Background: The adenoviral vector, AdCMVGRPr, has been used to induce the expression of the murine gastrin-releasing peptide receptor (GRPr) both in vitro and in vivo. A bombesin analogue ([125I]-mIP-bombesin) has been shown to bind with high affinity to GRPr and to localize to intraperitoneal (i.p.) ovarian tumors 2 days after induction of GRPr in an athymic nude mouse model. The present study was conducted to determine the level of localization of [(125/131)I]-mIP-bombesin in the tumors at 2, 4, and 7 days after AdCMVGRPr administration and to determine the feasibility of giving multiple doses of [131I]-mIP-bombesin for therapy.
Methods: Human ovarian cancer cells (SKOV3.ip1) were infected in vitro with AdCMVGRPr and were assayed for receptor expression at 2, 4, and 7 days after infection by using a radiolabeled bombesin-binding assay. Biodistribution studies utilized athymic nude mice inoculated i.p. with SKOV3.ip1 cells. The tumors were induced to express GRPr with an i.p. injection of AdCMVGRPr followed by administration of [125I]-mIP-bombesin 2 days later (AdCMVLacZ or saline was used for negative controls). In addition, the tumor localization of [125I]-mIP-bombesin was determined 4 and 7 days after AdCMVGRPr administration. The tumor localization of [131I]-mIP-bombesin was compared with [125I]-mIP-bombesin in this in vivo model.
Results: SKOV3.ip1 cells infected with AdCMVGRPr resulted in 80.3 +/- 5.9% binding of [125I]-Tyr4-bombesin at 2 days after infection, which decreased to 46.8 +/- 0.4% at 4 days and to 17.7 +/- 0.1% at 7 days. The biodistribution study showed that the tumor localization (14.9 +/- 8.2% injected dose/gram; ID/g) of [125I]-mIP-bombesin 2 days after administration of AdCMVGRPr was significantly greater than its localization in other organs (P < 0.003) and was significantly greater than in AcCMVLacZ- and saline-treated mice (P < 0.003). Injections of [125I]-mIP-bombesin at 4 and 7 days after a single AdCMVGRPr administration showed tumor localization of 4.5 +/- 3.0% ID/g at Day 4 and 3.9 +/- 3.5% ID/g at Day 7. The decreased localization at longer times after AdCMVGRPr infection correlated with in vitro results. The tumor uptake of [125I]-mIP-bombesin was comparable to the uptake of [131I]-mIP-bombesin (21.2 +/- 8.3% ID/g versus 15.4 +/- 5.6% ID/g, respectively), as was the normal tissue biodistribution.
Conclusions: The expression of GRPr in human ovarian cancer cells can be accomplished both in vitro and in vivo by using AdCMVGRPr, with the in vivo tumor localization of [125I]-mIP-bombesin being significantly greater than in control animals. The tumor localization of [125I]-mIP-bombesin and [131I]-mIP-bombesin at 2 days after AdCMVGRPr was comparable in a mouse model of human ovarian carcinoma. Injections of [125I]-mIP-bombesin at Days 4 and 7 after AdCMVGRPr infection resulted in tumor localization of [125I]-mIP-bombesin but at a level lower than 2 days. Thus, the total amount of radioactivity delivered to the tumor should be increased by multiple injections of [131I]-mIP-bombesin, which would be required for a therapeutic effect.