Section 4
`Cancer and Infectious Diseases
`
`Editor: Jacob J. Plattner
`Anacor Pharmaceuticals
`Palo Alto
`California
`
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`

`
`Recent Advances in Antimetabolite
`Cancer Chemotherapies
`
`James R. Henry and Mary M. Mader
`
`Lilly Research Laboratories, Indianapolis, IN 46285, USA
`
`Contents
`1. Introduction
`2. Mechanisms of antimetabolite cancer chemotherapy
`2.1. Thymidine biosynthesis
`2.2. Purine nucleotide synthesis
`2.3. Ribonucleoside reductase
`2.4. Intracellular transformations of antimetabolites
`3. Antimetabolites
`4. Clinical advances with combinations of antimetabolites
`5. Conclusions
`References
`
`1. INTRODUCTION
`
`161
`161
`161
`162
`163
`164
`166
`168
`170
`170
`
`An antimetabolite interferes with the formation or utilization of a normal cellular
`metabolite. Most antimetabolites interfere with the enzymes involved in the synthesis of
`new DNA, are incorporated into the newly formed DNA, or in some cases both processes
`are important to an agent's efficacy. As a result, many antimetabolites are derivatives of
`the building blocks of DNA itself, such as the nucleoside based inhibitors, or analogs of
`critical cofactors such as the antifolates. A variety of key cellular pathways have been
`disrupted with antimetabolite therapy, including inhibition of the thymidine and purine
`nucleotide biosynthesis pathway, and the inhibition of ribonucleoside reductase. Given
`their mechanism of action, it is not surprising that the observed benefits of
`antimetabolites are often accompanied by significant toxicity, due to the fact that the
`affected cellular metabolites are critical to both normal and cancer cells. Single
`antimetabolite agents can act on a single pathway, or on multiple pathways at once, but in
`either instance, they are often used in combination with other therapies in the clinic.
`
`2. MECHANISMS OF ANTIMETABOLITE CANCER
`CHEMOTHERAPY
`
`2.1. Thymidine biosynthesis
`
`Critical to the cell's process of replication is its ability to synthesize thymidine.
`This process involves several key enzymes including thymidylate synthase (TS),
`
`ANNUAL REPORTS IN MEDICINAL CHEMISTRY. VOLUME 39
`ISSN: 0065-7743 DO! IO.IOI6/S0065-7743(04)39013-5
`
`© 2004 Elsevier Inc.
`Al! rights reserved
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`
`HN:SM'
`oAN
`
`o
`
`I
`R
`
`dTMP
`
`N5,N10-CHr THF
`
`Glycine ....J SHMT
`
`7,8-DHF
`
`NADPH+H+
`
`DHFR
`
`PLP ) - - THF : {
`
`Serine
`
`NADP+
`
`o
`II
`HO-7-o~0"-lBase
`OH ~
`
`R'
`
`OH
`
`Fig. 1. Key steps in thymidine biosynthesis.
`
`dihydrofolate reductase (DHFR), and serine hydroxymethyl transferase (SHMT)
`(Fig. 1).
`The methylation of deoxyuridine 5' monophosphate (dUMP) to produce deoxythymi(cid:173)
`dine 5' monophosphate (dTMP) is mediated by TS [1]. The methyl group for dTMP is
`provided by N5,N1O-methylene tetrahydrofolate (N5,N1O-CHrTHF) through its
`conversion to 7,8-dihydrofolate (7,8-DHF). The 7,8-DHF must then be converted to
`tetrahydrofolate (THF) by DHFR [2], followed by further transformation back to N5,N1O(cid:173)
`CHr THF through the action of SHMT [3]. Therefore, inhibition of TS, DHFR, or SHMT
`with an appropriate agent would interrupt the process of thymidine biosynthesis. Low
`thymidine levels cause defects in DNA which in turn activates stress response elements,
`such as the Fas ligand/Fas death pathway leading to apoptosis [4]. It has also been
`proposed that defects in this Fas-dependent apoptotic signaling pathway are one cause of
`cellular resistance to drugs.
`
`2.2. Purine nucleotide synthesis
`
`The cell's ability to provide the needed purine nucleotides for DNA and RNA synthesis is
`also critical to its survival. The de novo biosynthesis of purine nucleotides involves 10
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`163
`
`AICAR
`
`Inosinic acid
`
`AMP
`
`GMP
`
`10-CHO-THF
`
`THF
`
`GR
`
`formylglycinamide
`ribon ucleotide
`
`Fig. 2. Key step in purine biosynthesis.
