Receptors for
somatostatin (SST) that are found on
prostate cancers might be used for targeting of chemotherapeutic agents. Thus,
doxorubicin derivative
2-pyrrolinodoxorubicin (AN-201) can be linked to SST analogue
RC-121 (D-Phe-Cys-Tyr-D-Trp-Lys-Val-Cys-Thr-NH2) to form targeted cytotoxic SST analogue
AN-238. In this study, we evaluated the effects of
AN-238 on the growth of SST receptor (SSTR)-positive
androgen-independent Dunning R-3327-AT-1
prostate cancers in Copenhagen rats. The dose range and
tumor growth-inhibitory effects of
AN-238 and
AN-201 were investigated in preliminary experiments. Administration of cytotoxic radical
AN-201 at single i.v. doses of 110, 125, and 150 nmol/kg resulted in 0, 77.7, and 100% mortality, respectively, within 6-10 days. Four weeks after the injection of 110 nmol/kg
AN-201, mean
tumor volume was reduced by 35.1 % (P < 0.05), as compared with controls. In contrast, a single i.v. injection of analogue
AN-238 at a dose of 300 nmol/kg was nontoxic and remarkably potent in inhibiting the growth of Dunning AT-1
tumors, resulting in a 85.9% (P < 0.01) reduction in
tumor volume after 4 weeks. Treatment with
AN-238 extended the survival time of
tumor-bearing rats from 52.0+/-3.75 to 91.8+/-3.70 days, corresponding to a 76.5% (P < 0.01) increase. In a comprehensive experiment, we compared the effects of radical
AN-201 at 115 nmol/kg, analogue
AN-238 at 115 and 300 nmol/kg, carrier SST analogue
RC-121 at 300 nmol/kg, and a mixture of
AN-201 and
RC-121 at doses of 300 nmol/kg administered i.v. Administration of
AN-201 at 115 nmol/kg led to 90.0% mortality in 12 days, but animals treated with 115 nmol/kg of
AN-238 showed no signs of toxicity, their
tumor volume was reduced by 40.0% (P < 0.05), and their
tumor weight was reduced by 42.8% (P < 0.01) after 4 weeks, as compared with controls. The dose of 300 nmol/kg of
AN-238 was also nontoxic and diminished
tumor volume by 80.9% (P < 0.01) and
tumor weight by 82.0% (P < 0.01). No reduction in
tumor growth or toxic effects was observed with carrier
RC-121, but after the injection of unconjugated mixture of
AN-201 and
RC-121 at doses of 300 nmol/kg, all rats died within 4 days. Specific high-affinity receptors for SST were found on Dunning R-3327-AT-1
tumor membranes by radioligand binding assay and were identified by reverse transcription-PCR as SSTR2. Our study indicates that cytotoxic SST analogue
AN-238 can be targeted to SSTRs on
tumors and produces a powerful inhibition of the growth of Dunning-AT-1 rat
prostate cancer at doses that are nontoxic, whereas its cytotoxic component,
2-pyrrolinodoxorubicin, is toxic and ineffective.