Amino acid utilization and urine protein excretion in children treated with succinylated Acinetobacter glutaminase-asparaginase.

Amino acid utilization was evaluated in seven children with acute lymphocytic leukemia treated with succinylated Acinetobacter glutaminase-asparaginase. All patients received food p.o. ad libitum and glucose-electrolyte solutions i.v.; four patients received an i.v. amino acid supplement (1.5 g/kg/day). Although all patients were in negative energy balance, there was a significant linear regression between nitrogen balance and nitrogen intake during Days 1 to 7 and Days 8 to 14 of the study. The slope of the regression line, reflecting exogenous nitrogen utilization, was not significantly different from that found in healthy young men ingesting adequate or subadequate energy intakes. The Y-intercept (-210 mg/kg/day) indicated an obligatory nitrogen loss that was much greater than normal. Most of the nitrogen loss was due to urinary excretion. Ammonia and urea accounted for 77 to 91% of the urine nitrogen. Urinary glutamate accounted for 4 to 10% of this loss. Urine protein excretion was abnormally high in each of the patients, ranging from 987 to 3440 mg/day. Urine excretion of N-acetyl-beta-glucosaminidase and beta 2-microglobulin was also abnormally high, despite normal blood urea nitrogen and serum creatinine, suggesting that these children had renal tubular dysfunction. The antileukemic effect of succinylated Acinetobacter glutaminase-asparaginase did not appear to be altered by amino acid supplementation. These data indicate that amino acid supplementation can improve nutritional status in patients treated with succinylated Acinetobacter glutaminase-asparaginase.

Nitrogen utilization in mice bearing Ehrlich ascites tumor treated with Acinetobacter glutaminase-asparaginase.

The effects of Acinetobacter glutaminase-asparaginase (AGA) on protein and energy requirements were evaluated in mice bearing Ehrlich ascites tumors. In an initial experiment with normal mice, a zero protein diet resulted in a significant decrease in carcass nitrogen, liver nitrogen, and carcass energy relative to the animals on a normal, low, or high protein diet. In a second experiment, mice bearing Ehrlich ascites tumors were randomized into diet groups (zero or normal protein) and treatment groups (daily injections of AGA or 0.9% NaCl solution). In both treatment groups, the zero protein diet resulted in significant decreases in weight, liver nitrogen, carcass nitrogen, and carcass energy. Neither tumor nor AGA treatment affected body composition or the efficiency of nitrogen utilization. By Day 8, either the zero protein diet or AGA treatment significantly reduced ascites volume and tumor nitrogen content relative to controls. In a modification of Experiment 2, AGA treatment was stopped on Day 8, and all animals were given a normal protein diet. AGA, but not the zero protein diet, significantly enhanced ultimate survival. These experiments indicate that the requirements and utilization of energy and nitrogen are normal in mice with Ehrlich ascites tumor whether or not they are treated with AGA.

Effect of Acinetobacter glutaminase-asparaginase treatment on free amino acids in mouse tissues.

Acinetobacter glutaminase-asparaginase (AGA) and Escherichia coli asparaginase were compared for their effects on plasma and tissue levels of amino acids, ammonia, and glutamyl transferase activity in the mouse. Free asparagine was depleted similarly in plasma and tissues by both enzymes. AGA treatment produced partial depletion of glutamine concentrations in muscle, spleen, small intestine, and liver. Brain and kidney glutamine concentrations actually rose with treatment. Despite over 100-fold increase in plasma glutamate, only the kidney showed a substantial increase in free glutamate levels during AGA treatment. Glutamine biosynthesis measured by glutamyl transferase activity showed an appreciable increase only in the kidney. Ammonia levels in tissues and plasma rose 1.3- to 4.3-fold. In general, E. coli asparaginase treatment had much less effect on these measurements than did AGA. The changes in these levels are discussed in relation to sites of possible toxicity and antitumor effects.

