Inotropic and Chronotropic Effects of Phosphodiesterase Inhibitors in The

Proceedings of the British Pharmacological Society at http://www.pA2online.org/Vol2Issue2abst088P.html

088P INOTROPIC AND CHRONOTROPIC EFFECTS OF PHOSPHODIESTERASE INHIBITORS (PDEI) IN THE CANINE ISOLATED SPONTANEOUSLY BEATING RIGHT ATRIA Alexandra Cook, Sidath Katugampola & Carolyn Napier. Candidate Research Group, Pfizer, Sandwich, Kent CT13 0EY

Phosphodiesterase inhibitors (PDEi) have clinical utility in a number of disease areas, however a potential safety concern for some inhibitors is their effect on cardiac function. In this study we compared the ability of a range of PDEi to alter force of contraction (FOC) and heart rate (HR) in the canine isolated spontaneously beating right atria (RA) preparation. RA tissues were removed from beagle dogs of either sex (10-20 kg), euthanised under anesthesia induced by pentobarbitone (0.5 mg/kg), as used in several pharmacological studies (protocols were ethically reviewed by Pfizer ethics committee). The RA was dissected longitudinally and the half containing the sino-atrial node mounted on frog heart clips and placed in an organ bath filled with oxygenated (95%O2, 5%CO2) 37ºC Kreb’s with a resting tension of 2.5 g. Following a 60 min equilibration period a 1 µM isoprenaline challenge was given to obtain the maximal isometric FOC and HR response for the tissue. The tissue was then washed so that responses returned to baseline and a cumulative concentration response curve (CRC) to PDEi constructed (10-9-10-4M). CRC’s were fitted (ordinary least squares, unconstrained) using an in-house non-linear curve-fitting program. Values for FOC and HR are expressed as a percentage of the maximal response (Emax) to the isoprenaline challenge. Data are mean ± s.e.mean, n values refer to the number of dogs. Table 1: The effect of isoprenaline and PDEi on FOC and HR. Compound Mechanism FOC HR n pEC50 Emax % pEC50 Emax % Isoprenaline β agonist 7.53±0.17 100±4 7.59±0.42 100±4 8 Milrinone PDE3i 5.86±0.06 37±5.9 5.93±0.28 31±4.2 4 Cilostazol PDE3i 5.60±0.31 <10 5.75±0.17 23±6.5 3+ Cilostamide PDE3i 6.10±0.39 20±10.6 6.33±0.11 50±5.0 3 Amrinone PDE3i N/C <10 N/C <10 3 Zardaverine PDE3/4i 5.38±0.22 30±4.8 5.45±0.02 28±4.2 3+ Rolipram PDE4i No effects up to 100 µM 3 ICI63197 PDE4i No effects up to 100µM 3 Ariflo PDE4i No effects up to 100 µM 3 Zaprinast PDE5/6i No effect up to 100 µM 3 IBMX Non-selective N/C 32±4.4 N/C 77±11.1 3

Mean EC50 and Emax values (± s.e.mean). N/C = curve not complete at 100 µM, therefore, EC50 values could not be calculated, + values derived from 2 tissues.

Table 1 shows that of the PDE3i tested, milrinone, cilostazol and cilostamide produced concentration related effects on FOC and HR with EC50 values in good agreement with their potency against PDE3 isolated enzyme (Shakur et al., 2002). The lack of activity of amrinone shown here is consistent with its weak activity at PDE3 (Adawia et al., 1986). Interestingly, both cilostamide and IBMX produced greater effects on HR compared to FOC, similar to results obtained in the rabbit (Endoh et al., 1980). Rolipram, ICI63197 and zaprinast were devoid of activity. The PDE4i, ariflo, used in the treatment of chronic obstructive pulmonary disease was also devoid of activity. The potency and efficacy of zardaverine is similar to milrinone and the effects seen with zardaverine may be attributed to PDE3 inhibition. In conclusion, these results suggest that the canine isolated spontaneously beating RA is a suitable preparation for the detection of pharmacological differences in both potency and efficacy of subtype selective PDEi.

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