Polymer-derivatized technetium 99mTc-labeled liposomal blood pool agents for nuclear medicine applications

Radiolabelling of nanomaterials for medical imaging and therapy

Nanomaterials offer unique physical, chemical and biological properties of interest for medical imaging and therapy. Over the last two decades, there has been an increasing effort to translate nanomaterial-based medicinal products (so-called nanomedicines) into clinical practice and, although multiple nanoparticle-based formulations are clinically available, there is still a disparity between the number of pre-clinical products and those that reach clinical approval. To facilitate the efficient clinical translation of nanomedicinal-drugs, it is important to study their whole-body biodistribution and pharmacokinetics from the early stages of their development. Integrating this knowledge with that of their therapeutic profile and/or toxicity should provide a powerful combination to efficiently inform nanomedicine trials and allow early selection of the most promising candidates. In this context, radiolabelling nanomaterials allows whole-body and non-invasive in vivo tracking by the sensitive clinical imaging techniques positron emission tomography (PET), and single photon emission computed tomography (SPECT). Furthermore, certain radionuclides with specific nuclear emissions can elicit therapeutic effects by themselves, leading to radionuclide-based therapy. To ensure robust information during the development of nanomaterials for PET/SPECT imaging and/or radionuclide therapy, selection of the most appropriate radiolabelling method and knowledge of its limitations are critical. Different radiolabelling strategies are available depending on the type of material, the radionuclide and/or the final application. In this review we describe the different radiolabelling strategies currently available, with a critical vision over their advantages and disadvantages. The final aim is to review the most relevant and up-to-date knowledge available in this field, and support the efficient clinical translation of future nanomedicinal products for in vivo imaging and/or therapy.

Nuclear imaging of liposomal drug delivery systems: A critical review of radiolabelling methods and applications in nanomedicine

The integration of nuclear imaging with nanomedicine is a powerful tool for efficient development and clinical translation of liposomal drug delivery systems. Furthermore, it may allow highly efficient imaging-guided personalised treatments. In this article, we critically review methods available for radiolabelling liposomes. We discuss the influence that the radiolabelling methods can have on their biodistribution and highlight the often-overlooked possibility of misinterpretation of results due to decomposition in vivo. We stress the need for knowing the biodistribution/pharmacokinetics of both the radiolabelled liposomal components and free radionuclides in order to confidently evaluate the images, as they often share excretion pathways with intact liposomes (e.g. phospholipids, metallic radionuclides) and even show significant tumour uptake by themselves (e.g. some radionuclides). Finally, we describe preclinical and clinical studies using radiolabelled liposomes and discuss their impact in supporting liposomal drug development and clinical translation in several diseases, including personalised nanomedicine approaches.

Detection of acute postoperative mediastinitis in mice using (99m)Tc-liposomes

RATIONALE AND OBJECTIVES

To evaluate the usefulness of size-modified (99m)Tc-labeled liposomes for the detection of acute postoperative mediastinitis in a mouse model.

METHODS

Fourteen mice underwent low-neck collar incision and had sterile abscesses induced in mediastinum with turpentine. Ten of these animals were injected intravenously with anionic liposomes of 516 +/- 20 nm containing poly(ethylene)glycol labeled with (99m)Tc; the remaining four mice were injected with (67)Ga citrate and used as positive controls. In addition, eight mice either underwent the same surgical procedure but without turpentine (n = 4) or were not operated (n = 4). These were used as negative controls. Therefore, scintigraphy using (99m)Tc-liposomes was performed in eighteen and (67)Ga citrate in four mice. Target area of interest was outlined, and target to background count density ratio and percentage-injected dose were measured.

RESULTS

Significant accumulation of radiolabeled liposomes was observed at sites of inflammation within 1 hour in abscess-bearing animals. This correlated well with the findings of the lower quality (contrast) of (67)Ga images at 24 and 48 hours. The radiopharmaceutical did not significantly accumulate in the mediastinum of negative control animals.

CONCLUSION

(99m)Tc-liposomes (size modified) may prove useful nonspecific agent for the early diagnosis of postoperative mediastinitis.