Tissue nitrogen-sparing effect of high protein diet in mice with or without ascites tumor treated with Acinetobacter glutaminase-asparaginase.

Forty-eight tumor-free mice and 32 mice bearing Ehrlich ascites tumor were randomized into 2 treatments, Acinetobacter glutaminase-asparaginase (AGA) (600 IU/kg/day for 7 days) and 0.9% NaCl controls, and into 2 or 3 isocaloric diets, normal protein (NP) (20 g protein/100 g diet), high protein (HP) (58 g protein/100 g diet), and zero protein (ZP) (tumor-free mice only). In tumor-free, NP-fed mice, AGA caused percentage reductions (P less than 0.01) in the nitrogen content of liver (50%), intestine (42%), thymus (89%), spleen (75%), and carcass (20%), but HP prevented this effect on intestine and carcass and caused percentage increases in the nitrogen content of liver (53%), intestine (36%), thymus (122%), and carcass (25%). In Ehrlich ascites tumor mice (NP or HP fed) AGA caused markedly lower (P less than 0.01) tumor burdens and increased nitrogen content of intestine (HP), kidney (NP and HP), and spleen (NP and HP). Ehrlich ascites tumor, AGA-treated, HP-fed mice ate 31% less food (P less than 0.01) (compared to NP) but HP resulted in percentage increases in the nitrogen content of liver (18%; P = 0.05), intestine (25%; P less than 0.05), and thymus (164%; P less than 0.01). In the Ehrlich ascites tumor, AGA group the HP diet caused higher hematocrit and serum total protein (both, P less than 0.05). Adverse nutritional effects of AGA seen in normal mice were markedly diminished in tumor-bearing animals. The observed nitrogen-sparing effects of the high protein: energy ratio may be relevant to humans and to other forms of neoplasia and chemotherapy.

Effect of 5-azacytidine and congeners on DNA methylation and expression of deoxycytidine kinase in the human lymphoid cell lines CCRF/CEM/0 and CCRF/CEM/dCk-1.

The human lymphoid cell lines CCRF/CEM/0 and the deoxycytidine kinase (dCk)-deficient CCRF/CEM/dCk- were treated with various 5-azacytidine (5-aza-C) nucleosides and the effect on DNA methylation and dCk activity were examined. 5-Azacytidine (5-aza-C), 5,6-dihydro-5-azacytidine (DHAC), 5-aza-2'-deoxycytidine (5-aza-Cdr), and 1-beta-D-arabinofuranosyl-5-azacytidine (ara-AC) reduced the DNA 5-methylcytosine level in the CEM/0 cells, down to approximately 10% of the level in untreated cells. The dCk activity was increased after treatment with the 5-aza-C nucleosides approximately 10% compared to untreated cells. In CEM/dCk- cells DNA hypomethylation between 50 and 25% of control was seen only after treatment with DHAC and 5-aza-C. No decrease in the methylation level was seen after treatment with 5-aza-Cdr or ara-AC. The dCk activity was increased up to 37% after treatment with DHAC or 5-aza-C but no increase was observed after treatment with 5-aza-Cdr or ara-AC. CEM/dCk- cells treated with DHAC showed a revertant frequency to cells expressing dCk activity of between 0.1 and 0.6%. Cloned revertant CEM/dCk- cells isolated from soft agar had dCk activity between 31 and 113% compared to the activity in untreated CEM/0 cells. This in vitro study indicates that DHAC and 5-aza-C induced dCk re-expression in the CEM/dCk- cells whereas 5-aza-Cdr and ara-AC did not.

Prevention of methotrexate cytotoxicity by asparaginase inhibition of methotrexate polyglutamate formation.

