Investigation On Targeted Therapy Of Small Cell Lung Cancer Cells Mediated By Somatostatin Receptor

BACKGROUND:Lung cancer is the leading cause of cancer related deaths in the world and 20% of which is small cell lung cancer (SCLC). Chemotherapy and radiotherapy are the main choice of therapy for SCLC because SCLC patients have either locally advanced disease or distant metastases. The discovery of specific receptors for peptide hormones on cancer cells has led to the development of cytotoxic hormone analogs that can be used for tumor targeted therapy. The somatostatin receptor (SSTR) is one such target. SCLC express SSTRs and many studies have suggested that these receptors are present in greater numbers in tumors than in normal tissues. Targeted chemotherapy can be based on somatostatin analogs (SSA), which will directly deliver the cytotoxic agents to tumor cells expressing SSTR.OBJECTIVE:We evaluate the inhibitory effect and targeting ability of paclitaxel-octreotide conjugates on H446 small cell lung cancer (SCLC) cells for the purpose to explore the effects of somatostatin receptor in the therapy in SCLC.METHODS:PTX-OCT, PTX-Tyr-OCT,2PTX-OCT and 2PTX-Tyr-OCT were synthesized by coupling paclixtel (PTX) to octreotide (OCT). And real-time PCR was used to detect the expression of mRNA for somatostatin receptor subtypes (SSTRs) in fibroblasts and H446 cells. The MTT assay was used to investigate the antiproliferative effect of paclitaxel and the conjugates at different treatment concentrations and for different periods. Cell apoptosis and cell cycle perturbations were analyzed by flow cytometer.RESULTS:H446 cells were tested for the expression of mRNA for SSTR1, SSTR2, SSTR3, SSTR4 and SSTR5. SSTR1 had the highest level of expression, followed by SSTR5. The mRNA expression of all five SSTRs was; SSTR1:2.65±0.34×10-2, SSTR2:9.25±1.58×10-5, SSTR3:2.42±0.23×10-4, SSTR4:4.26±0.75×10-5, SSTR5:7.24±1.26×10-3. No SSTR mRNA was detected in the fibroblasts. The conjugates had a similar inhibitory effect to paclitaxel, they suppressed the growth of H446 cells in a dose (1-1000 nmol/L) and time (24 h,48 h and 72h)-dependent manner. Following 72h exposure to 1000 nmol/L of PTX, PTX-OCT, PTX-Tyr-OCT, 2PTX-OCT or 2PTX-Tyr-OCT, the viability of H446 cells was 31.7±5.9%,37.3±6.2%,35.7±2.3%,29.3±7.8%,34.2±4.3%. The viability was lower than that after 24h treatment. The viability of fibroblast cells were 85.2±2.8%,84.8±1.6%,90.3±4.6%,90.5±3.8%, or 55.1±3.2% after being treated with 1000 nmol/L of PTX-OCT, PTX-Tyr-OCT,2PTX-OCT,2PTX-Tyr-OCT or paclitaxel. The inhibitory effect of the conjugates on fibroblast cells was significantly lower (P<0.05) than that of paclitaxel. After exposure of H446 cells to 1000 mol/L of PTX-OCT, PTX-Tyr-OCT,2PTX-OCT, 2PTX-Tyr-OCT for 24 h, the percent of cells in G2/M increased from 14.56±1.86% in untreated H446 cells to 74.58±4.31%,73.95±5.72%,76.13±1.84%,75.46±2.72% (P<0.01). This shows that the conjugates arrested H446 cells at G2/M phase.CONCLUSION:Paclitaxel-octreotide conjugates effectively inhibited the growth of SSTR-positive SCLC H446 cells in a dose-and time-dependent manner. The conjugates were less cytotoxic than paclitaxel in SSTR-negative fibroblasts, which showed their SSTR-mediated targeted therapy effect. Because SSTRs are more prevalent in tumors than in normal tissues, our results suggest that the paclitaxel-octreotide conjugates are a prospective SSTRs targeted therapy for treating human SCLC.