Radio-nanomaterials for biomedical applications: state of the art

Abstract

The incorporation of radioactive isotope(s) into conventional nanomaterials can bring extra properties which are not possessed by original materials. The resulting radioactive nanomaterials (radio-nanomaterials), with added physical/chemical properties, can be used as important tools for different biomedical applications. In this review, our goal is to provide an up-to-date overview on these applications using radio-nanomaterials. The first section illustrates the utilization of radionanomaterials for understanding of in vivo kinetics of their parent nano-materials. In the second section, we focus on two primary applications of radio-nanomaterials: imaging and therapeutic delivery. With various methods being used to form radio-nanomaterials, they can be used for positron emission tomography (PET), single-photon emission computed tomography (SPECT), and multimodal imaging. Therapeutic isotopes-loading radio-nanomaterials can possess selective killing efficacy of diseased cells (e.g. tumor cells) and can provide promises for certain isotopes which are not able to be used in a conventional manner. The successful and versatile biomedical applications of radio-nanomaterials warrants further investigations of those materials and their optimizations can pave the way to future imaging guidable, personalized treatments in patients.

Graphical abstract

Review: With proper functionalization and cargo loading, radioactive nano-materials can prove to be extremely useful tools for material pharmacokinetic determination, imaging (diagnosis) of diseases, and therapeutic delivery into the diseased sites.

Introduction

In the past several decades, numerous nanoscale materials (size range: several to a few 100 nm) have been developed for a wide range of biomedical applications including but not limited to drug delivery, sensing, disease detection, and tissue engineering (1). With unique physical/chemical properties, these materials provided unparalleled opportunities to improve diagnostic accuracy and add treatment alternatives available to patients, especially those with different types of cancer (2–4). Several drug nanoformulations have been approved clinically (e.g. Doxil) which possess optimized pharmacokinetic profiles when compared with the original drug: reduced side effects, improved therapeutic index, etc. (5). Despite the proven success, great efforts have been continuously invested in the development of novel nanomaterials with more potent disease targeting or therapeutic capability, optimized in vivo kinetics, and enhanced cargo-loading efficiency.

Compared with conventional nanomaterials, radioactive nanomaterials (radio-nanomaterials, Scheme 1) have more appealing characteristics due to the synergistic integration of unique physical/chemical properties from radionuclides into nanomaterials (6). On the one hand, the incorporation of radioisotope(s) bestows extra tracking/therapeutic ability to the nanomaterial where radioisotope(s) on nanomaterial acts as an energy donor. Most radio-nanomaterials are useful contrast agents for positron emission tomography (PET) or single-photon emission computed tomography (SPECT) imaging. On the other hand, suitable nanomaterials may serve as carriers to accommodate certain “unconventional” isotopes so that they can be used in biomedical applications otherwise very difficult to achieve. A few examples include radioactive arsenic (e.g. ⁷²As) (7, 8), gemanium-69 (⁶⁹Ge) (9), or sodium-22 (²²Na) (10). With careful design and optimization, radio-nanomaterials can be utilized as effective theranostic agents.

Scheme 1.

Biomedical applications of radio-nanomaterials.

Three elements have to be taken into consideration before the production of a radio-nanomaterial, which include the choice of a suitable isotope (or isotopes), the selection of a nanomaterial with desired properties, and a method to integrate the isotope effectively into the nanomaterial (11). For a given biomedical application, the isotope can be used as either an imaging label (e.g. for PET or SPECT), or a therapeutic moiety, or both (6). The integrity of radio-nanomaterial is extremely important for their biomedical applications, and herein this integrity refers to two aspects: the isotope should not be released prematurely from the nanomaterial, and the nanomaterial itself should possess sufficient stability. In addition, radio-nanomaterials should not pose significant acute or chronic toxicity to the test subject. The intrinsic properties from nanomaterial can sometimes boost the performance of the loaded isotope in vivo, e.g. stimuli-responsive release of the isotope, or energy transfer for imaging applications (12).

An optimal method for isotope incorporation should be fast, efficient, and should impose minimal changes to the characteristics of the chosen nanomaterial. For detailed information on how an isotope can be incorporated into the nanomaterial (the radiolabeling/radiochemistry methods), or distinct nuclear reactions to obtain appropriate isotopes for these applications, interested readers can resort to references (6, 11). In brief, four strategies are currently used for radio-nanomaterial production: 1) synthesis of a radio-nanomaterial with an isotope precursor; 2) post-synthesis chemical linking (e.g. using a chelating agent for radiometals) or physical absorption; 3) post-synthesis isotope exchange; 4) material activation/bombardment by neutron/proton beams. The choice of strategies may affect the amount of the isotope loadable on the material, and impact the stability of resulting nanomaterials.

In this review, we will try to summarize the recent progress in radio-nanomaterials for biomedical applications. The utilization of radio-nanomaterials for clarifying in vivo material distribution/pharmacokinetics will be introduced in the first place. In the following sections, the biomedical applications of radio-nanomaterials will be categorized into two primary aspects: imaging and therapeutic applications. In the last section, future perspectives of new biomedical applications of radio-nanomaterials are given. Although the research on radio-nanomaterials literally started <10 years ago, a large number of radio-nanomaterials have been produced.

In vivo kinetics of radio-nanomaterials

Fabrication of radio-nanomaterials can help to noninvasively monitor the pharmacokinetics of parent materials inside a living subject and collect relevant knowledge on their organ absorption, distribution, metabolism (clearance), stability, and drug release kinetics, etc. This information can be critical to judge whether these nanomaterials are suitable for a given application, e.g. therapeutic delivery to a given diseased site.

Metabolic study of nanomaterials

The first example to illustrate the usefulness of radio-nanomaterial for metabolic study is the rodent safety evaluation of zerovalent iron nanoparticles (nZVI) (13), which are used for ground water decontamination. Whole-body retention and metabolic activity of neutron-activated radioactive ⁵⁹Fe-nZVI was assessed in mice post oral gavage. The findings indicated that the majority of ⁵⁹Fe-nZVI was rapidly excreted via the feces in 24 h with a small amount of radioactivity retained in the liver after three repeated daily doses. This information is extremely helpful to adjust the nZVI dose during the water processing for human consumption. Proton activated, nitrogen-13 (¹³N) labeled aluminum oxide nanoparticles [¹³N-Al₂O₃ NPs, via the ¹⁶O(p, α) ¹³N reaction] were used within another study to determine the material distribution pattern in rats, monitored by PET (14). The appearance of ¹³N into Al₂O₃ NPs did not alter their surface or structural properties. Al₂O₃ NPs demonstrated a biexponential decay in vivo and the relationship between particle size and organ distribution pattern could be determined after intravenous administration (Figure 1A).

Figure 1.

The utilization of radio-nanomaterials to determine kinetics from parent materials. (A) Formation of ¹³N-labeled Al₂O₃ nanoparticles. The resulting radio-nanomaterials can help to determine the size-organ distribution relationship. Adapted from reference (14). (B) Use of ¹⁹⁸Au-incorporated gold nanomaterials to study the relationship of shape-tumor uptake by Cerenkov imaging. Four nanostructures were studied: nanospheres, nanodisks, nanorods, and cubic nanocages. Adapted from reference (15).

In certain situation, different radiolabels can be tethered on the same nanomaterial to provide complementary and more accurate information on its distribution profile. For instance, oleic acid functionalized iron-oxide nanoparticles (IONPs) were separately radiolabeled with ⁵⁹Fe, ¹⁴C-oleic acid, and ¹¹¹In for studying their organ distribution in mice (16). Although all the radioactive signals preferentially localized in liver, spleen and bone, stronger signal levels from ⁵⁹Fe were observed in liver and spleen than those from ¹¹¹In, but lower than those from ¹⁴C. This technique can be of great value to clarify the kinetics of different components from the same nanomaterial.

