**5. CD-Based CT Contrast Agents**

The initial development of CD-based contrast agents for CT was used to investigate multimodality imaging potential of phosphorescent-modified NaDyF<sup>4</sup> NPs (DyNPs) [53]. OA-coated DyNPs underwent complexation with α-CD followed by conjugation with Gd(III)-DTPA complex. The last stage of contrast agent formation was the loading of phosphorescent Ir(Dbz)2(Pbi) complex with in the hydrophobic layer of Gd-α-CD-DyNPs [53]. Besides the contrast creation potential for fluorescence and MRI, Zhou et al. considered the developed Ir-Gd-α-CD-DyNPs as a potential contrast agent for CT due to the presence of heavy atoms of Dy, Gd, and Ir. The measured Hounsfield units (HU) value of 10 mg/mL Ir-Gd-α-CD-DyNPs aqueous solution was equal to 158 at 80 kV X-ray energies. Following in vitro experiments, Ir-Gd-α-CD-DyNPs were used for CT scanning of tumor-bearing mice (Figure 11). The original HU value of 109 for the tumor increased to 212 HU after intratumoral injection of 100 ul of 3 mg/mL Ir-Gd-α-CD-DyNPs (Figure 11G) solution. As a result, approximately 100% signal enhancement was observed on the CT scans.

Another multimodal CD-based contrast agent that was developed for CT scanning was a core-shell-structured alkali ion-doped CaF2:Yb,Er UCNP [50]. Similar to the NPs discussed above, the core of NPs was coated with OA with subsequent complexation with α-CD. The final α-CD/UCNPs contrast agent was compared to well-known iopromide 300 CT clinical contrast agent. Yin et al. found that developed α-CD/UCNPs have 45% higher HU value at concentration 80 mM/L. This high X-ray absorption originates from the present high atomic number elements. In addition, investigated α-CD/UCNPs were loaded with doxorubicin, and the ability of simultaneous cancer imaging and treatment using this contrast agent was suggested [50].

The next achievement in the CT contrast agent development was obtained in 2017 by conjugating β-CD to poly(methyl vinyl ether-alt-maleic anhydride) (PVME-alt-MAH) with a subsequent reaction using modified dextran to create gel microspheres [104]. To make dextran microspheres visible for CT, they were loaded with iodine in *n*-hexane. Through the micro-CT phantom imaging, Zhu et al. observed the subsequent improvement of contrast after iodine loading.

**5. CD-Based CT Contrast Agents** 

The initial development of CD-based contrast agents for CT was used to investigate multimodality imaging potential of phosphorescent-modified NaDyF4 NPs (DyNPs) [53]. OA-coated DyNPs underwent complexation with α-CD followed by conjugation with Gd(III)-DTPA complex. The last stage of contrast agent formation was the loading of phosphorescent Ir(Dbz)2(Pbi) complex with in the hydrophobic layer of Gd-α-CD-DyNPs [53]. Besides the contrast creation potential for fluorescence and MRI, Zhou et al. considered the developed Ir-Gd-α-CD-DyNPs as a potential contrast agent for CT due to the presence of heavy atoms of Dy, Gd, and Ir. The measured Hounsfield units (HU) value of 10 mg/mL Ir-Gd-α-CD-DyNPs aqueous solution was equal to 158 at 80 kV X-ray energies. Following in vitro experiments, Ir-Gd-α-CD-DyNPs were used for CT scanning of tumorbearing mice (Figure 11). The original HU value of 109 for the tumor increased to 212 HU after

approximately 100% signal enhancement was observed on the CT scans.

**Figure 11.** In vivo 3D volume-rendered (**A**,**B**) and maximum intensity projections in axial (**C**,**D**) and coronal (**E**,**F**) view CT images of the tumor-bearing mouse obtained pre- (**A**,**C**,**E**) and post- (**B**,**D**,**F**) injection of Ir-Gd-α-CD-DyNPs contrast agent [53]. The images are reprinted with permission from the publisher [53]. The position of the tumor was marked by red circles. The chemical structure of the developed CT contrast agent is shown in (**G**). **Figure 11.** In vivo 3D volume-rendered (**A**,**B**) and maximum intensity projections in axial (**C**,**D**) and coronal (**E**,**F**) view CT images of the tumor-bearing mouse obtained pre- (**A**,**C**,**E**) and post- (**B**,**D**,**F**) injection of Ir-Gd-α-CD-DyNPs contrast agent [53]. The images are reprinted with permission from the publisher [53]. The position of the tumor was marked by red circles. The chemical structure of the developed CT contrast agent is shown in (**G**).

