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The topoisomerase I inhibitor 9-dimethylaminomethyl-10-hydroxycamptothecin (topotecan) was evaluated against a panel of xenografts comprising four lines of adult colon adenocarcinoma, three colon tumors derived from adolescents, six childhood rhabdomyosarcomas from previously untreated patients as well as sublines selected in vivo for resistance to vincristine and melphalan, and three lines of childhood osteogenic sarcoma. Efficacy was determined at maximal tolerated dose levels using intermittent i.p. administration [every 4 days for 4 doses (q4dx4)] or daily p.o. or i.p. administration 5 days per week for up to 20 courses. On a q4dx4 schedule, the maximum tolerated dose (MTD) was 12.5 mg/kg per administration, which caused marked weight loss and lethality in approximately 5% of the tumor-bearing mice. This schedule caused significant growth inhibition (but no tumor regression) in advanced adult colon adenocarcinomas. The minimal treated/control (T/C) ratios were 0.49, 0.54, and 0.3 for three of the tumor lines and were achieved at 18-21 days after the initiation of treatment. In contrast, rhabdomyosarcomas were considerably more sensitive, with T/C ratios being < 0.1 for three lines, whereas topotecan was less active against two other rhabdomyosarcoma xenografts (minimal T/C ratios, 0.17 and 0.14). As inhibitors of topoisomerase I have been demonstrated to have activity in the replication phase of the cell cycle (S-phase-specific), prolonged administration schedules were examined. Mice received topotecan 5 days per week for 3 weeks either by i.p. injection or by oral gavage (p.o.). In selected experiments, p.o. administration was continued for up to 20 weeks. Oral administration for 3 weeks (2 mg/kg per dose) resulted in complete regression of all six lines of rhabdomyosarcoma, with two lines demonstrating no regrowth during the period of observation (> or = 84 days). Similar results were obtained after i.p. administration, suggesting significant schedule dependency for these tumors. For colon tumors, the daily administration schedule (i.p. or p.o.) demonstrated some advantage over the intermittent schedule, resulting in partial regressions and significant inhibition of the growth of several colon adenocarcinoma lines. In rhabdomyosarcoma Rh12 and VRC5 colon adenocarcinoma, both of which demonstrated intermediate sensitivity to topotecan, and in osteosarcoma OS33, protracted p.o. administration for 13-20 weeks (1.0-1.5 mg/kg per dose given daily x 5 days) caused complete regression without regrowth in Rh12 and OS33 tumors and partial regression of all VRC5 tumors. No toxicity was observed using this schedule of administration. Topotecan demonstrated significant activity against all three osteosarcoma xenografts examined, with optimal schedules causing complete regression in two lines.(ABSTRACT TRUNCATED AT 400 WORDS) [ABSTRACT FROM AUTHOR]

Aim: We describe the feasibility of combining infusional 5-fluorouracil (5-FU) with intraoperative radiation therapy (IORT)., Methods: Patients with surgically resectable locally advanced gastrointestinal cancers were treated concurrently during surgery with IORT and a 72 h infusion of 5-FU. Patients without previous external beam radiation therapy (EBRT) were subsequently treated with EBRT (40-50Gy) concurrent with a 21-day continuous infusion of 5-FU. Pancreatic, gastric, duodenal, ampullary, recurrent colorectal, and recurrent anal cancer were included., Results: During IORT/5-FU, no chemotherapy-related grade III or IV hematologic or gastrointestinal toxicity was noted. Post-surgical recovery or wound healing was not affected. One of nine patients who received post-operative radiation required a treatment break. During follow-up, there were more complications in patients with pelvic tumours, especially those with previous radiation. Nine patients have had local and/or local regional recurrences, two of these in the IORT field., Conclusions: Treatment with a combination of IORT and 5-FU followed by EBRT and 5-FU is feasible. However, long-term complications may be increased in previously irradiated recurrent pelvic tumours.

The orphan nuclear receptor SXR coordinately regulates drug clearance in response to a wide variety of xenobiotic compounds. This signaling system protects the body from exposure to toxic compounds; however, it can also pose a severe barrier to drug therapy. We now demonstrate that the human immunodeficiency virus (HIV) protease inhibitor ritonavir binds SXR and activates its target genes. This represents an example of a commonly used therapeutic agent that effectively activates SXR. We also show that other protease inhibitors are weaker (saquinavir) or unable to activate SXR (nelfinavir, indinavir) thus defining analogs that fail to induce SXR-regulated clearance pathways. Interestingly, HIV protease inhibitors are distinct from previously known SXR ligands in that they are peptide mimetic compounds. This expands the ligand specificity of SXR to include this unique chemical class whose pharmaceutical significance is expanding. Finally, we show that SXR ligands activate expression of multiple resistance protein 2, a critical regulator of bile flow and biliary drug excretion. These findings have important implications for the role of SXR in regulating drug clearance and hepatic disorders associated with impaired bile flow.