Radio-nanomaterials were also useful for evaluation of cargo integrity or material stability. For instance, dasatinib, an inhibitor for platelet-derived growth factor receptor, was loaded into a micellar liposome nanoformulation for enhanced tumor retention. Instead of providing radiolabels on micelles, ¹⁸F was introduced into the molecular structure of dasatinib to produce ¹⁸F-SKI249380. The stability and distribution of dasatinib micelles was estimated in a genetically engineered mouse model of high-grade glioma (17). Better tumor accumulation and retention was observed for ¹⁸F-SKI249380-containing micelle formulations compared to free ¹⁸F-SKI249380, and ¹⁸F-SKI249380-containing micelle formulations possessed good stability judging from their kinetic behaviors. As intra-tumor drug concentrations can be calculated from this study, this method can facilitate treatment planning with dasatinib. In another study, the stability of titanium implants was evaluated in rats by coating with hydroxyapatite (HA) nanoparticles and labeled with calcium-45 (⁴⁵Ca) (18). At different time points post-implantation, the implants and surrounding bone were retrieved and analyzed by autoradiography to determine particle migration from the implant surface. Major tissues and metabolic excretions were also retrieved and analyzed by liquid scintillation counting. The radioactivity of ⁴⁵Ca decreased over time from the vicinity of the implant into the blood and eventually appeared in the animal excretions. After 8 weeks, only trace amount of ⁴⁵Ca could be found in the liver. The results confirmed the safety of the nano-coating instead of being considered a potential biologic risk factor.

With proper radiolabels, in vivo tracking of nanomaterials can prepare them for new applications. For example, a rare-earth cation exchange method was used to introduce samarium-153 (¹⁵³Sm) into lanthanide based upconversion nanoparticles (UCNPs) and enabled the tracking of their in vivo behaviors (19). By SPECT imaging, it was demonstrated that UCNPs were mainly captured by the mononuclear phagocyte system (MPS, primarily liver and spleen), and the accumulation of UCNP in liver was faster and was about 15-fold of that in spleen. This information about distribution/excretion of UCNPs in vivo can be extremely helpful before they can be used as a contrast agent for detection of a specific disease. Similar technique was used by the same group to monitor in vivo behavior of UCNPs with different chemical compositions and different synthesis method (20). In another study, ^99mTc-labeled mannosylated Gantrez nanoparticles were studied in mice for their organ distribution (post-loading with the Brucella ovis antigen) after ocular administration (21). The accumulation of these nanoparticles in nasal and ocular mucosa and gastrointestinal tract was observed, confirming its value as an effective antigenic delivery system through the ocular mucosa. These nanoparticles can trigger elevated immune response and provide protection against Brucella to serve as a vaccine candidate. A more recent case is to evaluate distribution and clearance profile of indium-111 (¹¹¹In) labeled albumin-based nanomaterials and prepare them for pulmonary drug delivery (22). ¹¹¹In labeled albumin nanoparticles [via coordination with diethylenetriaminepentaacetic acid (DTPA)] demonstrated a slow clearance from the mouse lung with significant retention the lung tissue rather than the lung fluid. Low amounts of ¹¹¹In activity were also detectable in the liver, kidneys, and intestine post 24 h, indicating clearance by translocation across the lung mucosal barrier. This study provides important information on the fate of albumin vehicles in the lungs, which may direct future design of inhaled nanomedicines.

Factors to affect nanomaterial distribution

The distribution profile of a given nanomaterial can be affected by different factors (e.g. administration routes, size, morphology, etc.), and radio-nanomaterials can help to determine the importance of each factor. For example, both fluorine-18 (¹⁸F) (23) and zinc-65 (⁶⁵Zn) (24) were used to study in vivo distribution of zinc oxide (ZnO) nanoparticles via different administration routes. In those studies, ¹⁸F-labeled ZnO nanoparticles (via click chemistry) showed observable radioactivity in the lung, liver, kidney, and the GI tract after oral administration (23). Increased size of ZnO nanoparticles (e.g. from 20 nm to 100 nm) resulted in higher accumulation in the liver and kidney. Compared with ¹⁸F-labeled ZnO nanoparticles, intravenous injection of ⁶⁵ZnO nanoparticles resulted in preferential accumulation in the liver and spleen at 24 h while radioactivity of ⁶⁵Zn was also detectable in bone, brain, lung, heart and kidneys after 4 weeks (80% excretion was observed at that time) (24). Although the size of the ZnO nanoparticles did not play a significant role in determination of in vivo kinetics, smallersized ZnO nanoparticles had longer retention time in some organs.

The impact of administration routes on material distribution profile can also be clarified, capitalizing on the localization of radio-nanomaterials. In an early study, the pharmacokinetic behaviors of ¹¹¹In-labeled, PEGylated liposomes were evaluated in a peritoneal tumor model in mice (25). In this particular tumor model, intraperitoneal (i.p.) injection could result in more effective retention of liposomes compared with intravenous (i.v.) injection, thus being considered as better treatment approach. More recently, copper-64 (⁶⁴Cu) was used as a radiolabel for gold nanoshells [via 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelation] in order to provide a systematic investigation on their distribution pattern in tumor-bearing rats via different injection routes (26), as some studies indicated that passive delivery of gold nanoshells through intravenous administration resulted in limited tumor accumulation. From the acquired pharmacokinetic curves, intratumor administration showed higher retention of ⁶⁴Cu-nanoshells in tumors and less concentration in other healthy organs. ⁶⁴Cu-labeled hollow gold nanospheres (HAuNS) were evaluated in a rabbit liver tumor model to compare tumor uptakes from different injection routes (27). Animals with hepatic intra-artery injection of arginine-glycine-aspartate (RGD) peptide-conjugated HAuNS showed significantly higher tumor uptake and higher tumor-to-normal liver ratios in all other groups.

The relationship between surface charges/material morphology and material pharmacokinetic behavior can also be studied with radio-nanomaterials. In one study, the tissue distribution of nanostructured lipid carriers (NLCs, radiolabeled with ^99mTc) with different surface charges and sizes was evaluated after their intravenous administration to rats (28). No differences on the mean residence time in blood among the NLCs were observed in spite of these differences. Higher accumulation in the kidney was observed for those NLCs with positive charges, while NLCs with negative charge accumulated preferably in the liver. NLCs with larger particle size showed a higher uptake in the lung and lower accumulation in liver and bone marrow, in comparison with the smaller ones. Another interesting study explored the impact of material shapes on their distribution pattern in vivo (15). Different Au nanostructures (i.e. nanospheres, nanodisks, nanorods, and cubic nanocages) with a similar size were synthesized with the incorporation of ¹⁹⁸Au into the crystal lattice for organ distribution evaluation in a murine EMT6 breast cancer model. Significantly higher tumor uptake was observed for the Au nanospheres and nanodisks relative to the Au nanorods and nanocages within the observing time frames (monitored by Cerenkov luminescence, Figure 1B). Furthermore, intratumoral distributions from autoradiography confirmed that nanospheres and nanodisks were only observed on the surfaces of the tumors, while nanorods and nanocages had a more even distribution throughout the tumors. Proper determination of nanomaterial pharmacokinetics from the usage of radio-nanomaterials will provide invaluable insights to guide the design of more effective imaging/therapy vectors discussed in the below sections.

