*2.10. Metallodendrimers*

Metallodendrimers has a monodisperse nature with facile preparation. This exclusive feature is demonstrated by Smith et al. to entrepreneur the Photo-CORMats (Figure 26) [161]. For this purpose, Photo-CORM Mn(CO)3 moiety scaffolds with polypyridyl dendritic. The general representation of polypyridyl dendritic is [DAB-PPI-{MnBr(bpyCH3,CH=N)(CO)3}n] (whereas DAB=1,4-diaminobutane, PPI=poly(propyleneimine), bpy=bipyridyl). Photo-activated metallodendritic CORMats has been observed to liberate CO molecules upon 410 nm visible light photons penetration.

**Figure 26.** Metallodendrimers photoactivated CORMats.

#### *2.11. Nanodiamond (ND)*

The tpm ligand of Photo-CORM [Mn(CO)3(tpm)]+ peptide material and nanoparticle [Mn(CO)3(tpm-L1)] could further be constructed to explore the azide-modified nanodiamond (ND) by CuAAC (copper-catalyzed 1,3-dipolar azide–alkyne cycloaddition) "click" formation as Manganese-MCCs Photo-CORMats (Figure 27) [159]. Dordelmann, G. et al. introduced the first-time CuAAC coupling to attach the CO-liberating agen<sup>t</sup> with ND as a biocompatible supporter. Photoactivatable CORM [Mn(CO)3(tpm)]+ retained at the ND's surface for CO biological services and therapeutic purposes and were evaluated through standard myoglobin assay.

**Figure 27.** The [Mn(CO)3(tpm)]+-functionalized nanodiamond (ND) immobilized on azidemodified ND'surface through CuAAC "click" reaction.

Different CORMats are compatible with special cellular environments and are free to perform their therapeutic activities. Certain conditions restrict CORMats activities; definitely it would directly affect the therapeutic performance. CORMats therapeutic potential relies on the material's nature such as solubility, compatibility and activation mechanism. Another advantage of CORMats is to modify the CORMats assembly according to respective disease cells, which could be more helpful in searching the selective targets. For cancer treatment, the redirected T-cells, i.e., chimeric antigen receptor (CAR) T-cell might be providing a governing principle for cancer therapy [169]. CAR-Tcell is easier to find its own therapeutic targets from the peculiar receptor configuration. This exclusive feature facilitates gene-therapy. Similarly, this morphology can be applied to the CORMats development for special tissue selectivity. Numerous CORMats with their biological significance are described in Table 2.


**Table 2.** Conjugate strategies for therapeutic CO release.

Briefly if the above intensive research discussion is summarized; it is evident that Ru-MCCs and Mn-MCCs are the right choices for nano-medicine due to bio-compatibility and tremendous prescribed feasibility analysis especially Ru-MCCs due to its lessened toxicity. Scrolling down from micellization to nanodiamond, all CO-prescriptions have CO liberation capability, but none of them could be claimed as safe therapeutic managemen<sup>t</sup> and can't be directly applied for exogenous CO-prodrug. During the CORMats administration, rather than focus on the exploration of new advanced materials, the researchers might be considering already existing pharmaceutical materials. A pharmaceutical drug like substance such as crystalline smectite clay; is one of the promising biocompatible and pharmaceutical composites, which could be transformed into CORMats after proper formulation and careful administration. There are two types of strategies that have been introduced for the CORMats production. One is exploiting from the already developed CORMs with biocompatible materials as CO

carriers. Another analogy is captured from the CO moiety in the vicinity of material specific akin to MOFs [138]. Already developed CO incorporating strategies are generalized in Table 3.


**Table 3.** Summarize the CO-releasing substrate along with their association.

#### **3. CO-Releasing Kinetic Profile**

CORMs and CORMats must have a CO utility to deliver in response to the biological system soon after trigger. The specified trigger plays a decisive role for therapeutic applications. Their kinetics is highly dependent on the trigger facility at which they are applied for. The CO discharging rate was exclusively committed for searching the affected sites of selected targets. The half-life (*<sup>t</sup>*1/2) of CORMs/CORMats (*<sup>t</sup>*1/2 is defined in time duration as half of the introduced CORMs/CORMats amount will be dis-integrated) is the key parameter for examining the CORMs/CORMats stability and sustainability. The fast CO-releasing rate is difficult to attain predetermined clinical objectives.