Dexrazoxane is a bidentate chelator of divalent cations. Pretreatment with short infusions of dexrazoxane prior to bolus doxorubicin has been shown to lessen the incidence and severity of anthracycline-associated cardiac toxicity. However, because of rapid, diffusion-mediated cellular uptake and the short plasma half-life of dexrazoxane, combined with prolonged cellular retention of doxorubicin, dexrazoxane may be more effective when administered as a continuous infusion. Thus, a Phase I pharmacokinetic trial of a 96-h infusion of dexrazoxane was performed. Dexrazoxane doses were escalated in cohorts of 3 to 6 patients per dose level. All patients received granulocyte-colony stimulating factor at a dose of 5 microg/kg/day starting 24 h after completion of the dexrazoxane infusion. Plasma samples were collected and analyzed for dexrazoxane by high-performance liquid chromatography. Urine collections were performed at baseline and during the infusion to determine the renal clearance of dexrazoxane and the excretion rate of divalent cations. Twenty-two patients were enrolled at doses ranging from 125 to 250 mg/m(2)/day. Grade 3 and 4 toxicities included grade 4 thrombocytopenia in 2 patients treated at 250 mg/m(2)/day, grade 3 thrombocytopenia and grade 4 nausea and vomiting in 1 patient treated at 221 mg/m(2)/day, grade 4 diarrhea and grade 3 nausea and vomiting in 1 patient treated at 221 mg/m(2)/day, and grade 3 hypertension in 1 patient treated at 166.25 mg/m(2)/day. Steady-state dexrazoxane levels ranged from 496 microg/l (2.2 microM) to 1639 microg/l (7.4 microM). Dexrazoxane plasma CL(ss) and elimination t(1/2) were 7.2 +/- 1.6 l/h/m(2) and 2.0 +/- 0.8 h, respectively. The mean percentage of administered dexrazoxane recovered in the urine at steady state was 30% (range, 10-66%). Urinary iron and zinc excretion during the dexrazoxane infusion increased in 12 of 18 and 19 of 19 patients by a median of 3.7- and 2.4-fold, respectively. These results suggest that dexrazoxane as a 96-h infusion can be safely administered with granulocyte-colony stimulating factor at doses that achieve plasma levels that have been demonstrated previously to inhibit topoisomerase II activity and to induce apoptosis in vitro. Additional studies will be required to determine whether the combination of continuous infusions of dexrazoxane and doxorubicin would provide enhanced cardioprotection compared with the currently recommended bolus or short infusion schedules.

In prior studies, it was demonstrated that the redox metabolism of doxorubicin leads to the formation of promutagenic oxidized DNA bases in human chromatin, suggesting a potential mechanism for doxorubicin-related second malignancies. To determine whether a similar type of DNA damage is produced in the clinic, peripheral blood mononuclear cell DNA from 15 women treated with infusional doxorubicin (165 mg/m(2)) as a single agent was examined for 14 modified bases by gas chromatography/mass spectrometry with selected ion monitoring. Prior to the 96-hour doxorubicin infusion, 13 different oxidized bases were present in all DNA samples examined. Chemotherapy, producing a steady-state level of 0.1 microM doxorubicin, increased DNA base oxidation up to 4-fold compared to baseline values for 9 of the 13 bases studied. Maximal base oxidation was observed 72 to 96 hours after doxorubicin treatment was begun; the greatest significant increases were found for Thy Gly (4.2-fold), 5-OH-Hyd (2.5-fold), FapyAde (2.4-fold), and 5-OH-MeUra (2.4-fold). The level of the promutagenic base FapyGua increased 1.6-fold (P < .02), whereas no change in 8-OH-Gua levels was observed in peripheral blood mononuclear cell DNA during the doxorubicin infusion. These results suggest that DNA base damage similar to that produced by ionizing radiation occurs under clinical conditions in hematopoietic cells after doxorubicin exposure. If doxorubicin-induced DNA base oxidation occurs in primitive hematopoietic precursors, these lesions could contribute to the mutagenic or toxic effects of the anthracyclines on the bone marrow.

