Modification of the hydantoin ring in the previously described lead compound 2a has led to the discovery of compound 12a, tadalafil, a highly potent and highly selective PDE5 inhibitor. The replacement of the hydantoin in compound 2a by a piperazinedione ring led to compound cis-11a which showed similar PDE5 inhibitory potency. Introduction of a 3,4-methylenedioxy substitution on the phenyl ring in position 6 led to a potent PDE5 inhibitor cis-11c with increased cellular potency. Optimization of the chain on the piperazinedione ring led to the identification of the racemic cis-N-methyl derivative 11i. High diastereospecificity for PDE5 inhibition was observed in the piperazinedione series with the cis-(6R,12aR) enantiomer displaying the highest PDE5 inhibitory activity. The piperazinedione 12a, tadalafil (GF196960), has been identified as a highly potent PDE5 inhibitor (IC(50) = 5 nM) with high selectivity for PDE5 vs PDE1-4 and PDE6. Compound 12a displays 85-fold greater selectivity vs PDE6 than sildenafil 1. 12a showed profound and long-lasting blood pressure lowering activity (30 mmHg/>7 h) in the spontaneously hypertensive rat model after oral administration (5 mg/kg).

The peroxisome proliferator activated receptors PPARalpha, PPARgamma, and PPARdelta are ligand-activated transcription factors that play a key role in lipid homeostasis. The fibrates raise circulating levels of high-density lipoprotein cholesterol and lower levels of triglycerides in part through their activity as PPARalpha agonists; however, the low potency and restricted selectivity of the fibrates may limit their efficacy, and it would be desirable to develop more potent and selective PPARalpha agonists. Modification of the selective PPARdelta agonist 1 (GW501516) so as to incorporate the 2-aryl-2-methylpropionic acid group of the fibrates led to a marked shift in potency and selectivity toward PPARalpha agonism. Optimization of the series gave 25a, which shows EC50 = 4 nM on PPARalpha and at least 500-fold selectivity versus PPARdelta and PPARgamma. Compound 25a (GW590735) has been progressed to clinical trials for the treatment of diseases of lipid imbalance.

A model for the pharmacophore of GABA-uptake inhibitors was established using published structure-activity data and molecular modeling. The model accounted for the activities of different classes of GABA-uptake inhibitors. Analogues of guvacine substituted at position 6 were synthesized in order to confirm the model. 6-(3,3-Di-phenylpropyl)guvacine (30f), which fit well with the pharmacophore, had an in vitro IC50 of 0.1 microM. This value is as good as those of the best GABA-uptake inhibitors known today.