Imaging

Many radio-nanomaterials are natural imaging contrast agents for PET and SPECT imaging due to the existence of radioisotopes. Image conspicuity can be enhanced from their utilization for lesion detection. Nanomaterials can be of great valuable for diagnostic applications since they can serve as selective carriers of a radioisotope to a particular region of interest (ROI) with good accuracy. In addition, the benefits of nanomaterials as imaging agents include large surface area to volume, versatile surface chemistry to integrate multiple targeting ligands, and excellent cargo-carrying capacity for multimodal imaging.

PET or SPECT

Nuclear imaging modalities (PET and SPECT) provide highly sensitive detection of biological events in vivo, particularly useful for disease diagnosis, staging, and treatment response evaluation (29). Conventional isotopes for PET or SPECT imaging can be seamlessly integrated into nanomaterials and create new applications for them, for example, zirconium-89 (⁸⁹Zr) (30), ⁶⁴Cu (31), or ¹¹¹In (32). Most of these radio-nanomaterials were confirmed to be sufficiently stable in vivo with acceptable biocompatibility.

Cancer imaging

Radio-nanomaterials are particularly suitable for imaging of cancer as most nanomaterials with diameters in the range of 5–200 nm have good enhanced permeability and retention (EPR) effect to tumor sites, which means that they can have a significant accumulation in tumor even without a targeting ligand (33). Different radiolabeled, inorganic [e.g. silica- or carbon-based, gold nanomaterials, quantum dots (QDs), etc.] or organic (polymer-based, liposomal nanomaterials, etc.) nanomaterials have been used for detection of primary/metastatic tumors and imaging guided therapeutic delivery (34, 35). The majority of nanomaterials have some unique physical properties which can be applicable for different imaging techniques (e.g. gold nanomaterials can be used for fluorescence or photo-acoustic imaging, QDs can be used for fluorescence imaging, or IONPs can be used for MRI). In addition, some nanomaterials can have inherent physicochemical properties to enable them to accumulate in certain types of cancer. For example, ^99mTc-labeled HA nanoparticles can be used for imaging of bone tumors (36). However, to enhance the capture of nanomaterials by cancerous tissues, most radio-nanomaterials need to be attached to “targeting” ligands (which usually possess strong affinity against an up-regulated biomarker during cancer progression) (37).

Good examples include graphene-based nanosheets labeled with ⁶⁶Ga (38), ⁶⁴Cu (39, 40), or ¹¹¹In (41) for PET (SPECT) imaging of breast cancer. With the attachment of different targeting ligands (e.g. antibodies, proteins, etc.), the tumor accumulation of those nano-graphene was sufficient to prepare them for future targeted photothermal therapy of tumors. Silicabased nanomaterials are another attractive contrast agents for PET (SPECT) imaging of cancer. For example, mesoporous silica nanoparticle (MSN) or hollow MSN (hMSN, Figure 2A) was labeled with ⁶⁴Cu and an antibody against an angiogenesis target CD105 for PET-guided, enhanced drug delivery to 4T1 murine breast cancer model (42, 44). A good overview on imaging applications with mesoporous silica nanomaterials can be found elsewhere (45).

Figure 2.

The utilization of radio-nanomaterials for detection of tumors or tumor-associated cell sub-population. (A) ⁶⁴Cu-labeled, antibody-conjugated hMSN for targeting of 4T1 tumor in vivo. Both PET imaging and NIRF imaging confirmed the enhanced accumulation hMSN post antibody conjugation. The improved drug delivery efficiency was also confirmed after loading of doxorubicin onto hMSN via fluorescence imaging in tumor. T, Targeted group; NT, non-targeted group. Adapted from from reference (42). (B) ⁸⁹Zr-labeled HDL nanoparticles for imaging of TAMs. Two types of ⁸⁹Zr-labeling strategies (on apoA-1 [AL] or on phospholipid [PL]) were adopted. The resulting HDL nanoparticles demonstrated strong accumulation in 4T1 tumors. Both flow cytometry and histology examination showed the preferred uptake of these ⁸⁹Zr-labeled HDL nanoparticles in TAMs. Adapted from reference (43).

Gold nanomaterials are a standout category of inorganic nanomaterials which are frequently used for cancer imaging. For example, water-soluble, maleimideterminated PEGylated gold nanoparticles (AuNPs) were radiolabeled by the [¹⁸F]-silicon-fluorine prosthetic group and attached by a cysteine-modified octreotate (binds to somatostatin receptor [SSTR] subtypes 2 and 5) for delineation of brain tumors (46). Aptamer- and antibody (C225)-coated HAuNS (both targeted to epidermal growth factor receptors [EGFR]) were evaluated in another report for targeting of human OSC-19 oral tumors (EGFR⁺) post labeling with ¹¹¹In (47). Both antibody-conjugated and aptamer-conjugated HAuNS demonstrated good tumor uptake, and SPECT/CT confirmed that there was even higher tumor uptake from aptamer-conjugated HAuNS. Using a chelator-free radiolabeling method, ⁶⁴Cu was chemically reduced onto the surface of PEG-stabilized Au nanorods (48). These ⁶⁴Cu-incorporated, RGD-conjugated Au nanorods showed potent tumor targeting ability in a U87MG glioblastoma xenograft model and were successfully used for image-guided photothermal tumor therapy.

Radioactive organic nanomaterials are also beneficial contrast agents for cancer detection. Among them, nanosized liposomes were frequently adopted for tumor imaging (49, 50). For example, PEG-coated liposome was modified with a monoclonal antibody 2C5 (against nucleosome overexpressed in tumors) for SPECT imaging of tumor (^99mTc was encapsulated as a radiolabel) (51). ^99mTc-2C5-PEG-liposomes demonstrated potent uptake in different tumor types, and the tumor uptake was 3–8 fold higher than non-targeted liposomes. Another study adopted a dual targeting strategy for ¹¹¹In-labeled liposomal nanoparticles via integrin (by RGD) and neurokinin-1 (by substance P peptide) overexpressed in glioma and melanoma (52). However, SPECT studies in tumor- bearing mice unexpectedly revealed only moderate tumor uptake and no observable synergistic effect using this dual-targeting approach. SSTR-targeted polyamidoamine (PAMAM) dendrimers were used recently for targeting of neuroendocrine tumors (53). ^99mTc-PAMAM-Tyr3-octreotide showed specific accumulation in AR42J tumors and the pancreas (both SSTR⁺), with a significant renal excretion.

Aside from traditional “tumor detection”, radio-nanomaterials are also useful for imaging of tumor associated markers/cell populations. One good example is tumorassociated macrophages (TAMs), which are considered to have high diagnostic and prognostic value for various cancer types (54). Imaging of TAM is considered as largely unexplored, before a recent study described the development of ⁸⁹Zr-labeled, reconstituted high-density lipoprotein (HDL)-nanoparticles for PET imaging of TAM in a murine model of breast cancer (43). Intravenous administration of ⁸⁹Zr-labeled HDL nanoparticles (with two formulations) resulted in potent tumor accumulation at 24 h after injection (Figure 2B). Histologic analysis showed good localization of radioactivity with TAM-rich areas in tumor sections. Thus, these ⁸⁹Zr-rHDL imaging agents could be valuable for noninvasive monitoring of TAM and assessing relevant therapeutic interventions. Other important applications for radio-nanomaterials include noninvasive lymph node (LN) mapping – especially for tumor-drained LNs. Their advantages and limitations have been thoroughly summarized in a recent review article (55).

