of the ¹¹¹In-DTPA-conjugated peptide.

RESULTS

In both cases, in vitro and in vivo, the tumor radioactivity levels of ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide were approximately 2-4 times higher than those of ¹¹¹In-DTPA-d-Phe¹-octreotide. On in vitro cellular uptake inhibition and radioreceptor assay, ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide exhibited a binding affinity to somatostatin receptor highly similar to that of ¹¹¹In-DTPA-d-Phe¹-octreotide. As the additional cellular uptake of ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide was significantly lower at low temperature than at 37°C, it was considered that a cellular uptake pathway is involved in energy-dependent endocytotic processes. In the radiometabolite analysis of ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide, ¹¹¹In-DTPA-d-Phe-Asp-OH was a major metabolite in the tumor at 24h post-injection.

CONCLUSION

¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide exhibited higher tumor accumulation and persistence of tumor radioactivity than ¹¹¹In-DTPA-d-Phe¹-octreotide. We reasoned that this higher tumor accumulation would not be based on the receptor affinity but on a receptor-mediated endocytotic process involved in temperature-dependent cellular uptake. The present study demonstrated the great potential of the pharmaceutical development of a new radiolabeled peptide with high tumor accumulation and low renal radioactivity by the chemical modification of ¹¹¹In-DTPA-d-Phe¹-octreotide.

Redesign of negatively charged 111In-DTPA-octreotide derivative to reduce renal radioactivity

INTRODUCTION

Radiolabeled octreotide derivatives have been studied as diagnostic and therapeutic agents for somatostatin receptor-positive tumors. To prevent unnecessary radiation exposure during their clinical application, the present study aimed to develop radiolabeled peptides which could reduce radioactivity levels in the kidney at both early and late post-injection time points by introducing a negative charge with an acidic amino acid such as L-aspartic acid (Asp) at a suitable position in ¹¹¹In-DTPA-conjugated octreotide derivatives.

METHODS

Biodistribution of the radioactivity was evaluated in normal mice after administration of a novel radiolabeled peptide by a counting method. The radiolabeled species remaining in the kidney were identified by comparing their HPLC data with those obtained by alternative synthesis.

RESULTS

The designed and synthesized radiolabeled peptide ¹¹¹In-DTPA-d-Phe^-1-Asp⁰-d-Phe¹-octreotide exhibited significantly lower renal radioactivity levels than those of the known ¹¹¹In-DTPA-d-Phe¹-octreotide at 3 and 24h post-injection. The radiolabeled species in the kidney at 24h after the injection of new octreotide derivative represented ¹¹¹In-DTPA-d-Phe-OH and ¹¹¹In-DTPA-d-Phe-Asp-OH as the metabolites. Their radiometabolites and intact ¹¹¹In-DTPA-conjugated octreotide derivative were observed in urine within 24h post-injection.

CONCLUSION

The present study provided a new example of an ¹¹¹In-DTPA-conjugated octreotide derivative having the characteristics of both reduced renal uptake and shortened residence time of radioactivity in the kidney. It is considered that this kinetic control was achieved by introducing a negative charge on the octreotide derivative thereby suppressing the reabsorption in the renal tubules and affording the radiometabolites with appropriate lipophilicity.

Toward in vivo imaging of heart disease using a radiolabeled single-chain Fv fragment targeting tenascin-C

Antibodies specific to a particular target molecule can be used as analytical reagents, not only for in vitro immunoassays but also for noninvasive in vivo imaging, e.g., immunoscintigraphies. In the latter case, it is important to reduce the size of antibody molecules in order to achieve suitable in vivo "diagnostic kinetics" and generate higher-resolution images. For these purposes, single-chain Fv fragments (scFvs; M(r) < 30 kDa) have greater potential than intact immunoglobulins (~150 kDa) or Fab (or Fab') fragments (~50 kDa). Our recent observation of enhanced tenascin-C (Tnc) expression at sites of cardiac repair after myocardial infarction prompted us to develop a radiolabeled scFv against Tnc for in vivo imaging of heart disease. We cloned the genes encoding the heavy and light chain variable domains of the mouse anti-Tnc monoclonal antibody 4F10, and combined them to create a single gene. The resulting scFv-4F10 gene was expressed in E. coli cells to produce soluble scFv proteins. scFv-4F10 has an affinity for Tnc (K(a) = 3.5 × 10(7) M(-1)), similar to the Fab fragment of antibody 4F10 (K(a) = 1.3 × 10(7) M(-1)) and high enough to be of practical use. A cysteine residue was then added to the C-terminus to achieve site-specific (111)In labeling via a chelating group. The resulting (111)In-labeled scFv was administered to a rat model of acute myocardial infarction. Biodistribution and quantitative autoradiographic studies indicated higher uptake of the radioactivity at the infarcted myocardium than the noninfarcted one. Single photon emission computed tomography (SPECT) provided in vivo cardiac images that coincided with the ex vivo observations. Our results will promote advances in diagnostic strategies for heart disease.

Intracellular reactions affecting 2-amino-4-([(11)C]methylthio)butyric acid ([(11)C]methionine) response to carbon ion radiotherapy in C10 glioma cells

PURPOSE

The response of 2-amino-4-([(14)C]methylthio)butyric acid ([(14)C]Met) uptake and [(125)I]3-iodo-alpha-methyl-l-tyrosine ([(125)I]IMT) uptake to radiotherapy of C10 glioma cells was compared to elucidate the intracellular reactions that affect the response of 2-amino-4-([(11)C]methylthio)butyric acid ([(11)C]Met) uptake to radiotherapy.

