MUC1-targeting small peptide radiopharmaceuticals for breast cancer.

Abstract

Background: Mucin 1 (MUC1) is a transmembrane glycoprotein overexpressed and underglycosylated in numerous epithelial cancers, including breast cancer. Reduced glycosylation leads to the exposure of the variable number tandem repeat (VNTR) region. To the best of our knowledge, all peptides previously described in the literature target the same epitope sequence of the VNTR. Given the high prevalence of breast cancer and the limited treatment options for the aggressive subtype triple-negative breast cancer (TNBC), due to its lack of oestrogen receptors (ER), progesterone receptors (PR), and human epidermal growth factor receptor 2 (HER2), we sought to develop a small peptide radiopharmaceutical targeting MUC1 by exploring all 3 minimal epitope sequences of the VNTR: RPAPGS, PPAHGVT and PDTRP. We also investigated the influence of linker lipophilicity on the binding affinity to MUC1.

Results: The reference compound 1 showed the highest cell uptake among all tested compounds. While some statistically significant differences were observed for the cell uptake between the different peptide sequences and the linkers, the uptake was so low that no reliable structure-activity relationships could be established. We then studied the specificity of all compounds for MUC1 by comparing the uptake in MUC1-expressing and MUC1-knockout (KO) cells, and unexpectedly observed no specificity for any compound. A saturation binding assay of several peptides showed their binding was too low to reliably determine their binding affinity (K_d). Surface plasmon resonance (SPR) further confirmed the absence of binding of all [^NatGa]- and [^NatLu]-labelled peptides tested. Given the discrepancies between our cell data and the previously reported results, we next assessed the specificity of the reference (1) in vivo in mice bearing MUC1-expressing and MUC1-knockdown (KD) tumours, which further proved its non-specificity.

Conclusions: While MUC1 is a very promising target for the development of breast cancer theranostics, designing peptidomimetics based on its minimal epitopes do not lead to high-affinity binders. Our ongoing efforts involve utilizing phage-display to identify new peptide sequences.