Development of a prolactin receptor-targeting fusion toxin using a prolactin antagonist and a recombinant form of Pseudomonas exotoxin A

Summary Human prolactin (hPRL) promotes the proliferation and differentiation of mammary epithelial cells during mammary gland development and has been linked to breast tumor development. The receptor for hPRL (hPRL-R) is elevated in a majority of human breast tumors, suggesting the overexpression of hPRL-R makes cancer cells highly sensitive to the mitogenic and anti-apoptotic activity of hPRL. These findings provide the rationale for the development of hPRL-R targeting breast cancer therapeutics. Previously, an hPRL-R antagonist, G129R, was developed that competitively binds to the hPRL-R resulting in growth inhibition and the induction of apoptosis in certain types of breast cancer cells. To further increase thepotency of G129R, we fused G129R to a truncated form of Pseudomonas exotoxin A (PE₄₀) that lacks the cell recognition domain of the toxin but retains the domains necessary for PE₄₀_ to translocate into the cytosol and inhibit protein synthesis. We postulated that the fusion of G129R with PE₄₀-KDEL would (1) deliver the recombinant toxin to breast cancer cells where hPRL-R is overexpressed; (2) block hPRL signaling via its G129R moiety; and (3) inhibit protein synthesis via its PE₄₀-KDEL moiety. We demonstrate that the fusion toxin can competitively bind to hPRL-Rs on T-47D human breast cancer cells and inhibit STAT5 phosphorylation induced by hPRL. In addition, we show that G129R-PE₄₀-KDEL is selectively cytotoxic to breast cancer cell lines expressing the hPRL-R and that cell death is associated with the inhibition of protein synthesis and does not involve caspase mediated apoptosis.