**5. TLRs modulation in cancer treatments**

TLR agonists play an important role in activation of immune system, both innate and adaptive. In *in vivo* models, TLR antagonism have been shown to reduce tumor growth in treatment group, receiving combination of therapeutic agents, such chemotherapy drugs, monoclonal antibodies (mAb), subunit or DNA vaccines [47, 48]. The selection of TLR agonists has been premised on their ability to activate antigen-presenting cells (APCs), particularly DCs. The involvement of specific TLRs on cancer cells may impact tumor growth by various mechanisms, such as inducing apoptosis and potentiating the effects of chemotherapy [45]. Inhibition of TLRs can be achieved either by 1) Preventing the interaction between TLR

and respective binding partner and 2) By blocking the interaction between TLRs and respective adaptor protein. TLR inhibitors can be broadly classified into (i) Small molecule inhibitors, (ii) Antibodies, (iii) Oligonucleotides, and (iv) Lipid-A analogs. The following section illustrates the anticancer effects of inhibiting TLR signaling pathways on tumor growth and developments.

**Small molecule inhibitors (SMI):** These are synthetic or naturally derived small molecules with the ability to cross plasma membranes due to their small size and amphipathic nature. Interestingly, one of the most commonly used antimalarial drug chloroquine has been shown to possess inhibitory effects against endosomal TLR7/8/9 [49]. Inhibition of TLR7 and 9 by chloroquine inhibits the growth of hepatocellular carcinoma in both cellular *in vitro* models and mouse xenograft models via down regulation of p-Akt [50]. However due to their non specific mode of actions continuous efforts have been made to develop more efficient and specific derivatives of chloroquine as anti-cancer agents by targeting TLRs. One such derivative, CpG-52,363 has immunosuppressor functions but its role in cancer therapeutics is yet to be discovered [51]. SM934, a derivative of another anti-malarial drug artemensin can inhibit the proliferation and metastasis in breast cancer probably via inhibition of TLR signaling [52]. TAK-242 specifically inhibits TLR-4 by binding to cysteine 747 in the intracellular domain and consequently suppresses the progression of breast cancer [53]. Therapeutic role of SM934, another artemensin derivative has been well documented in inflammatory disease [54], but its role in cancer prevention is still need to be explored.

**Antibodies:** Various antibodies with therapeutic potential have been raised against TLRs to treat a wide spectrum of inflammatory diseases and cancer. Therapeutic role of OPN-305, the first fully humanized IgG against TLR2, against Myelodysplastic Syndromes (MDS) has been reported in different clinical trials [55]. Several antibodies have been developed beside OPN-305, like NI-0101and T2.5, but their role in cancer has not been determined yet [56].

**Oligonucleotides:** Specific nucleotide sequences are known to inhibit the function of endosomal TLRs by blocking their binding with respective ligands. These includes immunoregulatory DNA sequence (IRS) 661 (TLR7 specific), IRS-869 (TLR9 specific) and IRS-954 (both TLR7 and 9 specific). Recent report suggest that TLR antagonism using immune modulator oligonucleotide-3100 (IMO-3100) can serve as a potential therapeutic for the management of pancreatic cancer associated cachexia [57].

**Lipid A analogs:** Eritoran, a syhnthetic analogue of lipid A from *Rhodobacter sphaeroides,* is known to inhibit TLR4 by binding to MD2 pocket and thereby preventing the interaction between TLR4 and lipid A. Bacterial LPS induced colon cancer can be prevented by the administration of Eritoran by mechanism involving inhibition TLR4 and induction of CD14/Src/PKCζ-mediated apoptosis [58].

It is important to mention here that TLR acts as double edged sword and its agonism can also prevents the progression of cancer by activating the immune response against cancer cells. The following section describes TLR agonists which had shown the potential to prevent cancer progression.

Calmette–Guerin strain (BCG) a live-attenuated *Mycobacterium bovis* can activate TLR2, TLR4, and TLR9. The activation of TLR in urothelial cell carcinomas with BCG induced cell death and decreased proliferation as well as metaststais. The anti-cancer effects of BCG have been associated with increased production of cytotoxic NO in cell lines, as well as in patients [46]. These studies also emphasize the development of vaccination strategies that incorporate TLR ligands to stimulate immune responses and make cancerous cells specific targets for immune system

#### *Role of Toll-Like Receptor (TLR)-Signaling in Cancer Progression and Treatment DOI: http://dx.doi.org/10.5772/intechopen.94423*

mediated death. In human colon cancer cells, TLR3 activation with Polyriboinosinicpolyribocytidylic acid (Poly I:C) can induce apoptosis alone or when used in synergy with 5-fluorouracil or IFN-α [16]. Poly I:C is a synthetic analogue of viral dsRNA. The expression of TLR5 on cancer cells has been shown to revoke cell growth in certain types of cancer [16]. For instance, in breast cancer, when TLR5 is over–expression with flagellin inhibits tumor cell proliferation and downregulates expression of cyclin B1, cyclin D1 and cyclin E2 in a murine model [59].

Irradiation along with activation of TLR9 signaling pathway in human glioma cell line can decrease cell proliferation by arresting cell-cycle, which is mediated by NF-κB and nitric oxide (NO) [60]. This therapeutic effect could be used to sensitize the cancerous cells to the toxic effects of radiation treatment [61]. Also, CpG-island mediated activation of TLR9 in neuroblastoma cell has been revealed to decrease cell proliferation and increase caspase-dependent apoptosis and leads to an increased survival in tumor-bearing mice. Several TLR agonists have been approved by the food and drug administration (FDA) for use in the treatment of cancer patients like BCG (which activate TLR2, TLR3, TLR4, and TLR9), MPL (TLR4 agonist) and imiquimod (TLR7 agonist) [62]. TLR agonists should be used in combination with other agents to synergistically increase their immune stimulatory response. An important TLR modulators are summarized in **Table 1** which having anticancer activity.

### **Future direction of TLRs based treatment of cancer:**

In this book chapter, we summarized the role of TLRs signaling in inflammation, cell proliferation, apoptosis and chemo-resistance, which are the major attributes of cancerous cells. Beside these, several TLRs agonists and antagonists have been developed and/or are in clinical trials as cancer therapeutics. TLRs play a critical role in imparting immunity against tumor, and their antitumor effects are noticeable as depicted from previous studies. It is quite interesting to note that activation of same TLR in one tumor type might induce cell death, and in a different tumor could exert pro-tumor effects. Using TLR agonists or antagonist as cancer therapeutics must be decided on the basis of TLR expression profile of tumor cells and resulting response within a specific cancer type [19]. The prospective approach for future cancer treatment will be the combination of specific TLR agonists or antagonists with traditional cancer treatments to improve treatment outcome. The role of TLRs in both promoting and inhibiting tumor growth and metastasis has been confirmed in various studies. However, the specific mechanism of action is still unclear as cancer is a multifactorial\disease, and the research of TLRs on tumor immunity is still in the nascent phase. Further in depth studies will help us to develop better


#### **Table 1.**

*Different TLR modulators having anticancer activity.*

understanding of TLRs role in tumorigenesis, tumor immunity, and tumor metastasis which in turn can provide new strategies and prospects for more effective cancer management. We anticipate that future studies on the role of TLRs in cancer progression and development will provide us a better insight into the mechanisms underplaying. Therefore, understanding the roles of TLRs in tumor biology may pave the way for the discovery of novel therapeutic targets in cancer therapy.
