Comparative biodistribution and metabolism of carbon-11-labeled N-[2-(dimethylamino)ethyl]acridine-4-carboxamide and DNA-intercalating analogues

The tricyclic carboxamide N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (DACA) is a DNA-intercalating agent capable of inhibiting both topoisomeras es I and LI and is currently in Phase IT clinical trial. Many related analo gues have been developed, but despite their potent in vitro cytotoxicities, they exhibit poor extravascular distribution. As part of an ongoing drug d evelopment program to obtain related "minimal inter-calators" with lower DN A association constants, we have compared the biodistribution and metabolit e profiles of the prototype compound, DACA, with three analogues to aid rat ional drug selection. All of these compounds share a common structural feat ure, N-dimethyl side chain, which was radiolabeled with the positron-emitti ng radioisotope, carbon-11. This strategy was selected because it allows pr omising candidates emerging from preclinical studies in animals to be evalu ated rapidly in humans using positron emission tomography (PET). The acridine DACA, the phenazine SN 23490, the pyridoquinoline SN 23719, an d the dibenzodioxin SN 23935 were found to be cytotoxic in ill vitro assays with an IC50 of 1.4-1.8 muM, 0.4-0.6 muM, 1.3-1.6 muM, and 24-36 muM, resp ectively, in HT29, U87MG, and A375M cell lines. Ex vivo biodistribution stu dies with carbon-11 radiolabeled compounds in mice bearing human tumor xeno grafts showed rapid clearance of C-11-radioactivity (parent drug and metabo lites) from blood and the major organs. Rapid hepatobiliary clearance and r enal excretion mere also observed. There was low [<5% of injected dose/gram (%ID/g)] and variable uptake of C-11-radioactivity in three tumor types fo r all of the compounds. Tumor (U87MG) to blood C-11-radioactivity for [C-11 ]DACA, [C-11](9-methoxy-phenazine-1-carboxamide (SN 23490), [C-11]2-(4-pyri dyl)quinoline-8-carboxamide (SN 23719), and [C-11]dibenzo[1,4]dioxin-1-carb oxamide (SN 23935) at 30 min were 2.9 <plus/minus> 1.1, 2.3 +/- 0.6, 2.6 +/ - 0.6, and 0.7 +/- 0.2, respectively. For SN 23719, the distribution of C-1 1-radioactivity in normal tissues and tumors determined ex vivo was in broa d agreement with that determined in vivo by whole body PET scanning. [C-11] DACA was rapidly and extensively metabolized to several plasma metabolites and a major tumor metabolite. In contrast, [C-11]SN 23935, [C-11]SN 23490, and [C-11]SN 23719 showed less extensive metabolism. In the tumor samples, the parent [C-11]DACA and [C-11]SN 23935 represented between 0.3 and 1.5%ID /g, whereas [C-11]SN 23490 and [C-11]SN 23719 represented between 1.5 and 2 .8%ID/g. In conclusion, by using a strategy with C-11-labeling, we have determined t he tissue distribution and metabolic stability of novel tricyclic carboxami des with the view of selecting analogues with potentially better in vivo ac tivity against solid tumors. SN 23490 and SN 23719 had more favorable distr ibution and metabolic stability compared with DACA and SN 23935 and may war rant further development, The radiolabeling strategy used allows ex vivo an d in vivo evaluation of promising anticancer agents in animals and offers t he potential of rapid translation to studies in humans using PET.