Drug Resistance and its Clinical Circumvention - MAFIADOC.COM

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41 Drug Resistance and its Clinica l Circumvention Jeffrey Moscow, MD Erasmus Schneider, PhD Branimir I. Sikic, MD Charles S. Morrow, MD, PhD Kenneth H. Cowan, MD, PhD

Systemic therapy with cytotoxic drugs is the basis for most effective treatments of disseminated cancers. Additionally, adjuvant chemotherapy can offer a significant survival advantage to selected patients, following the treatment of localized disease with surgery or radiotherapy, presumably by eliminating undetected minimal or microscopic residual tumor. However, the responses of tumors to chemotherapeutic regimens vary, and failures are frequent owing to the emergence of drug resistance. The phenomenon of clinical drug resistance has prompted studies to clarify mechanisms of drug action and to identify mechanisms of antineoplastic resistance. It is expected that through such information, drug resistance may be circw11vented by rational design of new noncross-resistant agents, by novel delivery or combinations of known drugs, and by the development of other treatments that might augment the activity of or reverse resistance to known antineoplastics. Multiple mechanisms of antineoplastic failure have been identified using in vitro (tissue culture) and in vivo (animal and xenograft) models of antineoplastic resistance (Table 41-1 ). These include anatomic and pharmacologic mechanisms that may be uniquely pertinent to individual patients as well as cellular mechanisms within tumor cells. Although mechanisms of drug resistance have been largely determined in experimental systems, many have been implicated in at least some examples of clinical studies of chemotherapeutic failure . Evidence that bears upon these mechanisms of resistance as well as strategies to circumvent them are discussed below. GENERAL MEC HANI SMS O F DR UG RESIST ANCE Experimental selection of drug resistance by repeated exposure to single antineoplastic agents will generally result in cross-resistance to some related agents of the same drug class. This phenomenon is explained on the basis of shared drug transport carriers, drug metabolizing pathways, and intracellular cytotoxic targets of these structurally and biochemically similar compounds. Generally, the resistant cells retain sensitivity to drugs of different classes with alternative mechanisms of cytotoxic action. 1•2 Thus, cells selected

for resistance to alkylating agents or antifolates will usually remain sensitive to Wlfelated drugs, such as anthracyclines. Exceptions include emergence of cross-resistance to multiple, apparently structurally and functionally unrelated drugs, to which the patient or cancer cells were never exposed during the initial drug treatment. Despite apparent differences in their presumed sites of action within cells, the drugs associated with multidrug resistance (MDR) phenotypes frequently share common metabolic pathways or efflux transport systems. In this section, the processes related to drug resistance will be described. A more comprehensive discussion of mechanisms of resistance to specific classes of drugs will be discussed in subsequent sections. lahlr 41-1 Gcm•ral \Jc,:huni\TUS of l>rug Rt•c,ic,;taucc

Cellular and biochemical mechanisms Decreased drug accumulation Decreased dmg influx Increased drug efflux Altered intracellular trafficking of drug Decreased drug activation Increased inactivation of drug or toxic intermediate Increased repair of or tolerance to drug-induced damage to: Deoxyribonucleic acid (DNA) Protein Membranes Drug targets altered (quantitatively or qualitatively) Altered cofactor or metabolite levels Altered downstream effectors of cytotoxicity Altered signaling pathway and/or apoptotic responses to drug insult Altered gene expression DNA mutation, amplification, or deletion Altered transcription, post-transcription processing, or translation Altered stability of macromolecules Mechanisms relevant in vivo Pharmacologic and anatomic drug barriers (tumor sanctuaries) Host-drug interactions lncreased drug inactivation by normal tissues Decreased drug activation by normal tissues Relative increase in nonnal tissue drug sensitivity (toxicity) Host·tumor interactions

DECREASED DRUG ACCUMULATION Decreased intracellular levels of cytotoxic agents is one of the most common mechanisms of drug resistance. Since polar, water-soluble drugs cannot penetrate the lipid bilayer of the cell membrane and require specific mechanisms of cell entry, resistance to these drugs is readily mediated by down-regulation of drug uptake mechanisms in tumor cells. For example, decreased influx via a high-affinity folate-transport system, 3 as well as via a reduced folate carrier, 4 are well described causes of methotrexate resistance .5 ·6 For hydrophobic, nonpo lar drugs that can easily diffuse across the cell membrane, decreased intracellular drug concentrations can be achieved by increasing the activities of drug efflux pun1ps. For example , overexpression of the ?-glycoprotein (MDRIIABCBI) drug efflux pump is an important example of this mechanism of resistance.7•8 ALTERED DRUG METABOLISM Decreased drug activa tion, increased drug inactivation, or alterations in necessary cofactors can also confer resistance to selected antineoplastic agents. For example, decreased conversion of nucleobase analogues to their cytotoxic nucleoside and nucleotide derivatives by alterations in specific k.inases and phosphoribosyl transferase salvage enzymes can lead to resistance to these anticancer drugs.9, to Another example associated with resistance is decreased levels of carboxyesterase- an activity necessary to convert a topoisomerase I inhibitor, CPT- I I, to its active metabolite, SN38.11·1 2 On the other hand, enhanced inactivation of pyrimidine and purine analogues by increased expression of deaminases is linked to resistance toward these agents. 13 • 14 Finally, alterations in cofactor levels can also modify drug toxicity. For example, optimal formation of inhibitory complexes between 5-fluorodeoxyuridine monophosphate (FdUMP) and its target enzyme, thymidylate synthase, require the cofactor 5, I 0methylene tetrahydrofolate. 15 Alterations in the intracellular levels of this cofactor can lead to resistance to fluoropyrimidines . INCREASED REPAIR OR CELLULAR TOLERANCE TO DRUG-INDUCED DAMAGE Cells contain multiple complex systems involved in the repair

