Comparative myocardial imaging in the baboon with 123I-labelled ortho and para isomers of 15-(iodophenyl)pentadecanoic acid (IPPA).

The use of various animal models for researching radioiodinated fatty acid cardiac metabolism has been proven to be risky. This fact is here confirmed in the higher primate. Six baboons were injected on different occasions with 123I-labelled para and ortho isomers of 15-(iodophenyl) pentadecanoic acid (123I-pPPA and 123I-oPPA, respectively) for dynamic scintigraphy, followed by a Na123I injection at 30 min for blood background correction. Times for maximum myocardial tracer uptake (Tmax), and half-times of clearance (two phases T1/2(1) and T1/2(2)) from biexponential fitting of the time-activity curves proved statistically not different for pPPA and oPPA (P > 0.05). A repeat study at a later stage on the same baboons included pretreatment (2 h pretracer) with a carnitine palmitoyl transferase I inhibitor (Extomoxir; 10 mg kg-1) to block myocardial beta-oxidation. Time-activity curves with Tmax, T1/2(1) and T1/2(2) followed as above and were compared to the initial uninhibited values. Blocking of the carnitine shuttle was demonstrated by significantly increased retention of 123I-pPPA and 123I-oPPA. Thus both the tracers are beta-oxidized in the baboon myocardium, which is different in rat and human studies. In rats both isomers wash out from the myocardium, but 123I-oPPA is not metabolized. In humans the 123I-pPPA is beta-oxidized as with rats, but the 123I-oPPA is retained in the lipid pool of the myocardium. Simultaneous multiple consecutive gated blood pool studies (six of 1 min each) provided encouraging perfusion-related, background corrected images, and corresponding uptake information.