Liposome biodistribution by time resolved fluorimetry of lipophilic europium complexes

Liposome Biodistribution via Europium Complexes

Vector biodistribution is a requirement prior pharmaceutical development. Radioactive tracers allow the most sensitive and quantitative assessment of biodistribution, and conventional fluorophores are widely used in academic laboratories. We propose here to use europium complexes as a label for nanoparticles or biotherapeutics taking liposomes as models. Time-resolved fluorimetry (TRF) has the tremendous advantage of taking into accounts the fluorescence decay time of the lanthanide chelates, resulting in an improved sensitivity in biological media. The work described aimed following liposome biodistribution by TRF. An octadecyl-DTPA.Eu compound has been prepared and incorporated into liposomes without altering its fluorescence signal. The method has been validated through a comparison with fluorophore-labeled liposomes. The way to proceed when using this method for liposome biodistribution assessment is detailed. It could obviously be applied to other nanosystems, such as lipid nanoparticles.

Liposome Biodistribution via Europium Complexes

The drug delivery field needs tools to follow vector biodistribution. Radioactive tracers and conventional fluorophores are widely used. We propose here to use europium complexes. Use of pulsed light source time-resolved fluorimetry takes into account the fluorescence decay time of the lanthanide chelates to gain sensitivity in biological media. The method was developed to follow liposome biodistribution. Octadecyl-DTPA.Eu compound has been prepared and incorporated into liposomes without alteration of its fluorescence signal. The method has been validated by comparison with fluorophore-labeled liposomes. The way to proceed to use this method for liposomes or other vectors is detailed.

A Simple and Sensitive Method to Quantify Biodegradable Nanoparticle Biodistribution using Europium Chelates

The biodistribution of biodegradable nanoparticles can be difficult to quantify. We report a method using time resolved fluorescence (TRF) from a lanthanide chelate to minimize background autofluorescence and maximize the signal to noise ratio to detect biodegradable nanoparticle distribution in mice. Specifically, antenna chelates containing europium were entrapped within nanoparticles composed of polylactic acid-polyethylene glycol diblock copolymers. Tissue accumulation of nanoparticles following intravenous injection was quantified in mice. The TRF of the nanoparticles was found to diminish as a second order function in the presence of serum and tissue compositions interfered with the europium signal. Both phenomena were corrected by linearization of the signal function and calculation of tissue-specific interference, respectively. Overall, the method is simple and robust with a detection limit five times greater than standard fluorescent probes.

Thermosensitive, near-infrared-labeled nanoparticles for topotecan delivery to tumors

Liposomal nanoparticles have proven to be versatile systems for drug delivery. However, the progress in clinic has been slower and less efficient than expected. This suggests a need for further development using carefully designed chemical components to improve usefulness under clinical conditions and maximize therapeutic effect. For cancer chemotherapy, PEGylated liposomes were the first nanomedicine to reach the market and have been used clinically for several years. Approaches toward targeted drug delivery using next generation "thermally triggered" nanoparticles are now in clinical trials. However, clinically tested thermosensitive liposomes (TSLs) lack the markers that allow tumor labeling and improved imaging for tissue specific applied hyperthermia. Here we describe the development of optically labeled TSLs for image guidance drug delivery and proof-of-concept results for their application in the treatment of murine xenograft tumors using the anticancer drug topotecan. These labeled TSLs also allow the simultaneous, real-time diagnostic imaging of nanoparticle biodistribution using a near-infrared (NIR; 750-950 nm) fluorophore coupled to a lipidic component of the lipid bilayer. When combined with multispectral fluorescence analysis, this allows for specific and high sensitivity tracking of the nanoparticles in vivo. The application of NIR fluorescence-labeled TSLs could have a transformative effect on future cancer chemotherapy.

Preparation and Evaluation of Multiple Nanoemulsions Containing Gadolinium (III) Chelate as a Potential Magnetic Resonance Imaging (MRI) Contrast Agent

PURPOSE

The objective was to develop, characterize and assess the potentiality of W1/O/W2 self-emulsifying multiple nanoemulsions to enhance signal/noise ratio for Magnetic Resonance Imaging (MRI).

METHODS

For this purpose, a new formulation, was designed for encapsulation efficiency and stability. Various methods were used to characterize encapsulation efficiency ,in particular calorimetric methods (Differential Scanning Calorimetry (DSC), thermogravimetry analysis) and ultrafiltration. MRI in vitro relaxivities were assessed on loaded DTPA-Gd multiple nanoemulsions.

RESULTS

Characterization of the formulation, in particular of encapsulation efficiency was a challenge due to interactions found with ultrafiltration method. Thanks to the specifically developed DSC protocol, we were able to confirm the formation of multiple nanoemulsions, differentiate loaded from unloaded nanoemulsions and measure the encapsulation efficiency which was found to be quite high with a 68% of drug loaded. Relaxivity studies showed that the self-emulsifying W/O/W nanoemulsions were positive contrast agents, exhibiting higher relaxivities than those of the DTPA-Gd solution taken as a reference.

