Comparative Evaluation of Thiol- and Amine-Conjugating Moieties for Endogenous Albumin Binding after Intravenous Administration

Maleimides are widely used in anticancer drug development for linking small-molecule drugs to macromolecules like antibodies or proteins via thiol-Michael addition reactions. Despite their widespread use, even in clinically approved therapeutics, they present significant drawbacks such as hydrolysis at physiological pH and instability of the formed thiosuccinimide bond. Hence, there is a growing need for more stable yet equally efficient binding units. This is particularly important for drug-delivery systems that bind to endogenous albumin in vivo, exploiting the ability of the protein to accumulate in tumor tissue. This study compares phenyloxadiazolyl methyl sulfone (PODS) and a 2,4-difluorophenyl sulfonamide (DFSA) derivative with maleimide as endogenous albumin binders. Of note, PODS and maleimide bind to Cys34, whereas DFSA targets Lys64 of albumin. The albumin binders were conjugated as axial ligands to oxaliplatin(IV) complexes (PODS-Ox-OAc and DFSA-Ox-OAc) and studied in comparison to a maleimide-bearing reference compound (Mal-Ox-OAc). Both PODS- and DFSA-complexes showed higher hydrolytic stability at pH 7.4 than the maleimide complex. Albumin-binding was highly efficient for the PODS and maleimide complexes. However, the DFSA derivative exhibited only slow conjugation. This was also reflected in the serum pharmacokinetic and organ distribution studies using CT26 colon cancer-bearing mice. Here, the PODS complex showed the highest platinum levels in both serum and tumor tissue. Additionally, PODS-Ox-OAc induced the most significant tumor regression and prolonged overall survival in this model. Together, our data highlight PODS as a promising alternative to maleimide as an endogenous albumin binder.