The combined 5-HT transporter and 5-HT1B receptor inhibitor LY393558 prevents pulmonary hypertension in two murine models of pulmonary hypertension

Ian Morecroft1, Louisa Pang1, Margaret Nilsen1, Lynn Loughlin1, Anthony Harmar2, Margaret MacLean1

1Glasgow University, Glasgow, UK, 2Edinburgh University, Edinburgh, UK

Pulmonary arterial hypertension (PAH) can be associated with increased activity/expression of the 5-HT transporter (SERT) (Eddahibi et al., 2001). Mice over-expressing the SERT (SERT+ mice) have established PAH (MacLean et al.,2004). The 5-HT1B receptor mediates constriction in human PAs (Morecroft et al., 1999) and proliferation of PA smooth muscle cells and is over-expressed in PAs of patients with PAH (Launay et al., 2002). There is synergy between the 5-HT1B receptor and SERT (Lawrie et al., 2005; Morecroft et al., 2005).

Here we compared the in vivo effects of 1-[2-[4-(6-fluoro-1H-indol-3-yl)-3,6-dihydro-1(2H)-pyridinyl]ethyl]-3-isopropyl-6-(methylsulphonyl)-3,4-dihydro-1H-2,1,3-benzothiadiazine-2,2-dioxide) (LY393558 (combined 5-HT transporter and 5-HT1B receptor inhibitor)) with the SERT inhibitor citalopram in SERT+ mice and their wildtype (WT) controls (C57BL/6, 5-6 months, 25-35g), under hypoxic conditions. Female SERT+ and WT mice received LY393558 (30mgkg-1day-1) or citalopram (20mgkg-1day-1) orally for 16 days. 2 days into the dosing regimen, some mice were subjected to 14 days of hypoxia (10% O2). Control groups received vehicle (1% carboxymethylcellulose). Under isoflurane anaesthesia, systolic right ventricular pressure (sRVP) was obtained by transdiaphragmatic right ventricular cannulation. Right ventricular (RV) / left ventricle+septum (LV+S) ratio was determined as described previously (MacLean et al., 2004). Statistical comparisons were made by one-way ANOVA with a Tukeys multiple comparison test. Data are expressed as mean ± s.e.mean.

Hypoxia, increased right ventricular hypertrophy (RV/LV+S ratio =0.323 ± 0.012 versus 0.244 ± 0.006; P<0.001; n=8-10), and sRVP in WT (sRVP =35.6 ± 3.1mmHg versus 18.0 ± 0.8mmHg; P<0.01; n=8-10) and SERT+ (RV/LV+S ratio = 0.445±0.012 versus normoxia: 0.310±0.014, n=8-10; P<0.001 (sRVP=39.4 ± 2.0mmHg; n=8 versus normoxic SERT+, P<0.001)) mice. These effects of hypoxia were greatly attenuated in the LY393558 treated mice (SERT+ sRVP= 23.2 ±1.7mmHg, n=8, P<0.001; WT sRVP= 21.1 ± 1.4mmHg, n=9, P<0.001(SERT+ RV/LV+S ratio =0.315±0.008, n=8, P<0.001; WT RV/LV+S ratio =0.294±0.008, n=10, P<0.05)). Under hypoxia, citalopram treatment attenuated the indices of PAH to a significantly lesser extent (SERT+ sRVP= 31.4 ±2.3mmHg, n=8, P<0.01; WT sRVP= 27.0 ± 1.2mmHg, n=9, P<0.01(SERT+ RV/LV+S ratio =0.385±0.020, n=10, P<0.01; WT RV/LV+S ratio =0.348±0.014, n=10)). Normoxic SERT+ mice have elevated sRVP (28.4 ± 2.2mmHg,n=9, P<0.001 vs WT), treatment with LY393558 (17.9 ± 1.5mmHg versus normoxic SERT+ mice, n=9, P<0.01) but not citalopram (sRVP = 29.2± 2.0 mmHg versus normoxic SERT+ mice, n=9)) markedly reversed this elevation to levels commensurate with WT mice. Compounds with the dual action of SERT inhibition and 5-HT1B receptor antagonism may be more beneficial than SERT inhibition alone in the treatment of PAH

Eddahibi et al., (2001) Journal of Clinical Investigation. 108: 1141-1150.

Launay et al., (2002) Nature Medicine. 8:1129-1135.

Lawrie et al., (2005) Circulation Research. 97: 227-235.

MacLean, M.R et al., (2004) Circulation. 109:2150-5.

Morecroft, I et al., (1999) British Journal Pharmacology. 128:730-734.

Morecroft, I et al., (2005) JPET. 313:539-45