Synthesis and in Vitro/in Vivo Evaluation of Novel 99mTc(CO)3-Folates

Design, characterization and evaluation of a new 99mTc-labeled folate derivative with affinity towards folate receptor

Folate receptors (FRs) are known to be over-expressed in several human malignancies and therefore serve as an important target for small radiolabeled folate derivatives for non-invasive imaging of tumor, which is an important tool for future treatment recourse. In the present article, we report the synthesis of a new ^99mTc-labeled radiotracer for the aforementioned application following the well-established ^99mTc-'4+1' chemistry. Formation of the desired [^99mTc]Tc-complex with >95% radiochemical purity was confirmed by radio-HPLC and its structure was ascertained by characterizing a natural rhenium analogue of the said complex. Although the ligand exhibited a weaker affinity towards FRs compared to native folic acid (IC50 8.09 µM vs 29.46 nM), the ^99mTc-labeled complex was found to bind folate receptor-positive KB cells with high specificity (∼90%). Similar studies in a folate receptor negative cell line viz. A549 further corroborated the receptor-specificity of the synthesized complex. In vivo studies in KB tumor xenograft showed moderate uptake of ∼2.6% upto 3 h post-injection with high specificity (∼80%). The favorable features observed warrant further screening of the current design towards achieving an improved molecular probe for the said application.

Novel Synthetic Strategies Enable the Efficient Development of Folate Conjugates for Cancer Radiotheranostics

The folate receptor (FR) is an interesting target for radiotheranostics due to its overexpression in several tumor types. The progress in developing novel folate radioconjugates is, however, slow due to the synthetic challenges that folate chemistry presents. The goal of this study was, thus, to establish versatile solid-phase synthetic strategies for a convenient preparation of novel folate conjugates. Two approaches were established based on an orthogonal fluorenylmethyloxycarbonyl (Fmoc)-protection strategy to enable a modular buildup of an albumin-binding DOTA conjugate (known as OxFol-1) using folic acid (oxidized folate version) as a targeting agent. The main difference between the two approaches was the sequence of conjugating the single structural units. The approach that introduced the folate entity as the last unit appeared particularly useful for the preparation of conjugates based on 6R- or 6S-5-methyltetrahydrofolic acid (5-MTHF; a reduced folate version) as targeting entity. Three types of folate conjugates were synthesized either with a p-iodophenyl-based albumin binder (OxFol-1, 6R-RedFol-1, and 6S-RedFol-1) or without an albumin-binding entity (OxFol-14, 6R-RedFol-14, and 6S-RedFol-14). All six conjugates were obtained with high chemical purity (>98%) after 9-13 synthesis steps and a single final HPLC purification. Radiolabeling with lutetium-177 was feasible at high molar activity, and the resulting radioconjugates were stable over at least 24 h. Biodistribution and SPECT/CT imaging studies confirmed the favorable effect of an albumin-binding entity to increase the tumor uptake and reduce kidney retention of folate radioconjugates. The increased tumor-to-kidney ratios obtained with [¹⁷⁷Lu]Lu-6R-RedFol-1 and [¹⁷⁷Lu]Lu-6S-RedFol-1 as compared to [¹⁷⁷Lu]Lu-OxFol-1 indicated that 5-MTHF is the preferred FR-targeting agent for albumin-binding radioconjugates. This was, however, not the case for folate radioconjugates without an albumin binder. Thanks to the established synthesis strategy, the preparation of further folate radioconjugates will be facilitated, potentially enabling the optimization of the tissue distribution characteristics even more.

Single Photon Emission Computed Tomography Tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g., peptide, antibody fragment) and a γ-radiation-emitting radionuclide (e.g., ^99mTc, ¹²³I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design, these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Radiosynthesis and evaluation of a 99mTc-folic acid radiotracer prepared using [99mTcN(PNP)]2+ metal fragment

