Chapter 54 Mechanisms of Action of Antineoplastic Drugs

Abbreviations
Ara-C	Cytarabine
DNA	Deoxyribonucleic acid
FH₄	Tetrahydrofolate
5-FU	5-Fluorouracil
IV	Intravenous
MESNA	Sodium 2-mercaptoethane sulfonate
6-MP	6-Mercaptopurine
MTX	Methotrexate
RNA	Ribonucleic acid
6-TG	6-Thioguanine

Therapeutic Overview

Antineoplastic agents are used to treat more than 100 types of neoplastic diseases, with the goal of destroying malignant cells. Additional drugs (see Chapter 55) are used to enhance host defense mechanisms to eradicate those tumor cells not killed by the antineoplastic drugs. In clinical practice nearly all neoplastic diseases are treated by using multiple drugs, although only individual drugs, which form the basis for multiple drug therapy, are discussed in this chapter; multiple-drug protocols are described in Chapter 53.

The effectiveness of antineoplastic drugs varies greatly with the following:

• Type of cancer

• Biological and physiological condition of the patient

• Extent to which the tumor has grown or spread

The end point used to evaluate effectiveness (e.g., tumor response, patient survival) is also important. Most antineoplastic agents, particularly chemotherapeutic agents, are more effective destroying cells that are progressing through the cell cycle (Fig. 54-1) than destroying cells that are resting in the G₀ phase. The “growth fraction,” defined in Figure 54-1, is the fraction of cells progressing through the cycle. Besides tumor cells that may be proliferating, there are also some non-neoplastic cells undergoing division, particularly those of hair follicles, bone marrow, and intestinal epithelium. These rapidly dividing cells are especially sensitive to antineoplastic drugs and account for many of their undesirable side effects. It is believed that most, if not all, anticancer drugs kill cells primarily through a programmed, energy-dependent process called apoptosis, rather than through necrosis.

FIGURE 54–1 Growth cycle for mammalian cells. The cells are dormant in the G₀ (resting) phase. A variety of stimulants, often of unknown origin in clinical situations, cause cycling of cells to begin by entry into the G₁ phase (pre-DNA synthesis). Here, precursors for DNA are formed. DNA synthesis occurs in the S, or synthetic, phase. This is followed by premitotic synthesis and structural developments in the G₂ phase. Mitosis occurs in the M phase to produce two cells, each of which can continue to cycle by entry again into G₁ or can enter the resting phase, G₀. Growth fraction is defined as the total cells in the growth cycle (G₁, S, G₂, M) divided by the total cells (G₁, S, G₂, M, G₀).

The number of cultured neoplastic cells that survive exposure to each drug typically shows a first-order relationship to the drug concentration (Fig. 54-2). This means that the same fraction of cells is killed with each drug dose and that a series of several doses does not kill 100% of them. This “log-cell kill” hypothesis is compatible with the clinical observation that a functional host immune system is needed for killing all neoplastic cells and curing a patient.

FIGURE 54–2 Decline in viable cells is first order with respect to drug concentration. Many antineoplastic agents and cultured tumor cells follow this relationship, thus establishing the principle of a fixed percentage of viable cells killed per concentration of drug. This same relationship appears to apply in vivo, although the actual situation may be more complex. A threshold concentration of drug is often required to cause a noticeable decrease in cell survival. This phenomenon, called “survival shoulder,” may reflect endogenous repair processes.

Endogenous cellular defenses, such as thiols or deoxyribonucleic acid (DNA) repair enzymes, however, can necessitate a threshold of drug concentration, or “shoulder,” in the survival curves of patients receiving these drugs (see Fig. 54-2).

Of the four major types of tumors, the faster growing hematological (non-solid) types (leukemias and lymphomas) are more responsive to treatment than are the

Therapeutic Overview
Goal
Kill tumor cells selectively with no side effects
Uses
Treatment of systemic disease (curative and palliative) Decrease tumor burden Treatment of carcinomas, sarcomas, leukemias, lymphomas
Effects
Some but not all tumors respond
Considerations
Drug-delivery problems to individual cells Cycling versus noncycling cells log-cell kill (same fraction of cells killed per dose) Need for active immune system (host defenses) to eradicate remaining neoplastic cells Problem of central hypoxic zone of tumors Drug resistance

slower-growing solid types (carcinomas and sarcomas). Factors responsible for this difference are the more rapid doubling times of hematological malignancies and the greater ease of drug distribution to hematological cells than to solid tumors. Typically, the outer, more recently synthesized portions of many solid tumors are well vascularized and readily accessible to drugs. This is attributable in part to the growth of new blood vessels by angiogenesis. However, the inner and older portions of many solid tumors are hypoxic and often necrotic because angiogenesis is inadequate, making them poorly accessible to drugs. The inner cells may be dead or merely in the resting phase of the cell cycle and therefore still capable of returning to cycling. Delivery of drugs to inner portions of solid tumors is a major unsolved problem.

