Monocarboxylate transporters (MCTs) are members of the solute carrier (SLC) family of proteins. They are expressed in a wide variety of normal tissues and take part in key metabolic pathways. Of the 14 isoforms, MCTs 1–4 have been extensively studied, because they can transport L-lactate and pyruvate across the plasma membrane. It is also well known that MCTs form metabolon systems, enhancing lactate transport, with different CA isozymes and even with noncatalytic CA-related proteins.
MCT inhibitors have been proposed as good candidates for antitumor drugs since MCT expression has been reported in various human tumors, including, e.g., breast, colon, bladder, and prostate carcinomas, and gliomas. Both in vitro and in vivo preclinical studies have shown that ablation of MCT1 or MCT4 can halt the proliferative capability of tumor cells, and MCT inhibition can be augmented with metformin and phenformin used widely in diabetes mellitus.