Just an illustration [170], the half-life (*<sup>t</sup>*1/2) of CORM-3 is 3.6 min only when anticipated with the human plasma. At that moment, CORM-3 dissolves in plasma configuration and suddenly reacting with albumin, supplies CO2 and Ru(CO)2 segment; and also makes an alliance with protein in vivo circulation, where CO serves slowly and nonspecifically [171]. Non-technical CO release is unable to deliver the necessary pharmaceutical features. Likewise slow and fast CO release molecules would be engineered to accommodate the distinct clinical trials (Figure 28).

The half-life of CORM-1 and CORM-2 is about 1 min in PBS (*phosphate bu*ff*ered saline*) at 37 temperature with pH~7.4 [37,172]. Such types of half-lives are considered very short intervals. The CORMs and CORMats deliberation must be regulated along with integral body fluids in order to communicate with victim organs and/or tissues before CORMs/CORMats (as CO-producer) consume entire CO quantity [173]. To improve the sustainability of CO carriers, the half-life (*<sup>t</sup>*1/2) should be extended for few minutes, but somehow few seconds and milliseconds extension will be more beneficial. A different designated strategy has promoted the transient CO releases. These mechanisms will be observed through ion-channel kinetic studies [174]. The extended pharmacokinetic qualities containing nanomaterials (NPs) and macromolecular models could be exploited for the managemen<sup>t</sup> of CO transporters or CO carriers.

**Figure 28.** The CO-releasing rate profile reflects the different characteristics: (**a**) Slow CO release has therapeutic significance; (**b**) fast CO release demonstrates the path of ion-channel kinetics.

#### **4. CORMs**/**CORMats Cytotoxicity and Tissue Accumulation**

In addition to CORMs and CORMats pharmaceutical advantages, it delivers some adverse effects too because of their toxicological profile or even proliferation of toxic metal residues (i-CORMs/i-CORMats) resulting soon after CORMs/CORMats launch the CO into the biological environment [95,175–177]. Subsequently, a particular deficiency of CORMs/CORMats is usually observed after the CO excretion; their CO-missing analogues tend to prevail in situ administration. Therefore the transition-heavy metal core usually harbors cofactors and is involved in some uncontrolled reactions/activities with neighboring tissues/cells, thereby contributing serious cellular impairment (Figure 29).

Wang et al. studied the in vivo toxicity, cytotoxicity, metabolism and bio-distribution of two carbonyl metal CORMs series including Ru(CO)3ClnL and M(CO)5L (M= Cr, Mo, W) [178]. The cytotoxic effect was monitored on murine macrophages through MTT colorimetric assay with respect to IC50 and LD50 values; the severely damaged kidney and liver were observed to picture both morphological and functional aspects. The cell culture RAW264.7 was incubated with CORMs/CORMats while examining cytotoxicity and demonstrates their bactericidal activity against a variety of microbes, including *Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa* [126,127]. In this study, they found the uneven distribution of metal complexes in organs and tissues that subsequent damage through metal ions oxidation such as Ruthenium complexes. It was oxidized from RuII to RuIII by P450 enzymes. This toxicity issue was elucidated by Winburn et al., they also performed the CORM's toxicological profile [179]. CORM-2 and its depleted form i-CORM-2 were studied and compared in two kidney cell lines (MDCK and HeK lines) and primary rat cardiomyocytes. This study explained

that the CORM-2 cytoprotective concentration (<20mM) is approaching to cytotoxic value (>100 mM). Moreover, both CORM-2 and i-CORM-2 exerted the cellular toxicity by means of the abnormal cell cytology, cell cycle arrest, reduced cell viability, increased apoptosis and inhibited mitochondrial enzyme activity [136]. These particular consequences were observed through the metal-core mediated toxicity. Different studies have also been explored that intensifying the polarity of CORMs/CORMats would be possibly limiting their penetration over the cellular membrane, and thus attenuating their toxicity [136].

**Figure 29.** The CORMs/CORMats integration exhibited the cytotoxicity of metal residue (i-CORMs/CORMats).