METHODS

After irradiation of cultured (3 Gy) or xenografted C10 glioma cells (25 Gy) using a carbon ion beam, the accumulation of [(14)C]Met and [(125)I]IMT in the tumors was investigated. The radiometabolites in xenografted tumors after radiotherapy were analyzed by size-exclusion HPLC.

RESULTS

[(14)C]Met provided earlier responses to the carbon ion beam irradiation than [(125)I]IMT in both cultured and xenografted tumors. While [(125)I]IMT remained intact in xenografted tumor before and after irradiation, the radioactivity derived from [(14)C]Met was observed both in high molecular fractions and intact fractions, and the former decreased after irradiation.

CONCLUSION

The earlier response of [(11)C]Met uptake to tumor radiotherapy could be attributable to the decline in the intracellular energy-dependent reactions of tumors due to radiotherapy.

Renal uptake and metabolism of radiopharmaceuticals derived from peptides and proteins

Radiolabeled anti-CD20 antibodies have demonstrated impressive efficacy in the treatment of relapsed non-Hodgkin lymphoma. This encourages the treatment of solid tumor with radiolabeled antibody fragments and peptides. However, both preclinical and clinical studies revealed that persistent localization of radioactivity in the kidney constitutes a major obstacle that compromises therapeutic efficacy. Recent extensive studies show that long residence times of radiolabeled end products from lysosomes are responsible for the renal radioactivity levels. Recent studies have also elucidated the involvement of megalin-cubilin in renal tubular reabsorption of radiolabeled antibody fragments and peptides. In light of these findings, efforts are being made to block tubular reabsorption of radiolabeled antibody fragments and peptides by competitive inhibitors, charge modification, and PEGylation. An interposition of an enzyme-cleavable linkage between antibody fragments and radiolabels would constitute an alternative approach to reduce renal radioactivity levels. Recent findings of these studies will be described.

Chemical design of a radiolabeled gelatinase inhibitor peptide for the imaging of gelatinase activity in tumors

Since elevated levels of gelatinases [matrix metalloproteinase (MMP)-2 and MMP-9] are associated with a poor prognosis in cancer patients, these enzymes are potential targets for tumor imaging. In the present study, a cyclic decapeptide, cCTTHWGFTLC (CTT), was selected as a mother compound because of its selective inhibitory activity toward gelatinases. For imaging gelatinase activity in tumors, we designed a CTT-based radiopharmaceutical taking into consideration that (1) the HWGF motif of the peptide is important for the activity, (2) hydrophilic radiolabeled peptides show low-level accumulation in the liver and (3) an increase in the negative charge of radiolabeled peptides is effective in reducing renal accumulation. Thus, a highly hydrophilic and negatively charged radiolabel, indiun-111-diethylenetriaminepentaacetic acid ((111)In-DTPA), was attached to an N-terminal residue distant from the HWGF motif ((111)In-DTPA-CTT). In MMP-2 inhibition assays, In-DTPA-CTT significantly inhibited the proteolytic activity in a concentration-dependent fashion. When injected into normal mice, (111)In-DTPA-CTT showed low levels of radioactivity in the liver and kidney. A comparison of the pharmacokinetic characteristics of (111)In-DTPA-CTT with those of other CTT derivatives having different physicochemical properties revealed that the increase in hydrophilicity and negative charge caused by the conjugation of (111)In-DTPA reduced levels of radioactivity in the liver and kidney. In tumor-bearing mice, a significant correlation was observed between the accumulation in the tumor as well as tumor-to-blood ratio of (111)In-DTPA-CTT and gelatinase activity. These findings support the validity of the chemical design of (111)In-DTPA-CTT for reducing accumulation in nontarget tissues and maintaining the inhibitory activity of the mother compound. Furthermore, (111)In-DTPA-CTT derivatives would be potential radiopharmaceuticals for the imaging of gelatinase activity in metastatic tumors in vivo.

Technetium-99m-Labeled Long Chain Fatty Acid Analogues Metabolized by β-Oxidation in the Heart

The development of 99mTc-labeled fatty acid analogues metabolized by beta-oxidation in the myocardium constitutes an unsolved challenge. On the basis of our recent findings that [188Re]tricarbonyl(cyclopentadienylcarbonate)rhenium ([188Re]CpTR-COOH) was recognized as an aromatic compound and was metabolized as such in the body, [99mTc]cyclopentadienyltricarbonyltechnetium ([99mTc]CpTT) was conjugated at the omega-position of pentadecanoic acid to prepare [99mTc]CpTT-PA. When injected into rats, [99mTc]CpTT-PA exhibited the maximum myocardial accumulation and heart-to-blood ratio of 3.85 %ID/g at 1 min and 4.60 at 10 min postinjection, respectively. The metabolic study using isolated Langendorff perfused rat hearts demonstrated that approximately 67% of perfused [99mTc]CpTT-PA was incorporated and [99mTc]CpTT-propionic acid, the metabolite after six cycles of beta-oxidation of [99mTc]CpTT-PA, was detected as the major radiometabolite in the perfusate and myocardium. These findings indicate that [99mTc]CpTT-PA was recognized, transported, and metabolized as a long chain fatty acid analogue for energy production in the myocardium.

