The positron-emitting radionuclide carbon-11 (11C, t1/2 = 20.3 minutes) possesses the unique potential for radiolabeling of any biological, naturally occurring, or synthetic organic molecule for in vivo positron emission tomography (PET) imaging. Carbon-11 is most often incorporated into small molecules by methylation of alcohol, thiol, amine or carboxylic acid precursors using [11C]methyl iodide or [11C]methyl triflate (generated from [11C]CO2). Consequently, small molecules that lack an easily substituted 11C-methyl group are often considered to have non-obvious strategies for radiolabeling and require a more customized approach. [11C]Carbon dioxide, [11C]carbon monoxide, [11C]cyanide, and [11C]phosgene represent alternative carbon-11 reactants to enable 11C-carbonylation. Methodologies developed for preparation of 11C-carbonyl groups have had a tremendous impact on the development of novel PET radiopharmaceuticals and provided key tools for clinical research. 11C-Carbonyl radiopharmaceuticals based on labeled carboxylic acids, amides, carbamates, and ureas now account for a substantial number of important imaging agents that have seen translation to higher species and clinical research of previously inaccessible targets, which is a testament to the creativity, utility, and practicality of the underlying radiochemistry.

Indenoisoquinolines with lactam substituents such as ethylamino, propylamino, and butylamino have previously demonstrated potent biological activity, but optimal length has never been established. In the present study, a series of simplified indenoisoquinoline analogues possessing a linker spacing of 0-12 carbon atoms between the lactam nitrogen and the terminal amino group have been prepared, determining that 2-4 atom lengths are optimal for topoisomerase I inhibition and cytotoxicity. Using these lengths, analogues were prepared with the amino group and portions of the linker replaced by a pyridine ring. A three-carbon spacer within the pyridine series still demonstrated potent topoisomerase I inhibition.