Growing vascular endothelial cells express somatostatin subtype 2 receptors

We hypothesized that non-proliferating (quiescent) human vascular endothelial cells would not express somatostatin receptor subtype 2 (sst 2) and that this receptor would be expressed when the endothelial cells begin to grow. To test this hypothesis, placental veins were harvested from 6 human placentas and 2 mm vein disks were cultured in 0.3% fibrin gels. Morphometric analysis confirmed that 50-75% of cultured vein disks developed radial capillary growth within 15 days. Sst 2 gene expression was determined by reverse transcription-polymerase chain reaction (RT-PCR) analysis of the RNA from veins before culture and from tissue-matched vein disks that exhibited an angiogenic response. The sst 2 gene was expressed in the proliferating angiogenic sprouts of human vascular endothelium. The presence of sst 2 receptors on proliferating angiogenic vessels was confirmed by immunohistochemical staining and in vivo scintigraphy. These results suggest that sst 2 may be a unique target for antiangiogenic therapy with sst 2 preferring somatostatin analogues conjugated to radioisotopes or cytotoxic agents.

Antitumor and antiangiogenic effects of somatostatin receptor-targeted in situ radiation with (111)In-DTPA-JIC 2DL

INTRODUCTION

Expression of somatostatin receptor subtype 2 (sst 2) in angiogenic tumor vessels appears to be homogeneous, while tumor cell expression of this receptor is often heterogeneous. We have developed a novel in vitro three-dimensional tumor angiogenesis model to study the antitumor and the antiangiogenic effects of radiolabeled somatostatin analogs. We hypothesized that targeted in situ radiation with an Auger electron-emitting radiolabeled somatostatin analog would produce receptor-specific cytotoxicity in sst 2-expressing cells.

MATERIALS AND METHODS

IMR-32 human neuroblastoma (sst 2-positive) and MDA MB-231 human breast cancer (sst 2-negative) xenografts were created in nude mice from monolayer cell cultures. Fragments of these tumors were embedded in three-dimensional fibrin gels supplemented with endothelial growth media and incubated for a period of 14 days. Tumor fragments were treated with 50 microCi/ml of (111)In-JIC 2DL, a sst 2-preferring somatostatin analog, or medium on Day 1. Initial angiogenic activity was determined at 48 h and the mean angiogenic score and tumoricidal responses were assessed on Day 14.

RESULTS AND CONCLUSION

Tumoricidal effects of (111)In-JIC 2DL were seen only in sst 2-positive IMR-32 tumors. However, the angiogenic response was inhibited in both IMR-32 and MDA MB-231 tumors independent of the tumor cells' sst 2 status. Somatostatin receptor-mediated in situ radiation therapy has profound cytotoxic effects on angiogenic blood vessels and sst 2-expressing tumor cells.

Synthesis and characterization of multiply-tyrosinated, multiply-iodinated somatostatin analogs

Radio-labeled somatostatin analogs have recently gained popularity as agents useful in intraoperative tumor localization, external scintigraphy and in situ radiotherapy. We have synthesized and characterized a series of novel N-terminally extended multiply-tyrosinated somatostatin analogs that possess high binding affinity for somatostatin receptors, exhibit biological activity comparable to the native peptide and retain these characteristics after iodination. These analogs can be radio-iodinated to high specific activities. Following radioiodination, these analogs exhibit minimal radiolysis and may be clinically useful for tumor localization, scanning and therapy.

Multiply radioiodinated somatostatin analogs induce receptor-specific cytotoxicity

BACKGROUND

Radiolabeled somatostatin analogs have gained popularity for tumor imaging and have recently been used for the treatment of somatostatin receptor-expressing tumors. We have developed a novel, N-terminally extended, multiply iodinated somatostatin analog, 125I-WOC 4a, that we hypothesize will be a useful tool for the detection of and therapy for somatostatin receptor-positive tumors. To evaluate the therapeutic potential of this agent, we compared the cytotoxicity of 125I-WOC 4a in a somatostatin receptor subtype-2 (sst 2)-expressing human neurobalstoma cell line to its cytotoxicity in a somatostatin receptor-negative human pancreatic carcinoma cell line.

METHODS

IMR-32 neuroblastoma cells (sst 2-positive) and PANC-1 human pancreatic cells (sst 2-negative) were incubated with 125I-WOC 4a at doses ranging from 0.1-100 CPM/cell for 48 h and cell viability was assessed by a colorimetric (MTT) cell viability assay. Subsequently, IMR-32 cells were incubated with either control medium, 125I-WOC 4a (1 cpm/cell) alone, 125I-WOC 4a with 10(-6) M octreotide acetate, 125I (1 cpm/cell) alone, 125I with octreotide acetate, or octreotide acetate alone for 48 h, washed, and cryopreserved for 4 weeks. Cells were then thawed, replated, and allowed to acclimate for 48 h. Cell viability was assessed by trypan blue exclusion and a colorimetric assay.

RESULTS

Following short-term exposure, 125I-WOC 4a induced dose-dependent cytotoxicity in IMR-32 cells (P < 0.05 by ANOVA), but not in the PANC-1 cells. After exposure to 125I-WOC 4a (1 cpm/cell) for 48 h followed by a 4-week cryopreserved exposure, significant cytotoxicity was induced in IMR-32 cells (P < 0.05 by ANOVA) which was not seen in cells treated with 125I alone or 125I with 10(-6) M octreotide acetate. Simultaneous exposure to 125I-WOC 4a and octreotide acetate was also cytotoxic.

CONCLUSION

125I-WOC 4a induces receptor-specific cytotoxicity following both short- and long-term drug exposures. This radiopharmaceutical may be useful for localizing or treating somatostatin receptor-positive tumors.