Design, Synthesis, and Evaluation of [188Re]Organorhenium-Labeled Antibody Fragments with Renal Enzyme-Cleavable Linkage for Low Renal Radioactivity Levels

Renal localization of radiolabeled antibody fragments constitutes a problem in targeted imaging and radiotherapy. We have reported that Fab fragments labeled with 3'-[131I]iodohippuryl Nepsilon-maleoyl-lysine (HML) showed markedly low renal radioactivity levels even shortly after injection, due to a rapid and selective release of m-[131I]iodohippuric acid by the action of brush border enzymes. To estimate the applicability of the molecular design to metallic radionuclides, [188Re]tricarbonyl(cyclopentadienylcarbonate)rhenium ([188Re]CpTR-COOH) was conjugated with Nepsilon-tert-butoxycarbonyl-glycyl-lysine or Nepsilon-maleoyl-glycyl-lysine to prepare [188Re]CpTR-GK-Boc or [188Re]CpTR-GK. The cleavage of the glycyl-lysine linkage of the two compounds generates a glycine conjugate of [188Re]CpTR-COOH ([188Re]CpTR-Gly), which possesses in vivo behaviors similar to those of m-iodohippuric acid. The hydrolysis rate of the peptide bond in [188Re]CpTR-GK-Boc was compared with that in 3'-[125I]iodohippuryl Nepsilon-Boc-lysine ([125I]HL-Boc) using brush border membrane vesicles (BBMVs) prepared from rat kidneys. [188Re]CpTR-GK was conjugated to thiolated Fab fragments to prepare [188Re]CpTR-GK-Fab. The biodistribution of radioactivity after injection of [188Re]CpTR-GK-Fab was compared with that of [125I]HML-Fab and [188Re]CpTR-Fab prepared by conjugating N-hydroxysuccinimidyl ester of [188Re]CpTR-COOH with antibody fragments. While [188Re]CpTR-GK-Boc liberated [188Re]CpTR-Gly in BBMVs, [125I]HL-Boc liberated m-[125I]iodohippuric acid at a much faster rate. In addition, although [125I]HL-Boc was hydrolyzed by both metalloenzymes and nonmetalloenzymes, metalloenzymes were responsible for the cleavage of the peptide linkage in [188Re]CpTR-GK-Boc. In biodistribution studies, [188Re]CpTR-GK-Fab exhibited significantly lower renal radioactivity levels than did [188Re]CpTR-Fab. However, the renal radioactivity levels of [188Re]CpTR-GK-Fab were slightly higher than those of [125I]HML-Fab. The analysis of urine samples collected for 6 h postinjection of [188Re]CpTR-GK-Fab showed that [188Re]CpTR-Gly was the major radiometabolite. In tumor-bearing mice, [188Re]CpTR-GK-Fab significantly reduced renal radioactivity levels without impairing the radioactivity levels in tumor. These findings indicate that the molecular design of HML can be applied to metallic radionuclides by using a radiometal chelate of high inertness and by designing a radiometabolite of high urinary excretion when released from antibody fragments following cleavage of a glycyl-lysine linkage. This study also indicates that a change in chemical structure of a radiolabel attached to a glycyl-lysine linkage significantly affected enzymes involved in the hydrolysis reaction. Since there are many kinds of enzymes that cleave a variety of peptide linkages on the renal brush border membrane, selection of a peptide linkage optimal to a radiometal chelate of interest may provide radiolabeled antibody fragments that exhibit renal radioactivity levels similar to those of [131I]HML-labeled ones. The in vitro system using BBMVs might be useful for selecting an appropriate peptide linkage.

Assessment of 186Re chelate-conjugated bisphosphonate for the development of new radiopharmaceuticals for bones

INTRODUCTION

The preferable pharmacokinetics of rhenium-186 (186Re)-monoaminemonoamidedithiol-conjugated or 186Re-mercaptoacetyltriglycine-conjugated bisphosphonates (BPs) suggested that the molecular design would be applicable to other radionuclides such as 68Ga, 99mTc, 153Sm and 177Lu. In this study, a key factor affecting the pharmacokinetics of a chelate-conjugated BP was investigated to estimate the validity and the applicability of molecular design.

METHODS

Chemically inert and well-characterized tricarbonyl[186Re][(cyclopentadienylcarbonyl amino)-acetic acid]rhenium ([186Re]CpTR-Gly) was conjugated with 3-amino-1-hydroxypropylidene-1,1-bisphosphonate and purified by high-performance liquid chromatography (HPLC) to prepare [186Re](1-{3-[tricarbonyl(cyclopentadienylcarbonyl amino)-acetylamido]-1-hydroxy-1-phosphono-propyl}-phosphonic acid)rhenium ([186Re]CpTR-Gly-APD). Plasma stability, plasma protein binding, hydroxyapatite (HA) binding and the pharmacokinetics of [186Re]CpTR-Gly-APD were compared with those of 186Re 1-hydroxyethylidene-1,1-diphosphonate (HEDP). The effect of HEDP coadministration and preadministration on the pharmacokinetics of [186Re]CpTR-Gly-APD was also determined.

RESULTS

The HPLC-purified [186Re]CpTR-Gly-APD showed higher plasma stability, higher HA binding, higher bone accumulation and lower plasma protein binding than did 186Re-HEDP. However, HA binding of [186Re]CpTR-Gly-APD decreased to levels slightly higher than that of 186Re-HEDP at similar HEDP concentrations. Bone accumulation of [186Re]CpTR-Gly-APD also decreased to levels similar to that of 186Re-HEDP when [186Re]CpTR-Gly-APD was coinjected with HEDP equivalent to that in 186Re-HEDP. In contrast, HEDP pretreatment did not impair bone accumulation of the two 186Re-labeled compounds. However, a delay in blood clearance and an increase in renal radioactivity levels were observed particularly with 186Re-HEDP.

