Exploring the Structure-Activity Relationships of Albumin-Targeted Picoplatin-Based Platinum(IV) Prodrugs

Platinum(II) complexes prevail as first-line treatment for many cancers but are associated with serious side effects and resistance development. Picoplatin emerged as a promising alternative to circumvent GSH-induced tumor resistance by introducing a bulky 2-picoline ligand. Although clinical studies were encouraging, picoplatin did not receive approval. Interestingly, the anticancer potential of prodrugs based on picoplatin is widely underexplored, and even less so the respective tumor-targeting approaches. We synthesized two new "hybrid" picoplatin(II) derivatives with an oxalate or cyclobutane dicarboxylate leaving group and their corresponding platinum(IV) prodrugs with an albumin-targeting maleimide moiety or a succinimide as reference. Picoplatin(II) and its derivatives indeed reacted much slower with GSH compared to the respective analogs cisplatin, carboplatin, or oxaliplatin. While PicoCarbo(IV) and PicoOxali(IV) were reduced slowly in the presence of ascorbic acid, picoplatin(IV) was extremely unstable. All three prodrugs were widely inactive in the MTT assays. The platinum(IV)-maleimide complexes rapidly bound to albumin with stable conjugates for >25 h. Albumin-binding resulted in elevated platinum plasma levels, prolonged blood circulation, and enhanced tumor accumulation of the prodrugs in mice bearing CT26 tumors. However, only maleimide-functionalized PicoCarbo(IV) and picoplatin(II) significantly inhibited tumor growth. One possible explanation is that for albumin-binding platinum(IV) prodrugs, the bulky 2-picoline moiety prevents sufficient activation/reduction to unlock their full anticancer potential.