Plasma pharmacokinetics and cerebrospinal fluid penetration of thioguanine in children with acute lymphoblastic leukemia: A collaborative Pediatric Oncology Branch, NCI, and Children's Cancer Group study

Purpose: In preclinical studies, thioguanine (TG) has been shown to be more potent than the standard acute lymphoblastic leukemia (ALL) maintenance agent, mercaptopurine (MP), suggesting that TG may be more efficacious than MP in the treatment of childhood ALL. As part of a pilot trial in which TG was used in place of MP, we studied the plasma pharmacokinetics of oral TG and measured steady-state plasma and CSF TG concentrations during a continuous intravenous infusion (CIVI) in children with newly diagnosed standard-risk ALL. Methods: Nine plasma samples were collected after each patient's first 60 mg/m² oral TG dose during maintenance. CIVI TG (20 mg/m²/h over 24 h) was administered during the consolidation phase of therapy, and simultaneous plasma and CSF samples were collected near the end of the infusion. TG was measured by reverse-phase HPLC with ultraviolet detection. Erythrocyte TG nucleotide (TGN) concentrations were measured 7 days after a course of CIVI TG and prior to the start of each maintenance cycle. Results: After oral TG (n = 35), the mean (± SD) peak plasma concentration was 0.46 ± 0.68 μM and the AUC ranged from 0.18 to 9.5 μM · h (mean 1.5 μM · h). Mean steady-state plasma and CSF TG concentrations during CIVI (n = 33) were 2.7 and 0.5 μM, respectively. The mean (± SD) TG clearance was 935 ± 463 ml/min per m². Plasma TG concentrations did not correlate with erythrocyte TGN concentrations after oral or CIVI TG. The 8-OH-TG metabolite was detected in plasma and CSF. Conclusions: TG concentrations that are cytotoxic to human leukemia cell lines can be achieved in plasma after a 60 mg/m² oral dose of TG and in plasma and CSF during CIVI of TG.