High-dose methotrexate therapy: Focus on glucarpidase

High-dose methotrexate chemotherapy is used to treat a variety of malignancies, including lymphoid leukemias, lymphomas and sarcomas. Methotrexate inhibits dihydrofolate reductase, depleting cells of reduced folates necessary for purine biosynthesis. Achieving high serum levels of high-dose methotrexate takes advantage of several pharmacologic characteristics, including entering the cell by passive diffusion; achieving higher central nervous system concentrations; better competition with reduced folates for dihydrofolate reductase binding; and increased production of high-dose methotrexate polyglutamates, which are more slowly eliminated from the cell.

Methotrexate (MTX) doses of more than 500 to 1,000 mg/m² are considered high dose. Adverse effects of high-dose MTX therapy include pancytopenia, mucositis, neurotoxicity, liver damage and renal failure. There are several key features of a successful strategy to manage high-dose MTX therapy: patient selection, hydration, alkalinization, leucovorin rescue and MTX serum concentration monitoring.

Methotrexate is primarily eliminated by the kidney; high-dose therapy should be reserved for patients with adequate baseline renal function. Concomitant nephrotoxins or medications that interfere with MTX excretion (salicylates, nonsteroidal anti-inflammatory agents, beta-lactams and sulfonamides) should be avoided. Supplemental IV or oral hydration, to maintain urine output, should be started before MTX administration and continued until MTX levels have cleared.

MTX has poor urine solubility at physiologic pH. Goal urine pH is at least 7. Alkalinization of the urine increases MTX solubility and promotes excretion. Alkalinization can be achieved with IV or oral sodium bicarbonate; the alkalinization process should be started before MTX administration and continued until the MTX levels have cleared.

Leucovorin for rescue

Deborah Blamble

Leucovorin, a reduced form of folic acid, competes with MTX for transport into the cell. Administering leucovorin after high-dose MTX administration rescues cells from MTX toxicity. Leucovorin is administered starting approximately 24 hours after administration of the MTX dose and continued until MTX levels have cleared. Importantly, leucovorin does not affect MTX excretion or decrease MTX levels.

Leucovorin bioavailability decreases with increasing doses. Doses of leucovorin more than 50 mg are best administered intravenously. Levoleucovorin (Fusilev, Spectrum Pharmaceuticals), an IV formulation of the active l-isomer, is also commercially available and given at one-half of the racemic leucovorin doses.

Serum MTX levels are typically drawn daily, starting within 24 hours of the MTX administration and continuing until the MTX is considered cleared from the body, usually considered at a level less than 0.05 mcM to 0.1 mcM. It is important that the turnaround time, from obtaining the serum sample until results are available, is short so that changes in patient care can occur in a timely fashion. Several nomograms exist that indicate the dose of leucovorin to be administered, given the MTX serum concentration and the time since the MTX dose. Higher doses of leucovorin will be needed to rescue cells as the MTX level increases.

Despite optimal management, some patients will have difficulty receiving high-dose MTX. Nephrotoxicity frequently occurs and leads to further delays in MTX excretion. Besides optimizing hydration and leucovorin, other strategies have been shown to rescue these patients, including dialysis, thymidine replacement and carboxypeptidase G2. Dialysis has been used with mixed success. A considerable amount of MTX is intracellular, so serum MTX levels frequently rebound after dialysis, necessitating continual or daily dialysis. Thymidine is no longer being clinically developed. The remainder of this article will focus on carboxypeptidase G2.

Studies of glucarpidase

Carboxypeptidase G2 (CPDG2, glucarpidase), a recombinant bacterial enzyme, hydrolyzes MTX into inactive metabolites, DAMPA (2,4-diamino-N10-methylpteroic acid) and glutamate. This bypasses the renal excretion of MTX, particularly relevant in patients with nephrotoxicity. Glucarpidase is a large compound with a high molecular weight; it does not achieve intracellular levels or cross the blood brain barrier. Therefore, glucarpidase should not interfere with the anti-tumor activity of MTX.

A compassionate and early use protocol was conducted in Europe. Patients had to meet one of three eligibility criteria: 1) serum MTX level of more than 10 mcM at 36 hours or more than 5 mcM at 42 hours or more than 3 mcM at 48 hours from the start of the MTX infusion, or 2) decreased urine output (less than 50% excretion of administered hydration), or 3) a serum creatinine of more than 1.5 times the upper limit of normal (ULN) and a documented increase during the MTX infusion period.

Glucarpidase was administered intravenously for 5 minutes at a dose of 50 U/kg. Eighty-two patients with a median age of 15 years were treated with glucarpidase. Data were presented on 65 patients. The median MTX level at enrollment was 18 mcM and the level before administering glucarpidase was 12 mcM. Fifteen minutes after administration, MTX levels were reduced by 87% (greater than 97% reduction by high-pressure liquid chromatography [HPLC]).

After glucarpidase administration, two patients experienced side effects of flushing, and one patient experienced shaking. Grade 3/4 toxicity due to MTX was observed in approximately 50% of patients, predominantly renal, hepatic and gastrointestinal toxicities. Hematologic toxicity was not reported.

