11.3 Novel Substances in the Treatment of Lung Cancer for the Radiation Oncologist

11.3 Novel Substances in the Treatment of Lung Cancer for the Radiation Oncologist

L. Chinsoo Cho, Hak Choy

L. C. Cho, M.D., Associate Professor H. Choy, M.D., Professor and Chairman

Department of Radiation Oncology, University of Texas, Southwestern Medical Center at Dallas, 5801 Forest Park, Dallas, TX 75390, USA

11.3.1 Introduction

Radiotherapy has been a major treatment modality for regionally confi ned lung cancers. The rate of treat- ment failure is still high, particularly for large tumors or advanced disease, which lowers the overall tumor response rate to radiotherapy and the patient sur- vival. Improvements in radiotherapy have been made both in technological innovations, allowing delivery of higher radiation doses to the tumor or lower doses to normal tissues, and in the implementation of strat- egies that modulate the biological response of tumors or normal tissues to radiation. The latter strategies include altered fractionation of radiotherapy, com-

bined modality therapy using systemic chemother- apy or biological agents, and, more recently, targeting of molecular processes and signaling pathways that have become dysregulated in cancer cells.

The combination of chemotherapeutic drugs with radiation has had a signifi cant impact on current ra- diotherapy practice for non-small cell lung cancer (NSCLC). This combination has been used for many years but has now become a common treatment op- tion for lung cancer patients. This is particularly true for concurrent chemoradiotherapy, which in many recent clinical trials has been shown to be superior to radiotherapy alone in controlling local-regional dis- ease and in improving patient survival. Combining chemotherapeutic drugs with radiotherapy has a strong biologic rationale. Such chemotherapeutic agents reduce the number of tumor cells undergoing radiotherapy by their independent cytotoxicity and often render tumor cells more susceptible to killing by ionizing radiation. An additional benefi t of combined treatment is that chemotherapeutic drugs, by virtue of their systemic activity, may also act on subclini- cal metastatic disease. Most chemotherapeutic drugs have been chosen for combination with radiotherapy based on their known effectiveness in lung cancer.

Alternatively, agents that are effective in overcoming resistance mechanisms associated with radiotherapy could be chosen. There have been improvements in clinical outcome with concurrent chemoradiotherapy using traditional drugs, such as cisplatin, and these studies have led to extensive research on exploring newer chemotherapeutic agents for their interactions with radiation. A number of new potent chemothera- peutic agents, including taxanes, nucleoside analogs, and topoisomerase inhibitors, have entered clini- cal trials or practice. Preclinical testing has shown that they are potent enhancers of radiation response and thus might improve the therapeutic outcome of chemoradiotherapy. Also, rapidly emerging molecu- lar targeting strategies are aimed at improving the ef- fi cacy of chemoradiotherapy.

This chapter reviews the biologic rationale and principles fundamental to the use of radiotherapy

CONTENTS

11.3.1 Introduction 447

11.3.2 Current Strategies in Chemoradiotherapy 448 11.3.3 Drug-Radiation Interactions 448

11.3.3.1 Radiation-Induced Damage 448

11.3.4 Inhibition of Cellular Radiation Injury Repair 448 11.3.5 Cell Cycle Redistribution 449

11.3.5.1 Tumor Hypoxia and Radioresistance 449 11.3.5.2 Inhibition of Tumor Cell Repopulation 450 11.3.5.3 Emerging Strategies for Improvement in Chemoradiotherapy 451

11.3.5.4 Timing of Therapy 451 11.3.5.5 Increasing Antitumor Effi cacy

of Existing Chemotherapeutic Drugs 451 11.3.6 Incorporation of Molecular Targeting 452 11.3.6.1 Epidermal Growth Factor Receptor 452 11.3.6.2 Cyclooxygenase-2 454

11.3.6.3 Tumor Angiogenesis Inhibitor 454 11.3.6.4 RAS Farnesylation Inhibitors 455 11.3.6.5 Promoters of Apoptosis 455 11.3.6.6 Normal Tissue Protection 455 11.3.6.7 Topoisomerase I Inhibitors 456 11.3.7 Conclusion 457

References 457

with systemic therapy, the knowledge of which is essential in developing the optimal treatment strat- egies. It also overviews novel biologic therapy com- bined with radiotherapy in the treatment of NSCLC.

11.3.2

Current Strategies in Chemoradiotherapy The goals of combining chemotherapy with radio- therapy in the management of lung cancer are to increase patient survival by improving local-regional tumor control and by reducing distant metastases.

Combined modality treatment can further improve positive therapeutic outcome of individual treat- ments through a number of specifi c strategies, which Steel and Peckham (1979) classifi ed into four groups: “spatial cooperation,” independent toxicity, enhancement of tumor response, and protection of normal tissues.

“Spatial cooperation” was the initial rationale for combining chemotherapy with radiotherapy, where the action of radiation and chemotherapeutic drugs is directed towards different anatomical sites. Localized tumors would be the domain of radiotherapy, as large doses of radiation can be given. On the other hand, chemotherapeutic drugs are likely to be more effec- tive in eliminating disseminated micrometastases.

Thus, the cooperation between radiation and chemo- therapy is achieved through the independent action of two modalities. The concept of “spatial coopera- tion” is also applied in the treatment of hematologi- cal malignancies that have spread to “sanctuary” sites such as the brain. These sites are poorly accessible to chemotherapeutic agents, and thus they are more ap- propriately treated with radiotherapy.

Independent toxicity is another important strat- egy for increasing the therapeutic ratio of chemo- radiotherapy. Normal tissue toxicity is the main dose-limiting factor for both chemotherapy and radiotherapy. Therefore, combinations of radiation and chemotherapy would be better tolerated if drugs were selected such that toxicities do not overlap with, or minimally add to, radiation-induced toxici- ties. This strategy requires a thorough knowledge of chemotherapy toxicity, underlying mechanisms, and drug pharmacokinetics. Careful drug selection based on these mechanisms may minimize normal tissue damage while retaining tumoricidal effi cacy when combined with radiotherapy.

Another strategy in chemoradiotherapy is to exploit the ability of chemotherapeutic agents to

enhance tumor response to radiotherapy. The en- hancement denotes the existence of interaction be- tween chemotherapeutic drugs and radiation at the molecular, cellular, or metabolic level, resulting in an antitumor effect greater than would be expected on the basis of additive actions of chemotherapy and radiotherapy. The enhancement must be selective or preferential to tumors compared to critical normal tissues in order to achieve therapeutic gain. The abil- ity of chemotherapeutic agents to enhance tumor radioresponse by counteracting factors associated with tumor radioresistance is a major rationale for concurrent radiotherapy.

An additional strategy is to protect normal tis- sues in order to deliver higher doses of radiation to the tumor. This protection can be achieved through technical improvements in radiation delivery or ad- ministration of drugs that exert increased protection of normal tissues against the damage by radiation or drugs.

11.3.3

Drug-Radiation Interactions 11.3.3.1

Radiation-Induced Damage

Radiation induces many different effects on DNA, which is the critical target for radiation damage.

The effect may include single-strand breaks, double- strand breaks (DSBs), base damage, and DNA-DNA and DNA-protein cross-links. DSBs and chromosome aberrations that occur in association with or as a con- sequence of DSBs are generally considered to be the principal damage that ultimately results in cell death (Radford 1986). Any agent that makes DNA more susceptible to radiation damage may enhance cell killing. Certain drugs, such as halogenated pyrimi- dines, incorporate into DNA and make it more sus- ceptible to radiation damage (Kinsella et al. 1987).