CONCLUSIONS

Although 186Re-HEDP possessed HA binding and bone accumulation similar to those of [186Re]CpTR-Gly-APD, the specific activity of 186Re-labeled BPs was found to play a crucial role in bone accumulation and blood clearance. Thus, the molecular design of chelate-conjugated BP would be useful for the development of bone-seeking radiopharmaceuticals with a variety of radionuclides by selecting chelating molecules that provide high specific activities.

In Vitro System To Estimate Renal Brush Border Enzyme-Mediated Cleavage of Peptide Linkages for Designing Radiolabeled Antibody Fragments of Low Renal Radioactivity Levels

Renal localization of radiolabeled antibody fragments presents a problem in targeted imaging and radiotherapy. We recently reported that Fab fragments labeled with 3'-[(131)I]iodohippuryl N(epsilon)-maleoyl-l-lysine (HML) demonstrated markedly low renal radioactivity levels from early postinjection in mice. Previous studies suggested that low renal radioactivity levels were attributable to cleavage of the glycyl-lysine sequence in HML by the action of renal brush border enzymes, followed by urinary excretion of the resulting m-iodohippuric acid. In this study, an in vitro system using brush border membrane vesicles (BBMVs) isolated from the rat kidney cortex was developed to estimate renal brush border enzyme(s)-mediated cleavage of the peptide linkage. Low molecular weight HML derivatives, 3'-[(125)I]iodohippuryl l-lysine (HL), 3'-[(125)I]iodohippuryl N(epsilon)-tert-butoxycarbonyl-l-lysine (HBL), and their d-amino acid counterparts, were synthesized and incubated in BBMVs. Both [(125)I]HL and [(125)I]HBL generated m-[(125)I]iodohippuric acid after incubation in BBMVs at 37 degrees C while the latter liberated significantly higher amounts of the metabolite. [(125)I]d-HL and [(125)I]d-HBL failed to release the metabolite under similar conditions. The liberation of m-[(125)I]iodohippric acid from [(125)I]HL was significantly facilitated or completely inhibited by the addition of an activator or an inhibitor for carboxypeptidase M. The release of m-[(125)I]iodohippuric acid from [(125)I]HBL increased by the addition of the activator, whereas the inhibitor partially inhibited the release of the metabolite from [(125)I]HBL. The BBMV-mediated release of m-[(125)I]iodohippuric acid from [(125)I]HBL was not impaired by the addition of inhibitors for neutral endopeptidase or renal dipeptidase. These findings showed that the glycyl-l-lysine sequence in HML would be recognized and cleaved by metalloenzymes and nonmetalloenzymes on the renal brush border even when iodine was incorporated into a benzene ring and the N(epsilon)-amine residue of lysine was chemically modified, which supported the hypothesis that low renal radioactivity levels of HML-conjugated Fab fragments would be attributed to the release of m-iodohippuric acid by renal brush border enzymes. This study suggested that this in vitro system using BBMVs would be useful to estimate radiolabeling reagents of antibody fragments or peptides designed to reduce renal radioactivity with a variety of radionuclides.

pi-Electron interaction of PAHs with anion-exchange silica gels modified with anionic metal-porphine and -phthalocyanine derivatives as HPLC stationary phase for preparative column in organic solvents

To develop easy-to-prepare stationary phases for HPLC, we investigated anion-exchange silica gels, Nucleosil 5SB (Nuc), modified with metal-porphines and -phthalocyanines (M-P). The modified silica gels (M-P(N)) were evaluated for the availability as a stationary phase of HPLC for the separation of pi-electron-rich polyaromatic hydrocarbons (PAHs) in polar and non-polar eluents. Separation ability of silica gels modified with Cu-phthalocyanine derivative (Cu-PCS(N)) was comparable to that of the silica gels binding Cu-PCS through sulfonamide bonds; however, the latter requires troublesome procedures for the preparation. The PAHs tested interact with Cu-PCS(N) in non-polar organic eluents through their pi-electrons similarly as in the case of the PYE column((R)), in which interaction with PAHs was reported to be only the pi-pi-electron interaction.

Evaluation of Cleavable (Tyr3)-octreotate Derivatives for Longer Intracellular Probe Residence

Radioligand targeting of somatostatin receptor subtype 2 (sstr2)-positive tumors with synthetic somatostatin analogues such as octreotide is subject to improvement in tumor to nontumor biodistribution, in part because internalization of such somatostatin analogues is limited by sstr2 recycling to the cell surface. We reasoned that it might be possible to prepare probe-carrying somatostatin analogues that would escape recycling, efficiently depositing probe molecules inside cells and ultimately increasing their intracellular concentration. We have incorporated cathepsin-B-cleavable linkers into (Tyr3)-octreotate chelate conjugates and examined these constructs as to cellular uptake, externalization, subcellular localization, and cleavage in the rat pancreatic tumor cell line AR42J in culture. Comparison of the cleavable radioligands with a noncleavable control indicates that scission of the constituent cathepsin B substrate occurs at a rate faster than ligand externalization, depositing virtually all internalized cleaved radiochelates within lysosomal compartments.

