lixazinone and Heart-Failure

There are now several new CT drugs undergoing clinical study for the treatment of CHF. While most of these new agents are like digitalis in that they possess significant inotropic activity, they differ from digitalis in that they tend to also have pronounced vasodilator properties. Such compounds, because they are not pure CT agents, should not be regarded as true digitalis replacements. Despite the increased understanding about the cAMP cascade and about cardiac muscle contraction in general, the long search for a specific digitalis replacement cannot be regarded as having been accomplished. It should also be noted that the initial clinical assessment for any of these new compounds will be complicated by the fact that they are tested in only the latter stages of CHF where prognosis is very poor and the possibility of showing significant improvement in mortality is extremely difficult. It will be unfortunate if the potential for positive inotropic agents (pure CT drugs) is compromised by clinical studies that employ mixed inotropic-vasodilator compounds in extremely sick patient populations. The continued use of the digitalis glycosides for such a long period of history implies that at least some subpopulation of CHF patients should derive benefit from therapy with an appropriately selective CT drug. Neither the appropriate drug nor the appropriate specific patient population, both of which are needed to properly address the eventual role of inotropic therapy in CHF, have, as yet, been identified.

Topics: 3',5'-Cyclic-AMP Phosphodiesterases; Cardiotonic Agents; Cyclic AMP; Digitalis Glycosides; Heart Failure; Humans; Myocardial Contraction; Receptors, Adrenergic, beta; Structure-Activity Relationship

Hybridization of structural elements of 1,2,3,5-tetrahydro-2-oxoimidazo[2,1-b]quinazoline ring system common to the cyclic AMP (cAMP) phosphodiesterase (PDE) inhibitors lixazinone (RS-82856, 1) and anagrelide (3) with complementary features of other PDE inhibitor cardiotonic agents prompted the design and synthesis of the title compounds 7a-d, 11, 12, and 13a,b. The necessary features of these compounds were determined within the framework of the proposed active-site models for the high affinity form of cAMP PDE inhibited by cGMP (type IV). Evaluation of these targets, both in vitro as inhibitors of platelet or cardiac type IV PDE or in vivo as inotropic agents in the pentobarbital-anesthetized dog model of congestive heart failure, showed that these structure possessed negligibly enhanced activities over the parent heterocyclic system, and remained significantly inferior to 1 in all respects. This difference is ascribed to the absence of the N-cyclohexyl-N-methylbutyramidyl-4-oxy side chain of 1. The proposal that the acidic lactam-type functionality, common to the type IV PDE inhibitor inotropic agents such as 4-6 and 8-10, mimics the polarizable cyclic phosphate moiety of cAMP suggested that the side chain of 1 may function as an effective surrogate for selected characteristics of the adenine portion of cAMP. However, the results of this study show that incorporation of adenine-like hydrogen-bonding functionalities common to other type IV PDE inhibitors into the 1,2,3,5-tetrahydro-2-oxoimidazo[2,1-b]quinazoline system did not enhance activity to the levels observed for 1 and analogues. These observations, coupled with the kinetic pattern of inhibition of type IV PDE observed for 1 and analogues, suggest that access to a secondary, lipophilic-tolerant binding site, possibly coincident with the adenine binding domain, and adjacent to the catalytic ribose-phosphate binding site of platelet and cardiac type IV PDE, is responsible for the increased potency of these compounds.

Topics: 3',5'-Cyclic-AMP Phosphodiesterases; Animals; Blood Platelets; Cardiotonic Agents; Dogs; Drug Evaluation; Heart Failure; Humans; Myocardium; Platelet Aggregation; Quinazolines; Structure-Activity Relationship