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Targeted Doxorubicin-containing Luteinizing Hormone-releasing Hormone Analogue AN-152 Inhibits the Growth of Doxorubicin-resistant MX-1 Human Breast Cancers¹

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 70112-2699

Purpose: The receptors for luteinizing hormone-releasing hormone receptor (LHRH-R) are found in >50% of human breast cancers. Doxorubicin (DOX) was linked to [D-Lys⁶]LHRH to form a cytotoxic conjugate, AN-152, which can be targeted to tumor cells expressing LHRH-R. We evaluated the effects of AN-152 on the estrogen-independent, DOX-resistant human mammary carcinoma line MX-1, xenografted into nude mice.

Experimental Design: Nude mice bearing MX-1 tumors were administered five i.v. injections of AN-152 or DOX at doses equivalent to 3 mg/kg DOX. Tumor growth was followed, and changes in the expression of LHRH-R on tumors were evaluated by radioreceptor assays, reverse transcription-PCR, and Western blotting. The effects of AN-152 on the expression of human epidermal growth factor receptor (HER)-2 were investigated. Because LHRH-R are coupled to various G proteins, which are involved in mitogenic signaling, we determined the outcome of treatment with AN-152 on the levels of mRNA for different G proteins.

Results: Treatment with AN-152 significantly (P < 0.05) decreased the final tumor volume to 978.56 ± 176.85 mm³, compared with the control tumors, which measured 2837.38 ± 515.38 mm³. Tumor doubling time was likewise significantly (P < 0.05) extended by AN-152 to 12.01 ± 1.99 days from 6.45 ± 0.36 days for the controls. Therapy with AN-152, but not with DOX, resulted in a significant decrease of LHRH-R levels on MX-1 tumors. The expression of mRNAs for HER-2, HER-3, G_i2, and G₁₁ and the levels of HER-2 and HER-3 proteins were also significantly reduced by AN-152.

Conclusions: Cytotoxic LHRH analogue AN-152 could be considered for targeted chemotherapy of DOX-resistant breast cancers expressing LHRH-R.

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