Abstract 2532: Triethylenetetramine metabolism and its implication in cancer chemotherapy

Abstract

Triethylenetetramine (TETA), a copper II chelator and a polyamine analog traditionally used for the treatment of Wilson's disease, is currently in Phase I clinical trials for cancer in combination with carboplatin. However, the metabolism of TETA has never been thoroughly investigated, which is crucial for its further development in cancer treatment1. The two major metabolites of TETA found in humans are N1-acetyl-TETA (MAT) and N1,N10-diacetyl-TETA (DAT)2. Traditionally, acetylation of drug is thought to be catalyzed by N-acetyltransferase (NAT2). FDA requires that pharmacological properties of any drug metabolized via acetylation route have to be investigated in population with different NAT2 phenotype. However, our recent clinical study showed that there was no significant difference in pharmacokinetic and metabolites profiles of healthy volunteers with fast or slow NAT2 acetylation phenotype3. We therefore hypothesize that the enzyme responsible to TETA acetylation is spermidine/spermine acetyltransferase (SSAT). We carried out in vitro drug metabolism assays to pin-point the enzyme responsible for TETA metabolism. TETA (as substrate) and acetyl-coenzyme A were incubated with human liver microsome and cytosol from 7 individuals in the presence or absence of pentamidine or acetaminophen separately. The formation of MAT from TETA was detected and measured using our LC-MS method published previously2. Both liver microsome and cytosol could catalyze the biotransformation of TETA. There is a huge inter-individual difference in the rate of metabolism in microsome and cytosol (Km range 181 – 12,457 μM, Vmax range 110 – 3,068 pg/min/mg protein). Pentamidine, a specific inhibitor for SSAT, could inhibit the reaction and increase Km by over 30%. While acetaminophen, a specific inhibitor for NAT2, did not show any inhibition effect. Same experiments had been performed in rat liver microsome and cytosol, and similar results were obtained apart from the Km and Vmax values. Two other analogs of polyamine, diethylnorspermine and diethylspermine, all of which are under investigation for the treatment of cancer, were assayed as well, and similar results were obtained. The identification of SSAT, which has never been before considered to be a xenobiotic metabolizing enzyme, as the enzyme to catalyze the actylation of polyamine analogs, have a significant importance in the cancer combination chemotherapy. Finding SSAT (instead of NAT2) as the metabolic enzyme of TETA suggests that TETA may be an ideal candidate for cancer combination chemotherapy, as cancer drugs are rarely metabolized via this route; thus competition for drug metabolizing enzyme is less likely to occur. However, it seems that individuals do have different SSAT phenotypes, and this needs to be taken into account when using TETA as a chemotherapeutic agent.
1. Lu, J. (2010) MCT 9: 2458-67.
2. Lu, J. et al. (2007) DMD 35: 221-7.
3. Lu, J. et al. (2010) JCP 50: 647-58.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2532. doi:10.1158/1538-7445.AM2011-2532