Carbon 11 labeled aliphatic amines in lung uptake and metabolism studies: potential for dynamic measurements in vivo

In order to assess the potential utility of amines labeled with short lived nuclides as agents for lung imaging and function studies in humans, a series of aliphatic amines (C₄-C₁₀ and C₁₃) labeled with carbon 11 (T(1/2) = 20.4 min), which decays by the emission of body penetrating radiation, has been used as a model for studying some basic parameters affecting amine uptake and metabolism by the lung and other tissues in mice. The lung uptake (percentage of dose per organ) of aliphatic amines at 1 min increased from 2.18 ± 0.13% for butylamine to 13.33 ± 0.84% for tridecylamine. Partition coefficients (between n-octanol and pH = 7 buffer) were measured for the C₄ through C₁₀ amines and for octanoic acid and octanenitrile. Within the amine series, the partition coefficient correlated with lung uptake. A comparison of a series of compounds all having a carbon chain length of eight but with different functional groups (-NH₂, -C (triple bond)N, -CO₂H, -OH) showed that the amino group as well as the relatively lipophilic alkyl group were required for lung specificity. The ¹¹C aliphatic amines were rapidly metabolized via monoamine oxidase (ultimately to ¹¹CO₂). Non amine metabolites in blood and lungs at 5 min postinjection were 95 and 50%, respectively. Pretreatment of mice with iproniazid and with pargyline decreased ¹¹CO₂ excretion, and iproniazid significantly increased the radioactivity retained by the brain, lungs and liver at 15 min. The rate of ¹¹CO₂ excretion depended on carbon chain length (C₄ < C₅ < C₆ > C₇ > C₈ > C₉> C₁₀ > C₁₃).