In Vivo FRET Imaging to Predict the Risk Associated with Hepatic Accumulation of Squalene-Based Prodrug Nanoparticles

In vivo fluorescence imaging of nanocarriers in near-infrared window II based on aggregation-caused quenching

Accurate fluorescence imaging of nanocarriers in vivo remains a challenge owing to interference derived mainly from biological tissues and free probes. To address both issues, the current study explored fluorophores in the near-infrared (NIR)-II window with aggregation-caused quenching (ACQ) properties to improve imaging accuracy. Candidate fluorophores with NIR-II emission, ACQ984 (λem = 984 nm) and IR-1060 (λem = 1060 nm), from the aza-BODIPY and cyanine families, respectively, were compared with the commercial fluorophore ICG with NIR-II tail emission and the NIR-I fluorophore P2 from the aza-BODIPY family. ACQ984 demonstrates high water sensitivity with complete fluorescence quenching at a water fraction greater than 50%. Physically embedding the fluorophores illuminates various nanocarriers, while free fluorophores cause negligible interference owing to the ACQ effect. Imaging based on ACQ984 revealed fine structures in the vascular system at high resolution. Moreover, good in vivo and ex vivo correlations in the monitoring of blood nanocarriers can be established, enabling real-time noninvasive in situ investigation of blood pharmacokinetics and dynamic distribution in various tissues. IR-1060 also has a good ACQ effect, but the lack of sufficient photostability and steady post-labeling fluorescence undermines its potential for nanocarrier bioimaging. P2 has an excellent ACQ effect, but its NIR-I emission only provides nondiscriminative ambiguous images. The failure of the non-ACQ probe ICG to display the biodistribution details serves as counterevidence for the improved imaging accuracy by NIR-II ACQ probes. Taken together, it is concluded that fluorescence imaging of nanocarriers based on NIR-II ACQ probes enables accurate in vivo bioimaging and real-time in situ pharmacokinetic analysis.

In vivo deposition of poorly soluble drugs

Administered drug molecules, whether dissolved or solubilized, have the potential to precipitate and accumulate as solid forms in tissues and cells within the body. This phase transition can significantly impact the pharmacokinetics of treatment. It is thus crucial to gain an understanding of how drug solubility/permeability, drug formulations and routes of administration affect in vivo behaviors of drug deposition. This review examines literature reports on the drug deposition in tissues and cells of poorly water-soluble drugs, as well as underlying physical mechanisms that lead to precipitation. Our work particularly highlights drug deposition in macrophages and the subcellular fate of precipitated drugs. We also propose a tissue permeability-based classification framework to evaluate precipitation potentials of poorly soluble drugs in major organs and tissues. The impact on pharmacokinetics is further discussed and needs to be considered in developing drug delivery systems. Finally, bioimaging techniques that are used to examine aggregated states and the intracellular trafficking of absorbed drugs are summarized.

In vitro and in vivo assessment of structural integrity for HPCD complex@Liposome nanocomposites from ocular surface to the posterior segment of the eye

Investigating the structural integrity of carriers in transit from ocular surface to ocular posterior segment is essential for an efficient topical drug delivery system. In this study, dual-carrier hydroxypropyl-β-cyclodextrin complex@Liposome (HPCD@Lip) nanocomposites were developed for the efficient delivery of dexamethasone. Förster Resonance Energy Transfer with near-infrared I fluorescent dyes and in vivo imaging system were used to investigate the structural integrity of HPCD@Lip nanocomposites after crossing Human conjunctival epithelial cells (HConEpiC) monolayer and in ocular tissues. The structural integrity of inner HPCD complexes was monitored for the first time. The results suggested that 23.1 ± 6.4 % of nanocomposites and 41.2 ± 4.3 % of HPCD complexes could cross HConEpiC monolayer with an intact structure at 1 h. 15.3 ± 8.4 % of intact nanocomposites could reach at least sclera and 22.9 ± 1.2 % of intact HPCD complexes could reach choroid-retina after 60 min in vivo, which showed that the dual-carrier drug delivery system could successfully deliver intact cyclodextrin complexes to ocular posterior segment. In conclusion, in vivo assessment of structural integrity of nanocarriers is greatly significant for guiding the rational design, higher drug delivery efficiency and clinical transformation for topical drug delivery system to the posterior segment of the eye.