11.3.4

Inhibition of Cellular Radiation Injury Repair

Both sublethal (SLDR) (Elkind and Sutton 1959)

and potentially lethal (PLDR) (Little et al. 1973)

damages caused by radiation can be repaired. SLDR

is rapid, with a half-time of about 1 hour, and is usu-

ally complete within 6 hours after irradiation. This

time between two radiation fractions allows radia- tion-induced DSBs in DNA to rejoin and repair. SLDR is expressed as the restitution of the shoulder on the cell survival curve for the second dose. PLDR occurs when environmental conditions prevent cells from dividing. Preventing cells from division allows completion of repair of DNA lesions that would have been lethal had DNA undergone replication shortly after irradiation. PLDR is considered to be a major determinant responsible for radioresistance in some tumor types, such as melanomas.

Many chemotherapeutic agents used in chemora- diotherapy interact with cellular repair mechanisms and inhibit repair, and hence may enhance cell or tis- sue response to radiation. For example, halogenated pyrimidines enhance cell radiosensitivity not only through increasing initial radiation damage but also by inhibiting cellular repair (Kinsella 1987; Wang et al. 1994). Nucleoside analogs, such as gemcitabine, are a potent in inhibitor of the repair of radiation-in- duced DNA and chromosome damage (Plunkett et al. 1995; Lawrence et al. 1997; Gregoire et al. 1999;

Milas et al. 1999a).

11.3.5

Cell Cycle Redistribution

Both chemotherapeutic agents and radiation are more effective against proliferating than nonproliferating cells. Their cytotoxicity further depends on the posi- tion of cells in the cell cycle. Terasima and Tolmach (1963) reported that radiosensitivity of cells varied widely depending on which phase of the cell cycle the cells were in at the time of irradiation. Cells in the G

and M cell cycle phases were about three times more sensitive than cells in the S phase. The exact mecha- nism for this variability is still unknown.

The infl uence of cytotoxic agents on the cell cy- cle can be therapeutically exploited in chemoradio- therapy by using cell cycle redistribution strategies.

For example, some chemotherapeutic drugs, such as taxanes, can block transition of cells through mitosis.

This results in accumulation of cells in the radiosen- sitive G

and M phases of the cell cycle. The enhanced radioresponse of cells can be demonstrated in vitro (Tishler et al. 1992; Choy et al. 1993) and of tumors in vivo (Milas et al. 1995, 1999b). However, this cell- cycle mechanism of taxane-induced enhancement of tumor radioresponse is dominant only in tumors that are resistant to paclitaxel or docetaxel as a single treatment. Although the drug does not substantially

affect tumor growth in taxane-resistant tumors, tu- mors do exhibit signifi cant transient accumulation of cells in mitosis 6–12 hours after the treatment (Milas et al. 1999b).

Elimination of the radioresistant S phase cells by the chemotherapeutic agents may be another cell- cycle redistribution strategy in chemoradiotherapy.

Nucleoside analogs, such as fl udarabine or gem- citabine, are good examples of the agents that be- come incorporated into S phase cells and eliminate them by inducing apoptosis (Gregoire et al. 1999;

Milas et al. 1999a). In addition to purging S-phase cells, the analogs induce the surviving cells to un- dergo parasynchronous movement to accumulate in G

and M phases of the cell cycle between 1 and 2 days after drug administration, a time when the highest enhancement of tumor radioresponse is ob- served (Milas et al. 1999a).

Tumors with a high cell growth fraction are likely to respond better to the cell-cycle redistribution strategy in chemoradiotherapy than tumors with a low cell growth fraction.

11.3.5.1

Tumor Hypoxia and Radioresistance

Solid malignant tumors often display abnormal vas- cularization, both in the number of blood vessels and vessel function. The blood supply to tumor cells may be inadequate, and multiple tumor microregions may be hypoxic, acidic, and eventually necrotic. Hypoxia occurs at distances from blood vessels >100–150 ìm.

The hypoxic cell content in tumors varies widely and can be more than 50%. The presence of hypoxia may induce aggressive and virulent tumor cell variants and stimulate metastatic spread (Brizel et al. 1996;

Brown and Giaccia 1998). Hypoxic cells are 2.5–3

times more resistant to radiation than well-oxygen-

ated cells. The fact that hypoxia may be a critical

factor in radiotherapy is suggested by the fi ndings

that reduced hemoglobin levels (Bush et al. 1978) and

low tumor pO

(Hockel et al. 1993; Nordsmark et

al. 1996) are associated with higher treatment failure

rates in some tumors. Also, there are reports show-

ing that local tumor control by radiotherapy can be

improved by the use of hypoxic cell radiosensitiz-

ers (Dische 1988) or hyperbaric oxygen (Henk and

Smith 1977). With respect to the effects of chemo-

therapy, hypoxic regions are less accessible to chemo-

therapeutic drugs; in addition, hypoxic tumor cells

are either nonproliferating or proliferate poorly and

as such do not respond well to chemotherapy.

Combining chemotherapeutic agents with ra- diotherapy can reduce or eliminate hypoxia or its negative infl uence on tumor radioresponse. Most chemotherapeutic drugs preferentially kill prolifer- ating cells, which are primarily found in well-oxy- genated regions of the tumor. Because these regions are located at a close proximity to blood vessels, they are easily accessible to chemotherapeutic agents.

Destruction of tumor cells in these areas will lead to an increased oxygen supply to hypoxic regions and hence reoxygenate hypoxic tumor cells. Massive loss of cells after chemotherapy lowers the interstitial pressure, which then allows reopening of previously closed capillaries and reestablishment of blood sup- ply. It also causes tumor shrinkage so that previously hypoxic areas are closer to capillaries and thus ac- cessible to oxygen. Finally, by eliminating oxygenated cells, more oxygen becomes available to cells that sur- vived chemotherapy. It was recently shown that tu- mor reoxygenation is a major mechanism underlying the enhancement of tumor radioresponse induced by taxanes in tumors sensitive to these drugs (Milas et al. 1995).

Another approach to counteract the negative im- pact of hypoxia is selective killing of hypoxic cells through bioreductive drugs, such as tirapazamine (Brown and Giaccia 1998), which undergo reduc- tive activation in a hypoxic milieu, rendering them cytotoxic. A related possibility is exploiting the acidic state (low pH) of tumors – which develops as a result of hypoxia-driven anaerobic metabolism that pro- duces lactic acid (Vaupel et al. 1989) – through the use of drugs that selectively accumulate in acidic en- vironments or become activated by low pH (Tannock and Rotin 1989).

One of the newer agents to counteract tumor hy- poxia is RSR13. RSR13 (efaproxiral) is a synthetic small molecule that enhances the diffusion of oxygen to hypoxic tumor tissues from hemoglobin. It allows the partial pressure of oxygen to increase in tumor tissues to enhance the effi cacy of radiation therapy and certain chemotherapeutic drugs. Preclinical stud- ies have demonstrated that RSR13 increases normal tissue oxygenation, reduces tumor hypoxia, and im- proves the effi cacy of radiation therapy. Preliminary clinical results using RSR13 have been reported for locally advanced inoperable stage IIIA-IIIB NSCLC (Hak Choy et al. 2001). All patients received two cy- cles of induction chemotherapy that consisted of pa- clitaxel 225 mg/m

and carboplatin AUC 6, followed by thoracic radiation therapy at a total dose of 64 Gy delivered in 32 fractions with concurrent daily infu- sion of RSR13 at a dose of 50–100 mg/kg. The over-

all response rate was 88.6%, with a 1-year survival of 64.7 %. There is currently a phase III randomized trial testing the role of RSR13 in stage III NSCLC.

11.3.5.2

Inhibition of Tumor Cell Repopulation

The constant balance between cell production and cell loss maintains the integrity of normal tissues.

When this balance is disturbed by cytotoxic action of chemotherapeutic drugs or radiation, the integ- rity of tissues is reestablished by an increased rate of cell production. The cell loss after each fraction of radiation during radiotherapy induces compensa- tory cell repopulation, the extent of which determines tissue tolerance to radiotherapy. In contrast to nor- mal tissues, malignant tumors are characterized by imbalance between cell production and cell loss in favor of cell production. And, as with normal tissues, tumors also respond to radiation- or drug-induced cell loss with a compensatory regenerative response.

