Improved Tumor Imaging Using dendrimer-based Nanoplatforms

Keywords:

• dendrimers
• dual mode imaging
• imaging sensitivity
• targeting specificity
• tumors

Traditional small molecular contrast agents used for medical imaging (e.g., iodine-based computed tomography [CT] contrast agent, Omnipaque) display their inherent shortcomings, such as kidney toxicity, lack of specificity, short imaging time and low signal-to-noise ratio. Consequently, exploring safe and effective contrast agents or constructing novel functionalized nanoprobes has been one of the focuses of medical imaging research in the past decades.

Dendrimers are a class of monodispersed and synthetic macromolecules with a well-defined and highly branched three-dimensional architecture [1,2]. The abundant terminal functional groups enable various surface modifications and the branched architecture can provide internal space for efficient loading of nanoparticles (NPs) and drugs. The unique properties and excellent biocompatibility of dendrimers after appropriate surface modification allow them to serve as versatile nanoplatforms for biomedical imaging and theranostic applications. Through architecture design, surface modification and integration of various contrast elements, dendrimer-based nanoplatforms have been developed for tumor imaging with improved biocompatibility, sensitivity, targeting specificity and precision.

Biocompatible dendrimer-based nanoplatforms for enhanced CT imaging

For enhanced tumor CT imaging, dendrimer-entrapped or -stabilized metal NPs have been synthesized. For instance, dendrimer-entrapped gold NPs (Au DENPs) can be used for enhanced CT imaging applications due to the fact that AuNPs entrapped within the dendrimers display a better x-ray attenuation property and longer blood circulation time than conventional iodinated contrast agents [3,4]. Moreover, compared with dendrimer-free AuNPs, the Au DENPs or dendrimer-stabilized AuNPs display excellent colloidal stability and easy surface functionalization capability [5]. Given the fact that the abundant surface amines of dendrimers would cause cytotoxicity and nonspecific binding with cell membranes, acetylation [6], PEGylation (PEG denotes polyethylene glycol) [7], mixed acetylation/PEGylation [8] or zwitterion functionalization/acetylation [9,10] of dendrimer periphery amines has been generally conducted to shield their positive surface potentials to improve the biocompatibility of dendrimer-based nanoplatforms for CT imaging applications, in particular CT imaging of tumors.

Dendrimer-based nanoplatforms with improved CT imaging sensitivity

In order to improve the CT imaging sensitivity of tumors, two radio-dense elements (iodine and Au) have been integrated within a single dendrimer platform. For instance, iodinated CT contrast agent of diatrizoic acid can be covalently conjugated onto the dendrimer surface via a 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) coupling reaction, followed by formation of dendrimer-stabilized AuNPs [11]. Alternatively, preformed Au DENPs can be post covalently linked with diatrizoic acid [12]. Apparently, the dendrimer platform with dual radio-dense elements incorporated exhibits significantly enhanced x-ray attenuation property as compared with Au DENPs or Omnipaque alone, thereby enabling improved CT imaging sensitivity.

Apart from the strategy of incorporating dual radio-dense elements within single dendrimer platform, improving the loading amount of AuNPs within the interior of dendrimers is another feasible approach for sensitive CT imaging. To achieve this goal, a stepwise reduction method was adopted. In this method, amine-terminated generation 5 poly(amidoamine) dendrimers (G5.NH₂) were used as templates to form Au DENPs via a stepwise Au salt complexation/reduction approach, followed by acetylation of the dendrimer terminal amines [13]. Through the stepwise reduction process, AuNPs were able to be tightly compacted within the dendrimer interior, thus contributing to the improvement of colloidal stability after acetylation. The imaging sensitivity could be improved by two to three folds using this strategy. In addition, PEGylation of dendrimer periphery is a powerful way to improve the Au loading content within the interior of dendrimers presumably due to the enlarged dendrimer geometry after PEGylation for sensitive CT imaging of blood pool and tumors [8,13,14]. What is more, with the prolonged circulation time in vivo, the PEGylated Au DENPs are able to effectively escape the reticuloendothelial system and be accumulated in the tumor site via the passive enhanced permeability and retention effect, thus allowing for effective CT imaging of tumors [13,14].