2 SECTION 11 I Chemotherapy

of damage to membranes and deoxyribonucleic acid (DNA), and changes in these repair processes can influence drug sensitivity. For example, resistance to cisplatin, a drug whose cytotoxic action involves intrastrand DNA crosslinkages (see below), is associated with increased DNA repair. Conversely, defects in mismatch repair are associated with tolerance to cisplatininduced DNA damage. 16 In this form of platinum resistance, the repair system is apparently unable to recognize platinum-DNA adducts and fails to activate the normal, appropriate programmed cell death response. ALTERED DRUG TARGETS Qualitative changes in the enzyme targets of antineoplastic drugs can compromise drug efficacy and have been associated with resistance to inhibitors of dihydrofolate reductase,l7·18 thymidylate synthase, 19 and topoisomerases I and 11 2 ()...25 Perhaps not surprisingly, alteration of the drug target has also been recognized as a mechanism of resistance to newer molecularly targeted chemotherapy. For example, clinical resistance to the BCR-ABL kinase inhibitor Gleevec (imatinib mesylate) results from the development of mutations in the kinase's drugbinding site.26 ALTERED GENE EXPRESSION Increased expression of target enzymes can also lead to drug resistance. These alterations may result from changes that occur at any point along the pathways of gene expression and regulation, including DNA deletion or amplification, altered transcriptional or post-transcriptional control of ribonucleic acid (RNA) levels, and altered post-translational modifications of proteins. In addition, the same molecular mechanisms that lead to oncogenesis can also lead to drug resistance through altered expression of drug targets. For example, loss of function of the retinoblastoma (Rb) gene leads to accumulation of the transcription factor E2F, which, in turn, activates transcription of at least two genes involved in antifolate drug resistance, dihydrofolate reductase and thymidylate synthase (reviewed by Banetjee and colleagues 27 ). Efforts to exploit these observations by using E2F as a marker of antifolate chemosensitivity have met with mixed results 28- 30

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Resistance associated with decreased drug accumulation ABC transporter-mediated resistance • P-Glycoprotein/MDR 1/ABCBl-mediated classic MDR • MRP family member-mediated MDR (currently at least 3 members, MRP I, 2, and 3 in ABCC 1, ABCC2, ABCC3 implicated in MDR drug efflux and detoxification) • BCRP (ABCG2)-mediated MDR (putative ABC half-transporter implicated in mitoxantrone and anthracycline resistance) LRP? (lung resistance protein, a major vault protein) Alterations in topoisomerases

seven subfamilies, ABCA - ABCG 31 Of these, at least three have been directly shown to cause MDR, namely MDR//P-glycoprotein (ABCBJ), MRPJ (ABCCJ) and BCRPIMXRIABC-P (ABCG2). Classic MDR associated with resistance to drugs listed in Table 41-3 is mediated by P-glycoprotein (MDRJ or ABCBJ). A similar but distinct MDR phenotype was attributed to the energy-dependent drug efflux activities of multidrug resistance protein (MRP) family members, including MRPJ or ABCC/.32-35 Another overlapping but discrete MDR phenotype is associated with increased expression of the recently isolated putative efflux transporter, breast cancer resistance protein (BCRP or ABCG2)36,37 MDR has also been described in association with overexpression of the lw1g resistance protein (LRP). The mechanism of LRP-associated resistance is unclear, and whether LRP alone is sufficient to confer resistance is unknown. It is speculated that as a major vault protein, LRP is involved in nucleocytoplasmic transport and may be able to prevent entry of drugs into the nucleus.38,39 Drug resistance defined by alterations in topoisomerases represents a third major category ofMDR.19-23,25,26 CLASS IC (MDRJIA B CB l-MEDIATED) MDR A model of MDR was described by Biedler and Riehm three decades ago.40 Exposure of cells to any of the drugs listed in Table 41-3 can generally result in MDR phenotype 7 ·8 Drug transport studies using parental and MDR cells demonstrate that the reduced cytotoxicity of these drugs is the result of decreased drug accumulation secondary to enhanced drug effiux.41 ·42 The emergence ofMDR

RESISTANCE TO MULTIPLE DRUGS De novo and acquired cross-resistance to multiple antineoplastic agents can result from several alternative factors and processes. Accordingly, we have grouped the major patterns of cross-resistance into several categories on the basis of their presumed underlying mechanisms (Table 41-2). First, MDR is frequently associated with decreased drug accumulation, usually because of increased drug efflux. MDR is generally thought to be caused by increased energy-dependent efflux of the anticancer drugs, mediated by the overexpression of ATP binding cassette (ABC) proteins. ABC proteins constitute a large family of 48 transport proteins organized into

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