Abstract
Owing to the high efficacy of L-asparaginase in the treatment of acute lymphatic leukaemia the enzyme was introduced into the chemotherapy schedules for remission induction of this disease shortly after results of large-scale clinical trials had become available. Since asparaginase monotherapy was associated with a high response rate but short remission duration, the enzyme is currently employed within the framework of combination chemotherapy schedules which achieve treatment response in about 90% and long-term remissions in the majority of patients. Recently initiated clinical trials have still confirmed the eminent value of asparaginase in the combination chemotherapy of acute lymphatic leukaemia and of some subtypes of non-Hodgkin lymphoma, and its important role as an essential component of multimodal treatment protocols. Despite the unique mechanism of action of this cytotoxic substance which shows relative selectivity with regard to the metabolism of malignant cells, some patients experience toxic effects during asparaginase therapy. Immunological reactions toward the foreign protein include enzyme inactivation without any clinical manifestations as well as anaphylactic shock. Severe functional disorders of organ systems result from the impaired homeostasis of the amino acids asparagine and glutamine. The changes affecting the proteins of the coagulation system have considerable clinical impact as they may induce bleeding as well as thromboembolic events and may be associated with life-threatening complications when the central nervous system is involved. Risk factors predisposing to thromboembolic complications are hereditary resistance against activated protein C and any other hereditary thrombophilia. Other organ systems potentially affected by relevant functional disorders are the central nervous system, the liver, and the pancreas, with patients who have a history of pancreatic disorders carrying an especially high risk of developing pancreatitis. Studies on the mechanisms of action and the occurrence of resistance phenomena have shown that a treatment response may only be expected if the malignant cells are unable to increase their asparagine synthetase activity to an extent providing enough asparagine to the cell; one may thus conclude that the enzyme-induced asparagine depletion of the serum constitutes the decisive cytotoxic mechanism. Independent of the asparagine depletion related cytotoxicity however, there are other mechanisms of clinical relevance like induction of apoptosis. Besides this, further influences on signal transduction cannot be excluded. Only few publications have dealt with the question of minimum trough activities to be ensured before each subsequent asparaginase dose in order to maintain uninterrupted asparagine depletion under treatment, and answers to this problem are not definitive. Clinical studies using enzymes from E. coli strains indicate that a trough activity of 100 U/l will suffice for complete asparagine depletion of the fluid body compartments with the preparations studied. These findings have been transferred to enzymes from other E. coli strains as well as those isolated from Erwinia chrysanthemi and to the PEG-conjugated E. coli asparaginases. It might be desirable to countercheck the results for confirmation or correction. The dosage and administration schedule of the various enzyme preparations required for complete asparagine depletion over a period of time have been insufficiently defined. While pharmacokinetic studies showed clinically relevant differences in biological activity and activity half-lives for enzymes from different biological sources, the findings of recently published clinical trials indicate that the therapeutic efficacy is affected when different asparaginase preparations are given by identical therapy schedules. (ABSTRACT TRUNCATED)
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