Preclinical studies have provided evidence demon- strating that the rate of cell proliferation in tumors treated by radiation or chemotherapeutic drugs is higher than that in untreated tumors (Hermens and Barendsen 1978; Stephens and Steel 1980; Milas et al. 1994). This increased rate of treatment-induced cell proliferation is commonly termed accelerated repopulation. Accelerated repopulation of tumor clonogens has been shown to occur during clinical radiotherapy as well. Withers et al. (1988) showed that the total dose of radiation needed to control 50% of head and neck carcinomas progressively in- creased with time whenever radiotherapy treatment was prolonged beyond 1 month. This increase in ra- diation dose required to achieve tumor control was greater than what would be anticipated based on the pretreatment tumor volume doubling time of about 60 days for head and neck tumors. The increase was attributed to accelerated repopulation, and it was es- timated to average about 0.6 Gy/day (Withers et al.

1988) but may be as high as 1 Gy/day (Taylor et al.

1990).

Although accelerated cell proliferation is benefi -

cial for normal tissues because it spares them from

radiation damage, it has an adverse impact on tumor

control by radiotherapy or chemotherapy. Therefore,

any approach that reduces or eliminates accelerated

clonogen repopulation in tumors would improve ra-

diotherapy. Chemotherapeutic drugs, because of their

cytotoxic or cytostatic activity, can reduce the rate of

proliferation when given concurrently with radio-

therapy, and hence increase the treatment’s effective- ness. Caution must be taken to select drugs that pref- erentially affect rapidly proliferating cells and that preferentially localize in malignant tumors. However, the main limitation of concurrent chemoradiother- apy is the enhanced toxicity of rapidly dividing nor- mal tissues because most available chemotherapeu- tic agents show poor tumor selectivity. Experimental evidence suggests that drug-induced accelerated cell repopulation can actually make the tumor more dif- fi cult to control with radiation (Stephens and Steel 1980; Milas et al. 1994).

11.3.5.3

Emerging Strategies for Improvement in Chemoradiotherapy

Despite increasing therapeutic achievements of chemoradiotherapy for patients with NSCLC, the use of this form of therapy is still very much restricted by its narrow therapeutic index. The available agents are either insuffi ciently effective on their own or in com- bination with radiation against tumors, or normal tissue toxicity prevents the use of effective doses of drugs or radiation. Signifi cant preclinical and clinical research has been undertaken to improve chemora- diotherapy, and includes the development of more se- lective and more effective chemotherapeutic agents.

In addition, drugs that either protect normal tissues from injury by drugs or radiation or that selectively target molecular processes responsible for tumor ra- dio- or chemoresistance have been developed.

11.3.5.4

Timing of Therapy

Induction chemotherapy has resulted in therapeutic improvement in a number of clinical trials when compared to radiotherapy, but in general the thera- peutic benefi ts are below expectations. A number of factors could account for this, including acceler- ated repopulation of tumor cell clonogens and se- lection or induction of drug-resistant cells that are cross-resistant to radiation. The preclinical fi ndings provide solid evidence for the existence of acceler- ated repopulation in tumors treated with chemo- therapeutic agents. Although development of drug resistance is a signifi cant problem in chemotherapy, a similar degree of evidence that cells that acquire chemotherapy resistance are also resistant to radia- tion is lacking.

Concurrent chemoradiotherapy consists of ad- ministering chemotherapeutic agents during a course of radiotherapy. Concurrent treatment is intended to treat both the metastatic foci and the primary tumor. In addition, it incorporates the advantage of drug-radiation interactions to maximize tumor ra- dioresponse. The drug scheduling in relation to ra- diation fractions is important. The selection of op- timal timing of drug administration must be based on multiple factors, including mechanisms of tumor radioenhancement by a given drug, the drug’s nor- mal tissue toxicity, and conditions under which the highest enhancement is achieved. The data from pre- clinical studies contribute to the selection of the most optimal schedules. For example, it has been demon- strated that murine tumors sensitive to taxanes show enhanced radioresponse when the drug treatment precedes radiation by 1–3 days (Milas et al. 1999b).

A major mechanism for tumor radioenhancement was reoxygenation of hypoxic cells. Based on this preclinical information, one would anticipate that in clinical protocols such tumors would best respond to a bolus of a taxane given once or twice weekly during radiotherapy. In contrast, tumors resistant to taxanes may benefi t from daily administration of a taxane, since they show accumulation of radiosensitive G2 and M cells 6–12 hours after drug administration. If the objective is to counteract rapid repopulation of tumor cell clonogens induced by radiation, then ad- ministration of cell-cycle-specifi c chemotherapeutic agents during the second half of radiotherapy might be more effective. Optimal scheduling is essential in concurrent chemotherapy, not only to maximize tu- mor radioresponse but also to minimize toxicity to critical normal tissues. The enhancement in normal tissue complications remains the major limitation of concurrent chemoradiotherapy. Nevertheless, con- current chemoradiotherapy has provided better clin- ical results both in terms of local tumor control and patient survival than have other modes of chemora- diotherapy combinations (Munro 1995; Morris et al. 1999). (See Table 11.3.1.)

11.3.5.5

Increasing Antitumor Effi cacy of Existing Chemotherapeutic Drugs

A number of chemotherapeutic agents are effective against lung cancer and are potent radiosensitizers.

Among these are taxanes, nucleoside analogs, and

topoisomerase inhibitors. However, normal tissue

toxicity is still a major limitation for the effective use

of these agents. One approach to make currently ex- isting chemotherapeutic drugs more effective against tumors and less toxic to normal tissues is to conjugate them with water-soluble polymeric drugs, such as polyglutamic acid. These conjugates accumulate in tumors and release the active drug into the tumor in high concentrations and for a longer time. This effect is thought to be due to the increased permeability and retention effect of macromolecular compounds in solid tumors (Maeda et al. 1992; Li et al. 2000a).

The abnormal vasculature in tumors is porous to macromolecules, but high concentrations of drug can build up in tumors because of inadequate lymphatic drainage, whereas polymer-drug conjugates are con- fi ned to the bloodstream in normal tissue (Maeda et al. 1992). A recently developed polyglutamic acid- paclitaxel conjugate (CT-2103) may be less toxic and more effective against tumors in preclinical studies than unconjugated paclitaxel (Li et al. 2000a, 2000b).

It is undergoing phase III clinical trials in the U.S.

(Phase III Randomized Study of Polyglutamate Paclitaxel ).

Another example is oxaliplatin, which is a third- generation cisplatin compound. Platinum chemo- therapy has been used widely in managing advanced NSCLC. Oxaliplatin is a new diamonicyclohexane that has a different toxicity profi le than that of other plati- num compounds such as cisplatin and carboplatin.

Oxaliplatin has almost no nephrotoxicity, produces mild nausea and vomiting, and has a mild to moder- ate hematologic toxicity. The dose-limiting toxicity of oxaliplatin includes dose-dependent and revers- ible peripheral neuropathy. Based on the favorable toxicity profi le of oxaliplatin as well as the potential for even higher activity against NSCLC, a combined modality therapy of oxaliplatin and thoracic radio- therapy will likely be started soon. Several studies

have reported the use of oxaliplatin in NSCLC, but none has incorporated radiotherapy (Monnet et al.

1998, 2001, 2002; Faivre et al. 2002; Kakolyris et al.

2002; Vittorio et al. 2003; Kourousis et al. 2003) The use of oxaliplatin has the potential advantage of reducing cisplatin- or carboplatin-associated tox- icity as well as potentiating the effects of radiation.

11.3.6

Incorporation of Molecular Targeting

Recent advances in molecular biology have identifi ed a number of molecular determinants that may be responsible for resistance of cancer cells to radiation or other cytotoxic agents. Among these determinants are epidermal growth factor receptor (EGFR), cyclo- oxygenase-2 (COX-2) enzyme, mutated ras, angio- genic molecules, and various other molecules that are involved in signal transduction pathways (Mason et al. 2001).

