Preparation, characterization, and in vivo biodistribution study of intranasal 131I-clonazepam-loaded phospholipid magnesome as a promising brain delivery system: Biodistribution and pharmacokinetic behavior of intranasal 131I-Clonazepam loaded phospholipid magnesome as a potential brain targeting system

OBJECTIVE

Clonazepam (CP) is a potent long-acting nitrobenzodiazepine derivative that could be used for targeting peripheral benzodiazepine receptors. Phospholipid magnesome is a new vesicular nanosystem recently developed for brain targeting. Improving the uptake of ¹³¹I-CP to the brain might be effective for the diagnosis and/or radiotherapy of certain brain diseases and/or tumors.

METHODS

CP was radiolabeled with ¹³¹I using direct electrophilic substitution reaction. Quality control of ¹³¹I-CP was performed using different techniques. Different formulas of ¹³¹I-CP were prepared and characterized according to particle size and polydispersity index. The structural features of the optimized formula were then interpreted using transmission electron microscopy and scanning electron microscopy, whereas pharmacokinetic and in vivo behaviors were estimated using the intravenous and intranasal delivery routes.

RESULTS

The heart and blood demonstrated lower uptake of ¹³¹I-CP, which inevitably decreased the nontarget effects of radioiodine. Intranasally administered ¹³¹I-CP-loaded magnesomes (INMg) had noticeably higher brain uptake (7.1 ± 0.09%ID/g) with rapid onset of action within 5 min and effective pharmacokinetic behavior. INMg had a drug targeting efficiency and nose-to-brain direct transport percentage of 121.1% and 94.6%, respectively as well as a relative bioavailability of 441.04 ± 75.5%.

CONCLUSION

The present study showed that ¹³¹I-CP-loaded magnesomes can be a beneficial brain-targeting approach for improving the diagnosis and/or radiotherapy of certain brain diseases.

Radiobromine and radioiodine for medical applications

The halogens bromine and iodine have similar chemical properties and undergo similar reactions due to their closeness in Group 17 of the periodic chart. There are a number of bromine and iodine radionuclides that have properties useful for diagnosis and therapy of human diseases. The emission properties of radiobromine and radioiodine nuclides with half-lives longer than 1 h are summarized along with properties that make radionuclides useful in PET/SPECT imaging and β/Auger therapy, such that the reader can assess which of the radionuclides might be useful for medical applications. An overview of chemical approaches that have been used to radiolabel molecules with radiobromine and radioiodine nuclides is provided with examples. Further, references to a large variety of different organ/cancer-targeting agents utilizing the radiolabeling approaches described are provided.

From Unorthodox to Established: The Current Status of (18)F-Trifluoroborate- and (18)F-SiFA-Based Radiopharmaceuticals in PET Nuclear Imaging

Unorthodox (18)F-labeling strategies not employing the formation of a carbon-(18)F bond are seldom found in radiochemistry. Historically, the formation of a boron- or silicon-(18)F bond has been introduced very early on into the repertoire of labeling chemistries, but is without translation into any clinical radiotracer besides inorganic B[(18)F]F4(-) for brain tumor diagnosis. For many decades these labeling methodologies were forgotten and have just recently been revived by a handful of researchers thinking outside the box. When breaking with established paradigms such as the inability to obtain labeled compounds of high specific activity via isotopic exchange or performing radiofluorination in aqueous media, the research community often reacts skeptically. In 2005 and 2006, two novel labeling methodologies were introduced into radiochemistry for positron emission tomography (PET) tracer development: RBF3(-) labeling reported by Perrin et al. and the SiFA methodology by Schirrmacher, Jurkschat, and Waengler et al. which is based on isotopic exchange (IE). Both labeling methodologies have been complemented by other noncanonical strategies to introduce (18)F into biomolecules of diagnostic importance, thus profoundly enriching the landscape of (18)F radiolabeling. B- and Si-based labeling strategies finally revealed that IE is a viable alternative to established and traditional radiochemistry with the advantage of simplifying both the labeling effort as well as the necessary purification of the radiotracer. Hence IE will be the focus of this contribution over other noncanonical labeling methods. Peptides for tumor imaging especially lend themselves favorably toward one-step labeling via IE, but small molecules have been described as well, taking advantage of these new approaches, and have been used successfully for brain imaging. This Review gives an account of both radiochemistries centered on boron and silicon, describing the very beginnings of their basic research, the path that led to optimization of their chemistries, and the first encouraging preclinical results paving the way to their clinical use. This side by side approach will give the reader the opportunity to follow the development of a new basic discovery into a clinically applicable radiotracer including all the hurdles that have had to be overcome.