HPLC Retention Behavior of Poly-Aromatic-Hydrocarbons on Aminopropyl Silica Gels Modified with Cu(II)- and Ni(II)-Phthalocyanine Derivatives in Non-polar Eluent

A comparative study was conducted to elucidate the mechanism underlying the separation of poly-aromatic-hydrocarbons (PAHs) and related compounds thereof on a column packed with silica gels modified with Ni(II)- or Cu(II)-phthalocyanine derivatives (PCS) (Ni- or Cu-PCS(D) column) and commercially available PYE and NPE columns with a non-polar eluent, such as n-hexane. It has been revealed that the dominant interaction responsible to the separation of PAHs on the Cu-PCS(D) and the PYE columns with n-hexane is the pi-pi interaction; however, in the separation of PAHs having 4 rings such as pyrene on the Ni-PCS(D) column, participation of pi-d interaction was indicated. The predominant role of pi-pi interaction in the separation of PAHs of less than three rings on the Ni-PCS(D) column was demonstrated using anthracene. All the columns possessed planar recognition ability and were estimated to be potentially useful in the separation and the analysis of PAHs.

Effect of Carboxylation of N-Terminal Phenylalanine of 111In-DTPA (Diethylenetriaminepentaacetic Acid)-Octreotide on Accumulation of Radioactivity in Kidney

For purpose of reducing renal accumulation of radioactivity of a known radiopharmaceutical agent, i.e., (111)In-DTPA (diethylenetriaminepentaacetic acid)-D-Phe(1)-octreotide, a derivative in which p-carboxy-L-phenylalanine is substituted for D-Phe(1) was synthesized. Biodistribution study of the resultant compound having carboxy-substituted L-Phe(1) revealed that the renal accumulation was significantly lower than that of control compound having unsubstituted L-Phe(1), demonstrating that the presence of negative charge on the N-terminal amino acid of octreotide is effective to reduce the renal accumulation. This effect can be attributed to the reduction of lipophilicity and also the repulsive force arisen from the negative charge of renal brush border membrane.

Flow injection analysis of hydrogen peroxide using glass-beads modified with manganese(III)-tetra(4-carboxyphenyl)porphine derivative and its analytical application to the determination of serum glucose

A flow injection analysis system of hydrogen peroxide was developed. The present system is based on measuring of the absorbance of a quinoid dye formed by the following reaction catalyzed by peroxidase: Phenol + 4-Aminoantipyrine + 2H2O2 --> Peroxidase --> Quinoid dye + 4H2O. A column packed with aminopropyl-glass beads modified with amanganese(III)-tetra(4-carboxyphenyl)porphine derivative (Mn-TCPP(G) column), which has peroxidase-like activity, was used in place of an immobilized peroxidase column in the above reaction. The linear range of the calibration curve was 0.4-80 microg/ml hydrogen peroxide. The relative standard deviation of this system was 2.97% (n = 100, 10 microg/ml hydrogen peroxide 20 microl injection). The Mn-TCPP(G) column has sufficiently stability for the continuous injection of hydrogen peroxide untill 100 times. The advantageous feature of the Mn-TCPP(G) column was a less-electrostatic interaction between the mother glass beads and the anionic chromogen or quinoid dye formed and the stability in terms of the storage, temperature and moisture. The determination of serum glucose was achieved by attaching an immobilized glucose oxidase column to this system without deproteinization.

Flow injection analysis of ascorbic acid using carriers modified with metal-porphine as oxidative solid catalyses

To investigate one of practical applications of the supports modified with metal-porphines as artificial solid-catalysts, columns into which the supports were packed were supplied to catalytic columns for a flow injection analysis (FIA) system for determination of ascorbic acid (AsA) by the following reactions: AsA+O(2)-->dehydoroAsA+H(2)O(2),H(2)O(2)+chromogen-->2H(2)O+Dye. Among the columns tested, the column containing silica gels modified with Co-tetrakis(carboxyphenyl)porphine catalyzed most rapidly the oxidation reaction of AsA that is accompanied by the formation of hydrogen peroxide. The resulting hydrogen peroxide was determined by FIA system equipped with the column containing glass beads modified with Mn-tetrakis(carboxyphenyl)porphine, which gave a linear calibration curve and large peak-areas of the range corresponding to AsA concentration between 0.2 and 10 micromol/ml. The results indicated that some supports modified with metal-porphine would be applicable to the FIA for AsA as the solid catalyses which function as if the immobilized enzymes.

Photoreaction generating active oxygens of In(3+)-tetrakis(4-methylpyridyl)-porphine in the presence of albumins

The complex formation of In(3+)-tetrakis(4-N-methylpyridyl)-porphine (In-TMPyP) with albumin was studied by resonance Raman spectroscopy. Albumin coordinated to In(3+) through the -S(-) group(s). The photoreaction was investigated using the visible spectral change and In-TMPyP-thiourea complex was used as a model. It was demonstrated that the complex in a weak basic solution (pH 8.5) is excited by light and the excited complex converts oxygen to superoxide anion, which finally cleavages the porphine ring of In-TMPyP.

Altering pharmacokinetics of radiolabeled antibodies by the interposition of metabolizable linkages. Metabolizable linkers and pharmacokinetics of monoclonal antibodies

Monoclonal antibodies, their fragments and low molecular weight oncophilic molecules such as synthetic somatostatin derivatives have been used to deliver radioactivity to target cells for both diagnostic and therapeutic purposes. Clinical studies demonstrated high abilities of the radiolabeled antibodies and peptides for nuclear medicine applications. However, high and persistent localization of radioactivity was observed in the liver or kidney especially when these molecules are labeled with metallic radionuclides, which reduce diagnostic accuracy and compromise therapeutic effectiveness. Thus, radiolabeled antibodies and peptides would become much more useful in both targeted imaging and radiotherapy if the undesirable radioactivity localization can be diminished. As a means to reduce the undesirable radioactivity, interposition of a metabolizable linkage between an antibody and a radiolabel was proposed to generate radiolabeled small molecules of urinary excretion from the parental antibody by enzymatic cleavage of the linkage. In this paper, after indicating the rationale behind the radiopharmaceutical design, a significant role played by the interposition of the metabolizable linkage in altering pharmacokinetics of radiolabeled antibodies is described from a variety of studies so far reported with an emphasis being laid on the importance of radiometabolite-based design of metabolizable linkages.