Long-circulating gadolinium-loaded liposomes: potential use for magnetic resonance imaging of the blood pool

In our previous paper, we reported a method of liposome loading with Gadolinium (Gd) via so called polychelating amphiphilic polymer (PAP). A novel Gd-containing polymeric probe, suitable for the incorporation into the liposomal membrane, was prepared from a low-molecular-weight DTPA-polylysine by linking its N-terminus to a lipid anchor, NGPE-PE. When compared with known membranotropic MR probes, such as Gd-DTPA-SA and Gd-DTPA-PE, liposomes containing new membrane-bound polychelator possess enhanced relaxivity for water protons resulting in an increase of tissue signal intensity on MR images. In this study, we developed the optimized protocol to prepare a liposomal MR contrast agent with high relaxivity and narrow size distribution. Gd-containing liposomes were additionally modified with PEG to provide longevity in vivo. We also demonstrated that upon intravenous administration in rabbit and dog, the new preparation causes a prolonged decrease in the blood T(1) value (reflecting the proton relaxation rate in the blood) and may be considered as a potential contrast agent for MRI of the blood pool.

Radiolabeled liposomes for scintigraphic imaging

Liposomes have been investigated extensively as carriers for drugs in attempts to achieve selective deposition and/or reduced toxicity. Liposomes radiolabeled with gamma emitters such as (67)Ga, (111)In and (99m)Tc, can be used for imaging purposes. Liposomes as formulated in the past, are rapidly taken up by cells of the mononuclear phagocyte system (MPS), primarily those located in liver and spleen. The recent development of long-circulating liposomes (LCLs), yielded liposomes that oppose recognition by the MPS. The development of these LCLs with enhanced circulatory half-lives has broadened the potential of liposomes to scintigraphically visualize pathologic processes in vivo. Liposomes have been proposed for tumor imaging, infection imaging and blood pool imaging. Strategies have been developed that allow rapid, easy and efficient labeling of preformed liposomes with (111)In and (99m)Tc. There is now a vast body of preclinical evidence showing that LCLs can be used to image a wide variety of tumors as well as inflammatory lesions. The first studies in patients show that radiolabeled liposomes can image tumor and inflammatory lesions with good sensitivity and good specificity. Here, the present status of liposome-based radiopharmaceuticals for scintigraphic application is reviewed.

Anticancer therapy using glucuronate modified long-circulating liposomes

Since conventional liposomes tend to be trapped by the reticuroendothelial systems (RES), their use as drug carriers is limited when the targets are not RES cells. Therefore, many attempts have been made to avoid the RES-trapping of liposomes. Favorable results were obtained by a modification of liposomes with a glucuronic acid derivative, PGlcUA, and polyethyleneglycol. These liposomes have a long-circulating character, and showed the further advantage for passive targeting to tumor tissues, since the vasculature in tumor tissues is leaky enough for small-sized liposomes to extravasate. Thus long-circulating liposomes are useful for tumor imaging and treatment. PGlcUA-modified liposomes were actually found to accumulate effectively in tumor tissue, and showed enhanced efficacy of antitumor agents, such as adriamycin and vincristine when they were encapsulated into the liposomes. Usefulness of PGlcUA liposomes as drug carriers was also observed in photodynamic therapy and in treatment of cancer by amphiphilic novel antitumor agents.

Approaches and agents for imaging the vascular system

Several classes of vascular imaging agents are described: (1) liposome-based blood cell mimetics; (2) plasma protein mimetics; (3) small molecules that bind to plasma proteins in the circulation. The characteristic features of the different agents are described and critically compared, including the advantages and potential pitfalls of each individual type.

Polymeric micelles as carriers of diagnostic agents

This review deals with diagnostic applications of polymeric micelles composed of amphiphilic block-copolymers. In aqueous solutions these polymers spontaneously form particles with diameter 20-100 nm. A variety of diagnostic moieties can be incorporated covalently or non-covalently into the particulates with high loads. Resulting particles can be used as particulate agents for diagnostic imaging using three major imaging modalities: gamma-scintigraphy, magnetic resonance imaging and computed tomography. The use of polyethyleneoxide-diacyllipid micelles loaded with chelated (111)In/Gd(3+) as well as iodine-containing amphiphilic copolymer in percutaneous lymphography and blood pool/liver imaging are discussed as specific examples.

Development and evaluation of a kit formulation for the preparation of 99mTc-DMP-HSA, a new tracer agent for radionuclide ventriculography

This study presents the development of a kit formulation for the preparation of 99mTc-DMP-HSA, followed by a comparison of such kit-prepared 99mTc-DMP-HSA to 99mTc-RBCs in a volunteer. Reconstitution of the labeling kits with up to 5.55 GBq 99mTc afforded 99mTc-DMP-HSA preparations with a > 95% radiochemical purity for up to 8 h. Only minor differences were observed in the global distribution of both tracer agents, whereas the calculated ejection fractions were almost identical. The effective dose equivalent of 99mTc-DMP-HSA is 8.68 microSv/MBq.

