Recent progress in small molecules targeting the acidic tumor microenvironment

The acidic tumour microenvironment (pHe 6.5–6.9) is sustained by the Warburg effect and pH regulators, such as monocarboxylate transporters 1 and 4, Na+/H + exchanger 1, vacuolar ATPase, and carbonic anhydrases IX and XII. This environment facilitates tumour invasion, immune evasion, and resistance to therapy in solid tumours. Recent advancements in small molecule inhibitors targeting these pathways have demonstrated potential in molecular design, mechanisms of action, and preclinical studies. However, practical applications face challenges, including metabolic compensation, insufficient target selectivity, and clinical translation difficulties. This article reviews the structural design, structure–activity relationships, biological activity, and clinical trial progress of small molecule inhibitors. It also summarises acid-targeted delivery strategies, such as pH-responsive prodrugs and pHLIP peptides. The aim is to highlight the opportunities and challenges in acid-base regulation within the tumour microenvironment and offer insights for developing a new generation of antitumor drugs with high selectivity and low toxicity.