`
`separate enzyme catalyzed reactions starting with 5-phosphoribosyl-l-pyrophosphate
`and leading to inosinic acid [5]. Both adenosine monophosphate (AMP) and guanine
`monophosphate (GMP) are then derived from inosinic acid (Fig. 2). The third step in this
`process is the biosynthesis of formylglycinamide ribonucleotide from glycinamide
`ribonucleotide (GR) via glycinamide ribonucleotide formyltransferase (GARFT). The
`last two steps in the synthesis of inosinic acid occur via a bifunctional enzyme having
`both aminoimidazolecarboxamide ribonucleotide formyltransferase (AlCARFT) and
`inosine monophosphate cyclohydrolase (IMPCH) activity. This enzyme has been shown
`to be made up of a 39 kDa carboxy-terminal AICARFT active fragment along with a
`25 kDa amino-terminal IMPCH active fragment [6]. Both GARFT and AICARFT
`catalyze the transfer of a formyl group from lO-CHO-tetrahydrofolate (lO-CHO-THF) to
`GR or aminoimidazolecarboxamide ribonucleotide (AICAR) respectively, returning
`THF as the second product of the reaction.
`
`2.3. Ribonucleoside reductase
`
`The synthesis of new DNA within a cell requires the production of deoxynucleotides.
`The four required deoxynucleotides (adenosine, guanosine, cytidine, and thymidine) are
`produced as by reduction of the appropriate ribonucleotide substrate with ribonucleoside
`reductase, also referred to a nucleoside diphosphate reductase (NDPR) [7]. The resulting
`oxidized form of NDPR can then be reduced back to NDPR by the action of
`glutaredoxin, which is in tnrn oxidized to thioredoxin [8]. NDPR is a dimer, with each
`monomer made up of two subunits: a larger (Ml) and a smaller (M2) subunit. The Ml
`subunit contains two allosteric sites involved in regulation of the overall activity of the
`enzyme and the enzyme's substrate specificity. The deoxynucleoside triphosphates bind
`to this allosteric site, and regulate their own synthesis. The M2 subunit is responsible for
`the key reduction reaction, carrying a tightly bound iron atom that stabilizes the tyrosyl
`free radical critical to reduction. Deoxynucleotide pools in proliferating cells are
`
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`164
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`J.R. Henry and M.M. Mader
`
`sufficient for only a few minutes of DNA synthesis without regeneration, thus making
`NDPR inhibition an attractive candidate for cancer chemotherapy [9].
`o
`II
`
`Ho-ro-rOtt
`
`0
`II
`
`o
`0
`II
`II
`HO -r~o-r~O~o"se
`)-l
`
`NDPR
`-------I.~
`
`HO
`
`OH
`
`HO
`
`2.4. Intracellular transformations of antimetabolites
`
`Most antimetabolites must undergo modification within the cell before they are active
`agents, and so in essence are prodrugs. Methotrexate (MTX, 1), an antifolate agent
`targeting DHFR, and pemetrexed (Alimta, 2), a multi-targeted antifolate, exert much
`of their pharmacological effects as a polyglutamate, as do most classical antifolates
`[10]. This transformation is carried out by the enzyme folylpolyglutamate synthetase
`(FPGS). Formation of the polyglutamate of antifolate drugs can cause a dramatic
`increase in the activity of the agent toward its intended target. Further, polyglutamates
`(above diglutamate) are less susceptible to cellular efflux, thus providing a long-lived
`pool of drug within the cell [11]. Most natural folate cofactors exist as polyglutamates,
`and so the beneficial action of FPGS on antifolate drugs is not surprising. It also
`follows that any cellular change leading to decreased FPGS activity could lead to
`antifolate resistance.
`
`o
`
`C02H
`
`N~C02H
`
`C02Na
`N~C02Na
`
`H
`
`/
`
`H
`
`"
`
`site of polyglutamate
`formation
`
`Many nucleoside derived antimetabolite analogs also undergo intracellular trans(cid:173)
`formations to become active agents (Fig. 3). The earliest of these agents, 5-fluorouracil
`(5-FU, 3) is converted into three major metabolites that are responsible for its activity.
