Life Sciences, Vol. 43, pp. 793-800 Pergamon Press Printed in the U.S.A. INVOLVEMENT OF MONOAMINE OXIDASE AND DIAMINE OXIDASE IN THE METABOLISM OF THE CELL DIFFERENTIATING AGENT I-IEXAMETHYLENE BISACETAMIDE (HMBA) Barbara A. Conley, Patrick S. Callery, Merrill J. Egorin, Babu Subramanyam Linda A. Geelhaar, and Su-Shu Pan Division of Developmental Therapeutics, University of MD Cancer Center (BAC, MJE and SSP), Division of Medical Oncology, Department of Medicine, University of MD School of Medicine (BAC, MJE), and Department of Medicinal Chemistry, University of MD School of Pharmacy (PSC, BS, and LAG), Balto., MD 21201. Correspondence: Dr. B. A. Conley, UMCC, Div. Develop. Therap., Rm 9-019, Bressler Research Laboratories, 655 West Baltimore Street, Baltimore, MD 21201 (Received in final form July 8, 1988) Summary We have previously demonstrated a number of metabolites of hexamethylene bisacetamide (HMBA) in the urine of patients treated with HMBA. These include N-acetyl-l,6-diaminohexane (NADAH), 6-acetamido- hexanoic acid (6AclIA), 1,6-diaminohexane(DAH) and 6-aminohexanoicacid (6AmHA). Because these compounds have potential roles in the dose- limiting metabolic acidosis and neurotoxicity associated with HMBAtherapy, and are similar in structure to known substrates of monoamine oxidase (MAO) and diamine oxidase (DAO), we investigated the activities of these enzymes in the metabolic interconversion of HMBAmetabolites. NADAH (5 raM) was incubated with MAO and aldehyde dehydrogenase. 6AcHAproduc- tion was verified by gas chromatography-mass spectrometry and quantified by gas chromatography. 6AeHA production was linear for up to 4 hr. Complete inhibition of MAO activity was observed with 2 mM tranyl- cypromine or pargyline. Mouse liver microsomes, which do not contain MAO, did not convert NADAHto 6AcHAand, in control experiments, did not degrade 6AcIIA. The HMBA metabolite, DAH, was a substrate for DAO, producing 3,4,5,6-tetrahydro-2H-azepine. Participation of DAO in the metabolism of HMBAimplies potential interaction of HMBAand metabolites with polyamine metabolism and may represent a mechanism for HMBA's effects on cellular growth and differentiation. Metabolismof NADAH, also a differentiator, by MAO impliesthat concurrent use of HMBA and an MAO inhibitor may be clinically useful. Hexamethylene bisacetamide (HMBA) is a polar, planar compound capable of inducing cellular differentiation in vitro (I-4). This in vitro activity is both time- and concentration-dependent (2-4), although the meehanism-()finduction is unknown. Studies using HMBA in tumor-bearing animals and phase I clinical trials in patients with solid tumors have thus far failed to show significant responses (5-7). It is not known whether this lack of response is in any way related to the inability to maintain sufficient plasma concentrations of HMBA for a sufficient duration of exposure. Dose limiting toxicities in phase I trials, using 5-day continuous infusions of HMBA, have consisted of anion-gap metabolic acidosis and neurotoxicity (obtundation, confusion, agitation, hallucinations) (6,7). These toxicities, the mechanisms of which are unknown, occur at plasma HMBA concentrations considered to be the minimal HMBA concentrations required to induce differentiation in vitro (2-5). Since at least one HMBA metabolite, NADAH, is also a differentiator (f9",20-'~'s, strategies to optimize differentiating activity of this agent, and/or to decrease toxicity would have clinical utility. 0024-3205/88 $3.00 + .00 Copyright (c) 1988 Pergamon Press plc