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`
`MTA: Summary and Conclusions
`
`Hllar7" C.alvert
`
`BACKGROUND
`
`S h ’qCE the inttodu~xion of methotrexate into
`
`clinical practice in the late 1940s, there have
`been continua[ attempts to introduce improved or
`different antifolates. AJthough antifohtes have
`been successfully inttoduced as. antbinfective
`agents (for example, pyfimethamine for malaria
`and trimethoprim for bacterial infections), the dis-
`covezy of additional fohte-based therapeutic
`agents against cancer has been more elusive. A
`number of dihydrofolate reductase (DH~) inhib-
`itozs have been tested, including metoprine~ (2,4-
`diamino.5-(3’,4’-dichlorophenyl)-6-mer.hylpyrl-
`midine), trimetrexate$ and edacrexate.~ While tri-
`metrexate shows some activity as an any.ica~cer
`agent and as an antiFungal agent (Pneunm~s~
`carirdi pneumonia) and edatrexare has demon-
`~wated activity in lung cancer, none of these com-
`pounds has so far achieved a mainsaem’a role in
`cancer treatment. The development of antifohte
`clings targeted agaimt ahemative eazlane~ of fohte
`metabolism has met with rather more success.
`has been argued that a selective fohte-based
`hibitor of thymidylate synthase (TS) would pro-
`vide a therapeutic advantage comv~ed with com-
`pounds inhibiting DHI~.4 The first clinically
`tested foloxe-based "IS inhibitor (CB 3717)
`showed antitumor activity, but clinical devdop-
`ment was discontinued due to sporadic and
`dictable toxicity,s However, this experience led to
`the development of rakitmxed, a drug that has
`activity in colon cancees and is licensed for this
`indication in a number of countries. Several fo-
`late-based inhibitors of glycinamide zibonucle.
`otide formyl tmmferase (GAKF’r) have entered
`early clinical studies, but so far none has pro-
`g~essed beyond the phase I/early phase II stage.TM
`The lesson to be learned Eora almost 50 yeazs of
`experience with antifolate drags must be that it is
`difficult to discover new agents that pt~lu~ suf-
`ilciendy corapdling clinical results to give them a
`role in everyday practice. This review wilt examine
`the results presented in this supplement on btTA
`to project whether this drag wiLl be a bzoadly
`use.hi .antlcance~ agent and whether it wLli differ
`substantially frora those, akeady avaihble.
`k has been argued that antifolates capable of
`
`inhibiting two lod in folate metabolism could
`offer advantages compared with those inhibiting
`ordy one,e since the development of drug resis-
`tance might be rendered less likely. If zesistance
`were to develop by an increase in the level of one
`target enzyme, the folate pathwa~ would still be
`inhibited by the action of the drug on th~ alter-
`native target. Counter to such an argument is the
`observation that different folate-dependent en-
`zymes are present in differing activities. For exam-
`ple, a 50% inhibition of the flux through a rate-
`limiting enzyme such as TS will result in a
`corresponding inhibition of the rate of DNA syn-
`r.hesis, while a greater than 90% inhibition o~
`DI-I~ will be necessan/to achieve a similar re-
`duction in the tare of DlqA synthesis, since this
`latter enz~e is normally pztsent in. excess.~o
`Thus, in oider for a cling to be capable d recl~cing
`the rate of DNA synthesis due to inhibiting either
`TS or DH~, it would be necessaw for its K~ for
`DHFfl. to be considerab|y bwer thun that for "IS.
`However, such ~m acgumen~ does not t~ke into
`account the idea that the level dex~te~ion o~TS
`and other ez~,mes of folate metabolism may he
`both variable between cell tyges and inducible in
`re~onse to exposure to an inhibitor. In this event,
`loci other than the primary target of an.antifolate
`could become important in response to exposure
`¯ to the drug.
`
`MTA AS A NEW’ DRUG
`
`Is MTA Funcaondly More Than a Pure
`Th:ymid~/am Ss~rtthas~ Inhibimff ¯ ¯
`
`As has been described in this supplement, MTA
`w-as discovered during the evaluation of a series of
`compounds originally intended to be Lnhibitors of
`OAPffrr. Initial testing suggested that it was in
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`
`fact functionally a TS. in_hibitor, but further eval-
`uation showed’that it also inhibited DHFR, as well
`as GARFT, and aminoimidazo~e carboxamide ri-
`bonucleorlde fonnyhxansferese. The Kas for TS,
`DHFR, GARFT, and aminoimidazole carboxam-
`ide ribonucleotide for the Ghs derivative are m-
`ported by Chen et al in this supplement as 13, 7.1,
`65, and 260 nmol/L, respectively. These figur~
`lead one to believe that the dominant locus, of
`MTA would be TS, a supposition that is supposed
`by the observation that in most of the studies
`presented, end-product reversal of a mildly toxic
`concentration MTA cart be achieved in vitro by
`the addition of thymidine alone (Chen et al and
`Smith ee al, this stipplement). However, MTA at
`a concentration 10-foId higher than the ICs0 (7
`~mol/L; Smith et el, this .supplement) also re.
