Effective conversion of irinotecan to SN-38 after intratumoral drug delivery to an intracranial murine glioma model in vivo

Laboratory investigation

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Object

Irinotecan (CPT-11), a topoisomerase I inhibitor, is a cytotoxic agent with activity against malignant gliomas and other tumors. After systemic delivery, CPT-11 is converted to its active metabolite, SN-38, which displays significantly higher cytotoxic potency. However, the achievement of therapeutically effective plasma levels of CPT-11 and SN-38 is seriously complicated by variables that affect drug metabolism in the liver. Thus the capacity of CPT-11 to be converted to the active SN38 intratumorally in gliomas was addressed.

Methods

For in vitro studies, 2 glioma cell lines, U87 and U251, were tested to determine the cytotoxic effects of CPT-11 and SN-38 in a dose-dependent manner. In vivo studies were performed by implanting U87 intracranially into athymic/nude mice. For a period of 2 weeks, SN-38, CPT-11, or vehicle was administered intratumorally by means of an osmotic minipump. One series of experiments measured the presence of SN-38 or CPT-11 in the tumor and surrounding brain tissues after 2 weeks' exposure to the drug. In a second series of experiments, after 2 weeks' exposure to the drug, the animals were maintained, in the absence of drug, until death. The survival curves were then calculated.

Results

The results show that the animals that had CPT-11 delivered intratumorally by the minipump expressed SN-38 in vivo. Furthermore, both CPT-11 and SN-38 accumulated at higher levels in tumor tissues compared with uninvolved brain. Intratumoral delivery of CPT-11 or SN-38 extended the average survival time of tumor-bearing animals from 22 days to 46 and 65 days, respectively.

Conclusions

These results demonstrate that intratumorally administered CPT-11 can be effectively converted to SN-38 and this method of drug delivery is effective in extending the survival time of animals bearing malignant gliomas.

Abbreviations used in this paper: AED = antiepileptic drug; CES2 = carboxylesterase 2; CPT-11 = irinotecan; HPLC = high-performance liquid chromatography; IC50 = 50% inhibiting concentration; MTT = methylthiotetrazole; PBS = phosphate-buffered saline.

Article Information

Address correspondence to: Thomas C. Chen, M.D., Ph.D., Department of Neurosurgery, University of Southern California, Los Angeles, California 90033. email: tcchen@usc.edu.

Please include this information when citing this paper: published online April 23, 2010; DOI: 10.3171/2010.2.JNS09719.

© AANS, except where prohibited by US copyright law.

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Figures

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    Cytotoxic activity of CPT-11 and SN-38 in vitro. Both U87 and U251 glioblastoma cells were treated with increasing concentrations of CPT-11 or SN-38. After 48 hours, cell viability was determined by MTT assay. Shown is the average survival (mean ± SD), where viability of untreated cell cultures was set at 100%. The SN-38 was substantially more cytotoxic than CPT-11, with an IC50 of 750 nM versus 85 μM in U87, and 10 nM versus 10 μM in U251, respectively.

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    Survival in an orthotopic tumor model after drug treatment. Mice with orthotopically implanted U87 glioblastoma cells received intratumoral delivery of either PBS (control), CPT-11, or SN-38 for 2 weeks via an osmotic minipump. Thereafter, their survival was monitored and the results plotted in a Kaplan-Meier survival curve. Compared with control animals, the survival of both groups of drug-treated animals was prolonged significantly (p < 0.0001). The results also illustrated a significant difference in survival times in animals treated with SN-38 in comparison with those treated with CPT-11 (p < 0.0001).

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    Presence of SN-38 in tumor tissue after treatment with CPT-11 in vivo. Mice with orthotopically implanted U87 glioblastoma cells received intratumoral delivery of PBS (control), CPT-11, or SN-38 via an osmotic minipump. At the end of the 2-week treatment, the animals were killed and the tumors were harvested. The amount of SN-38 in tumor extracts was quantified by HPLC. Shown is the average amount of SN-38 (mean ± SD, 5 animals per group) in the 3 treatment groups. ***p < 0.005.

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    Biodistribution of SN-38 after intratumoral delivery. Mice with orthotopically implanted U87 glioblastoma cells received intratumoral delivery of either PBS (control), CPT-11, or SN-38. At the end of the 2-week treatment, all animals were sacrificed and tumors, as well as cerebellum and other normal brain tissue, were harvested. The amount of SN-38 in tissue extracts was quantified by HPLC. Shown is the average amount of SN-38 (mean ± SD; n = 5) in the 3 treatment groups.

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    Biodistribution of SN-38 compared with CPT-11. Mice with orthotopically implanted U87 glioblastoma cells received intratumoral delivery of PBS (control), CPT-11, or SN-38. At the end of the 2-week treatment, all animals were sacrificed and tumors, as well as cerebellum and other normal brain tissues, were harvested. The amount of SN-38, as well as CPT-11, in tissue extracts was quantified by HPLC. Shown is the average amount of SN-38 (right bar graph) and CPT-11 (left bar graph) as mean ± SD (n = 5) in the 3 treatment groups.

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