A second study was subsequently conducted in Germany. Patients were eligible if their MTX level was more than 5 mcM at 42 hours or more from the start of MTX therapy. Patients with renal failure (serum creatinine greater than 1.5 times ULN and/or oliguria) were eligible if 1) the renal failure occurred less than 42 hours after the start of MTX, or 2) the MTX level was more than 1 mcM at 42 hours or more from the start of MTX therapy or more than 0.4 mcM at 48 hours or more from the start of MTX therapy.

Glucarpidase was administered intravenously for 5 minutes at a dose of 50 U/kg. Forty-three patients with a median age of 54 years were treated with glucarpidase. The median MTX level at enrollment was 10.5 mcM. In the 24 patients who had levels analyzed by HPLC, MTX concentrations were reduced by greater than 97% after glucarpidase administration. Two patients experienced toxicity thought to be due to glucarpidase: allergic skin reaction (one patient) and fever (one patient). Grade 3/4 MTX toxicities included hematologic (60% of patients), mucositis (35%), renal (19%) and liver (18%).

The US data were recently updated and published. Patients could receive glucarpidase if they met either of the two eligibility criteria: 1) MTX level was at least 10 mcM at least 42 hours after the start of the MTX or 2) serum creatinine was at least 1.5 times ULN (or creatinine clearance ¡Ü 60 mL/min/m²) and the MTX level was at least two standard deviations above the mean at 12 hours or more after MTX administration.

Glucarpidase, administered intravenously at a dose of 50 mg/kg for 5 minutes, was given by one of three regimens: a single dose, two doses 24 hours apart or every 4 hours for three doses. One hundred patients with a median age of 17 years received at least one dose of glucarpidase. The median MTX level at enrollment was 17 mcM. Glucarpidase reduced MTX concentrations by 98%. Giving more than a single dose of glucarpidase did not result in substantially lower MTX levels.

Patients who received glucarpidase more than 96 hours after the start of the MTX experienced more grade 4/5 toxicities compared with those patients who received glucarpidase earlier. Seven patients had grade 1 adverse events possibly attributable to glucarpidase. Severe MTX-related toxicities, including neutropenia, mucositis, transaminitis and nephrotoxicity, were observed.

Summary

A single dose of glucarpidase is able to significantly reduce serum MTX levels in patients with delayed MTX elimination. Significant grades 3 to 5 MTX toxicity still occurred in treated patients. Because these were not randomized or controlled studies, it is unclear if the toxicity rates would have been higher without the rapid MTX clearance provided by glucarpidase. The optimal interval between MTX and glucarpidase administration, occurring a median of 52 to 96 hours in the above studies, is not known and should be the focus of future research.

There are some practical considerations with glucarpidase use. Glucarpidase clears leucovorin; leucovorin doses should be held 4 hours before and 1 to 2 hours after the glucarpidase dose. Glucarpidase only clears extracellular MTX. MTX will continue to leave the intracellular fluids resulting in some rebound in serum MTX levels. Leucovorin will still be required to help rescue normal cells. One of the MTX metabolites, DAMPA, is also partially cleared renally. Therefore, hydration and alkalinization should continue, as well. Commercially available MTX immunoassays may detect DAMPA, potentially overestimating MTX levels. MTX serum measurements by HPLC are preferred.

Glucarpidase was granted orphan drug status in the US in 2003. It is being developed by BTG/Protherics Inc., and will have the brand name of Voraxaze. FDA approval may be expected as early as 2011, per the manufacturer¡¯s website.

Currently, glucarpidase for IV use can be obtained on an open-label treatment protocol. Institutions will be charged for the cost of the drug. Interested providers should contact the Voraxaze 24-hour access call center at 877-398-9829. There is also an emergency use protocol for intrathecal methotrexate overdose. Glucarpidase is provided at no charge on this protocol. Providers with eligible patients should contact Protherics (888-327-1027) and the FDA (301-796-2320).

High-dose MTX therapy is an important therapeutic modality in a number of malignancies. Key features of high-dose MTX therapy include patient selection, hydration, alkalinization, MTX serum concentration monitoring and leucovorin rescue. Despite optimal management, patients continue to experience delayed methotrexate clearance and nephrotoxicity.

Glucarpidase is a recombinant enzyme that rapidly cleaves MTX into inactive metabolites, thereby decreasing overall MTX exposure. Additional studies will help determine the role of glucarpidase in the management of high-dose MTX therapy.

Deborah Blamble, PharmD, BCOP, is an Oncology Clinical Pharmacy Specialist at The University of Texas MD Anderson Cancer Center.

For more information:

• Buchen S. Br J Cancer. 2005;92:480-487.
• Patterson DM. Expert Opin Biol Ther. 2010;10:105-111.
• Schwartz S. Oncologist. 2007;12:1299-1308.
• Widemann BC. J Clin Oncol. 2010;28:3979-3986.
• Widemann BC. Oncologist. 2006;11:694-703.