A bisphosphonate for 19F-magnetic resonance imaging

19F-magnetic resonance imaging (MRI) is a promising technique that may allow us to measure the concentration of exogenous fluorinated imaging probes quantitatively in vivo. Here, we describe the synthesis and characterisation of a novel geminal bisphosphonate (19F-BP) that contains chemically-equivalent fluorine atoms that show a single and narrow 19F resonance and a bisphosphonate group that may be used for labelling inorganic materials based in calcium phosphates and metal oxides. The potential of 19F-BP to provide contrast was analysed in vitro and in vivo using 19F-MRI. In vitro studies demonstrated the potential of 19F-BP as an MRI contrast agent in the millimolar concentration range with signal-to-noise ratios (SNR) comparable to previously reported fluorinated probes. The preliminary in vivo MRI study reported here allowed us to visualise the biodistribution of 19F-BP, showing uptake in the liver and in the bladder/urinary system areas. However, bone uptake was not observed. In addition, 19F-BP showed undesirable toxicity effects in mice that prevent further studies with this compound at the required concentrations for MRI contrast. This study highlights the importance of developing 19F MRI probes with the highest signal intensity achievable.

Cardiac hypoxia imaging: second-generation analogues of 64Cu-ATSM

Myocardial hypoxia is an attractive target for diagnostic and prognostic imaging, but current approaches are insufficiently sensitive for clinical use. The PET tracer copper(II)-diacetyl-bis(N4-methylthiosemicarbazone) ((64)Cu-ATSM) has promise, but its selectivity and sensitivity could be improved by structural modification. We have therefore evaluated a range of (64)Cu-ATSM analogs for imaging hypoxic myocardium.

METHODS

Isolated rat hearts (n = 5/group) were perfused with normoxic buffer for 30 min and then hypoxic buffer for 45 min within a custom-built triple-γ-detector system to quantify radiotracer infusion, hypoxia-dependent cardiac uptake, and washout. A 1-MBq bolus of each candidate tracer (and (18)F-fluoromisonidazole for comparative purposes) was injected into the arterial line during normoxia, and during early and late hypoxia, and their hypoxia selectivity and pharmacokinetics were evaluated. The in vivo pharmacokinetics of promising candidates in healthy rats were then assessed by PET imaging and biodistribution.

RESULTS

All tested analogs exhibited hypoxia sensitivity within 5 min. Complexes less lipophilic than (64)Cu-ATSM provided significant gains in hypoxic-to-normoxic contrast (14:1 for (64)Cu-2,3-butanedione bis(thiosemicarbazone) (ATS), 17:1 for (64)Cu-2,3-pentanedione bis(thiosemicarbazone) (CTS), 8:1 for (64)Cu-ATSM, P < 0.05). Hypoxic first-pass uptake was 78.2% ± 7.2% for (64)Cu-ATS and 70.7% ± 14.5% for (64)Cu-CTS, compared with 63.9% ± 11.7% for (64)Cu-ATSM. Cardiac retention of (18)F-fluoromisonidazole increased from 0.44% ± 0.17% during normoxia to 2.24% ± 0.08% during hypoxia. In vivo, normoxic cardiac retention of (64)Cu-CTS was significantly lower than that of (64)Cu-ATSM and (64)Cu-ATS (0.13% ± 0.02% vs. 0.25% ± 0.04% and 0.24% ± 0.03% injected dose, P < 0.05), with retention of all 3 tracers falling to less than 0.7% injected dose within 6 min. (64)Cu-CTS also exhibited lower uptake in liver and lung.

CONCLUSION

(64)Cu-ATS and (64)Cu-CTS exhibit better cardiac hypoxia selectivity and imaging characteristics than the current lead hypoxia tracers, (64)Cu-ATSM and (18)F-fluoromisonidazole.

PEGylation improves the receptor-mediated transfection efficiency of peptide-targeted, self-assembling, anionic nanocomplexes

Non-viral vector formulations comprise typically complexes of nucleic acids with cationic polymers or lipids. However, for in vivo applications cationic formulations suffer from problems of poor tissue penetration, non-specific binding to cells, interaction with serum proteins and cell adhesion molecules and can lead to inflammatory responses. Anionic formulations may provide a solution to these problems but they have not been developed to the same extent as cationic formulations due to difficulties of nucleic acid packaging and poor transfection efficiency. We have developed novel PEGylated, anionic nanocomplexes containing cationic targeting peptides that act as a bridge between PEGylated anionic liposomes and plasmid DNA. At optimized ratios, the components self-assemble into anionic nanocomplexes with a high packaging efficiency of plasmid DNA. Anionic PEGylated nanocomplexes were resistant to aggregation in serum and transfected cells with a far higher degree of receptor-targeted specificity than their homologous non-PEGylated anionic and cationic counterparts. Gadolinium-labeled, anionic nanoparticles, administered directly to the brain by convection-enhanced delivery displayed improved tissue penetration and dispersal as well as more widespread cellular transfection than cationic formulations. Anionic PEGylated nanocomplexes have widespread potential for in vivo gene therapy due to their targeted transfection efficiency and ability to penetrate tissues.

