# An Updated Review of Smac Mimetics, LCL161, Birinapant, and GDC-0152 in Cancer Treatment
## 4 Anti-Cancer Molecular Mechanism of LCL161  
LCL161 is an orally bioavailable monovalent Smac mimetic developed by Novartis Pharmaceuticals. LCL161 shows both pro-apoptotic and anti-proliferation effects in  
Figure 2: Chemical structure of Smac mimetics for cancer treatment. BV6 was modified with trifluoroacetic acid (xCF3 COOH) to improve the solubility of BV6 to DMSO and water.  
cancer cells [47]. In many preclinical studies, the anti-cancer potency of LCL161 has been established in multiple myeloma, glioblastoma, hepatocellular carcinoma, oral squamous carcinoma, neuroblastoma, osteosarcoma, sarcoma, triple-negative breast cancer (TNBC), leukemia, cervical cancer, non-small cell lung cancer, and head and neck cancer cells [24; 54; 55; 56]. At the molecular level, LCL161 binds to the BIR3 domain of cIAP1 and cIAP2 with high affinity, and induces cIAP1 and cIAP2 autobiquitination and proteasome degradation, resulting in the activation of non-canonical NF-\(\kappa\)B signaling pathways and production of tumor necrosis factor \(\alpha\) (TNF-\(\alpha\)) (Figure 3) [57]. LCL161 also induces apoptosis and necroptosis through the RIP1/RIP3/MLKL signaling pathway (Figure 3) [58; 59]. Surprisingly, a few studies showed that LCL161 does not induce apoptosis, but instead it induces G2/M phase arrest by downregulating cIAP1 and activating the p21 signaling pathway in medulloblastoma and neuroblastoma cells (Figure 3) [60; 61]. Moreover, it has been shown that LCL161 does not kill cancer cells directly, but it promotes the activity of immune cells and enhances cytokine secretion instead. Mechanistically, LCL161 promotes the activity of T cells by inducing cytokine secretion and dendritic cell maturation (Figure 3) [62; 63; 64]. In multiple myeloma, cIAP1 and cIAP2 are usually deleted, but LCL161 still inhibits the growth of multiple myeloma through the upregulation of tumor cell autonomous type I interferon signaling, and strong acute inflammatory signaling (promoting macrophages, M-spike, and dendritic cell maturation) in transgenic Vk*MYC mice [65]. Recent studies demonstrated the anti-cancer potency of LCL161 is correlated with the protein expression levels of B-cell lymphoma 2 (BCL-2) and TNF-\(\alpha\) and NF-\(\kappa\)B activity in cancer cells. In neuroblastoma, [PERSON] et al. suggested that the anti-cancer potency of LCL161 is based on the cells' sensitivity to TNF-\(\alpha\) and modulation of NF-\(\kappa\)B [61]. Interestingly, in our recent study, we found that LCL161 interacts with p-glycoprotein (also called multidrug resistance protein 1 (MDR1)) and inhibits the MDR1 multi-drug efflux activity, resulting in increased sensitivity to the MDR1 substrates, such as paclitaxel and YM155 in cervical and bladder cancer cells [66]. Moreover, we found that LCL161 downregulates the protein expression level of survivin in MDR1-expressing cervical and bladder cancer cells [66].  
Figure 3: Anti-cancer effects and the molecular mechanisms of action of LCL161 (black) and birnapant (blue) in cancer cells.  
Combination therapy of LCL161 with chemotherapeutic drugs such as paclitaxel and gemcitabine has been evaluated in patients with cancer (Table 1). [PERSON] et al. found that (1) LCL161 combined with paclitaxel can improve the pathologic complete response of TNF-\(\alpha\)-positive TNBC, (2) the expression level of TNF-\(\alpha\) might be a biomarker for predicting the anti-cancer effects of LCL161 and paclitaxel combination treatment, and (3) the anti-cancer potency of this combination is not correlated with the alternation of breast cancer type 1 susceptibility protein (BRCA1) and breast cancer type 2 susceptibility protein (BRCA2) in TNBC [75]. Some adverse effects were observed during combination treatment, including pyrexia (5 of 209 patients studied), pneumonia (4 of 209 patients studied), and pneumonitis (4 of 209 patients studied).