Pemetrexed disodium (Alimta, LY231514) is a novel, multitargeted antifolate that inhibits thymidylate synthase, dihydrofolate reductase, and glycinamide ribonucleotide formyl transferase. This agent is broadly active in a wide
ABSTRACT: Pemetrexed disodium (Alimta, LY231514) is a novel,multitargeted antifolate that inhibits thymidylate synthase, dihydrofolatereductase, and glycinamide ribonucleotide formyl transferase. This agent isbroadly active in a wide variety of solid tumors, including breast cancer.Pemetrexed disodium has also shown clinically relevant activity in combinationwith gemcitabine (Gemzar). This combination is being evaluated for the treatmentof metastatic breast cancer. [ONCOLOGY 15(Suppl 3):34-37, 2001]
Gemcitabine (Gemzar) is a pyrimidine antimetabolite that isanabolized sequentially to the nucleoside mono-, di-, and triphosphateintracellularly. Difluorodeoxycytidine triphosphate (dFdCTP) is incorporatedinto DNA, resulting in chain termination. In addition, gemcitabine inhibitsribonucleotide reductase, an enzyme that catalyzes the formation ofdeoxynucleosides required for DNA synthesis. Gemcitabine has broad activityin a variety of solid tumors, including breast cancer, and is approved forthe treatment of pancreatic and non-small-cell lung cancers. The clinicalpharmacology of gemcitabine has been reviewed recently.
Pemetrexed disodium (Alimta, LY231514) is a novel antimetabolitethat targets at least three enzymes involved in the synthesis of purines andpyrimidines. A primary enzyme target is thymidylate synthase (TS),[4-6] afolate-dependent enzyme, whose inhibition results in decreased thymidinenecessary for DNA synthesis.
The other known enzyme targets of pemetrexed disodium aredihydrofolate reductase (DHFR) and glycinamide ribonucleotide formyl transferase(GARFT). The dependence of the cytotoxicity of pemetrexed disodium on bothDHFR and GARFT is supported by the fact that thymidine and hypoxanthine are bothrequired to rescue cells from the cytotoxic effects of this agent in vitro.
Key Enzyme Targets
The three enzyme targets for pemetrexed disodium are shown in Figure 1. As illustrated, pemetrexed disodium is not only similar tomethotrexate (a DHFR inhibitor), fluorouracil (5-FU), and raltitrexed (Tomudex,a TS inhibitor), but also inhibits GARFT, which currently has no clinicallyrelevant inhibitor. Because of its ability to inhibit multiple enzymes,pemetrexed disodium may prove to have superior clinical activity in comparisonto other antifolates and TS inhibitors.
Pemetrexed is transported into cells via the reduced folatecarrier, and is polyglutamated in a reaction that is catalyzed byfolylpolyglutamate synthase (FPGS). The predominant intracellular glutamatedform of pemetrexed disodium is the pentaglutamate, which is greater than 60-foldmore potent in its inhibition of TS than the monoglutamate. The pharmacologyand clinical activity of pemetrexed disodium has been comprehensivelyreviewed.
Folate Status and Toxicity
Folic acid and its derivatives have been traditionally utilizedto reverse the toxicity of antifolate agents. However, there has not been aconsistent correlation between cellular or serum folate levels of patients andthe incidence of toxicity from antifolate chemotherapeutic agents. Recent datahave provided some insight into this apparent contradiction by indicating thatplasma homocysteine is a much more sensitive measure of the functional folatestatus of patients than the traditionally used measures of red blood cell countsor serum folate levels. In view of these data, serum homocysteine andvitamin metabolite levels have been evaluated as predictors of pemetrexeddisodium toxicity in phase I and phase II studies.
The s-adenosylmethionine (SAM) cycle responsible for criticalsingle-carbon transfer reactions (through methyl groups) in mammalian systems isillustrated in Figure 2. The transfer of a methyl group from N5-methyltetrahydrofolate(CH3FH4) to homocysteine generates methionine, whichin turn generates SAM. When humans are folate-deficient, the lack of CH3FH4leads to an increase in the levels of plasma homocysteine, which is an early,sensitive, and reliable indicator of folate deprivation.
Vitamins B12 and B6
Cobalamin (vitamin B12) and pyridoxine (vitaminB6) deficienciescan also lead to an increase in plasma homocysteine levels. Cobalamin is acofactor for two synthetic processes in mammals. The first process is thesynthesis of methionine from homocysteine and CH3FH4, catalyzed by methioninesynthase. The second process is the formation of succinyl co-enzyme A fromL-methylmalonic acid co-enzyme A. Serum methylmalonic acid levels are thereforeelevated in cobalamin deficiency, but not in folate deficiency, and are usefulin the differential diagnosis of cobalamin and folate deficiency in the settingof elevated plasma homocysteine levels.
