THE P-GLYCOPROTEIN MULTIDRUG TRANSPORTER

1. P-glycoprotein (P-gp) is a transmembrane protein involved in ATP-de pendent efflux of various structurally unrelated anticancer drugs. Its overexpression in cancer cells decreases intracellular drug concentra tions and, thus, confers a multidrug resistance phenotype. 2. P-gp is encoded by MDR genes, which constitute a small gene family comprising two genes in humans and three genes in rodents. Only the MDR1 gene in humans and mdr1 and mdr3 genes in rodents have been demonstrated to be involved in drug resistance. 3. P-gp encoded by the human MDR1 gene i s a phosphorylated and glycosylated protein 1289 amino acids long, and consists of 2 halves that share a high degree of similarity. 4. A wid e variety of cancers have been shown to express P-gp, including solid tumors and hematological malignancies. This P-gp positivity can be evi denced at the time of diagnosis prior to chemotherapy or at relapse af ter treatment, and has been correlated with treatment failure and poor prognosis in several types of cancer. In addition, P-gp is also expre ssed by some normal tissues, such as liver and kidney. 5. P-gp express ion is regulated by various factors, including xenobiotics and hormone s. 6. P-gp-mediated multidrug resistance can be reversed by various un related compounds called chemosensitizers or reversing agents. These d rugs act through inhibition of P-gp function and have entered clinical trials. Copyright (C) 1996 Elsevier Science Inc.