Plasma pharmacokinetics and urinary excretion of hexamethylene bisacetamide metabolites

In order to further understand the clinical toxicities of hexamethylene bisacetamide (HMBA) and to allow appropriate in vitro studies, we developed a suitable gas chromatographic assay and quantified plasma concentrations and urinary excretion of four metabolites which we had previously identified in urine of patients receiving 5-day HMBA infusions at 4.8-43.2 g/m2/day. 6-Acetamidohexanoic acid (AcHA) was the major plasma metabolite and reached steady state concentration (Css) by 24 h. AcHA Css increased from 0.12 +/- 0.02 (SD) mM at 4.8 g/m2/day to 0.72 mM at 43.2 g/m2/day. The Css AcHA:Css HMBA ratio decreased with increasing HMBA dosage. At dosages below 24 g/m2/day plasma Css of N-acetyl-1,6-diaminohexane (NADAH), the initial metabolite of HMBA, were below the limit of detection of our assay. With HMBA infusions of 24, 33.6, and 43.2 g/m2/day, Css of NADAH were 0.16 +/- 0.05, 0.14 +/- 0.06, and 0.19 +/- 0.04 mM, respectively. Css NADAH:Css HMBA ratios at 24, 33.6, and 43.2 g/m2/day were 0.18 +/- 0.06, 0.08 +/- 0.02, and 0.31 +/- 0.05, respectively. Plasma Css of 1,6-diaminohexane and 6-aminohexanoic acid were below the limit of detection of our assay. Each patient's urinary excretion of NADAH, AcHA, and 1,6-diaminohexane was consistent from day to day. The fraction of dose excreted in urine as AcHA was not affected by HMBA dosage and accounted for 12.7 +/- 3.9% of the daily dose. The percentage of daily HMBA dose accounted for by excretion of NADAH decreased with increasing HMBA dosage (10.8 +/- 6.0% at 4.8 g/m2/day to 4.2 +/- 1.2% at 33.6 g/m2/day). Urinary excretion of 1,6-diaminohexane always accounted for less than 3% of the daily dose. Our results indicate that: (a) plasma concentrations of AcHA alone cannot explain the degree of acidosis observed with toxic doses of HMBA; (b) NADAH is present in plasma at concentrations that we have found to cause differentiation in vitro; and (c) the probable rate-limiting step in HMBA metabolism is the initial deacetylation.