Colorectal cancer (CRC) is the third most common type of cancer and the second leading cause of cancer deaths worldwide. Surgery or surgery plus radiotherapy and/or chemotherapy for patients with metastatic CRC (mCRC) were accepted as the main therapeutic strategies until the early 2000s, when targeted drugs, like cetuximab and bevacizumab, were developed. The use of targeted drugs in clinical practice has significantly increased patients’ overall survival. To date, the emergence of several types of targeted drugs has opened new possibilities and revealed new prospects for mCRC treatment. Therapeutic strategies are continually being updated to select the most suitable targeted drugs based on the results of clinical trials that are currently underway. This review discusses the up-to date molecular evidence of targeted therapy for mCRC and summarizes the Food and Drug Administration-approved targeted drugs including the results of clinical trials. We also explain their mechanisms of action and how these affect the choice of a suitable targeted therapy.

Gold compounds have a long history of use in medicine. Auranofin was developed more than 30 years ago as an oral therapy for rheumatoid arthritis. Now, however, auranofin is rarely used in clinical practice despite its efficacy for treating rheumatoid arthritis because more novel antirheumatic medications are available. Although its use in clinical practice has decreased, studies on auranofin have continued and it shows promise for the treatment of several different diseases, including cancer and bacterial and parasitic infections. Several potential novel applications of auranofin for treating human disease have been proposed. Auranofin inhibits the activity of thioredoxin reductase (TrxR), an enzyme of the thioredoxin (Trx) system that is important for maintaining the intracellular redox state. Particularly in cancers, TrxR inhibition leads to an increase in cellular oxidative stress and induces apoptosis. TrxR overexpression is associated with aggressive tumor progression and poor survival in patients with breast, ovarian, and lung cancers. The Trx system may represent an attractive target for the development of new cancer treatments. Therefore, the TrxR inhibitor auranofin may be a potent anticancer agent. This review summarizes the current understanding of auranofin for cancer therapy.

Prostate cancer shows high expression of type I insulin-like growth factor (IGF-I) receptor (IGF-IR) and prostate stromal cells (PrSC) produce IGF-I. Although high plasma level of IGF-I is a risk factor of prostate cancer, the significance of the prostate stromal IGF-I in the regulation of prostate cancer remains elusive. Here we show that the stromal IGF-I certainly regulates the development of prostate cancer. Coinoculation of PrSC increased the growth of human prostate cancer LNCaP and DU-145 tumors in severe combined immunodeficient mice. The conditioned medium of PrSC, as well as IGF-I, induced phosphorylation of IGF-IR and increased the growth of LNCaP and DU-145 cells. PrSC, but not LNCaP and DU-145 cells, secreted significant amounts of IGF-I. Coculture with PrSC increased the growth of DU-145 cells in vitro but the pretreatment of PrSC with small interfering RNA of IGF-I did not enhance it. Furthermore, various chemical inhibitors consisting of 79 compounds with ∼60 different targets led to the finding that only IGF-IR inhibitor suppressed the PrSC-induced growth enhancement of DU-145 cells. Thus, these results show that the prostate stromal IGF-I mediates tumor-stromal cell interactions of prostate cancer to accelerate tumor growth, supporting the idea that the IGF-I signaling is a valuable target for the treatment of prostate cancer. (Cancer Res 2006; 66(8): 4419-25)

Targeting stroma in tumor tissues is an attractive new strategy for cancer treatment. We developed in vitro coculture system, in which the growth of human prostate cancer DU‐145 cells is stimulated by prostate stromal cells (PrSC) through insulin‐like growth factor I (IGF‐I). Using this system, we have been searching for small molecules that inhibit tumor growth through modulation of tumor‐stromal cell interactions. As a result, we have found that leucinostatins and atpenins, natural antifungal antibiotics, inhibit the growth of DU‐145 cells cocultured with PrSC more strongly than that of DU‐145 cells alone. In this study we examined the antitumor effects of these small molecules in vitro and in vivo. When DU‐145 cells were coinoculated with PrSC subcutaneously in nude mice, leucinostatin A was found to significantly suppress the tumor growth more than atpenin B. The antitumor effect of leucinostatin A in vivo was not obtained against the tumors of DU‐145 cells alone. RT‐PCR experiments revealed that leucinostatin A specifically inhibited IGF‐I expression in PrSC without effect on expressions of other IGF axis molecules. Leucinostatins and atpenins are known to abrogate mitochondrial functions. However, when we used mitochondrial DNA‐depleted, pseudo‐ρ⁰ cells, we found that one of leucinostain A actions certainly depended on mitochondrial function, but it actually inhibited the growth of DU‐145 cells more strongly in coculture with pseudo‐ρ⁰ PrSC and reduced IGF‐I expression in pseudo‐ρ⁰ PrSC. Taken together, our results suggested that leucinostatin A inhibited prostate cancer cell growth through reduction of IGF‐I expression in PrSC.

Recent studies have shown that integrin αvβ3, a receptor for vitronectin, plays an important role in tumor‐induced angiogenesis and tumor growth and that antagonists of αvβ3 inhibit angiogenic processes including endothelial cell adhesion and migration. On the other hand, most inhibitors of integrin αvβ3 are peptide antagonists that include the Arg‐Gly‐Asp (RGD) motif. We therefore reasoned that non‐peptide inhibitors of endothelial cell adhesion to vitronectin might be useful for inhibition of tumor angiogenesis in vivo. We screened for low‐molecular‐weight natural products able to inhibit adhesion of human umbilical vein endothelial cells (HUVECs) to vitronectin, and pyrrothine group compounds including aureothricin, thioaurin and thiolutin were isolated from microbial culture broths. Of these compounds, thiolutin inhibited adhesion of HUVECs to vitronectin the most effectively (IC₅₀, 0.83 μM). In vivo experiments showed that thiolutin significantly suppressed angiogenesis induced by tumor cells (S‐180), a pathological form of neovascularization, in a mouse dorsal air sac assay system. To explore the mechanism of inhibition of HUVEC adhesion to vitronectin by thiolutin, we examined the effect of this agent on intracellular cell adhesion signaling. We found that the amount of paxillin in HUVECs was significantly reduced by thiolutin treatment, while those of other focal adhesion proteins including vinculin and focal adhesion kinase (FAK) were not. Metabolic labeling experiments showed that thiolutin enhanced degradation of paxillin in HUVECs. Protease inhibitors (MG115 and E64‐D) decreased the rate of degradation of the paxillin induced by thiolutin and partially restored thiolutin‐induced inhibition of HUVEC adhesion to vitronectin. Based on these findings, we concluded that thiolutin, an inhibitor of HUVEC adhesion to vitronectin, reduces the paxillin level in HUVECs and suppresses tumor cell‐induced angiogenesis in vivo.