Comparison of Linear and Hyperbranched Polyether Lipids for Liposome Shielding by 18F-Radiolabeling and Positron Emission Tomography.

Multifunctional and highly biocompatible polyether structures play a key role in shielding liposomes from degradation in the bloodstream, providing also multiple functional groups for further attachment of targeting moieties. In this work hyperbranched polyglycerol ( hbPG) bearing lipids with long alkyl chain anchor are evaluated with respect to steric stabilization of liposomes. The branched polyether lipids possess a hydrophobic bis(hexadecyl)glycerol membrane anchor for the liposomal membrane. hbPG was chosen as a multifunctional alternative to PEG, enabling the eventual linkage of multiple targeting vectors. Different hbPG lipids ( Mn = 2900 and 5200 g mol-1) were examined. A linear bis(hexadecyl)glycerol-PEG lipid ( Mn = 3000 g mol-1) was investigated as well, comparing hbPG and PEG with respect to shielding properties. Radiolabeling of the polymers was carried out using 1-azido-2-(2-(2-[18F]fluoroethoxy)ethoxy)ethane ([18F]F-TEG-N)3 via copper-catalyzed alkyne-azide cycloaddition with excellent radiochemical yields exceeding 95%. Liposomes were prepared by the thin-film hydration method followed by repeated extrusion. Use of a custom automatic extrusion device gave access to reproducible sizes of the liposomes (hydrodynamic radius of 60-94 nm). The in vivo fate of the bis(hexadecyl)glycerol polyethers and their corresponding assembled liposome structures were evaluated via noninvasive small animal positron emission tomography (PET) imaging and biodistribution studies (1 h after injection and 4 h after injection) in mice. Whereas the main uptake of the nonliposomal polyether lipids was observed in the kidneys and in the bladder after 1 h due to rapid renal clearance, in contrast, the corresponding liposomes showed uptake in the blood pool as well as in organs with good blood supply, that is, heart and lung over the whole observation period of 4 h. The in vivo behavior of all three liposomal formulations was comparable, albeit with remarkable differences in splenic uptake. Overall, liposomes shielded by the branched polyglycerol lipids show a favorable biodistribution with greatly prolonged blood circulation times, rendering them promising novel nanovesicles for drug transport and targeting.

Fate of linear and branched polyether-lipids in vivo in comparison to their liposomal formulations by 18F-radiolabeling and positron emission tomography.

In this study, linear poly(ethylene glycol) (PEG) and novel linear-hyperbranched, amphiphilic polyglycerol (hbPG) polymers with cholesterol (Ch) as a lipid anchor moiety were radiolabeled with fluorine-18 via copper-catalyzed click chemistry. In vivo investigations via positron emission tomography (PET) and ex vivo biodistribution in mice were conducted. A systematic comparison to the liposomal formulations with and without the polymers with respect to their initial pharmacokinetic properties during the first hour was carried out, revealing remarkable differences. Additionally, cholesterol was directly labeled with fluorine-18 and examined likewise. Both polymers, Ch-PEG27-CH2-triazole-TEG-(18)F and Ch-PEG30-hbPG24-CH2-triazole-TEG-(18)F (TEG: triethylene glycol), showed rapid renal excretion, whereas the (18)F-cholesten displayed retention in lung, liver, and spleen. Liposomes containing Ch-PEG27-CH2-triazole-TEG-(18)F revealed a hydrodynamic radius of 46 nm, liposomal Ch-PEG30-hbPG24-CH2-triazole-TEG-(18)F showed a radius of 84 nm and conventional liposomes with (18)F-cholesten 204 nm, respectively. The results revealed fast uptake of the conventional liposomes by liver, spleen, and lung. Most importantly, the novel hbPG-polymer stabilized liposomes showed similar behavior to the PEG-shielded vesicles. Thus, an advantage of multifunctionality is achieved with retained pharmacokinetic properties. The approach expands the scope of polymer tracking in vivo and liposome tracing in mice via PET.

HPMA-LMA copolymer drug carriers in oncology: an in vivo PET study to assess the tumor line-specific polymer uptake and body distribution.

Polymeric drug carriers aim to selectively target tumors in combination with protecting normal tissue. In this regard polymer structure and molecular weight are key factors considering organ distribution and tumor accumulation of the polymeric drug delivery system. Four different HPMA based copolymer structures (random as well as block copolymers with lauryl methacrylate as hydrophobic block) varying in molecular weight, size and resulting architecture were analyzed in two different tumor models (AT1 prostate carcinoma and Walker-256 mammary carcinoma) in vivo. Polymers were labeled with (18)F and organ/tumor uptake was followed by µPET imaging and ex vivo biodistribution. Vascular permeability was measured by dextran extravasation and vascular density by immunohistochemistry. Cellular polymer uptake was determined in vitro using fluorescence-labeled polymers. Most strikingly, the high molecular weight HPMA-LMA random copolymer demonstrated highest tumor uptake and blood pool concentration. The molecular structure (e.g., amphiphilicity) is holding a higher impact on desired in vivo properties than polymer size. The results also revealed pronounced differences between the tumor models although vascular permeability was almost comparable. Accumulation in Walker-256 carcinomas was much higher, presumably due to a better cellular uptake in this cell line and a denser vascular network in the tumors. These investigations clearly indicate that the properties of the individual tumor determine the suitability of polymeric drug carriers. The findings also illustrate the general necessity of a preclinical screening to analyze polymer uptake for each individual patient (e.g., by noninvasive PET imaging) in order to individualize polymer-based chemotherapy.