Folate receptors (FR) are over-expressed on a wide variety of tumor cells and are a potential molecular target for radiolabeled folates. In this respect, several SPECT and PET based radiofolates have been evaluated in the past albeit with their high renal uptake posing limitation towards their clinical use. To overcome this, a new ^99mTc labeled folic acid was synthesized via the use of [^99mTcN(PNP)]²⁺ metal fragment, where the presence of the latter pharmacophore redirects in vivo clearance via the hepatobiliary pathway. In this respect, folic acid was derivatized at the γ-acid group with a cysteine BFCA (bifunctional chelating agent) and subsequently reacted with the preformed [^99mTcN]²⁺ intermediate in presence of PNP2 (bisphosphine) ligand, to yield the final complex. While preliminary, in vivo distribution of the complex exhibited high association of activity with liver and intestines and provided support to the rationality of the present design as clearance of labeled folic acid could be effected via the hepatic route, the in vitro studies of the folic acid-cysteine conjugate carried out in KB-31 cells, did not show much promise with reduction in receptor affinity in comparison with the native folic acid. The route followed herein to prepare a folic-acid based radiotracer constitutes the first report of radiolabeling folic acid using the [^99mTcN(PNP)]²⁺ as a radiosynthon. Modification in the structure of conjugate by linking the BFCA through a long-chain linker can be envisaged to improve the affinity of [^99mTcN(PNP)]-folic acid complex towards FRs.

Clickable, hydrophilic ligand for fac-[M(I)(CO)3](+) (M = Re/(99m)Tc) applied in an S-functionalized α-MSH peptide

The copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) click reaction was used to incorporate alkyne-functionalized dipicolylamine (DPA) ligands (1 and 3) for fac-[M^I(CO)₃]⁺ (M = Re/^99mTc) complexation into an α-melanocyte stimulating hormone (α-MSH) peptide analogue. A novel DPA ligand with carboxylate substitutions on the pyridyl rings (3) was designed to increase the hydrophilicity and to decrease in vivo hepatobiliary retention of fac-[^99mTc^I(CO)₃]⁺ complexes used in single photon emission computed tomography (SPECT) imaging studies with targeting biomolecules. The fac-[Re^I(CO)₃(3)] complex (4) was used for chemical characterization and X-ray crystal analysis prior to radiolabeling studies between 3 and fac-[^99mTc^I(OH₂)₃(CO)₃]⁺. The corresponding ^99mTc complex (4a) was obtained in high radiochemical yields, was stable in vitro for 24 h during amino acid challenge and serum stability assays, and showed increased hydrophilicity by log P analysis compared to an analogous complex with nonfunctionalized pyridine rings (2a). An α-MSH peptide functionalized with an azide was labeled with fac-[M^I(CO)₃]⁺ using both click, then chelate (CuAAC reaction with 1 or 3 followed by metal complexation) and chelate, then click (metal complexation of 1 and 3 followed by CuAAC with the peptide) strategies to assess the effects of CuAAC conditions on fac-[M^I(CO)₃]⁺ complexation within a peptide framework. The peptides from the click, then chelate strategy had different HPLC t_R’s and in vitro stabilities compared to those from the chelate, then click strategy, suggesting nonspecific coordination of fac-[M^I(CO)₃]⁺ using this synthetic route. The fac-[M^I(CO)₃]⁺-complexed peptides from the chelate, then click strategy showed >90% stability during in vitro challenge conditions for 6 h, demonstrated high affinity and specificity for the melanocortin 1 receptor (MC1R) in IC₅₀ analyses, and led to moderately high uptake in B16F10 melanoma cells. Log P analysis of the ^99mTc-labeled peptides confirmed the enhanced hydrophilicity of the peptide bearing the novel, carboxylate-functionalized DPA chelate (10a′) compared to the peptide with the unmodified DPA chelate (9a′). In vivo biodistribution analysis of 9a′ and 10a′ showed moderate tumor uptake in a B16F10 melanoma xenograft mouse model with enhanced renal uptake and surprising intestinal uptake for 10a′ compared to predominantly hepatic accumulation for 9a′. These results, coupled with the versatility of CuAAC, suggests this novel, hydrophilic chelate can be incorporated into numerous biomolecules containing azides for generating targeted fac-[M^I(CO)₃]⁺ complexes in future studies.

