The best way to use asparaginase in childhood acute lymphatic leukaemia – still to be defined?

For over 40 years, the enzyme asparaginase has held an established position in therapy concepts employed in the treatment of lymphoproliferative disease of childhood. Owing to the fact that asparaginase is the only therapeutically applied enzyme preparation and is not associated with any remarkable haematotoxicity, it is considered an appropriate partner in combination chemotherapy regimens of many highly active treatment protocols. Nevertheless, or quite possibly for this very reason, there continues to be considerable uncertainty as to the best application scheme. This uncertainty covers relevant aspects of clinical practice, such as dosage, dose intervals, optimal positioning within the treatment schedule, optimal duration of one treatment course, or interchangeability of and appropriate indications for different asparaginase preparations. The expectations implied in the title of this paper must thus be qualified, in that current knowledge supplies no general standard for the ‘best way’ of using asparaginase. It is, however, possible to weigh the options in a rational manner and approach the definition of a ‘proper way of use’ via a largely consistent construct, which adds a high degree of plausibility to the data available on asparaginase. Surrogate parameters of the pharmacokinetic (PK) and pharmacodynamic (PD) behaviour of asparaginase play an important role, as they represent the interface between preclinical and clinical findings, and reflect the treatment intensity achievable with asparaginase. The present review is thus intended to approach the ‘best way of use’, starting with the clinico-pharmacological aspects. First, the conceptual frame will be described, which enables us to develop definitions for a rational use of asparaginase. The second part will deal with the asparaginase preparations used in clinical practice – with a focus on the pharmacological surrogates – and will attempt to draw relevant conclusions from available findings. Asparaginase catalyses the hydrolytic decomposition of the amino acid asparagine to aspartic acid and ammonia. When applied for medication, the enzyme leads to a reduction in the asparagine level and, when appropriately scheduled, to persistent depletion of this amino acid which is non-essential under physiological conditions. According to the conceptions on the mechanism of action proposed around the middle of the 20th century, and still valid today, the anti-leukaemic effect of asparaginase is associated with a particular characteristic of the malignant lymphoblastic cells’ metabolic deficiency. Malignant blasts, in contrast to most of the healthy tissues, show a reduced expression of the enzyme asparagine synthetase and are thus unable to synthesize sufficient amounts of asparagine. As a consequence, the blasts depend on extracellular sources of asparagine to maintain protein biosynthesis (Haley et al, 1961; Prager & Bachynsky, 1968; reviews: Cooney & Handschumacher, 1970; Wriston & Yellin, 1973). An asparaginase-induced interruption of the asparagine supply leads to a depletion of the substrate needed by the malignant blasts. The resulting impairment of protein biosynthesis and subsequent cell cycle arrest finally lead to cell death through cellular dysfunction (Becker & Broome, 1967, 1969). Alternative approaches to explain the mechanism of action primarily discuss the induction of apoptosis by way of signal transduction (Story et al, 1993; Ueno et al, 1997). As those ideas have not yet been developed into a differentiated concept and still lack relevance for clinical application, they will not be considered in this review. So far, there is neither in vivo nor in vitro knowledge about the duration of asparagine depletion necessary for blasts to undergo irreversible changes. The time of exposure that is needed for killing cells strongly depends on the protocol-specific context of the drug. As the different study protocols have their own empirically based, complex histories of development, it is hardly possible to state anything about a cumulative single drug exposure. The preparations approved for clinical application are based on asparaginases derived from bacterial sources of either Escherichia coli or Erwinia chrysanthemi (Mashburn & Wriston, 1964; Wade et al, 1968). The enzyme is available either chemically unchanged (native asparaginases) or conjugated to polyethylene glycol (PEG asparaginase). Many older publications even fail to name the particular asparaginase preparation used. On the contrary, E. coli asparaginase preparations from different sources do in fact differ in their physicochemical and PK properties (e.g. review: Wriston & Yellin, 1973). As a consequence, it is fairly difficult to devise general recommendations based on those older reports. In clinical practice, the