| Somatostatin(SST) and the somatostatin analogue(SSA) are small regulating polypeptide inhibiting the action of the growth hormone(GH), SST are distributed universally in human body, not only in hypothalamus and GH secreting pituitary gland, but also in tissues that are not involved in GH secretion. SST has multiple physiological and pharmaceutical actions, studies suggested that SST/SSA delivered antiproliferation function to tumor cells. All these functions are achieved through mediation of somotastatin receptor (SSTR) on cell membrane. High level of SSTR has been observed inAPUD, neuroendocrinal tumors, including small cell lung cancer. Since 1990's, 5 subtypes of SSTR have been cloned.Recently, the regulation of SSTR by SST/SSA has been the hot point in this field. Lamberts et al reported first of all that the biological efficiency of sst-14, a agonist of SSTR, displayed a time-dependent fashion, the agonistic efficiency at early stage could be replaced by desensitization due to subsequent SST-14 administration, which was related to internalization and phosphorylation of SSTR. Hukovic et al transfected CHO-K1 with SSTR1-5 to study regulation rule of subtypes of SST-14, SST-28, and suggested that the dynamic regulation included internalization and upregulation. Except SSTR1, other subtypes could be internalized to different extent, after 15-60 min exposure to SST; Obvious upregulation was observed at 22h. The internalization and upregulation of SSTR induced by SSA can enhance the targeted binding efficiency of SSA to SSTR, extend action time of radio nuclide in tumors, so as to improve efficiency of targeted radiotherapy. Up to date, studies on regulation of SSTR are limited to in vitro studies of transfected cells and solid tumor cells. In this paper, we investigated the regulation of SSTR by RC-160 on SCLC cell line in vitro to provide basis for design a protocol of intervenient enhancement of targeted tumor inhibition.SSA has better anti-degradation ability than SST, so multiple SSA have been developed in abroad several laboratories. Among these, RC-160 has excellent physiochemical and biological properties, so it is the first choice in this study.We investigated the upregulation and internalization of SSTR on SCLC cells, NCI-H446, induced by RC-160, to provide theoretical basis for increasing the SCLC uptake of radionuclide, and extension of action time of radionuclide in tumor, and enhancement of tumor inhibition function. We also explored the biodistribution and imaging features of SSTR in SCLC bearing nude mice by radio labeled RC-160. The above studies may provide guidance for preclinical study of SSTR mediated targeted radiotherapy.Methods1,125I-RC-160 was prepared with Ch-T method, 188Re-RC-160 and 99Tcm-RC-160 with stannous tartrate method.2, Binding character of 125I-RC-160 with NCI-H446 and RC-160 induced internalization and regulation of SSTR on NC1-H446 were analyzed with receptor radio assay in vitro determined.3, Primary experiments on biodistribution of 125I-RC-160 and receptor radio imaging of SSTR in normal mice and tumor-bearing mice were performed according to methods reported by ZAMORA.P.OResults1, After purification, labeling efficiency and specific activity of 125I-RC-160 were 92.0%and 1.85TBq/mmol/L respectively; Labeling efficiency of 188Re-RC-160 was 96.2%, radiochemical purity at 24h after labeling at room temperature in ascorbic acid group was 85.0%: Labeling efficiency of 99Tcm-RC-160 was 84.0%, radiochemical purity at room temperature after 4h was more than 80.0%.2, The appropriate binding condition between I25I-RC-160 andNCI-H446 cells obtained at 37 C and 60min; In vitro receptor radio analysis revealed that binding efficiency of 125I-RC-160 and NCI-H446 cells augmented with increase of radiolabeled ligand, and reached saturation finally. Scatchard analysis of 125I-RC-160 and NCI-H446 revealed a high affinity (Kd=1.71+10-10mol/L, Bmax=1.06 +105/cell). The IC50 of RC-160 was 1.58+10-3mol/L in competitive binding assay.3,...
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