Antineoplastic drugs must enter the cell to produce cytotoxic effects. Some drugs can pass through the membrane by passive diffusion, with the concentration gradient driving uptake. Other drugs must bind to carrier proteins that transport the drug through the membrane and release it in the cytoplasm; this is especially common with antimetabolites. Such carrier-mediated transport is an active process that is not concentration driven, and the rate of transport is often limited by the fixed number of carrier molecules available.

Once the drug enters the cell and diffuses into the nucleus or other sites, the drug can react with target molecules to disrupt key processes necessary for cell viability. Considerations in the use of antineoplastic agents are in the Therapeutic Overview Box.

Mechanisms of Action

Basic Approaches

The basic mechanisms by which antineoplastic drugs kill tumor cells are summarized in Figure 54-3. Only compounds that show some selectivity for neoplastic cells are used clinically. In general, antimetabolites inhibit DNA synthesis, whereas alkylating agents, intercalators, and antibiotics damage or disrupt DNA, interfere with topoisomerase activity, or alter ribonucleic acid (RNA) structure. Steroids interfere with transcription, several plant alkaloids disrupt mitosis, agents such as asparaginase destroy essential amino acids needed for translation, and other drugs act through important growth factor signal transduction pathways. Many clinically used antineoplastic drugs must undergo either chemical or enzymatic modification to become actively cytotoxic.

FIGURE 54–3 Basic mechanisms by which antineoplastic drugs selectively kill tumor cells. E stands for enzymes, some of which are inhibited by these drugs. Inhibition of DNA or RNA synthesis or replication, production of miscoded nucleic acids, and formation of modified proteins are key mechanisms of action for many of these drugs.

Alkylating Agents

Alkylation refers to the covalent attachment of alkyl groups to other molecules. Alkylating agents came to be used for cancer therapy as a result of observations of the effects of the mustard gases on cell growth. Although these compounds are too toxic for clinical use in cancer, the first effective antineoplastic agents, including mechlorethamine, were developed from related nitrogen mustard alkylating agents and are still used today.

Alkylation takes place through chemical formation of a positively charged carbonium ion that reacts with an electron-rich site, particularly on DNA or RNA, to form modified nucleic acids. Most clinically used alkylating drugs have two active groups, which enable them to form covalent links between adjacent nucleic acid strands that are more difficult to repair than monofunctional adducts. These cross-links also prevent separation of the dual strands of DNA during cell cycling. For maximal kill, it is important to administer the maximally tolerated dose. The alkylation sequence for mechlorethamine reacting with the N-7 position of deoxyguanylate is shown in Figure 54-4. Although many other nucleophilic constituents, including RNA, proteins, and membrane components, become alkylated within cells, it is generally believed that the primary cytotoxic events occur through alkylation of DNA, especially by coupling to the N-7 position of the deoxyguanylates of either single- or double-stranded DNA.

FIGURE 54–4 Alkylation by mechlorethamine, showing positively charged intermediate ion and its covalent attachment to the N-7 position of two deoxyguanylate nucleotides of DNA.

Structures of several clinically used alkylating agents are shown in Figure 54-5. Cyclophosphamide undergoes a combination of enzymatic and chemical activation to form the active phosphoramide mustard alkylating agent (Fig. 54-6). Exposure of cells to cyclophosphamide and other alkylating agents can also lead to carcinogenesis. For example, leukemia is a well-known long-term complication in patients with Hodgkin’s disease treated with a regimen including mechlorethamine.

FIGURE 54–5 Structures of nitrogen mustards and busulfan.

FIGURE 54–6 Mechanism of enzymatic and chemical activation of cyclophosphamide to form active phosphoramide mustard. Acrolein has some antitumor activity but much less than that of the phosphoramide mustard.

Another group of antineoplastic alkylating agents in clinical use is the nitrosoureas. The structures and primary mechanisms of activation for these compounds are shown in Figure 54-7. In addition to alkylation of DNA, the nitrosoureas also cause carbamoylation of proteins, which may play a role in cytotoxicity. The alkylation route, however, is the major source of cytotoxicity. Nitrosoureas are lipophilic and can cross the blood-brain barrier, so they are often used to treat brain tumors.