Imaging of other diseases

Radio-nanomaterials have also been used for imaging/ diagnosis of other diseases, for example, inflammation, or cardiovascular diseases. In one study, ⁸⁹Zr-labeled dextran nanoparticle (DNP) was used for imaging of monocytes and macrophages, as biomarkers of atherosclerotic plaques progression (56). PET imaging revealed higher uptake of ⁸⁹Zr-DNP in the aortic root of apolipoprotein E knock out (ApoE^−/−) mice than that of wild-type controls (corroborated by autoradiography) while silencing of monocytes decreased ⁸⁹Zr-DNP accumulation in the plaque. ⁸⁹Zr-DNP enabled noninvasive assessment of inflammation in atherosclerotic plaques and could provide feedback on therapeutic efficacy of anti-inflammatory therapy. Poly (methyl methacrylate)-core/ polyethylene glycol-shell amphiphilic comblike nanoparticles were used in another study for detection of atherosclerotic lesions in ApoE^−/− wire-injury mice (57). These nanoparticles were labeled with ⁶⁴Cu and conjugated with D-Ala1-peptide T-amide (DAPTA) peptide (a selective ligand for chemokine receptor 5 [CCR5]), and they showed better accumulations in the injury lesions compared with those at the sham-operated sites (Figure 3A). This nanoplatform can provide sensitive and specific detection of CCR5 for clarification of its biological functions in atherosclerosis. In addition, ^99mTc-labeled, methoxypolyethylene glycol-graft-poly (L-lysine) copolymer (mPEG-gPLL) stabilized AuNPs was also produced recently for detection of induced inflammation (Figure 3B) (58).

Figure 3.

Imaging of cardiovascular disease and inflammation with radio-nanomaterials. (A) Use of ⁶⁴Cu-labeled, DAPTA-conjugated comb nanoparticles for imaging of wire-injury induced atherosclerosis. Significantly higher uptake of these nanoparticles was found in the injury area compared with sham-operated area. Histology examination confirmed the progressive atherosclerotic plaque in the injury group. Adapted from reference (57). (B) The structure and morphology of mPEG-gPLL grafted AuNPs and their application in detection of an experimental inflammation via SPECT/CT post ^99mTc labeling. Arrows indicated the location of inflammation. Adapted from reference (58).

Chitosan hydrogel nanoparticles loaded with VEGF fragments (labeled with ^99mTc) are considered as a good example for targeting of ischemic myocardium (59). Rats with myocardial ischemia were injected with chitosan hydrogel nanoparticles labeled with VEGF₁₆₅ or VEGF_81–91 peptides via apical puncture. Ischemic hearts receiving chitosan without tethering VEGF fragments served as the control group. SPECT imaging revealed that both chitosan-VEGF₁₆₅ and chitosan VEGF_81–91 possessed more potent absorption in the ischemic sites and reduced the degree of perfusion defects.

Imaging of other biological process

Radio-nanomaterials can also be used for studying some complex biological process. For example, ¹⁵³Sm-labeled UCNPs coated with polyphosphoric acid ligand were used for SPECT imaging of blood pool (60). These radiolabeled UCNPs demonstrated superior circulation time in mice, which is possibly due to the adhesion of these UCNPs on the membrane of red blood cells. Another interesting study used ⁶⁸Ga-labeled, glucose-coated AuNPs [which carried blood-brain barrier (BBB)-permeable peptides] to visualize the drug permeability across the BBB (61). Organ distribution of AuNPs was evaluated in rats by PET after intravenous administration. One peptide candidate-conjugated ⁶⁸Ga-AuNPs managed to show beneficial BBB crossing (near 3-fold higher) compared to non-targeted AuNPs.

Cerenkov imaging

Radioactive nanomaterials can prove to be a cost-effective alternative for laboratories equipped with optical imaging instruments by using the Cerenkov imaging technique. This imaging method is based on the electromagnetic radiation produced when a charged particle (e.g. positron, electron, etc.) travels faster than the light speed in an insulating medium (62). When radioisotopes are incorporated onto nanomaterials with optical properties (e.g. QDs or AuNPs), Cerenkov luminescence emitted from radioisotopes can serve as an excitation source (an energy donor) to trigger fluorescence from these nanomaterials, and it is named Cerenkov radiation energy transfer (CRET).

Based on this concept of CRET, self-illuminating QDs (structure: CuInS/ZnS) were developed in one study by using ⁶⁴CuCl₂ as a synthesis precursor (63). With excellent radiochemical stability, PEGylated ⁶⁴Cu-QDs showed high tumor uptake in U87MG mouse xenografts and were successfully applied as an efficient PET/CRET agent for in vivo usage. Self-illuminating QDs can also be constructed by doping ⁶⁴Cu into CdSe/ZnS core/shell QDs via a cationexchange reaction (64). The resulting ⁶⁴Cu-doped CdSe/ ZnS QDs also exhibited strong CRET properties and good tumor-targeting ability in U87MG xenografts. Another interesting study used different lengths of DNA as a linker between ⁶⁴Cu and QDs (65). Cerenkov radiation from ⁶⁴Cu was able to excite the QDs in a distance-dependent manner to make this radiolabeled QD a so-called “nano-ruler”.

Radiolabeled gold nanomaterials are attractive as CRET contrast agents due to their better biocompatibility (than QDs) and optimal fluorescence excitation/emission. ¹⁹⁸Au was used recently to form radioluminescent Au nanocages via a precursor-based method (66). Under biological conditions these ¹⁹⁸Au nanocages can emit light in the wavelengths of visible and near-infrared regions, enabling luminescence imaging of the whole mice in vivo. With a similar design, ⁶⁴Cudoped gold nanoclusters (AuNCs) were developed for PET/CRET dual modal imaging (67), in which AuNCs acted as the energy acceptor to produce NIR fluorescence. In U87MG xenografts, ⁶⁴Cu-doped AuNCs produced prominent tumor accumulation, confirmed from self-illuminating NIR images in the absence of external excitation.

Multimodality imaging

Different imaging labels can be integrated into one nanomaterial to be detectable by different imaging techniques simultaneously. Each imaging modality has its own pros and cons (68), for example PET and SPECT have excellent detection sensitivity (down to 10⁻¹² M)/depth and quantitative capacity, while they possess relatively low image spatial resolution. MRI gives fine anatomical information however its detection sensitivity is not optimal. Optical imaging is safe and cost-effective, and can provide surgical guidance, however the narrow tissue penetration depth (usually <1 cm) is the primary limiting factor for detection of deep-tissue biological events. Thus, combination of these imaging modalities will provide synergistic benefits to facilitate more accurate disease diagnosis and longitudinal therapeutic response assessment.

PET (SPECT)/MRI

Since the debut of PET/MRI instruments (69), significant research interests have been devoted in this area. Three types of nanomaterials are useful for PET (SPECT)/MRI: radioactive magnetic iron oxide nanoparticles (IONPs), gadolinium (Gd)-containing nanomaterials, and manganese (Mn)-containing nanomaterials. The applications of radiolabeled IONPs in imaging applications (particularly PET/MRI and SPECT/MRI) have already been elucidated (70).

