Abstract 2601: What do we actually measure during therapeutic drug monitoring of thiopurine treatment in acute lymphoblastic leukemia and inflammatory bowel disease
Authors: Vikingsson Svante , Carlsson Björn , Almer Sven , Peterson Curt 
Published: February 2011

Abstract
The thiopurines have been used in the treatment of acute lymphoblastic leukemia (ALL), and inflammatory bowel disease (IBD) for the last 50 years. They are pro-drugs and undergo a complex metabolism. The main consequence is cell death induced by incorporation of thioguanine bases into DNA. However, more than 10% of patients experience adverse events leading to modification or discontinuation of therapy. To help individualize treatment, monitoring of the active metabolites in erythrocytes has been carried out in many centers for the last 20 years; however, the clinical benefit of such methods has been questioned due to poor predictability of treatment effect and side effects.
Both of the most commonly used methods (Lennard et. al (1992). and Dervieux et. al (1998)) depend on the degradation of thiopurine metabolites to bases before quantification. Normally, the base representing thioguanosine nucleotides (TGN) and the base representing methylthioinosine nucleotides (meTIN) (the latter quoted as methylmercaptopurine (MMP) or methylthioinosine monophosphate (meTIMP)), are reported. However, there are at least 9 different known metabolites of thiopurines that could contribute to TGN and as many that could contribute to meTIN. We believe that different clinical effects of these metabolites might explain, in part, the lack of predictability with these methods. The purpose of this study was to determine exactly which metabolites are being measured.
Patient samples were analyzed using Lennard's method and our novel HPLC method which measures mono- di-, and triphosphates of thioguanosine and methylthioinosine. Our technique is based on liquid extraction with dichloromethane and ion-pair chromatography on an Ascentis Express C18 column, using UV and fluorescent detection.
Pilot data from patient samples and publically available data indicate that TGN and meTIN are composed of the mono-, di-, and triphosphates of thioguanosine and methylthioinosine, respectively.
For TGN the mono-, di-, and triphosphate contribute around 1, 14, and 85 % of the total TGN, respectively, thus the triphosphate contributes the most to the total TGN levels, as reported by others. In the case of meTIN the mono-, di-, and triphosphates contribute more equally towards the total, 32, 14, and 55 %, respectively. However, these ratios have to be confirmed at an inter-individual level in more patients.
Based on this data we believe that the nomenclature: meTIMP and MMP are misleading and suggest that meTIN should be used instead. We believe that measuring the individual nucleotides separately during therapeutic drug monitoring offers an advantage over previous methods, especially for meTIN which are believed to play a role in predicting side effects of thiopurine treatment. Clinical studies on both IBD patients and ALL patients are ongoing.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2601.