99mTc-labelled diphytanoylphosphatidylcholine liposomes: in vitro and in vivo studies

The uniquely structured diphytanoylphosphatidylcholine (DphPC) forms liposomes more stable than conventional straight chain phospholipids. In this study DphPC and pegylated DphPC (DphPC-PEG) liposomes were radiolabelled and evaluated in vitro and in vivo. 99mTc-DphPC liposomes were found to be nontoxic to human white blood cells in vitro. In addition 99mTc labelled DphPC-PEG liposomes were evaluated as a nonspecific infection imaging agent in a mouse model. Infection sites were imaged within 30 min postinjection, and the radiopharmaceutical exhibited a remarkable in vivo stability. As their biodistribution and pharmacokinetic patterns can be size-modulated, DphPC-based lipsomes are excellent candidates for diagnostic and therapeutic applications.

Liposomes as delivery agents for medical imaging

Medical imaging requires an appropriate intensity of signal from the area of interest in order to differentiate certain structures from surrounding tissues, regardless of the modality used. In the majority of cases, contrast agents specific for each imaging modality are necessary to achieve a sufficiently intense signal. To facilitate the accumulation of contrast in the required zone, various microparticulates have been suggested as carriers for contrast agents. Among these carriers, liposomes-microscopic artificial phospholipid vesicles-draw special attention because of their easily controlled properties and useful pharmacological characteristics. This review will discuss how the advantages of liposomes have been used so far in the rapidly growing field of diagnostic medical imaging.

Antitumor activity of vincristine encapsulated in glucuronide-modified long-circulating liposomes in mice bearing Meth A sarcoma

Liposomes modified with the uronic acid derivative palmityl-D-glucuronide (PGlcUA) have a long circulation time and tend to accumulate in the tumors of tumor-bearing mice. Taking advantage of this character, we investigated the therapeutic effect of vincristine (VCR) encapsulated in liposomes containing PGlcUA (dipalmitoylphosphatidylcholine/cholesterol/PGlcUA = 4:4:1 as a molar ratio) on tumor-bearing mice. VCR was loaded into liposomes by a remote loading method, and then free or liposomal VCR was injected intravenously into BALB/c mice bearing Meth A sarcoma implanted subcutaneously 5 days before hand. Single-dose administration of VCR (3.0 mg/kg) in PGlcUA-liposomes significantly suppressed tumor growth, and prolonged the survival time (T/C = 1.37). Furthermore, two-dose administration of the liposomes cured one third of the animals. The therapeutic effect of PGlcUA-liposomes was greater than that of control liposomes containing dipalmitoylphosphatidylglycerol instead of PGlcUA. PGlcUA-liposomes might thus be a useful tool for delivering antitumor agents to tumor tissues.

Polyethylene glycol modification: relevance of improved methodology to tumour targeting

Of all the polymers applied to molecule altering structural chemistry, polyethylene glycol (PEG) modification has numerous benefits and relatively few drawbacks. PEG is now increasingly being applied to the problems of tumour targeting, both in the context of the passive targeting of PEG-liposomes and in active targeting strategies using PEGylated anti-tumour antibodies. PEG can also serve as a useful linker molecule between targeting moieties and other agents, including cytotoxic or imaging agents and targeted liposomes. Despite these demonstrated benefits and the level of attention which PEGylation has received, relatively little consideration has been given to two key areas: first, the extent to which the coupling method has an impact on both the functionality of the PEG-adduct and the acquisition of beneficial properties; second, that the impact of PEGylation on biodistribution is complex, thus any attempt to optimise a PEG-peptide or PEG-liposome for a particular task must involve an examination of all the individual facets of the effects of PEGylation. Studies investigating the underlying principles of tumour targeting suggest that current views concerning the optimisation of PEGylated vehicles for tumour localisation need to be re-examined.

