Michael J. Ashton

Suppression of eosinophil function by RP 73401, a potent and selective inhibitor of cyclic AMP‐specific phosphodiesterase: comparison with rolipram

1 We have investigated the inhibitory potency of RP 73401, a novel, highly selective and potent inhibitor of cyclic AMP‐specific phosphodiesterase (PDE IV), against partially‐purified PDE isoenzymes from smooth muscle and the particulate PDE IV from guinea‐pig eosinophils. The inhibitory effects of RP 73401 on the generation of superoxide (.O2−), major basic protein (MBP) and eosinophil cationic protein (ECP) from guinea‐pig eosinophils have also been studied. 2 RP 73401 potently inhibited partially‐purified cyclic AMP‐specific phosphodiesterase (PDE IV) from pig aortic smooth muscle (IC50 = 1.2 nM); it was similarly potent against the particulate PDE IV from guinea‐pig peritoneal eosinophils (IC50 = 0.7 nM). It displayed at least a 19000 fold selectivity for PDE IV compared to its potencies against other PDE isoenzymes. Rolipram was approximately 2600 fold less potent than RP 73401 against pig aortic smooth muscle PDE IV (IC50 = 3162 nM) and about 250 times less potent against eosinophil PDE IV (IC50 = 186 nM). 3 Solubilization of the eosinophil particulate PDE IV increased the potency of rolipram 10 fold but did not markedly affect the potency of RP 73401. A similar (10 fold) increase in the PDE IV inhibitory potency of rolipram, but not RP 73401, was observed when eosinophil membranes were exposed to vanadate/glutathione complex (V/GSH). 4 Reverse transcription polymerase chain reaction (RT‐PCR), using primer pairs designed against specific sequences in four distinct rat PDE IV subtype cDNA clones (PDE IVA‐D), showed only mRNA for PDE IVD in guinea‐pig eosinophils. PDE IVD was also the predominant subtype expressed in pig aortic smooth muscle cells. 5 RP 73401 (Kiapp = 0.4 nM) was 4 fold more potent than (±)‐rolipram (Kiapp =1.7 nM) in displacing [3H]‐(±)‐rolipram from guinea‐pig brain membranes. 6 In intact eosinophils, RP 73401 potentiated isoprenaline‐induced cyclic AMP accumulation (EC50 = 79 nM). RP 73401 also inhibited leukotriene B4‐induced generation of O2− (IC50 = 25 nM), and the release of major basic protein (IC50‐ 115 nM) and eosinophil cationic protein (IC50 = 7nM). Rolipram was 3–14 times less potent than RP 73401. 7 Thus RP 73401 is a very potent and selective PDE IV inhibitor which suppresses eosinophil function suggesting that it may be a useful agent for the treatment of inflammatory diseases such as asthma. The greatly different inhibitory potencies of rolipram against PDE IV from smooth muscle and eosinophils (in contrast to the invariable effects of RP 73401) are unlikely to be attributable to diverse PDE IV subtypes but suggest distinct interactions of the two inhibitors with the enzyme.

New perspectives in lead generation II: Evaluating molecular diversity

The identification of biological targets was discussed in part I. The next major hurdle is to discover a compound that selectively interacts with the target. In the second part of a two-part article, the authors focus on the problem of how to define and measure diversity within and between compound libraries in order to optimize molecular diversity for lead generation.

RP 73163: A bioavailable alkylsulphinyl-diphenylimidazole ACAT inhibitor☆

RP 73163 5 the major metabolite of the ACAT inhibitor RP 76076 3 retains ACAT activity. This alkylsulphinyl-4,5-diphenyl-1H-imidazole has higher systemic bioavailability than the parent thioether, with plasma levels of parent compound in certain species exceeding the IC50 required for inhibition of hepatic, intestinal, artery, adrenal and macrophage ACAT for up to twelve hours after oral dosing.

RP 70676: A potent systematically available inhibitor of acyl-CoA:cholesterol O-acyl transferase (ACAT)

Abstract RP 70676 (3d) is a potent inhibitor of ACAT. It is an effective hypocholesterolaemic agent in the cholesterol-fed rabbit, and reduces the accumulation of both cholesterol and cholesterol ester in rabbit aorta and thoracic artery. The compound is readily bioavailable in rabbits with significant levels of parent compound present in plasma up to 6 hours after an oral dose.

New perspectives in lead generation I: Discovery of biological targets

Future success for most pharmaceutical companies will depend on innovation leading to the discovery of new drugs that provide clear medical benefit to patients when compared to current therapies. There are three key steps in the discovery process; discovery of relevant biological targets, generation of ‘lead’ compounds and the optimization of leads to give potent, efficacious and safe drugs. There are, increasingly, many ‘biological’ approaches to treatment, such as gene therapy or antisense therapy. This article is focused solely on key aspects of lead generation for compounds of low molecular weight 1 . Part 1 focuses on aspects of biological target identification. Part 2, to be published in the February issue of Drug Discovery Today , will address lead generation, with special emphasis on the measurement of diversity within and between compound libraries.