`quired a purina source for revemal, suggesting that
`an alternative locus, most likely GARFT, was
`coming into play. Pharmacokinetic dam show that
`clinically achieved plasma levels of MTA follow-
`i~g the administration of 600 mg/mz exceed 200
`t~md/L at the peak and remain over 7 t~raol/I, for
`many hours (Robert D. Johnson, PhD, Eli Lilly
`and C~mpany, pewoonal communtcat[on),t* Fur-
`thermore, biochemical evidence shows a direct
`effect of MTA on put~.ne s~nthesis (Mendelsohn et
`al, this supplement) and a difference in the accu-
`muhtion of deo~adenosine trlphosphare (Chert
`et el, this supplement) compared with the more
`specific TS inhibitor, ~alrltrexed. All these obser-
`vations lend weight to the idea that when admin-
`istered in clinically relevant doses to humans, the
`alternative targets of MTA will phy a significant
`part ih its actions. Also of great interest are the
`data on resistant cell lines (Schultz et el, this
`supplement) in which ceil lines axe described that
`are significantly more resistant to rakitrexed than
`to MTA, in which a purine source is requ~d to
`protect from MTA toxicity in the resistant line.
`These data again suggest that a second biochemb
`cal target may be important in circumventing drug
`resistance.
`In vivo antitumor dam derived in mice, while
`not directly establishing a mechanism of action for
`MTA, are encouraging, k is well-known that mice
`have higher circulating levels of th~anidine than
`humam*z; this fact leads them to be poor models
`for the antitumor efficacy of TS inhibito~s, unless
`the hsmors concerned are low in thlanidine kinase
`and therefore unable to use the citmalating thlmd-
`
`H~a~RY CALVERT
`
`dine. Nevertheless, MTA showed significant ac-
`tivity in a range of thymidine kinase-compet~nt
`human tumor xenografts grown in mice.
`
`DOES HTA HAVE CLINICAL
`CHARACTERISTICS LIKELY TO blAKE IT
`A PRACTICAL DRUG?
`To be broadly applicable to the t~eatmertt of
`hum~ cancer, a drag needs to have a reasonably
`convenient mode of administration, reasonably
`consistent and predictable toxlcities, and amena-
`ble to drug combinations.
`The phase I experience with MTA (Rinaldi,
`this suppIement) reports three schedules of admin-
`istration. The weekly ×4 schedule was excluded
`from phase II evaluation on account of the pnssi-
`bility of cumulative toxicity, but the other two
`schedules (single dose every 3 weeks or five daily"
`doses repeated every 3 weeks) were both judged
`feasible. The single 3.weekly schedule was chosen
`for the phase II work on account of its conve-
`nience. All the trials showed similar toxicities,
`with leucoperda and thrombocytopenia being
`dose-limiting. Non-dose-limiting toxicgfies com-
`prised transient transamtnase d~vatio~, rashes,
`mucesal toxicity, general malaise, diarrhea, and
`skin rashes. Symptomatic and ~patienr unfriendly"
`toxicities, such as acute nausea and vomiting or
`alopecia, were noticeably infrequent. Sporadic
`cases of severe myelosuf~pression with severe gas- .
`trointestinal toxicity and sepsis were seen in all
`the phase I studies. However, such toxicities have
`not been a serious problem in those phase II stud-
`ies in which the patients were in general of a good
`performance and nutritional status.*~ The ~cently
`presenzed study of the use of phsma homoc~steine
`as a marker for folate deficienc~*~ shows a’corre-
`lation between elevated pretreatment homocys-
`teine levelsand the subsequent oceunence of
`grade 3 or 4 toxicity. Thus, it seems that it will be
`possible to administer MTA every 3 weeks without
`significant slanptomatlc, toxicities and with a good
`level of safe~.
`So far in oncdogy practice the um of cytotoxic
`drags as single agents has been unusual. The opti-
`mal therapeutic regimens derived for the common
`tumors have nearly always been combinations of
`active drugs for the disease concerned. In order for
`a drug to be gen..eraliy useful it should’be amenable
`to use in combinations with other major agents.
`At the predinical, level (Teicher et el, this sup-
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`
`SUP/MARY AND CONCLUSIONS
`
`107
`
`plement)~ MTA was tested incombination with
`cisplatin, methotrexate, 5-fluoroutacil, paclimxe|,
`docetaxel, doxorubicin, irinotecan, and fraction-
`ated radiation therapy, in vivo using the EMT-6
`mammar~ carcinoma, the human HCT 116 Colon
`carcinoma, and the human H460 non-small cell
`lung carcinoma grown as xenografts in nude mice.
