[Cancer Biology & Therapy 2:4:Suppl. 1, S2-S4, July/August 2003]; ©2003 Landes Bioscience
`
`Models of Anti-Cancer Therapy
`Classical Chemotherapy
`Mechanisms, Toxicities and the Therapeutic Window
`
`Vikas Malhotra
`Michael C. Perry
`
`Ellis Fischel Cancer Center; University of Missouri; Columbia, Missouri USA
`
`*Correspondence to: Vikas Malhotra, M.D. Assistant Professor of Internal Medicine;
`Division of Hematology/Medical Oncology; University of Missouri—Ellis Fischel
`Cancer Center; 115 Business Loop 70 West, DC 116.71; Columbia, Missouri 65203
`USA; Tel.: 573.882-6163; Fax: 573.884.6051; Email: malhotrav@health.mis-
`souri.edu
`
`Received 02/14/03; Accepted 02/14/03
`
`Previously published online as a CB&T E-publication at:
`http://www.landesbioscience.com/journals/cbt/toc.php?volume=2&issue=0
`
`©2003 Landes Bioscience. Not for distribution.
`
`ABSTRACT
`Chemotherapy can be best used by understanding the principles of pharmacology,
`tumor biology, cellular kinetics and drug resistance. Here we try to focus on the major
`classes of chemotherapeutic drugs, their mechanisms of action, toxicities; and the thera-
`peutic window.
`
`THE THERAPEUTIC WINDOW
`The therapeutic window is the range of plasma drug concentrations with a high
`probability of therapeutic success, defined as tumor shrinkage. The therapeutic window
`for a typical population is sometimes inappropriate for an individual patient. This is related
`to the differing metabolism of chemotherapeutic agents in different individuals. The
`coexistence of genetic polymorphisms in drug metabolizing enzymes, targets, receptors,
`and transporters, in the context of drug and non-drug influences, may result in high
`frequencies of unusual drug toxicities. This has given rise to the new field of pharmacoge-
`netics. The importance of the therapeutic window is related to the fact that to be effective
`the drug concentrations have to be in the appropriate range. Levels too high will increase
`toxicity without adding to clinical benefit and levels too low may not produce optimum
`benefit.
`Normal Cell Kinetics. The cell cycle is composed of four phases. Cells that are
`committed to divide enter the G1 phase. Preliminary synthetic cellular processes occur that
`prepare the cell to enter the DNA synthetic (S) phase. Specific protein signals regulate the
`cell cycle and allow replication of the genome where the DNA content becomes tetraploid
`(4N). After completion of the S phase, the cell enters a second resting phase, G2, prior to
`undergoing mitosis. The cell progresses to the mitotic (M) phase, in which the chromosomes
`condense and separate and the cell divides, producing two daughter cells.
`Chemotherapeutic agents may be cell cycle specific or cell cycle-nonspecific. Cell cycle
`non-specific drugs, like the alkylating agents have a linear dose-response curve; that is, the
`fraction of cell kill increases linearly with the dose of drug. However, cell cycle-phase-specific
`drugs have a plateau with respect to cell killing ability. For example cytarabine is active
`only in the S phase
`Tumor Cell Kinetics. The growth of a tumor depends on several closely related factors:
`1. Cell cycle time, or the average time for a cell that has just completed mitosis to grow,
`re-divide and again pass through mitosis, determines the maximum growth rate of a tumor.
`2. Growth fraction is the fraction of cells undergoing cell division. This fraction is usually
`vulnerable to chemotherapy.
`3. The total number of cancer cells in the population is an indicator of total cancer burden.
`As the number of cells increases, so does the number of resistant cells, which leads to
`decreased curability. Large tumors also have greater compromise of blood supply and
`therefore impaired drug delivery to the tumor.
`Variations in these three factors are responsible for the variable rates of tumor growth
`observed among tumors of differing histologies, as well as among metastatic and primary
`tumors of the same histology. Tumors characteristically exhibit a sigmoid-shaped Gomp-
`ertzian growth curve, in which tumor doubling time varies with tumor size. Tumors grow
`most rapidly at small tumor volumes. As tumors become larger, growth slows based on a
`complex process dependent on cell loss and tumor blood and oxygen supply. In order to
`have the best chance for cure, chemotherapy must be given that can achieve a fractional
`cell kill in a logarithmic fashion (i.e., 1-log-kill is 90% of cells, 2-log-kill 99% of cells).
`From these concepts, chemotherapy models have been developed utilizing alternating
`non-cross-resistant therapies, induction-intensification approaches, and adjuvant
`chemotherapy regimens.