CONCLUSION

New self-emulsifying multiple nanoemulsions that were able to load satisfactory amounts of contrasting agent were successfully developed as potential MRI contrasting agents. A specific DSC protocol was needed to be developed to characterize these complex systems as it would be useful to develop these self-formation formulations.

Cholesterol-diethylenetriaminepentaacetate complexed with thulium ions integrated into bicelles to increase their magnetic alignability

Lanthanides have been used for several decades to increase the magnetic alignability of bicelles. DMPE-DTPA (1,2-dimyristoyl-sn-glycero-3-phospho-ethanolamine-diethylenetriaminepentaacetate) is commonly applied to anchor the lanthanides into the bicelles. However, because DMPE-DTPA has the tendency to accumulate at the highly curved edge region of the bicelles and if located there does not contribute to the magnetic orientation energy, we have tested cholesterol-DTPA complexed with thulium ions (Tm(3+)) as an alternative chelator to increase the magnetic alignability. Differential scanning calorimetric (DSC) measurements indicate the successful integration of cholesterol-DTPA into a DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) bilayer. Cryo transmission electron microscopy and small-angle neutron scattering (SANS) measurements show that the disklike structure, that is, bicelles, is maintained if cholesterol-DTPA·Tm(3+) is integrated into a mixture of DMPC, cholesterol, and DMPE-DTPA·Tm(3+). The size of the bicelles is increased compared to the size of the bicelles obtained from mixtures without cholesterol-DTPA·Tm(3+). Magnetic-field-induced birefringence and SANS measurements in a magnetic field show that with addition of cholesterol-DTPA·Tm(3+) the magnetic alignability of these bicelles is significantly increased compared to bicelles composed of DMPC, cholesterol, and DMPE-DTPA·Tm(3+) only.

Heterobivalent ligands target cell-surface receptor combinations in vivo

A challenge in tumor targeting is to deliver payloads to cancers while sparing normal tissues. A limited number of antibodies appear to meet this challenge as therapeutics themselves or as drug-antibody conjugates. However, antibodies suffer from their large size, which can lead to unfavorable pharmacokinetics for some therapeutic payloads, and that they are targeted against only a single epitope, which can reduce their selectivity and specificity. Here, we propose an alternative targeting approach based on patterns of cell surface proteins to rationally develop small, synthetic heteromultivalent ligands (htMVLs) that target multiple receptors simultaneously. To gain insight into the multivalent ligand strategy in vivo, we have generated synthetic htMVLs that contain melanocortin (MSH) and cholecystokinin (CCK) pharmacophores that are connected via a fluorescent labeled, rationally designed synthetic linker. These ligands were tested in an experimental animal model containing tumors that expressed only one (control) or both (target) MSH and CCK receptors. After systemic injection of the htMVL in tumor-bearing mice, label was highly retained in tumors that expressed both, compared with one, target receptors. Selectivity was quantified by using ex vivo measurement of Europium-labeled htMVL, which had up to 12-fold higher specificity for dual compared with single receptor expressing cells. This proof-of-principle study provides in vivo evidence that small, rationally designed bivalent htMVLs can be used to selectively target cells that express both, compared with single complimentary cell surface targets. These data open the possibility that specific combinations of targets on tumors can be identified and selectively targeted using htMVLs.

Liposome biodistribution via europium complexes

The drug-delivery field needs tools to follow vector biodistribution. Radioactive tracers and conventional fluorophores are widely used. We propose here to use europium complexes. Use of pulsed light source time-resolved fluorimetry takes into account the fluorescence decay time of the lanthanide chelates to gain sensitivity in biological media. The method was developed to follow liposome biodistribution. Octadecyl-DTPA.Eu compound has been prepared and incorporated into liposomes without alteration of its fluorescence signal. The method has been validated by comparison with fluorophore-labelled liposomes. The way to proceed to use this method for liposomes or other vectors is detailed.

Noncovalent functionalization of carbon nanotubes with amphiphilic gd3+ chelates: toward powerful t1 and t2 MRI contrast agents

An amphiphilic gadolinium (III) chelate (GdL) was synthesized from commercially available stearic acid. Aqueous solutions of the complex at different concentrations (from 1 mM to 1 microM) were prepared and adsorbed on multiwalled carbon nanotubes. The resulting suspensions were stable for several days and have been characterized with regard to magnetic resonance imaging (MRI) contrast agent applications. Longitudinal water proton relaxivities, r1, have been measured at 20, 300, and 500 MHz. The r1 values show a strong dependence on the GdL concentration, particularly at low field. The relaxivities decrease with increasing field as it is predicted by the Solomon-Bloembergen-Morgan theory. Transverse water proton relaxation times, T2, have also been measured and are practically independent of both the frequency and the GdL concentration. An in vivo feasibility MRI study has been performed at 300 MHz in mice. A negative contrast could be well observed after injection of a suspension of functionalized nanotubes into the muscle of the leg of the mouse.