Peptide Drug Conjugates: An Exploration for Antimalarial and Anticancer Treatments

Abstract

Small molecule drugs are essential in the treatment and management of acute and chronic diseases. However, they often possess disadvantages such as nonspecific interactions with healthy cells and can suffer from loss of efficacy as pathogens or diseased cells gain drug resistance. The approval of antibody-drug conjugates (ADCs) for cancer treatment has highlighted targeted drug delivery as a novel therapeutic approach to overcome some of the limitations of small molecules. Inspired by the success of ADCs, peptide-drug conjugates (PDCs) provide an attractive and cheaper alternative for the exploration of targeted drug delivery. In this thesis, three small molecule drugs, primaquine (PQ), camptothecin (CPT) and vemurafenib (Vem), are conjugated onto bioactive cell-penetrating peptides to understand if certain drug liabilities, such as off-target toxicity or drug resistance, can be mitigated through peptide conjugation. The exploration of PDCs in both an infectious disease context (malaria) and a noncommunicable disease context (cancer) will highlight these emerging therapeutic modalities as a versatile tool for targeted treatments. Chapter One introduces PDCs as a promising tool for targeted therapy and highlights the need for such an approach in the treatment of both infectious and noncommunicable diseases. The different design aspects of PDCs are discussed, including the types of targeting peptides, different linker technologies and alternative conjugation strategies. The design, synthesis and biological evaluation of a suite of PDCs in different disease contexts will be underlined as the key theme explored in this thesis, which is subdivided into chapters based on the identity of the PDC drug cargo. Chapter Two discusses the synthesis of six PQ-containing antimalarial PDCs, each with varied design characteristics, and the subsequent biological evaluation of their killing ability against Plasmodium parasites. The insights provided by changes in the low micromolar activity for different PDC analogues revealed that conjugation site and linker type play an important role in PDC potency. These first-generation conjugates highlighted PDCs as a feasible approach to antimalarial therapy, an area that remains underexplored for this treatment type. Chapter Three investigates the synthesis of four anticancer PDCs containing CPT, a small molecule drug with off-target toxicity. The knowledge of the relationship between design characteristics and potency from the previous chapter was used to inform PDC construction. Biological evaluation of the PDCs against a melanoma cancer cell line revealed nanomolar to low micromolar potency, albeit with toxicity against a noncancerous cell line. The non-selective nature of the PDCs reinforced the importance of design in PDC development, including matching drug cargo with the correct carrier molecule. Chapter Four explores the synthesis of five Vem-containing anticancer PDCs and details their killing ability against melanoma cell lines that are sensitive and resistant to the drug cargo. The resulting PDCs had selective low micromolar activity against the drug-sensitive cell line. Evaluation of peptide-linker controls containing no drug highlighted the contribution of each PDC component to PDC activity and indicated the importance of the peptide carrier to overall potency. One of the PDCs was also active against the drug-resistant cell line, emphasising this approach as a feasible means to reinvigorate interest in small molecule drugs with resistance issues. Chapter Five summarises the above projects on antimalarial and anticancer PDCs utilising bioactive cell-penetrating peptides. A perspective is provided on the prospects of future generations of PDCs and how the lessons learned throughout this thesis will inform design considerations.