`It was possible to use full doses of each of these
`agents in the combination, and additive or syner-
`gistic antitumoi re~uks were seen. Two phase I
`clinicaI trials of MTA in combination are pre-
`sented in this supplement. The combination wifla
`cisplatin allowed the administration of a ~ dose
`(600 mg/mz) of MTA and a dc~e of 75 mg/mz of
`cisphtirL to be given on a repeated basis. The
`cisplatin dose is the same as that generally used in
`a large number of combinations and cannot be
`considered subopdrmd. The combination with
`gemcitabine (Adjei and Edichman, this supple-
`men0 is ongoing, but show that a full dose of
`gemcimbine (1 g/mz da~s I. and 8) can be com-
`bined with at leas’r a dose of 400 mg/mz MTA,
`again suggesting that full doses of the c~mbination
`will he possible.
`
`Does MTA/-/a~ Promising
`
`The phase II studies summarized here (O’I~er
`et al, this supplement) report responses in six
`tumor types. In metastatic breast cancer responses
`were seen in 31% of 36 patients. The previous
`treatment of responding patients included both
`taxa~es and antb~acydines and there was no evi-
`dence for a lower response rate in those with more
`extensive pretreatment. In previously untreated
`non-small ceR lung cancer, two studies have
`shown response rates of 23% and 17%. In previ-
`ously untreated colon cancer, response rates of
`20% and 17% have been reported in two indepen-
`dent studies. The early results of a bladder cancer
`study sugge.sr acdvioA with 7 of 25 tmtient* being
`reported as showing responses. Lesser levels of
`activiW also have been seen in h1~zrnephroma
`and cervical cancer. Of particular interest is the
`observation made in the combination phase I
`study Of MTA and cisphtin in which four of seven
`patients with mesothelioma have been tetmrted as
`responding. If cordlzmed in a larger study this is a
`truly exceptional result in a very reEactor~ tumor.
`Overall the breadth and co’nsisrencT of the phase
`11 activit,/reported with MTA is remarkable and
`
`unusual in a new drag Of any dam at this stage of
`its development..
`
`CONCLUSIONS
`MTA is dearly mechanistically distinct from
`existing antffotates, The biochemical data make a
`strong case for the role of. more than one locus in
`its cytotoxic action and this feature may inhibit or
`preclude the development of certain rae~anisms
`of drug zesisu~ce. The phase I and II experienc~
`suggest that although MTA displays typical "anti-
`folate" to:deities, tlmse are manageable and pre-
`dictable so that broad scale clinical use will be
`feasible. Of interest Is the rehtive lack of toxicities
`that induce unpleasant’s~aptoms of concern to
`tmtients, such as nausea, vomiting, and alopecia.
`Also noteworthy are the early clinical results
`showing evidence of significant activtw in a broad
`range of common tumors, some of which were
`resistant to the major agents currently "available.
`These observations token together suggest that
`MTA will become a major addition to the atma-
`mentarium of drags cttrrenfly available to oncob
`ogy practice.
`
`REFERENCES
`1. Calvert AH, Price LA, Hill BT: DDMP (2,@diamtna*5-
`(3’,4’-dtchlorophen~,l)-6.methylp~tratdLae) In the treatment
`of metastatic hypemephtoma. Proc_~d__Lngs of the lOth Inter-
`national Congress on Chemotherapy, Zurich S~tzedamL Am
`See Mlcrobiol 1270-1272, 19"/13
`2. IE~xino JR: Tdraetzeaa~ Overall cli~..~ tesultn. 8emin
`Oacol 15:50-5l, 1988 (~up01 2)
`3. Chndara DR, gddman MJ, C-~owley J], et al: Phase II trial
`ofeda~mxate plm cazboplatin in metastatic non-small.cell lung
`ca~et: A Southwest Or~olol~ Gmu~ studv. Cancer Che-
`mother Ph~macoi 41:75-78, 1997
`4. Jone~ TP,, Caivm AH, ]ada~n AL,.~t ~ A potent
`antitumour quinamline i_n.hlbitor of th~mldylam ~ath~tme:
`S~nthesls, blologlcal pmlma~s and thetalx’utk r~ul~ tn mice.
`Eur ] Caneer 12:11-19, 1981
`5. Jaclama AL Calvert AH: Folate-based th~aid~hate
`daam inhth|to~ as antlcaneer ’dmg~ Aria Oncol &871-881,
`1995
`6. Z~lr.b~ Jl~ Cum~nd~ D, Vm Cumin E,
`ZD1694: A hOWl ~raidyhte slrnd~ase ir~bitor with,
`6al activity in the treatment of patienta with advanc~l colo-
`~ eaneex. J Clio Oneo114:’/I6-721, 1996
`7. Lsohavinlj S, Wedge 8R, Lind MJ, e~ al: A phase I
`clinical r~dy ~ ~e antipurir, e antifolate Mme.tre.xo[
`(DDA’FHI~ givm wlth oral fdic acid. Itwcst New Drags 14:
`395-~35, I~96
`8. Pearce I-R.: Anr~cancer drag development at LI[1~,
`seat~ Labor~to~e~ Ana Oncd 6:55-62, 1995 (su~p| I~
`9. Calwrt AH, Jon~ "I’g, Dad,, I>.I, et ah Quinazollne and-
`folates with dual hlod.hemlcal ~ o~ a~don. Blochemlca[ and
`
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