Escherichia coli asparaginase (Asnase) pretreatment of Asnase-sensitive L5178Y cells in vitro is thought to antagonize methotrexate (MTX) cytotoxicity through nonspecific inhibition of protein synthesis and MTX uptake. We have reexamined the mechanism of this interaction in view of recent data demonstrating the importance of MTX metabolism to polyglutamate derivatives (MTXPGs) in the cytotoxic effects of the antifolate. After a 3-hr exposure to 0.5 microM MTX, 67% of intracellular drug was in the form of MTXPGs containing a total of 2 to 5 glutamyl residues (MTX-Glu2-5), and cloning efficiency in drug-free medium was only 7% of untreated control. After a 3-hr pretreatment with E. coli Asnase (0.1 unit/ml), [3H]thymidine incorporation dropped by 29%, MTXPG formation during subsequent MTX exposure decreased by more than one-half (MTX-Glu2 unchanged; MTX-Glu3 and 4 decreased to 51.7 and 18.5% of levels achieved in cells not pretreated with Asnase; no MTX-Glu5 formed), and cloning efficiency increased to 71% of untreated control. This effect was not due to decreased MTX uptake into L5178Y cells or to decreased intracellular free L-glutamate or L-glutamine levels. A 3-hr exposure of L5178Y cells to media lacking L-isoleucine, an essential amino acid for cell growth, prior to MTX exposure inhibited [3H]thymidine incorporation by 37%, decreased subsequent MTXPG formation by 62%, and increased subsequent cloning in drug-free medium to control levels. Decreased MTXPG formation was responsible for the prevention of MTX cytotoxicity seen after both pretreatments. Unmetabolized MTX rapidly left L5178Y cells after removal of extracellular MTX. Consequently, lower levels of unbound intracellular drug, a prerequisite of drug activity, were maintained in pretreated than in control cells after passage in drug-free medium. Asnase pretreatment protects L5178Y cells from the cytotoxic effects of MTX, possibly through inhibition of cell growth which nonspecifically decreases MTXPG formation.

L-asparaginase pharmacokinetics and asparagine levels in cerebrospinal fluid of rhesus monkeys and humans.

L-Asparaginase has been widely used for the treatment of acute lymphoblastic leukemia. Therapeutic and toxic effects in the central nervous system have been noted with systemic treatment. In order to better define the relationship between L-asparaginase administration and cerebrospinal fluid (CSF) asparagine levels, L-asparaginase and asparagine were measured in the CSF of rhesus monkeys following intrathecal and i.v. administration. Following intrathecal injection, the enzyme activity of Escherichia coli L-asparaginase in the CSF demonstrated a more rapid terminal half-life than did that of 111In-labeled diethylenetriaminepentaacetic acid, a marker of CSF bulk flow [4 +/- 0.7 (S.D.) hr versus 5.8 +/- 0.2 hr]. Intrathecal injection of E. coli asparaginase resulted in complete depletion of CSF asparagine for at least 5 days. A similar period of CSF asparagine depletion was observed following i.v. administration of L-asparaginase. Similar results were found in seven patients undergoing systemic L-asparaginase therapy. The minimal plasma level of L-asparaginase necessary to deplete CSF asparagine in both species was 0.1 IU/ml. Two other enzymes, Erwinia L-asparaginase and succinylated Acinetobacter glutaminase-asparaginase, were cleared from the CSF at the same rate as bulk flow. These data indicate that systemic L-asparaginase therapy may be a feasible means of treating central nervous system involvement in patients with acute lymphoblastic leukemia and that there is no therapeutic advantage to using intrathecal L-asparaginase.

Intrathecal 6-mercaptopurine: preclinical pharmacology, phase I/II trial, and pharmacokinetic study.