`
`Michael C. Perry
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`Ex. 1098-0001
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`CLASSICAL CHEMOTHERAPY
`
`Principles of Combination Chemotherapy. Combination
`chemotherapy provides maximum cell kill within the range of toxicity
`tolerated by the host for each drug. It also offers a broader range of
`coverage of resistant cell lines in a heterogeneous tumor population
`and prevents or slows the development of new drug-resistant cell
`lines. Drugs with different mechanisms of action and differing dose-
`limiting toxicities should be combined in order to allow for additive
`or synergistic effects on the tumor with minimum toxicity.
`Dose Intensity. Kinetic principles predict that, for drug-sensitive
`cancers, the factor limiting the capacity to cure is proper dosing. A
`dose reduction of approximately 20% can lead to a loss of up to 50%
`of the cure rate. Conversely, a 2-fold increase in dose can be associated
`with a 10-fold (1-log) increase in tumor cell kill in animal models.
`Drug Resistance. The most common mechanism of drug resist-
`ance is related to altered gene expression. Cells in the G0 phase are
`generally resistant to all drugs active in the S phase. Chemothera-
`peutic agents may be unable to kill tumor cells if there is insufficient
`drug concentration due to their presence in body locations where it
`is difficult to achieve effective drug concentrations or if there is some
`alteration in the metabolism of the drug after it is administered.
`MDR-1-Mediated Multidrug Resistance. Repeated exposure of
`a tumor to a single anti-neoplastic agent will generally result in
`cross-resistance to the drug and agents of the same drug class as the
`original drug. This can be due to the over expression of the MDR-1
`gene, which encodes a 170-kd transmembrane P-glycoprotein.
`P-glycoprotein is an energy-dependent pump that serves to remove
`exogenous toxins or endogenous metabolites from the cell. It is
`found in a wide range of normal tissues, including adrenal tissue,
`cells lining the renal tubule, cells lining the jejunum and colon, cells
`lining the bile canaliculi, and endothelial cells of capillaries in the
`brain and testes. A high level of MDR expression is reliably correlated
`with resistance to cytotoxic agents. Tumors that intrinsically express
`the MDR1 gene prior to chemotherapy characteristically display
`poor durable responses. MDR1 expression represents one of the
`most important mechanisms of acquired drug resistance. Rates of
`P-glycoprotein are commonly increased at relapse when compared to
`rates at diagnosis. Chemotherapeutic agents subject to MDR-1-
`mediated resistance include the anthracyclines, vinca alkaloids,
`paclitaxel , etoposide and mitomycin.
`
`CHEMOTHERAPEUTIC AGENTS CLASSIFIED BY MECHANISM
`OF ACTION
`Alkylating Agents. The alkylating agents impair cell function by
`forming covalent bonds with the amino, carboxyl, sulfhydryl,
`and phosphate groups in biologically important molecules. The
`important sites of alkylation are on DNA, RNA, and proteins. The
`chemotherapeutic and cytotoxic effects are directly related to the
`alkylation of nitrogen at the 7 position of guanine in DNA.
`Alkylating agents depend on cell proliferation for activity but are not
`cell cycle phase-specific. A fixed percentage of cells are killed at a
`given dose. Tumor resistance probably occurs through efficient
`glutathione conjugation or by enhanced DNA repair mechanisms.
`Alkylating agents are classified according to their chemical structures
`and mechanisms of covalent bonding; this drug class includes the
`nitrogen mustards, nitrosoureas, and platinum complexes, among
`other agents.
`Nitrogen Mustards. The nitrogen mustards, which include such
`drugs as mechlorethamine, mitomycin-C, vinca alkaloids, platinum
`and the anthracyclines are powerful local vesicants; as such, they can
`
`cause problems ranging from local tissue necrosis, to pulmonary
`fibrosis, to hemorrhagic cystitis. The metabolites of these compounds
`are highly reactive in aqueous solution, in which an active alkylating
`moiety, the ethylene immonium ion, binds to DNA. The hemato-
`poietic system is especially susceptible to these compounds. Dose
`limiting toxicity (DLT) includes myelosuppression and severe nausea
`and vomiting are common side effects. Occasionally alopecia, sterility,
`diarrhea and thrombophlebitis may be seen.
`Nitrosoureas. The nitrosoureas are distinguished by their high
`lipid solubility and chemical instability. These agents rapidly and
`spontaneously decompose into two highly reactive intermediates:
`chloroethyl diazohydroxide and isocyanate. They are thought to act
`through alkylation as well. The lipophilic nature of the nitrosoureas
`enables free passage across membranes; therefore, they rapidly
`penetrate the blood-brain barrier, achieving effective CNS concen-
`trations. As a consequence, these agents are used for a variety of
`brain tumors. Their dose limiting toxicity is myelosuppression.
