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Preclinical Evaluation of Targeted Cytotoxic Luteinizing Hormone-Releasing Hormone Analogue AN-152 in Androgen-Sensitive and Insensitive Prostate Cancers

Endocrine, Polypeptide, and Cancer Institute, Veterans Affairs Medical Center and Section of Experimental Medicine, Department of Medicine, Tulane University School of Medicine, New Orleans, Louisiana 70112 [M. L., A. V. S., K. S., G. H., A. N.]

Purpose and Experimental Design: To improve conventional chemotherapy, we developed cytotoxic analogues of luteinizing hormone-releasing hormone (LH-RH), which can be targeted to prostate cancers expressing LH-RH receptors. In view of pending clinical trials on cytotoxic LH-RH analogue AN-152, containing doxorubicin (DOX) linked to [D-Lys^6]-LH-RH, we investigated the effects of AN-152 on tumor growth of s.c. implanted androgen-sensitive LNCaP and MDA-PCa-2b prostate cancers, as well as androgen-independent C4-2 prostate cancers xenografted into the tibiae of nude mice. In the C4-2 study, serum prostate-specific antigen (PSA) levels were also measured. LH-RH receptors were analyzed by reverse transcription-PCR and ligand competition assay. We also evaluated whether AN-152 can affect mRNA expression of human epidermal growth factor receptor and HER-2 and -3 oncogenes

Results: After 32 days of treatment with AN-152, the growth of LNCaP cancers in castrated nude mice was strongly inhibited by 83% versus intact controls (P < 0.01) and 62% versus castrated controls (P < 0.05). In animals bearing MDA-PCa-2b prostate cancers, therapy with AN-152 for 25 days resulted in a 69% inhibition of tumor growth (P < 0.01 versus controls) and was more effective (P < 0.05) than equimolar doses of DOX or microcapsules of LH-RH agonist Decapeptyl. In nude mice bearing intraosseous C4-2 prostate cancers, treatment with AN-152 decreased serum PSA levels (P < 0.01) to 10.3 ± 3.4 ng/ml from 24.8 ± 4 ng/ml in controls, whereas DOX had no effect on PSA. The inhibitory effects of AN-152 on C4-2 tumors was accompanied by an increase in apoptosis and a decrease in tumor proliferation. Binding sites for LH-RH and the expression of mRNA for LH-RH receptors were found on s.c. C4-2 and MDA-PCa-2b tumors. The inhibition of MDA-PCa-2b tumors by AN-152 was associated with a significant decrease in mRNA expression for epidermal growth factor receptor, HER-2, and 3.

Conclusions: Our findings suggest that cytotoxic analogue AN-152 could be considered for therapeutic trials in patients with advanced prostate carcinoma.

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