PEGylation enhances the tumor selectivity of melanoma-targeted conjugates

In the development of our melanoma-selective delivery approach, three preselected conjugates of 5-iodo-2'-deoxyuridine (IUdR) to the ICF01012 melanoma-carrier were radiolabelled with iodine-125, and their in vivo distribution profile was determined. A radioiodination method for the conjugate 1a and its PEGylated derivatives 1b-c was developed via electrophilic iododestannylation in good radiochemical yield with excellent radiochemical purity (>99%). When administered to melanoma-bearing mice, the PEGylated conjugates exhibited an increased tumour uptake with a prolonged residence time. PEGylation also resulted in enhanced tumour selectivity compared with the non-PEGylated parent. These characteristics support further development of this model to achieve maximal concentration of anticancer therapeutics at the local site of action and minimize distribution to non-targeted sites.

Preparation and biological evaluation of 3-[(76)Br]bromo-α-methyl-L-tyrosine, a novel tyrosine analog for positron emission tomography imaging of tumors

INTRODUCTION

3-[(18)F]fluoro-α-methyl-l-tyrosine ([(18)F]FAMT) is a useful amino acid tracer for positron emission tomography (PET) imaging of malignant tumors. FAMT analogs labeled with (76)Br, a positron emitter with a long half-life (t(1/2)=16.1 h), could potentially be widely used as amino acid tracers for tumor imaging. In this study, 3-[(76)Br]bromo-α-methyl-l-tyrosine ([(76)Br]BAMT) was designed, and its usefulness was evaluated as a novel PET tracer for imaging malignant tumors.

METHODS

In this study, both [(76)Br]BAMT and [(77)Br]BAMT were prepared. The in vitro and in vivo stability of [(77)Br]BAMT was evaluated by HPLC analysis. Cellular uptake and retention of [(77)Br]BAMT and [(18)F]FAMT were evaluated using LS180 colon adenocarcinoma cells. Biodistribution studies were performed in normal mice and in LS180 tumor-bearing mice, and the tumors were imaged with a small-animal PET scanner.

RESULTS

[(77)Br]BAMT was stable in vitro but was catabolized after administration in mice. Cellular accumulation and retention of [(77)Br]BAMT were significantly higher than those of [(18)F]FAMT. In biodistribution studies, the tumor accumulation of [(77)Br]BAMT was higher than that of [(18)F]FAMT. However, some level of debromination was seen, which caused more retention of radioactivity in the blood and organs than was seen with [(18)F]FAMT. PET imaging with [(76)Br]BAMT enabled clear visualization of the tumor, and the whole-body image using [(76)Br]BAMT was similar to that using [(18)F]FAMT.

CONCLUSIONS

[(77)Br]BAMT showed high levels of tumor accumulation, and [(76)Br]BAMT enabled clear visualization of the tumor by PET imaging. Although an improvement in stability is still needed, (76)Br-labeled FAMT analogs could potentially serve as PET tracers for the imaging of malignant tumors.

Radiobromination of closo-dodecaborate anion. Aspects of labelling chemistry in aqueous solution using Chloramine-T

Closo-dodecaborate dianion is a three-dimensional aromatic inorganic molecule, which can be easily halogenated forming a stable halogen-boron bond. Derivatives of closo-dodecaborate were considered as a convenient chemical form of delivery of enriched 10B to malignant tumors for boron neutron capture therapy (BNCT). Some properties of closo-dodecaborate (hydrophilicity, strength of halogen-boron bond, charge at lysosomal pH) make it attractive as a potential prosthetic group for attachment of radioactive halogens to tumor-targeting proteins. Bromine radioisotopes possess a variety of useful nuclear characteristics, and can be used in different areas of clinical diagnostics and therapy. In this work, a basic chemistry of closo-dodecaborate radiobromination was studied. It was found, that di(triethylamonium) dodecahydro-closo-dodecaborate can be labelled in high yield, more then 90%, in a wide pH range. By decreasing the pH, the bromination can be directed to closo-dodecaborate in the presence of phenolic compounds. The results of the study indicate a possibility of using the radioactive bromine label for investigation of pharmacokinetics of boronated compounds for BNCT.