enzyme is currently given by the intravenous (i.v.; e.g. Schrappe et al, 2000) or intramuscular (i.m.) route (e.g. Gaynon et al, 2000), or subcutaneously (e.g. Eden et al, 2000). Although this might theoretically result in different PK profiles, information on the comparability of the routes of administration is limited with regard to the currently available preparations. All asparaginases, due to their high molecular weight, are distributed intravascularly and are generally unable to cross the blood–brain barrier (Ohnuma et al, 1970). Only after the application of very high doses of an E. coli asparaginase could low levels of enzyme activity be detected in the cerebrospinal fluid (CSF) (Schwartz et al, 1970; Riccardi et al, 1981). The ratio between the enzyme activity measured in the CSF and the activity in the blood was found to be <0·2% (Riccardi et al, 1981). Conclusive findings on the metabolism and elimination of asparaginase have not yet been produced. Various studies indicate that it might be eliminated via the reticulo-endothelial system (Ohnuma et al, 1970; Brueck et al, 1989). PK models describe first order elimination (Schwartz et al, 1970; Asselin et al, 1993; Keating et al, 1993; Albertsen et al, 2001a; Avramis et al, 2002). As regards PEG asparaginase, elimination following Michaelis–Menten kinetics is alternatively favoured (Müller et al, 2000). All studies, however, unvaryingly show high intra-individual variability of the kinetics, which is why prospective statements on the course of activity in individual patients are hardly possible. Apart from the classical PK variables, such as bodyweight, height and sometimes co-medication, the suppression of the reticulo-endothelial system and the presence of asparaginase antibodies will most probably exert some influence on the elimination process. Generally, the immune reaction and the formation of specific antibodies are essential factors in the discussion on the use of asparaginase (e.g. Woo et al, 1998). The probability of those reactions occurring is considered to be related to the number of doses within one treatment phase and, above all, to repeat application following an extended asparaginase-free interval (review: Müller & Boos, 1998). Clinically, the formation of antibodies may manifest itself by hypersensitivity reactions (Killander et al, 1976; Evans et al, 1982; Müller et al, 2001) or may lead to a faster decrease of enzymatic activity, an effect which is of relevance from a pharmacological point of view (Kurtzberg et al, 1993; Kurtzberg, 1994; Asselin, 1999). The latter phenomenon has been appropriately termed ‘silent inactivation’ for its lack of clinical symptoms. While the described reactions of the immune system may develop independently of each other, each one necessitates the discontinuation of the particular asparaginase used. Owing to comparatively low immunological cross reactivity between the available Erwinia chrysanthemi- and E. coli-based preparations, treatment may be continued after switching to an asparaginase preparation from a different bacterial source (Ohnuma et al, 1972; Billett et al, 1992), or to a pegylated preparation when the immune reaction occurred with a native E. coli preparation (review: Holle, 1997). The cross reactivity of the different native E. coli sources has never been investigated. The first results of current in vitro studies suggest cross reactivity between antibodies against native and pegylated E. coli preparations and no cross reactivity with regard to Erwinia asparaginase (Wang et al, 2003). Previous studies focussing on the interplay between the PK and PD of asparaginase have probably supplied the most relevant stimuli for the eventual definition of a ‘best way of use’. Findings from a primarily animal experimental study provided the basis for postulating that the peripheral application of asparaginase will lead to asparaginase activities in the serum of >100 U/l and the subsequent complete depletion of asparagine from both plasma and CSF (Riccardi et al, 1981). At present, there are no publications in the literature giving clear information on the mechanisms that induce the depletion of asparagine from CSF under asparaginase treatment. The balancing of amino acid concentrations by diffusion into the systemic circulation due to the different gradients of CSF and plasma has been proposed as one explanation (Schwartz et al, 1970; Müller & Boos, 1998). Minute levels of asparaginase in the CSF below the detection level might also be responsible for the depletion of the physiologically low levels of CSF asparagine (around 3–8 μmol/l) (Gerrits et al, 1989; Woo et al, 1999), especially, as they are not exposed to the sketched elimination processes in this particular compartment. Faced with complex treatment protocols that regularly employ combination chemotherapy, findings on the surrogates of asparaginase activity and