11.3.6.1

Epidermal Growth Factor Receptor

EGFR is a transmembrane glycoprotein with a ty- rosine kinase activity. There are in fact four trans- membrane receptor tyrosine kinases in the epi- dermal growth factor receptor class: EGFR, HER2, HER3, and HER4. On binding to a ligand, such as epidermal growth factor (EGF) or transforming growth factor-_ (TGF-_), EGFR undergoes auto- phosphorylation and initiates transduction signals regulating cell division, proliferation, differentiation, and death (see Fig. 11.3.1). EGFR plays an impor-

Table 11.3.1 Advantages and disadvantages of different chemoradiation sequencing strategies

Strategy Advantages Disadvantages

Sequential chemoradiation • Least toxic

• Maximize systemic therapy

• Smaller radiation fi elds if induction shrinks tumor

• Increased treatment time

• Lack of local synergy

Concurrent Chemoradiation • Shorter treatment time

• Radiation enhancement

• Compromised systemic therapy

• Increased toxicity

• No cytoreduction of tumor Concurrent chemoradiation

and posterior chemotherapy

• Maximize systemic therapy

• Radiation enhancement

• Both local and distant therapy delivered upfront

• Increased toxicity

• Increased treatment time

• Diffi cult to complete chemotherapy after chemoradiation

tant role in tumor growth and tumor response to cytotoxic agents, including ionizing radiation. The receptor is frequently expressed in many types of cancers. It is often associated with aggressive tumors, poor patient prognosis, and tumor resistance to treat- ment with cytotoxic agents (Mendelsohn and Fan 1997; Schmidt-Ullrich et al. 2000). In vitro stud- ies have provided evidence linking EGFR with drug resistance. While incorporation of EGFR into tumor cells increases their drug resistance (Dickstein et al. 1995), the blockade of the EGFR-mediated path- way with antibodies to EGFR enhances the sensitiv- ity of tumor cells to chemotherapy (Mendelsohn and Fan 1997) and radiotherapy (Huang et al. 1999).

In vivo studies have shown that blockade of EGFR with anti-EGFR monoclonal antibody, such as with C225, or interference with its signaling processes can improve tumor treatment with chemotherapy and radiotherapy (Milas et al. 2000; Huang and Harari 2000). C225 (Cetuximab; Erbitux, Imclone, NJ, USA) is a highly specifi c monoclonal antibody that binds to EGFR and blocks its activation. Synergistic effects with radiotherapy have been seen in NSCLC cell lines that expressed EGFR (Salek et al. 1999; Raben et al. 2002).

A phase I study of C225 and radiotherapy com- bination in advanced head and neck cancer was

generally well tolerated by the patients. The C225- related morbidity included fever, nausea, hepatic transaminase elevation, and skin reaction. Thirteen of 15 evaluable patients achieved complete response (Robert et al. 2001). A study found that EGFR lev- els were inversely correlated with radiation-induced apoptosis (Akimoto et al. 1999). There are also data to suggest that EGFR expression may be increased as a defense mechanism against the harmful ef- fects of ionizing radiotherapy (Schmidt-Ullrich et al. 1997). In addition, EGFR inhibition induces cell-cycle arrest at theG

/G

phase of the cell cycle, which reduces the proportion of cells in the S phase, which are more resistant to the effects of radiation.

ZD 1839 (Iressa) is an example of an orally active inhibitor of EGFR-tyrosine kinase (EGFR-TK).

Administration of ZD 1839 enhanced apoptosis, re- duced tumor cell growth, and was synergistic with radiation (Solomon et al. 2002; Huang et al. 2002).

The optimal combination of ZD 1839 and radiother- apy is unknown, but ongoing studies including the Southwest Oncology Group (SWOG) 0023 trial administer ZD 1839 as a maintenance therapy for locally advanced NSCLC after chemoradiotherapy.

The Cancer and Leukemia Group B (CALGB) is also studying the role of ZD 1839 in the management of stage III NSCLC. The CALGB study administers

Fig. 11.3.1 Potential benefi ts of blocking the EGFR. Modifi ed from Harari and Huang (2000) Copyright American Association for Cancer Research

Other enzyme or adaptor

Tyrosine kianase Intracellular

domain

SOS RAS RAF1 MEK TGFα

GRB2

P P

P P

MAPK

Cytoplasm Gene activation, cell cycle progression

G2 M

G1 S

DNA damage

and repair Nucleus

Growth Arrest or

Apoptosis Radiation

or selected chemotherapy agents

Growth eff ects proliferation, diff erentiation

Angiogenesis eff ects blood vessels recruitment, invasion, metastases Extracellular

EGFR

domain

Cell membrane

ZD 1839 concurrently with chemoradiotherapy, fol- lowed by additional administration of ZD 1839. (See Table 11.3.2.)

COX-2 is observed within human tumor neovascu- lature, suggesting that COX-2-derived prostaglandins contribute to formation of new tumor blood vessels.

Indeed, Celecoxib, a COX-2 inhibitor, is a potent in- hibitor of angiogenesis and has been shown to inhibit neoplastic cells.

11.3.6.3

Tumor Angiogenesis Inhibitor

Inhibitors of tumor angiogenesis have been investi- gated extensively for possible tumor treatment. The formation of tumor vasculature, which is a prerequi- site for tumor growth, is initiated and sustained by angiogenic mediators secreted by tumor cells and cells from the surrounding tissues. Many different angiogenic factors have been identifi ed, including vascular endothelial growth factor/vascular perme- ability factor (VEGF/VPF), members of the fi broblast growth factor (FGF) family, platelet-derived growth factor (PDGF), interleukin-8, and PGs. In addition to angiogenic factors, tumors secrete substances that inhibit angiogenesis, such as angiostatin, endostatin, thrombospondin-1, and interferons. The fi nal out- come of angiogenesis depends on the balance be- tween proangiogenic and antiangiogenic activities.

Radiotherapy may induce angiogenesis by increas- ing the levels of angiogenesis stimulator, transform- ing growth factor `1(TGF `-1). Increased levels of both VEGF and TGF `-1 have been observed after radiotherapy (Canney and Dean 1990; Gorski et al.

1999). There are also data supporting an antiangio- genesis effect of radiotherapy, which implies a dual effect of radiotherapy (Hartford et al. 2000). Thus angiogenesis is a complex interaction of multiple factors, including VEGF, TGF `-1, ionizing radiation, and others. Angiogenesis inhibitors are undergoing extensive testing for tumor therapy purposes, used alone or in combination with chemotherapy or radio- therapy. Angiogenesis inhibitors, such as angiostatin, inhibit the growth or even cause temporary regres- sion of established murine tumors or human tumor xenografts in mice (O’Reilly et al. 1996). Angiostatin selectively inhibits proliferation of endothelial cells and indirectly induces tumor cell loss through apop- tosis. Angiostatin has been shown to potentiate the effects of radiotherapy in lung cancer xenograft if given concurrently, but not when given after radio- therapy. However, other inhibitors of angiogenesis such as SU5416 and SU 6668 exhibited no difference in response according to the sequence of administra- tion (Ning et al. 2002).