Cytochrome P450 3A4 is an important mediator of drug catabolism that can be regulated by the steroid and xenobiotic receptor (SXR). We show here that SXR also regulates drug efflux by activating expression of the gene MDR1, which encodes the protein P-glycoprotein (ABCB1). Paclitaxel (Taxol), a commonly used chemotherapeutic agent, activated SXR and enhanced P-glycoprotein-mediated drug clearance. In contrast, docetaxel (Taxotere), a closely related antineoplastic agent, did not activate SXR and displayed superior pharmacokinetic properties. Docetaxel's silent properties reflect its inability to displace transcriptional corepressors from SXR. We also found that ET-743, a potent antineoplastic agent, suppressed MDR1 transcription by acting as an inhibitor of SXR. These findings demonstrate how the molecular activities of SXR can be manipulated to control drug clearance.

Dolastatin-10 is a novel pentapeptide agent originally isolated from the marine mollusk Dolabella auricularia with a mechanism of antitumor activity that involves the inhibition of microtubule assembly. We performed a Phase II trial of Dolastatin-10, 400 microg/m2 in patients with advanced melanoma who had received no prior chemotherapy. Dolastatin-10 pharmokinetics were evaluated in a subset of patients following courses 1 and 2. Twelve patients were treated with a median of 2 cycles of Dolastatin-10, and no patient experienced an objective response. The only grade >2 toxicities were grade 3 neutropenia uncomplicated by infection, occurring in 4 patients following the first treatment cycle. The total systemic clearance and volume of distribution at steady-state were 2.61 +/- 1.9 L/h/m2 and 28.4 +/- 13 L/m2, respectively. Due to prolonged terminal elimination. Dolastatin-10 plasma concentrations of greater than 1 nM were sustained for 24 h in all patients studied. Dolastatin-10 is unlikely to have substantial activity in the treatment of melanoma.

Glycinamide ribonucleotide formyltransferase (GARFT) is a component of the de novo purine synthesis pathway. AG2034 is a specific inhibitor of GARFT that was designed based on the GARFT crystal structure. In conjunction with Phase I studies at four clinical centers in the United States and United Kingdom, AG2034 pharmacology was evaluated in 54 patients receiving 1-11 mg/m2 AG2034 as a 2-5 min injection. Blood samples were obtained just prior to and 5, 15, 30, and 45 min, and 1, 1.5, 2, 4, 6, 8, 12, 24, 48, 72, and 96 h after bolus injection during course 1. Limited sampling was also performed on course 3. Plasma AG2034 was measured using a sensitive and reproducible ELISA assay. AG2034 demonstrated a trimodal elimination pattern over 24 h, with median half-life (t(1/2))alpha = 8.7 min, t(1/2)beta = 72.6 min, and t(1/2)gamma = 364.2 min. AG2034 systemic clearance ranged from 9.4-144.5 ml/min/m2, and volume of distribution was 1.2-7.6 liters/m2. Course 1 AG2034 area under the concentration versus time curve (AUC) had a linear relationship with dose (r(s) = 0.86). Accumulation of AG2034 was evident, because course 3 AUC was higher than course 1 in 23 of 23 evaluable patients, but was not associated with an increase in erythrocyte AG2034. AG2034 systemic exposure had an impact on toxicity, because course 1 and course 3 AG2034 AUCs were significantly higher for patients with grade III/IV toxicity than patients with less than grade II toxicity (P < 0.001 and P = 0.001 for course 1 and course 3, respectively). This study demonstrates rapid systemic clearance of AG2034 and suggests pharmacokinetic approaches that may minimize patient toxicity and aid the development of this interesting class of anticancer agents.