`5-FU is changed into 5-fluoro-2'-deoxyuridine monophosphate (FdUMP, 4) that acts as a
`mimic for the natnral substrate of TS, dUMP, thus inhibiting TS activity. 5-FU can also
`be tranformed to 5-fluoro-2'-deoxyuridine triphosphate, which is eventually incorporated
`into DNA causing DNA damage, and finally to 5-fluorouridine triphosphate (5), which is
`incorporated into RNA leading to impaired RNA function [12]. Deoxyribonucleoside
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`
`165
`
`cellular
`
`Hi
`
`:Y F
`
`o
`I metabolism.
`~
`0 :
`
`F
`
`Hi
`
`F
`
`Hi
`
`~O
`:yo
`O~N + 1. O~N
`I
`I
`
`H,opO ~ HO,P J ~~
`
`HO
`
`OH
`HO
`40 5
`
`NH2
`
`NH2
`
`HN~O~
`F~N
`lN~O
`H'C~
`
`OH
`
`HO
`
`6
`
`HO
`
`F
`
`7
`
`HO
`
`OH
`
`8
`
`Fig. 3. Metabolism of 5-FU 3 and other nucleoside antimetabolites.
`
`analogs such as cytarabine (6) and gemcitabine (7) are converted to their triphosphate
`derivatives in vivo before showing their effects [l3,14] , while capecitabine (8) is
`metabolized in vivo to 3 (Fig. 4). 8 is readily absorbed in the gastrointestinal tract, then
`passes intact through the intestinal mucosa. It is subsequently converted to 5'(cid:173)
`deoxycytidine 9 by carboxylesterase in the liver, 5'-deoxy-5-fluorouridine 10 by cytidine
`deaminase, and finally to 5-FU 3 by thymidine phosphorylase.
`
`NH-Z.
`
`o
`F~N OR
`H3CHN~O
`
`carboXYleste~se
`
`NH
`
`0
`
`H
`
`F~N' F 'n
`H3CHN~O H3CHN~O F~O
`
`cytidine
`deaminase.
`
`thymine
`phosphoryl,;,e
`
`1/
`NH
`I N~
`
`HO OH
`8
`
`HO OH
`
`9
`
`HO OH
`10
`
`0
`
`H
`3
`
`Fig. 4. Metabolism of capecitabine to 5-FU.
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`J.R. Henry and M.M. Mader
`
`3. ANTIMETABOLITES
`
`Historically, antimetabolites have been useful agents in hematopoietic therapy. However,
`in recent years, they have been shown to be effective in the treatment of solid tumors
`(Table 1). Three antimetabolites have been approved for clinical use since 1996:
`gemcitabine, capecitabine and pemetrexed. A new method for the delivery of cytarabine,
`DepoCyt, was given accelerated approval in 1999. Additional oncolytic indications for
`some of the approved antimetabolites are in clinical trials, and other antimetabolites,
`including nolatrexed and decitabine, are currently undergoing Phase III study. An
`antifolate, raltitrexed, has been approved for use outside of the US, but its development as
`a single agent, as well as that of the nucleoside analog eniluracil has been discontinued in
`the USA [15,16].
`Although 5-FU is widely used, its pharmacokinetic profile is not ideal. Its optimal
`method of delivery is by continuous intravenous infusion, as its bioavailability after oral
`administration is variable. 5-FU is rapidly metabolized, with a mean half-life of
`elimination of approximately 16 min. Within 3 h, no intact drug can be detected in
`plasma. 5-FU is more effective when co-administered with leucovorin, a prodrug of
`5, lO-CHr THF. Inhibition of TS by FdUMP is dependent on the cofactor 5, lO-CHr THF,
`which combines with TS and FdUMP to form a covalent teruary complex. Excess
`cofactor decreases the dissociation rate of this complex, and consequently addition of
`
`Table 1. Selected oncolytic antimetabolites in clinical use
`
`Clinical agent
`
`Trade name
`
`Year approved
`(USA)
`
`Indication
`
`Ref.
