**5. PDT CRC clinical challenges**

*Multidisciplinary Approach for Colorectal Cancer*

Porfimer sodium (PII) and 2-[1-hexyloxyethyl]-2-

Tetra-α-(4-carboxyphenoxy) phthalocyanine zinc

*In vivo* **PDT CRC research**

butylimide methyl ester

Metalloporphyrin Ga-4cisPtTPyP (5,10,15,20-tetrakis{cis-diammine-chloroplatinum(II)}(4-pyridyl)-porphyrinato gallium(III) hydroxide tetranitrate)

Porphyrazine platform with gadolinium (III) cation chelated by tetrapyrrole macrocycles (GdPz1 and GdPz2)

Bacteriochlorin analogues: 3-(1′-butyloxy) ethyl-3-deacetyl-bacteriopurpurin-18-N-

devinylpyropheophorbide-a (HPPH)

**Photosensitizer Remarks Ref.**

Protoporphyrin IX (PpIX) Enhanced the apoptosis in HCT116 CRC cell line

Sulfonated zinc phthalocyanine (ZnPcSmix) Within CRC DLD-1 and CaCo-2 cells the PS

δ-aminolevulinic acid (ALA) CRC cell lines SW480 and SW620 were treated

Hydrophilic bacteriochlorin (F2 BOH) PDT enabled long-term cures of BALB/c

Photosan-II (PS-II) and chloroquine Significantly reduced the tumor size in a

Redaporfin Single dose was well tolerated by male BALB/c

tumor immunity.

carcinoma cells.

tumor inhibition.

PDT controlled metastatic tumor growth in murine colon 26-HA cells and enhanced anti

localized in multiple organelles and noted significant apoptotic PDT induced cell death.

Noted interaction between p38 MAPK and caspase-9 regulated mitochondria-PDT mediated apoptosis in LoVo human colon

in sublethal doses with ALA PDT in hypoxialike conditions with cobalt chloride and noted decreases release of VEGF and significant

High tumor uptake and long-term cure within BALB/c mice bearing Colon 26 tumors.

mice with subcutaneously implanted CT26 tumors, and the cured mice rejected tumor re-inoculation 1 year after the treatment.

xenograft mice model and induced apoptotic and autophagy cell death within *in vitro*

Selective *in vivo* accumulation within murine colon carcinoma CT26 models was observed, with significant inhibition of tumor growth.

mice with subcutaneously implanted colon (CT26) tumors and PDT led to the complete tumor regression in 83% of the mice.

High tumor accumulation and almost completely inhibited tumor growth over 2 weeks in BALB/c mice bearing Colon 26

SW620 and HCT116 cells.

[52]

[53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

[62]

**48**

**Table 1.**

reoccurrence [35].

small-molecular inhibitor system capable of blocking any tumor survival pathways post PDT, in order to halt possible tumor reoccurrence [35]. However, in relation to fourth-generation PSS this form of PDT treatment research is limited to only being able to target and inhibit VEGFs, in order to promote PS drugs uptake and so deter the neovascularization of tumors, preventing CRC tumor metastatic spread and

*Current PDT studies which utilize different types of PS for the in vitro, in vivo or clinical treatment of CRC.*

tumors.

At the moment clinically FDA approved first and second generation PSs for PDT oncology include: Porfimer sodium (Photofrin), 5-Aminolevulinic acid

Despite the many positive features of CRC PDT, within clinical settings this form of treatment has experienced some drawbacks in relation to PS drug solubility, mode of delivery and selective tumor uptake [64, 65].

In order to ensure the maximum levels of ROS are generated during a PDT treatment, as to ensure complete tumor destruction, the highest possible concentrations of PS drugs must be able to be successfully delivered and localize in target tumor tissues [27]. Within PDT clinical settings using first and second generation PS drugs, poor outcomes and effectiveness has been noted, as only minor amounts of PS drugs are able to overcome the human bodies biological barriers and so passively accumulate (due to the EPR effect) in tumor cells, generating very low levels ROS and tumor destruction [2, 31]. Additionally, due to this passivation process sometimes PS drugs can accumulate in healthy tissues inducing unwanted PDT side effects such as patients' photosensitivity and damage to normal tissues [26].

### **Figure 3.**

*Passive PS NP drug delivery versus active targeting moiety conjugated PS NP drug delivery, which shows targeted and enhanced CRC tumor PS drug uptake for more effective PDT treatment outcomes.*

Another issue sometimes noted in clinical settings is that PS drugs have limited solubility and so tend to aggregate during administration, limiting their overall uptake and effectivity [2]. Moreover, a PS drugs concentrated subcellular localization in a tumors mitochondria, lysosomes, endoplasmic reticulum, plasma membrane etc., is of utmost importance since ROS have only a very short half-life and so will only induce effective cell death in tumor cells if they are proximately localized within these organelles [29, 32].

Thus, shortcomings such as poor solubility, bioavailability, maximum ROS generation and tumor subcellular localization targeting need to be overcome in order to ensure the effectivity of PDT [26]. Nevertheless third generation PS drug nanoparticle (NP) drug carriers are currently being investigated to ensure PS drug solubility and improved passive uptake, with functionalized active targeting abilities (e.g. overexpressed peptides), as to ensure specific uptake in tumor cells only to enhance the overall efficacy of PDT (**Figure 3**) [29, 32].
