Development of bioactive gemcitabine-D-Lys 6 -GnRH prodrugs with linker-controllable drug release rate and enhanced biopharmaceutical profile.

Peptide-drug conjugates have emerged as a potent approach to enhance the targeting and pharmacokinetic profiles of drugs. However, the impact of the linker unit has not been explored/exploited in depth. Gemcitabine (dFdC) is an anticancer agent used against a variety of solid tumours. Despite its potency, gemcitabine suffers mostly due to its unspecific toxicity, lack of targeting and rapid metabolic inactivation. To minimize these limitations and enable its targeting to tumours overexpressing the GnRH receptor, we examined the peptide-drug conjugation approach. Our design hypothesis was driven by the impact that the linker unit could have on the peptide-drug conjugate efficacy. Along these lines, in order to exploit the potential to manipulate the potency of gemcitabine through altering the linker unit we constructed three different novel peptide-drug conjugates assembled of gemcitabine, the tumour-homing peptide D-Lys6 -GnRH and modified linker building blocks. Specifically, the linker was sculpted to either allow slow drug release (utilizing carbamate bond) or rapid disassociation (using amide and ester bonds). Notably, the new analogues possessed up to 95.5-fold enhanced binding affinity for the GnRH receptor (GnRH-R) compared to the natural peptide ligand D-Lys6 -GnRH. Additionally, their in vitro cytotoxicity was evaluated in four different cancer cell lines. Their cellular uptake, release of gemcitabine and inactivation of gemcitabine to its inactive metabolite (dFdU) was explored in a representative cell line. In vitro stability and the consequent drug release were evaluated in cell culture medium and human plasma. In vivo pharmacokinetic studies were performed in mice, summarizing the relative stability of the three conjugates and the released levels of gemcitabine in comparison with dFdU. These studies suggest that the fine tuning of the linkage within a peptide-drug conjugate affects the drug release rate and its overall pharmaceutical profile. This could eventually emerge as an intriguing medicinal chemistry approach to optimize bio-profiles of prodrugs.