Tricarbonyltechnetium(I) and tricarbonylrhenium(I) complexes of amino acids: crystal and molecular structure of a novel cyclic dimeric Re(CO)3-amino acid complex comprised of the OON donor atom set of the tridentate ligand

Radiolabeled complexes of monoamino polycarboxylic, polyamino monocarboxylic and thiol containing amino acid ligands were prepared from a fac-[(99m)Tc(CO)3(H2O)3](+) precursor.The overall radiochemical yield was 94-98%. The complexes exhibited substantial in vitro and in vivo stability. The corresponding Re(I) complexes of the ligands DAPA, Asp and CysH were prepared and characterized by means of IR, NMR, and MS spectroscopic studies, as well as X-ray crystallography (for those containing D,L-DAPA and D,L-Asp). The rhenium complexes have been structurally correlated with the technetium complexes by means of HPLC studies. The reaction of Re(CO)5Cl with D,L-Asp in presence of triethylamine led to the formation of a new class of cyclic dimeric complexes formed by the OON donor atom set of the tridentate ligands. The amino carboxylate ligand system formed well defined complexes with a fac-[M(CO)3(H2O)3](+) core and shows good promise in (99m)Tc(CO)3 tracer development.

Single photon emission computed tomography tracer

Single photon emission computed tomography (SPECT) is the state-of-the-art imaging modality in nuclear medicine despite the fact that only a few new SPECT tracers have become available in the past 20 years. Critical for the future success of SPECT is the design of new and specific tracers for the detection, localization, and staging of a disease and for monitoring therapy. The utility of SPECT imaging to address oncologic questions is dependent on radiotracers that ideally exhibit excellent tissue penetration, high affinity to the tumor-associated target structure, specific uptake and retention in the malignant lesions, and rapid clearance from non-targeted tissues and organs. In general, a target-specific SPECT radiopharmaceutical can be divided into two main parts: a targeting biomolecule (e.g. peptide, antibody fragment) and a γ-radiation emitting radionuclide (e.g. (99m)Tc, (123)I). If radiometals are used as the radiation source, a bifunctional chelator is needed to link the radioisotope to the targeting entity. In a rational SPECT tracer design these single components have to be critically evaluated in order to achieve a balance among the demands for adequate target binding, and a rapid clearance of the radiotracer. The focus of this chapter is to depict recent developments of tumor-targeted SPECT radiotracers for imaging of cancer diseases. Possibilities for optimization of tracer design and potential causes for design failure are discussed and highlighted with selected examples.

Tumor targeting using 67Ga-DOTA-Bz-folate--investigations of methods to improve the tissue distribution of radiofolates

INTRODUCTION

Use of folic acid radioconjugates for folate receptor (FR) targeting is a promising strategy for imaging purposes as well as for potential therapy of cancer and inflammatory diseases due to the frequent FR overexpression found on cancer cells and activated macrophages. Herein, we report on preclinical results using a novel DOTA-Bz-EDA-folate conjugate radiolabeled with [(67)Ga]-gallium.

METHODS

DOTA-Bz-EDA-folate was prepared by conjugation of ethylenediamine-(γ)-folate with 2-(p-isothiocyanobenzyl)-DOTA. Radiolabeling was carried out with (67)GaCl(3) according to standard procedures. Biodistribution studies of the tracer were performed in mice bearing FR-positive KB tumor xenografts. The effects on radiofolate biodistribution with coadministered renal uptake-blocking amino acids, diuretic agents, antifolates as well as different routes of administration were likewise investigated. Supportive imaging studies were performed using a small-animal single photon emission computed tomography (SPECT)/CT scanner.

RESULTS

(67)Ga-DOTA-Bz-EDA-folate showed a high and specific accumulation in tumors (6.30%±0.75% ID/g, 1 h pi and 6.08%±0.89% ID/g, 4 h pi). Nonspecific radioactivity uptake in nontargeted tissues was negligible, but significant accumulation was found in FR-positive kidneys, which resulted in unfavorably low tumor-to-kidney ratios (<0.1). Coadministered amino acids or diuretics did not effectively reduce renal accumulation; in contrast, predosed pemetrexed did significantly reduce kidney uptake (<29% of control values). The SPECT/CT studies confirmed the excellent tumor-to-background contrast of (67)Ga-radiofolate and the favorable reduction in kidney uptake (with improved imaging quality) resulting from pemetrexed administration.

CONCLUSION

Conventional methods to reduce kidney uptake of radiofolates fail. However, the novel (67)Ga-radiolabeled DOTA-Bz-EDA-folate can effectively be used to image FR-positive cancer and potentially inflammatory diseases. Due to its rapid blood clearance properties, this tracer is also a promising candidate for positron emission tomography imaging if radiolabeled with the short-lived [(68)Ga]-gallium radionuclide.