For over 30 years, oral 6-mercaptopurine (6-MP) has been a mainstay of systemic maintenance therapy for acute lymphoblastic leukemia. Despite its efficacy as an antileukemic agent, 6-MP has not been previously administered by the intrathecal (IT) route. In anticipation of a clinical trial of IT 6-MP, preclinical cytotoxicity and pharmacology studies were performed to define a safe, effective dose. The optimal concentration (greater than 1 microM) and duration of exposure (greater than 12 h) to 6-MP required for cytotoxicity were determined in vitro using human leukemia cell lines. The dose required to achieve the desired cerebrospinal fluid concentrations in humans was derived from pharmacokinetic parameters determined in rhesus monkeys. A phase I/II study was then performed in pediatric patients with refractory meningeal leukemia. Nine patients (aged 3.5 to 16 years) with chronic meningeal leukemia (2 to 6 central nervous system relapses) were entered onto the study. All had previously failed, at a minimum, IT methotrexate, IT cytarabine, and cranial (+/- spinal) radiation. A 10-mg IT dose of 6-MP (calculated to produce cytotoxic cerebrospinal fluid levels for 12 h) was administered twice weekly for 4 weeks. There were four complete responses and three partial responses. The duration of complete responses ranged from 7 to 22 weeks. Observed toxicities were not dose limiting and included mild headache (three patients) and minimal nausea (two patients). Pharmacokinetic studies performed in patients confirmed that cerebrospinal fluid concentrations of 6-MP were greater than 1 microM for 12 h. These results indicate that the IT administration of 6-MP is feasible, is not associated with significant toxicity, and has definite activity in patients with refractory meningeal leukemia.

Pharmacology and phase I trial of high-dose oral leucovorin plus 5-fluorouracil in children with refractory cancer: a report from the Children's Cancer Study Group.

Because of the synergy seen in adult trials when 5-fluorouracil is combined with leucovorin, we initiated a Phase I trial of this combination in children's refractory cancer. Leucovorin, an equal mixture of the (6R,S)-diastereoisomers, was administered p.o. for 6 consecutive days as 4 equal doses at 0, 1, 2, and 3 h totaling 500 mg/m2/day. 5-Fluorouracil was given daily on days 2 to 6 as an i.v. bolus immediately following the last dose of leucovorin. The leucovorin dose was held constant while the 5-fluorouracil dose was escalated in cohorts of patients from 300 mg/m2/day to its maximally tolerated dose. Thirty-five patients (19 with acute leukemia and 16 with solid tumors) were evaluable for toxicity. The maximally tolerated dose of FUra was 450 mg/m2/day for 5 treatments for patients with solid tumors and 650 mg/m2/day for 5 treatments for the children with leukemia. The dose-limiting toxicities were myelosuppression and stomatitis. Other side effects included transient, mild elevations of serum transaminases, mild nausea, vomiting, and diarrhea. The pharmacokinetics of high-dose p.o. leucovorin was studied in 23 children. There was considerable interpatient variability in the plasma concentrations of total bioactive folates (TBAF), (6S)-leucovorin, and (6S)-5-methyltetrahydrofolic acid. The maximum plasma concentration (Cmax) of TBAF was 821 +/- 97 (SE) nM, occurring at a median of 8 h; the Cmax of (6S)-leucovorin was 77 +/- 11 nM, occurring at 4 h. The TBAF concentration fell to 146 +/- 42 nM by 24 h. (6S)-5-Methyltetrahydrofolic acid accounted for 90 +/- 7% of the TBAF at the Cmax. The plasma concentration of (6R)-leucovorin, the unnatural isomer, was equal to that of TBAF. Thus, p.o. leucovorin reduced the 5-fold excess of (6R)-leucovorin over TBAF seen after i.v. doses. The relative amounts of the three major plasma species were approximately the same as in adults, even though the Cmax of each compound was lower.

Biochemical pharmacology of high dose 1-beta-D-arabinofuranosylcytosine in childhood acute leukemia.