Table 11.3.2. Ongoing trials of ZD 1839 and radiotherapy in NSCLC

ZD 1839 regimen Eligibility Study group

Maintenance Unresectable stage III SWOG Concurrent Unresectable stage III CALGB

11.3.6.2

Cyclooxygenase-2

COX-2 relates to prostaglandins (PGs). PGs are me- tabolites of arachidonic acid that possess diverse biologic activities, including vasoconstriction, vaso- dilatation, platelet aggregation, and immunomodu- lation. They are also implicated in the development and growth of malignant tumors as well as in the response of tumor and normal tissues to cytotoxic agents, including radiation (Milas and Hanson 1995; Milas 2001; Koki et al. 1999). Two cyclooxy- genase enzymes, COX-1 and COX-2, mediate produc- tion of PGs. Whereas COX-1 is ubiquitous and has physiological roles in maintaining homeostasis, such as the integrity of gastric mucosa, normal platelet function, and regulation of renal blood fl ow, COX-2 is nonphysiological but induced by diverse infl amma- tory stimuli, mitogens, and carcinogens. Increasing evidence shows that COX-2 expression is upregulated in many human tumors. This selective or preferential expression of COX-2 in tumors makes this enzyme a potential target for cancer therapy. Selective inhibi- tors of COX-2 have recently been developed for use as anti-infl ammatory and analgesic agents, but the availability of these inhibitors has provided a tool for evaluating the role of COX-2 in cancer. Selective COX- 2 inhibitors are reported to enhance tumor response to chemotherapeutic drugs or radiation (Milas 2001;

Koki et al. 1999; Milas et al. 1999c). One of the effects

of COX-2 overexpression in malignant cells is inhi-

bition of apoptosis. Several preclinical studies have

shown that inhibition of COX-2 results in restoration

of apoptosis and increased cell death. The mecha-

nisms of the enhancement seem to be multiple, in-

cluding increases in intrinsic cell radiosensitivity and

inhibition of tumor neoangiogenesis. For example,

Kishi et al reported that inhibition of COX-2 resulted

in enhancement of radioresponse of murine sarcoma

with little effect on COX-2 protein expression (2000).

Because antiangiogenesis therapy inhibits forma- tion of new blood vessels, such therapy may not result in rapid tumoricidal response. A number of antian- giogenic agents have been shown to improve the an- titumor effi cacy of chemotherapeutic drugs or radia- tion, either by additive or synergistic effect (Teicher et al. 1995; Mauceri et al. 1998). The mechanisms of action include direct effect on tumor cells, rendering them more sensitive to killing by radiation or drugs, and indirect effect through the damage of tumor vas- culature.

Antiangiogenic agents, such as TNP-470, may enhance tumor radioresponse by increasing tumor oxygenation (Teicher et al. 1995). It has been sug- gested that this increase in oxygenation could result from decreased oxygen consumption as a result of the reduced number of endothelial cells. In addition, decreased tight junctions between endothelial cells may allow more oxygen to diffuse to tumor cells more distantly located from vasculature.

Ongoing clinical studies will investigate the role of angiogenesis inhibitors in NSCLC. The University of Texas M.D. Anderson Cancer Center will lead an investigation into the role of AE-941 (Neovastat) in stage III NSCLC patients (MD Anderson Cancer Center ). The Eastern Cooperative Oncology Group (ECOG) has an ongoing trial investigating the role of thalidomide in patients with stage III NSCLC. In addition, the Radiation Therapy Oncology Group (RTOG) will study the role of ce- lecoxib in patients with locally advanced NSCLC. (See Table 11.3.3.)

K, and N) are often mutated in many types of human cancer, including pancreatic, lung, and head and neck carcinomas. This mutation often causes persistent ac- tivation of ras oncogenes, alters tumor cell growth, and makes cells resistant to radiation (Bernhard et al. 2000). Therefore, blocking mutated ras oncogenes may also improve chemoradiotherapy. Because ras proteins must be farnesylated by farnesyltransferase in order to be active, inhibition of farnesylation could counteract the negative impact of mutated ras on- cogene. A number of farnesyl transferase inhibitors have been developed and shown to increase tumor cell radiosensitivity in vitro and increase tumor ra- dioresponse in vivo (Bernhard et al. 1998; Cohen- Jonathan et al. 2001). These compounds selectively affect tumors because the ras genes in normal tis- sues are not mutated and may result in an improved therapeutic index.

11.3.6.5

Promoters of Apoptosis

Complex interactions of molecular events determine the proliferation and death of tumor cells. Bcl-2 is an important modulator of apoptosis. It functions to in- hibit apoptosis, and studies have shown that overex- pression of Bcl-2, for instance by loss of p53, results in suppression of apoptosis induced by cytotoxic agents (Rafi et al. 2000; Eliopoulos et al. 1995). An example of manipulation of bcl-2 is the use of G3139, which is an antisense oligonucleotide preventing the expres- sion of Bcl-2. It has been used to treat low-grade non-Hodgkin’s lymphomas, with overexpressed Bcl- 2. Hematologic toxicity was not seen, but the effect on the tumor volume was also not signifi cant (Kuss and Cotter 1997). Whether an initial modulation of Bcl-2 can lead to increased response to subsequent cytotoxic therapy, including radiotherapy, remains to be determined. The development of an effective pro- apoptosis regimen in combination with radiotherapy will require better defi nition of a preclinical model of the synergy between an apoptosis promoter and ionizing irradiation.

11.3.6.6

Normal Tissue Protection

Normal tissue toxicity represents a major limita- tion of concurrent chemoradiotherapy. Preventing or minimizing normal tissue complications is an important strategy. This could be achieved through

Table 11.3.3. Ongoing trials of angiogenesis inhibitors and radiotherapy in NSCLC

Angiogenesis inhibitor Eligibility Study group

Thalidomide Stage III ECOG

AE 941(Neovastat) Stage III MD Anderson

Celecoxib Stage III RTOG

11.3.6.4

RAS Farnesylation Inhibitors

The ras nucleotide binding proteins relay signals from growth factor receptors such as EGFR and regulate transcription of genes required for prolif- eration (Radiation Therapy Oncology Group;

Grunicke and Maly 1993). The ras oncogenes (H,

incorporating radio- or chemoprotective agents into the treatment or through improving radia- tion delivery. A number of chemical and biologi- cal compounds are available that in preclinical in vivo testing exhibited either selective or preferential protection of normal tissues (Milas and Hanson 1995; Yuhas and Storer 1969; Hahn et al. 1994).

The most commonly tested radioprotectors are thiol compounds, such as WR2721 (amifostine).

The principal mechanisms of protection by these agents include scavenging of free radicals gener- ated by ionizing radiation and some chemotherapy agents, such as alkylating agents, and donating hy- drogen atoms to facilitate direct chemical repair of DNA damage. WR-2721 must be converted in vivo into its active metabolite WR-1065. The protector is taken up preferentially by normal tissues, where the entry into cells is accomplished by active trans- port. In contrast, the cytotoxic drugs diffuse pas- sively into tumors, where its availability is further reduced by defi cient tumor vasculature. Amifostine has been shown to reduce normal tissue toxicity in a number of clinical settings, including protection of salivary glands in head and neck radiotherapy (Brizel et al. 1999) and of the esophagus in chemo- radiotherapy of lung cancer (Komaki et al. 2000), without adversely affecting tumor response to treat- ment. Technical improvements in radiotherapy, such as three-dimensional treatment planning, confor- mational radiotherapy, or use of protons, are other approaches likely to minimize the toxicity, and con- sequently enhance the effectiveness, of chemoradia- tion. The use of either radioprotective compounds or implementation of technical advances may en- able administration of higher doses of radiation, chemotherapeutic drugs, or both, which may result in superior treatment outcome.

11.3.6.7

Topoisomerase I Inhibitors

The camptothecins are potent radiation sensitizers that are increasingly incorporated in clinical stud- ies. Camptothecin is a plant alkaloid obtained from the tree Camptotheca acuminata. Its initial clinical evaluation in the 1960s and 1970s was abandoned because of severe and unpredictable hemorrhagic cystitis (Moertel et al. 1972; Muggia et al. 1972).

Camptothecin and its derivatives (irinotecan, topo- tecan, 9-aminocamptothecin, SN-38, etc.) target DNA topoisomerase I (Hsiang et al. 1985; Hsiang and Liu 1988; Andoh et al. 1987). This enzyme

relaxes both positively and negatively supercoiled DNA and allows processes such as replication and transcription to proceed. In the presence of camp- tothecin, a camptothecin-topoisomerase I-DNA complex becomes stabilized with the 5’-phospho- ryl terminus of the enzyme-catalyzed DNA single- strand break bound covalently to a tyrosine resi- due of topoisomerase I. These stabilized cleavable complexes interact with the advancing replication fork during S phase or during unscheduled DNA replication after genomic stress and cause the con- version of single-strand breaks into irreversible DNA double -strand breaks, resulting in cell death (Iliakis 1988).