Synthesis and in vitro/in vivo evaluation of 99mTc-labeled folate conjugates for folate receptor imaging

INTRODUCTION

Folate receptor (FR) is a potential molecular target for radionuclide imaging since it is overexpressed in many human epithelial tumor cells. In this study, a novel folate conjugate was synthesized and labeled with (99m)Tc using different coligands. In vitro and in vivo evaluations of these complexes have been done to explore the effect of coligands on the stable, affinity and pharmacokinetic properties.

METHODS

A novel folate conjugate, HYNIC-NHHN-FA, was synthesized and characterized. This conjugate was radiolabeled with (99m)Tc using tricine, tricine/diphenylphosphinobenzene-3-sulfonic acid sodium (TPPMS) and tricine/trisodium triphenylphosphine-3,3',3''-trisulfonate (TPPTS) as coligands, respectively. The complexes were purified by high-pressure liquid chromatography (HPLC). In vitro and in vivo evaluations were performed with FR-positive KB cells, normal Kunming mice and athymic nude mice bearing KB tumors.

RESULTS

Labeling with (99m)Tc using different coligands resulted in three complexes, (99m)Tc (HYNIC-NHHN-FA)(tricine), 5, (99m)Tc (HYNIC-NHHN-FA)(tricine/TPPMS), 6 and (99m)Tc (HYNIC-NHHN-FA)(tricine/TPPTS), 7. Complex 5 showed at least two isomers and was unstable after being purified by HPLC. Complexes 6 and 7 displayed high stability and similar affinity to FR in vitro. Biodistribution results in athymic nude mice bearing KB tumor showed that complex 7 had a high uptake in FR-positive tumor (9.79±1.66%ID/g at 4 h postinjection), and the results of blockade studies confirmed the specific accumulation of the radiotracer in vivo. However, complex 6 showed a low tumor uptake due to its fast excretion via the gastrointestinal tract.

CONCLUSION

The modification of the coligands can significantly alter the pharmacokinetic properties of the corresponding (99m)Tc-HYNIC complexes. (99m)Tc (HYNIC-NHHN-FA)(tricine/TPPTS), 7 could be a promising radiotracer for FR imaging.

In vivo assay of folate receptors in nonfunctional pituitary adenomas with 99mTc-folate SPECT/CT

The objective of this study was to evaluate the in vivo assay of folate receptors in nonfunctional pituitary adenomas using preoperative (99m)Tc-folate SPECT/CT and Western blot analysis (WBA) of surgical specimens as the standard.

METHODS

Fifty-six patients (29 men, 27 women; age range, 29-82 y) with clinically nonfunctional pituitary adenomas on MRI underwent preoperative imaging using 666 MBq (18 mCi) of (99m)Tc-folate. SPECT/CT images and whole-body and lateral head planar images were acquired approximately 2 h after injection. Surgical resection took place within a week. WBA on a portion of the excised specimens assessed folate receptor expression in 49 patients. Attenuation-corrected (99m)Tc-folate SPECT/CT images were assessed qualitatively and quantitatively (maximal adenoma counts to background), with WBA as a standard.

RESULTS

Integrated CT was useful for uptake localization and assisted region-of-interest placement. Qualitative interpretation of planar imaging yielded a sensitivity of 81% and specificity of 72%. Qualitative SPECT/CT yielded a sensitivity of 94% and specificity of 61%. Receiver-operating-characteristic curve analysis of quantitative uptake yielded a tumor-to-background cutoff ratio of 3.5, with a sensitivity of 81% and specificity of 83%. Scalp uptake yielded consistent results (over the brain, neck, and choroid plexus) for background when SPECT/CT misalignment artifacts were avoided. Detection of pituitary uptake on anterior-posterior and lateral images was hampered by facial uptake, which varied between patients.

CONCLUSION

SPECT/CT of (99m)Tc-folate is an accurate method of assaying folate receptors in vivo and may provide a quantitative marker for identifying folate receptor-positive tumors. This method may also prove beneficial in selecting patients for folate-targeted therapy of clinically nonfunctional pituitary adenomas, for which there is currently no medical therapy.