The pharmacodynamic parameters of 1-beta-D-arabinofuranosylcytosine (ara-C) in patient plasma and its active anabolite 1-beta-D-arabinofuranosylcytosine-5-triphosphate (ara-CTP) in circulating and bone marrow blast cells were studied in 20 pediatric patients with acute leukemia. ara-C (3 g/m2) was administered as a short-term infusion over 3 h every 12 h for a total of eight doses. The peak plasma concentration of ara-C ranged from 0.02 to 5.6 microM after the first dose of ara-C. The area under the concentration-time curve (AUC) of ara-C in plasma ranged from 302 to 20,298 microMh after the first dose of ara-C. The half-life of elimination (t1/2,el) of ara-C from plasma was 2.4 +/- 1.5 h in three patients with acute nonlymphoblastic leukemia (ANLL) and 4.78 +/- 4.1 h in 9 patients with acute lymphoblastic leukemia (ALL). The intracellular peak concentration of ara-CTP in circulating blast cells averaged 432.2 +/- 14.5 microM and 544.3 +/- 330 microM in patients with ANLL and ALL, respectively. The elimination kinetics of ara-CTP was monoexponential with t1/2,el of 3.30 +/- 0.8 h and 6.9 +/- 2.8 h in patients with ANLL and ALL. DNA synthetic capacity (DSC) of the blast cells was inhibited to between 24 and 64% of control after the first dose of ara-C and it declined further to between 1 and 32% after four doses of ara-C. The AUC of ara-CTP in leukemic cells ranged from 1,073 to 14,751 microMh and it was not related to the AUC of ara-C in plasma. The pharmacodynamic parameters of ara-CTP in circulating blast cells were more homogeneous in patients with ANLL than in patients with ALL. Four of six patients (67%) with ANLL and six of 14 patients (43%) with ALL achieved either complete remission or partial remission with high dose ara-C. We conclude that treatment of pediatric patients with leukemia in relapse with high dose ara-C is tolerable and moderately successful. Inhibition of DSC is positively correlated with the probability of having zero nadir peripheral blast cells. In turn there is a trend for a zero nadir peripheral blast cell count to be related to achievement of a response to therapy. This latter result is consistent with the results of larger studies in adults with leukemia.

Pediatric phase I trial and pharmacokinetic study of trimetrexate.

Trimetrexate, a new nonclassical antifolate, was evaluated in a phase I trial in children with refractory cancer including nine with acute leukemia and 21 with solid tumors. The drug was administered as an i.v. bolus injection weekly for three doses, and courses were repeated every 28 days. The dose ranged from 35 to 145 mg/m2. Thirty patients who received a total of 33 courses were evaluable for toxicity, including 19 who were evaluable for hematological toxicity. The maximally tolerated dose for patients with a solid tumor and leukemia was 110 mg/m2. The dose-limiting toxicities were myelosuppression, mucositis and a pruritic, diffuse maculopapular rash. Other side effects observed included transient, mild elevations of serum transaminases, mild nausea and vomiting, and a local phlebitis at the site of injection at higher dose levels. A single patient with delayed drug clearance had evidence of renal toxicity with a transient increase in serum creatinine. The pharmacokinetics of trimetrexate were studied in 25 patients over the entire dose range. There was considerable interpatient variability in total drug clearance (range 9.2 to 215 ml/min/m2) and half-life (2.1 to 20 h). There was a suggestion of a correlation between plasma concentration at 24 h and the development of hematological toxicity at the highest dose level. Trimetrexate was cleared primarily by biotransformation with renal clearance accounting for only 10% of total clearance. Two metabolites of trimetrexate which inhibit the enzyme dihydrofolate reductase were identified in the urine. One of these appears to be a glucuronide conjugate.

Effects of single-dose and fractionated cranial irradiation on rat brain accumulation of methotrexate.