Several investigators have reported that campto- thecin enhances the cytotoxic effect of radiation in vitro and in vivo (Omura et al. 1997; Chen et al.

1997). Chen et al. (1997) showed that cells exposed to 20(S)-10,11 methylenedioxycamptothecin before or during radiation had sensitization ratios of 1.6, whereas those treated with the drug after radiation had substantially less enhancement of radiation-in- duced DNA damage. There are several hypotheses regarding the mechanism of interaction between radiation and irinotecan, which is perhaps the best studied of the camptothecin derivatives. The fi rst hypothesis suggests that inhibition of topoisom- erase I by irinotecan leads to inhibition of repair of radiation-induced DNA strand breaks. The sec- ond hypothesis suggests that irinotecan causes re- distribution of cells into the more radiosensitive G2 phase of the cell cycle. The third hypothesis is that topoisomerase I-DNA adducts are trapped by irinotecan at the sites of radiation-induced single- strand breaks, leading to their conversion into dou- ble-strand breaks (Amorino et al. 2000). The pri- mary mechanism involved with radiosensitization may depend on which camptothecin derivative is being used; there is currently insuffi cient evidence to identify the underlying mechanism with cer- tainty. Data from in vivo experiments demonstrate that the 9-aminocamptothecin (9-AC) and irradia- tion is more effective when fractionated compared with single doses (Kirichenko and Rich 1999).

The integration of this group of drugs into clini-

cal treatments with radiation is ongoing. Much of

the current experience with irinotecan has been

accumulated in NSCLC (Takeda et al. 1999; Choy

and MacRae 2001), whereas much of the experi-

ence with topotecan has occurred in brain tumors

(Fisher et al. 2001; Grabenbauer et al. 1999).

11.3.7 Conclusion

The combination of chemotherapy and radiation has become a common strategic practice in the therapy of locally advanced cancers, with recent empha- sis on the concurrent delivery of both modalities.

Improvements in treatment outcome both in terms of local control and patient survival have been achieved with traditional chemotherapeutic agents such as cis- platin and 5-fl uorouracil. Nonetheless, the cure rates of the majority of solid tumors remain poor, and the addition of combined treatments is frequently associ- ated with increased normal tissue toxicity. Therefore, there is considerable room for improvement of the combined treatment strategies. However, selection of the most effective drug or the optimal treatment ap- proach remains a signifi cant challenge.

Newer chemotherapies, such as the taxanes, nucle- oside analogs, and topoisomerase inhibitors, which interfere with one or more tumor radioresistance mechanisms, are becoming increasingly available.

These agents have high potential for increasing the therapeutic effectiveness of radiotherapy; therefore, their evaluation in combination with radiotherapy, both in the laboratory and in the clinic, is essential for improving cancer treatment. Preclinical studies provide not only a biologic rationale for the use of a given drug with radiation but are able to generate in- formation critical to the design of effective treatment schedules in clinical settings. Studies of the mecha- nisms of chemotherapy-radiotherapy interaction at the genetic-molecular, cellular, and tumor (or nor- mal tissue) microenvironmental levels are essential for obtaining clear insight into the radiomodulating potential of chemotherapeutic agents and their abil- ity to increase radiotherapeutic effects.

Signifi cant progress has been made in our under- standing of the basic mechanisms of radiation injury as well as the injury infl icted by chemotherapeutic agents and cellular processing of these injuries in both normal and malignant cells. Recent advances in molecular biology have exposed many potential tar- gets for augmentation of radioresponse or chemore- sponse, including EGFR, cox-2, angiogenic molecules, and various components of the signal transduction pathways that these molecules initiate. It has become possible to intervene actively in some molecular pathways in order to improve the therapeutic ratio, and the incorporation of molecular targeting strat- egies into chemoradiotherapy is becoming increas- ingly used for therapeutic intervention in many types of human cancer.

References

Akimoto T, Hunter NR, Buchmiller L, et al. (1999) Inverse relationship between epidermal growth factor receptor expression and radiocurability of murine carcinomas. Clin Cancer Res 5:2884–2890

Amorino GP, Hercules SK, Mohr PJ, et al. (2000) Preclini- cal evaluation of the orally active camptothecin analog, RFS-2000 (9-Nitro-20(S)-Camptothecin) as a radiation enhancer. Int J Radiat Oncol Biol Phys 47:503–509

Andoh T, Ishii K, Suzuki Y, et al. (1987) Characterization of a mammalian mutant with a camptothecin-resistant DNA topoisomerase I. Proc Natl Acad Sci U S A 84:5565–5569 Bernhard EJ et al. (1998) Inhibiting ras prenylation increases

the radiosensitivity of human tumor cell lines with acti- vating mutations of ras oncogenes. Cancer Res 58:1754–

1761

Bernhard EJ et al. (2000) Direct evidence for the contribution of activated N-ras and K-ras oncogenes to increased intrin- sic radiation resistance in human tumor cell lines. Cancer Res 60:6597–6600

Brizel DM et al. (1996) Tumor oxygenation predicts for the likelihood of distant metastases in human soft tissue sar- coma. Cancer Res 56:941

Brizel D, Wasserman TH, Strnad V, et al. (1999) Final report of a phase III randomized trial of amifostine as a radioprotec- tant in head and neck cancer. Int J Radiat Oncol Biol Phys 45(suppl 3):147–148

Brown JM, Giaccia AJ (1998) The unique physiology of solid tumors: opportunities (and problems) for cancer therapy.

Cancer Res 58:1408–16

Bush RS et al. (1978) Defi nitive evidence for hypoxic cells infl uencing cure in cancer therapy. Br J Cancer 37:302 Cancer and Leukemia Group B. Phase II study of paclitaxel,

carboplatin, and gefi tinib followed by radiotherapy with or without paclitaxel and carboplatin followed by gefi tinib in patients with stage III non-small cell lung cancer – CALGB- 30106

Canney PA, Dean S. (1990) Transforming growth factor beta: A promoter of late connective tissue injury following radio- therapy? Br J Radiol 63:620–623

Chen AY, Okunieff P, Pommier Y, et al. (1997) Mammalian DNA topoisomerase I mediates the enhancement of radia- tion cytotoxicity by camptothecin derivatives. Cancer Res 57:1529–1536

Choy H, et al. (1993) Investigation of taxol as potential radia- tion sensitizer. Cancer 71:3774–3778

Choy H, MacRae R (2001) Irinotecan in combined-modality therapy for locally advanced non-small-cell lung cancer.

Oncology (Huntingt) 15(1 suppl 1):31–36

Cohen-Jonathan E et al (2001) The farnesyltransferase inhibi- tor L744,832 reduces hypoxia in tumors expressing acti- vated H-ras. Cancer Res 61:2289–2293

Dickstein BN, Wosikowski K, Bates S (1995) Increased resis- tance to cytotoxic agents in ZR75B human breast cancer cells transfected with epidermal growth factor receptor.