asparagine concentration have gained relevance in the assessment of asparaginase treatment. While those parameters have not yet been validated for efficacy, they are still considered to be useful tools in estimating the treatment intensity achieved. Using those surrogates is supported by considerations of practicability. The target ranges to be achieved by asparaginase treatment can be defined for each parameter. Treatment objectives may be considered to be met when the asparagine concentration in the blood or CSF is no longer detectable or when the asparagine activity in the blood exceeds 100 U/l. Moreover, a range of analytical methods is available that cover the relevant ranges for all parameters. The lower limit of quantification is c. 0·2 μmol/l for the asparagine concentration (e.g. Boos et al, 1996) and c. 2·5 U/l for asparaginase activity (Lanvers et al, 2002). With a reference range of 40–80 μmol/l for plasma asparagine, depletion by more than two log steps can thus be detected (Lepage et al, 1997). However, the minimum level not to be exceeded for a ‘therapeutic’ effect has not been determined yet. Considering that low physiological CSF levels of 3–8 μmol/l suffice for blast complete asparagine depletion on the basis of the above of quantification Asparaginase treatment the complete depletion of the amino acid asparagine. asparagine depletion can be achieved with serum asparaginase activities >100 U/l. methods are available for the surrogates of asparaginase activity and asparagine The time course of asparaginase activity may be by formation in a of and there may be ‘silent inactivation’ clinical symptoms. inactivation’ results in reduced treatment a the treatment In the of a hypersensitivity asparaginase treatment may be continued after switching preparations. of efficacy, the native from E. coli and Erwinia have been available the as to the ‘proper way of use’ of asparaginase, however, as the first clinical on of In single drug application of E. asparaginase was found to result in of to with around et al, et al, 1970; et al, 1970; et al, et al, For the most those studies in patients with and employed the E. enzyme in different treatment The first clinical studies, which on a approach the best of asparaginase application and dosage, within the of treatment protocols of the between and et al, et al, 1981). The with chemotherapy for their disease and of of application, with regard to dose and of application, the E. coli preparation no longer The from the results of this was that a doses of over could be as the most scheme. With this of c. a that could not be by The findings of the studies on all considerations the definition of the ‘best way of use’. In the the asparaginases from E. coli and Erwinia chrysanthemi applied to results from dose studies for the Erwinia chrysanthemi asparaginase, different asparaginases been found to different properties (Ohnuma et al, 1970; review: Wriston & Yellin, and the first data on in the PK of preparations been (Schwartz et al, 1970). The of the particular asparaginase preparation used for therapy in a given treatment was primarily by aspects such as and individual clinical stimuli in the discussion of a ‘proper way of use’ of the native preparations from studies using the parameters of asparaginase activity and asparagine concentration from the Treatment approaches that applied different asparaginases in with the first to of their PK the E. coli preparation was found to result in a of enzyme activity of in the the of the applied Erwinia chrysanthemi asparaginase was standard in both also found with to the duration of asparagine depletion with with et al, 1993; Asselin, 1999). on the time course of the asparagine concentration in the CSF have been for the E. coli preparation in patients et al, 1999). doses of to asparagine concentrations below the detection limit treatment and to after the The of patients with CSF levels below the detection limit under treatment was c. of findings on the ‘proper way of use’ of asparaginase from a drug established within the of the protocols of the on the course over time of asparaginase activity and asparagine concentration for drug available the time and by from E. from Erwinia and different and recommendations et al, from those the between the preparations enzyme activity and the effect on the plasma asparagine The established which was for all asparaginases, the application of It was found that both of the E. coli and when given for to complete depletion of plasma asparagine as measured to the however, a enzyme activity in the serum than U/l enzyme activities U/l for and U/l for on after the In those the was to as a result of hypersensitivity the activity was below the analytical limit of quantification of U/l. The asparagine concentration in the plasma a complete depletion was detected in more than of the and over of the as to only of the results