`Platinum Agents. Cisplatin is an inorganic heavy metal complex
`that has activity typical of a cell cycle-phase-nonspecific alkylating
`agent. The compound produces intra-strand and inter-strand DNA
`cross-links and forms DNA adducts, thereby inhibiting the synthesis of
`DNA, RNA, and proteins. Carboplatin has the same active diamine
`platinum moiety as cisplatin, but this is bonded to an organic
`carboxylate group that allows increased water solubility and slower
`hydrolysis to the alkylating aqueous platinum complex, thus altering
`toxicity profiles. Dose limiting toxicities for cisplatin are renal insuf-
`ficiency, peripheral sensory neuropathy and ototoxicity. For carbo-
`platin the DLT is myelosuppression, especially thrombocytopenia.
`Antimetabolites. Antimetabolites are structural analogs of the
`naturally occurring metabolites involved in DNA and RNA synthesis.
`Their major effect is interfering with the building blocks of DNA
`synthesis and they are therefore most active in the S phase of the cell
`cycle and have little effect on the cells in G0. Consequently, these
`drugs are most effective in tumors that have a high growth fraction.
`Antimetabolites have a nonlinear dose-response curve, such that,
`after a certain dose, no more cells are killed despite increasing doses
`(fluorouracil [5-FU] is an exception). The antimetabolites can be
`divided into folate analogs, purine analogs, pyrimidine analogs,
`adenosine analogs, and substituted ureas. These include methotrexate,
`fluorouracil, cytarabine, gemcitabine, pentostatin, fludarabine and
`cladiribine.
`Natural Products. A wide variety of compounds possessing anti-
`tumor activity have been isolated from natural substances, such as
`plants, fungi, and bacteria. Likewise, selected compounds have
`semisynthetic and synthetic designs based on the active chemical
`structure of the parent compounds, and these, too, have cytotoxic
`effects.
`Antitumor Antibiotics. Bleomycin preferentially intercalates
`DNA at guanine-cytosine and guanine-thymine sequences, resulting
`in spontaneous oxidation and formation of free oxygen radicals that
`cause strand breakage. Major DLT is pulmonary toxicity occurring
`in 10–40% of the treated patients usually 4 to 10 weeks after starting
`therapy. Fevers, chills, anorexia and dermatologic toxicity are also
`frequently seen.
`Anthracyclines. The anthracycline antibiotics are products of the
`fungus Streptomyces percetus var caesius. They are chemically very
`similar, with a basic anthracycline structure containing a glycoside
`bound to an amino sugar, daunosamine. The anthracyclines have
`several modes of action. Most notable is intercalation between DNA
`base pairs and inhibition of DNA topoisomerases I and II. Oxygen
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`CLASSICAL CHEMOTHERAPY
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`free radical formation from reduced doxorubicin intermediates is
`thought to be a mechanism associated with cardiotoxicity.
`Epipodophyllotoxins. Etoposide is a semisynthetic epipodo-
`phyllotoxin extracted from the root of Podophyllum peltatum
`(mandrake). It inhibits topoisomerase II activity by stabilizing the
`DNA-topoisomerase II complex; this ultimately results in the
`inability to synthesize DNA, and the cell cycle is stopped in G1
`phase. Myelosuppression is the DLT for this class of agents.
`Vinca Alkaloids. The vinca alkaloids are derived from the
`periwinkle plant, Vinca rosea. This category includes vincristine,
`vinblastine and vinorelbine.Upon entering the cell, vinca alkaloids
`bind rapidly to the tubulin and inhibits its assembly.This binding
`occurs in S phase at a site different from that associated with paclitaxel
`and colchicine. Thus, polymerization of microtubules is blocked,
`resulting in impaired mitotic spindle formation in the M phase.
`Peripheral neurotoxicity is the DLT for this class of agents.
`Taxanes. Paclitaxel and docetaxel are semisynthetic derivatives of
`extracted precursors from the needles of yew plants. These drugs
`have a novel 14-member ring, the taxane. Unlike the vinca alkaloids,
`which cause microtubule disassembly, the taxanes promote micro-
`tubule assembly and stability, therefore blocking the cell cycle in
`mitosis. Docetaxel is more potent in enhancing microtubule assembly
`and also induces apoptosis. The major side effects of paclitaxel
`include myelosuppression, peripheral neurotoxicity, myalgias and
`acute hypersensitivity reactions. In addition, docetaxel can cause a
`syndrome of cumulative fluid retention characterized by peripheral
`edema and occasionally pericardial and pleural effusions.
`Camptothecin analogs. include irinotecan and topotecan. These
`semi-synthetic analogs of the alkaloid camptothecin, derived from
`the Chinese ornamental tree, Camptotheca acuminata, inihibit topoi-
`somerase I and interrupt the elongation phase of DNA replication.
`Topotecan’s DLT is myelosuppression, and in addition, irinotecan
`can cause acute and delayed onset diarrhea.
`
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