In vitro and in vivo evaluation of Bombesin-MMAE conjugates for targeted tumour therapy.

Targeted tumour therapy has proved to be an efficient alternative to overcome the limitations of conventional chemotherapy. The upregulation of the bombesin receptor 2 (BB2) in several malignancies and the advantages offered by peptide drug conjugates over antibody drug conjugates in terms of production and tumour targeting motivated us to synthesise and test bombesin conjugates armed with the tubulin binder monomethyl auristatin E. The widely used Val-Cit-PABC was initially included as cathepsin cleavable self-immolative linker for the release of the free drug. However, the poor stability of the Val-Cit-conjugates in mouse plasma encouraged us to consider the optimised alternatives Glu-Val-Cit-PABC and Glu-Gly-Cit-PABC. Conjugate BN-EVcM1, featuring Glu-Val-Cit-PABC, combined suitable stability (t _(½) in mouse and human plasma: 8.4 h and 4.6 h, respectively), antiproliferative activity in vitro (IC ₅₀  = 29.6 nM on the human prostate cancer cell line PC-3) and the full release of the free payload within 24 h. Three conjugates, namely BN-EGcM1, BN-EVcM1 and BN-EVcM2, improved the accumulation of MMAE in PC-3 human prostate cancer xenograft mice models, compared to the administration of the free drug. Among them, BN-EVcM1 also stood out for the significantly extended survival of mice in in vivo acute efficacy studies and for the significant inhibition of the growth of a PC-3 tumour in mice in both acute and chronic efficacy studies.
Competing Interests: Declaration of competing interest Gabor Mezo reports financial support was provided by National Research, Development and Innovation Fund of Hungary. Gabor Halmos reports financial support was provided by National Research, Development and Innovation Fund of Hungary. Jozsef Tovari reports financial support was provided by National Research, Development and Innovation Fund of Hungary. Christian Steinkuhler reports financial support was provided by European Commission. Gabor Mezo reports financial support was provided by European Commission. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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