with in studies on asparagine depletion within the of an the was used as therapy et al, Using a with the one employed by the this study complete depletion of plasma asparagine in of of patients in this study complete depletion the induction study associated with the which the application of both and no between the two of application et al, 2000). of the and c. of the enzyme activities in the serum above the of 100 U/l to the of the drug achieved complete depletion of asparagine from plasma and CSF in all The drug described above et al, 1996) studies on the of and in with the protocols. a treatment intensity that was with the one by the employed The studies on which high and intra-individual variability of serum asparaginase activities to the of the dose to this into that the complete depletion of asparagine from blood and CSF been after a dose of in and patients et al, 1997). With the intended treatment intensity was achieved with a dose of and dose et al, 1999). findings on the use of within the protocols the on this et al, of given or associated with serum enzyme activities above U/l to the dose in over of When was used for using two doses about of levels in the serum below the of 100 U/l. of plasma asparagine was in about of those The clinico-pharmacological findings above are fairly consistent in of the in analytical and clinical conditions. gained as they to the of a clinical study on asparaginase that was within the of the protocols of the for and Treatment of et al, et al, 2002). This study on the clinical aspects of and of different asparaginase preparations, the E. coli preparations The by and by with the Erwinia chrysanthemi of more than patients treatment in the with a time of The was to the one employed by between the two treatment with regard to both and As to the E. coli a of However, the of patients was also in this than in the Erwinia chrysanthemi The results of the studies clinico-pharmacological studies in a manner and indicate a between the effect and the treatment intensity achieved by different asparaginase preparations. Moreover, they that the application of the therapy to all asparaginases in an manner will lead to possibly The that the PK properties are to be considered when it all a dose intensity with the empirically treatment by on serum activity and CSF is the most basis for treatment with different asparaginase preparations The clinical of native asparaginases has been by single drug for native asparaginases are based on studies on the E. coli preparation asparaginases from different bacterial sources and different native asparaginase preparations from one single source specific PK As a consequence, native asparaginases are not in of a treatment intensity may be achieved by the application to the PK data on different for native the of PEG to therapeutically is a of the that is primarily used to the of the (e.g. & 2003). Moreover, the of the molecular protein essential of the PK In the of PEG asparaginase, to an extended of enzyme activity in the serum et al, Asselin et al, 1993; et al, which – with the native E. enzyme – was in patients et al, As a consequence, the pegylated enzyme can be given a lower and longer when for the native a point of patients will have two from First, it is that the therapy may be reduced to the lower number of exposure to lower amounts of bacterial protein may be to a reduced of hypersensitivity reactions et al, et al, 1981). the of PEG asparaginase in the its into established treatment was by a lack of data recommendations on the ‘proper way of use’. While first studies on the of PEG asparaginase in patients with in fact which comparability with the native et al, review: Holle, the and number of patients those findings from The of the PEG asparaginase by into the protocols for the treatment of or was by a drug enzyme activity in the The of a treatment intensity with that of native preparations in all treatment was by single application of doses between and within a drug (Müller et al, et al, 2002). The conclusions from the of c. of more than can be as In the treatment intensity was considered with that from the application of the native asparaginase. This however, be in that a relevant number of patients not even the For of PEG asparaginase for under the not show any serum enzyme activity above the of 100 after the application (Müller et al, 2000). high variability was with the application of PEG asparaginase for However, the no results that a between the of treatment with native preparations or any manifest hypersensitivity reaction and in the activity course of PEG asparaginase over time et al, 2002). application of PEG asparaginase within treatment of the was not associated with any of enzyme to a possible ‘silent inactivation’ the intra-individual activity over time fairly et al, the PEG asparaginase dose not lead to a longer duration of the when the serum activity