Synthesis, biodistribution, and microsingle photon emission computed tomography (SPECT) imaging study of technetium-99m labeled PEGylated dendrimer poly(amidoamine) (PAMAM)-folic acid conjugates

Three conjugates based on dendrimer PAMAM generation five were synthesized and radiolabeled successfully. To investigate their tumor targeting, the in vitro and in vivo stability, cell uptake, in vivo biodistribution, and micro-SPECT imaging were evaluated, respectively. The conjugate of (99m)Tc labeled PEGylated dendrimer PAMAM folic acid conjugate ((99m)Tc-G5-Ac-pegFA-DTPA) shows much higher uptake in KB cancer cells and accumulated more in the tumor area than that of the other two conjugates. The uptake in KB cells depends on the incubation time. The results of in vivo biodistribution agree with the data obtained from micro-SPECT imaging. These studies show that PEGylation of PAMAM dendrimer folic acid conjugate improves the tumor targeting. Folate-conjugated dendrimer maybe developed to be potential radiopharmaceuticals and targeted drug delivery systems.

Synthesis and antimicrobial evaluation of bile acid tridentate conjugates

Two series of novel bile acid tridentate conjugates with different linkers were synthesized and characterized, and their biological activities in vitro were evaluated. The procedure was straightforward and efficient to be carried out with high overall yield. The antimicrobial activity of the synthesized compounds against Saccharomyces cerevisiae, Aspergillus niger, Escherichia coli and Staphylococcus aureus was investigated in vitro. The best activity of minimal inhibitory concentrations (MICs) for 1c, 1c', 2c and 2c' against S. cerevisiae was up to 0.125 microg/mL.

Radioactive synthesis and biodistribution study of beta-elemene-99mTc(CO)3 conjugates

Beta-elemene, (5S,7R,10S)-(-)-(1-methyl-1-vinyl-2,4-diisopropenylcyclohexane), is an anticancer agent from traditional Chinese herbal medicine. Three novel (99m)Tc(CO)(3)-beta-elemene conjugates were synthesized successfully, and compared with beta-elemene exhibited improved water solubility. A biodistribution and micro single photon emission computed tomography image study showed there is a visible accumulation in Lewis lung cancer tumors.

Tumor imaging using P866, a high-relaxivity gadolinium chelate designed for folate receptor targeting

The objective of this study was to evaluate the potential of a high-relaxivity macromolecular gadolinium (Gd) chelate to target folate receptors (FRs). P866 is a dimeric high-relaxivity Gd chelate coupled to a folate moiety. Binding affinity, in vivo biodistribution studies in KB tumor-bearing mice at 1, 4, and 24 h, and dynamic contrast-enhanced (DCE)-MRI (2.35 T) over 4 h were assessed. Binding and internalization of P866 through the FR was demonstrated. Due to the high molecular volume of P866, the binding affinity compared to free FA was decreased (K(D) = 59.3 +/- 1.8 nM and 5.9 +/- 0.2 nM, respectively). Tumor/muscle (T/M) uptake was 5.4 +/- 1.0, 4 h after injection of 15 micromol/kg. Competition with free FA was less effective when the dose was increased due to a saturation of FR. At a dose of 5 micromol/kg, a 70% difference in signal enhancement was observed between P866 and the nonspecific reference compound, thus demonstrating the specificity of FR targeting. While this high-relaxivity folate-Gd chelate has demonstrated its potential capacity to target in vivo FR on tumors, the sensitivity is probably limited to a certain extent by the saturation of the FR and by the decrease in the apparent relaxivity of the internalized part of P866 in the tumor cells.

Synthesis and in vitro anti-proliferative activity of beta-elemene monosubstituted derivatives in HeLa cells mediated through arrest of cell cycle at the G1 phase

Beta-elemene monosubstituted amine, ether and rhenium coordinated complex were synthesized. Their structures were characterized by IR, (1)H NMR, (13)C NMR, HRMS or EA. Their IC(50) on HeLa cell lines, cell cycle and protein expression of G(1) phase (Cyclin D(1), Rb, P-Rb) were detected respectively by the method of WST-1, Flow Cytometry and Western Blot. The Results showed that the in vitro anti-proliferative activity of beta-elemene monosubstituted amine and Re(CO)(3)-beta-elemene derivatives in human cervix epitheloid carcinoma HeLa cells were improved significantly compared with both of ether derivatives and parent beta-elemene. These derivatives could reduce Rb phosphorylation and cyclin D(1) protein expression to arrest the cell cycle at G(1) phase.