The effects of single-dose and fractionated whole-brain irradiation on brain methotrexate (MTX) has been studied in a rat model. The amount of MTX present in the brain 24 hr after a single i.p. dose (100 mg/kg) was the same wether animals were sham irradiated or given a single dose of 2000 rads 6 or 48 hr prior to the drug (6.9, 8.3, and 6.8 pmol MTX/g, wet weight, respectively). Animals sham irradiated or given 2000 rads in 10 fractions over 11 days and treated with an average dose of 1.2 mg MTX/kg i.p. twice a week for 24 weeks did not differ significantly in their brain MTX concentration (7.9 and 8.3 pmol MTX/g, wet weight, respectively). Chronically MTX-treated animals became folate deficient whether they were irradiated or not (450 and 670 pmol folate/g, wet weight, brain in MTX-treated and control animals). Thus, MTX accumulates in the brain with acute or chronic administration, and this accumulation is not altered by this amount of brain irradiation.

Levels of 2'-deoxycoformycin, adenosine, and deoxyadenosine in patients with acute lymphoblastic leukemia.

2'-Deoxycoformycin (dCF), a potent inhibitor of adenosine deaminase, has recently undergone Phase I clinical trials and has been found to be therapeutically active in acute lymphoblastic leukemia. In this report, levels of dCF in plasma, plasma concentrations of adenosine and deoxyadenosine, and urine levels of deoxyadenosine were measured in leukemic patients undergoing treatment with dCF during a Phase I clinical trial. dCF was administered i.v. at a dose of 0.25 to 1.0 mg/kg (7.5 to 30 mg/sq m) for 3 consecutive days. Plasma drug levels of 2 to 6 microM were observed following the third dose of dCF, and drug accumulation occurred only at the 1-mg/kg dosage. In this limited series of patients, the plasma concentrations of adenosine and deoxyadenosine and the urine concentrations of deoxyadenosine did not show an obvious correlation with dCF dose, therapeutic response, or toxicity.

Pharmacology of fludarabine phosphate after a phase I/II trial by a loading bolus and continuous infusion in pediatric patients.

Fludarabine phosphate is a nucleotide analogue of adenine arabinoside with antitumor activity in murine and human lymphoid malignancies; it has occasional, unpredictable neurotoxicity after high dose bolus injections in adults. To avoid this toxicity, we studied a loading dose plus 5-day continuous infusion in 47 evaluable pediatric patients. Dose limiting myelosuppression was seen in children with solid tumors after a loading dose of 8 mg/m2 followed by 23.5 mg/m2/day for 5 days. In children with leukemia, no dose limiting toxicity was seen at dose level 6, consisting of a loading dose of 10 mg/m2 and an infusion of 30.5 mg/m2/day for 5 days. One complete and 3 partial remissions were seen in 26 evaluable children with acute lymphoblastic leukemia. 9-beta-D-arabinofuranosyl-2-fluoroadenine plasma concentrations and the area under the moment curve increased linearly with dose. The terminal half-life was similar, while the total body clearance was shorter than that reported for adults receiving bolus or continuous doses. Lymphoblasts isolated from 2 patients during fludarabine phosphate (9-beta-D-arabinofuranosyl-2-fluoroadenine) treatment increased their ability to convert 1-beta-D-arabinofuranosylcytosine to 1-beta-D-arabinofuranosylcytosine 5'-triphosphate by more than 10-fold. The antileukemic activity of 9-beta-D-arabinofuranosyl-2-fluoroadenine 5'-phosphate and its ability to alter the metabolism of 1-beta-D-arabinofuranosylcytosine indicate that timed combinations of these 2 agents should be tested.

Phase I and clinical pharmacological study of mercaptopurine administered as a prolonged intravenous infusion.