Purpose: In recognition of the variety of available chemotherapeutic modulating agents and their potential to enhance the efficacy of platinum-based therapy, we embarked upon a phase I study to investigate the feasibility of combining fixed doses of carboplatinum (CBDCA) and etoposide (VP-16) with 24-h concurrent infusions of dipyridamole (DP), prochlorperazine (PCZ) and cyclosporine A (CSA) administered in escalating doses., Methods: Patients received intravenous VP-16 (200 mg/m2) and CBDCA (300 mg/m2), each over 30 min, starting at hour 6 of the modulator infusions. Resistance modulators were escalated sequentially to determine their respective maximally tolerated doses (MTDs). The pharmacokinetics (PK) of VP-16, CBDCA, and the three drug resistance (DR) modifiers were studied in eight patients., Results: A total of 59 patients were entered on study. The MTD was established at DP 5 mg/kg per day, PCZ 24 mg/h, and CSA 9.5 mg/kg per day. Dose-limiting toxicities included hypotension and severe sedation, presumably related to PCZ. No objective responses were seen. PK studies were performed when PCZ and DP doses were 24 mg/h and 3.3 mg/kg, and the CSA dose was either 8.5 mg/kg (five patients) or 9.5 mg/kg (three patients). The median clearance of VP-16 was 0.96 l/h per m2 (range 0.8-1.5 l/h per m2), which is lower than for VP-16 alone and similar to previously reported effects of CSA on VP-16 elimination. The median measured CBDCA AUC was 3.0 mg/ml x min (range 2.4-4.8 mg/ml x min). CBDCA AUC predicted by the Calvert formula using measured creatinine clearance underestimated the actual AUC in seven of the eight patients, in one case by as much as twofold. The median end of infusion PCZ and total DP plasma concentrations were 1.2 microM (range 0.5-2.2 microM) and 4.4 microM (range 1.3-5.9 microM), respectively, consistent with in vitro resistance modulatory levels. However, free DP was only 0.02 microM (range 0.004-0.04 microM). The median CSA level at 24 h of 1450 microg/l (range 1075-1640 microg/l) is in agreement with concentrations required for partial DR reversal in vitro, although it is much lower than levels achieved in our previous phase I study of CBDCA + CSA alone using similar doses of CSA. The CSA dose on the current trial was escalated beyond the MTD for the previous phase I study, suggesting that there may be an interaction between CSA and one of the other modulators., Conclusion: These results demonstrate that in vitro DR-reversing levels of two of the three agents used in this study can be achieved in vivo, and that this combination of DR modulators has significant effects on the pharmacokinetics of VP-16.

Lometrexol inhibits the first folate-dependent enzyme in de novo purine biosynthesis and is avidly polyglutamated and retained in tissues expressing folylpolyglutamate synthetase. Although clinical studies have been limited by cumulative toxicity, preclinical studies show that pretreatment with folic acid can protect normal tissue while maintaining tumor cytotoxicity. Therefore, a Phase I study of lometrexol every 21 days preceded by i.v. folic acid was initiated. Lometrexol was studied in six patients at 15 mg/m2, in six patients at 20 mg/m2, in three patients at 25 mg/m2, and in nine patients at 30 mg/m2. Patients received either 5 mg of folic acid 1 h before or 25 mg/m2 3 h before lometrexol. Blood samples were obtained around the first course and weekly thereafter for determination of plasma and erythrocyte (RBC) lometrexol concentrations. Bioactive folates in plasma and RBCs were determined in a subset of patients. Lometrexol pharmacokinetics were best described by a three-compartment model. Mean clearance and volume of distribution were 1.6 +/- 0.6 liters/h/m2 and 8.9 +/- 4.1 liters/m2. Mean half-lives were 0.23 +/- 0.1, 2.9 +/- 1.4, and 25.0 +/- 48.7 h. Pharmacokinetics were independent of either lometrexol or folic acid dose. In the weekly blood samples, RBC lometrexol levels rose, long after plasma lometrexol was undetectable. RBC lometrexol levels were independent of folic acid or lometrexol dose. Bioactive folates measured in plasma and RBCs during this same time period did not accumulate. Rising RBC levels were correlated with a fall in hematocrit, hemoglobin, and platelet count. This study indicates that the cumulative toxicity of lometrexol is related to tissue concentration and not plasma pharmacokinetics. RBC lometrexol may be an indicator of cumulative drug exposure and effect.

Although originally developed as an antitumor agent in the 1970s, dexrazoxane (DEX) is currently used as a cardioprotective agent in combination with doxorubicin (DOX). Due to concerns about anthracycline-induced cardiotoxicity at higher cumulative doses, many investigators have chosen to administer DOX by prolonged infusion. Therefore, with the ultimate goal of combining infusional DEX and DOX, we performed a phase I study of intravenous DEX alone as a 96-hour infusion. Surprisingly, the maximum tolerated dose of DEX identified in this study was 10- to 15-fold lower than previously determined using different schedules of administration. Results of pharmacokinetic studies in support of the trial have found that steady-state DEX plasma concentrations in the range of 4 to 5 micromol/L can be achieved safely. Because previous experiments have explored the ability of DEX to inhibit the catalytic activity topoisomerase II at low micromolar concentrations and due to a lack of in vitro cytotoxicity data for long-term exposures, we performed further laboratory studies to provide a context for our pharmacokinetic findings. As a result of these correlative studies, we have found that prolonged exposures to DEX are cytotoxic to human leukemic cells at concentrations that are clinically achievable.

