Tumor Targeting with Peptide-Drug Conjugates: Showcasing Key Progress and Hurdles

Peptide-drug conjugates (PDCs) are modular, targeted therapeutics composed of a homing peptide linked to a cytotoxic or modulating drug payload via a cleavable/non-cleavable linker. PDCs utilize peptide targeting to enhance the delivery of potent drugs to tumors, providing advantages such as superior tissue penetration, reduced immunogenicity, and simpler manufacture compared to antibody-drug conjugates (ADCs). A comparison of PDCs versus ADCs highlights that PDCs' small size (~1-3 kDa) enables deeper tumor penetration and faster clearance, whereas ADCs (~150 kDa) benefit from prolonged circulation but suffer from limited tissue diffusion. This review surveys recent advances in PDC design and application. We discuss key design elements (targeting peptides, cleavable/non-cleavable linkers, and payloads) and how these drive mechanisms of tumor delivery and intracellular drug release. Mechanistically, PDCs bind receptors or translocate across membranes, undergo endocytosis, and exploit stimuli-responsive linkers or cell-penetrating peptides to release drugs. Many PDCs can self-assemble into nanoscale structures in aqueous environments. We illustrate PDC concepts through specific instances, such as the brain-penetrant paclitaxel trevatide (ANG1005, paclitaxel-Angiopep-2), the radiotherapeutic lutetium (¹⁷⁷Lu)-DOTATATE (Lutathera), and the LyP-1-conjugated doxorubicin-loaded liposomes (LyP-1-doxorubicin conjugate) for triple-negative breast cancer. Persistent challenges include in vivo stability (premature drug release and metabolic clearance), tumor heterogeneity (variable receptor expression), and manufacturing scale-up. We also address regulatory hurdles that have limited PDC clinical success; for example, currently, only lutetium (¹⁷⁷Lu)-DOTATATE is FDA-approved (others, like melphalan flufenamide (melflufen), have faced setbacks). Finally, we outline future directions, including theranostic PDCs, AI-assisted peptide optimization, dual-stimuli linkers, and integration with nanomaterials, to further enhance targeting and efficacy. This comprehensive review integrates findings from recent literature and provides an in-depth perspective on the design, advantages, limitations, and future prospects of PDCs in cancer therapy.