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.

Effect of molecular charges on renal uptake of 111In-DTPA-conjugated peptides

The effect of molecular charges on renal accumulation of 111In-DTPA-labeled low molecular weight (LMW) peptides was investigated using 111In-DTPA-octreotide derivatives as models to design radiolabeled peptides that are taken up less by renal cells. The N-terminal D-phenylalanine (Phe) of 111In-DTPA-D-Phe(1)-octreotide was replaced with L-aspartic acid (Asp), L-lysine (Lys), L-methionine (Met) or L-Phe. Cellulose acetate electrophoresis indicated that both 111In-DTPA-L-Phe(1)-octreotide and 111In-DTPA-L-Met(1)-octreotide showed similar net charges, whereas 111In-DTPA-L-alphaLys(1)-octreotide and 111In-DTPA-L-Asp(1)-octreotide had more positive and negative charges, respectively, at pH values similar to those in blood and glomerular filtrate. When injected into mice, significant differences were observed in the renal radioactivity levels. 111In-DTPA-L-alphaLys(1)-octreotide showed the highest radioactivity levels from 10 min to 6 h postinjection, whereas the lowest radioactivity levels were observed with 111In-DTPA-L-Asp(1)-octreotide at all the postinjection intervals. These findings indicated that the replacement of only one amino acid in 111In-DTPA-D-Phe(1)-octreotide significantly altered net molecular charges of the resulting peptides and that the net charges of the 111In-DTPA-octreotide derivatives significantly affected their renal uptake. Thus, an increase of negative charges in peptide molecules may constitute a strategy for designing 111In-DTPA-conjugated LMW peptides with low renal radioactivity levels.

Significance of (111)In-DTPA chelate in renal radioactivity levels of (111)In-DTPA-conjugated peptides

Metabolic studies of (111)In-DTPA-labeled polypeptides and peptides showed that the radiolabeled (poly)peptides generated (111)In-DTPA-adducts of amino acid that possess long residence times in the lysosomal compartment of the tissues where (poly)peptides accumulated. However, a recent study suggested that metal-chelate-methionine (Met) might possess in vivo behaviors different from metal-chelate adducts of other amino acids. In this study, to elucidate whether some biological characteristics of Met may accelerate the renal elimination rate of (111)In-DTPA-adduct of Met into urine, (111)In-DTPA-Met(1)-octreotide was synthesized and the renal handling of (111)In-DTPA-Met was investigated using (111)In-DTPA-L-Phe(1)-octreotide (Phe represents phenylalanine), which was reported previously, as a reference. Both (111)In-DTPA-conjugated octreotide analogs were stable against 3-h incubation in murine serum at 37 degrees C. Both (111)In-DTPA-octreotide analogs also showed rapid clearance of the radioactivity from the blood and similar accumulation of the radioactivity in the kidney. No significant differences were observed in the renal radioactivity levels from 10 min to 24 h postinjection between the two. Metabolic studies indicated that (111)In-DTPA-Met(1)-octreotide and (111)In-DTPA-L-Phe(1)-octreotide generated (111)In-DTPA-adducts of Met and Phe, respectively, as the final radiometabolites at similar rates. These findings suggested that the long residence times of the radioactivity in tissues after administration of (111)In-DTPA-labeled peptides and polypeptides would be attributed to inherent characteristics of (111)In-DTPA chelate.

Species difference in radioactivity elimination from liver parenchymal cells after injection of radiolabeled proteins

To elucidate the cause for the different levels of hepatic radioactivity among mammals after injection of protein radiopharmaceuticals, the metabolism of radiolabeled proteins and the fate of their radiometabolites in the parenchymal cells of rat liver were investigated and compared with those of mice. We used galactosyl-neoglycoalbumin (NGA) as a carrier protein, and NGA was labeled with 111In via 1-(4-isothiocyanatobenzyl)ethylenediaminetetraacetic acid (SCN-Bz-EDTA) or 1-[p-(5-maleimidopentyl)aminobenzyl]ethylenediaminetetraacetic acid (EMCS-Bz-EDTA) and with 125I via direct iodination. All radiolabeled NGAs exhibited rapid accumulation in liver parenchymal cells after intravenous injection into rats. Radioactivity was eliminated following NGA-125I injection at similar rates from rat and mouse liver. In contrast, both 111In-labeled NGAs demonstrated much slower elimination of radioactivity in rat when compared with mouse liver. Analyses of radioactivity in bile and liver indicated that both SCN-Bz-EDTA and EMCS-Bz-EDTA rendered mono-amino acid adducts as the final radiometabolites, which were generated in rat liver within 1 h postinjection. Subcellular distribution studies suggested that these radiometabolites were copurified with lysosome in rat liver. Because similar results were observed in mice previously, the difference between rats and mice in radioactivity elimination from liver parenchymal cells would be predominantly attributable to the different efflux rate of the 111In-labeled metabolites from the lysosome between these species. Such differences in the efflux rates of radiometabolites from the lysosome among mammals may also account for the different hepatic radioactivity levels of radiolabeled proteins between animal and clinical studies.

