Design, Synthesis, and Evaluation of Cyclopropyl-Based PD-1/PD-L1 Interaction Inhibitors Bearing a Dipeptide Tail with Improved Drug-likeness

Small-molecule blockade of PD-1/PD-L1 offers a promising strategy for cancer immunotherapy, although combining potent inhibition with favorable drug-like properties remains challenging. Herein, hydrophilic substituents were introduced into the solvent-exposed region of a biphenyl scaffold using a stereochemically defined cyclopropyl linker. The cyclopropyl linker enabled systematic variation of amino alcohol, amino acid, dipeptide, and peptidomimetic polar groups. Systematic structure−activity relationship studies identified the dipeptide-derived (1S,2S)-F33 as a potent inhibitor of PD-1/PD-L1 binding (IC50 = 1.6 nM) with high affinity for PD-L1 (KD = 0.16 μM). The compound exhibited improved aqueous solubility and liver microsomal stability, with an oral bioavailability of 28.29% in mice. Oral administration of (1S,2S)-F33 dose-dependently suppressed tumor growth in a B16−F10 melanoma mouse model, achieving 73.69% tumor growth inhibition, accompanied by enhanced antitumor immune responses and favorable tolerability. Collectively, these findings establish (1S,2S)-F33 as a promising lead for the further development of drug-like small-molecule PD-1/PD-L1 inhibitors.