FRET as the tool for in vivo nanomedicine tracking

Advanced drug delivery system utilizing a nanocarrier is the major application of nanotechnology on pharmacotherapeutics. However, despite the promising benefits and a leading trend in pharmaceutical research, nanomedicine development suffers from a poor clinical translation problem as only a handful of nanomedicine products reach the market yearly. The conventional pharmacokinetic study generally focuses only on monitoring the level of a free drug but ignores the nanocarrier's role in pharmacokinetics. One hurdle is that it is difficult to directly track intact nanocarriers in vivo to explore their pharmacokinetics. Although several imaging techniques such as radiolabeling, nuclear imaging, fluorescence imaging, etc., have been developed over the past few years, currently, one method that can successfully track the intact nanocarriers in vivo directly is by Förster resonance energy transfer (FRET). This review summarizes the application of FRET as the in vivo nanoparticle tracker for studying the in vivo pharmacokinetics of the organic nanocarriers and gives elaborative details on the techniques utilized.

FRET Ratiometric Nanoprobes for Nanoparticle Monitoring

Fluorescence labelling is often used for tracking nanoparticles, providing a convenient assay for monitoring nanoparticle drug delivery. However, it is difficult to be quantitative, as many factors affect the fluorescence intensity. Förster resonance energy transfer (FRET), taking advantage of the energy transfer from a donor fluorophore to an acceptor fluorophore, provides a distance ruler to probe NP drug delivery. This article provides a review of different FRET approaches for the ratiometric monitoring of the self-assembly and formation of nanoparticles, their in vivo fate, integrity and drug release. We anticipate that the fundamental understanding gained from these ratiometric studies will offer new insights into the design of new nanoparticles with improved and better-controlled properties.

Gemcitabine lipid prodrug nanoparticles: Switching the lipid moiety and changing the fate in the bloodstream

A simple approach to achieve a lipoprotein (LP)-mediated drug delivery is to trigger the spontaneous drug insertion into endogenous lipoproteins in the bloodstream, by means of its chemical modification. Nanoparticles (NPs) made of the squalene-gemcitabine (SQGem) conjugate were found to have a high affinity for plasma lipoproteins while free gemcitabine did not, suggesting a key role of the lipid moiety in this event. Whether the drug conjugation to cholesterol, one of the major lipoprotein-transported lipids, could also promote an analogous interaction was a matter of question. NPs made of the cholesterol-gemcitabine conjugate (CholGem) have been herein thoroughly investigated for their blood distribution profile both in vitro and in vivo. Unexpectedly, contrarily to SQGem, no trace of the CholGem prodrug could be found in the lipoprotein fractions, nor was it interacting with albumin. The investigation of isolated NPs and NPs/LPs physical mixtures provided a further insight into the lack of interaction of CholGem NPs with LPs. Although essential for allowing the self-assembly of the prodrug into nanoparticles, the lipid moiety may not be sufficient to elicit interaction of the conjugated drug with plasma lipoproteins but the whole NP physicochemical features must be carefully considered.

Recent advances in FRET-Based biosensors for biomedical applications

Fluorescence resonance energy transfer (FRET)-based biosensors are effective analytical tools extensively used in fields of biomedicine, pharmacology, toxicology, and food sciences. Ratiometric imaging of substantial cellular processes, molecular components, and biological interactions is widely performed by these biosensors. A variety of FRET-based biosensors have provided comprehensive insights into underlying mechanisms of pathological conditions in live cells, tissues, and organisms. Moreover, integration of FRET-based biosensors with the current bioanalytical techniques allows for accurate, rapid, and sensitive diagnosis and proposes the advanced strategies for treatment. Precise analysis of ligand-receptor interactions by FRET-based biosensors has presented a basis for determination of novel therapeutic agents. Therefore, this study was designed to review the recent developments in FRET-based biosensors and their biomedical applications. In addition, characteristics, challenges, and outlooks of these biosensors were discussed.

