Synthesis and Investigation of Peptide–Drug Conjugates Comprising Camptothecin and a Human Protein-Derived Cell-Penetrating Peptide

Drug targeting strategies, such as peptide–drug conjugates (PDCs), have arisen to combat the issue of off-target toxicity that is commonly associated with chemotherapeutic small molecule drugs. Here we investigated the ability of PDCs comprising a human protein-derived cell-penetrating peptide—platelet factor 4-derived internalization peptide (PDIP)—as a targeting strategy to improve the selectivity of camptothecin (CPT), a topoisomerase I inhibitor that suffers from off-target toxicity. The intranuclear target of CPT allowed exploration of PDC design features required for optimal potency. A suite of PDCs with various structural characteristics, including alternative conjugation strategies (such as azide–alkyne cycloaddition and disulfide conjugation) and linker types (non-cleavable or cleavable), were synthesized and investigated for their anticancer activity. Membrane permeability and cytotoxicity studies revealed that intact PDIP-CPT PDCs can cross membranes, and that PDCs with disulfide- and protease-cleavable linkers liberated free CPT and killed melanoma cells with nanomolar potency. However, selectivity of the PDIP carrier peptide for melanoma compared to noncancerous epidermal cells was not maintained for the PDCs. This study emphasizes the distinct role of the peptide, linker, and drug for optimal PDC activity and highlights the need to carefully match components when assembling PDCs as targeted therapies.