Pharmacology zyxwvutsrqpon & Toxicology zyxwvutsrqpo 1989,65, 214-216. zyxwvutsr Brain and Plasma Kinetics of the Opioid zy "C- Hydromorphone zyx in Two Macaque Species Per Hartvig', Anders Neil*, Lars Terenius*, Gunnar Anton?, Annike Rimland3, Joban Uh3 and Ben@ Llngstr6m3 'Hospital Pharmacy, University Hospital, S-75185 Uppsala, ,Department of Pharmaceutical Pharmacology and )Organic Chemistry, University of Uppsala, Uppsala, Sweden (Received February 20, 1989; Accepted April 19, 1989) Abstract: The regional brain kinetics of the opioid receptor agonist, hydromorphone, labelled with "C, were studied with positron emission tomography in two Macaque species, Rhesus and Cynomolgus monkeys. The plasma kinetics of "C- hydromorphone were simultaneously determined. Rapid plasma elimination was obvious in both monkey species. Brain radioactivity was generally low in both, but 1.4 times higher in the Rhesus than in the Cynomolgus monkey brain. This observation might explain the more rapid development of tolerance in the Rhesus monkey. Of two Macaque species, the Rhesus monkey (Macaca mu- latta) acquired tolerance more rapidly than the Cynomolgus monkey (Macaca fascicularis) to low intravenous doses of the opioid receptor agonist, hydromorphone (Ternes et al. 1983). After prolonged exposure to hydromorphone, toler- ance was also observed in the Cynomolgus monkey. Apart from the pharmacodynamic differences between the species, differences in pharmacokinetics and brain distribution of hydromorphone may explain the difference in rate of deve- lopment of tolerance between the two Macaca species. Positron emission tomography (PET) is a non-invasive tracer technique, which measures the kinetics of a radioac- tive tracer in a physiological process in the tissues of the living object (Hartvig et al. 1984). An estimation of the concentration of drug in the effect organ can be obtained by combination of PET with blood or plasma analysis. In the present study, the brain and plasma kinetics of "C- hydromorphone were measured in the Rhesus and the Cy- nomolgus monkey in order to assess species differences in hydromorphone distribution. Materials and Methods Radionuclide production and radiopharmaceutical syntheses. The radionuclide, "C, was produced in the Tandem Accelerator of the The Svedberg Laboratory, Uppsala university, and obtained as "C- COP The "C-CO, was used in the synthesis of "C-hydromorphone as described (Rimland et al. 1987). Analysis of identity, chemical and radiochemical purity was done using HPLC. A dose of 50-150 MBq of "C-hydromorphone in sterile phosphate buffer pH= 7.4 was given intravenously to the monkey. The amount of hydromor- phone given vaned between 10 to 20 pg. Animals. Three Rhesus monkeys, Macaca mulatta, weighing 6-8 kg from the Primate Laboratory of Reproductive Research, University of Uppsala, and four Cynomolgus monkeys, Macaca fascicularis, (n =4) weighing 3-4 kg from the Department of Medical Pharma- cology, University of Uppsala, or from the State Bacteriological Laboratory, Solna, were used. After an overnight fast, they were anaesthetized with ketamine, 50-100 mg, which was supplemented by more ketamine and diazepam intravenously when required. Forty min. after injection, 0.02 and 0.01 mg of naloxone was given to the Rhesus and Cynomolgusmonkeys, respectively. Studies were also performed after pretreatment of the monkeys with the same naloxone dose. Positron emission tomography. Immediately after the administration of "C-hydromorphone, imaging of the head of the monkey was commenced using a PC 384-3B positron emission tomograph (Scan- ditronix AB, Uppsala, Sweden). The monkey was placed in a fixed position in a cradle so that the lowest tomographic transsection of the head included cerebellum. Emission images were collected over periods of 12, 40 and 200 sec. at predetermined intervals. Data collection and reconstruction of images were done as described by Eriksson el al. (1982). The distribution and kinetics of L'C-hydromorphone-derived radioactivity were analyzed in the following regions: areas corre- sponding to the frontal cortex, midbrain, cerebellum, total brain and extracranial soft tissue. The radioactivity measured was cor- rected for physical decay from the time of administration of the radioactive dose. Radioactivity in the various regions was calculated as uptake from the corrected radioactivity per cm3 and divided by the administered dose given per g body-weight of the monkey. Furthermore, blood borne radioactivity in the brain was subtracted assuming a blood volume of 6% of the brain volume (Hartvig et al. 1984). A calculated uptake of 1.0 corresponded to an even distribution of radioactivity in the body. The values given are mean values. The half-life of total blood radioactivity was calculated from a plot in the lin-log scale of uptake versus time. Blood and plasma analysis. Blood samples were collected from a venous catheter in the leg opposite to where the dose was given. Samples were taken at 0, 2, zyxw 5, 10, 20, 30, 45 and 60 min. after injection and measured for total radioactivity in a well counter. Plasma was separated from the samples collected at 10 and 30 min. and 50 p1 was injected directly into a Hewlett Packard 1090 liquid chromatograph with variable wavelength detection in series with a p-flow detector. The chromatographic system contained a precol- umn (50 x 4.6 mm) filled with Alltech C 18 and an analytical column (200 x 4.6 mm) also filled with Alltech C 18. The mobile phase was a linear gradient of 0.1 M ammonium formiate buffer pH = 3.5 and methanol. Fractions of the eluted mobile phase were collected be- fore, after and corresponding to intact "C-hydromorphone. The fractions were measured for radioactivity in the well counter. Results Positron emission tomography "C-Hydromorphone-derived radioactivity corrected for blood borne radioactivity reached a plateau level in the