Compared with IONPs, Gd-containing nanomaterials are more preferred choice for PET (SPECT)/MRI applications since they are optimal T₁ contrast agents (positive contrast). As optimal image contrast agents, the applications of gadolinium oxide nanoparticles in PET/MRI has been reviewed recently (71). Maintaining the material integrity and preventing Gd dissociation are the main themes for successful imaging with Gd-containing nanomaterials. In an early report, ¹⁸F-labeled, Gd-doped NaYF₄ nanophosphors were developed possessing PET, MRI, and upconversion luminescent properties (72). It showed a good paramagnetic longitudinal relaxivity and strong radiochemical stability (via strong interaction between Y and ¹⁸F). From cells and tissue slide studies, these ¹⁸F-labeled NaYF₄ nanophosphors could produce good contrast enhancement in PET/MRI. More recently, ⁶⁴Cu-labeled and Eu-doped GdVO₄ nanosheets (NSs) were fabricated with radioactivity, fluorescence, and paramagnetic properties. The carboxyl-functionalized GdVO₄ NSs were further modified by DOTA for ⁶⁴Cu labeling and Asp-Gly-Glu-Ala (DGEA) peptide for integrin α₂β₁ targeting (73). ⁶⁴Cu-DOTA-GdVO₄:Eu-DGEA demonstrated good accumulation in PC-3 tumor (integrin α₂,β₁⁺) in vivo, confirmed by PET and T₁-weighted MRI.

As a well-known accommodating vector for Gd with good biocompatibility, fullerene can be combined with radionuclides to produce PET (SPECT)/MRI probes with good clinical future (74), although more research attention should be devoted to this type of application. A PET/MRI probe based on ¹²⁴I labeled Gd₃N@C₈₀ fullerene derivate was developed to avoid potential Gd leakage by caging the gadolinium ions firmly inside the fullerene structure (75). Not only can this biocompatible Gd₃N@C₈₀ be used as a T¹-weighted MRI agent and PET probe, it can also serve as a “radical sponge” to ameliorate inflammatory responses. Gd-containing organic nanomaterials were also appealing choices as PET/MRI contrast agents. For example, Gdcontaining liposome (via DTPA coordination) was labeled with ⁸⁹Zr by lipid membrane adsorption (76). By coupling with a SSTR2-targeted peptide, accumulation in SSTR2⁺ tumors was more obvious compared with those in SSTR2⁻tumors. Despite the popularity of Gd-containing nanomaterials, manipulating Gd complexes inevitably changes their behavior with respect to Gd release and relaxivity. This has also be taken into serious consideration during the design of Gd-based nanomaterials.

Excellent T₁ shortening characteristics qualify manganese for MRI applications while in vivo toxicity is the primary reason which hampers this otherwise useful contrast agent (77). Unlike gadolinium, an effective chelating agent for manganese with satisfactory in vivo stability has not been identified. Incorporation of manganese into stable nanostructures can provide new opportunities for imaging applications with manganese. Surprisingly, the research effort devoted on manganese-containing radio-nanomaterials is quite limited. To the best of our knowledge, only one existing report has used ⁶⁴Cu-labeled, human serum albumin (HSA) coated MnO nanoparticles for PET/MRI of glioblastoma (78). The coating of HSA in this report can boost material solubility and r₁ relaxivity. These ⁶⁴Cu-labeled MnO@HSA nanoparticles possessed good physiological stability, optimal distribution profile, and superior T₁ contrast. Good tumor delineation has been achieved with ⁶⁴Cu-labeled MnO@HSA in both PET and MRI. There are plenty of opportunities ahead for manganese-containing nanomaterials to be used in PET/MRI studies, since the production of ⁵²Mn (t_1/2 =5.6 days) has been optimized for PET applications (79).

PET (SPECT)/optical imaging

For radio-nanomaterials to be used for PET (SPECT)/ optical imaging, the material should either possess intrinsic optical emissions (fluorescence, photoacoustic signals, or Raman emissions), or be labeled by both a radioisotope and an optical emitter. With tunable fluorescence emission, QDs have a wide applicability in PET/fluorescence imaging. In one study, a dual receptor-targeting PET/NIRF probe was developed from QDs by further modification with β-Glu-RGD-BBN (BBN stands for bombesin) peptides and ¹⁸F (80). QD-RGD-BBN exhibited strong NIRF emission with the maximum fluorescence wavelength at 705 nm. The functionalized QD probe has great potential as a universal dual-targeting probe for detecting tumors in living subjects with improved tumor-targeting efficacy.

Post-synthesis incorporation of both optical emitters and radioisotopes are most prevalent to produce PET/ optical imaging-suited nanomaterials. For example, MSNs conjugated with ⁶⁴Cu, 800 CW (a NIRF dye), and a monoclonal antibody against tumor vasculature, were adopted for PET/NIRF imaging of 4T1 breast tumors in one study (81). Good tumor targeting efficacy and specificity in tumor-bearing mice was achieved for these ⁶⁴Cu-labeled MSNs. A lot of attentions have been devoted on ultra-small silica-based Cornell dots (C-dots). C-dots are the first PET/fluorescence nano-probe that entered clinical testing. After conjugation with ¹²⁴I, an NIRF fluorophore (Cy5), and RGD peptide, C-dots were used for imaging of melanoma metastasis with improved SLN localization and retention, target-to-background ratios, and fast clearance from the body (primarily from renal pathways) (82). The specificity of this C-Dots platform, when compared with ¹⁸F-FDG, for metastasis/inflammation discrimination, was also satisfactory in the setting of surgery and therapeutic intervention. These radiolabeled C-dots were later used in a first-in-human clinical trial for lesion detection, cancer staging, and treatment management of patients with metastatic melanoma (Figure 4A) (83). ¹²⁴I-RGD-C-dots(Cy5) exhibited superior in vivo stability, good tolerance in patients, and sensitive detection of small metastatic lesions. Silica-based hybrid nanostructures are another category of PET/fluorescence contrast agents. For example, PAMAM-coated silica nanoparticles loaded with ^99mTc and indocyanine green (ICG) were used for SLN mapping (85). NIR fluorescence imaging provided realtime clear fluorescent images of the lymph nodes with anatomical resolution. Although further studies are needed to determine the appropriate dose of the dual-imaging nanoparticle probe for effective sensitivity and safety, the results acquired in this study confirmed the value of this hybrid nanostructure for sentinel lymph node biopsy.

Figure 4.

Radio-nanomaterials for multi-modal imaging. (A) First-in-human clinical trial using ¹²⁴I-RGD-C-dots(Cy5) for detection of pituitary lesion from metastatic melanoma by PET/MRI. Extremely high tumor-to-background ratios were achieved in these patients. Adapted from reference (83). (B) Hexamodal imaging with a radiolabeled, core-shell structured UCNP-liposome complex. In vitro analysis and in vivo LN mapping were both achieved with these nanocomplex in CT, PET, UCL, Cerekov luminescence, PAI, and fluorescence imaging. Adapted from reference (84).

Photoacoustic imaging (PAI), based on the photoacoustic effect, is another attractive optical imaging technique with good resolution and contrast, portable instrumentation, and the ability to partially quantify the signal (86). PAI has been applied to the imaging of cancer, neurological disorders, vasculature function, and gene expression, among others. A ⁶⁴Cu-labeled, anisotropic branched gold nanomaterial (Au-tripod) was proved to be useful to generate good contrast in PET/PAI (87). Linear correlation between PAI signals and Au-tripods concentration was confirmed in vivo. Intravenous administration of ⁶⁴Cu-labeled, RGD peptide conjugated Au-tripods (RGD-Au-tripods) to U87MG tumor-bearing mice showed PAI contrast in tumors almost three-fold higher than for the blocking group, and PAI results correlated well with corresponding PET images. Melanin nanoparticle (MNP) was another attractive choice with unique photoacoustic property and natural binding ability with metal ions (e.g. ⁶⁴Cu), which enables them to be used as PET/PAI agents. With further conjugation of RGD peptide, ⁶⁴Cu-MNPs demonstrated potent accumulation in U87MG tumor, and this observation validated the value of MNPs as a theranostic platform for potential clinical translation (88). This radiolabeled MNPs can also be used as a PET/PAI guidable drug delivery platform in living mice (89). With apofer-ritin conjugation for transferrin receptor 1 (TfR1) targeting, ⁶⁴Cu-MNPs showed excellent stability and presented good tumor uptake and high tumor contrast in HT29 tumor (TfR1⁺) with significantly lower accumulation in HepG2 (TfR1⁻).