Chemical design of radiolabeled antibody fragments for low renal radioactivity levels

The renal uptake of radiolabeled antibody fragments presents a problem in targeted imaging and therapy. We hypothesized that the renal radioactivity levels of radiolabeled antibody fragments could be reduced if radiolabeled compounds of urinary excretion were released from glomerularly filtered antibody fragments before they were incorporated into renal cells by the action of brush border enzymes, present on the lumen of renal tubules. 3'-[131I]Iodohippuryl N(epsilon)-maleoyl-L-lysine ([131I]HML) was conjugated with a thiolated Fab fragment because the glycyl-lysine sequence in HML is a substrate for a brush border enzyme and metaiodohippuric acid is released by cleavage of the linkage. Fab fragments were also radiolabeled by direct radioiodination (125I-Fab) or by conjugation with meta-[125I]-iodohippuric acid via an amide bond [N-(5-maleimidopentyl) 3'-iodohippuric acid amide ([125I]MPH-Fab)] or an ester bond [maleimidoethy 3'-iodohippurate ([125I]MIH-Fab)] by procedures similar to those used for [131I]HML-Fab. In biodistribution experiments in mice, [131I]HML-Fab demonstrated markedly low renal radioactivity levels with kidney:blood ratios of radioactivity of 1 from 10 min to 1 h due to rapid release of meta-[131I]iodohippuric acid. [125]MIH-Fab and 1251-Fab reached their peak ratios of 3.8 and 7.3 at 1 h, respectively, and [125I]MPH-Fab showed the maximum ratio of 16.8 at 6 h. In subcellular distribution studies, both [125I]MIH-Fab and 125I-Fab showed migration of radioactivity from the membrane to the lysosomal fraction of the renal cells from 10 to 30 min postinjection, whereas the majority of the radioactivity was detected only in the membrane fraction after administration of [131I]HML-Fab at both time points. In nude mice, [131I]HML-Fab showed one-quarter of the renal radioactivity of simultaneously administered 125I-Fab without impairing the target radioactivity levels 3 h after injection. These findings indicated that HML is a useful reagent for targeted imaging and therapy using antibody fragments as vehicles. These findings also suggested that the radiochemical design of radiolabeled antibody fragments that liberate radiometabolites of urinary excretion from antibody fragments by the action of brush border enzymes may constitute a new strategy for reducing the renal radioactivity levels of antibody fragments.

Renal metabolism of 111In-DTPA-D-Phe1-octreotide in vivo

The persistent localization of radioactivity in the kidney after administration of 111In-DTPA-D-Phe1-octreotide impairs the diagnostic accuracy of this radiopharmaceutical. To better understand the mechanisms responsible for the renal radioactivity levels of 111In-DTPA-D-Phe1-octreotide, the renal metabolism of this compound was compared with 111In-DTPA-L-Phe1-octreotide, where the N-terminal D-phenylalanine was replaced with L-phenylalanine to facilitate metabolism. DTPA-D-Phe1-octreotide and DTPA-L-Phe1-octreotide were synthesized by solid-phase methods. Both 111In-DTPA-conjugated octreotide analogues were prepared with radiochemical yields of over 96%, and both remained stable after a 3 h incubation in murine serum at 37 degreesC. When injected into mice, the two 111In-DTPA-conjugated octreotide analogues showed similar radioactivity elimination rates from the blood and accumulation in the kidney with about 60% injected radioactivity being excreted in the urine by 24 h postinjection. Over 85% of the radioactivity in the urine existed as intact peptides for both analogues. Despite the similar renal radioactivity levels, significant differences were observed in the radiolabeled species remaining in the kidney between the two; while 111In-DTPA-L-Phe1-octreotide was rapidly metabolized to the final radiometabolite, 111In-DTPA-L-Phe, the metabolic rate of 111In-DTPA-D-Phe1-octreotide was so slow that various intermediate radiolabeled species were observed. However, both 111In-DTPA-D-Phe and 111In-DTPA-L-Phe remained in the lysosomal compartment of the renal cells as the final radiometabolites for long periods. These findings indicated that although the metabolic stability of 111In-DTPA-D-Phe1-octreotide in the renal cells may be partially involved, the slow elimination rate of the radiometabolite derived from 111In-DTPA-D-Phe1-octreotide from the lysosomal compartment of renal cells would be predominantly attributable to the persistent renal radioactivity levels of 111In-DTPA-D-Phe1-octreotide.

Assessment of the radiochemical design of antibodies with a metabolizable linkage for target-selective radioactivity delivery

Interposition of a metabolizable linkage has been performed to reduce the hepatic radioactivity levels of radiolabeled antibodies. To estimate the validity of this strategy, a radioiodination reagent (HML) that provides a stable attachment for m-iodohippuric acid with proteins in plasma while facilitating rapid and selective release of the compound after lysosomal proteolysis in the liver was conjugated with a monoclonal antibody (mAb) against osteogenic sarcoma (OST7, IgG1). Radiolabeled OST7 conjugates with a plasma-labile ester bond for releasing m-iodohippuric acid (MIH), plasma-stable amide bonds for releasing radiometabolites of hepatobiliary excretion (MPH), or slow elimination rates from hepatocytes ([111In]EMCS-Bz-EDTA) were prepared with similar conjugation chemistry. The four radiolabeled OST7 conjugates were characterized both in vitro and in vivo. All the radiolabeled OST7 conjugates had similar radiochromatograms on size-exclusion HPLC and similar antigen binding affinities. While MIH-OST7 indicated accelerated clearance of radioactivity from the blood due to the release of m-iodohippurate, the rest of the three radiolabeled OST7 conjugates remained stable in serum incubation studies and had similar radioactivity elimination from the blood in vivo. When injected into normal mice, HML-OST7 demonstrated tissue-to-blood ratios of radioactivity similar to those of MIH-OST7 and significantly lower than those of the other two radiolabeled OST7 conjugates. In biodistribution studies in nude mice, both HML-OST7 and MIH-OST7 exhibited tumor-to-liver or tumor-to-intestine ratios of radioactivity higher than those of [111In]EMCS-Bz-EDTA-OST7 or MPH-OST7, respectively. HML-OST7, MPH-OST7, and [111In]EMCS-Bz-EDTA-OST7 indicated there were no changes in the radioactivity levels in the tumor between 24 and 48 h postinjection, whereas MIH-OST7 significantly decreased the radioactivity levels in the tumor at these time points. HML reduced the radioactivity levels in nontarget tissues without impairing the tumor radioactivity levels delivered by OST7. These findings indicated that the design of a radiolabeled mAb that is stable in plasma and liberates the radiometabolite of rapid urinary excretion constitutes an effective strategy for achieving target-selective radioactivity delivery.

