**5. Conclusion**

74 Biomedicine

that is overexpressed on the tumor cell membrane, thereby triggering the entry of the nanoparticles into tumor cells through receptor-mediated endocytosis. The target gene is ribonucleotide reductase subunit 2 (RRM2). After systemic administration by i.v. injection, siRNA efficiently inhibited tumor growth by down-regulating RRM2 expression (Davis,2009). It was also found that the levels of RRM2 mRNA and RRM2 protein were both decreased in the tumor. To detect the efficiency of RNAi, researchers found that there existed large amounts of RRM2 mRNA degradation products at tumor site containing the nanoparticles(Davis *et al,* 2010). This is the first clinical trial on RNAi medical treatment of

SiRNA agent Company/Institution Target Disease Clinical

ALN-VSP02 Alnylam KSP, VEGF hepatocarcinoma Phase II

HIV

Alnylam Pharmaceuticals (Alnylam Pharmaceuticals,2011a,2011b) conducted Phase I clinical trial by intravenous administration of ALN-VSP02 for the treatment of HCC, where the siRNA therapeutic was enveloped in liposome. A total of 58 patients were recruited in the study and administered with the therapeutic at an interval of two weeks. The therapeutic agent targets kinesin spindle protein (KSP) and VEGF that are highly expressed in tumor cells. KSP plays a key role in cell mitosis and is the dynein necessary for bipolar spindle formation and maintenance. Inhibition of KSP would arrest the cell cycle. VEGF promotes angiogenesis, and therefore inhibition of VEGF would suppress tumor growth. Currently, Phase I clinical trial with ALN-VSP02 has been done, and Phase II clinical trial is to be

Recently, Silence Therapeutics has started Phase I clinical trial on Atu027(Aleku et al, 2008; Santel et al, 2010). Atu027 is a lipid-based siRNA complex containing target protein kinase N3 (PKN3), which participates in tumor occurrence and plays an important role in tumor metastasis. Atu027 can inhibit tumor progression by silencing the expression of PKN3. Preclinical experiments demonstrated that Atu027 could prevent breast cancer cells from

Tat & Rev from

Trials

Phase I

Phase I

Single patient

Phase I

Phase I

Phase I

RRM2 Solid tumor Phase II

AIDS-related lymphoma

carcinoma

leukemia

leukemia

melanoma

cancer

Bcr-able Chronic myeloid

Bcr-abl Chronic myeloid

Proteasome Metastatic

malignant tumors.

Anti-tat/rev shRNA

BCR-ABL siRNA

siRNA

immunotherapy

started in 2012.

CALAA-01 Calando

Pharmaceuticals

University of Duisburg-Essen

Organization

Hospital

Duke University

Table 1. A summary of current siRNA agent for clinical trials

SV40/BCR-ABL Hadassah Medical

Benitec&The city of Hope National Medical Center

Atu027 Silence Therapeutics PKN3 Pulmonary

FANG Gradlis, Inc. Furin Advanced

The understanding of signaling pathways participating in the development and progression of malignant tumors and the development of specific target RNAi technique have offered hope to conduct individualized regimens for tumor treatment. However, current RNAi therapeutic approaches are immature and many problems need to be tackled, such as targeting and off-target effects, drug delivery systems, drug administrations and safety. As siRNA with a sequence of only 20-nucleotides could induce a tremendous effect of gene silencing, the sequence of siRNA must be selected accurately in the process of designing siRNA sequences to avoid the off-target effect. Additionally, a mRNA target sequence may hide in the mRNA secondary structure or folding region, making it impossible for siRNA to silence the mRNA. It is therefore necessary to conduct experiments repeatedly to select a suitable target site. Before RNAi medical treatment can be used clinically, it is necessary to seek an efficient delivery system and rational drug administration. In addition, as nucleotide sequences used for RNAi have no cell targetability and are likely to degrade, it is necessary to design and synthesize a drug delivery system that is non- or low toxic to normal cells and can deliver siRNA to tumor cells efficiently.