was above 100 U/l. of the the of asparaginase activity below the target range within following application et al, 2002). a pharmacological point of the findings may be to indicate that the elimination kinetics are described by a et al, 1993; Avramis & 2002). In of clinical practice, the findings indicate that repeat application than might to the exposure Owing to the highly time point of ‘silent the course of pharmacological parameters over time may be most as it is the only way to this phenomenon on and treatment As with the asparaginases, studies the relevance of a ‘proper way of use’ are also available for PEG of application was in patients with et al, 2000). as the the on the a The application of PEG asparaginases has been by data from two clinical The PEG asparaginase for doses and E. coli asparaginase for doses the phase of a as part of et al, not differ and was to be for native and for PEG In in with the protocols of the the single application of PEG asparaginase was with doses of of the native E. coli preparation in c. patients treatment et al, 2002). an of in c. of data however, be with the point of this study was the of and the of on which the data are based, been with a view to this results on the interplay of PK and PD within the study the first indications that the pegylated asparaginase was used in a asparaginase activity in the serum in found to above 100 U/l for a of or there was no complete asparagine depletion in the serum and asparagine to of the level of asparaginase activity in the around μmol/l asparagine found in the serum and The on this which on than the of PEG asparaginase, no on those however, be as as they the the ‘proper way of use’ of PEG asparaginase and also on the concept which for the native far, the activities described been associated with lower asparagine concentrations and CSF depletion et al, et al, et al, et al, 2000). On the contrary, one be that the pegylated asparaginases used in and in the differ in the E. coli source & is thus in those studies which may have employed preparations. This is for particular which may be by in PK such as or related in the current discussion on PEG asparaginase is based on data from a study associated with the protocols et al, 2003). This study the course of asparaginase activity in the serum and asparagine concentration in the CSF after a single dose of of the PEG asparaginase preparation As the enzyme activity above 100 U/l over a minimum of and was associated with complete peripheral asparagine depletion While asparagine in the CSF was also found to be reduced (e.g. μmol/l on after of the the under the application of PEG asparaginase to two different it can be that asparagine in the CSF within a range associated with the continued of blasts in vitro & Handschumacher, 1970). The of asparaginase in the treatment of a possible of the system has not been yet. a pharmacological point of asparaginase might be as the most active for the time of exposure to the effect of this is for blasts. clinical studies on PEG asparaginase are for a of the and for appropriately the pegylated enzyme the asparaginase PEG asparaginase has a longer than the native asparaginase. The pegylated lower and longer intervals, can be for native PEG asparaginase, when used as therapy to native asparaginases as part of combination chemotherapy, dose is not associated with a time of asparaginase activity in the serum above 100 U/l. a pharmacological point of it is with dose of PEG asparaginase than the when treatment is different have been unable to the PD of complete asparagine depletion in the data on different for PEG The described aspects may a number of which might to conclusions on the ‘proper way of use’ of asparaginase. The application of asparaginase in treatment protocols has been by high variability due to the use of different drug with different and application on the PK and PD is in order to the treatment intensity achieved in a given of the surrogates of asparaginase activity and asparagine concentration or the of in the treatment protocols may relevant findings. Considering the most findings on CSF asparagine, studies of PEG asparaginase with to the interplay between PK and PD are needed to its comparability with native the immune which may any and all asparaginase preparations, may also be considered highly relevant for clinical results in this of on the and of et al, et al, Albertsen et al, Avramis et al, et al, that this may be useful in the ‘proper way of use’ of asparaginase. of the asparaginase treatment given to the patients and the of the resulting dose intensity in of serum activity and depletion of asparagine from CSF and plasma are important they are to be used possible in order to the of application in the individual those that employ asparaginase preparations, have to those methods and are to in order to their treatment and the clinico-pharmacological each dose