Real-time analysis of liposomal trafficking in tumor-bearing mice by use of positron emission tomography

Long-circulating liposomes are known to accumulate passively in tumor tissues of tumor-bearing animals. To evaluate the in vivo behavior of such liposomes, we investigated the real-time liposomal trafficking by a non-invasive method using position emission tomography (PET). Liposomes composed of dipalmitoylphosphatidylcholine, cholesterol, and palmityl-D-glucuronide (PGlcUA) in a molar ratio of 4:4:1 were prepared in the presence of 2-[18F]fluoro-2-deoxyglucose ([2-18F]FDG). [2-18F]FDG-labeled liposomes sized by extrusion through a filter with various-sized pores were administered to mice bearing Meth A sarcoma, and a PET scan was performed for 120 min. Small-sized, long-circulating liposomes (100 nm in diameter) constructed with PGlcUA tended to accumulate in the tumor tissues. On the contrary, control liposomes (100 nm in diameter) containing dipalmitoylphosphatidylglycerol instead of PGlcUA accumulated in the liver. Large-sized PGlcUA-containing liposomes (> 300 nm) also accumulated in the liver, as well as in the spleen. Time-activity curves indicated that the small long-circulating liposomes (< 200 nm) transiently accumulated in the liver right after the injection but that the accumulation there decreased time-dependently. These data suggest that, although the majority of small long-circulating liposomes remain in the bloodstream, some extravasate once into the interstitial spaces in the liver re-enter the bloodstream again, and finally accumulate in the tumor tissues. This PET technique might be useful for studying real-time liposomal trafficking and for tumor imaging.

Lyophilization and rehydration of polymer-coated lipid vesicles containing a lipophilic chelator in the presence of sucrose: labeling with 99mTc and biodistribution studies

In this paper we describe studies of the effect of lyophilization and rehydration of polymer-coated lipid vesicles bearing a lipophilic surface chelator upon subsequent labeling with technetium-99m and in vivo biodistribution behavior. Unilamellar vesicles of average diameter 100 nm were prepared containing 2 mol% of the lipophilic chelator phosphatidylethanolamine-diethylenetriaminetetraacetic acid (PE-DTTA) and either 0 or 3 mol% of the lipid-polymer conjugate, dipalmitoyl-phosphatidyl-ethanolamine-monomethoxy polyethylene glycol 5000 (PE-MPEG) in 0.9% sodium chloride to which was added varying amounts of sucrose to a final concentration of 100-500 mM. The size of the vesicles in sucrose was determined before lyophilization and after rehydration and the effect of sucrose on the ability to label the vesicles with pertechnetate in the presence of stannous chloride was determined. Biodistribution studies were done in rabbits with vesicles before lyophilization and after rehydration in order to determine whether the rate of clearance from the blood pool was affected by the lyophilization/rehydration procedure. Results demonstrate that vesicles containing PE-DTTA and without PE-MPEG can be lyophilized and rehydrated with no change in average size or size distribution so long as the external sucrose concentration is greater than approx. 250 mM. When PE-MPEG is also present in the membrane the average vesicle size increases from approx. 140 to 200 nm, consistent with vesicle fusion. However, this small change in vesicle size makes no difference to the resulting circulation half-life (approximately 2 h at a lipid dose of approximately 10 mumol/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

PEG-coated lipid vesicles with encapsulated technetium-99m as blood pool agents for nuclear medicine

Unilamellar lipid vesicles of average diameter 200 nm containing 30 mM glutathione were internally labeled with 99mTc using exametazime (HMPAO) to transport technetium across the lipid bilayer. Vesicles were prepared both in the absence and presence of the lipid-polymer conjugate phosphatidylethanolamine-monomethoxy polyethylene glycol 5000 (PE-MPEG). Labeling efficiency both in the absence and presence of surface polymer was greater than 95% and the vesicles retained greater than 95% of their contents when incubated against 50% human serum at 37 degrees C for 12 h. When introduced intravenously into rabbits at a total lipid concentration of 4 mumol/kg (3.5 mg/kg), radiolabeled vesicles without surface polymer were rapidly cleared from the circulation with a half-life of approx. 30 min and delivered to liver and spleen, however if the lipid vesicles were prepared containing 3 or 4.5 mol percent PE-MPEG the circulation half-life of the label was approx. 5 and 10 h, respectively, and RES uptake was suppressed. These studies confirm a previous report of the utility of exametazime for preparing 99mTc-labeled lipid vesicles and demonstrate that extended circulation half-lives are achievable for 99mTc-labeled vesicles without recourse to high lipid doses and reticuloendothelial blockade. Applications to use exametazime as a blood pool marker in nuclear medicine are discussed.