Modifying the body distribution of HPMA-based copolymers by molecular weight and aggregate formation.

There is a recognized need to create well-defined polymer probes for in vivo and clinical positron emission tomography (PET) imaging to guide the development of new generation polymer therapeutics. Using the RAFT polymerization technique in combination with the reactive ester approach, here we have synthesized well-defined and narrowly distributed N-(2-hydroxypropyl)methacrylamide homopolymers (pHPMA) (P1* and P2*) and random HPMA copolymers consisting of hydrophilic HPMA and hydrophobic lauryl methacrylate comonomers (P3* and P4*). The polymers had molecular weights below (P1* and P3*) and above the renal threshold (P2* and P4*). Whereas the homopolymers dissolve in isotonic solution as individual coils, the random copolymers form larger aggregates above their critical micelle concentration (∼ 40 nm), as determined by fluorescence correlation spectroscopy. Structure-property relationships of the pharmacokinetics and biodistribution of the different polymer architectures were monitored in the living organism following radiolabeling with the positron emitter (18)F via fluoroethylation within a few hours. Ex vivo organ biodistribution and in vivo µPET imaging studies in male Copenhagen rats revealed that both size and the nature of the aggregate formation (hydrophobically modified copolymers) played a major role in blood clearance and biodistribution, especially concerning liver and kidney accumulation. The high-molecular-weight random copolymer P4* (hydrophobically modified), in particular, combines low liver uptake with enhanced blood circulation properties, showing the potential of hydrophobic interactions, as seen for the represented model system, that are valuable for future drug carrier design.

HPMA based amphiphilic copolymers mediate central nervous effects of domperidone.

In this study we give evidence that domperidone encapsulated into amphiphilic p(HPMA)-co-p(laurylmethacrylate) (LMA) copolymer aggregates is able to cross the blood-brain barrier, since it affected motor behaviour in animals, which is a sensitive measure for CNS actions. Carefully designed copolymers based on the clinically approved p(HPMA) were selected and synthesized by a combination of controlled radical polymerization and post-polymerization modification. The hydrodynamic radii (R(h) ) of amphiphilic p(HPMA)-co-p(LMA) alone and loaded with domperidone were determined by fluorescence correlation spectroscopy.

Radioactive labeling of defined HPMA-based polymeric structures using [18F]FETos for in vivo imaging by positron emission tomography.

During the last decades polymer-based nanomedicine has turned out to be a promising tool in modern pharmaceutics. The following article describes the synthesis of well-defined random and block copolymers by RAFT polymerization with potential medical application. The polymers have been labeled with the positron-emitting nuclide fluorine-18. The polymeric structures are based on the biocompatible N-(2-hydroxypropyl)-methacrylamide (HPMA). To achieve these structures, functional reactive ester polymers with a molecular weight within the range of 25,000-110,000 g/mol were aminolyzed by 2-hydroxypropylamine and tyramine (3%) to form (18)F-labelable HPMA-polymer precursors. The labeling procedure of the phenolic tyramine moieties via the secondary labeling synthon 2-[(18)F]fluoroethyl-1-tosylate ([(18)F]FETos) provided radiochemical fluoroalkylation yields of ∼80% for block copolymers and >50% for random polymer architectures within a synthesis time of 10 min and a reaction temperature of 120 °C. Total synthesis time including synthon synthesis, (18)F-labeling, and final purification via size exclusion chromatography took less than 90 min and yielded stable (18)F-labeled HPMA structures in isotonic buffer solution. Any decomposition could be detected within 2 h. To determine the in vivo fate of (18)F-labeled HPMA polymers, preliminary small animal positron emission tomography (PET) experiments were performed in healthy rats, demonstrating the renal clearance of low molecular weight polymers. Furthermore, low metabolism rates could be detected in urine as well as in the blood. Thus, we expect this new strategy for radioactive labeling of polymers as a promising approach for in vivo PET studies.

Long-term biodistribution study of HPMA-ran-LMA copolymers in vivo by means of 131I-labeling.