Bromination from the macroscopic level to the tracer radiochemical level: (76)Br radiolabeling of aromatic compounds via electrophilic substitution

No-carrier-added (NCA) (76)Br labeling of 4-(5-acetoxy-7-bromobenzoxazol-2-yl)phenyl acetate, a diacetate-protected estrogen-receptor beta (ERbeta) selective ligand, was carried out successfully using [(76)Br]bromide ion. The labeling was achieved via oxidative electrophilic destannylation of an organotin precursor molecule by modification of the leaving group (from Bu(3)Sn to Me(3)Sn) and the addition of methanol to the reaction mixture. The differences between the oxidative bromination reaction under small-scale macroscopic vs tracer level radiochemical conditions were explored in terms of effective brominating agents, which depend greatly on the nature of the solvent during the radiochemical bromination, and the potential interference by trace levels of highly reactive impurities in the reaction that compete for the desired bromination at the NCA level. Our observations, and our development of experimental protocols for successful radiobromination at the tracer NCA-scale, should be applicable to the synthesis of other radiobromine-labeled organic compounds of potential interest as PET radiopharmaceuticals and radiotherapy agents.

Optimized indirect (76)Br-bromination of antibodies using N-succinimidyl para-[76Br]bromobenzoate for radioimmuno PET

Monoclonal antibody 38S1 was radiobrominated with the positron emitter (76)Br (T(1/2) = 16.2 h). Indirect labeling was performed using N-succinimidyl para-(tri-methylstannyl)benzoate (SPMB) as the precursor molecule. SPMB was labeled using Chloramine-T yielding N-succinimidyl para-[(76)Br]bromobenzoate, which was then conjugated to the antibody. Optimization of the labeling conditions and further conjugation gave a total yield ( mean+/-max error) of 49+/-2%. The immunoreactivity of the antibodies was retained after labeling. Thus, antibodies intended for positron emission tomography can be labeled with (76)Br, which gives high yields and preserved immunoreactivity when using the SPMB technique described.

High yield direct 76Br-bromination of monoclonal antibodies using chloramine-T

Monoclonal antibody (MAb) A33 was labeled with the positron emitter 76Br (T(1/2) = 16.2 h). Direct labeling was done using the conventional chloramine-T method. After optimization of the labeling conditions, a maximum yield (mean +/- max error) of 77 +/- 2% was obtained at pH 6.8. In vitro binding of 76Br-A33 to SW1222 colonic cancer cells showed that the immunoreactivity was retained. Also, the MAbs 38S1 and 3S193 and the peptide hEGF were 76Br-labeled, resulting in labeling yields (mean +/- max error) of 75 +/- 3%, 63 +/- 4%, and 73 +/- 0.1%, respectively. We conclude that antibodies and peptides can be labeled conveniently with 76Br for the purpose of whole-body tumour imaging by positron emission tomography.

76Br-labeled monoclonal anti-CEA antibodies for radioimmuno positron emission tomography

For the application of anti-tumor monoclonal antibodies (MAbs) in positron emission tomography (PET), labeling radionuclides with half-lives allowing a suitable time frame for imaging are required. The anti-CEA MAb 38S1 was labeled with the positron emitting nuclide 76Br (t1/2 16 h) using bromoperoxidase (BPO), and subsequently affinity purified. A procedure was devised to allow reproducible production of MAb-preparations of high immunoreactivity and with acceptable bromination yield. The biological activity of 76Br-38S1 was retained and comparable to that of chloramine-T labeled 125I-38S1, as tested in vitro.

Synthesis of the 123I- and 125I-labeled cholinergic nerve marker (-)-5-iodobenzovesamicol

The highly toxic curraremimetic and cholinergic neuron marker (-)-5-iodobenzovesamicol (IBVM) has been labeled with iodine-125 and iodine-123. [125I]IBVM, suitable for animal distribution and ex vivo autoradiographic studies, was synthesized by solid-state exchange; isolated yields were 65-89% with specific activities in the range of 130-200 Ci/mmol. The synthesis of no-carrier-added (-)-5-[125I]IBVM from the corresponding chiral (-)-5-(tri-n-butyltin) derivative using Na125I was evaluated using the oxidants H2O2, peracetic acid and chloramine-T. Both peracetic acid and chloramine-T gave good yields (70-95%). However, when Na123I was utilized, acceptable yields of [123I]IBVM were obtained only with chloramine-T. Use of the latter oxidant did produce 5-chlorobenzovesamicol which was eliminated during HPLC purification. After optimization of the reaction parameters, [123I]IBVM in batch sizes of 10-27 mCi, is routinely obtained with a specific activity of 30-70,000 Ci/mmol, radiochemical purity (> 97%) and chiral purity (> 98%). Isolated radiochemical yields have averaged 71% (N = 40). Distribution analyses of [125I]IBVM and [123I]IBVM in mice 4 h following intravenous administration show essentially equivalent concentrations of the two tracers in the four brain regions sampled. The exceptionally high specific activity of [123I]IBVM has made possible the evaluation of this radiotracer in humans.