A novel radioiodination reagent for protein radiopharmaceuticals with L-lysine as a plasma-stable metabolizable linkage to liberate m-iodohippuric acid after lysosomal proteolysis

Radiochemical design of polypeptides using metabolizable linkages would be attractive to enhance target-selective localization of radioactivity for diagnostic and therapeutic nuclear medicine. However, while use of ester bonds as the linkage allows selective release of the designed radiometabolite from covalently conjugated polypeptides after lysosomal proteolysis in nontarget tissues, low plasma stability of ester bonds causes a decrease in radioactivity levels of the target. In pursuit of new metabolizable linkages that provide stable attachment of radiolabels with polypeptide in plasma while facilitating rapid and selective release of designed radiometabolites of rapid urinary excretion in lysosomes, a new radioiodination reagent with L-lysine as the metabolizable linkage to liberate m-iodohippuric acid (L-HML) was designed and synthesized. Stabilities of the metabolizable linkage in serum and cleavabilities of the linkage in lysosomal proteolysis in hepatic cells were investigated after conjugation of [131I]-L-HML with galactosyl-neoglycoalbumin (NGA). For comparison, a radioiodination reagent with an ester bond to release m-iodohippuric acid (MIH) was conjugated with NGA under similar conditions. When incubated in human serum, [131I]-L-HML-NGA liberated less than 3% of the initial radioactivity after 24 h, whereas [125I]MIH-NGA released more than 60% of its radioactivity during the same interval. In biodistribution studies, [131I]-L-HML-NGA demonstrated radioactivity elimination from murine liver at a rate and excretion route similar to [125I]MIH-NGA. Analyses of murine urine after injection of [131I]-L-HML-NGA indicated a single radioactivity peak at fractions identical to those of m-iodohippuric acid. Biodistribution studies of radioiodinated NGAs with D-lysine or cadaverine as the linkages demonstrated a delayed elimination rate from murine liver with significantly higher radioactivity being excreted in the feces at 24 h postinjection. Thus, L-HML is the first reagent that allows stable attachment of radiolabel with polypeptide in serum while facilitating selective release of a radiometabolite with rapid urinary excretion from covalently conjugated polypeptides after lysosomal proteolysis at a rate similar to that of ester bonds. Thus, L-HML is potentially useful for the radioiodination of polypeptides for diagnostic and therapeutic purposes.

Conventional and high-yield synthesis of DTPA-conjugated peptides: application of a monoreactive DTPA to DTPA-D-Phe1-octreotide synthesis

Successful imaging of somatostatin receptor-positive tumors with 111In-DTPA-D-Phe1-octreotide has stimulated development of peptide radiopharmaceuticals using DTPA as the chelating agent. However, use of cyclic DTPA dianhydride (cDTPA) resulted in low synthetic yields of DTPA-peptide by either solution or solid-phase syntheses. This paper reports a novel high-yield synthetic procedure for DTPA-D-Phe1-octreotide that is applicable to other peptides of interest using a monoreactive DTPA derivative. A monoreactive DTPA that possesses one free terminal carboxylic acid along with four carboxylates protected with tert-butyl ester (mDTPA) was synthesized. Fmoc-Thr(tBu)-ol, prepared from Fmoc-Thr(tBu)-OH, was loaded onto 2-chlorotrityl chloride resin. After construction of the peptide chains by Fmoc chemistry, mDTPA was coupled to the alpha amine group of the peptide on the resin in the presence of 1,3-diisopropylcarbodiimide and 1-hydroxybenzotriazole. Treatment of the mDTPA-peptide-resin with trifluoroacetic acid-thioanisole removed the protecting groups and liberated [Cys(Acm)2,7]-octreotide-D-Phe1-DTPA from the resin. Iodine oxidation of the DTPA-peptide, followed by the reversed-phase HPLC purification, produced DTPA-D-Phe1-octreotide in overall 31.8% yield based on the starting Fmoc-Thr(tBu)-ol-resin. The final product gave a single peak on analytical HPLC, and amino acid analysis and mass spectrometry confirmed the integrity of the product. 111In radiolabeling of the product provided 111In-DTPA-D-Phe1-octreotide with > 95% radiochemical yield, as confirmed by analytical reversed-phase HPLC, TLC, and CAE. These finding indicated that use of mDTPA during solid-phase peptide synthesis greatly increased the synthetic yield of DTPA-D-Phe1-octreotide, due to the absence of nonselective reactions that are unavoidable when cDTPA is used. These results also suggested that mDTPA would be a versatile reagent to introduce DTPA with high yield into peptides of interest.