Multimodal imaging (with more than two modalities)

Nanoplatforms that combine more than two different imaging modalities have come into a research focus (90, 91). To achieve this, nanomaterials used are usually in a hybrid structure or a core/shell architecture to embrace more contrast capacity from different components (92–94).

A MRI/PET/fluorescence probe based the integration of IONP and UCNP was developed recently (95). The nanoparticles are composed of a core/shell Fe₃O₄@NaYF₄ nanoparticles with different metal ions doped (Yb, Er, Tm, etc.). With the stabilization from polyethylene glycol, the obtained nanoparticles showed high transverse relaxivity (r₂), good radiolabel stability, and strong upconversion luminescence. LNs in live mice could be clearly visualized by using ¹⁸F labeled Fe₃O₄@NaYF₄ (Yb, Tm) nanoparticles in PET, MRI, and up-conversional luminescence (UCL). With a similar design, hybrid gold-IONP nanoparticles were made, in which IONPs worked as a T₂ MRI contrast agent, and the gold component acted as a strong fluorescence emitter and functionalization site [modified with 1,4,7-triazacyclononane-1,4,7-trisacetic acid (NOTA) for ⁶⁴Cu labeling] (92). Anti-EGFR affibody was also included to provide tumor targeting capabilities. As expected, the gold-IONP platform gave very sharp tumor contrast in PET, MRI, and fluorescence imaging. More recently, IONPs were assembled on the surface of two-dimensional MoS₂ nanosheets, forming MoS₂-IONP nanocomposites, which were then modified with PEG. ⁶⁴Cu could be adsorbed on the surface of MoS₂ without a chelator to enable PET imaging. The strong NIR emission and paramagnetic signals of MoS₂-IONP-PEG could also be utilized for PAI and MRI. Under the guidance by such triple-modal imaging, efficient tumor retention of MoS₂-IONP-PEG was confirmed, and effective tumor ablation was achieved in an image guidable manner in vivo. The same research group conducted hexamodal imaging using porphyrin-phospholipids coated UCNP (PoP-UCNP) (84). To more fully utilize the imaging capacity of this nanomaterial, it was characterized in vitro and in vivo for imaging via fluorescence, UCL, PET, CT, Cerenkov luminescence, and PAI (Figure 4B).

Therapeutic applications

Radiation therapy

Radio-nanomaterials were frequently used for delivering therapeutic isotopes to desired disease sites. The primary goal is to maximize the radiation dose to given cell population while decrease the radiation to normal tissues. Two types of isotopes are primarily used for therapeutic applications with radio-nanomaterials: α-emitters, and β-emitters.

β-emitter loaded radio-nanomaterials

β emitting radio-nuclides have found widespread use in cancer therapy due to their good availability and low production cost (96). Three β emitters are primarily used for production of radio-nanomaterials, namely, lutethium-177 (¹⁷⁷Lu), yttrium-90 (⁹⁰Y), and rhenium-188 (¹⁸⁸Re).

¹⁷⁷Lu

As a therapeutic isotope, ¹⁷⁷Lu emits β-particles (E_β =0.50 MeV, t_1/2=6.73 days) with moderate energy and a maximum travel range of 2 mm in tissues. AuNPs were radiolabeled with ¹⁷⁷Lu via DOTA and attached to RGD for targeting of integrin α_vβ₃ in the C6 glioma (α_vβ₃⁺) (97). The therapeutic response of ¹⁷⁷Lu-AuNP-RGD was compared with that of ¹⁷⁷Lu-AuNP or ¹⁷⁷Lu-RGD. In this study, ¹⁷⁷Lu-AuNP-RGD delivered the highest tumor radiation absorbed dose (63.8±7.9 Gy) and triggered observable tumor regression. There was low uptake in non-target organs and no evidence of renal toxicity. More recently, ¹⁷⁷Lu-conjugated AuNPs were attached to panitumumab, an antibody against EGFR, and able to deliver even higher radiation dose (73.2±6.7 Gy) to breast tumors (Figure 5A) (98). To further increase the loading efficiency of ¹⁷⁷Lu onto nanomaterials, first (G1-) or fourth (G4-) generation PAMAM dendrimer were both used as the delivery vectors (100). It was shown that the G1- and G4-dendrimer conjugates, modified in average with 7.5 or 57 ¹⁷⁷Lu chelating units, respectively, can be labeled with ¹⁷⁷Lu with a significantly improved specific activity and radiochemical purity. G1-dendrimer conjugate demonstrated faster clearance in normal organs.

Figure 5.

Therapeutic applications with β-emitter and α-emitter containing radio-nanomaterials. (A) ¹⁷⁷Lu-labeled, panitumumab-conjugated AuNPs for radiation delivery into breast cancer cells with different EGFR level. MDA-MB-468: EGFR-high; MDA-MB-231: EGFR-medium; MCF-7: EGFR-low. Both cellular fluorescence images and radioactivity measurement confirmed that conjugation of panitumumab could help AuNPs to deliver high dose of radiation (up to 73.2±6.7 Gy) into the cell nucleus. NT, non-targeted; T, targeted; B, blocking. Adapted from reference (98). (B) Gold-coated GdPO nanoparticles were successfully used to trap ²²⁵Ac and its daughter isotopes for targeted alpha therapy. By conjugation of an antibody against thrombomodulin receptors (expressed in lung endothelium), these nanocomplexes demonstrated selective accumulation in the lungs, which was monitored by SPECT/CT. Adapted from reference (99).

⁹⁰Y is a high energy β-emitter with a decay half-life of 64.1 h and E_β of 2.27 MeV, which can affect tumor cells up to a maximum depth of 11 mm in the soft tissue. Recently, ⁹⁰Y labeled N-(2-hydroxypropyl) methacrylamide (HPMA) nanoparticle was developed and its treatment efficacy was evaluated in prostate tumor (DU145) bearing mice (101). HPMA copolymer-DOTA conjugates demonstrated efficient labeling and stability for ⁹⁰Y and a marked accumulation of these radiolabeled copolymers was observed within prostate tumors. Histological analysis confirmed treatment efficacy and safety.

The relatively high energy from ⁹⁰Y requires more careful design of delivering method to limit its radiation to surrounding healthy tissues. In a clinical scenario, usually only several radioactive atoms (e.g. ⁹⁰Y) are linked to a targeting ligand (e.g. an antibody) and the deposited dose is often insufficient to eradicate solid tumors or tumors with radiation resistance. By replacing the single radionuclide by a radioactive nanoparticle containing hundreds of radioactive atoms, increased radiation dose can be delivered inside tumors. A study confirmed the benefit from ⁹⁰Y₂O₃ nanoparticles for this application (102).

