Abstract 1696: Metabolism of (S)-NNAL and (R)-NNAL in the isolated perfused rat lung system
Authors: Maertens Laura A. , Upadhyaya Pramod , Hecht Stephen S. , Zimmerman Cheryl L. 
Published: February 2011

(S)-NNAL and (R)-NNAL are metabolites of NNK, a tobacco-specific pulmonary carcinogen. The purpose of the current study was to compare the pulmonary metabolism of the NNAL enantiomers to determine if differences in metabolism are responsible for the greater carcinogenicity of (S)-NNAL compared to (R)-NNAL.

The pulmonary metabolism of (S)-NNAL and (R)-NNAL was examined in the isolated perfused rat lung system (IPRL) to eliminate the contribution of other metabolizing organs, such as the liver. The lungs of F344 rats were perfused with [5-3H] (S)-NNAL and [5-3H] (R)-NNAL at an initial concentration of 1.2 µM in 50 mL of Ringer's buffer. The perfusions were conducted for 360 min at 37 °C with a perfusate flow rate of 8 mL/min.

The major perfusate metabolite for both enantiomers was NNAL-N-oxide, which accounted for approximately 40% of the final perfusate radioactivity. Unmetabolized NNAL contributed to roughly 20-30% of the final perfusate radioactivity. Hydroxy acid (∼10-15%) and diol (∼9%) were both observed in the perfusate, but NNK and its subsequent metabolites were not detected. NNAL-N-oxide was also the major metabolite in the tissue (∼70-80%), while NNAL contributed to less than 3% of the radioactivity in the tissue. Both (S)-NNAL and (R)-NNAL had apparent volumes of distribution similar to the volume of the reservoir, indicating that substantial partitioning of NNAL into the tissue did not occur. The apparent clearance, half-life, volume of distribution, and extraction ratio for each enantiomer was estimated, but no significant difference was observed for any of the pharmacokinetic parameters.

There was no significant difference in perfusate metabolite concentrations or apparent pharmacokinetic parameters between (S)-NNAL and (R)-NNAL in the IPRL system. These results contradict the hypothesis that differences in the lung metabolism of (S)-NNAL and (R)-NNAL may account for the difference in the lung carcinogenicity of the two enantiomers. This also contradicts previous in vitro metabolism studies in rat lung microsomes and in vivo pharmacokinetic studies in the rat. It is possible that the liver is responsible for the metabolic differences of (S)-NNAL and (R)-NNAL in vivo, a process which is absent in the IPRL. Alternatively, it may be that diffusional barriers exist in the intact lung, inhibiting the ability of preformed (S)-NNAL and (R)-NNAL to reach the site of metabolism. The microsomal studies would lack these diffusional barriers. The long perfusion time required for metabolism of the NNAL enantiomers and the low concentration of NNAL in the tissue support the existence of diffusional barriers to NNAL metabolism.

This work was supported by the Public Health Service [Grants NCI CA-81301 to SSH]; and University of Minnesota Fellowships [3M Science and Technology Fellowship, Ted Rowell Graduate Fellowship, and Edward Rippie Fellowship to LAM].

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 1696.