Targeted single-wall carbon nanotube-mediated Pt(IV) prodrug delivery using folate as a homing device

Most low-molecular-weight platinum anticancer drugs have short blood circulation times that are reflected in their reduced tumor uptake and intracellular DNA binding. A platinum(IV) complex of the formula c, c, t-[Pt(NH 3) 2Cl 2(O 2CCH 2CH 2CO 2H)(O 2CCH 2CH 2CONH-PEG-FA)] ( 1), containing a folate derivative (FA) at an axial position, was prepared and characterized. Folic acid offers a means of targeting human cells that highly overexpress the folate receptor (FR). Compound 1 was attached to the surface of an amine-functionalized single-walled carbon nanotube (SWNT-PL-PEG-NH 2) through multiple amide linkages to use the SWNTs as a "longboat delivery system" for the platinum warhead, carrying it to the tumor cell and releasing cisplatin upon intracellular reduction of Pt(IV) to Pt(II). The ability of SWNT tethered 1 to destroy selectively FR(+) vs FR(-) cells demonstrated its ability to target tumor cells that overexpress the FR on their surface. That the SWNTs deliver the folate-bearing Pt(IV) cargos into FR(+) cancer cells by endocytosis was demonstrated by the localization of fluorophore-labeled SWNTs using fluorescence microscopy. Once inside the cell, cisplatin, formed upon reductive release from the longboat oars, enters the nucleus and reacts with its target nuclear DNA, as determined by platinum atomic absorption spectroscopy of cell extracts. Formation of the major cisplatin 1,2-intrastrand d(GpG) cross-links on the nuclear DNA was demonstrated by use of a monoclonal antibody specific for this adduct. The SWNT-tethered compound 1 is the first construct in which both the targeting and delivery moieties have been incorporated into the same molecule; it is also the first demonstration that intracellular reduction of a Pt(IV) prodrug leads to the cis-{Pt((NH 3) 2} 1,2-intrastrand d(GpG) cross-link in nuclear DNA.

"Click-to-chelate": in vitro and in vivo comparison of a 99mTc(CO)3-labeled N(tau)-histidine folate derivative with its isostructural, clicked 1,2,3-triazole analogue

A side-by-side comparison of the synthesis, radiolabeling, and in vitro and in vivo characterization of two new and isostructural (99m)Tc-tricarbonyl folic acid radiotracers comprising either a N(tau)-functionalized histidine (His) chelator or a 1,4-bifunctionalized 1,2,3-triazole His analogue is described. The 1,2,3-triazole-containing folic acid derivative was synthesized in approximately 80% yield by a short reaction sequence including application of click chemistry (the Cu(I)-catalyzed cycloaddition of azides and terminal alkynes). The synthesis of the ligand system and the functionalization of the folic acid derivative were accomplished simultaneously, which prompted us to call this approach "click-to-chelate". In comparison, the reported regioselective synthesis of the N(tau)-His compound provided the final product in only very low yields (<1%). While the efficiency of the syntheses differs considerably, the two isostructural folate derivatives exhibit virtually identical properties with respect to Tc-99m radiolabeling and in vitro and in vivo characteristics as shown by experiments performed with FR-positive KB cells and xenografted mice bearing folate receptor overexpressing tumors. We have demonstrated herein for the first time that a ligand system known to be an excellent chelator for the stable complexation of the organometallic core [M(CO)3] (+) (M = Tc-99m, Re) can be replaced by an isostructural 1,2,3-triazole analogue without influencing the characteristics of the radiometal conjugate. The "click-to-chelate" strategy provides a highly efficient and convenient entry to metal conjugates suitable for diagnostic and potentially therapeutic applications. The described procedures should be readily applicable to any azide-functionalized (bio)molecule and, thus, are likely to represent the method of choice for the future development of radiopharmaceuticals radiolabeled with the organometallic precursors [M(CO)3(H2O)3] (+) (M = (99m)Tc, (188)Re).

Exploitation of the folate receptor in the management of cancer and inflammatory disease

Over the last 25+ years, the folate receptor (FR) has emerged as an attractive tumor biomarker with the potential to be exploited for therapeutic purposes. Increasing evidence suggests that this endocytosing protein can functionally mediate the cellular uptake and retention of natural folates, certain antifolates, and folate-drug conjugates; the consequences of the latter two events could result in biological modulation, including (but not limited to) tumor-targeted cytotoxicity. Because its tissue expression profile appears to be somewhat limited to either tissues responsible for whole body retention of folates (e.g., kidney and placenta), or certain pathologic tissues (e.g., tumors or activated macrophages), the FR is believed to be a useful biological target for disease management. Indeed, recent years have been peppered with reports of novel FR-targeted therapies, and many have demonstrated impressive in vivo potency, particularly against tumor xenografts, without the undesirable toxicity that often accompanies nontargeted drug regimens. This chapter will provide essential details on the properties of the FR, including where it is expressed and how it has been successfully manipulated for therapeutic benefit.