Study Objective: To validate the utility of a previously reported 3-point limited sampling model (LSM) for determining etoposide area under the curve to infinity (AUC(infinity))., Design: Secondary analysis of data from two clinical trials of etoposide., Setting: University medical center clinical research center., Patients: Thirty-four patients with different malignancies., Interventions: Etoposide was administered as a 2-hour infusion to 34 patients. Serial plasma samples were drawn over 24 hours after the infusion and analyzed for etoposide by high-performance liquid chromatography., Measurements and Main Results: The 3-point LSM AUC was compared with a 14-point actual AUC calculated by the linear trapezoidal rule. Actual and predicted AUC(infinity) by the LSM were highly correlated (r=0.97, p<0.0001). The LSM predictions had a mean absolute error of 10.9% (95% CI -14.1, -5.3) and a mean error of -9.7% (95% CI 6.9, 14.9). Nine patients with poor AUC(infinity) estimations by the LSM (error > 12%) tended to have abnormally low or high peak concentrations., Conclusion: Our findings suggest the development of more robust LSM using other techniques, such as pharmacostatistical models, that can accommodate a greater degree of pharmacokinetic variability.

Cellular accumulation of methotrexate polyglutamates (MTXPGs) is recognized as an important determinant of the cytotoxicity and selectivity of methotrexate in acute lymphoblastic leukemia (ALL). We identified a significantly lower cellular accumulation of MTXPGs in T-lineage versus B-lineage lymphoblasts in children with ALL, which is consistent with the worse prognosis of T-lineage ALL when treated with conventional antimetabolite-based therapy. Maximum MTXPG accumulation in leukemic blasts in vivo was 3-fold greater in lymphoblasts of children with B-lineage ALL (129 children) compared with those with T-lineage ALL (20 children) (p < 0.01) and was characterized by a saturable (Emax) model in both groups. The human leukemia cell lines NALM6 (B-lineage) and CCRF/CEM (T-lineage) were used to assess potential mechanisms for these lineage differences in MTX accumulation, revealing i) greater total and long-chain MTXPG accumulation in NALM6 over a wide range of methotrexate concentrations (0.2-100 microM), ii) saturation of MTXPG accumulation in both cell lines, with a higher maximum (Emax in NALM6, iii) 3-fold higher constitutive FPGS mRNA expression and enzyme activity in NALM6 cells, iv) 2-fold lower levels of DHFR mRNA and protein in NALM6 cells, and v) 4-6 fold lower extracellular MTX concentration and 2-fold lower intracellular MTXPG concentration to produce equivalent cytotoxicity (LC50) in NALM6 versus CEM. There was a significant relationship between FPGS mRNA and enzyme activity in lymphoblasts from children with newly diagnosed ALL, and blast FPGS mRNA and activity increased after methotrexate treatment. These data indicate higher FPGS and lower DHFR levels as potential mechanisms contributing to greater MTXPG accumulation and cytotoxicity in B-lineage lymphoblasts.

In this review, the clinical pharmacology of high-dose doxorubicin is examined, and the pharmacokinetics of infusional doxorubicin at standard and high dose are compared. The data developed here, including recent clinical trials of high-dose infusional doxorubicin, demonstrate the safety and efficacy of doxorubicin administration at doses > or = 150 mg/m2 for patients with breast cancer and strongly suggest that the utility of this drug can be substantially increased with appropriate hematopoietic support.

Even though folate antimetabolites were introduced over forty years ago, they continue to be the backbone of many active chemotherapeutic regimens used by medical and pediatric oncologists. The recognition of polyglutamylation by folylpolyglutamate synthetase (FPGS) as an important metabolic step in the "activation" of classical antifolates and novel drugs aimed at thymidylate synthase (TS) and de novo purine synthesis, has resulted in renewed interest in this class of drugs. In addition, the emergence of secondary neoplasms in patients treated with alkylating agents and topoisomerase inhibitors in contrast to the exceptional safety record of antimetabolites, underscores the need for clinical trials that incorporate new strategies with known active antimetabolites and novel promising agents. In that context, FPGS is an important target for further laboratory and clinical investigations.