A rapid high yield synthesis of no-carrier-added (-)-[123I]iodocyanopindolol

A convenient and efficient radiosynthesis of no-carrier-added [123I]labeled (-)iodocyanopindolol, (-)-[123I]ICYP, a high affinity beta adrenergic antagonist, is described. (-)-[123I]ICYP was synthesized by a modified chloramine-T radioiodination of (-)cyanopindolol followed by a novel reversed-phase HPLC purification that provided the radiopharmaceutical as a directly injectable solution. The total synthesis time was typically less than 45 min and provided (-)-[123I]ICYP in a 59% radiochemical yield (not corrected for decay). In view of its high affinity for the beta adrenergic receptor, (-)-[123I]ICYP is a potentially useful probe for SPECT evaluation of cardiac adrenergic receptor density.

Distribution and stability in the rat of a 76Br/125I-labelled polypeptide, epidermal growth factor

A positron-emitting isotope of bromine, 76Br, with a half-life of 16.2 h, was produced using the reaction natBr(p, xn)76Kr. Labelling of mouse epidermal growth factor (EGF) with 76Br was optimized, using the chloramine-T method, obtaining a maximal radiochemical yield of 53%. In tests with receptor-rich, cultured glioma cells, [76Br]EGF and [125I]EGF bound equally well. A study of the distribution and stability of [76Br]EGF and [125I]EGF in normal rat was carried out. The distribution of both radioisotopes was similar, however, the percentage of 76Br bound to the high molecular weight fraction in the plasma, liver and kidney was greater than that of 125I.

Bromination, no-carrier-added radiobromination and simultaneously-occurring chlorination by chloramine T

Factors regulating initial rates of bromination, no-carrier-added radiobromination and simultaneously-occurring chlorination by chloramine T were studied using a neuroleptic drug spiperone as the substrate. Besides the factors such as initial concentrations of chloramine T, substrate and bromide ions, upon which the rates were dependent in first, second or zero-order, the water-acetic acid composition or the hydrogen ion concentration of the solutions, where the reactions took place, was found to play a key role. By controlling these factors, optimal radiobromination conditions, where radiobromination proceeds effectively whilst simultaneously-occurring chlorination is kept from proceeding at a high rate, thus resulting in radiobrominated radiopharmaceuticals of high specific activity (10 Ci/mumol) and high radiochemical and chemical purity, could be fulfilled.

Synthesis of 2-deoxy-2-[82Br]bromo-D-mannose and related compounds and their biodistributions in mice

The synthesis of 2-deoxy-2-[82Br]bromo-3,4,6-tri-O-acetyl-alpha-D-mannopyranosyl chloride (compound 3b) and 2-deoxy-2-[82Br]bromo-D-mannose (compound 4b), and their biodistributions in mice are described. The reaction of 3,4,6-tri-O-acetyl-D-glucal (compound 2) with unlabeled and labeled bromine chloride (compounds 1a and 1b) generated in situ from the oxidation of bromide with N-chloro-succinimide gave unlabeled and labeled 2-deoxy-2-bromo-3,4,6-tri-O-acetyl-alpha-D-mannopyranosyl chloride (compounds 3a and 3b) with a radiochemical yield of 58% (chemical yield, 63%). The hydrolysis of compounds 3a and 3b with 2N HCl gave 2-deoxy-2-bromo-D-mannose (compounds 4a and 4b) with a radiochemical yield of 72%. The biodistribution of compounds 3b and 4b after injection in mice indicated that 2% of the total injected radioactivity rapidly accumulated in the brain, while 6% of the total injected radioactivity accumulated in the heart; however, the radioactivity started to decline in these two organs after 15 min.

Synthesis of 131I, 125I and 82Br labelled (E)-5-(2-halovinyl)-2'-deoxyuridines

Radiohalogenated (E)-5-(2-iodovinyl)-2'-deoxyuridine (IVDU, 4) and (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU, 5) were synthesized by reaction of (E)-5-(2-carboxyvinyl)-2'-deoxyuridine (1) with radiolabelled iodide or bromide in the presence of chloramine-T. A "no-carrier-added" synthesis of [131I]IVDU was completed within 30 min providing a radiochemical yield of 65%. Alternatively, radioactive iodine was incorporated into IVDU using a halogen isotope exchange reaction catalyzed by cuprous ion. [82Br]BVDU was also prepared by direct neutron activation of unlabelled BVDU.