Insight into the in vivo translocation of oral liposomes by fluorescence resonance energy transfer effect

Liposomes have been broadly used in pharmaceutical field to overcome oral absorption barriers, such as gastric acid, tenacious mucus or intestinal epithelia. However, the concrete in vivo absorption mechanisms of liposomes are still indistinct. This study aims to visually elucidate the effect of particle size and surface characteristics on in vivo translocation of oral liposomes by fluorescence resonance energy transfer (FRET) effect. We fabricated liposomes of various sizes (100 nm, 200 nm and 500 nm) and surface characteristics (anionic, cationic and PEGylated) which are also labeled with FRET probes for discriminating the intact liposomes. We then investigated the in vivo fate of those different liposomes upon oral administration. Results showed that smaller conventional liposomes, cationic and PEGylated liposomes had longer retention time in digestive tract. Few intact liposomes were taken up by intestinal epithelial cells and none were found in circulation. In vivo pharmacokinetics revealed that the smaller, cationic or PEGylated liposomes had higher relative bioavailability. Similar retention time of various liposomes in blood circulation to control solution indicated that liposomes improved oral drug absorption by either prolonging contact time with gastrointestinal tract or increasing penetration ability through mucus barrier, instead of being absorbed integrally into circulation. This study offered new insight into developing highly effective liposomes for oral delivery.

Multiscale Live Imaging Using Förster Resonance Energy Transfer (FRET) for Evaluating the Biological Behavior of Nanoparticles as Drug Carriers

To develop targeted drug delivery systems using nanoparticles for treating various diseases, the evaluation of nanoparticle behavior in biological environments is necessary. In the present study, the biological behavior of polymeric nanoparticles was directly traced in living mice and cells. The dissociation of nanoparticles was detected by Förster resonance energy transfer (FRET) imaging. DiR and DiD were encapsulated in the nanoparticles for near-infrared FRET imaging, and they were traced using in vivo FRET imaging and intravital FRET imaging at the whole-body and tissue scales, respectively. In vivo FRET imaging revealed that the nanoparticles dissociated over time following intravenous administration. Intravital FRET imaging revealed that the nanoparticles dissociated in the liver and blood vessels following intravenous administration. DiI and DiO were encapsulated in nanoparticles for FRET imaging using confocal microscopy, and they were traced using in vitro FRET imaging in HepG2 cells. In vitro FRET imaging revealed that the nanoparticles dissociated and released fluorescent dyes that distributed in the cell membrane. Finally, live imaging was performed using FRET at the whole-body, tissue, and cellular scales. This method is suitable for obtaining information regarding the biological kinetic properties of nanoparticles and their use in targeted drug delivery.

Investigation of the in vivo integrity of polymeric micelles via large Stokes shift fluorophore-based FRET

Polymeric micelles hold great potential for anticancer drug delivery. Sufficient integrity of polymeric micelles after intravenous injection is critical for successful drug delivery to solid tumors, but accurate measurement of the in vivo micellar integrity remains challenging. Methods based on Förster resonance energy transfer (FRET) to monitor the in vivo micellar integrity are frequently used. However, the self-quenching effect of these FRET fluorophores used has been improperly ignored and has caused inaccurate measurements. Herein, we report a FRET-based approach using the large Stokes shift (LSS) fluorophores NBD-X and MS735 as the donor and acceptor, respectively, to investigate the integrity of polyethylene glycol-block-poly(ε-caprolactone) (PEG-PCL) micelles. We established a mathematical formula for the integrity calculation, and an in vitro verification experiment showed that the formula results exactly matched the simulated results. Our results demonstrated that PEG-PCL micelles gradually dissociated in blood circulation, but approximately 60% of the micelles in plasma remained intact 72 h after intravenous (i.v.) injection. This LSS fluorophore-based FRET approach can be used to accurately monitor the integrity of nanoparticles, and this study demonstrates that most of PEG-PCL micelles maintain their aggregation state during blood circulation for anticancer drug delivery.