`
`Folate Antagonists
`
`Methotrexate (MTX)
`Pemetrexed
`Nolatrexed
`Nucleoside analogs
`
`5-Fluorouracil
`Cytarabine
`Fludarabine
`Pentostatin
`Cladribine
`Gemcitabine
`
`Cytarabine
`lyposomal
`Capecitabine
`
`Decitabine
`
`Alimta®
`Thymitaq®
`
`1953, 1959, 1971
`2004
`(Phase III)
`
`Leukemia
`Mesothelioma
`Liver cancer
`
`Adrucil®
`Cytosar-U®
`Fludara®
`Nipent®
`Leustatin®
`Gemzar®
`
`DepoCyt®
`
`Xeloda®
`
`Dacogen®
`
`1962
`1969
`1991
`1991
`1993
`1996
`1998
`
`1999
`
`1998
`200l
`(Phase III)
`
`Colorectal cancer
`Leukemia
`Leukemia
`Leukemia
`Leukemia
`Pancreatic cancer
`Non-small cell
`lung cancer
`Lymphomatous
`meningitis
`Breast cancer
`Colorectal cancer
`Leukemia
`
`[18]
`[19]
`[20]
`
`[21]
`[22]
`[23]
`[24]
`[25]
`[26]
`[27]
`
`[28]
`
`[29]
`[30]
`[31]
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`
`leucovorin increases the cytotoxicity of 5-FU. The major toxicities of 5-FU are to bone
`marrow and mucous membranes [17].
`Capecitabine (8) is an orally administered fluoropyrimidine carbamate that is
`metabolized in vivo to 5-FU (3) [32] . Pharmacokinetic studies in patients showed rapid
`gastrointestinal absorption of capecitabine, followed by extensive conversion to 10, with
`only low systemic levels of 5-FU [33]. Significantly, capecitabine appears to deliver drug
`selectively to tumors. Analysis of the tumor:plasma AUC ratios of capecitabine vs. 5-FU
`in four human tumor xenografts in mice (HCTlI6, CXF280, COL0205, and WiDr) at the
`maximum tolerated dose (p.o.) showed that although the half-life of 5-FU was similar in
`all four tumors, the tumor:plasma AUC ratio of 5-FU was significantly higher for animals
`dosed with capecitabine. For example, in HCTI16, 5-FU exposure was 127-fold higher in
`tumor than plasma in animals treated with capecitabine, and 209-fold higher in CXF280
`[34]. In the clinic, the efficacy of capecitabine equals or exceeds 5-FU, although its
`pharmacology in the murine models might have predicted consistently superior efficacy.
`Capecitabine is approved for the treatment of breast and colorectal cancer, and its ease of
`dosing by mouth is attractive to patients.
`Gemcitabine (7) is a nucleoside analog that exhibits cell phase specificity, primarily
`killing cells undergoing DNA synthesis (S-phase) and also blocking the progression of
`cells through the Gl/S-phase boundary. The cytotoxic effect of gemcitabine is attributed
`to a combination of two actions of the diphosphate and the triphosphate nucleosides,
`which leads to inhibition of DNA synthesis. First, gemcitabine diphosphate inhibits
`NDPR, which causes a reduction in the concentrations of deoxynucleotides, including
`deoxycytidine triphosphate (dCTP) [35] . Second, gemcitabine triphosphate competes
`with dCTP for incorporation into DNA. The reduction in the intracellular concentration
`of dCTP (by the action of the diphosphate) enhances the incorporation of gemcitabine
`triphosphate into DNA (self-potentiation). After the gemcitabine nucleotide is
`incorporated into DNA, only one additional nucleotide is added to the growing DNA
`strands. After this addition, there is inhibition of further DNA synthesis. Because of the
`addition of this final nucleotide, DNA polymerase epsilon is unable to remove the
`gemcitabine nucleotide and repair the growing DNA strands (masked chain termination).
`First approved by the FDA in 1996, 7 was demonstrated to have a significant clinical
`benefit response in advanced pancreatic cancer patients compared to 5-FU, with a
`survival advantage of 5.6 months vs. 4.4 months in the 5-FU-treated patients [26]. The
`clinical benefit was measured as improvement in three symptoms present in most
`pancreatic cancer patients: pain, functional impairment and weight loss. Gemcitabine has
`become accepted as the standard of care for the treatment of advanced pancreatic cancer,
`and in 1998 the FDA approved its combination with cisplatin for the treatment of non(cid:173)
`small cell lung cancer (NSCLC).
`DepoCyt is an injectable, sustained release form of cytarabine (ara-C; 6), for the
`treatment of antineoplastic meningitis (NM) arising from lymphoma (lymphomatous
`meningitis) [28]. Ara-C acts by inhibiting DNA polymerase as well as through
`incorporation of its triphosphate into DNA. A phase III trial of DepoCyt in
`lymphomatous NM showed it to be more convenient and associated with a higher
`positive response rate than ara-C. The DepoCyt formulation of ara-C is encapsulated in
`the aqueous chambers of a spherical 20 fLM matrix comprised of lipids biochemically
`similar to normal human cell membranes (phospholipids, triglycerides, and cholesterol).