Mol Cell Endocrinol 110:205–211

Dische S (1988) Modifying radiosensitivity to improve clinical radiotherapy. In: Progress in radio-oncology, IV. Interna- tional Club of Radio-Oncology, Vienna

Eastern Cooperative Oncology Group (ECOG). Phase III ran- domized study of carboplatin, paclitaxel, and chemora- diotherapy with or without thalidomide in patients with

stage III non-small cell lung cancer. http://www.cancer.

gov/clinicaltrials/

Eliopoulos AG, Kerr DJ, Herod J, et al. (1995) The control of apoptosis and drug resistance in ovarian cancer: infl uence of p53 and bcl-2. Oncogene 11:1217–1228

Elkind MM, Sutton HF (1959) X-ray damage and recovery in mammalian cells in culture. Nature 184:1293

Faivre S, Le Chevalier T, Monnerat C, et al. (2002) Phase I/II and pharmacokinetic study of gemcitabine combined with oxaliplatin in patients with advanced non-small-cell lung cancer and ovarian carcinoma. Ann Oncol 13:1479–1489 Fisher BJ, Scott C, Macdonald DR, et al. (2001) Phase I study

of topotecan plus cranial radiation for glioblastoma multi- forme: results of Radiation Therapy Oncology Group Trial 9507. J Clin Oncol 19:1111–1117

Gorski DH, Beckett MA, Jaskowiak NT, et al. (1999) Blockade of the vascular endothelial growth factor stress response increases the antitumor effects of ionizing radiation.

Cancer Res 59:3374–3378

Grabenbauer GG, Buchfelder M, Schrell U et al. (1999) Topo- tecan as a 21-day continuous infusion with accelerated 3D-conformal radiation therapy for patients with glioblas- toma. Front Radiat Ther Oncol 33:364–368

Gregoire V et al. (1999) Chemo-radiotherapy: radiosensitizing nucleoside analogues. Oncol Rep 6:949–957

Grunicke HH, Maly K (1993) Role of GTPases and GTPase regulatory proteins in oncogenesis. Crit Rev Oncog 4:389–

402

Hahn SM et al. (1994) Potential use of nitroxides in radiation oncology. Cancer Res 54(suppl 7):2006s–2010s

Hak Choy, A Nabid, B Stea, W Roa, et al. (2001) Positive phase II results of RSR13 and concurrent radiation therapy after induction chemotherapy with paclitaxel and carboplatin for locally advanced inoperable non-small cell lung cancer.

ASCO annual meeting , abstract#1248

Harari PM, Huang SM (2000) Modulation of molecular targets to enhance radiation. ClinCancer Res 6:323–325

Hartford AC, Gohongi T, Fukumura D, et al. (2000) Irradia- tion of a primary tumor, unlike surgical removal, enhances angiogenesis suppression at a distal site: Potential role of host-tumor interaction. Cancer Res 60:2128–2131

Henk JM, Smith CW (1977) Radiotherapy and hyperbaric oxygen in head and neck cancer. Lancet 2:104–105 Hermens AF, Barendsen GW (1978) The proliferative status

and clonogenic capacity of tumour cells in a transplantable rhabdomyosarcoma of the rat before and after irradiation with 800 rad of X-rays. Cell Tissue Kinet 11:83–100 Hockel M et al. (1993) Intratumoral pO₂ predicts survival in

advanced cancer of the uterine cervix. Radiother Oncol 26:45

Hsiang YH, Liu LF (1988) Identifi cation of mammalian DNA topoisomerase I as an intracellular target of the anticancer drug camptothecin. Cancer Res 48:1722–1726

Hsiang YH, Hertzberg R, Hecht S, et al. (1985) Camptothecin induces protein-linked DNA breaks via mammalian DNA topoisomerase I. J Biol Chem 260:14873–14878

Huang SM, Harari PM (2000) Modulation of radiation response after epidermal growth factor receptor blockade in squamous cell carcinomas: inhibition of damage repair, cell cycle kinetics, and tumor angiogenesis. Clin Cancer Res 6:2166–2174

Huang SM, Bock JM, Harari PM (1999) Epidermal growth factor receptor blockade with C225 modulates prolifera-

tion, apoptosis, and radiosensitivity in squamous cell carci- nomas of the head and neck. Cancer Res 59:1935–1940 Huang SM, Li J, Armstrong EA, et al. (2002) Modulation of

radiation response and tumor induced angiogenesis after epidermal growth factor receptor inhibition by ZD 1839 (Iressa). Cancer Res 62:4300–4306

Iliakis G (1988) Radiation-induced potentially lethal damage:

DNA lesions susceptible to fi xation. Int J Radiat Biol Relat Stud Phys Chem Med 53:541–584

Kakolyris S, Kouroussis C, Koukourakis M, et al. (2002) A dose- escalation study of oxaliplatin and vinorelbine in patients with advanced solid tumors.

Oncology 63:213–218

Kinsella TJ et al. (1987) Enhancement of X ray induced DNA damage by pre-treatment with halogenated pyrimidine analogs. Int J Radiat Oncol Biol Phys 13:733–739

Kirichenko AV, Rich TA (1999) Radiation enhancement by 9-aminocamptothecin: the effect of fractionation and timing of administration. Int J Radiat Oncol Biol Phys 44:659–664

Kishi K, Petersen S, Petersen C, et al. (2000) Preferential enhancement of tumor radioresponse by a cylooxygenase- 2 inhibitor. Cancer Res 60:1326–1331

Koki A, Leahy KM, Masferrer JL (1999) Potential utility of COX-2 inhibitors in chemoprevention and chemotherapy.

Expert Opin Invest Drugs 8:1623–1638

Komaki R, Seiferheld W, Curran W, et al. (2000) Sequential vs.

concurrent chemotherapy and radiation therapy for inop- erable phase III study (RTOG 9410). Int J Radiat Oncol Biol Phys 48:113

Kourousis C, Agelaki S, Mavroudis D, et al. (2003) A dose esca- lation study of docetaxel and oxaliplatin combination in patients with metastatic breast and non-small cell lung cancer. Anticancer Res 23:785

Kuss B, Cotter F (1997) Antisense therapy in patients with non- Hodgkin lymphoma. Lancet 349:1137–1141

Lawrence TS, Eisbruch A, Shewach DS (1997) Gemcitabine- mediated radiosensitization. Semin Oncol 24: S-7,24–

S7,28

Li C et al. (2000a) Tumor irradiation enhances the tumor- specifi c distribution of poly(L-glutamic acid)-conjugated paclitaxel and its antitumor effi cacy. Clin Cancer Res 6:2829–2834

Li C et al. (2000b) Potentiation of ovarian OCa-1 tumor radio- response by poly(L-glutamic acid)-paclitaxel conjugate. Int J Radiat Oncol Biol Phys 48:1119–1126

Little JB, et al. (1973) Repair of potentially lethal radiation damage in vitro and in vivo. Radiology 106:689

Maeda H, Seymour LW, Miyamoto Y (1992) Conjugates of anti- cancer agents and polymers: advantages of macromolecu- lar therapeutics in vivo. Bioconjug Chem 3:351–362 Mason KA et al. (2001) Biology-based combined-modality

radiotherapy: workshop report. Int J Radiat Oncol Biol Phys 50: 1079–1089

Mauceri H.J et al. (1998) Combined effects of angiostatin and ionizing radiation in antitumour therapy. Nature 394:287–

291

MD Anderson Cancer Center. Phase III randomized study of induction platinum-based chemotherapy and radiotherapy with or without Æ-941 (Neovastat) in patients with unresect- able stage IIIA or IIIB non-small cell lung cancer. http://www.

cancer.gov/clinicaltrials/

Mendelsohn J, Fan Z (1997) Epidermal growth factor receptor

family and chemosensitization. J Ntl Cancer Inst 89:341–

343

Milas L (2001) Cyclooxygenase-2 (COX-2) enzyme inhibitors as potential enhancers of tumor radioresponse. Semin Radiat Oncol 11:296–299

Milas L, Hanson WR (1995) Eicosanoids and radiation. Eur J Cancer 31A:1580–1585

Milas L, et al. (1994) Dynamics of tumor cell clonogen repopu- lation in a murine sarcoma treated with cyclophosphamide.