In Vivo Biosensing Using Resonance Energy Transfer

Solution-phase and intracellular biosensing has substantially enhanced our understanding of molecular processes foundational to biology and pathology. Optical methods are favored because of the low cost of probes and instrumentation. While chromatographic methods are helpful, fluorescent biosensing further increases sensitivity and can be more effective in complex media. Resonance energy transfer (RET)-based sensors have been developed to use fluorescence, bioluminescence, or chemiluminescence (FRET, BRET, or CRET, respectively) as an energy donor, yielding changes in emission spectra, lifetime, or intensity in response to a molecular or environmental change. These methods hold great promise for expanding our understanding of molecular processes not just in solution and in vitro studies, but also in vivo, generating information about complex activities in a natural, organismal setting. In this review, we focus on dyes, fluorescent proteins, and nanoparticles used as energy transfer-based optical transducers in vivo in mice; there are examples of optical sensing using FRET, BRET, and in this mammalian model system. After a description of the energy transfer mechanisms and their contribution to in vivo imaging, we give a short perspective of RET-based in vivo sensors and the importance of imaging in the infrared for reduced tissue autofluorescence and improved sensitivity.

Application of Förster Resonance Energy Transfer (FRET) technique to elucidate intracellular and In Vivo biofate of nanomedicines

Extensive studies on nanomedicines have been conducted for drug delivery and disease diagnosis (especially for cancer therapy). However, the intracellular and in vivo biofate of nanomedicines, which is significantly associated with their clinical therapeutic effect, is poorly understood at present. This is because of the technical challenges to quantify the disassembly and behaviour of nanomedicines. As a fluorescence- and distance-based approach, the Förster Resonance Energy Transfer (FRET) technique is very successful to study the interaction of nanomedicines with biological systems. In this review, principles on how to select a FRET pair and construct FRET-based nanomedicines have been described first, followed by their application to study structural integrity, biodistribution, disassembly kinetics, and elimination of nanomedicines at intracellular and in vivo levels, especially with drug nanocarriers including polymeric micelles, polymeric nanoparticles, and lipid-based nanoparticles. FRET is a powerful tool to reveal changes and interaction of nanoparticles after delivery, which will be very useful to guide future developments of nanomedicine.

Hybrid drug nanocrystals

Nanocrystals show promise to deliver poorly water-soluble drugs to yield systemic exposure. However, our knowledge regarding the in vivo fate of nanocrystals is in its infancy, as nanocrystallization is simply viewed as an approach to enhance the dissolution of drug crystals. The dying crystal phenomenon inspired the development of hybrid nanocrystals by physically embedding fluorophores into the crystal lattice. This approach achieved concurrent therapy and bioimaging and is well-established to study pharmacokinetics and nanocrystal dissolution in vivo. Nanocrystals also offer the advantage of long-term durability in the body for interacting with biological tissues and cells. This review introduces the hybrid nanocrystal technique, including the theoretical concepts, preparation, and applications. We also discuss the latest development in self-discriminative hybrid nanocrystals utilizing environment-responsive probes. This review will stimulate further development and application of nanocrystal-based drug delivery systems for theranostic strategies.

From poly(alkyl cyanoacrylate) to squalene as core material for the design of nanomedicines

This review article covers the most important steps of the pioneering work of Patrick Couvreur and tries to shed light on his outstanding career that has been a source of inspiration for many decades. His discovery of biodegradable poly(alkyl cyanoacrylate) (PACA) nanoparticles (NPs) has opened large perspectives in nanomedicine. Indeed, NPs made from various types of alkyl cyanoacrylate monomers have been used in different applications, such as the treatment of intracellular infections or the treatment of multidrug resistant hepatocarcinoma. This latest application led to the Phase III clinical trial of Livatag^®, a PACA nanoparticulate formulation of doxorubicin. Despite the success of PACA NPs, the development of a novel type of NP with higher drug loadings and lower burst release was tackled by the discovery of squalene-based nanomedicines where the drug is covalently linked to the lipid derivative and the resulting conjugate is self-assembled into NPs. This pioneering work was accompanied by a wide range of novel applications which mainly dealt with the management of unmet medical needs (e.g. pancreatic cancer, brain ischaemia and spinal cord injury).

