Drug–Drug Conjugates as Intelligent Preclinical Platforms for Targeted Cancer Therapy

Drug–drug conjugates (DDCs), in which two active pharmaceutical ingredients (APIs) are linked covalently within a single molecular structure to produce a synergistic therapeutic effect, represent an emerging category of single-molecule therapeutics. These conjugates differ from conventional combination therapies by utilising well-defined covalent chemical linkages (such as amide, ester, disulfide, and carbamate bonds), usually coupled with self-immolative spacers. This structural architecture ensures synchronised intracellular delivery and controlled, stimulus-responsive release of the parent APIs in a fixed stoichiometry. In oncology, this strategy is especially attractive because it reduces systemic toxicity, improves intratumoral accumulation, and helps bypass resistance mechanisms that limit conventional chemotherapy. Recent advances in linker chemistry, tumour-responsive cleavage, and self-immolative systems have shifted these constructs from simple mutual prodrugs toward intelligent, adaptive delivery platforms. This review summarizes how DDCs mitigate off-target effects, overcome cellular resistance, exploit tumor-specific triggers, and evolve into next-generation translational cancer therapeutics.