Pyridoxine is involved in the conversion of homocysteine tocystathionine, and the subsequent conversion of cystathionine to cysteine andalphaketoglutaric acid. Cystathionine levels are elevated to a much greater extent invitamin B6 deficiency, compared to folate and B12 deficiency.[15,16] Isolatedvitamin B6 deficiency, however, occurs very rarely.
Phase I and Phase II Studies
With the emerging information described above, plasmahomocysteine, cystathionine, and methylmalonic acid levels have been measuredand included in a multivariate analysis of potential prognostic factors thatpredict serious toxicity in patients treated with pemetrexed disodium. Inphase II trials, 139 patients with a variety of solid tumors were treated withpemetrexed disodium at doses of 600 mg/m2 every 3 weeks.
Plasma homocysteine, other vitamin deficiency markers, serumalbumin, and hepatic enzymes were measured at baseline and once each cyclethereafter. Preliminary data find baseline plasma homocysteine concentrations tobe the only statistically significant prognostic factors for serious toxicities,predominantly myelosuppression. Mucositis and diarrhea were also correlated withplasma concentrations of methylmalonic acid. A threshold baseline homocysteinevalue of 10 µmol/L was used to differentiate between high- and low-riskpopulations.
Plasma concentrations > 10 µmol/L predicted a greatlyincreased rate of toxicities, even though a continuum existed for various serumlevels. These studies support the notion that individuals who arefolate-deficient are at subclinically or clinically increased risk ofsevere toxicity when treated with standard doses of pemetrexed disodium.
In an ongoing phase I trial, pemetrexed disodium is administered with high-dose intermittent folic acidsupplementation (5 mg orally, days -2 to +2). Available results suggest thatthis folic acid supplementation schedule permits marked dose escalation of pemetrexed disodium with minimal toxicity. Minimally pretreated patients havetolerated pemetrexed disodium doses of up to 925 mg/m2.
A strict interpretation of these findings would suggest thatpatients with reduced folate pools should receive folic acid supplementationbefore receiving pemetrexed disodium. Since plasma homocysteine is a continuouspredictor for severe toxicities, it may be assumed that folic acidsupplementation significantly improves the tolerability of pemetrexed disodium in all patients. Early evidence that supplementation withlow-dose daily oral folic acid and quarterly IM vitamin B12 significantlyreduced toxicities lends support to this idea.
Pemetrexed disodium has been investigated as a single agent inadvanced breast cancer without folic acid and vitamin B12 support. Preliminaryresults from two completed phase II studies in Europe have been reported.
In one study, most patients (33 out of 38) had been previouslytreated with chemotherapy. Of these, 26 patients had previously receivedanthracyclines, while 4 had been treated with taxanes; 16 patients receivedtherapy in the adjuvant setting. Out of 36 evaluable patients, 10 achieved apartial response, and 1 patient achieved a complete response, for an overallresponse rate of 31% (95% confidence interval [CI]: 16%-46%). Responses wereseen in three out of the four patients previously treated with taxanes and inthree out of the 26 patients previously treated with anthracyclines. The mediansurvival was 13 months, and median time to disease progression was 5 months. Themedian response duration was > 9 months.
In the second study, all enrolled patients had been previouslytreated with anthracyclines. In all, 26 patients had documented diseaseprogression ≤ 30 days after anthracycline treatment (anthracycline refractory)and 43 patients developed progressive disease > 30 days after stoppingtreatment (anthracycline failures); 29 patients had also received a taxane. Theoverall response rate in 69 evaluable patients was 23% in anthracycline failuresand 19% in anthracycline refractory patients.
In the cohort of patients who received both anthracyclines andtaxanes, objective responses were documented in 8 out of a total of 29 patientsfor an overall response rate of 28%. The median response duration was 6 months,the median time to progression was 4 months, and the 1-year survival was46%.
These early results indicate thatpemetrexed disodium may not demonstrate complete cross resistance with taxanesor anthracyclines. To test this hypothesis, a phase II study of pemetrexed disodium in patients with stage IV breast cancer who have previouslyreceived anthracyclines and taxanes is underway in the United States andEurope. This study enrolled 40 patients prior to a protocol amendment thatadded folic acid and vitamin B12 supplementation to the remaining patientsenrolled. Preliminary data report a 19% objective response rate with grade 3/4neutropenia as the most frequent toxicity. Final reports of the analysis fromthis study are awaited.
Gemcitabine has broad activity in a variety of solid tumors.In several phase II studies, the response rate of patients with metastaticbreast cancer who were treated with gemcitabine ranged from 29% in second-linetherapy to 38% in first-line therapy.[22,23] The role of gemcitabine in breastcancer is discussed further in this supplement.