Radiother Oncol 30:247–253

Milas L, et al. (1995) Role of reoxygenation in induction of enhancement of tumor radioresponse by paclitaxel. Cancer Res 55:3564–3568

Milas L, et al. (1999a) Enhancement of tumor radioresponse in vivo by gemcitabine. Cancer Res 59:107–114

Milas L, Milas MM, Mason KA (1999b) Combination of tax- anes with radiation: preclinical studies. Semin Radiat Oncol 9:12–26

Milas L, et al. (1999c) Enhancement of tumor response to g- radiation by an inhibitor of cyclooxygenase-2 enzyme. J Natl Cancer Inst 91:1501–1504

Milas L, et al. (2000) In vivo enhancement of tumor radio- response by C225 antiepidermal growth factor receptor antibody. Clin Cancer Res 6:701–708

Moertel CG, Schutt AJ, Reitemeier RJ, et al. (1972) Phase II study of camptothecin SC100880) in the treatment of advanced gastrointestinal cancer. Cancer Chemother Rep 56:95–101.

Monnet I, Brienza S, Hugret F, et al (1998) Phase II study of oxaliplatin in poor prognosis non-small cell lung cancer (NSCLC). Eur J Cancer 34:1124–1127

Monnet I, Soulié P, de Cremoux H, et al. (2001) Phase I/II study of escalating doses of vinorelbine in combination with oxaliplatin in patients with advanced non–small-cell lung cancer. J Clin Oncol 19:458–463

Monnet I, de Cremoux H, Soulie´ P, et al. (2002) Oxaliplatin plus vinorelbine in advanced non-small-cell lung cancer:

fi nal results of a multicenter phase II study. Ann Oncol 13:103–107

Morris M, et al. (1999) Pelvic radiation with concurrent chemo- therapy compared with pelvic and para-aortic radiation for high-risk cervical cancer. New Engl J Med 340:1137–1143 Muggia FM, Creaven PJ, Hansen HH, et al. (1972) Phase I clini-

cal trial of weekly and daily treatment with camptothecin (NSC-100880): correlation with preclinical studies. Cancer Chemother Rep 56:515–521

Munro AJ (1995) An overview of randomised controlled trials of adjuvant chemotherapy in head and neck cancer. Br J Cancer 71:83–91

Ning S, Laird D, Cherrington JM, et al. (2002) The antiangio- genic agents SU 5416 and SU6668 increase the antitumor effects of fractionated irradiation. Radiat Res 157:45–51 Nordsmark M, Overgard M, Overgard J (1996) Pretreatment

oxygenation predicts radiation response in advanced squamous cell carcinoma of the head and neck. Radiother Oncol 41:31

Omura M, Torigoe S, Kubota N (1997) SN-38, a metabolite of the camptothecin derivative CPT-11, potentiates the cyto- toxic effect of radiation in human colon adenocarcinoma cells grown as spheroids. Radiat Oncol 43:197–201 O’Reilly MS et al. (1996) Angiostatin induces and sustains

dormancy of human primary tumors in mice. Nat Med 2:689–692

Phase III randomized study of polyglutamate paclitaxel (CT- 2103) versus docetaxel as second-line therapy in patients with progressive non-small cell lung cancer. http://www.

cancer.gov/clinicaltrials/

Plunkett W et al. (1995) Gemcitabine: metabolism, mechanisms of action, and self-potentiation. Semin Oncol 22(suppl 11):

3–10

Raben D, Helfrich BA, Chan D, et al. (2002) ZD 1839, a selective epidermal growth factor receptor tyrosine kinase inhibi- tor, alone, and in combination with radiation and chemo- therapy as new therapeutic strategy in non-small cell lung cancer. Semin Oncol 29(suppl 4):37–46

Radford IR (1986) Evidence for a general relationship between the induced level of DNA double-strand breakage and cell- killing after X-irradiation of mammalian cells. Int J Radiat Biol Relat Stud Phys Chem Med 49:611–620

Radiation Therapy Oncology Group. RTOG-0213. Phase I/II study of celecoxib and limited-fi eld radiotherapy in inter- mediate-prognosis patients with locally advanced non-small cell lung cancer. http://www.cancer.gov/clinicaltrials/

Rafi MM, Rosen RT, Vassil A, et al. (2000) Modulation of Bcl-2 and cytotoxicity by licochalcone-A, a novel estrogenic fl a- vonoid. Anticancer Res 20:2653–2658

Robert F, Ezekiel MP, Spencer SA, et al. (2001) Phase I study of anti-epidermal growth factor receptor antibody cetux- imab in combination with radiation therapy in patients with advanced head and neck cancer. J Clin Oncol 19:3234–

3243

Salek MN, Raisch KP, Strackhouse MA, et al. (1999) Combined modality therapy of A431 human epidermoid cancer using anti-EGFr antibody C225 and radiation. Cancer Biothera Radiopharm 14:451–463

Schmidt-Ullrich RK, Mikkelsen RB, Dent P, et al. (1997) Radia- tion-induced proliferation of the human A431 squamous carcinoma cells dependent on EGFR tyrosine phosphoryla- tion. Oncogene 15:1191–1197

Schmidt-Ullrich RK, et al. (2000) Signal transduction and cel- lular radiation responses. Radiat Res 153:245–257 Solomon B, Hagekyriakou M, Trivett M, et al. (2002) Poten-

tiation of the antitumor effect of ionizinf radiation by ZD 1839 (‘Iressa’) in vitro and in vivo A431 cells. Proc Am Assoc Cancer Res 43:1002, (abstr 4966)

Southwest Oncology Group. Phase III randomized study of cisplatin, etoposide, radiotherapy, and docetaxel with or without gefi tinib in patients with unresectable stage iii non-small cell lung cancer. a substance that is being studied as a treatment for cancer. It belongs to the family of drugs called epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitors. Also called ZD 1839. SWOG-S0023 Steel GG, Peckham MJ (1979) Exploitable mechanisms in com-

bined radiotherapy-chemotherapy: the concept of additiv- ity. Int J Radiat Oncol Biol Phys 5:85–91

Stephens T, Steel GG (1980) Regeneration of tumors after cytotoxic treatment. In: Meyn R, Withers HR (eds.) Radia- tion biology in cancer research. Raven Press, New York, pp 385–295

Takeda K, Negoro S, Kudoh S, et al. (1999) Phase I/II study of weekly irinotecan and concurrent radiation therapy for locally advanced non-small cell lung cancer. Br J Cancer 79:1462–1467

Tannock IF, Rotin D (1989) Acid pH in tumors and its potential for therapeutic exploitation. Cancer Res. 49:4373–4384 Taylor J, Withers HR, Mendenhall WM (1990) Dose-time con-

siderations of head and neck squamous cell carcinomas treated with irradiation. Radiother Oncol 17:95–102 Teicher BA et al. (1995) Potentiation of cytotoxic therapies by

TNP-470 and minocycline in mice bearing EMT-6 mam- mary carcinoma. Breast Cancer Res Treat 36:227–236 Terasima T, Tolmach .J (1963) Variations in survival responses

of HeLa cells to x- irradiation during the division cycle.

Biophys J 3:11–33

Tishler RB, et al. (1992) Taxol sensitizes human astrocytoma cells to radiation. Cancer Res 52:3495–3497

Vaupel P, Kallinowski F, Okunieff P (1989) Blood fl ow, oxygen and nutrient supply, and metabolic microenvironment of human tumors: a review. Cancer Res 49:6449–6465 Vittorio F, Roberto B, Enrico A, et al. (2003) Gemcitabine and

oxaliplatin: a safe and active regimen in poor prognosis advanced non-small cell lung cancer patients. Lung Cancer 41:101–106

Wang Y, Pantelias GE, Iliakis G (1994) Mechanism of radiosen- sitization by halogenated pyrimidines: the contribution of excess DNA and chromosome damage in BrdU radiosensi- tization may be minimal in plateau-phase cells. Int J Radiat Biol 66: 133–142

Withers HR, Taylor JM, Maciejewski B (1988) The hazard of accelerated tumor clonogen repopulation during radio- therapy. Acta Oncol. 27:131–146

Yuhas JM, Storer JB (1969) Differential chemoprotection of normal and malignant tissues. J Natl Cancer Inst 42:331–

335 .