| Nelarabine is a pro-drug of the deoxyguanosine analogue ara-G. Nelarabine is rapidly demethylated by adenosine deaminase (ADA) to ara-G and then phosphorylated intracellularly by deoxyguanosine kinase and deoxycytidine kinase to its 5'-monophosphate metabolite. The monophosphate metabolite is subsequently converted to the active 5'-triphosphate from, ara-GTP. Accumulation of ara-GTP in leukaemic blasts allows for preferential incorporation of ara-GTP into deoxyribonucleic acid (DNA) leading to inhibition of DNA synthesis. This results in cell death. Other mechanisms may contribute to the cytotoxic effects of nelarabine. In vitro, T-cells are more sensitive than B-cells to the cytotoxic effects of nelarabine.In a cross-study analysis using data from four Phase I studies, the pharmacokinetics of nelarabine and ara-G were characterized in patients aged less than 18 years and adult patients with refractory leukaemia or lymphoma. Absorption Adults Plasma ara-G Cmax values generally occurred at the end of the nelarabine infusion and were generally higher than nelarabine Cmax values, suggesting rapid and extensive conversion of nelarabine to ara-G. After infusion of 1,500 mg/m2 nelarabine over two hours in adult patients, mean (%CV) plasma nelarabine Cmax and AUCinf values were 13.9 µM (81 %) and 13.5 µM.h (56 %) respectively. Mean plasma ara-G Cmax and AUCinf values were 115 µM (16 %) and 571 µM.h (30 %), respectively.Intracellular Cmax for ara-GTP appeared within 3 to 25 hours on day 1. Mean (%CV) intracellular ara-GTP Cmax and AUC values were 95.6 µM (139 %) and 2214 µM.h (263 %) at this dose.Paediatric patients After infusion of 400 or 650 mg/m2 nelarabine over one hour in 6 paediatric patients, mean (%CV) plasma nelarabine Cmax and AUCinf values, adjusted to a 650 mg/m2 dose, were 45.0 µM (40 %) and 38.0 µM.h (39 %), respectively. Mean plasma ara-G Cmax and AUCinf values were 60.1 µM (17 %) and 212 µM.h (18 %), respectively.Distribution Nelarabine and ara-G are extensively distributed throughout the body based on combined Phase I pharmacokinetic data at nelarabine doses of 104 to 2,900 mg/m2. Specifically, for nelarabine, mean (%CV) VSS values were 115 l/m2 (159 %) and 89.4 l/m2 (278 %) in adult and paediatric patients, respectively. For ara-G, mean VSS/F values were 44.8 l/m2 (32 %) and 32.1 l/m2 (25 %) in adult and paediatric patients, respectively.Nelarabine and ara-G are not substantially bound to human plasma proteins (less than 25 %) in vitro, and binding is independent of nelarabine or ara-G concentrations up to 600 µM. No accumulation of nelarabine or ara-G was observed in plasma after nelarabine administration on either a daily or a day 1, 3, 5 schedule.Intracellular ara-GTP concentrations in leukaemic blasts were quantifiable for a prolonged period after nelarabine administration. Intracellular ara-GTP accumulated with repeated administration of nelarabine. On the day 1, 3, and 5 schedule, Cmax and AUC(0-t) values on day 3 were approximately 50 % and 30 %, respectively, greater than Cmax and AUC(0-t) values on day 1.Metabolism The principal route of metabolism for nelarabine is O-demethylation by adenosine deaminase to form ara-G, which undergoes hydrolysis to form guanine. In addition, some nelarabine is hydrolysed to form methylguanine, which is O-demethylated to form guanine. Guanine is N-deaminated to form xanthine, which is further oxidized to yield uric acid. Elimination Nelarabine and ara-G are rapidly eliminated from plasma with a half-life of approximately 30 minutes and 3 hours, respectively. These findings were demonstrated in patients with refractory leukaemia or lymphoma given a dose of 1,500 mg/m2 nelarabine (adults) or a 650 mg/m2 (paediatrics).Combined Phase 1 pharmacokinetic data at nelarabine doses of 104 to 2,900 mg/m2 indicate that mean (%CV) clearance (Cl) values for nelarabine are 138 l/h/m2 (104 %) and 125 l/h/m2 (214 %) in adult and paediatric patients, respectively, on day 1 (n = 65 adults, n = 21 paediatric patients). The apparent clearance of ara-G (Cl/F) is comparable between the two groups [9.5 l/h/m2 (35 %) in adult patients and 10.8 l/h/m2 (36 %) in paediatric patients] on day 1.Nelarabine and ara-G are partially eliminated by the kidneys. In 28 adult patients, 24 hours after nelarabine infusion on day 1, mean urinary excretion of nelarabine and ara-G was 5.3 % and 23.2 % of the administered dose, respectively. Renal clearance averaged 9.0 l/h/m2 (151 %) for nelarabine and 2.6 l/h/m2 (83%) for ara-G in 21 adult patients.Because the timecourse of intracellular ara-GTP was prolonged, its elimination half-life could not be accurately estimated.Children Limited clinical pharmacology data are available for patients below the age of 4 years.Combined Phase 1 pharmacokinetic data at nelarabine doses of 104 to 2,900 mg/m2 indicate that the clearance (Cl) and Vss values for nelarabine and ara-G are comparable between the two groups. Further data with respect to nelarabine and ara-G pharmacokinetics in the paediatric population are provided in other subsections.Gender Gender has no effect on nelarabine or ara-G plasma pharmacokinetics. Intracellular ara-GTP Cmax and AUC(0t) values at the same dose level were 2 to 3 fold greater on average in adult female than in adult male patients.Race The effect of race on nelarabine and ara-G pharmacokinetics has not been specifically studied. In a pharmacokinetic/pharmacodynamic cross study analysis, race had no apparent effect on nelarabine, ara-G, or intracellular ara-GTP pharmacokinetics.Renal Impairment The pharmacokinetics of nelarabine and ara-G have not been specifically studied in renally impaired or haemodialysed patients. Nelarabine is excreted by the kidney to a small extent (5 to 10 % of the administered dose). Ara-G is excreted by the kidney to a greater extent (20 to 30 % of the administered nelarabine dose). Adults and children in clinical studies were categorized into the three groups according to renal impairment: normal with Clcr greater than 80 ml/min (n = 56), mild with Clcr equalling 50 to 80 ml/min (n = 12), and moderate with Clcr less than 50 ml/min (n = 2). The mean apparent clearance (Cl/F) of ara-G was about 7 % lower in patients with mild renal impairment than in patients with normal renal function (see section 4.2). No data are available to provide a dose advice for patients with Clcr less than 50 ml/min.Elderly Age has no effect on the pharmacokinetics of nelarabine or ara-G. Decreased renal function, which is more common in the elderly, may reduce ara-G clearance (see section 4.2). | |