Dose-dependent effects of (anti)folate preinjection on 99mTc-radiofolate uptake in tumors and kidneys

INTRODUCTION

The folate receptor (FR) is frequently overexpressed in tumors and can be targeted with folate-based (radio)pharmaceuticals. However, significant accumulation of radiofolates in FR-positive kidneys represents a drawback. We have shown that preadministration of the antifolate pemetrexed (PMX) significantly improved the tumor-to-kidney ratio of radiofolates in mice. The aim of this study was to investigate the dose dependence of these effects and whether the same results could be achieved with folic acid (FA) or 5-methyl-tetrahydrofolate (5-Me-THF).

METHODS

Biodistribution was assessed 4 h postinjection of the organometallic (99m)Tc-picolylamine monoacetic acid folate in nude mice bearing FR-positive KB tumor xenografts. PMX (50-400 microg/mouse) was injected 1 h previous to radioactivity. The effects of FA and 5-Me-THF (0.5-50 microg/mouse) were investigated likewise. Tissues and organs were collected and counted for radioactivity and the values tabulated as percentage of injected dose per gram tissue (% ID/g).

RESULTS

PMX administration reduced renal retention (<1.6% ID/g vs. control: >10% ID/g), while the tumor uptake (average 1.35%+/-0.40% ID/g vs. control: 1.79%+/-0.49% ID/g) was only slightly affected independent of the PMX dose. Replacement of PMX by FA or 5-Me-THF (50 microg/mouse) resulted in a significant renal blockade (<0.1% ID/g) but at the same time in an undesired reduction of tumor uptake (<0.2% ID/g).

CONCLUSIONS

Selective reduction of radiofolate uptake in kidneys under retention of high tumor accumulation could be achieved in combination with PMX over a broad dose range but not with FA or 5-Me-THF.

Isostructural folate conjugates radiolabeled with the matched pair 99mTc/188Re: a potential strategy for diagnosis and therapy of folate receptor-positive tumors

(99m)Tc-technetium ((99m)Tc) and (188)Re-rhenium ((188)Re) represent an interesting pair of radionuclides for diagnosis and therapy. The aim of this study was to synthesize and characterize in vitro/in vivo the first (188)Re-folate derivative [(188)Re(CO)(3)-picolylamine monoacetic acid 188/Re-OANA-folate (2)] for potential targeted radionuclide therapy of FR-positive tumors. The data were compared with those of the isostructural (99m)Tc-analog [(99m)Tc-PAMA folate (1)] reported previously.

METHODS

In vitro stability of compound 2 was tested in phosphate-buffered saline and human plasma. Cell binding experiments were performed with FR-positive human KB cells. Biodistribution was assessed in female nude mice, bearing KB tumor xenografts.

RESULTS

Cell binding experiments showed high and FR-specific uptake. In vivo, compound 2 accumulated specifically in the FR-positive tumors with maximal values 4 h post injection (p.i.) ['2: 1.87+/-0.04 percent injected dose per gram of weight tissue (% ID/g) vs. '1: 2.33+/-0.36% ID/g]. Unfavorably high retention of radioactivity was found in FR-positive kidneys (12.04+/-0.62% ID/g; 4 h p.i.). Tumor-to-blood ratio of radioactivity ('2: 14.5+/-1.32, 4 h p.i.) was lower than for compound '1 (58.0+/-12.2, 4 h p.i.), whereas tumor-to-kidney ratios were in the same range ('2: 0.15+/-0.01 vs. '1: 0.13+/-0.02, 4 h p.i.). Preadministration of the antifolate pemetrexed significantly improved the tumor-to-kidney ratio (2: 1.59+/-0.30, 4 h p.i.).

CONCLUSIONS

The isostructural radiofolates 1 and '2 displayed almost identical pharmacokinetic profiles and accumulated both specifically in FR-positive tumors. However, only the coapplication of the antifolate pemetrexed improved the biodistribution of the radiotracers in such ways that a potential therapeutic application of compound '2 can be envisaged in the future.