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`
`When injected into the cerebral spinal fluid (CSF) at room temperature, the particles
`spread throughout the neuroaxis and slowly release ara-C. A single injection of free
`unencapsulated ara-C maintains cytotoxic concentrations in the CSF for <24 h [36] ,
`whereas a single injection of 50 mg of DepoCyt maintains cytotoxic concentrations of
`ara-C in most patients in the CSF for> 14 days [37]. As cytotoxicity is a function of both
`drug concentration and duration of exposure, this formulation maintains high
`concentrations of ara-C in the cancer cell for prolonged periods of time and increases
`the efficacy of the agent.
`Pemetrexed is an antifolate that inhibits multiple folate-requiring enzymes including
`TS, DHFR, GARFT and to a lesser extent, AICARFT [38,39] . Having multiple sites of
`inhibition results in an activity profile that differs from the TS inhibitor, 5-FU, or the
`DHFR inhibitor, MTX. Folic acid and vitamin B-12 supplementation modulate
`pemetrexed's overall toxicity while enhancing its cytotoxic effects, and pre-treatment
`with folic acid is a component of the clinical regimen [40] . In a Phase II study of
`pemetrexed as a single agent in patients with malignant pleural mesothelioma (MPM),
`a 17% response rate was observed (9 of 64 patients). In combination with cisplatin,
`however, a Phase III trial found a response rate of 41.3% and a median survival of 12.3
`months [19] . The control arm of the study received cisplatin monotherapy, and the
`response rate in these patients was 16.7% with median survival of9.3 months [41]. Based
`on these findings, the pemetrexed/cisplatin combination was approved by the FDA in
`February 2004 as a treatment for MPM. Clinical trials of pemetrexed are underway as a
`therapy for solid tumors including non-small cell lung, pancreatic, metastatic breast,
`colorectal, and gastric cancers [42].
`
`4. CLINICAL ADVANCES WITH COMBINATIONS
`OF ANTIMETABOLITES
`
`Anti-cancer agents are rarely given singly, as combinations of drugs have proven to be
`far superior to single agent therapy for a variety of cancers. Antimetabolites are no
`exception; they have been combined with other antimetabolites and with other
`chemotherapeutic agents. The goal of combination therapy is to find agents whose
`activities are synergistic, i.e., a regime where the combined effect is greater than what
`would be expected from the sum of the two individual agent's activities, and have non(cid:173)
`overlapping toxicities. Since most antimetabolites interfere with the process of DNA
`synthesis or growth, many combinations with drugs that react with DNA have been used
`[43]. An illustration of this is the combination of pemetrexed with cisplatin. Other
`potential combinations would be with compounds targeted towards inducing apoptosis,
`preventing angiogenesis, or with antimetabolites targeting different enzymes in the same
`pathway. An example is the combination of cyclophosphamide (a DNA alkylating agent),
`MTX, and 5-FU. This regimen, referred to as CMF, was an early standard of care in the
`treatment of metastatic breast cancer [44]. 5-FU targets the thymine biosynthesis
`pathway by inhibiting TS, while MTX targets the same general pathway by inhibiting
`DHFR. Combinations are first studied preclinically in cellular and animal models, but
`application of the models to patients in the clinical setting is complicated by ADMET
`phenomena that are not well predicted by the models [45] . Clinical success for therapy
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`
`(combination or single agent) is assessed by a variety of measures relative to the standard
`of care for the indication, including toxicities, survival benefit, response rate, median
`survival, and survival to a determined point in time (e.g. 12 months). Our focus will be on
`recent clinical outcomes with combinations of antimetabolites and the biochemical
`rationale behind them.
