Background
Extragonadal follicle-stimulating hormone (FSH) receptor (FSHR) expression in various cancers and their endothelial vessel cells has highlighted novel opportunities for targeted FSHR therapy.

Methods
We investigated the specificity/cytotoxicity of Phor21 fusion lytic peptide, conjugated to 12 different FSHβ-chain fragments to ablate FSHR-expressing cancer cells in vitro and in vivo. Additionally, the use of the gonadotropin-releasing hormone (GnRH) antagonist cetrorelix (CTX) alone or with the Phor21-FSHβ33-53 C/S conjugate for anticancer therapy was analyzed.

Results
Phor21 linked to the FSHβ33–53 fragment with cysteine (Cys) replaced by serine (Ser) (Phor21-FSHβ33-53 C/S) demonstrated the highest specific cytotoxicity towards FSHR possessing cancer cells vs. other compounds. Recombinant human FSH treatment significantly decreased the cytotoxicity of Phor21-FSHβ33-53 C/S conjugate in FSHR-positive cancer cells. Phor21-FSHβ33-53 C/S (further addressed as Phor21-FSHβ) treatment in vivo significantly inhibited the growth of FSHR-positive cancer xenografts, resulting in necrosis. The efficacy of the Phor21-FSHβ was enhanced by co-treatment with the gonadotropin-releasing hormone (GnRH) antagonist cetrorelix (CTX). CTX alone exerted pro-apoptotic effects. CTX significantly inhibited the growth of prostate cancer LNCaP cell xenografts. Although FSHR-positive tumor vessel endothelial cells were previously reported in LNCaP cell xenografts, we were unable to reproduce FSHR expression. Consequently, Phor21-FSHβ had no effect on tumor destruction because of the lack of Fshr transcripts in the endothelium of these tumor vessel cells.

Conclusion
This novel functional evidence shows that any cancer cell expressing FSHR can be specifically targeted and destroyed by the conjugated lytic peptide Phor21-FSHβ33–53 (Phor21-FSHβ). FSHR expression was not detected in the tumor vessel endothelial cells, which needs further re-evaluation.