Abstract LB-287: Spermidine/spermine actyltransferase is responsible for the metabolism of triethylenetetramine and other polyamine analogs in humans

Abstract

Abstract Triethylenetetramine (TETA), a copper II chelator traditionally used for the treatment of Wilson's disease, and a number of structural analogs are currently in clinical trials for cancer. However, the metabolism of TETA has never been thoroughly investigated, which is crucial for its further development in cancer treatment. The two major metabolites of TETA found in humans are N1-acetyl-TETA (MAT) and N1, N10-diacetyl-TETA (DAT). 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, pharmacodynamic and metabolites profiles of healthy volunteers with fast or slow NAT2 acetylation phenotype. We therefore hypothesize that the enzyme responsible to TETA acetylation is spermidine/spermine acetyltransferase (SSAT), which has never before been considered as a drug metabolizing enzyme, rather than the NAT2. We carried out in vitro drug metabolism assays to pin-point the enzyme responsible for TETA metabolism. TETA and MAT (as substrate) and acetyl-coenzyme A were incubated with human liver microsome and cytosol from 5 individuals in the presence or absence of pentamidine or acetaminophen separately. The formation of MAT from TETA and DAT from MAT was detected and measured using our LC-MS method published previously. Both liver microsome and cytosol could catalyze the biotransformation of TETA and MAT. The rate of metabolism in microsome and cytosol within each individual varies. The rates of metabolism in microsome and cytosol also varied between individuals (Km range 223 - 909 M, Vmax range 96 - 482 pg/min/mg protein). Pentamidine, a specific inhibitor for x 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. TETA and two other analogs of polyamine, diethylnorspermine and diethylspermine, all of which are under investigation for the treatment of cancer, were incubated with pure whole-recombinant human SSAT protein, and formation of acetylated metabolites were found. 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 clinic, especially in clinical trial situations. Many polyamine analogs have been developed as potential cancer drugs and many of them, including TETA, are being investigated in clinical trials. Finding the right enzyme (i.e. SSAT instead of traditional NAT2) to study their biotransformation is crucial for understanding their pharmacology, which is essential for the overall drug study. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. 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 LB-287.