Allopurinol is used to prevent hyperuricemia in newly diagnosed patients with acute lymphoblastic leukemia (ALL). Although allopurinol has been shown to inhibit de novo purine synthesis (DNPS) in fibroblasts in vitro, this effect has not been assessed in ALL lymphoblasts. We assessed DNPS in ALL lymphoblasts in 46 consecutive patients with ALL. DNPS was determined by 14C-formate incorporation in purine bases both at diagnosis (n = 46) and 44h after MTX therapy +/- allopurinol (n = 31). The 27 patients who had received no allopurinol prior to the diagnostic bone marrow aspirate had significantly higher rates of DNPS (median, 102 fmol new purines/nmol total purines/h) compared to the 12 patients who had received more than one dose of allopurinol (100 mg/m2 orally) (median, 2.3 fmol/nmol/h; P < 0.001); the seven patients who received one dose of allopurinol had intermediate rates of DNPS (median, 58.5 fmol/nmol/h). Among patients who were evaluable for MTX effects at 44h (n = 31), the percent inhibition of DNPS was greater in the eight patients who received concomitant allopurinol (median, 100% inhibition) compared to the 23 patients who received only methotrexate therapy (median, 89% inhibition, P = 0.03). These data indicate that allopurinol suppresses may contribute to the decrease in circulating blasts in patients with newly diagnosed acute leukemias.

Study Objectives: To determine whether glomerular filtration rate (GFR) changes during induction chemotherapy in children with leukemia, and to examine GFR as a determinant of pharmacokinetic variability of methotrexate clearance., Design: Prospective, unblinded observational study in consecutive patients., Setting: A research hospital., Patients: Thirty-eight children newly diagnosed with acute lymphoblastic leukemia., Interventions: The patients received either high-dose methotrexate 1 g/m2 intravenously over 24 hours or low-dose methotrexate 30 mg/m2 orally every 6 hours for six doses; both regimens were followed by an intensive six-drug chemotherapy regimen given over 6 weeks. Glomerular filtration rate was determined in each subject before and at the conclusion of induction therapy., Measurements and Main Results: The GFR was determined from 99mTc-DTPA serum clearance in all patients, and methotrexate clearance was estimated from serial serum concentrations in 18 of these children who received high-dose methotrexate. Median values for GFR at diagnosis (131 ml/min/1.73 m2) and after induction therapy (120 ml/min/1.73 m2) were not significantly different (p = 0.26) but were highly variable (range 49-274 ml/min/1.73 m2). Body size, age, and serum creatinine were correlated significantly with GFR at diagnosis. Amphotericin B therapy (6 patients) significantly decreased GFR (p = 0.046) without a corresponding increase in serum creatinine. Methotrexate clearance (58-155 ml/min/m2) was significantly (p = 0.007) correlated with GFR, but GFR accounted for only 37% of the variability of methotrexate clearance., Conclusions: The GFR was normal but highly variable in these children with leukemia and was significantly altered by amphotericin. Our results explain little of the intersubject variability in methotrexate clearance.

Folylpolyglutamate synthetase (FPGS) is responsible for the metabolism of natural folates and a broad range of folate antagonists to polyglutamate derivatives. Recent studies indicated increased accumulation of methotrexate (MTX) polyglutamates (MTX-PG) in blast cells as a predictor of favorable treatment outcome in childhood acute lymphoblastic leukemia (ALL). We determined the expression of FPGS activity in blasts from children with ALL at diagnosis and after treatment with MTX as a single agent, before conventional remission induction therapy. The levels of enzyme activity in ALL blasts at diagnosis (median of 689 pmol/h/mg protein) were significantly higher (P = .003) than those found in acute nonlymphoblastic leukemia (ANLL) blasts (median of 181 pmol/h/mg protein). Comparable lineage differences in normal lymphoid versus nonlymphoid cells suggest a lineage-specific control of FPGS expression, FPGS activity increased in ALL blasts after in vivo exposure to MTX. The median increase in FPGS activity was significantly higher (P = .003) in B-lineage ALL (188%) than in T-lineage ALL (37%). Likewise, the percentage of intracellular long chain MTX-PG (Glu3-6) was significantly higher (P = .02) in B-lineage ALL (92%) than in T-lineage ALL (65%), consistent with higher FPGS activity in B-lineage blasts. This finding could explain, at least in part, the superior outcome in children with B-lineage ALL treated with antimetabolite therapy.