¹⁸⁸Re

Rhenium-188 is one of the most readily available generator-derived and useful radionuclides emitting β particles (2.12 MeV, 71.1% and 1.965 MeV, 25.6%, t_1/2=17 h) and gamma photons (155 keV, for imaging applications). The ¹⁸⁸W/¹⁸⁸Re generator is a good source for the long term (4–6 months) continuous availability of no-carrieradded ¹⁸⁸Re for radionuclide therapy (103). ¹⁸⁸Re-labeled magnetic nanoparticles (functionalized with albumin) were also loaded with cisplatin, and these nanoparticles could provide triple killing effects via hyperthermal therapy, chemotherapy, and radiotherapy (104). In vivo studies in tumor-bearing mice revealed that tumor uptake of these ¹⁸⁸Re-labeled magnetic albumin nanoparticles increased gradually after injection, peaked at 8 h and decreased gradually over time. This study confirmed the potential of ¹⁸⁸Re-labeled nanomaterials for tumor treatment, but until 2 years later the same group released their findings with the same material on ovarian tumor treatment (105). Acquired results indicated that the combination of magnetic induced hyperthermia, chemotherapy and targeted radionuclide of radiation exposure could effectively inhibit the growth of ovarian cancer. In addition, G5 PAMAM dendrimer was radiolabeled with ¹⁸⁸Re, potentially useful for boosting isotope loading and facilitating subsequent functionalization (106).

Some unconventional β-emitting isotopes were also used for radio-nanomaterial production including ⁶⁷Ga (107), ¹⁶⁶Ho (108), and ¹⁹⁸Au (109). The incorporation chemistry and therapeutic application is very similar to what we have discussed above.

α-emitter loaded radio-nanomaterials

Compared with a variety of isotope choices of β-emitters, therapeutic application of α-emitter loaded nanomaterials is at a relatively preliminary stage. The smaller travel range of a-particles demands more precise delivery of radio-nanomaterials (110). The major challenge with α-emitter radiotherapies is that traditional chelating moieties are unable to sequester all the radioactive daughters, and this “detachment” can pose toxicity to healthy tissue. To provide more strict control of daughter radioisotopes, different methods have been proposed.

A number of targeted alpha therapy (TAT) agents based on the single α-emitting radionuclides ²¹¹At (t =7.2 h), ²¹³Bi (t =46 months), ²¹²Pb (t =10.6 h), and ²¹²Bi (t =61 months) have been developed and showed promise in pre-clinical and clinical trials (111). The therapeutic efficacy of TAT could, however, be further enhanced by use of in vivo a-generator radionuclides like actinium-225 (²²⁵Ac). ²²⁵Ac can produce 4 net a-particles during its decay, dramatically amplifying the radiation dose delivered to the target (112). Moreover, the longer half-life of ²²⁵Ac (t_1/2= 10 dsay) reduces activity loss during radiopharmaceutical production and allows longer time for localization of antibodies to the target sites. Radio-nanomaterials we mentioned in this section are primarily based on ²²⁵Ac incorporation.

For effective TAT with ²²⁵Ac, daughter radioisotopes (i.e. ²²¹Fr, ²¹⁷At, ²¹³Bi, and ²⁰⁹Tl, Figure 5B) from the parent emissions should be sufficiently stable in the nanomaterials as the recoil energy of the ²²⁵Ac daughters following α-decay can compromise metalligand bond used to stabilize ²²⁵Ac. To solve this dilemma, a multilayered nanoparticle based on the structure of La_0.5 Gd_0.5 (²²⁵Ac) PO₄@ GdPO₄@Au was used to constrain the decay daughters of ²²⁵Ac (99). The stability of ²²⁵Ac on these multi-shell nanoparticles was sufficiently high, and these nanoparticles demonstrated good receptor targeting efficacy after conjugation of an antibody (Figure 5B), which was monitored by SPECT/CT. Similarly structured layered nanoparticles were used in another study for treatment of lung tumors in mice (113). Retention of daughter ²¹³Bi in lung was about 90% at 24 h post-injection. Treatment of mice with lung-targeted ²²⁵Ac NP significantly reduced EMT-6 lung colonies. Single-walled carbon nanotubes (SWNTs) was another useful carrier to bring ²²⁵Ac into therapeutic applications (114). After loading of both ²²⁵Ac and Gd³⁺ via bath sonication, the ²²⁵Ac@GNTs were tested for stability against heat, time, and human serum. SWNTs could successfully sequester ²²⁵Ac in the presence of Gd³⁺, rendering ²²⁵Ac@GNTs candidates for delivery of ²²⁵Ac at higher concentrations.

Combinational therapies

Due to the evolving nature of cancer, monotherapies rarely catch up with the disease progression, especially for cancer. By tackling multiple targets, combinatorial treatments aim to improve the therapeutic index either through increased efficacy and overcoming resistance or through similar efficacy with reduced systemic toxicity. Most importantly, the combination of multiple therapeutic agents must generate a synergistic effect, ideally at lower doses than typical doses of each individual compound (115). The aforementioned ¹⁸⁸Re-containing, cisplatinloaded magnetic nanoparticles were good examples for combinational radio-/chemo- therapies (104, 105). Moreover, ⁹⁰Y-labeled albumin microspheres encapsulated with magnetite nanoparticles have been investigated for possible applications as a bimodal radionuclide-hyperthermia cancer therapy agent (116). In a later study, IONPs were also used to load ⁹⁰Y, functionalized with PEG (117). With reproducible high radiolabeling yield (>97%) labeling and good stability, these ⁹⁰Y-IONPs are also suitable candidates for MRI-magnetic hyperthermia and regional radiotherapy. Despite the foreseeable benefits of combinational therapy, the effort devoted in this area is very limited, and in some situation, the loaded isotope did not seem to play a synergistic role with other therapeutic elements. Another very unique nanomaterial type for combinational therapeutic application is boron-containing/enriched nanocomposites, which can emit α-particles after absorbing slow neutrons, although they are not radioactive before neutron activation (118). The nuclear capture reaction concept is known as boron neutron capture therapy (BNCT). BNCT is a potentially promising treatment for malignant brain tumors as well as other cancers, despite the fact that neutron sources may not always be available.

Summary and future perspectives

Employing nanotechnology in delivery of diagnostic/therapeutic agents offers significant advantages, including normalized pharmacokinetics and pharmacodynamics, sustained bioavailability, targeted accumulation, and controllable drug release. Radio-nanomaterials, with synergistic integration of physical/chemical/functional properties from the material(s) and the isotope(s), are important players for different biomedical applications (Table 2). There are two critical composing elements for a radioactive nanomaterial, i.e. the radioisotope, and the nanomaterial. To make the radioactive nanomaterial readily applicable for different biomedical applications, the suitable selection of both components should be synergistic. On the one hand, incorporation of radioisotope(s) bestows extra tracking/therapeutic ability to the nanomaterial which cannot be acquired by loading of other cargos. On the other hand, the utilization of suitable nanomaterials may serve as an isotope carrier and enable some unconventional isotopes to be used in specific biomedical applications which might otherwise be very difficult to achieve, for example, gemanium-69 (⁶⁹Ge) (9), or sodium-22 (²²Na) (10).

Table 2.

Representative radio-nanomaterials for biomedical applications.