Receptor-mediated gene delivery by folate-PEG-baculovirus in vitro

Gene delivery using baculovirus is a promising approach for efficient and safe gene therapy compared with animal viruses. However, obstacles of baculovirus-mediated gene delivery include inactivation of baculovirus in human serum and whole blood and the lack of specificity in targeted delivery. Therefore, chemical modification of the viral surface with poly(ethylene glycol) (PEG) and a targeting ligand, such as folate, is necessary for stable and targeted gene delivery via receptor-mediated endocytosis. In this study, folate-PEG (F-PEG) was attached on the baculovirus surface to obtain efficiency and specificity of gene delivery. Composition of F-PEG and degree of capsid modification with F-PEG was determined using (1)H nuclear magnetic resonance ((1)H NMR) and fluorescamine assay, respectively. Folate-PEG-Baculovirus (F-P-Bac) showed enhanced transduction efficiency compared to PEG-Baculovirus (P-Bac) in folate receptor (FR)-positive KB cells. Moreover, this enhanced transduction was not observed in FR-negative HepG2 cells. Presence of free folate in the medium blocked the transduction of F-P-Bac, whereas transduction efficiency of P-Bac in the presence or absence of free folate was not changed significantly. This study thus suggests that F-P-Bac can be used as a receptor-mediated gene delivery system.

Rhenium analogues of promising renal imaging agents with a [99mTc(CO)3]+ core bound to cysteine-derived dipeptides, including lanthionine

The coordination chemistry of lanthionine (LANH2) and cystathionine (CSTH2) dipeptides, which respectively consist of two cysteines and one cysteine and one homocysteine linked by a thioether bridge, is almost unstudied. Recently for fac-[99mTc(CO)3(LAN)]- isomers, the first small 99mTc(CO)3 agents evaluated in humans were found to give excellent renal images and to have a high specificity for renal excretion. Herein we report the synthesis and characterization of Re complexes useful for interpreting the nature of tracer 99mTc radiopharmaceuticals. Treatment of [Re(CO)3(H2O)3]OTf with commercially available LANH2 (a mixture of meso (d,l) and chiral (dd,ll) isomers) gave three HPLC peaks, 1A, 1B, and 1C, but treatment with CSTH2 (l,l isomer) gave one major product, Re(CO)3(CSTH) (2). Crystalline Re(CO)3(LANH) products were best obtained with synthetic LANH2, richer in meso or chiral isomers. X-ray crystallography showed that these dipeptides coordinate as tridentate N2S-bound ligands with two dangling carboxyls. The LANH ligand is meso in 1A and 1C and chiral in 1B. While 1A (kinetically favored) is stable at ambient temperature for days, it converted into 1C (thermodynamically favored) at 100 degrees C; after 6 h, equilibrium was reached at a 1A:1C ratio of 1:2 at pH 8. The structures provide a rationale for this behavior and for the fact that 1A and 1C have simple NMR spectra. This simplicity results from fluxional interchange between an enantiomer with both chelate rings having the same delta pucker and an enantiomer with both chelate rings having the same lambda pucker. Agents with the [99mTc(CO)3]+ core and N2S ligands show promise of becoming an important class of 99mTc radiopharmaceuticals. The chemistry of Re analogues with these ligands, such as the LAN2- complexes reported here, provides a useful background for designing new small agents and also tagged large agents because two uncoordinated carboxyl groups are available for conjugation with biological molecules such as proteins.

One-pot syntheses, coordination, and characterization of application-specific biodegradable ligand-polymers

Syntheses and chelation of tailored biodegradable polymers to rhenium for medicinal applications are described. A group of bifunctional ligand-initiators consisting of a chelating end for metal complexation and a hydroxyl end suitable to initiate polymerization was utilized in the ring-opening polymerization of l-lactide. The resulting biodegradable ligand-polymers were equipped with a tridentate donor set to coordinate specific metal ions. All synthesized compounds were characterized by IR spectroscopy, 1D/2D NMR spectroscopy and MALDI-TOF mass spectrometry, confirming successful polymerization and coordination to the [Re(CO)(3)](+) core. The pliability of designing application-specific polymers with respect to the nature of the metal ion facilitates extending the application of these biodegradable polymers to early detection of diseases (imaging) and radiotherapy of cancers.