Gen. Pharmac. Vol. 27, No. 2, pp. 251-255, 1996 Elsevier Science Inc. Printed in the USA. ELSEVIER ISSN 0306-3623/96 0306-3623(95)02013-4 All rights reserved Cellular Resistance to Anthracyclines Dorte Nielsen, Christian Maare and Torben Skovsgaard DEPARTMENTOF ONCOLOGY R, HERLEV HOSPITAL,UNIVERSITY OF COPENHAGEN, DENMARK ABSTRACT. The antracyclines induce muhiple intraceUular effects; however, inhibition of the nuclear enzyme topoisomerase II (TOPO II) is the main mechanism of action. Resistance to anthracyclines in tumor ceils is muhifactorial. The main mechanisms are: (1) the classic muhidrug resistance (MDR) phenotype, which is due to the presence of P-glycoprotein (PGP) in plasma membrane, that is, a Upump" that can extrude a wide range of anticancer drugs. Membrane-active drugs (e.g., verapamil) have been found in vitro to reverse this phenotype. Most clinical studies including chemosensitizers have, however, been disappointing. (2) Non.PGP-mediated MDR: this phenotype is characterized by expression of other proteins in the plasma membrane which are also able to extrude anticancer drugs. (3) Changes in the intracellular distribution of drug: this mechanism has been demonstrated in several cell lines, most often in combination with PGP or non-PGP-mediated resistance. (4) Glutathione transferases (GST) and detoxification mechanisms: these represent a muhigene family of enzymes that conjugate glutathione to chemically reactive groups. Direct evidence for a causative role of GST in anthracycline resistance is missing. (5) Alterations in TOPO II (at-MDR): DNA topoisomerases are involved in several aspects of DNA metabolism, in particular genetic recombination, DNA transcription, and chromosome segregation. Low levels of expression or alterations in TOPO II are associated in vitro with resistance. (6) Increased DNA repair: in several cell lines, an increase in the efficacy of DNA repair has been associated with resistance to doxorubicin (DOX). So far, only classic MDR has been shown to contribute to resistance in clinical conditions, whereas evidence for the other mechanisms of resistance is still missing. GEN PrtARMAC27;2:251-255, 1996. KEY WORDS. Antrinoplastic agents, drug resistance INTRODUCTION The anthracyclines are some of the most valuable anticancer drugs in present day clinical use. Among these, DOX is the best known, being an integral part of the treatment of malignancies such as carcinoma of the breast, lung, and ovary, and soft tissue sarcomas and leukemias. Currently, however, only a minority of patients with neoplastic dis- eases are cured. Reasons for clinical resistance may include pharmacoki- netic factors such as absorption, distribution, metabolism, and excretion of drugs. It is generally accepted, however, that cellular drug resistance is one of the major reasons for failure of treatment. Resistance may be inherent or acquired, either as a stable change within the cell or induced following drug administration. The purpose of this review is to examine the phenomenon of resistance to classic anthracyclines (DOX and daunorubicin [DNR]), and to assess those means by which the different resistance mechanisms may be overcome, at least in the laboratory. DRUGS The anthracyclines are antibiotics produced from the streptomyces species. They are weak bases and have a planar cyclic anthraquinone nucleus attached to an amino sugar (Fig. 1). MECHANISMS OF ACTION The anthracyclines induce multiple intracellular effects such as free oxygen radical formation, covalent binding to DNA, DNA intercala- Address reprint requests to: Dr. Dorte Nielsen, Department of Oncology ONK, Finsen Center, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, DK- 2100 Copenhagen 0 , Denmark. Received 4 May 1995. tion, and direct membrane toxicity, but inhibition of the nuclear enzyme TOPO II seems to be the final target leading to cell death (Cunnings et al., 1991). TOPO II alters the topology of DNA by passing an intact double-strand of DNA through a transient double-strand break. During this process, an intermediate DNA-enzyme complex (the cleavable complex) is formed. By covalent binding to and thereby stabilizing this complex, anthracyclines and other TOPO II-interacting drugs inhibit the rejoining action of the enzyme, resulting in DNA double-strand breaks. RESISTANCE MECHANISMS Most information regarding mechanisms of resistance derives from in vitro models of cells selected by exposure to extremely high levels of drugs which cannot be achieved clinically. These studies have shown that there are many mechanisms of resistance to anthracyclines and that resistance is often multifactorial. The main mechanisms recognized so far are" (1) the classic MDR phenotype; (2) non-PGP-mediated MDR; (3) changes in intracellular distribution of drug; (4) increased GST content and detoxification mechanisms; (5) alterations in TOPO II; and (6) increased DNA repair. CLASSIC MDR (PGP.MEDIATED MDR) Structure of PGP, the mdrl Gene The classic MDR phenotype is a well-characterized experimental phe- nomenon (Gottesman and Pastan, 1993, Skovsgaard et al., 1994). MDR cells are found to be resistant to several natural product drugs (e.g., anthracyclines, vinca alkaloids, and epipodophyllotoxins)and are char- acterized by a decrease in drug accumulation and by the expression of a transmembrane 170-kD glycoprotein, called P-glycoprotein (PGP). This protein has 12 hydrophobic domains and contains two putative