`Thymidylate synthase inhibitors including 5-FU, capecitabine, and raltitrexed have
`been tested in the clinic in combination with gemcitabine. Gemcitabine and the TS
`inhibitors inhibit DNA and RNA synthesis by different mechanisms and possess almost
`no overlapping toxicity profiles. Gemcitabine inhibits NDPR, depleting cellular dUMP
`pools, thereby decreasing the dUMP competition with 5-FdUMP at TS. Raltitrexed
`prevents binding of the folate cofactor on TS, and also has little overlapping toxicity with
`gemcitabine. At least additive, and possibly synergistic cytotoxic effects for
`gemcitabine/5-FU were anticipated based on preclinical results with the combination
`in HT29 colon cancer cells [46] . To date, several Phase III trials in pancreatic cancer have
`found gemcitabine/5-FU regimens to be tolerable in terms of toxicities, but an optimal
`dosing schedule has yet to be found which improves the median survival of patients with
`advanced pancreatic carcinoma compared with single-agent gemcitabine [47]. A Phase II
`trial of bi-weekly high dose gemcitabine plus capecitabine in pancreatic cancer similarly
`found a good therapeutic index for the combination, but no advantage was observed
`in terms of efficacy parameters compared to mono therapy with gemcitabine [48,49] .
`The combination of gemcitabine with raltitrexed has been investigated in a Phase II trial
`involving pancreatic cancer patients, and was found to be convenient with little
`symptomatic toxicity. However, like the combinations with the nucleoside TS inhibitors,
`the dual therapy of gemcitabine with the antifolate TS inhibitor was no more efficacious
`in terms of survival benefit than single gemcitabine monotherapy [50].
`Gemcitabine has also been investigated with pemetrexed in pancreatic cancer and
`NSCLC. Pemetrexed depletes the intracellular supply of both purine and thymidine
`deoxynucleotides, while gemcitabine is incorporated into nascent DNA strands
`ultimately resulting in strand termination. Thus, the two agents together would
`interfere with DNA replication at both the nucleotide and strand synthesis level. Early
`in vitro cell assays and tumor xenograft models indicated that gemcitabine/pemetrexed
`would show synergism in vivo, but the degree of activity was dependent on cell type
`and dosing schedule [40,51,52]. Each drug is active as a single agent in pancreatic
`cancer (Table 2) [26,53] , and a Phase I trial of the gemcitabine/pemetrexed combination
`in advanced pancreatic cancer was initiated. In patients, the recommended schedule and
`doses were found to be gemcitabine on days 1 and 8 @ 1250 mg/m2 with pemetrexed
`on day 8 only @ 500 mg/m2 [51]. Phase II results for the gemcitabine/pemetrexed
`combination have been reported, with patients showing a 15% partial response rate,
`with 29% of the evaluable patients surviving for 12 months. As the three measures
`(partial response, median survival, and l-yr survival) for the combination showed
`improvement relative to therapy with gemcitabine or pemetrexed alone, a Phase III trial
`was initiated. Enrollment for the Phase III trial has concluded, and a final data analysis
`is anticipated in the coming year. The combination has demonstrated efficacy in an
`ongoing Phase II trial for NSCLC, employing the same schedule, with evidence of
`improved median survival and l-yr survival in comparison to the gemcitabine/cisplatin
`combination or single agent pemetrexed.
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`J.R. Henry and M.M. Mader
`
`Table 2. Comparisons of selected anti-metabolite single agent and combination clinical
`trials
`
`Patients
`totalJeval.
`
`Ref.
`
`Treatment
`
`Agent
`
`PRa n (%)
`
`Median
`survival
`(mo)
`
`1-yr
`survival
`(%)
`
`Advanced pancreatic cancer
`
`[26]
`[53]
`[54]
`
`63/56
`42/35
`42/40
`
`Phase III
`Phase II
`Phase II
`
`Non-small cell lung cancer
`
`[55]
`
`3011288
`
`Phase III
`
`[56]
`[57]
`
`33/30
`60/54
`
`Phase II
`Phase II
`
`apR = partial response.
`b Objective response.
`
`5. CONCLUSIONS
`
`Gemcitabine
`Pemetrexed
`Gemcitabinel
`pemetrexed
`
`Gemcitabinel
`cisplatin
`Pemetrexed
`Gemcitabinel
`pemetrexed
`
`3 (5.4)
`2 (5.7)
`6 (15)
`
`60 (21)
`
`7 (23)
`9 (17)b
`
`5.7
`6.5
`6.5
`
`8.1
`
`9.2
`1l.3
`
`18
`28
`29
`
`36
`
`25
`46
`
`Much progress has been made in the last decade in the development and clinical use of
`antimetabolites as chemotherapeutics for the treatment of solid tumors. Both mono- and
`combination therapies have been found to be efficacious, and clinical trials are underway
`to determine efficacies against a greater variety of tumor types, and of regimens involving
`two, three and four-drug combinations.
`
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