Dendrimer-based nanoplatforms for targeted tumor CT imaging

To endow the contrast agents with targeting specificity, targeting ligands (e.g., folic acid [FA], RGD peptide, and lactobionic acid [LA]) can be covalently linked onto the dendrimer-based nanoplatforms [15–18]. For instance, FA has been linked onto the surface of Au DENPs through EDC chemistry, and the formed FA-functionalized Au DENPs can be applied for targeted CT imaging of human lung adenocarcinoma overexpressing FA receptor in vitro and in vivo [15]. Furthermore, dendrimers prelinked with PEGylated LA [16] could be used as templates to synthesize Au DENPs for targeted CT imaging of hepatocellular carcinoma via a receptor-mediated manner. In this case, the PEG spacer between targeting ligands and dendrimers not only enlarges the dendrimer periphery for improved loading of AuNPs, but also improves the flexibility of the targeting ligands for enhanced targeting of cancer cells. Similarly, Au DENPs linked with RGD peptide via a PEG spacer have also been synthesized for targeted imaging of orthotopic brain gliomas [17].

Dendrimer-based nanoplatforms for dual mode precision imaging of tumors

To improve the imaging precision, dendrimer-based bimodal or multimodal contrast agents have been developed. Through the versatile dendrimer nanotechnology, it is possible to integrate two types of radiodense imaging elements within one single system for dual mode bioimaging, which significantly improves the detection accuracy [17,19]. CT imaging can provide high spatial and density resolution as well as three-dimensional tomography information of the anatomic structure, while magnetic resonance (MR) imaging can give excellent resolution for soft tissue and functional information of the lesions. Thus, developing nanoplatforms with dual mode CT/MR imaging capacity should be a feasible approach to achieve precision diagnosis of tumors. For instance, Au DENPs can be modified with chelator/Gd(III) complexes for dual mode CT/MR imaging applications [10,17]. As a typical example, Liu et al. [10] constructed gadolinium (III)-complexed Au DENPs with zwitterion functionalization that can be used for enhanced CT/MR imaging of lung cancer metastasis due to the zwitterion-enabled antifouling property of the materials.

In addition, dual-mode single-photon emission computed tomography (SPECT)/CT has been identified as a powerful molecular imaging technology due largely to the fact that it can obtain the metabolic information from SPECT and the anatomic diagnostic information from CT at the same time, thus significantly improving the accuracy of diagnosis [1]. In our previous work, Au DENPs were labeled with ^99mTc and modified with FA for targeted dual-mode SPECT/CT imaging of tumors [20]. Similarly, a multifunctional dendrimer-based nanoplatform was designed to have both Mn(II) and ^99mTc loaded via chelation for targeted dual-mode SPECT/MR imaging of tumors [21].

Moreover, multimode imaging contrast agents have also been developed using dendrimer-based nanoplatforms. In our recent work [22], we established a multifunctional theranostic nanoplatform based on G5.NH₂ dendrimer-stabilized gold nanoflowers embedded with ultrasmall iron oxide NPs for multimode MR/CT/photoacoustic imaging and combination therapy of tumors. The formed dendrimer-based nanoplatform was proven to have precision imaging capability and improved therapeutic efficacy of tumors.

Conclusion & future perspective

The unique properties of dendrimers have inspired the rapid development of dendrimer-based nanoplatforms for improved tumor imaging applications. With the highly branched internal cavity and rich chemistry for tunable surface functionalization and conjugation, dendrimer-based nanoplatforms can be designed for enhanced CT imaging with improved biocompatibility via acetylation, PEGylation and mixed functionalization of dendrimer periphery amines, for sensitive CT imaging through integration of two radiodense elements or increase of entrapped Au content, for targeted tumor imaging via surface conjugation of targeting ligands, for precision imaging using dual-mode or multimode imaging strategies by incorporating different types of imaging elements within one single dendrimer-based system.

Even though much effort has been made to achieve improved tumor imaging using dendrimer-based nanoplatforms, challenges still exist. For instance, considering the microenvironment of tumor tissues, more effective strategies such as zwitterion functionalization, cell membrane coating, nanocluster effect and stimuli-responsive linkage chemistry can be introduced to endow the dendrimer-based platforms with antifouling properties for prolonged blood circulation time, and with enhanced accumulation, permeation and retention in solid tumors. Furthermore, dendrimer-based platforms can be exploited for multifunctional theranostic applications of cancer or many other diseases (e.g., arthritis, liver cirrhosis, cardiovascular disease, etc.) through loading of imaging agents as well as anticancer drugs, photothermal agents, photosensitizers or therapeutic genes.

Supplementary data

To view the supplementary data that accompany this paper please visit the journal website at:www.tandfonline.com/doi/full/10.2217/epi-2016-0184

Financial & competing interests disclosure

This work was financially supported by the National Natural Science Foundation of China (21773026 and 81761148028), the Science and Technology Commission of Shanghai Municipality (19XD1400100, 17540712000 and 18520750400) and the Shanghai Education Commission through the Shanghai Leading Talents Program (ZX201903000002). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.