New insights on asparaginase

Expression, purification and crystallization of Helicobacter pylori L-asparaginase

The L-asparaginases from Escherichia coli and Erwinia chrysanthemi are effective drugs that have been used in the treatment of acute childhood lymphoblastic leukaemia for over 30 years. However, despite their therapeutic potential, they can cause serious side effects as a consequence of their intrinsic glutaminase activity, which leads to L-glutamine depletion in the blood. Consequently, new asparaginases with low glutaminase activity, fewer side effects and high activity towards L-asparagine are highly desirable as better alternatives in cancer therapy. L-Asparaginase from Helicobacter pylori was overexpressed in E. coli and purified for structural studies. The enzyme was crystallized at pH 7.0 in the presence of 16-19%(w/v) PEG 4000 and 0.1 M magnesium formate. Data were collected to 1.6 A resolution at 100 K from a single crystal at a synchrotron-radiation source. The crystals belong to space group I222, with unit-cell parameters a = 63.6, b = 94.9, c = 100.2 A and one molecule of L-asparaginase in the asymmetric unit. Elucidation of the crystal structure will provide insight into the active site of the enzyme and a better understanding of the structure-activity relationship in L-asparaginases.

Structural and functional insights into Erwinia carotovora L-asparaginase

Bacterial L-asparaginases are enzymes that catalyze the hydrolysis of l-asparagine to aspartic acid. For the past 30 years, these enzymes have been used as therapeutic agents in the treatment of acute childhood lymphoblastic leukemia. Their intrinsic low-rate glutaminase activity, however, causes serious side-effects, including neurotoxicity, hepatitis, coagulopathy, and other dysfunctions. Erwinia carotovora asparaginase shows decreased glutaminase activity, so it is believed to have fewer side-effects in leukemia therapy. To gain detailed insights into the properties of E. carotovora asparaginase, combined crystallographic, thermal stability and cytotoxic experiments were performed. The crystal structure of E. carotovoral-asparaginase in the presence of L-Asp was determined at 2.5 A resolution and refined to an R cryst of 19.2 (R free = 26.6%) with good stereochemistry. Cytotoxicity measurements revealed that E. carotovora asparaginase is 30 times less toxic than the Escherichia coli enzyme against human leukemia cell lines. Moreover, denaturing experiments showed that E. carotovora asparaginase has decreased thermodynamic stability as compared to the E. coli enzyme and is rapidly inactivated in the presence of urea. On the basis of these results, we propose that E. carotovora asparaginase has limited potential as an antileukemic drug, despite its promising low glutaminase activity. Our analysis may be applicable to the therapeutic evaluation of other asparaginases as well.

Clinical pharmacology of asparaginases in the United States: asparaginase population pharmacokinetic and pharmacodynamic (PK-PD) models (NONMEM) in adult and pediatric ALL patients

In the past 25 years, effective new drugs along with better treatment decisions based on disease factors have resulted in significantly improved clinical outcomes in acute lymphoblastic leukemia. Despite these successes in the last 2 decades, 15% to 25% of acute lymphoblastic leukemia patients relapse. Therefore, better dosing therapies are still needed. Insights in the pharmacokinetic and pharmacodynamic (PK-PD) contributions of licensed drugs may guide us into better protocol design and optimal use of existing combination drug regimens. Currently, 3 asparaginase formulations are available in the United States, Escherichia coli native asparaginase (ASNase), Pegaspargase, and Erwinase. On the basis of these clinical studies, PK and PD population modeling (NONMEM) have been used to delve into new insights as to the optimal dose, formulation, and time intervals of ASNases that may be used in future clinical trials. Pegaspargase 2500 IU/m2 Q2week dosing seems to be the "golden standard" as far as being safe and effective. Lower doses of this formulation Qweek may achieve better PK "steady-state" profiles in serum. Native E. coli or Erwinia ASNase at 6000 IU/m2 showed inferior PK parameters (peak, trough, and area under the curve) than Pegaspargase. Assuming linear handling of ASNase modeling, simulations of higher doses of these ASNase formulations on a daily or Q48 hours regimen are showing bioequivalency with Pegaspargase PK-PD parameters. Future clinical trial designs may prove these efforts useful.