A Scalable Method for Squalenoylation and Assembly of Multifunctional 64Cu-Labeled Squalenoylated Gemcitabine Nanoparticles

Squalenoylation of gemcitabine, a front-line therapy for pancreatic cancer, allows for improved cellular-level and system-wide drug delivery. The established methods to conjugate squalene to gemcitabine and to form nanoparticles (NPs) with the squalenoylated gemcitabine (SqGem) conjugate are cumbersome, time-consuming and can be difficult to reliably replicate. Further, the creation of multi-functional SqGem-based NP theranostics would facilitate characterization of in vivo pharmacokinetics and efficacy.

Methods: Squalenoylation conjugation chemistry was enhanced to improve reliability and scalability using tert-butyldimethylsilyl (TBDMS) protecting groups. We then optimized a scalable microfluidic mixing platform to produce SqGem-based NPs and evaluated the stability and morphology of select NP formulations using dynamic light scattering (DLS) and transmission electron microscopy (TEM). Cytotoxicity was evaluated in both PANC-1 and KPC (Kras^LSL-G12D/+; Trp53^LSL-R172H/+; Pdx-Cre) pancreatic cancer cell lines. A ⁶⁴Cu chelator (2-S-(4-aminobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid, NOTA) was squalenoylated and used with positron emission tomography (PET) imaging to monitor the in vivo fate of SqGem-based NPs.

Results: Squalenoylation yields of gemcitabine increased from 15% to 63%. Cholesterol-PEG-2k inclusion was required to form SqGem-based NPs using our technique, and additional cholesterol inclusion increased particle stability at room temperature; after 1 week the PDI of SqGem NPs with cholesterol was ~ 0.2 while the PDI of SqGem NPs lacking cholesterol was ~ 0.5. Similar or superior cytotoxicity was achieved for SqGem-based NPs compared to gemcitabine or Abraxane® when evaluated at a concentration of 10 µM. Squalenoylation of NOTA enabled in vivo monitoring of SqGem-based NP pharmacokinetics and biodistribution.

Conclusion: We present a scalable technique for fabricating efficacious squalenoylated-gemcitabine nanoparticles and confirm their pharmacokinetic profile using a novel multifunctional ⁶⁴Cu-SqNOTA-SqGem NP.

Highly Sensitive MoS2-Indocyanine Green Hybrid for Photoacoustic Imaging of Orthotopic Brain Glioma at Deep Site

Photoacoustic technology in combination with molecular imaging is a highly effective method for accurately diagnosing brain glioma. For glioma detection at a deeper site, contrast agents with higher photoacoustic imaging sensitivity are needed. Herein, we report a MoS₂-ICG hybrid with indocyanine green (ICG) conjugated to the surface of MoS₂ nanosheets. The hybrid significantly enhanced photoacoustic imaging sensitivity compared to MoS₂ nanosheets. This conjugation results in remarkably high optical absorbance across a broad near-infrared spectrum, redshifting of the ICG absorption peak and photothermal/photoacoustic conversion efficiency enhancement of ICG. A tumor mass of 3.5 mm beneath the mouse scalp was clearly visualized by using MoS₂-ICG as a contrast agent for the in vivo photoacoustic imaging of orthotopic glioma, which is nearly twofold deeper than the tumors imaged in our previous report using MoS₂ nanosheet. Thus, combined with its good stability and high biocompatibility, the MoS₂-ICG hybrid developed in this study has a great potential for high-efficiency tumor molecular imaging in translational medicine.

Energy transfer-based biodetection using optical nanomaterials

This review focuses on recent progress in the development of FRET probes and the applications of FRET-based sensing systems.