The bioavailability of oral mercaptopurine (MP) is poor, and plasma levels following p.o. dosing are highly variable. In an attempt to circumvent these problems, we conducted a Phase I trial and clinical pharmacological study of MP administered as a prolonged i.v. infusion. An infusion rate of 50 mg/sq m/h, which was designed to achieve therapeutic drug levels in plasma, was used in all patients. The infusion duration was escalated in 12-h increments. Thirty-eight patients were evaluated. The dose-limiting toxicity was mucositis. Other reversible toxicities were myelosuppression and hepatotoxicity. An infusion duration of 48 h was found to be safe, unassociated with dose-limiting toxicity. Objective responses were seen in five patients. The mean plasma steady-state MP concentration achieved was 6.9 microM with little interpatient variability seen. Allopurinol coadministration had no effect on the plasma pharmacokinetics of i.v. MP. However, allopurinol did alter the urinary metabolite pattern, decreasing thiouric acid and increasing MP and thioxanthine levels. The steady-state cerebrospinal fluid:plasma ratio for MP was 0.27, suggesting that this approach may be of value in the treatment of central nervous system cancer. MP can be safely administered as a 48-h i.v. infusion at a dose rate which reliably achieves MP levels associated with optimal antileukemic activity in vitro.

Pediatric phase I trial and pharmacokinetic study of tiazofurin (NSC 286193).

Tiazofurin (2-beta-D-ribofuranosylthiazole-4-carboxamide), a new nucleoside antimetabolite, was evaluated in a phase I trial involving children with refractory cancers. The drug was administered i.v. as a 10-min infusion daily for 5 consecutive days repeated at 3-week intervals. The dose ranged from 550 to 3300 mg/sq m/day. Seventeen patients received 23 courses and were evaluable for toxicity. The maximally tolerated dose was 2200 mg/sq m/day. The major dose-limiting toxicities were nonhematological. Neurotoxicity, including headache, drowsiness, and irritability, was common and was the principal dose-limiting toxicity at the higher doses. Severe myalgias were also dose limiting in one patient. Other side effects were mild, reversible elevations in serum transaminases; nausea, vomiting, and diarrhea; mild hypertension; dysphagia; and exfoliative dermatitis of the hands and feet. Myelotoxicity was not significant. The pharmacokinetics of tiazofurin was studied in 16 patients. Plasma disappearance was triphasic with half-lives of 9.7 min, 1.6 h, and 5.5 h. Clearance was dose related, ranging from 120 ml/min/sq m at 550 mg/sq m/day to 70 ml/min/sq m at 3300 mg/sq m/day. The primary route of elimination was renal with 85% of the drug recoverable in the urine as the parent compound in the 24 h following administration.

Kinetics of uptake and activity in mouse liver of glutaminase coupled to desialated orosomucoid.

Desialised orosomucoid (alpha-1-acidic glycoprotein) was coupled to Pseudomonas 7A glutaminase-asparaginase by glutaraldehyde, iodinated and injected into mice. The half-life of radioactivity and glutaminase activity in plasma was about 7 min. Radioactivity and glutaminase activity in the liver reached a peak at about 20 min. The radioactivity in liver then declined with a half-life of about 20 min. Enzyme activity in liver declined with a half-life of about 10 min. The ratio of enzyme activity to radioactivity was lower in the liver than in plasma at all times during the experiment, indicating rapid hepatic inactivation of the enzyme. Uptake into the liver could be blocked by excess desialised orosomucoid. Glutamine levels in the liver were about 10% of normal for 44 min but returned to 50% of normal by 93 min. Intestines, kidney and spleen failed to exhibit any appreciable uptake of desialated orosomucoid glutaminase-asparaginase.

High-dose trimetrexate and minimal-dose leucovorin: a case for selective protection?

