As already said, neoplastic cell growth and proliferation is driven via PI3K, Akt, mTOR and MAPK pathways after IGF-I binding to IGF1R [203]. The primary strategy in treating colon cancer includes the arrest of IGF1R overexpression by small inhibitors, antibodies or the inhibition of its ligands. A discovery of OSI-906, later termed linsitinib, a drug with a promising inhibitory effect on two receptor kinases, IR and IGF1R, was reported in 2009 [204]. This dual inhibitor intervenes in the process of autophosphorylation, demonstrating an antiproliferative effect on different tumor cell lines, including colorectal. One possibility to treat metastatic CRC is to use regorafenib (multi-kinase inhibitor) together with linsitinib and aspirin (both being IGF1R inhibitors) [205]. Linsitinib and aspirin reduce the resistance of CRC cells to regorafenib, enabling body weight gain and an increase in the survival rate of model animals (i.e., male mice). Leiphrakpam et al. reported on the down-regulation of X-linked inhibitor of apoptosis (XIAP) in CRC xenografted male mice driven by MK-0646 (mAb that blocks IGF1R and IGF2R, also known as dalotuzumab) and linsitinib [206].

An interplay of various factors keeps cancer cells alive. Hyperactivation of IGF1R, for example, results in the resistance to epidermal growth factor receptor (EGFR) inhibition in RAS wild-type metastatic CRC via up-regulation of PI3K/AKT pathway, implying that targeting both receptors might be a promising therapy for metastatic CRC. Unfortunately, a combination of cetuximab (mAb targeting EGFR) and MK-0646 or IMC-A12 (mAb that blocks IGF1R, also known as cixutumumab) did not lead to the expected results, as suggested by two studies both involving male and female participants [207,208]. A phase I trial employing a combination of cixutumumab (anti-IGF1R antibody) and selumetinib (MEK ½ inhibitor) obtained promising results offering the evidence of the health benefit and target inhibition in a cohort of 30 patients, including those with CRC [209]. A combination of ganitumab (IGF1R mAb) and conatumumab (a pro-apoptotic death receptor 5 agonist) also exerted promising effects in the Colo-205 xenograft model, but caused no response in approximately 80 tested individuals (both male and female), some of whom were patients with CRC [210].

MEDI-573 (mAb to IGF-I and IGF-II) application inhibited tumor growth in a CRC female mouse model that over-expresses IGF-II, and this effect was enhanced if MEDI-573 was combined with other known therapeutics such as trastuzumab, AZD2014, AZD5363, selumetinib or cetuximab [211]. AvFc lectibody, a combination of the human immunoglobulin G1 Fc and Avaren (high mannose glycan-binding lectin), selectively recognizes a range of cancer cell lines including colon cells [212]. Although the observed cytotoxic effects were thoroughly investigated only in the lung cancer cell lines, it was confirmed that targets of this lectibody are EGFR and IGF1R. Since aberrant glycosylation is a hallmark of cancer, and altered glycosylation of IGFRs in the colon tissue of CRC patients was reported [64], the lectibody treatment may be considered as a novel approach in cancer therapy.

IGF1R depletion/inhibition sensitizes CRC cells to radiotherapy (converting them to radiosensitive), as shown in HT-29 and SW480 cell lines where IGF1R was inhibited by NVP-ADW742 [213] and in HT-29, SW480 and DLD-1 cells pretreated with BMS-754807 [214]. The inhibitory effect of BMS-754807 on colon cancer cell growth was stronger compared to the effect of linsitinib, and the anti-neoplastic effect was mostly independent of IGF1R [215]. A prolonged treatment of colon cancer cells with BMS-754807 and GSK1838705A (inhibitors of IGF1R and IR) leads to cell survival, due to the activation of ribosomal protein S6 kinase 1 [216]. In GEO tumors, BI885578 administration (inhibitor of both IGF1R and IR) induces apoptosis and inhibition of cell proliferation in female mice [217]. Zhu et al. reported on an anti-proliferative, anti-migration, anti-invasion and inhibitory effect of isovitexin on EMT in human colon cancer cell lines and xenograft tumor model on female mice [218]. The levels of signaling molecules involved in the IGF1R signaling pathway were decreased after this treatment, pointing to the mechanism of the isovitexin action. Simvastatin down-regulates IGF1R expression and pro-apoptotic ERK activation in human HT-29 cells [219]. Phloroglucinol treatment inhibits or decreases the expression of various IGF1R downstream signaling molecules, such as RAS, MAPK, ERK, PI3K, Akt and mTOR, in HT-29 cells [220].

In addition to synthetic molecules and antibodies, numerous plant metabolites were investigated as potential therapeutics in the treatment of cancer. Curcumol, isolated from Rhizoma Curcumae, inhibited proliferation and induced apoptosis in LoVo cells, and inhibited CRC in xenograft models of nude mice, via inhibition of IGF1R and activation of p38 MAPKs [221]. Application of manuka honey (alone or in combination with 5-fluorouracil) on HCT-116 cell line decreased physical parameters of colonospheres and the survival ability of cancer cells, but also induced apoptosis via down-regulation of apoptosis inhibitors, including IGFs and IGF1R [222]. The expression levels of IGF-I and IGF1R were reduced, whereas the level of IGFBP-3 was normalized after Bifidiobacterium longum BAA-999 (with or without lycopene) was administrated to CD-A male mice in an azoxymethane-dextrane sulfate sodium-induced CRC model [223]. Polypeptides from Arca subcrenata Lischke inhibited growth of HT-29 cells and suppressed tumor growth in male mouse xenograft by reducing IGF1R phosphorylation and inhibiting IGF-I/IGF1R signaling activation [224]. Carnosic acid treatment also suppressed the growth of HT-29 cells; decreased the number of tumors and circulating concentrations of leptin, adiponectin, insulin and IGF-I; and reduced the expression of IR in a male mice model [225]. Fucoidan, sulfated polysaccharide from brown algae, inhibited IGF1R signaling through the IRS-1/PI3K/Akt pathway in HT-29 cells [226]. Laminarin, another polysaccharide from brown algae, decreased phosphorylation of ERK and MAPK and, consequently, IGF1R-dependent proliferation in the same cells [227]. Curcumin decreased the expression of IR and IGF1R in 5-fluorouracil-treated SW480 cells, and this down-regulation correlated with decreased proliferation and migration of cells [228]. Luteolin, a fruit and vegetable flavone, decreased IGF-II production in HT-29 cells and down-regulated the activation of PI3K/Akt and ERK1/2 pathways [229]. Cinnamaldehyde, isolated from stem bark of Cinnamomum cassia, inhibited PI3K/Akt signaling in SW480, HCT116 and LoVo cells [230]. Considering the incidence and mortality rates of CRC, together with limitations of the existing therapeutic procedures, the search for both natural and synthetic anti-CRC agents is expected to intensify.

It is sometimes necessary to apply multiple approaches, i.e., a combination of different therapeutics and therapies. A serious limitation in examining therapeutic potentials of various substances can be found in translation from cell line models to xenografts and further to humans as the results are unsatisfactory or the cancer cells are nonresponsive to just one therapeutic. It must also be noted that the gender equality was neglected in most of the mentioned studies, as some of the experiments were performed only on male or female animals (rarely both). Furthermore, the number of studies on humans is still low, which is understandable due to the limited number of potentially effective therapeutics, their toxicity and effective dose, as well as ethical principles related to studies on humans.