Phase I and II Combination Trials
Adjei and coworkers utilized in vitro clonogenic assays in HCT-8human colon cancer cell lines to demonstrate synergistic cytotoxicity whengemcitabine exposure preceded pemetrexed disodium exposure by 4 hours. Basedon these data, a phase I study of the combination regimen pemetrexeddisodium/gemcitabine was performed.
Initially, gemcitabine was administered on days 1 and 8 withpemetrexed disodium given 90 minutes after gemcitabine on day 1. However, due toneutropenia on this schedule, the day-8 dose of gemcitabine was reduced oromitted in 57% of courses. Therefore, after testing a second cohort of patients,an alternate schedule with pemetrexed disodium administered on day 8 rather thanday 1 proved to be better tolerated.
Promising clinical activity in a variety of solid tumors wasdocumented in 13 out of 55 evaluable patients. Durable objective responses wereseen in non-small-cell lung cancer (3), cholangiocarcinoma (2), ovariancarcinoma (2), colorectal cancer (3), breast cancer (1), mesothelioma (1), andan adenocarcinoma of unknown primary site (1). The most common and dose-limitingtoxicity was neutropenia; thrombocytopenia and anemia were rare. Commonnonhematologic toxicities included arthralgia, nausea, fatigue, fever, rash, andelevated levels of hepatic transaminases. In limited studies, there was nopharmacokinetic interaction between gemcitabine and pemetrexed disodium.
The recommended doses and sequence for phase II studies weregemcitabine at 1,250 mg/m2 administered on days 1 and 8, with pemetrexeddisodium at 500 mg/m2 given 90 minutes after gemcitabine on day 8. Because ofthe inconvenience of waiting 90 minutes between administration of gemcitabineand pemetrexed disodium, a study of this combination has been completed in which pemetrexed disodiumwas administered immediately after gemcitabine. This study evaluated toxicity,efficacy, and possible pharmacokinetic interactions between these two agents,and the results are awaited with interest.
Pemetrexed Disodium/Gemcitabine in Breast Cancer
In the phase I study described above, one out of three heavilypretreated breast cancer patients with soft-tissue disease achieved a durablepartial response. In addition, both pemetrexed disodium and gemcitabine havesingle-agent activity in breast cancer. Based on these factors, a phase II studyof pemetrexed disodium in combination with gemcitabine in metastatic breastcancer is open for accrual through the North Central Cancer Treatment Group. Theaccrual goal is 60 patients within 1 year.
Eligible patients must have previously received an anthracyclineand a taxane, which could have been administered in the adjuvant or metastaticsetting, or a combination of both. Patients must not have received more than onechemotherapy regimen for metastatic disease (unless these were a taxane andanthracycline). For example, a patient could have received cyclophosphamide(Cytoxan, Neosar), methotrexate, and 5-FU for adjuvant therapy, followed by ananthracycline and followed by a taxane for metastatic disease.
The end points of this study are: (1) assessment of antitumoractivity of the pemetrexed disodium and gemcitabine combination regimen, (2)determination of toxicity, and (3) determination of time to progression andoverall survival. The study follow up is 5 years, with patients examined every 3months.
Gemcitabine is administered at 1,250 mg/m2 on days 1 and 8.Pemetrexed disodium is administered at 500 mg/m2 on day 8, 90 minutes aftergemcitabine. Courses are repeated every 21 days for a minimum of six courses, inthe absence of unacceptable toxicity or progression of disease. Patients whoachieve a complete response receive two additional courses. To ameliorate thetoxicity of pemetrexed disodium, 350-600 µg of oral folic acid taken daily isstarted 7 to 14 days before treatment and continued for the duration of thestudy. Vitamin B12 is administered intramuscularly, 7 to 14 days beforetreatment and repeated at 9 weeks.
The phase I and II studies have investigated the role of thefolate status of patients in the toxicity of pemetrexed disodium. These studiessuggest that patients should receive folic acid and vitamin B12 supplementationprior to pemetrexed disodium therapy. Folicacid supplementation significantly improves the tolerability of pemetrexeddisodium in all patients; early evidence that supplementation with low-dosedaily oral folic acid and quarterly IM vitamin B12 significantly reducedtoxicities lends support to this idea.
Moreover, single-agent pemetrexed disodium possesses broadantitumor activity against a variety of tumors, including breast cancer. It isalso associated with clinically relevant activity in a wide variety of tumorswhen used in combination with gemcitabine. In light of these promising data, thecombination regimen of pemetrexed disodium/gemcitabine is under activeinvestigation for the management of metastatic breast cancer.
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