Multifunctional receptor-targeted nanocomplexes for the delivery of therapeutic nucleic acids to the brain

Convection enhanced delivery (CED) is a method of direct injection to the brain that can achieve widespread dispersal of therapeutics, including gene therapies, from a single dose. Non-viral, nanocomplexes are of interest as vectors for gene therapy in the brain, but it is essential that administration should achieve maximal dispersal to minimise the number of injections required. We hypothesised that anionic nanocomplexes administered by CED should disperse more widely in rat brains than cationics of similar size, which bind electrostatically to cell-surface anionic moieties such as proteoglycans, limiting their spread. Anionic, receptor-targeted nanocomplexes (RTN) containing a neurotensin-targeting peptide were prepared with plasmid DNA and compared with cationic RTNs for dispersal and transfection efficiency. Both RTNs were labelled with gadolinium for localisation in the brain by MRI and in brain sections by LA-ICP-MS, as well as with rhodamine fluorophore for detection by fluorescence microscopy. MRI distribution studies confirmed that the anionic RTNs dispersed more widely than cationic RTNs, particularly in the corpus callosum. Gene expression levels from anionic formulations were similar to those of cationic RTNs. Thus, anionic RTN formulations can achieve both widespread dispersal and effective gene expression in brains after administration of a single dose by CED.

Lipid peptide nanocomplexes for gene delivery and magnetic resonance imaging in the brain

Gadolinium-labelled nanocomplexes offer prospects for the development of real-time, non-invasive imaging strategies to visualise the location of gene delivery by MRI. In this study, targeted nanoparticle formulations were prepared comprising a cationic liposome (L) containing a Gd-chelated lipid at 10, 15 and 20% by weight of total lipid, a receptor-targeted, DNA-binding peptide (P) and plasmid DNA (D), which electrostatically self-assembled into LPD nanocomplexes. The LPD formulation containing the liposome with 15% Gd-chelated lipid displayed optimal peptide-targeted, transfection efficiency. MRI conspicuity peaked at 4h after incubation of the nanocomplexes with cells, suggesting enhancement by cellular uptake and trafficking. This was supported by time course confocal microscopy analysis of transfections with fluorescently-labelled LPD nanocomplexes. Gd-LPD nanocomplexes delivered to rat brains by convection-enhanced delivery were visible by MRI at 6 h, 24 h and 48 h after administration. Histological brain sections analysed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) confirmed that the MRI signal was associated with the distribution of Gd(3+) moieties and differentiated MRI signals due to haemorrhage. The transfected brain cells near the injection site appeared to be mostly microglial. This study shows the potential of Gd-LPD nanocomplexes for simultaneous delivery of contrast agents and genes for real-time monitoring of gene therapy in the brain.

A pyrophosphate-responsive gadolinium(III) MRI contrast agent

This study shows that the relaxivity and optical properties of functionalised lanthanide-DTPA-bis-amide complexes (lanthanide=Gd(3+) and Eu(3+) , DTPA=diethylene triamine pentaacetic acid) can be successfully modulated by addition of specific anions, without direct Ln(3+) /anion coordination. Zinc(II)-dipicolylamine moieties, which are known to bind strongly to phosphates, were introduced in the amide "arms" of these ligands, and the interaction of the resulting Gd-Zn(2) complexes with a range of anions was screened by using indicator displacement assays (IDAs). Considerable selectivity for polyphosphorylated species (such as pyrophosphate and adenosine-5'-triphosphate (ATP)) over a range of other anions (including monophosphorylated anions) was apparent. In addition, we show that pyrophosphate modulates the relaxivity of the gadolinium(III) complex, this modulation being sufficiently large to be observed in imaging experiments. To establish the binding mode of the pyrophosphate and gain insight into the origin of the relaxometric modulation, a series of studies including UV/Vis and emission spectroscopy, luminescence lifetime measurements in H(2) O and D(2) O, (17) O and (31) P NMR spectroscopy and nuclear magnetic resonance dispersion (NMRD) studies were carried out.

Novel multifunctional nanoparticle mediates siRNA tumour delivery, visualisation and therapeutic tumour reduction in vivo

RNA interference (RNAi) is being widely explored as a means of tumour therapy due to the specific and potent silencing of targeted genes. However, in vivo delivery of RNAi effectors, such as small interfering RNA (siRNA) and detection of delivery is fraught with problems. Here, we describe novel theranostic PEGylated siRNA nanoparticles termed liposome-entrapped siRNA (LEsiRNA) nanoparticles. Our LEsiRNA nanoparticles are MR sensitive, contain labels for fluorescence microscopy/histology and promote functional siRNA delivery to tumours in mice leading to a significant reduction in both Survivin expression and tumour growth. LEsiRNA nanoparticles, administered by intravenous injection, were shown to accumulate in xenograft tumours by MR contrast image enhancements 24h post-administration. Fluorescence microscopy was used to corroborate the MR results and simultaneously demonstrate co-localisation of nanoparticles and siRNA within the tumours. The LEsiRNA nanoparticle-mediated delivery of the anti-cancer Survivin siRNA causes significant reduction in tumour growth when compared to controls. Our results suggest that LEsiRNA nanoparticles can be valuable as an in vivo delivery agent for siRNA therapy to tumours.