Another study, conducted in 2017, demonstrated synthesis of β-CD-{poly(ε-caprolactone)-poly (2-aminoethyl methacrylate)-poly[poly(ethylene glycol) methyl ether methacrylate]}<sup>21</sup> (β-CD-(PCL-PAEMA-PPEGMA)21) with stable unimolecular micelles formed in aqueous solution [51]. β-CD-(PCL-PAEMA-PPEGMA)<sup>21</sup> had used a template for the creation of gold NPs (AuNPs) with uniform sizes, followed by the encapsulation of doxorubicin (DOX). The CT performance of the final β-CD-(PCL-PAEMA-PPEGMA)21/AuNPs/DOX contrast agent was compared to the clinically available Omnipaque both in vitro and in vivo. At a concentration of 800 uM, Lin et al. found the X-ray absorption of β-CD-(PCL-PAEMA-PPEGMA)21/AuNPs/DOX solution to be approximately 23% higher compared to Omnipaque. In vivo imaging of β-CD-(PCL-PAEMA-PPEGMA)21/AuNPs/DOX contrast agent in HepG2 mice tumor model demonstrated significant enhancement of CT signal compared to the clinical iodine analogue [51].

Another step in the development of CD-based CT contrast agents was done by creation an unimolecular micelle system synthesized from 21-arm star-like polymer β-CD-{poly(lactide)-poly(2-(dimethylamino) ethyl methacry late)-poly[oligo(2-ethyl-2-oxazoline) methacrylate]}<sup>21</sup> (β-CD-(PLA-PDMAEMA-PEtOxMA)21) followed by production of β-CD-(PLA-PDMAEMA-PEtOxMA)21/AuNPs/DOX [105]. The developed Au-loaded β-CD-based contrast agent was tested in the animal HepG2 tumor model. Lin et al. found that intravenously injected β-CD-(PLA-PDMAEMA-PEtOxMA)21/AuNPs/DOX produces substantially higher CT contrast compared to iodine-based Omnipaque, which was used for control scans.

Following the previous studies, the most recent advance in the field of CD-based CT contrast agents was achieved by the same group [106]. The authors synthetized 21-arm star-like polymers β-CD-g-{poly(2-(dimethylamino)ethyl methacrylate)-poly(2-hydroxyethyl methacrylate) poly[poly(ethylene glycol) methyl ether methacrylate]} (β-CD-g-(PDMA-b-PHEMA-b-PPEGMA)). By adding HAuCl<sup>4</sup> solution into β-CD-g-(PDMA-b-PHEMA-b-PPEGMA) aqueous solution and triggering subsequent reduction with DMA, the AuNPs at the core of unimolecular micelles were formed. The CT contrast of β-CD-g-(PDMA-b-PHEMA-b-PPEGMA)/AuNPs agent was compared to contrast created by Omnipaque in vitro. Lin et al. found that, at a concentration of 800 uM, the X-ray attenuation of β-CD-g-(PDMA-b-PHEMA-b-PPEGMA)/AuNPs was approximately 37% bigger than that of Omnipaque. The developed β-CD-g-(PDMA-b-PHEMA-b-PPEGMA)/AuNPs demonstrated slightly higher X-ray absorption compared to previously synthetized β-CD- (PCL-PAEMA-PPEGMA)21/AuNPs [51,106]. However, the average HU values of β-CD-g-(PDMA-b-PHEMA-b-PPEGMA)/AuNPs at each concentration were slightly lower than previously developed β-CD-g-(PLA-b-PDMA-b-PEtOxMA)21/AuNPs [105,106].