Radio-material	Labels	Material characteristics	Major application(s)	Research stage	Production cost
Silica-based nanomaterials	124I 64Cu 89Zr	Biocompatibility, biodegradability, superior cargo-loading capacity	Cancer imaging, therapeutic delivery	Clinical test (Cornell dots); small-animal test	$
Carbon nanomaterials	64Cu, 124I, 111ln, 66Ga, 225Ac, etc	Photothermal effect, Gd-loading (fullerene)	Cancer imaging, stimulus- responsive therapeutic delivery, radiotherapy	Mostly in small animals. SWNTs were tested for 225Ac loading. Clinical test can be foreseen	$$ (fullerene can be $$$)
IONPs	64Cu, 69Ge, 111ln, 14C, 72As, etc	Paramagnetic properties, biocompatibilitytd	Disease imaging	Mostly in small animals. Radiolabeled lONPs have not been tested in human	$$
Gold nanomaterials	68Ga, 64Cu, 198Au, 177Lu, etc	Optical emissions (e.g. for CRET), biocompatibility	Disease imaging, radiotherapy	Mostly in small animals	$$
QDs	64Cu, 18F, etc	Optical emissions	Cancer imaging	Mostly in small animals	$$
UCNPs	18F, 153Sm, etc	UCL emissions, magnetic properties (with doping)	Cancer imaging	Cell or rodents-based studies	$$$
Metal oxide nanomaterials (other than lONPs)	64Cu, 99mTc, 225Ac, etc	Stable cargo-loading, unique physical properties	Cancer imaging, radiotherapy	Cell or rodents-based studies	$ — $$$
Polymer-based nanomaterials	99mTc, 89Zr, 64Cu, 188Re, 90Y, etc	Biocompatible, chemically versatile	Cancer/inflammation imaging, radiotherapy	Cell or rodents-based studies	$$
Lipid-containing nanomaterials	89Zr, 64Cu, etc	Biocompatible, chemically versatile	Cancer/inflammation imaging, therapeutic delivery	Cell or rodents-based studies	$

The ability to track the kinetics of nanomaterials in humans and research animals on a whole body basis can provide invaluable information to researchers for development of efficient diagnostic or therapeutic agents. The attachment of radionuclides to nanomaterials can facilitate the tracking of nanomaterials in vivo and help to clarify their pharmacokinetic behaviors (stability, organ absorption/distribution, clearance, etc.). Different imaging “labels” can be integrated into a single nanoplatform for combining the strengths of different imaging modalities, which can synergistically improve the overall value of imaging in the context of basic research or patient care. In addition, nanomaterials with appropriate functionalization can evade attack from the immune system and thus create prolonged imaging time (11). Moreover, as most nanomaterials have large surface areas which result in superior cargo accommodating capacity, they can help to increase local imaging contrast in selected areas. In addition, loading of imaging labels (isotopes/ fluorophores, etc.) in nanomaterials can cause alterations of the in vivo pharmacokinetics of the labels, which can be tunable for image optimization in most cases.

One of the major challenges associated with radionuclide therapies is the inherent toxicity from the radiation damage to normal, healthy tissues. For example, although β emitting radionuclides have found widespread use in cancer therapy, the long travel range of β particles (e.g. 1.5 mm for ¹⁷⁷Lu and 12 mm for ⁹⁰Y) tends to pose damage to the surrounding tissues. Radiotherapies based on α particle emission can serve as a promising alternative since α particles deposit all of their energy within a few cell diameters (50–100 µm), thus accurate delivery of α emitters (and potentially daughter isotopes after decay) to the diseased sites is the prerequisite for effective cell killing. For both α and β emitters, incorporation into nanomaterials can increase effective dose delivered to diseased locations (particularly cancer). With or without the company of an “active targeting” ligand, radio-nanomaterials can alter the pharmacokinetics of the loaded α-/β- emitters, usually resulting in decreased toxicity. Significantly more research efforts will be devoted to prepare nanomaterials with better radiochemical stability, more optimal isotope loading efficiency, and more beneficial clearance profile.

Using one drug for cancer therapy is usually considered as ineffective: tackling on a specific target pathway often result in the activation of more pathogenic pathways as a compensation (119, 120). Therefore, combinatorial therapies using multiple therapeutic agents together for generating synergistic effects is a logical approach to combat cancer since they can respond to the dynamic nature of cancer during the treatment. By using radio-nanomaterials, multi-dimensional therapeutic options can be tested easily for a specific cancer type or in a given individual. Keep in mind that it is indispensable to ensure that these treatments are synergistic and complementary. The potential adverse effects from radio-nanomaterials need to be also considered when incorporating other therapeutic elements. Finally, the radio-nanomaterials usable for combinational therapy should not involve complex manufacturing process, and should be highly assessable to the majority of the research community and clinics.

Selection of radio-nanomaterial candidates to be eventually useful in clinics is extremely challenging, and the balance between cost and benefit must be judiciously mapped out. Radio-nanomaterials offer researchers and clinicians new tools to visualize the fate of these materials, evaluate their interaction for a given disease, and provide improvement to current disease therapies. Looking forward, these radio-nanomaterials are expected to play an even more important role in shaping the future of nanotheranostics.

Table 1.

Commonly used isotopes for production of radio-nanomaterials.

Isotope	Half-life	Incorporation method into nanomaterials	Production method	Availability	Cost	Utilization
18F	110 min	Post-production labeling; precursor	18O(p, n) 18F	On-site cyclotron	$$	Imaging
64Cu	12.7 h	Post-production labeling; precursor; ion exchange	64Zn(n, p)64Cu	Specific cyclotron production needed	$$	Imaging; radiotherapy; auger-electron therapy
89Zr	78.4 h	Post-production labeling; precursor	89Y(p, n) 89Zr	Specific cyclotron production needed	$$$	Imaging
13N	9.97 min	Proton activation	160(p, α) 13N	Specific cyclotron production needed	$$$	Imaging
68Ga	68 min	Post-production chelation	68Ge/68Ga generator	Generator	$$	Imaging
59Fe	44.6 years	Post-production labeling	Neutron activation	Nuclear reactor needed	$$$	Material distribution measurement
111In	2.8 days	Chelation	111Cd(p, n)111ln	Commercially available	$$	Imaging; radiotherapy
99mTC	6h	Chelation	99Mo/99mTc generator	Generator	$	Imaging
177Lu	6.7 days	Chelation	176Yb(n,γ)177Yb→177Lu	Commercially available	$$$	β-therapy
90γ	64.1 h	Chelation	90Sr/90Y generator	Generator	$$	β-therapy
225Ac	10 days	Embedded inside the nanomaterials	226Ra (γ, n) 225Ac	Specific cyclotron production needed	$$$	α-therapy

Biographies

Weifei Lu

Weifei Lu acquired his PhD degree in Biochemistry from Henan Agriculture University (HAU), China in 2014 and is currently an Associate Professor in HAU. As an active member of Chinese Society of Biochemistry and Molecular Biology, his research interest is focused on the development of gene therapy vectors, study of gene functions, protein engineering, and targeted drug delivery. He is now a visiting professor under the supervision of Dr. Hao Hong for design and synthesis of novel PET tracers based on biodegradable nanomaterials and engineered proteins. He also participates actively in different major research projects funded by NSF China.

Hao Hong

Hao Hong started his postion as a Research Assistant Professor of Radiology at the University of Michigan in 2014. He received his PhD degree in Biochemistry and Molecular Biology from Nanjing University (P.R. China) in 2008. He acquired his postdoctoral training from 2008 to 2013 in the Department of Radiology, University of Wisconsin, Madison. His research interest is in the design and optimization of new imaging tracers for cancer as well as modification/application of nanomaterials for image-guided therapeutic delivery. Dr. Hong has published >90 peer-reviewed manuscripts (citation >3400 in Google scholar) and received many awards.

Weibo Cai

Weibo Cai is currently an Associate Professor of Radiology and Medical Physics at the University of Wisconsin-Madison. He received his PhD in Chemistry from UC San Diego in 2004. After postdoctoral training at Stanford University, he launched his career at UW-Madison in early 2008. His research is primarily focused on molecular imaging and nanotechnology (http://mi.wisc.edu/), investigating the biomedical applications of various agents developed in his laboratory for imaging and therapy of cancer and cardiovascular diseases. Dr. Cai has authored ~200 peer-reviewed publications with H-index of >50, received many awards, and served on numerous grant review panels.