Introduction: Receptors of gonadotropin-releasing hormone (GnRH), also regarded as luteinizing hormone-releasing
hormone (LHRH), are found to be expressed in triple negative breast cancer (TNBC),
which contributes to approximately 10 to 20% of breast cancer. Selective targeting
of the GnRH receptors is a promising approach in breast cancer therapy. AEZS-125 is
a Disorazol-Z conjugate linked to the GnRH receptor agonist D-Lys6-LHRH. Disorazol-Z is a novel natural cytotoxic compound isolated from the myxobacterium
Sorangium cellulosum. Owing to the binding affinity of the D-Lys6-LHRH peptide to the GnRH receptors, it was hypothesized that AEZS-125 demonstrates
GnRH receptor specific cell growth inhibition when the compound is directed to the
GnRH receptors by the peptide ligand. Study design: The expression of GnRH receptors in the TNBC cell lines HCC1806 and MDA-MB-231 was
evaluated by RT-PCR and Western blot. The fibroblast cell line LTK(-), which does
not express GnRH receptors, served as a negative control. The cytotoxic activities
of Disorazol-Z and AEZS-125 were investigated in the LTK(-) cells in comparison to
the TNBC cell lines HCC1806 and MDA-MB-231. Receptor blocking experiments were carried
out to study the receptor mediated activity of AEZS-125. 100µM Triptorelin, a GnRH
agonist that has high receptor affinity, was used to block the GnRH receptors on the
HCC1806 cells and therefore may attenuate the binding of AEZS-125 to the receptors.
Results: GnRH receptors were expressed in both mRNA and protein levels in HCC1806 and MDA-MB-231,
whereas no expression was detected in LTK(-). Disorazol-Z showed a better performance
than AEZS-125 in cell growth inhibition in LTK(-), while AEZS-125 had a comparable
anti-proliferative activity to Disorazol-Z in HCC1806 and MDA-MB-231, despite the
outstanding performance of Disorazol-Z in low concentrations (< 1µM), which could
be explained by the small size and the subsequent high penetrating ability of the
substrate. Both LTK(-) and HCC1806 were found to show similar dosage responses to
Disorazol-Z. However, the anti-proliferation effect of AEZS-125 was significantly
more pronounced in HCC1806 than in LTK(-), suggesting that the performance of AEZS-125
may be directly related to the expression of GnRH receptors. In the receptor blocking
experiments, AEZS-125 could effectively inhibit cell growth, but its anti-proliferation
effect was significantly decreased in the presence of Triptorelin showing that the
binding and uptake of AEZS-125 into the HCC1806 cells was mediated by GnRH receptors.
Conclusions: The above data demonstrated that AEZS-125 underwent GnRH receptor mediated entry
and anti-proliferative activities in GnRH receptor positive TNBC cells. AEZS-125 is
thus a candidate for targeted treatment of TNBC. In vivo experiments are planned further
evaluate the cytotoxic potential of AEZS-125 in TNBC.