BACKGROUND: For the evaluation of macromolecular drug delivery systems suitable pre-clinical monitoring of potential nanocarrier systems is needed. In this regard, both short-term as well as long-term in vivo tracking is crucial to understand structure-property relationships of polymer carrier systems and their resulting pharmacokinetic profile. Based on former studies revealing favorable in vivo characteristics for 18F-labeled random (ran) copolymers consisting of N-(2-hydroxypropyl)methacrylamide (HPMA) and lauryl methacrylate (LMA) - including prolonged plasma half-life as well as enhanced tumor accumulation - the presented work focuses on their long-term investigation in the living organism. METHODS: In this respect, four different HPMA-based polymers (homopolymers as well as random copolymers with LMA as hydrophobic segment) were synthesized and subsequent radioactive labeling was accomplished via the longer-lived radioisotope 131I. In vivo results, concentrating on the pharmacokinetics of a high molecular weight HPMA-ran-LMA copolymer, were obtained by means of biodistribution and metabolism studies in the Walker 256 mammary carcinoma model over a time-span of up to three days. Besides, a direct comparison with the 18F-radiolabeled polymer was drawn. To consider physico-chemical differences between the differently labeled polymer (18F or 131I) on the critical micelle concentration (CMC) and the size of the polymeric micelles, those properties were determined using the 19F- or 127I-functionalized polymer. Special emphasis was laid on the time-dependent correlation between blood circulation properties and corresponding tumor accumulation, particularly regarding the enhanced permeability and retention (EPR) effect. RESULTS: Studies revealed, at first, differences in the short time (2h) body distribution, despite the very similar properties (molecular structure, CMC and size of the micellar aggregates) of the non-radioactive 19F- and 127I-functionalized polymers. Long-term investigations with the 131I-labeled polymer demonstrated that, despite a polymer clearance from the blood within 72h, there was still an increase in tumor uptake observed over time. Regarding the stability of the 131I-label, ex vivo biodistribution experiments, considering the uptake in the thyroid, indicated low metabolism rates. CONCLUSION: The observed in vivo characteristics strongly underline the EPR effect. The findings illustrate the need to combine information of different labeling approaches and in vivo evaluation techniques to generate an overall pharmacokinetic picture of potential nanocarriers in the pre-clinical setting. ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENTS: The in vivo behavior of the investigated HPMA-ran-LMA copolymer demonstrates great potential in terms of an effective accumulation in the tumor.

PEGylation of HPMA-based block copolymers enhances tumor accumulation in vivo: a quantitative study using radiolabeling and positron emission tomography.

This paper reports the body distribution of block copolymers (made by controlled radical polymerization) with N-(2-hydroxypropyl)methacrylamide (HPMA) as hydrophilic block and lauryl methacrylate (LMA) as hydrophobic block. They form micellar aggregates in aqueous solution. For this study the hydrophilic/hydrophobic balance was varied by incorporation of differing amounts of poly(ethylene glycol) (PEG) side chains into the hydrophilic block, while keeping the degree of polymerization of both blocks constant. PEGylation reduced the size of the micellar aggregates (Rh=113 to 38 nm) and led to a minimum size of 7% PEG side chains. Polymers were labeled with the positron emitter (18)F, which enables to monitor their biodistribution pattern for up to 4h with high spatial resolution. These block copolymers were investigated in Sprague-Dawley rats bearing the Walker 256 mammary carcinoma in vivo. Organ/tumor uptake was quantified by ex vivo biodistribution as well as small animal positron emission tomography (PET). All polymers showed renal clearance with time. Their uptake in liver and spleen decreased with size of the aggregates. This made PEGylated polymers--which form smaller aggregates--attractive as they show a higher blood pool concentration. Within the studied polymers, the block copolymer of 7% PEGylation exhibited the most favorable organ distribution pattern, showing highest blood-circulation level as well as lowest hepatic and splenic uptake. Most remarkably, the in vivo results revealed a continuous increase in tumor accumulation with PEGylation (independent of the blood pool concentration)--starting from lowest tumor uptake for the pure block copolymer to highest enrichment with 11% PEG side chains. These findings emphasize the need for reliable (non-invasive) in vivo techniques revealing overall polymer distribution and helping to identify drug carrier systems for efficient therapy.

Predicting the in vivo release from a liposomal formulation by IVIVC and non-invasive positron emission tomography imaging.

This study aimed to predict the in vivo performance from the in vitro release of a low-molecular weight model compound, [(18)F]-2-fluoro-2-deoxy-d-glucose ([(18)F]FDG), from liposomes and by means of positron emission tomography (PET). Liposomes composed of hydrogenated phosphatidylcholine (HPC) were prepared by a freeze-thaw method. Particle size distribution was measured by dynamic light scattering (DLS). In vitro release was examined with a dispersion method detecting the radioactivity of [(18)F]FDG. In vivo release of [(18)F]FDG, following i.p. injection of the liposomes in rats, was determined by using a Micro-PET scanner. Convolution was performed to predict the in vivo profiles from the in vitro data and to establish an in vitro-in vivo correlation (IVIVC). The in vivo predictions slightly underestimated the experimentally determined values. The magnitude of the prediction errors (13% and 19%) displayed a satisfactory IVIV relationship leaving yet room for further improvement. This study demonstrated for the first time the use of PET in attaining an IVIVC for a parenterally administered modified release dosage form. It is therefore possible to predict target tissue concentrations, e.g., in the brain, from in vitro release experiments. IVIVC using non-invasive PET imaging could thus be a valuable tool in drug formulation development, resulting in reduced animal testing.