The revolutionary progress in cancer immunotherapy, particularly the advent of immune checkpoint inhibitors, marks a significant milestone in the fight against malignancies. However, the majority of clinically employed immune checkpoint inhibitors are monoclonal antibodies (mAbs) with several limitations, such as poor oral bioavailability and immune-related adverse effects (irAEs). Another major limitation is the restriction of the efficacy of mAbs to a subset of cancer patients, which triggered extensive research efforts to identify alternative approaches in targeting immune checkpoints aiming to overcome the restricted efficacy of mAbs. This comprehensive review aims to explore the cutting-edge developments in targeting immune checkpoints, focusing on both small molecule- and peptide-based approaches. By delving into drug discovery platforms, we provide insights into the diverse strategies employed to identify and optimize small molecules and peptides as inhibitors of immune checkpoints. In addition, we discuss recent advances in nanomaterials as drug carriers, providing a basis for the development of small molecule- and peptide-based platforms for cancer immunotherapy. Ongoing research focused on the discovery of small molecules and peptide-inspired agents targeting immune checkpoints paves the way for developing orally bioavailable agents as the next-generation cancer immunotherapies.

The cysteine protease cathepsin S (CatS) is overexpressed in many tumors. It is known to be involved in tumor progression as well as antigen processing in antigen‐presenting cells (APC). Recent evidence suggests that silencing CatS improves the anti‐tumor immune response in several cancers. Therefore, CatS is an interesting target to modulate the immune response in these diseases. Here, we present a series of covalent‐reversible CatS inhibitors based on the α‐fluorovinylsulfone and ‐sulfonate warheads. We optimized two lead structures by molecular docking approaches, resulting in 22 final compounds which were evaluated in fluorometric enzyme assays for CatS inhibition and for selectivity towards the off‐targets CatB and CatL. The most potent inhibitor in the series has subnanomolar affinity (K_i=0.08 nM) and more than 100,000‐fold selectivity towards cathepsins B and L. These new reversible and non‐cytotoxic inhibitors could serve as interesting leads to develop new immunomodulators in cancer therapy.