Our objective was to find the minimum dose of leucovorin (LV; 5-formyltetrahydrofolate) needed to potentially provide selective protection of normal tissue in patients with tumors resistant to methotrexate (MTX) by virtue of transport during prolonged therapy with high-dose trimetrexate (TMTX). Based upon the known daily requirement for folate, that tumors are often resistant to methotrexate via a transport-based mechanism, and that large doses of trimetrexate can be given with large doses of leucovorin for the treatment of patients with Pneumocystis carinii, a protocol was designed to find the minimum LV dose required to allow the administration of large doses of TMTX. Patients were treated in 28-day cycles consisting of 14 consecutive days of oral TMTX (45 mg/m2 every 12 h), followed by 14 days of rest. The dose of concurrent LV was started at 5 mg/m2 twice daily. Cohorts of patients received successive half doses of LV so long as three consecutive patients had less than or equal to grade 3 toxicity. Ten patients received 29 courses of therapy. The most common toxicities encountered were thrombocytopenia (38%), mucositis (14%), and neutropenia (10%). At a LV dose of 2.5 mg/m2, toxicities were consistently limited to less than or equal to grade 3 and only one episode of grade 4 hematological toxicity. Although there was marked interpatient variability, the minimally effective LV dose for selective protection seems to be 2.5 mg/m2. If tumors are resistant to methotrexate because of decreased transport of drug (and also folate), then the same pharmacological principle used to develop TMTX/LV for the treatment of P. carinii may be applied to treatment of some patients with cancer.

Phase I study of topotecan administered as a 21-day continous infusion in children with recurrent solid tumors: a report from the Children's Cancer Group.

The purpose of this study was to determine the toxicity, maximum tolerated dose, and pharmacokinetics of a 21-day continuous infusion of topotecan in children with relapsed solid tumors. Fifteen patients received 40 courses of continuous ambulatory infusions of topotecan every 28 days or when there was resolution of hematological toxicity and any grade 2 or greater nonhematological toxicity. The starting dose was 0.4 mg/m2/day. Total topotecan levels were measured on days 1, 7, 14, and 21. Three of four patients who received a starting dose of 0.4 mg/m2/day experienced dose-limiting myelosuppression. At the reduced dose of 0.3 mg/ m2/day, only two of the seven patients experienced dose-limiting myelosuppression. Subsequently, four patients with more limited prior therapy were treated with 0.4 mg/m2/ day; three had dose-limiting myelosuppression. Two patients with a dose-limiting toxicity at 0.4 mg/m2/day tolerated additional courses at 0.3 mg/m2/day. An equal number of patients had grade 4 neutropenia or thrombocytopenia. Other adverse events were rare. Two patients with ependymoma, one with rhabdomyosarcoma, and one with retinoblastoma metastatic to the brain had objective responses. The steady state plasma concentration and clearance of topotecan (Css) was achieved by day 1. Css in six patients with complete data were 1.44 +/- 0.50 and 2.13 +/- 0.83 ng/ml at 0.3 and 0.4 mg/m2/day, respectively. Thus, a 21-day topotecan infusion was well-tolerated at 0.3 mg/m2/day. Myelo-suppression was the dose-limiting toxicity at 0.4 mg/m2/day. The steady state and clearance of topotecan in this study are similar to those reported in adult patients.

The effect of methotrexate on the bioavailability of oral 6-mercaptopurine.

Fourteen children (aged 3 to 14 years) with average-risk acute lymphoblastic leukemia were studied after an oral dose of 6-mercaptopurine (6-MP) (75 mg/m2) administered alone and, on the next day, concurrently with oral methotrexate (20 mg/m2). When 6-MP was administered alone, both the peak plasma concentration (15 to 150 ng X ml-1) and the AUC (36 to 340 ng X ml-1 X hr) were highly variable. Concurrent methotrexate resulted in a 31% increase in the AUC (P less than 0.01) and a 26% increase in peak plasma levels (P less than 0.05) of 6-MP. The AUC of methotrexate correlated with the degree of increase in 6-MP plasma concentrations. These findings are consistent with previous in vitro studies demonstrating that methotrexate is an inhibitor of xanthine oxidase, the enzyme that catabolizes 6-MP to the inactive metabolite thiouric acid. Although the increases in 6-MP AUC and peak plasma concentrations resulting from concurrent methotrexate administration were statistically significant, this interaction is probably not clinically significant at standard low oral doses of methotrexate in light of the wide interpatient variability in these pharmacokinetic parameters of 6-MP.